JP2024014734A - Gas sensor element and gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor element which features a form of diffusion of No_x reaching a measurement electrode, which has been changed from molecular diffusion form to a form of diffusion involving repetitive collision with a wall surface of a sufficiently narrow flow channel.

SOLUTION: A gas sensor element according to an aspect of the present invention comprises a porous diffusion layer occupying 70% or more of a surface of a flow channel of a measurement target gas perpendicular to a flow direction of the measurement target gas, and having a porosity in a range of 5% to 25%, inclusive, the porous diffusion layer being disposed at a distance of 0.15 mm or less from a measurement target on an upstream side of the measurement electrode.

SELECTED DRAWING: Figure 3

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to a gas sensor element and a gas sensor.

Conventionally, for a gas sensor element used to measure the concentration of a specific gas component contained in a gas to be measured, a predetermined diffusion resistance is imparted to the gas to be measured introduced into an internal space provided in the gas sensor element. Various attempts to do so are known. For example, Patent Document 1 listed below discloses a gas sensor element including a diffusion rate controlling section that imparts a predetermined diffusion resistance to the gas to be measured introduced into the internal space.

Japanese Patent Application Publication No. 2011-102793

The inventors of the present invention have discovered that the conventional gas sensor element including the above-mentioned diffusion-limiting section has the following problems. That is, in a gasoline vehicle, the concentration of H ₂ O in the exhaust gas is higher than that in a diesel vehicle. Further, hydrogen engine vehicles are expected to be used under highly lean conditions in consideration of the environment, and the H ₂ O concentration in the exhaust gas is also expected to be high. Furthermore, H ₂ O has a smaller molecular weight than NO _x and O ₂ . The inventors of the present invention have found that the following problem occurs in such a high H ₂ O concentration environment.

FIG. 15 is a diagram showing an example of molecular diffusion in which a certain molecule diffuses due to collision with another molecule. The inventors of the present invention considered that the following phenomenon occurs in a region where molecular diffusion is dominant as illustrated in FIG. 15. In other words, as illustrated in Fig. 15, in molecular diffusion, the diffusion of a certain molecule progresses when it collides with another molecule, so the diffusion coefficient changes depending on the molecule that a certain molecule collides with. In other words, the diffusion coefficient changes depending on the gas composition of the gas to be measured. Therefore, if H ₂ O with a small molecular weight is present in the gas to be measured, NO _x and O ₂ will easily diffuse between the H ₂ O, making it difficult to measure the concentration of a specific gas component contained in the gas to be measured. It is thought that the amount of NO _x and O ₂ gas reaching the measurement electrode increases. As a result, the inventors thought that the NO _x output may fluctuate depending on the H ₂ O concentration (for example, when the H ₂ O concentration increases), and the measurement electrode may deteriorate more easily. Ta. The inventors of the present invention have confirmed through experiments that when the H ₂ O concentration is high, the NO _x output fluctuates more easily and the measurement electrode deteriorates more quickly than when the H 2 O concentration is low.

FIG. 16 is a diagram showing an example of Knudsen diffusion, which is a diffusion form different from molecular diffusion. As illustrated in Fig. 16, in Knudsen diffusion, the diffusion of a certain molecule progresses by colliding with the pore wall (wall surface of the flow path), and the pore diameter of the wall surface is determined, for example, during firing, so that the gas to be measured increases. Even if the gas composition changes, the diffusion coefficient remains the same. Therefore, the inventors of the present invention have found that the following method is useful as a method for solving the above-mentioned problem considered to be caused by molecular diffusion of NO _x under high H ₂ O concentration. That is, a method of changing the diffusion form of NO _x reaching the measurement electrode from molecular diffusion to a form in which it diffuses while repeatedly colliding with the wall of a sufficiently narrow channel, such as the Knudsen diffusion illustrated in FIG. 16. was found to be useful.

One aspect of the present invention has been made in view of these circumstances, and its purpose is to change the diffusion form of NO _x reaching the measurement electrode from molecular diffusion to a sufficiently narrow channel wall surface. It is an object of the present invention to provide a gas sensor element or the like that is changed to a form that diffuses while repeating collisions.

The present invention adopts the following configuration in order to solve the above-mentioned problems.

The gas sensor element according to the first aspect includes an element base, into which the gas to be measured is introduced into the internal space through a gas inlet opened on the surface, and at least a surface of the element base where the gas inlet is opened. A tip protection layer, a measurement electrode provided in the internal space, and provided at a position upstream of the measurement electrode in the flow direction of the gas to be measured so that the distance to the measurement electrode is 0.15 mm or less. and a porous diffusion layer. The porous diffusion layer has a porosity of 5% or more and 25% or less, and has a porosity lower than that of the tip protection layer. Further, the porous diffusion layer has a surface (area of the surface) perpendicular to the flow direction of the gas to be measured, and a surface (area of the surface) perpendicular to the flow direction of the gas to be measured in the flow path of the gas to be measured. occupies more than 70% of the area (area). In addition, when the porous diffusion layer includes a plurality of surfaces (layers) having different porosities, the average porosity of the porous diffusion layer may be 5% or more and 25% or less; The average porosity of the porous diffusion layer may be lower than the porosity of the tip protection layer.

In this configuration, the porous diffusion layer, which has a porosity of 5% or more and 25% or less and has a lower porosity than the tip protective layer, is located upstream of the measurement electrode in the direction in which the gas to be measured flows. , is provided at a position where the distance to the measurement electrode is 0.15 mm or less. A surface (area of the surface) of the porous diffusion layer perpendicular to the flow direction of the gas to be measured is a surface (area of the surface) perpendicular to the flow direction of the gas to be measured of the flow path of the gas to be measured. occupies more than 70% of the total area (area). That is, the porous diffusion layer is provided upstream of the measurement electrode at a position where the distance to the measurement electrode is 0.15 mm or less, and the porous diffusion layer is provided in a predetermined position of the flow path in the direction in which the gas to be measured flows. It occupies (occupies) the range (more than 70%).

The porous diffusion layer allows the diffusion pattern around the measurement electrode, that is, the diffusion pattern of the gas to be measured to reach the measurement electrode through the flow path, to a sufficiently narrow flow rate such as Knudsen diffusion. It can be configured to diffuse while repeatedly colliding with the road wall. Therefore, in the gas sensor element, even if H ₂ O gas is present in the gas to be measured, the porous diffusion layer reduces the influence of H ₂ O gas on NO _x gas (and O ₂ gas). can be made smaller. Specifically, the gas sensor element suppresses fluctuations in NO _{x output and deterioration of the measurement electrode, which are considered to be caused by molecular diffusion of NO x under} high H ₂ O concentration, by the porous diffusion layer. I can do it.

Here, if the porous diffusion layer having a large diffusion resistance is provided around the measurement electrode, the porous diffusion layer may become clogged with poisonous substances or the like. Therefore, the gas sensor element includes the tip protection layer that covers at least the surface of the element base where the gas inlet is open. Therefore, the gas sensor element can trap poisonous substances and the like by the tip protection layer, that is, can capture (capture) poisonous substances and the like by the tip protection layer.

Particularly, in the gas sensor element, the porosity of the tip protection layer is greater than the porosity of the porous diffusion layer that closes a predetermined range of the flow path at a position where the distance to the measurement electrode is 0.15 mm or less. (expensive). In the gas sensor element, by making the porosity of the tip protection layer larger than the porosity of the porous diffusion layer, "the tip protection layer is clogged with poisonous substances and the NO _x output is reduced." It is possible to avoid situations such as "doing".

The gas sensor element according to the second aspect is the gas sensor element according to the first aspect, and may further include a diffusion-limiting section that imparts a predetermined diffusion resistance to the gas to be measured in the internal space. For example, the diffusion-limiting portion has a porosity lower than that of the porous diffusion layer, and is provided upstream of the measurement electrode in the direction in which the gas to be measured flows. The flow path has at least one surface defined by the diffusion-limiting section.

In this configuration, the gas sensor element is provided upstream of the measurement electrode, and further includes the diffusion-limiting portion having a porosity lower (smaller) than the porosity of the porous diffusion layer. The flow path has at least one surface defined (divided) by the diffusion-limiting portion. In other words, at least one surface of the flow path of the gas to be measured that reaches the measurement electrode is defined by the diffusion-limiting portion that is denser (has a lower porosity) than the porous diffusion layer. Therefore, the gas sensor element guides the gas to be measured to the measurement electrode by using the flow path in which at least one surface is defined by the diffusion-limiting portion that is denser than the porous diffusion layer. I can do it. In the gas sensor element, for example, there is a possibility that a situation may occur in which the gas to be measured leaks out in the middle of the flow path and reaches the measurement electrode without having its diffusion form changed by the porous diffusion layer. can be suppressed. Therefore, the gas sensor element can control the diffusion mode of the gas to be measured to the measurement electrode by using the porous diffusion layer that closes a predetermined range of the flow path through which the gas to be measured is guided, such as Knudsen diffusion. It is possible to get closer to the diffusion form.

In the gas sensor element according to the third aspect, in the gas sensor element according to the second aspect, the porous diffusion layer may be in contact with the diffusion-limiting portion and a surface defining the internal space.

In this configuration, the porous diffusion layer is in contact with the diffusion-limiting section and a surface defining the internal space, that is, between the porous diffusion layer and the diffusion-limiting section and between the porous diffusion layer and the surface defining the internal space. There is no space (gap) between the quality diffusion layer and the surface defining the internal space. The gas sensor element achieves the following effects by eliminating the gap between the porous diffusion layer and the diffusion control section and the gap between the porous diffusion layer and the surface defining the internal space. be able to. That is, in the gas sensor element, the gas to be measured is detected in at least one of the gap between the porous diffusion layer and the diffusion control section, and the gap between the porous diffusion layer and a surface defining the internal space. can be prevented from reaching the measurement electrode. In other words, in the gas sensor element, for example, a situation occurs in which the gas to be measured leaks in the middle of the flow path and reaches the measurement electrode without having its diffusion form changed by the porous diffusion layer. possibility can be reduced. Therefore, the gas sensor element can control the diffusion mode of the gas to be measured to the measurement electrode by using the porous diffusion layer that closes a predetermined range of the flow path through which the gas to be measured is guided, such as Knudsen diffusion. It is possible to get closer to the diffusion form.

In the gas sensor element according to the fourth aspect, in the gas sensor element according to the second or third aspect, at least two surfaces of the flow path may be defined by the diffusion controlling section.

In this configuration, at least two surfaces of the flow path are defined by the diffusion-limiting section, and as described above, the diffusion-limiting section is denser than the porous diffusion layer (the porosity is smaller than the porous diffusion layer). low). In other words, the gas sensor element defines at least two surfaces of the flow path that guides the gas to be measured to the measurement electrode by using the diffusion-limiting portion that is denser than the porous diffusion layer. do). Therefore, the gas sensor element guides the gas to be measured to the measurement electrode by using the flow path in which at least two surfaces are defined by the diffusion-limiting portion that is denser than the porous diffusion layer. I can do it. In the gas sensor element, for example, there is a possibility that a situation may occur in which the gas to be measured leaks out in the middle of the flow path and reaches the measurement electrode without having its diffusion form changed by the porous diffusion layer. can be further suppressed. Therefore, the gas sensor element can control the diffusion mode of the gas to be measured to the measurement electrode by using the porous diffusion layer that closes a predetermined range of the flow path through which the gas to be measured is guided, such as Knudsen diffusion. It is possible to get closer to the diffusion form.

In the gas sensor element according to a fifth aspect, in the gas sensor element according to any one of the first to fourth aspects, the distance from the outermost surface of the tip protective layer to the gas inlet is 0.2 mm or more. It's okay.

In this configuration, in the gas sensor element, the distance from the outermost surface of the tip protective layer to the gas inlet is 0.2 mm or more. By making the distance from the outermost surface of the tip protection layer to the gas inlet port sufficiently long (specifically, 0.2 mm or more), that is, by making the thickness of the tip protection layer sufficiently thick. The gas sensor element can achieve the following effects. That is, even in a harsh environment where there are many poisonous substances, the gas sensor element can reliably trap (capture) such poisonous substances in the tip protective layer and prevent poisoning near the gas inlet. By suppressing clogging caused by substances, etc., it is possible to avoid a decrease in NO _x sensitivity.

A gas sensor element according to a sixth aspect is a gas sensor element according to any one of the first to fifth aspects, wherein the tip protective layer is arranged at least on the surface of the element base where the gas inlet is opened. an inner tip protection layer in contact with the outer tip protection layer, and an outer tip protection layer constituting the outermost surface of the tip protection layer, the inner tip protection layer has a porosity larger than the outer tip protection layer, and the inner tip protection layer has a porosity larger than that of the outer tip protection layer; The thickness of the tip protection layer may be 30% or more and 90% or less of the thickness of the tip protection layer.

In this configuration, the tip protection layer includes at least the inner tip protection layer that is in contact with the surface of the element substrate where the gas inlet is open, and the outer tip protection layer that constitutes the outermost surface of the tip protection layer. including. The porosity of the inner tip protection layer is greater than the porosity of the outer tip protection layer, and the thickness of the inner tip protection layer is 30% or more and 90% or less of the thickness of the tip protection layer. be.

By making the porosity of the inner tip protection layer larger than the porosity of the outer tip protection layer, the gas sensor element can suppress clogging caused by poisonous substances in the vicinity of the gas inlet and reduce NO. _x Decrease in sensitivity can be avoided.

In particular, by increasing the thickness of the inner tip protection layer having a higher porosity than the outer tip protection layer, that is, by increasing the ratio of the thickness of the inner tip protection layer to the thickness of the tip protection layer, The gas sensor element can achieve the following effects. That is, by ensuring a sufficient thickness of the inner tip protective layer having a large porosity, clogging caused by poisonous substances etc. near the gas inlet can be suppressed, and in particular, the layer near the gas inlet (i.e. , the inner tip protection layer) can be prevented from clogging. In other words, by setting the ratio of the thickness of the inner tip protection layer having a large porosity to the thickness of the tip protection layer from 30% to 90%, the inner tip protection layer in contact with the gas inlet is poisoned. It is possible to avoid clogging caused by substances, etc.

Moreover, the gas sensor according to one aspect of the present invention may be configured to measure the amount of a specific gas component in the gas to be measured using the gas sensor element according to each of the above aspects. Such a gas sensor changes the diffusion mode of NO _x that reaches the measurement electrode from molecular diffusion to a mode in which it diffuses while repeatedly colliding with the wall surface of a sufficiently narrow channel. Therefore, in this gas sensor, the porous diffusion layer can suppress fluctuations in the NO _x output and deterioration of the measurement electrode, which are considered to be caused by molecular diffusion of NO _x under high H ₂ O concentration.

According to the present invention, it is possible to provide a gas sensor element, etc., in which the diffusion form of NO _x reaching a measurement electrode is changed from molecular diffusion to a form in which NO x diffuses while repeatedly colliding with the wall surface of a sufficiently narrow channel. can.

FIG. 1 is a schematic cross-sectional view schematically showing an example of the configuration of a sensor element according to an embodiment. FIG. 2 is a schematic cross-sectional view showing an example of the configuration of an element substrate included in the sensor element of FIG. 1. FIG. FIG. 3 is an enlarged view for explaining essential parts of the element substrate of FIG. 2. FIG. FIG. 4 is a diagram showing an example of a cross section of the element substrate shown in FIG. 3 taken along the line II-II. FIG. 5 is an enlarged view for explaining a main part of an element substrate including a porous diffusion layer according to a modification. FIG. 6 is an enlarged view for explaining a main part of an element substrate including a diffusion-limiting section according to a modification. FIG. 7 is an enlarged view for explaining the main parts of an element substrate including a porous diffusion layer and a diffusion control section according to a modification. FIG. 8 is an enlarged view for explaining a main part of an element substrate including a flow path according to a modified example. FIG. 9 is a diagram showing an example of a cross section of the element substrate shown in FIG. 8 taken along the line II-II. FIG. 10 is an enlarged view illustrating a main part of an element substrate including a flow path similar to that illustrated in FIG. 8 and a porous diffusion layer different from that illustrated in FIG. 8. FIG. 11 is an enlarged view illustrating a main part of an element substrate including a flow path not defined by a diffusion-limiting portion. FIG. 12 is a schematic cross-sectional view schematically showing an example of the configuration of a sensor element including a tip protection layer according to a modification. FIG. 13 is a graph showing the difference in the change in NO _x output over time depending on the presence or absence of a porous diffusion layer that closes a predetermined range of the channel CH around the measurement electrode. FIG. 14 is a graph showing the difference in the H ₂ O dependence of the NO _x output depending on the presence or absence of a porous diffusion layer that closes a predetermined range of the channel CH around the measurement electrode. FIG. 15 is a diagram showing an example of molecular diffusion. FIG. 16 is a diagram showing an example of Knudsen diffusion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment (hereinafter also referred to as "this embodiment") according to one aspect of the present invention will be described below based on the drawings. However, this embodiment described below is merely an illustration of the present invention in all respects. It goes without saying that various improvements and modifications can be made without departing from the scope of the invention. That is, in implementing the present invention, specific configurations depending on the embodiments may be adopted as appropriate.

The present inventors have confirmed that when the H ₂ O concentration in the gas to be measured is high, the NO _x output tends to fluctuate and the measurement electrode deteriorates more quickly. For example, in an environment with a high H ₂ O concentration (under high H ₂ O concentration), where the H ₂ O concentration in the gas to be measured is 20% or more (specifically, about 25%), NO _x It was confirmed that the output fluctuated more easily and the measurement electrode deteriorated more quickly. One of the causes of such problems such as fluctuations in NO _x output and deterioration of the measurement electrode under such high H ₂ O concentration is considered to be that the diffusion form around the measurement electrode 44 is molecular diffusion. The present inventors then changed the diffusion mode around the measurement electrode 44 from molecular diffusion to "a mode in which the diffusion occurs while repeatedly colliding with the walls of a sufficiently narrow channel" such as Knudsen diffusion. It was confirmed that the above problem could be solved by doing so.

Therefore, in the gas sensor element 101 according to the present embodiment, a porous diffusion layer 91 having a porosity of 5% or more and 25% or less is provided around the measurement electrode 44. Specifically, the porous diffusion layer 91 is provided upstream of the measurement electrode 44 in the flow direction DR of the gas to be measured at a position where the distance to the measurement electrode 44 is 0.15 mm or less. The porous diffusion layer 91 closes a predetermined range of the flow path CH of the gas to be measured leading to the measurement electrode 44. Specifically, the area of the surface of the porous diffusion layer 91 perpendicular to the flow direction DR of the gas to be measured is equal to the area of the surface of the flow path CH of the gas to be measured perpendicular to the flow direction DR of the gas to be measured. It occupies (occupies) more than 70%. The gas sensor element 101 is provided upstream of the measurement electrode 44 at a position where the distance to the measurement electrode 44 is 0.15 mm or less, and the measurement electrode 44 is The diffusion form around the . Specifically, the gas sensor element 101 uses the porous diffusion layer 91 to change the diffusion form around the measurement electrode 44 from molecular diffusion to Knudsen diffusion, such as ``repeated collisions with the walls of a sufficiently narrow channel.'' change to a form that spreads while As a result, even if H ₂ O gas is present in the gas to be measured, the gas sensor element 101 suppresses the influence of H ₂ O gas on NO _x gas (and O ₂ gas) by using the porous diffusion layer. 91, it is possible to suppress fluctuations in the NO _x output and deterioration of the measurement electrode 44. That is, the gas sensor element 101 suppresses deterioration of the measurement electrode 44 under high H ₂ O concentration, and suppresses deterioration of the measurement electrode 44 when driven for a long period of time under high H ₂ O concentration, for example. Furthermore, the gas sensor element 101 suppresses fluctuations in NOx output under high H ₂ O concentration, for example, suppresses the H ₂ O dependence of NOx output when NO _x gas is flowing, and improves NOx concentration measurement. Improve accuracy. Although the details will be described later, in this embodiment, the porosity is calculated using a known image processing method (binarization) on an image (SEM image) obtained by observation using a scanning electron microscope (SEM). This is a value derived by applying processing (such as processing). For example, the gas sensor element 101 is cut so that the cross section of a certain layer serves as an observation surface, and the cut surface is filled with resin and polished to obtain an observation sample. Next, the observation surface of the observation sample was taken using a SEM photograph (secondary electron image, acceleration voltage 15 kV, magnification 1000 times; however, if 1000 times magnification is inappropriate, use a magnification greater than 1000 times and less than 5000 times). By photographing, a SEM image of the certain layer is obtained. Next, by analyzing the obtained image, a threshold value is determined from the brightness distribution of brightness data of pixels in the image using a discriminant analysis method (Otsu's binarization). Thereafter, each pixel in the image is binarized into an object part and a pore part based on the determined threshold value, and the area of the object part and the area of the pore part are calculated. Then, the ratio of the area of the pores to the total area (the total area of the object part and the pores) is derived as the porosity [%] of the certain layer.

The gas sensor element 101 according to the present embodiment further includes a tip protection layer 200 covering at least the surface of the element base 100 where the gas inlet 10 is open. The gas sensor element 101 uses the tip protection layer 200 to trap (capture) poisonous substances and the like that cause clogging of the porous diffusion layer 91 that closes a predetermined range of the flow path CH around the measurement electrode 44 . Specifically, the gas sensor element 101 uses the tip protection layer 200 whose porosity is larger than that of the porous diffusion layer 91 to trap poisonous substances and prevent clogging around the measurement electrode 44, for example. , the clogging of the porous diffusion layer 91 is suppressed. Therefore, in the gas sensor element 101, the porous diffusion layer 91 that closes a predetermined range of the flow path CH around the measurement electrode 44 is clogged with poisonous substances, resulting in a decrease in NO _x output and a decrease in measurement accuracy. You can prevent it from happening. The details of the gas sensor element 101 according to this embodiment will be explained below with reference to FIG. 1 and the like.

[Configuration example]
FIG. 1 is a schematic cross-sectional view schematically showing an example of the configuration of a gas sensor element 101 according to the present embodiment. As illustrated in FIG. 1, the gas sensor element 101 includes an element base 100 and a tip protection layer 200. The element substrate 100 has a gas inlet 10 opened on its surface, and the gas to be measured is introduced from the gas inlet 10 into the gas to be measured flow section 7 which is an internal space of the element substrate 100 . In the example shown in FIG. 1, a gas introduction port 10 is opened on the front side (tip side) surface of the element substrate 100. In the following description, the front side (tip side) surface of the element substrate 100 may be referred to as the "tip surface" of the element substrate 100. In FIG. 1, the front side (tip side) of the element base 100 is on the left side of the paper.

<Tip protective layer>
The tip protection layer 200 covers at least the surface of the element substrate 100 where the gas inlet 10 is open (the tip surface of the element substrate 100). In the example shown in FIG. 1, the tip protection layer 200 is provided to cover the tip surface of the element substrate 100 and four side surfaces of the element substrate 100 that are continuous with the tip surface.

Although the details will be described later, by providing the tip protection layer 200, poisonous substances that cause clogging of the porous diffusion layer 91 provided around the measurement electrode 44 are trapped (captured) in the tip protection layer 200. can do. That is, the gas sensor element 101 can avoid clogging of the porous diffusion layer 91 by trapping (capturing) poisonous substances and the like in the tip protection layer 200. Further, the porosity of the tip protection layer 200 is higher than that of the porous diffusion layer 91 provided around the measurement electrode 44. Therefore, the gas sensor element 101 can prevent a situation in which the tip protection layer 200 itself becomes clogged with poisonous substances or the like and the NO _x output of the gas sensor element 101 decreases.

The tip protection layer 200 has a predetermined thickness, and specifically, the distance d1 from the outermost surface of the tip protection layer 200 to the gas inlet 10 is 0.2 mm or more. By making the distance d1 from the outermost surface of the tip protection layer 200 to the gas inlet 10 sufficiently long (specifically, 0.2 mm or more), that is, the thickness of the tip protection layer 200 is made sufficiently thick. Thus, the gas sensor element 101 can achieve the following effects. That is, even in a harsh environment where there are many poisonous substances, etc., such poisonous substances can be reliably trapped (captured) in the tip protective layer 200, thereby preventing clogging caused by poisonous substances, etc. near the gas inlet 10. can be suppressed to avoid a decrease in NO _x sensitivity.

<Element base>
FIG. 2 is a schematic cross-sectional view schematically showing an example of the configuration of the element base 100 included in the gas sensor element 101. The element substrate 100 has, for example, an elongated plate-like shape extending along the longitudinal direction (axial direction), and is also formed, for example, in the shape of a rectangular parallelepiped. The element base 100 illustrated in FIG. 2 has a front end and a rear end as ends in the longitudinal direction, and in the following description, the front end will be referred to as the left end in FIG. The rear end is the right end in FIG. 2 (that is, the rear end). However, the shape of the element substrate 100 does not need to be limited to such an example, and may be appropriately selected depending on the embodiment. In the following description, the back side of the page in FIG. 2 is the right side of the element base 100, and the front side of the page is the left side of the element base 100.

As illustrated in FIG. 2, the element substrate 100 includes a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6. The device includes a laminate formed by stacking layers in order from the bottom. Each layer 1-6 is composed of a solid electrolyte layer having oxygen ion conductivity such as zirconia (ZrO ₂ ). The solid electrolyte forming each layer 1-6 may be dense. Dense refers to a porosity of 5% or less.

The element substrate 100 is manufactured by, for example, performing steps such as predetermined processing and printing a wiring pattern on ceramic green sheets corresponding to each layer, then laminating them, and then firing them to integrate them. As an example, the element substrate 100 is a laminate of a plurality of ceramic layers. In this embodiment, the upper surface of the second solid electrolyte layer 6 constitutes the upper surface of the element substrate 100, the lower surface of the first substrate layer 1 constitutes the lower surface of the element substrate 100, and each side surface of each layer 1 to 6 constitutes the upper surface of the element substrate 100. , constitute each side of the element substrate 100.

In this embodiment, one tip of the element substrate 100, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, is configured to receive the gas to be measured from an external space. An internal space is provided in which the The internal space according to this embodiment includes a gas inlet 10, a first diffusion-limiting section 11, a buffer space 12, a second diffusion-limiting section 13, a first internal space 15, a third diffusion-limiting section 16, and a second internal space. The space 17, the fourth diffusion-limiting part 18, and the third internal cavity 19 are formed adjacent to each other in this order so as to communicate with each other. That is, the internal space according to this embodiment has a three-chamber structure (first internal space 15, second internal space 17, and third internal space 19).

In one example, this internal space is provided by hollowing out the spacer layer 5. The upper part of the internal space is defined by the lower surface of the second solid electrolyte layer 6. The lower part of the internal space is defined by the upper surface of the first solid electrolyte layer 4 . The sides of the internal space are defined by the sides of the spacer layer 5.

The first diffusion-limiting section 11 is a member (part) that imparts a predetermined diffusion resistance to the gas to be measured. In the example shown in FIG. A slit (flow path CH through which the gas to be measured flows) is formed. For example, the first diffusion controlling part 11 is a bridging part (first bridging part) that bridges the space hollowed out in the spacer layer 5, and between the first diffusion controlling part 11 and the layer 6 and the first The space between the diffusion rate controlling part 11 and the layer 4 becomes a slit, that is, a flow path CH through which the gas to be measured flows. Similarly, the second diffusion control section 13, the third diffusion control section 16, and the fourth diffusion control section 18 are members that provide a predetermined diffusion resistance to the gas to be measured. Each of the second diffusion-limiting section 13, the third diffusion-limiting section 16, and the fourth diffusion-limiting section 18 has a length extending in the direction perpendicular to the drawing, which is the first internal space 15, the second internal space 17, and a hole (flow path CH through which the gas to be measured flows) shorter than each of the third internal space 19 is formed.

As illustrated in FIG. 2, the second diffusion-limiting section 13 and the third diffusion-limiting section 16 each have two horizontally long openings (the openings are long in the direction perpendicular to the drawing), similarly to the first diffusion-limiting section 11. A slit (channel CH) having a side direction may be formed. On the other hand, the fourth diffusion rate-controlling part 18 is formed by a single horizontally long slit (the opening has a longitudinal direction perpendicular to the drawing) formed as a gap between the lower surface of the second solid electrolyte layer 6 and the lower surface of the second solid electrolyte layer 6. A channel CH) may also be formed. That is, the fourth diffusion-limiting section 18 may be in contact with the upper surface of the first solid electrolyte layer 4. For example, the second diffusion-limiting section 13 is a bridge section (second bridge section) that bridges the space hollowed out in the spacer layer 5, and the second diffusion-limiting section 13 is a bridge section (second bridge section) that bridges the space hollowed out in the spacer layer 5. The space between the diffusion rate controlling part 13 and the layer 4 becomes a slit, that is, a flow path CH through which the gas to be measured flows. For example, the third diffusion-controlling part 16 is a bridging part (third bridging part) that bridges the hollow space of the spacer layer 5, and between the third diffusion-controlling part 16 and the layer 6 and the third The space between the diffusion rate controlling part 16 and the layer 4 becomes a slit, that is, a flow path CH through which the gas to be measured flows. For example, the fourth diffusion-limiting section 18 is a bridge section (fourth bridge section) that bridges the hollow space of the spacer layer 5, and the space between the fourth diffusion-limiting section 18 and the layer 6 becomes a slit, In other words, it becomes a flow path CH through which the gas to be measured flows. Each of the second diffusion controlling section 13, the third diffusion controlling section 16, and the fourth diffusion controlling section 18 will be explained in detail later. A portion (internal space) from the gas inlet 10 to the third internal space 19 is referred to as a gas distribution section 7 to be measured.

A side surface of the first solid electrolyte layer 4 is located between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5 at a position farther from the tip side (the front side of the element substrate 100) than the gas flow section 7 to be measured. A reference gas introduction space 43 is provided at a position partitioned on the side by . For example, a reference gas such as the atmosphere is introduced into the reference gas introduction space 43 . However, the configuration of the element substrate 100 does not have to be limited to this example. As another example, the first solid electrolyte layer 4 may be configured to extend to the rear end of the element substrate 100, and the reference gas introduction space 43 may be omitted. In this case, the air introduction layer 48 may be configured to extend to the rear end of the element substrate 100.

An atmosphere introduction layer 48 is provided on a portion of the upper surface of the third substrate layer 3 adjacent to the reference gas introduction space 43 . The atmosphere introduction layer 48 is made of porous alumina, and is configured so that the reference gas is introduced through the reference gas introduction space 43. In addition, the atmosphere introduction layer 48 is formed to cover the reference electrode 42.

The reference electrode 42 is formed to be sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and an atmosphere introduction layer 48 connected to the reference gas introduction space 43 is provided around the reference electrode 42. ing. The reference electrode 42 is used to measure the oxygen concentration (oxygen partial pressure) in the first internal cavity 15 and the second internal cavity 17. Details will be described later.

The gas introduction port 10 is a part of the gas distribution section 7 to be measured that is open to the external space. The element base 100 is configured to introduce a gas to be measured into the interior from the external space through the gas inlet 10 . In this embodiment, as illustrated in FIG. 2, the gas introduction port 10 is arranged on the tip surface (front surface) of the element base 100. That is, the gas flow section 7 to be measured is configured to have an opening at the distal end surface of the element base 100. However, it is not essential that the gas flow section 7 to be measured be configured to have an opening on the front end surface of the element base 100, that is, that the gas inlet 10 be arranged on the front end surface of the element base 100. The element substrate 100 only needs to be able to take in the gas to be measured from the external space into the gas to be measured flow section 7, and the gas inlet 10 may be arranged, for example, on the right side or the left side of the element substrate 100. You may.

The first diffusion rate controlling part 11 is a part that imparts a predetermined diffusion resistance to the gas to be measured taken in from the gas inlet 10.

The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate-limiting unit 11 to the second diffusion rate-limiting unit 13 .

The second diffusion rate controlling section 13 is a section that provides a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 15 .

When the gas to be measured is introduced from the external space of the element substrate 100 into the first internal cavity 15, the pressure fluctuation of the gas to be measured in the external space (if the gas to be measured is automobile exhaust gas, the exhaust pressure) pulsation), the gas may be rapidly taken into the element substrate 100 from the gas inlet 10. Even in this case, in this configuration, the sample gas to be taken in is not directly introduced into the first internal space 15, but is introduced into the first diffusion-limiting section 11, the buffer space 12, and the second diffusion-limiting section 13. After the concentration fluctuations of the gas to be measured are canceled out, the measured gas is introduced into the first internal space 15. Thereby, the concentration fluctuation of the gas to be measured introduced into the first internal space 15 becomes almost negligible.

The first internal space 15 is for adjusting the partial pressure of oxygen in the gas to be measured introduced through the second diffusion-limiting section 13 (that is, through the flow path CH formed by the second diffusion-limiting section 13). It is set up as a space for The oxygen partial pressure is adjusted by operating the main pump cell 21.

The main pump cell 21 is an electrochemical pump cell composed of an inner pump electrode 22, an outer pump electrode 23, and a second solid electrolyte layer 6 sandwiched between these electrodes. The inner pump electrode 22 has a ceiling electrode portion 22 a provided on substantially the entire lower surface of the second solid electrolyte layer 6 adjacent to (facing) the first inner space 15 . The outer pump electrode 23 is provided on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22a so as to be adjacent to the external space.

The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that partition the first internal cavity 15, and the spacer layer 5 that provides a side wall. . Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 15, and a bottom electrode portion 22a is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Electrode portion 22b is formed. Then, side electrode portions (not shown) are connected to the ceiling electrode portion 22a and the bottom electrode portion 22b on the side wall surface (inner surface) of the spacer layer 5 constituting both side wall portions of the first internal space 15. is formed. In other words, the inner pump electrode 22 is disposed in a tunnel-like structure at the location where the side electrode portion is disposed.

The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, cermet electrodes made of Pt and ZrO ₂ containing 1% Au). Note that the inner pump electrode 22 that comes into contact with the gas to be measured is formed using a material that has a weakened ability to reduce nitrogen oxide (NO _x ) components in the gas to be measured.

The element substrate 100 (gas sensor element 101) applies a desired pump voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23 in the main pump cell 21, and creates a positive voltage between the inner pump electrode 22 and the outer pump electrode 23. By flowing the pump current Ip0 in the direction or in the negative direction, oxygen in the first internal space 15 can be pumped out to the external space, or oxygen in the external space can be pumped into the first internal space 15.

Further, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal cavity 15, the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third The substrate layer 3 and the reference electrode 42 constitute an oxygen partial pressure detection sensor cell 80 (namely, an electrochemical sensor cell) for controlling the main pump.

The element base 100 (gas sensor element 101) is configured to be able to identify the oxygen concentration (oxygen partial pressure) in the first internal space 15 by measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Ru. Further, the pump current Ip0 is controlled by feedback controlling Vp0 so that the electromotive force V0 is constant. Thereby, the oxygen concentration within the first internal cavity 15 can be maintained at a predetermined constant value.

The third diffusion rate controlling section 16 applies a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal space 15, and controls the gas to be measured. This is the part that leads to the second internal space 17.

The second internal space 17 further adjusts the oxygen partial pressure in the gas to be measured introduced through the third diffusion-limiting section 16 (that is, through the flow path CH formed by the third diffusion-limiting section 16). It is set up as a space for The oxygen partial pressure is adjusted by operating the auxiliary pump cell 50.

The auxiliary pump cell 50 includes an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, any suitable electrode outside the element substrate 100 is sufficient), and a second solid electrolyte layer 6. This is an auxiliary electrochemical pump cell. The auxiliary pump electrode 51 has a ceiling electrode portion 51 a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal cavity 17 .

The auxiliary pump electrode 51 has a tunnel-like structure similar to the inner pump electrode 22 provided in the first inner space 15 and is disposed in the second inner space 17 . That is, the ceiling electrode portion 51a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 17, and the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 17. A bottom electrode portion 51b is formed on the upper surface of the substrate. Side electrode portions (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 that provide the side walls of the second internal cavity 17, respectively. Thereby, the auxiliary pump electrode 51 has a tunnel-shaped structure.

Note that, like the inner pump electrode 22, the auxiliary pump electrode 51 is also formed using a material that has a weakened ability to reduce nitrogen oxide components in the gas to be measured.

The element base 100 (gas sensor element 101) removes oxygen from the atmosphere in the second internal space 17 by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 in the auxiliary pump cell 50. It is configured such that it can be pumped into the external space or pumped into the second internal cavity 17 from the external space.

Further, in order to control the oxygen partial pressure in the atmosphere in the second internal space 17, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and The third substrate layer 3 constitutes an oxygen partial pressure detection sensor cell 81 (that is, an electrochemical sensor cell) for controlling the auxiliary pump.

The auxiliary pump cell 50 performs pumping using a variable power source 52 whose voltage is controlled based on the electromotive force V1 detected by the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump. As a result, the oxygen partial pressure in the atmosphere within the second internal space 17 is controlled to a low partial pressure that does not substantially affect the measurement of _NOx .

In addition, the pump current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor cell 80 for controlling the main pump, and the electromotive force V0 is controlled, so that the pump current Ip1 is inputted as a control signal to the oxygen partial pressure detection sensor cell 80 for controlling the main pump. The gradient of the oxygen partial pressure in the gas to be measured introduced into the measuring gas 17 is controlled to be always constant. When used as a NO _x sensor, the main pump cell 21 and the auxiliary pump cell 50 work together to maintain the oxygen concentration within the second internal cavity 17 at a constant value of about 0.001 ppm.

The fourth diffusion rate controlling section 18 applies a predetermined diffusion resistance to the gas to be measured, the oxygen concentration (oxygen partial pressure) of which is controlled by the operation of the auxiliary pump cell 50 in the second internal space 17. This is the part that leads to the third internal space 19. The fourth diffusion rate controlling part 18 forms a flow path CH for the gas to be measured from the second internal space 17 to the third internal space 19, and in the example shown in FIG. A flow path CH is formed as a slit (gap) between them. That is, the fourth diffusion rate controlling section 18 controls the measurement gas that reaches the measurement electrode 44 on the upstream side of the measurement electrode 44 in the flow direction DR of the measurement gas (in the example shown in FIG. 2, from the left side to the right side of the page). defines the lower surface of the flow path CH.

The fourth diffusion rate controlling portion 18 has a lower porosity than a porous diffusion layer 91 described below, that is, it is denser than the porous diffusion layer 91. Although details will be described later using FIGS. 8 and 10, when the measurement electrode 44 is provided on the surface (for example, the bottom surface) of the fourth diffusion-limiting section, the fourth diffusion-limiting section is made of zirconia (ZrO ₂ ) or the like. It is composed of a solid electrolyte layer with oxygen ion conductivity of . That is, when the measurement electrode 44 is provided on the surface of the fourth diffusion-limiting section, the fourth diffusion-limiting section is configured as a dense layer (that is, the porosity is 5% or less) having oxygen ion conductivity. Ru.

A porous diffusion layer 91 is disposed in the channel CH, and in the example shown in FIG. A quality diffusion layer 91 is provided in contact with the upper surface of the fourth diffusion control section 18 .

The porous diffusion layer 91 has a porosity of 5% or more and 25% or less, and is a porous body with a lower porosity than the tip protection layer 200, for example, the main component is alumina (Al ₂ O ₃ ). It may be constituted by a porous membrane.

The porous diffusion layer 91 is arranged upstream of the measurement electrode 44 in the flow direction DR of the gas to be measured, and makes the diffusion form of the gas to be measured to the measurement electrode 44 suitable. Specifically, the porous diffusion layer 91 makes the diffusion form around the measurement electrode 44 "a form in which it diffuses while repeatedly colliding with the wall surface of a sufficiently narrow channel" such as Knudsen diffusion.

The porous diffusion layer 91 is provided on the flow path CH on the upstream side of the measurement electrode 44 in the flow direction DR of the gas to be measured. In the example shown in FIG. 2, the porous diffusion layer 91 is in contact with the upper surface of the fourth diffusion-limiting section 18 that forms the flow path CH for the gas to be measured from the second internal space 17 to the third internal space 19. In this state, it is provided on the flow path CH formed by the fourth diffusion rate controlling section 18.

Although details will be described later using FIG. 3 and the like, the porous diffusion layer 91 is provided on the channel CH at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less. Further, the area of the surface of the porous diffusion layer 91 perpendicular to the flow direction DR of the gas to be measured occupies 70% or more of the area of the surface of the flow path CH perpendicular to the flow direction DR of the gas to be measured. It is provided upstream from the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less, and covers 70% or more of the area of the surface of the flow path CH perpendicular to the flow direction DR of the gas to be measured. Due to the porous diffusion layer 91, the gas sensor element 101 achieves the following effects. In other words, the gas sensor element 101 uses the porous diffusion layer 91 to change the diffusion form around the measurement electrode 44 to "a form in which the diffusion occurs while repeatedly colliding with the wall surface of a sufficiently narrow channel" such as Knudsen diffusion. do. Therefore, the gas sensor element 101 can suppress fluctuations in NOx output and deterioration of the measurement electrode under high H ₂ O concentration, which occur when the diffusion form around the measurement electrode 44 is molecular diffusion.

Note that an example will be described below in which the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout. However, it is not essential that the porosity of the porous diffusion layer 91 be constant throughout the porous diffusion layer 91. The porous diffusion layer 91 may include a plurality of surfaces (layers) having different porosity. That is, the porosity of the two surfaces facing opposite to each other in the porous diffusion layer 91 may be different from each other. For example, the porosity may be different between the upstream side surface and the downstream side of the porous diffusion layer 91. You can leave it there. In particular, the porous diffusion layer 91 has a surface that opposes (faces) the measurement electrode 44 and a surface that does not oppose (faces) the measurement electrode 44 (for example, a surface that faces the gas flow section 7 to be measured). The porosity may be different. When the porous diffusion layer 91 includes a plurality of surfaces (layers) with different porosities, the average porosity of the porous diffusion layer 91 is 5% or more and 25% or less, and the pores of the tip protection layer 200 are lower than the rate.

As described above, the gas sensor element 101 includes the fourth diffusion-limiting section 18 (diffusion-limiting section) that imparts a predetermined diffusion resistance to the gas to be measured in the gas-to-be-measured flow section 7 (internal space). The fourth diffusion rate controlling section 18 is provided upstream of the measurement electrode 44 in the flow direction DR of the gas to be measured. Further, the porosity of the fourth diffusion-limiting section 18 is lower than that of the porous diffusion layer 91, that is, the fourth diffusion-limiting section 18 is denser than the porous diffusion layer 91. In the gas sensor element 101, at least one surface of the flow path CH whose predetermined range is blocked by the porous diffusion layer 91 is defined (divided) by the fourth diffusion control section 18, and in the example shown in FIG. The lower surface of CH is defined by the fourth diffusion-limiting section 18 .

In this configuration, the gas sensor element 101 is provided upstream of the measurement electrode 44 and further includes a fourth diffusion-limiting section 18 having a lower (smaller) porosity than the porosity of the porous diffusion layer 91 . The flow path CH whose predetermined range is blocked by the porous diffusion layer 91 has at least one surface defined (divided) by the fourth diffusion rate controlling section 18 . That is, at least one surface of the flow path CH of the gas to be measured leading to the measurement electrode 44 is defined by the fourth diffusion-limiting section 18 which is denser than the porous diffusion layer 91 (having a lower porosity). Therefore, the gas sensor element 101 guides the gas to be measured to the measurement electrode 44 by using the flow channel CH in which at least one surface is defined by the fourth diffusion-limiting section 18 which is denser than the porous diffusion layer 91. be able to. The gas sensor element 101 prevents the possibility that, for example, the gas to be measured that leaks out in the middle of the flow path CH will reach the measurement electrode 44 without being changed in its diffusion form by the porous diffusion layer 91. It can be suppressed. Therefore, the gas sensor element 101 uses a porous diffusion layer 91 that closes a predetermined range of the channel CH through which the gas to be measured flows to the measurement electrode 44 to achieve a suitable diffusion pattern such as Knudsen diffusion. You can get closer to the form.

The third internal cavity 19 contains a gas to be measured introduced through the fourth diffusion-limiting section 18 (that is, through the channel CH formed by the fourth diffusion-limiting section 18 and provided with the porous diffusion layer 91). It is provided as a space for performing processing related to measurement of the nitrogen oxide (NO _x ) concentration inside the chamber. The NO _x concentration is measured by the operation of the measurement pump cell 41 . In this embodiment, after the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 15, the measured gas introduced through the third diffusion rate controlling part 16 is adjusted in the second internal space 17. , the oxygen partial pressure is further adjusted by the auxiliary pump cell 50. Thereby, the oxygen concentration of the gas to be measured introduced from the second internal space 17 to the third internal space 19 can be kept constant with high accuracy. Therefore, the element substrate 100 according to this embodiment enables highly accurate measurement of NO _x concentration.

The measurement pump cell 41 measures the nitrogen oxide concentration in the gas to be measured in the third internal space 19 . The measurement pump cell 41 is an electrochemical pump cell composed of a measurement electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, a spacer layer 5, and a first solid electrolyte layer 4. In the example of FIG. 2, the measurement electrode 44 is provided on the lower surface of the second solid electrolyte layer 6 adjacent to (facing) the third internal cavity 19. As will be described later with reference to FIGS. 8, 10, etc., the measurement electrode 44 may be provided on the surface (for example, the bottom surface) of the fourth diffusion-limiting section.

The measurement electrode 44 is a porous cermet electrode, and may contain at least one of silica (SiO ₂ ) and alumina (Al ₂ O ₃ ). For example, the measurement electrode 44 contains 80 to 90% by weight of Pt (platinum), 9.5 to 19.8% by weight of the constituent material of the first solid electrolyte layer 4 (for example, ZrO ₂ ), and at least one of silica and alumina. Contains 0.2 to 0.5% by weight of a mixture containing. The content ratio of the measurement electrode 44 is set so that the noble metal content is greater than that of the constituent material of the first solid electrolyte layer 4 . Therefore, the adhesive force between the first solid electrolyte layer 4 and the measurement electrode 44 is strengthened. Moreover, the measurement electrode 44 in this embodiment contains 0.2 to 0.5% by weight of a mixture containing at least one of silica and alumina. Here, when measuring NO _x at high temperatures (eg, 700 to 800 degrees Celsius), the measurement electrode 44 constantly undergoes repeated expansion and contraction. Even under such an environment, the following effects can be achieved by the measurement electrode 44 containing at least one of silica and alumina. That is, the expansion and contraction of the measurement electrode 44 is suppressed, and the phenomenon that the measurement electrode 44 peels off from the first solid electrolyte layer 4 does not occur. Furthermore, when the measurement electrode 44 and the porous diffusion layer 91 are in contact with each other, especially when the porous diffusion layer 91 covers the measurement electrode 44 as illustrated in FIGS. By including at least one of alumina, the following effects can be achieved. That is, by suppressing expansion and contraction in the measurement electrode 44, cracks, cracks, etc. can be prevented from forming in the porous diffusion layer 91 in contact with the measurement electrode 44. In particular, when the porous diffusion layer 91 contacts the measurement electrode 44 and covers the measurement electrode 44, the effect of being able to prevent cracks, cracks, etc. from entering the porous diffusion layer 91 in contact with the measurement electrode 44 is useful. It is. However, in the gas sensor element 101, it is not essential that the measurement electrode 44 and the porous diffusion layer 91 come into contact with each other. In the gas sensor element 101, the porous diffusion layer 91 is provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less, and closes a predetermined range of the flow path CH of the gas to be measured. It's fine as long as you are.

The measurement electrode 44 also functions as a NO _x reduction catalyst that reduces NO _x present in the atmosphere within the third internal space 19 . In the example of FIG. 2, the measurement electrode 44 is exposed within the third internal cavity 19. In another example, the measurement electrode 44 may be covered with a diffusion-limiting portion or a porous diffusion layer (for example, any of the porous diffusion layers 91A, 91C, and 91E illustrated in FIGS. 5, 7, and 10). Such a diffusion-limiting portion may be constituted by a porous film containing alumina (Al ₂ O ₃ ) as a main component. The diffusion-limiting portion or porous diffusion layer that covers (covers) the measurement electrode 44 plays a role in limiting the amount of NO _x flowing into the measurement electrode 44 and also acts as a protective film for the measurement electrode 44 .

The element substrate 100 is configured to pump out oxygen generated by decomposition of nitrogen oxide in the atmosphere around the measurement electrode 44 in the measurement pump cell 41, and to detect the amount of oxygen generated as a pump current Ip2.

In addition, in order to detect the oxygen partial pressure around the measurement electrode 44, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42 are used. , an oxygen partial pressure detection sensor cell 82 (that is, an electrochemical sensor cell) for controlling the measurement pump is configured. The variable power supply 46 is controlled based on the voltage (electromotive force) V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82.

The gas to be measured guided into the third internal space 19 reaches the measurement electrode 44 under conditions where the oxygen partial pressure is controlled. Nitrogen oxides in the gas to be measured around the measurement electrode 44 are reduced (2NO→N ₂ +O ₂ ) to generate oxygen. Then, the generated oxygen is pumped by the measurement pump cell 41, but at this time, a variable power source is used so that the control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 is kept constant. voltage Vp2 is controlled. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured, the pump current Ip2 in the measurement pump cell 41 is used to measure the concentration of nitrogen oxides in the gas to be measured. The concentration will be calculated.

Furthermore, the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form an oxygen partial pressure detection means as an electrochemical sensor cell. It is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by reduction of NO _x components in the surrounding atmosphere and the amount of oxygen contained in the reference atmosphere. Thereby, it is also possible to determine the concentration of nitrogen oxide components in the gas to be measured.

Further, an electrochemical sensor cell 83 is constituted by the second solid electrolyte layer 6 , the spacer layer 5 , the first solid electrolyte layer 4 , the third substrate layer 3 , the outer pump electrode 23 , and the reference electrode 42 . The element base 100 is configured to be able to detect the oxygen partial pressure in the gas to be measured outside the sensor using the electromotive force Vref obtained by the sensor cell 83.

In the element substrate 100 having the above configuration, by operating the main pump cell 21 and the auxiliary pump cell 50, the oxygen partial pressure is always maintained at a constant low value (a value that does not substantially affect the measurement of NO _x ). A gas to be measured can be supplied to the measurement pump cell 41. Therefore, the element substrate 100 operates based on the pump current Ip2 that flows when oxygen generated by the reduction of _NOx is pumped out from the measurement pump cell 41 in approximately proportion to the concentration of nitrogen oxides in the gas to be measured. It is configured to be able to specify the concentration of nitrogen oxides in the gas to be measured.

Further, the element base 100 includes a heater 70 that plays a role of temperature adjustment to heat and keep the element base 100 warm in order to improve the oxygen ion conductivity of the solid electrolyte. In the example of FIG. 2, the heater 70 includes a heater electrode 71, a heat generating section 72, a lead section 73, a heater insulating layer 74, and a pressure dissipation hole 75. The lead portion 73 may be formed of a through hole.

In this embodiment, the heater 70 is disposed at a position closer to the bottom surface of the element substrate 100 than the top surface of the element substrate 100 in the thickness direction (vertical direction/layering direction) of the element substrate 100. However, the arrangement of the heater 70 is not limited to this example, and may be appropriately selected depending on the embodiment.

The heater electrode 71 is an electrode formed so as to be in contact with the lower surface of the first substrate layer 1 (the lower surface of the element base 100). By connecting the heater electrode 71 to an external power source, power can be supplied to the heater 70 from the outside.

The heat generating portion 72 is an electrical resistor formed between the second substrate layer 2 and the third substrate layer 3 from above and below. The heat generating section 72 is connected to the heater electrode 71 via a lead section 73, and generates heat by being supplied with power from the outside through the heater electrode 71, thereby heating and keeping the solid electrolyte forming the element substrate 100 warm.

Further, the heat generating portion 72 is buried throughout the entire area from the first internal space 15 to the second internal space 17, and it is possible to adjust the temperature of the entire element substrate 100 to the temperature at which the solid electrolyte is activated. It has become.

The heater insulating layer 74 is an insulating layer formed of an insulator such as alumina on the upper and lower surfaces of the heat generating portion 72. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heat generating section 72 and between the third substrate layer 3 and the heat generating section 72.

The pressure dissipation hole 75 is a portion provided to penetrate the third substrate layer 3 and communicate with the reference gas introduction space 43, and is for the purpose of alleviating an increase in internal pressure due to a rise in temperature within the heater insulating layer 74. It is formed.

<Porous diffusion layer>
FIG. 3 is an enlarged view for explaining the main parts of the element substrate 100. Specifically, FIG. 3 is a diagram showing details of the porous diffusion layer 91 disposed on the flow path CH formed (defined) by the fourth diffusion rate-limiting section 18. The porous diffusion layer 91 is a porous layer (porous body) having a porosity of 5% or more and 25% or less, and has a lower porosity than the tip protection layer 200. The porous diffusion layer 91 is located on the flow path CH on the upstream side of the measurement electrode 44 in the flow direction DR of the gas to be measured (from the left side to the right side of the page in the example shown in FIG. It is provided at a position where the distance is 0.15 mm or less. In the example shown in FIG. 3, the porous diffusion layer 91 is arranged on the flow path CH formed between the upper surface of the fourth diffusion regulating section 18 and the lower surface of the second solid electrolyte layer 6. It is provided at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less while in contact with the upper surface of the measurement electrode 44. For example, the porous diffusion layer 91 is arranged so that the distance d2 from the surface closest to the measurement electrode 44 (the surface facing (opposing) the measurement electrode 44) to the measurement electrode 44 is 0.15 mm or less. It is provided on the flow path CH on the more upstream side. In the example shown in FIG. 3, the porous diffusion layer 91 has a distance d2 from the downstream surface (the surface facing the measurement electrode 44) in the flow direction DR of the gas to be measured to the measurement electrode 44 of 0.15 mm or less. The measurement electrode 44 is provided on the flow path CH on the upstream side.

Further, although details will be described later using FIG. 4, the area of the surface of the porous diffusion layer 91 perpendicular to the flow direction DR of the gas to be measured is perpendicular to the flow direction DR of the gas to be measured of the flow path CH. It occupies more than 70% of the surface area. With such a porous diffusion layer 91, the diffusion form around the measurement electrode 44 can be made into "a form of diffusion while repeatedly colliding with the wall surface of a sufficiently narrow channel" such as Knudsen diffusion. Therefore, it is possible to suppress fluctuations in NOx output and deterioration of the measurement electrode under high H2O concentration, which occur when the diffusion form around the measurement electrode 44 is molecular diffusion.

As explained using FIG. 2, the element substrate 100 illustrated in FIG. 3 has an internal space provided by hollowing out the spacer layer 5 between the first solid electrolyte layer 4 and the second solid electrolyte layer 6. (Measurement gas flow section 7). The gas flow section 7 to be measured is partitioned (defined) by the lower surface of the second solid electrolyte layer 6 at the upper part (upper surface) and the upper surface of the first solid electrolyte layer 4 at the lower part (lower surface). The measured gas flow section 7 includes a first internal space 15 , a second internal space 17 , and a third internal space 19 .

The first internal space 15 is a main body composed of an inner pump electrode 22 (ceiling electrode part 22a and bottom electrode part 22b), an outer pump electrode 23 and a second solid electrolyte layer 6 (not shown in FIG. 3). This is a space for adjusting the oxygen partial pressure in the gas to be measured using the pump cell 21.

The third diffusion rate controlling section 16 applies a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal space 15, and controls the gas to be measured. It leads to the second internal cavity 17. That is, the third diffusion rate-limiting section 16 forms a flow path for the gas to be measured from the first internal space 15 to the second internal space 17.

In the second internal space 17, the oxygen partial pressure in the gas to be measured is further adjusted by an auxiliary pump cell 50. The auxiliary pump cell 50 includes an auxiliary pump electrode 51 (a ceiling electrode section 51a and a bottom electrode section 51b), an outer pump electrode 23, and a second solid electrolyte layer 6, which are not shown in FIG.

The fourth diffusion rate controlling section 18 applies a predetermined diffusion resistance to the gas to be measured, the oxygen concentration (oxygen partial pressure) of which is controlled by the operation of the auxiliary pump cell 50 in the second internal space 17. It leads to the third internal cavity 19. The fourth diffusion rate controlling part 18 forms a flow path CH for the gas to be measured from the second internal space 17 to the third internal space 19, and in the example shown in FIG. A flow path CH is formed as a slit (gap) between them.

A porous diffusion layer 91 having a porosity of 5% or more and 25% or less is arranged in the channel CH. The porous diffusion layer 91 is located upstream of the measurement electrode 44 in the flow direction DR of the gas to be measured (from the left side to the right side of the page in the example shown in FIG. 3), and has a distance d2 of 0.15 mm or less to the measurement electrode 44. It is installed in a certain position. The porous diffusion layer 91 provided at such a position closes a predetermined range (specifically, 70% or more) of the channel CH.

The nitrogen oxide concentration of the gas to be measured whose oxygen partial pressure has been adjusted by the auxiliary pump cell 50 in the second internal space 17 is measured by the measuring pump cell 41 in the third internal space 19 . The measurement pump cell 41 includes a measurement electrode 44, a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, and an outer pump electrode 23 not shown in FIG.

Around the measurement electrode 44, specifically, "a position upstream of the measurement electrode 44 and where the distance d2 to the measurement electrode 44 is 0.15 mm or less" is a flow path CH for the gas to be measured. A porous diffusion layer 91 having a porosity of 5% or more and 25% or less is disposed so as to close a predetermined range of . With such a porous diffusion layer 91, the diffusion form of the gas to be measured (particularly NO _x gas) around the measurement electrode 44 is diffused through repeated collisions with the walls of a sufficiently narrow channel, such as Knudsen diffusion. It can be in the form of That is, the gas sensor element 101 has a porous diffusion layer 91 that closes a predetermined range (specifically, 70% or more) of the flow path CH of the gas to be measured leading to the measurement electrode 44. The gas diffusion mode is set to a suitable one such as Knudsen diffusion.

Here, if the porous diffusion layer 91 having a large diffusion resistance is provided around the measurement electrode 44, the porous diffusion layer 91 may become clogged with poisonous substances or the like. In order to prevent this, the gas sensor element 101 includes a tip protection layer 200 that covers at least the surface of the element base 100 where the gas inlet 10 is open, as illustrated in FIG. The porosity of the porous diffusion layer 91 is lower than that of the tip protection layer 200, that is, the porosity of the tip protection layer 200 is higher than the porosity of the porous diffusion layer 91.

By adopting such a configuration, poisonous substances that cause clogging of the porous diffusion layer 91 are captured in the tip protection layer 200 in advance, and therefore do not reach the porous diffusion layer 91 and the measurement electrode 44. The amount of poisonous substances in the gas to be measured that reaches the sensor is very small. Therefore, the possibility of clogging of the porous diffusion layer 91 due to poisonous substances or the like can be suppressed. Further, even if a poisonous substance or the like reaches the measuring electrode 44 and adheres to the measuring electrode 44, it hardly affects the oxidation/reduction ability of the electrode metal.

Therefore, the gas sensor element 101 according to the present embodiment suppresses clogging of the porous diffusion layer 91 due to poisonous substances, etc., and suppresses the influence of poisonous substances etc. on the oxidation/reduction ability of the measurement electrode 44. can be done. Specifically, the gas sensor element 101 is configured such that deterioration in measurement accuracy due to use is suitably suppressed, that is, measurement accuracy is maintained stably even after repeated use. .

FIG. 4 is a diagram showing an example of a cross section of the element substrate 100 shown in FIG. 3 taken along the line II-II. The area of the surface of the porous diffusion layer 91 perpendicular to the direction DR in which the gas to be measured flows (in the example shown in FIG. occupies more than 70% of the area of the plane perpendicular to . In the example shown in FIG. 4, the flow path CH of the gas to be measured from the second internal space 17 to the third internal space 19 is formed (defined) by the fourth diffusion rate controlling section 18, and specifically, A channel CH is formed as a slit (gap) between the fourth diffusion rate-limiting section 18 and the second solid electrolyte layer 6. That is, the flow path CH illustrated using the two-dot chain line in FIG. It is divided (regulated) by. 70% or more of the area of the surface of the flow path CH perpendicular to the flow direction DR of the gas to be measured is occupied by the area of the surface of the porous diffusion layer 91 perpendicular to the flow direction DR of the gas to be measured. ing. That is, the porous diffusion layer 91 provided in contact with the upper surface of the fourth diffusion rate controlling section 18 occupies 70% or more of the area of the surface of the flow path CH perpendicular to the flow direction DR of the gas to be measured. There is (blocked).

In the example shown in FIG. 4, there is a space (gap) between the upper surface of the porous diffusion layer 91 and the lower surface of the second solid electrolyte layer 6. That is, in the example shown in FIG. 4, the area of the surface of the porous diffusion layer 91 perpendicular to the flow direction DR of the gas to be measured is relative to the area of the surface of the flow path CH perpendicular to the flow direction DR of the gas to be measured. The area ratio is not 100%. However, the ratio of the area of the surface of the porous diffusion layer 91 perpendicular to the flow direction DR of the gas to be measured to the area of the surface perpendicular to the flow direction DR of the gas to be measured is 70 % or more, and may be 100%. In other words, the surface of the porous diffusion layer 91 that is orthogonal to the flow direction DR of the gas to be measured may occupy the entire surface of the flow path CH that is orthogonal to the direction DR that the gas to be measured flows. In accordance with the example shown in FIG. 4, the porous diffusion layer 91 and the second solid electrolyte layer 6 may be in contact with each other. Specifically, the upper surface of the porous diffusion layer 91 and the second solid electrolyte layer 6 may be in contact with each other. It may be in contact with the bottom surface of.

Further, the porous diffusion layer 91 illustrated in FIGS. 3 and 4 has a constant porosity throughout, and specifically, the porosity of the porous diffusion layer 91 is 5% or more and 25% or less. The porosity was lower than that of the tip protection layer 200. However, in the gas sensor element 101 (element substrate 100), the porosity of the porous diffusion layer provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less It is not essential that it be constant throughout the diffusion layer.

In the gas sensor element 101, the porous diffusion layer provided upstream from the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less includes a plurality of surfaces (layers) with different porosity. You can stay there. In this case, of the two surfaces facing opposite to each other in the porous diffusion layer, the porosity of "the surface that opposes (faces) the measurement electrode 44" is defined as "the surface that does not oppose (does not face) the measurement electrode 44". It is desirable that the porosity be higher than the porosity of the surface, particularly by 10% or more. For example, in the example shown in FIG. 3, of the two faces facing opposite to each other in the porous diffusion layer 91, the porosity of the face facing the downstream side and facing the measurement electrode 44 is the same as that of the second face facing the upstream side. The porosity may be higher than the porosity of the surface facing (facing) the internal space 17. In particular, of the two faces facing opposite to each other in the porous diffusion layer 91, the porosity of the face that opposes (faces) the measurement electrode 44 is 10% higher than the porosity of the face that does not face the measurement electrode 44. It may be higher than that. By making the porosity of "the surface facing the measurement electrode 44" higher than the porosity of "the surface not facing the measurement electrode 44" among the two surfaces facing opposite to each other in the porous diffusion layer, the gas sensor element 101 can achieve the following effects. In other words, the gas sensor element 101 suppresses the effect of H ₂ O adhering to the surface of the measurement electrode 44 being decomposed and H ₂ is generated, and reduces the light-off time required from when the gas sensor is activated until it enters a steady operating state. can be shortened. In particular, by making the porosity of the "surface facing the measurement electrode 44" 10% higher than the porosity of the "surface not facing the measurement electrode 44", the gas sensor element 101 can further shorten the light-off time. can.

Among the two faces facing opposite to each other in the porous diffusion layer, the porosity of the "face facing (facing) the measurement electrode 44" is made higher than the porosity of the "face not facing the measurement electrode 44". In this case, the porous diffusion layer satisfies the following conditions. That is, the porous diffusion layer has an average porosity of 5% or more and 25% or less, and the average porosity is lower than the porosity of the tip protection layer 200. The details of the porous diffusion layer including a plurality of surfaces (layers) having different porosity will be described later using FIG. 5 and the like.

(Necessity of contact between porous diffusion layer and measurement electrode)
In order to make the diffusion form around the measurement electrode 44 suitable, the porous diffusion layer 91 is provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less, A predetermined range (specifically, 70% or more) of the channel CH is blocked. The porous diffusion layer 91 may be in contact with the measurement electrode 44, or may be placed upstream of the measurement electrode 44 so that the distance d2 to the measurement electrode 44 is 0.15 mm or less without contacting the measurement electrode 44. may be provided. That is, the porous diffusion layer 91 illustrated in FIGS. 2 and 3 is provided upstream of the measurement electrode 44 so that the distance d2 to the measurement electrode 44 is 0.15 mm or less without contacting the measurement electrode 44. was. However, the porous diffusion layer 91 provided upstream from the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less may be in contact with the measurement electrode 44. An example of the porous diffusion layer 91 in contact with the measurement electrode 44 will be described below with reference to FIG.

(Example where the porous diffusion layer and measurement electrode are in contact)
FIG. 5 is an enlarged view for explaining essential parts of an element substrate 100 (gas sensor element 101) including a porous diffusion layer 91A according to a modification. Specifically, FIG. 5 shows a sensor that is provided upstream of the measurement electrode 44 in the flow direction DR of the gas to be measured (from the left side to the right side of the page in the example shown in FIG. FIG. 3 is a diagram showing an example of a porous diffusion layer 91A in contact with a measurement electrode 44. Instead of the porous diffusion layer 91, the gas sensor element 101 may include a porous diffusion layer 91A, which will be described in detail below. Note that the first solid electrolyte layer 4, spacer layer 5, second solid electrolyte layer 6, etc. in FIG. 5 are the same as those described using FIGS. 2, 3, etc., so the description will not be repeated.

The porous diffusion layer 91 illustrated in FIGS. 2 and 3 is placed on the flow path CH on the upstream side of the measurement electrode 44 so that the distance d2 to the measurement electrode 44 is 0.15 mm or less without contacting the measurement electrode 44. It was set up in However, in the gas sensor element 101, a porous diffusion layer is provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less, and closes a predetermined range of the flow path CH. 44 is not essential. In the gas sensor element 101, the porous diffusion layer that is provided upstream from the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less and closes a predetermined range of the flow path CH is illustrated in FIG. The porous diffusion layer 91A may be in contact with the measurement electrode 44, like the porous diffusion layer 91A.

The porous diffusion layer 91A illustrated in FIG. 5 is similar to the porous diffusion layer 91 illustrated in FIGS. 2 and 3 in that it is arranged upstream of the measurement electrode 44. However, the porous diffusion layer 91A disposed upstream of the measurement electrode 44 extends downstream, and specifically extends to and is in contact with the measurement electrode 44. In particular, the porous diffusion layer 91A illustrated in FIG. 5 extends to the downstream side of the measurement electrode 44 and covers the measurement electrode 44.

Here, the porous diffusion layer 91A in contact with the measurement electrode 44 covers the measurement electrode 44 in the example shown in FIG. However, in the gas sensor element 101, it is not essential that the porous diffusion layer 91A in contact with the measurement electrode 44 covers the measurement electrode 44. In the gas sensor element 101, the porous diffusion layer 91A may be in contact with the measurement electrode 44 without covering it.

Further, in the example shown in FIG. 5, the porous diffusion layer 91A that covers the measurement electrode 44 is in contact with the measurement electrode 44, that is, the surface of the porous diffusion layer 91A that opposes (faces) the measurement electrode 44. , are in contact with the measurement electrode 44. However, the gas sensor element 101 may include a porous diffusion layer that covers the measurement electrode 44 without being in contact with the measurement electrode 44. In the gas sensor element 101, it is not essential that the porous diffusion layer covering the measurement electrode 44 be in contact with the measurement electrode 44, and the porous diffusion layer covering the measurement electrode 44 may not be in contact with the measurement electrode 44. That is, the gas sensor element 101 may include a porous diffusion layer that covers the measurement electrode 44, and there may be a space (gap) between the porous diffusion layer and the measurement electrode 44. However, if a gap is provided between the porous diffusion layer covering the measurement electrode 44 and the measurement electrode 44, the distance between the surface of the porous diffusion layer covering the measurement electrode 44 facing the measurement electrode 44 and the measurement electrode 44 is d2 shall be 0.15 mm or less. That is, in the gas sensor element 101, the porous diffusion layer that provides a suitable diffusion form around the measurement electrode 44 has a distance d2 upstream from the measurement electrode 44 to the measurement electrode 44 of 0.15 mm or less. It suffices if it is provided in the correct position.

To summarize the explanation so far, in the gas sensor element 101, the porous diffusion layer that is provided upstream of the measurement electrode 44 and closes a predetermined range (specifically, 70% or more) of the flow path CH is shown in FIG. It may be in contact with the measurement electrode 44 like the porous diffusion layer 91A illustrated. In the gas sensor element 101, the porous diffusion layer that is provided upstream of the measurement electrode 44 and closes a predetermined range of the flow path CH is provided at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less. It may or may not be in contact with the measurement electrode 44. Furthermore, like the porous diffusion layer 91A, a porous diffusion layer that is provided upstream of the measurement electrode 44 in the gas sensor element 101 and closes a predetermined range of the channel CH may cover the measurement electrode 44.

That is, in the gas sensor element 101, the porous diffusion layer "having a porosity lower than the porosity of the tip protection layer 200, 5% or more and 25% or less" may be one that satisfies the following conditions. That is, such a porous diffusion layer is provided in the flow path CH of the gas to be measured at a position upstream of the measurement electrode 44 such that the distance d2 to the measurement electrode 44 is 0.15 mm or less, and is provided at a predetermined position in the flow path CH. It is sufficient to cover the range (70% or more). In the gas sensor element 101, the porous diffusion layer and the measurement electrode 44 may or may not be in contact with each other. Furthermore, the gas sensor element 101 may or may not cover the measurement electrode 44 with such a porous diffusion layer. The surface of the porous diffusion layer that opposes (faces) the measurement electrode 44 may or may not be in contact with the measurement electrode 44. However, if the surface of the porous diffusion layer facing the measurement electrode 44 is not in contact with the measurement electrode 44, the distance d2 between the surface of the porous diffusion layer facing the measurement electrode 44 and the measurement electrode 44 shall be 0.15 mm or less.

The porous diffusion layer 91A illustrated in FIG. The electrolyte layer 6 (in particular, the lower surface of the second solid electrolyte layer 6) is in contact with the electrolyte layer 6. In other words, there is no space (gap) between the porous diffusion layer 91A and the fourth diffusion control section 18, and between the porous diffusion layer 91A and the surface defining the gas distribution section 7 to be measured. The gas sensor element 101 achieves the following by eliminating the gap between the porous diffusion layer 91A and the fourth diffusion rate controlling section 18 and the gap between the porous diffusion layer 91A and the surface defining the gas distribution section 7 to be measured. It is possible to achieve the following effects. That is, in the gas sensor element 101, the gas to be measured flows through the gap between the porous diffusion layer 91A and the fourth diffusion control section 18, and between the porous diffusion layer 91A and the surface defining the gas distribution section 7. This can prevent at least one of the gaps from reaching the measurement electrode 44. In other words, in the gas sensor element 101, for example, a situation may occur in which the gas to be measured leaks out in the middle of the flow path CH and reaches the measurement electrode 44 without having its diffusion form changed by the porous diffusion layer 91A. You can control your sexuality. Therefore, the gas sensor element 101 uses a porous diffusion layer 91A that closes a predetermined range of the flow path CH through which the gas to be measured flows to the measurement electrode 44 to achieve a suitable diffusion pattern such as Knudsen diffusion. You can get closer to the form.

Note that, like the porous diffusion layer 91 illustrated in FIGS. 2 and 3, the porous diffusion layer 91A illustrated in FIG. 5 has a porosity lower than that of the tip protection layer 200, and is 5% or more. and 25% or less. The porous diffusion layer 91A is provided upstream of the measurement electrode 44 in the flow direction DR of the gas to be measured at a position where the distance to the measurement electrode 44 is 0.15 mm or less, and extends to the measurement electrode 44. More than 70% of the flow path CH of the gas to be measured is blocked. In particular, the porous diffusion layer 91A illustrated in FIG. It occupies the entirety (that is, 100%) of the channel CH formed as a slit (gap). That is, the surface of the porous diffusion layer 91A that is perpendicular to the flow direction DR of the gas to be measured occupies 70% or more of the area of the surface of the flow path CH that is perpendicular to the flow direction DR of the gas to be measured.

The porous diffusion layer 91A illustrated in FIG. 5 has a constant porosity of 5% or more and 25% or less throughout. However, in the gas sensor element 101 (element substrate 100), the porosity of the porous diffusion layer provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less It is not essential that it be constant throughout the diffusion layer. In the gas sensor element 101, the porous diffusion layer provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less may be configured as follows. That is, among the two surfaces facing opposite to each other in the porous diffusion layer, "the surface that opposes (faces) the measurement electrode 44" and "the surface that does not oppose (face) the measurement electrode 44". , the porous diffusion layer may be configured to have different porosities.

Specifically, of the two faces facing opposite to each other in the porous diffusion layer 91A, the porosity of the face facing (in contact with) the measurement electrode 44 is the same as the porosity of the face not facing (in contact with) the measurement electrode 44. It may be higher than the rate. For example, the porous diffusion layer 91A may have two surfaces facing opposite to each other configured as follows. That is, the porous diffusion layer 91A may have a surface facing the gas distribution section 7 to be measured that does not face the measurement electrode 44, and may be constituted by a first porous diffusion layer that is a porous layer. The surface facing the measurement electrode 44 may be constituted by a second porous diffusion layer that is a porous layer. The first porous diffusion layer and the second porous diffusion layer have different porosity from each other, and the porosity of the second porous diffusion layer may be higher than the porosity of the first porous diffusion layer.

Of the two faces facing opposite to each other in the porous diffusion layer 91A, the porosity of the face facing (in contact with) the measurement electrode 44 is made higher than the porosity of the face not facing (in contact with) the measurement electrode 44. By doing so, the following effects can be achieved. In other words, it is possible to suppress the effect of H ₂ O adhering to the surface of the measurement electrode 44 being decomposed and generate H ₂ , and to shorten the light-off time required from when the gas sensor is activated until it enters a steady operating state. can. This is due to the reason explained below.

Immediately after the gas sensor is driven, when H ₂ O attached to the surface of the measurement electrode 44 is decomposed and H ₂ is generated, the potential difference (that is, the oxygen concentration difference) between the measurement electrode 44 and the reference electrode 42 increases. Therefore, pumping oxygen into the measurement electrode 44 may result in an undershoot waveform, resulting in a longer light-off time.

However, for a porous diffusion layer that has a suitable diffusion form around the measurement electrode 44, the porosity of the surface facing (contacting) the measurement electrode 44 is the same as the porosity of the surface not facing (contacting) the measurement electrode 44. The gas sensor element 101 achieves the following effects by making the rate higher than the rate. That is, by making the porosity of the surface facing the measurement electrode 44 higher than the porosity of the surface not facing the measurement electrode 44, the gas sensor element 101 quickly diffuses H ₂ generated near the surface of the measurement electrode 44. be able to. In other words, H ₂ generated by the decomposition of H ₂ O attached to the surface of the measurement electrode 44 is transferred to the measurement electrode 44 which has a larger porosity than the surface that does not face (does not touch) the measurement electrode 44. The "opposing (contacting) surfaces" allow rapid diffusion. Therefore, in the gas sensor element 101, the potential difference between the measurement electrode 44 and the reference electrode 42 does not become too large during constant control, and the gas sensor element 101 can shorten the light-off time. In other words, even if H ₂ O attached to the surface of the measurement electrode 44 is decomposed and H ₂ is generated, the gas sensor element 101 suppresses the influence of the H ₂ and the light-off time becomes longer. can be avoided.

In particular, when no space is provided between the porous diffusion layer 91A and the measurement electrode 44, that is, when they are in contact with each other, it is desirable that the porous diffusion layer 91A be configured as follows. That is, of the two faces facing opposite to each other in the porous diffusion layer 91A, the porous diffusion layer is formed so that the porosity of the face in contact with the measurement electrode 44 is higher than the porosity of the face not in contact with the measurement electrode 44. Layer 91A is preferably configured. Specifically, when the porous diffusion layer 91A and the measurement electrode 44 are in contact, the porosity of the surface of the porous diffusion layer 91A that is in contact with the measurement electrode 44 is the same as that of the surface that is not in contact with the measurement electrode 44 (for example, the porosity of the surface of the porous diffusion layer 91A that is in contact with the measurement electrode 44). It is desirable that the porosity be higher than that of the surface facing the flow section 7). In the porous diffusion layer 91A, the porosity of the surface in contact with the measurement electrode 44 is made higher than the porosity of the surface not in contact with the measurement electrode 44. However, the porous diffusion layer 91A can achieve the following effects. In other words, the porous diffusion layer 91A can suppress the influence when H ₂ O adhering to the surface of the measurement electrode 44 is decomposed to generate H ₂ and shorten the light-off time.

Note that the porosity can be derived, for example, by applying a known image processing method (such as binarization processing) to an image obtained by observation using a scanning electron microscope (SEM) (SEM image). This is the value. Specifically, the porosity of "the surface of the porous diffusion layer 91A facing (in contact with) the measurement electrode 44" was derived, for example, as follows. That is, first, an SEM image is taken near the center of the measurement electrode 44 and within a range of 10 to 15 μm from the interface between the measurement electrode 44 and the porous diffusion layer 91A when viewed in the length direction (axial direction of the sensor element). Obtained. Next, by applying a known image processing method such as binarization processing to the acquired SEM image, the porosity of "the surface of the porous diffusion layer 91A facing (in contact with) the measurement electrode 44" is determined. I asked for The porosity of "the surface of the porous diffusion layer 91A that does not face (does not touch) the measurement electrode 44" was also determined by applying the same method. That is, first, a SEM image is obtained in a range of 10 to 15 μm from the surface (surface, for example, the top surface) of the porous diffusion layer 91A that does not face (does not touch) the measurement electrode 44. Then, by applying a known image processing method to the SEM image, the porosity of the surface of the porous diffusion layer 91A that does not face (does not touch) the measurement electrode 44 was derived.

As described above, among the two surfaces facing opposite to each other in the porous diffusion layer 91A, the porosity of the surface that faces (contacts) the measurement electrode 44 is equal to the porosity of the surface that does not oppose (contact) the measurement electrode 44. It may be higher than In particular, in the porous diffusion layer 91A, the porosity of the surface facing (in contact with) the measurement electrode 44 may be 10% or more higher than the porosity of the surface not facing (in contact with) the measurement electrode 44.

Here, the present inventors believe that when the porosity of the surface facing (in contact with) the measurement electrode 44 is 10% or more higher than the porosity of the surface not facing (in contact with) the measurement electrode 44, It was confirmed that the light-off time was shorter than in the conventional case. Therefore, in the porous diffusion layer 91A, the difference between the porosity of the surface facing (in contact with) the measurement electrode 44 and the porosity of the surface not facing (in contact with) the measurement electrode 44 may be 10% or more. desirable. Specifically, of the two faces facing opposite to each other in the porous diffusion layer 91A, the porosity of the face facing (in contact with) the measurement electrode 44 is the same as the porosity of the face not facing (in contact with) the measurement electrode 44. It is desirable that the ratio be 10% or more higher than the ratio. The gas sensor element 101 has a structure in which the porosity of the surface of the porous diffusion layer 91A that faces (contacts) the measurement electrode 44 is made 10% or more higher than the porosity of the surface that does not oppose (contact) the measurement electrode 44. , the light-off time can be made shorter than in the case of less than 10%. For example, in the gas sensor element 101, the porosity of the "surface facing the measurement electrode 44" of the porous diffusion layer 91A is determined by the porosity of the "surface facing the measured gas flow section 7 without facing the measurement electrode 44". The light-off time can be further shortened by increasing the light-off rate by 10% or more.

As explained so far, of the two faces facing opposite to each other in the porous diffusion layer 91A, the porosity of the face that faces (contacts) the measurement electrode 44 is different from the porosity of the face that does not face (does not touch) the measurement electrode 44 ) may be higher than the porosity of the surface, for example, 10% or more. Here, the manner in which the porosity changes from the surface facing (in contact with) the measurement electrode 44 to the surface not facing (in contact with) the measurement electrode 44 in the porous diffusion layer 91A is not particularly limited. .

In the porous diffusion layer 91A, the porosity changes from the surface facing (in contact with) the measurement electrode 44 to the surface not facing (in contact with) the measurement electrode 44 (for example, the surface facing the gas flow section 7 to be measured). It may change stepwise (discontinuously). Specifically, the porous diffusion layer 91A may include the following two surfaces having different porosity from each other as two surfaces facing opposite to each other. That is, the porous diffusion layer 91A is a "first porous diffusion layer that is a porous layer that faces the measured gas flow section 7 without facing the measurement electrode 44" and a "porous layer that faces the measurement electrode 44". The second porous diffusion layer may also include a second porous diffusion layer. In this case, the porosity of the first porous diffusion layer is desirably lower than the porosity of the second porous diffusion layer, and particularly desirably 10% or more lower than the porosity of the second porous diffusion layer. . In other words, the porous diffusion layer 91A includes a plurality of layers having different porosity from each other, and the porosity of the porous diffusion layer 91A varies from the surface facing (contacting) the measurement electrode 44 to the surface not facing (contacting) the measurement electrode 44. It is also possible to change stepwise (discontinuously) to a surface that does not change.

Furthermore, in the porous diffusion layer 91A, the porosity changes from the surface facing (in contact with) the measurement electrode 44 to the surface not facing (in contact with) the measurement electrode 44 (for example, the surface facing the gas flow section 7 to be measured). It may change continuously. In other words, among the two surfaces facing opposite to each other in the porous diffusion layer 91A, the porosity is continuous from the surface facing (in contact with) the measurement electrode 44 to the surface not facing (in contact with) the measurement electrode 44. It may change to For example, the porosity gradually decreases from the surface facing the measurement electrode 44 to the surface not facing the measurement electrode 44 (for example, the surface facing the gas flow section 7 to be measured), and as a result, the porosity of both The porous diffusion layer 91A may be configured such that the difference between the two is 10% or more.

As explained above, of the two surfaces facing opposite to each other in the porous diffusion layer 91A, the porosity of the surface facing (in contact with) the measurement electrode 44 is the same as that of the surface not facing (in contact with) the measurement electrode 44. It may be higher than the porosity, particularly by 10% or more. The manner in which the porosity changes from the surface facing (in contact with) the measurement electrode 44 to the surface not facing (in contact with) the measurement electrode 44 is not particularly limited, and may be, for example, stepwise ( It may change discontinuously or continuously.

Note that a porous diffusion layer that is provided upstream from the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less and closes a predetermined range of the flow path CH has a plurality of porous diffusion layers having different porosity. When the porous diffusion layer has a surface (layer), the average porosity of the porous diffusion layer satisfies the following conditions. That is, when the porous diffusion layer has a plurality of surfaces having different porosities, the porous diffusion layer has an average porosity of 5% or more and 25% or less, and the average porosity is equal to or less than the tip protection layer. The porosity is lower than that of layer 200. For example, if the porosity of the porous diffusion layer 91A is different between the surface facing (in contact with) the measurement electrode 44 and the surface not facing (in contact with) the measurement electrode 44, the average porosity of the porous diffusion layer 91A is 5. % or more and 25% or less, which is lower than the porosity of the tip protection layer 200. Specifically, the average porosity of the porous diffusion layer 91A calculated from the porosity of each of the first porous diffusion layer and the second porous diffusion layer is 5% or more and 25% or less. , is lower than the porosity of the tip protection layer 200. That is, when the porous diffusion layer 91A includes a plurality of surfaces (layers), particularly when it includes a plurality of surfaces with different porosities, the average porosity of the porous diffusion layer 91A is 5% or more and 25% or less. Therefore, the porosity is lower than that of the tip protection layer 200.

(Length in the axial direction of the porous diffusion layer and diffusion controlling part)
The porous diffusion layer 91 illustrated in FIGS. 2 and 3 and the porous diffusion layer 91A illustrated in FIG. The width was greater than the width of the portion 18 (the length in the axial direction of the gas sensor element 101). That is, the width of the porous diffusion layer 91 and the width of the fourth diffusion rate controlling part 18 were similar, and the width of the porous diffusion layer 91A was longer than the width of the fourth diffusion rate controlling part 18. However, in the gas sensor element 101 (element substrate 100), the width of the porous diffusion layer provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less is the fourth diffusion rate limiting layer. It is not essential that the width be equal to or greater than the width of the portion 18. In the gas sensor element 101, the width of the porous diffusion layer provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less is shorter than the width of the fourth diffusion-limiting section 18. It's okay. That is, in the gas sensor element 101, the width of the diffusion-limiting part (for example, the fourth diffusion-limiting part 18) forming the flow path CH whose predetermined range is blocked by the porous diffusion layer is wider than the width of the porous diffusion layer. It can be long. Below, with reference to FIG. 6, an example of the fourth diffusion-limiting section 18 having a width longer than the width of the porous diffusion layer provided for optimizing the diffusion form around the measurement electrode 44 will be explained. do.

(Example of a diffusion-limiting part having a width longer than the width of the porous diffusion layer)
FIG. 6 is an enlarged view for explaining essential parts of an element substrate 100 (gas sensor element 101) including a fourth diffusion rate-limiting section 18B according to a modification. Specifically, FIG. 6 shows the width of the fourth diffusion-limiting section 18B having a width longer than the width of the fourth diffusion-limiting section 18 (the length in the axial direction of the gas sensor element 101) illustrated in FIGS. 2, 3, and 5. It is a figure showing an example. Note that the first solid electrolyte layer 4, spacer layer 5, second solid electrolyte layer 6, etc. in FIG. 6 are the same as those described using FIGS. 2, 3, etc., so the description will not be repeated.

The basic configuration of the fourth diffusion control section 18B is the same as that of the fourth diffusion control section 18. That is, the fourth diffusion rate controlling section 18B imparts a predetermined diffusion resistance to the gas to be measured in the gas to be measured distribution section 7 (internal space). Further, the porosity of the fourth diffusion-limiting portion 18B is lower than that of the porous diffusion layer 91, that is, the fourth diffusion-limiting portion 18B is denser than the porous diffusion layer 91. Furthermore, the fourth diffusion rate-limiting section 18B is provided upstream of the measurement electrode 44 in the flow direction DR of the gas to be measured.

However, the fourth diffusion control section 18B differs from the fourth diffusion control section 18 in the following points. That is, the fourth diffusion-limiting section 18B arranged upstream of the measurement electrode 44 extends downstream, that is, toward the measurement electrode 44, and particularly extends to the downstream side of the measurement electrode 44. In the example shown in FIG. 6, the fourth diffusion rate-limiting section 18B arranged upstream of the measurement electrode 44 extends to the downstream side of the measurement electrode 44 and is in contact with the spacer layer 5. Therefore, the width of the fourth diffusion-limiting portion 18B (the length in the axial direction of the gas sensor element 101) is longer than the width of the porous diffusion layer 91.

In the example shown in FIG. 6, the flow path CH(B) defined by the fourth diffusion-limiting part 18B and the second solid electrolyte layer 6 is integrated with the third internal space 19B in which the measurement electrode 44 is arranged. There is. That is, the fourth diffusion rate-limiting portion 18B extends downstream, and in particular extends downstream from the measurement electrode 44 and is in contact with the spacer layer 5. The flow path CH(B) of the gas to be measured that reaches the measurement electrode 44 is defined by the fourth diffusion rate-limiting section 18B and the second solid electrolyte layer 6. Specifically, in the example shown in FIG. 6, the flow path CH(B) (that is, the third internal space 19B) has an upper surface formed by the second solid electrolyte layer 6, a lower surface formed by the fourth diffusion-limiting section 18B, The side and rear surfaces are defined (divided) by a spacer layer 5. A porous diffusion layer 91 whose distance d2 to the measurement electrode 44 is 0.15 mm or less closes the entrance of the channel CH(B) (that is, the third internal space 19B).

That is, the fourth diffusion rate-limiting section 18B defines (divides) at least one surface of the flow path CH(B) of the gas to be measured leading to the measurement electrode 44, and in the example shown in FIG. B) defines the lower surface. Then, the predetermined range of the flow path CH(B) formed (defined) by the fourth diffusion rate controlling part 18B is located upstream of the measurement electrode 44, and the distance d2 to the measurement electrode 44 is 0.15 mm or less. It is closed by a porous diffusion layer 91 provided at the position.

The gas sensor element 101 includes a flow path CH (B) whose predetermined range (specifically, 70% or more) is blocked by the porous diffusion layer 91 instead of the flow path CH described using FIG. 2 and the like. You can leave it there. In the gas sensor element 101, the diffusion-limiting part that defines the flow path CH(B) may extend downstream, like the fourth diffusion-limiting part 18B, and in particular, may extend to the downstream side of the measurement electrode 44. It's okay. In the example shown in FIG. 6, the fourth diffusion-limiting section 18B, which is arranged upstream of the measurement electrode 44, extends to the downstream side of the measurement electrode 44. Then, a flow path CH(B) defined by the second solid electrolyte layer 6 and the fourth diffusion-limiting section 18B, which is arranged upstream of the measurement electrode 44 and extends to the downstream side of the measurement electrode 44, is formed. (That is, the entrance of the third internal cavity 19B) is closed by the porous diffusion layer 91.

As explained so far, like the flow path CH, at least one surface of the flow path CH(B) is defined (divided) by the fourth diffusion rate controlling part 18B, and in the example shown in FIG. , the lower surface of the flow path CH(B) is defined by the fourth diffusion-limiting portion 18B. The gas sensor element 101 can guide the gas to be measured to the measurement electrode 44 by using the flow path CH(B) in which at least one surface is defined by the fourth diffusion rate limiting part 18B. Therefore, the gas sensor element 101 allows the diffusion form of the gas to be measured to reach the measurement electrode 44 to be controlled by the Knudsen diffusion type by the porous diffusion layer 91 that closes a predetermined range of the flow path CH(B) through which the gas to be measured is guided. It is possible to get closer to a suitable diffusion form.

Further, a surface of the flow path CH(B) different from the surface defined by the fourth diffusion-limiting portion 18B (in the example shown in FIG. 6, the upper surface) is defined (divided) by the second solid electrolyte layer 6. ing). The porous diffusion layer 91 is arranged at the entrance of the channel CH(B), that is, on the upstream side of the measurement electrode 44, and particularly at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less. It is located in By the porous diffusion layer 91, a predetermined range of the entrance of the channel CH(B) is blocked, specifically, 70% or more is blocked.

In particular, the porous diffusion layer 91 in the example shown in FIG. The second solid electrolyte layer 6 (particularly the lower surface of the second solid electrolyte layer 6) is in contact with the second solid electrolyte layer 6. That is, there is no space (gap) between the porous diffusion layer 91 and the fourth diffusion-limiting section 18B and between the porous diffusion layer 91 and the surface defining the gas distribution section 7 to be measured. Therefore, in the gas sensor element 101, the gas to be measured is distributed between the gap between the porous diffusion layer 91 and the fourth diffusion control section 18B and the surface defining the porous diffusion layer 91 and the gas distribution section 7 to be measured. It is possible to prevent at least one of the gaps from reaching the measurement electrode 44. Therefore, the gas sensor element 101 allows the diffusion form of the gas to be measured to reach the measurement electrode 44 to be controlled by the Knudsen diffusion type by the porous diffusion layer 91 that closes a predetermined range of the flow path CH(B) through which the gas to be measured is guided. It is possible to get closer to a suitable diffusion form.

(Porous diffusion layer and diffusion controlling section according to modified example)
As described above, in the gas sensor element 101, the porous diffusion layer provided to make the diffusion form around the measurement electrode 44 suitable may be in contact with the measurement electrode 44. Furthermore, the flow path whose predetermined range is blocked by such a porous diffusion layer may be defined by a diffusion rate controlling section. The diffusion-limiting portion may extend downstream, for example, downstream of the measurement electrode 44. Below, an example of a gas sensor element 101 (element substrate 100) including a porous diffusion layer in contact with the measurement electrode 44 and a diffusion control section extending downstream from the measurement electrode 44 will be described using FIG. 7.

FIG. 7 is an enlarged view for explaining the main parts of an element substrate 100 (gas sensor element 101) including a porous diffusion layer 91C and a fourth diffusion rate controlling part 18C according to a modification. Specifically, FIG. 7 shows "the porous diffusion layer 91C and the fourth diffusion-limiting section 18C" which are arranged upstream of the measurement electrode 44 and extend downstream (that is, toward the measurement electrode 44), respectively. It is a figure showing an example. Note that the first solid electrolyte layer 4, spacer layer 5, second solid electrolyte layer 6, etc. in FIG. 7 are the same as those described using FIGS. 2, 3, etc., so the description will not be repeated. Instead of the porous diffusion layer 91, the gas sensor element 101 may include a porous diffusion layer 91C, which will be described in detail below.

Like the porous diffusion layer 91A, the porous diffusion layer 91C is arranged upstream of the measurement electrode 44 and extends downstream. Specifically, the porous diffusion layer 91C extends toward the measurement electrode 44 and performs measurement. It is in contact with the electrode 44. In particular, the porous diffusion layer 91C illustrated in FIG. 7 extends to the downstream side of the measurement electrode 44 and covers the measurement electrode 44. Further, like the porous diffusion layers 91 and 91A described above, the porous diffusion layer 91C has a porosity lower than that of the tip protection layer 200, and is 5% or more and 25% or less. be. Like the porous diffusion layers 91 and 91A, the porous diffusion layer 91C is located upstream of the measurement electrode 44 and covers a predetermined range (specifically, 70% or more) of the flow path CH(C) of the gas to be measured. is blocking the

The fourth diffusion-limiting section 18C, like the fourth diffusion-limiting section 18B, is arranged upstream of the measurement electrode 44 and extends downstream (that is, toward the measurement electrode 44), and is particularly illustrated in FIG. The fourth diffusion-limiting section 18C extends to the downstream side of the measurement electrode 44. Specifically, the fourth diffusion rate-limiting section 18C arranged upstream of the measurement electrode 44 extends downstream and is in contact with the spacer layer 5. Further, the fourth diffusion-limiting section 18C has a porosity lower than that of the porous diffusion layer 91, similar to the fourth diffusion-limiting sections 18 and 18B. It is denser than 91C. The fourth diffusion rate controlling section 18C imparts a predetermined diffusion resistance to the gas to be measured in the gas to be measured distribution section 7 (internal space).

In the example shown in FIG. 7, the flow path CH(C) defined by the fourth diffusion-limiting part 18C and the second solid electrolyte layer 6 is integrated with the third internal space 19C in which the measurement electrode 44 is arranged. There is. That is, the fourth diffusion-limiting section 18C extends downstream, and in particular extends downstream from the measurement electrode 44 and is in contact with the spacer layer 5. The flow path CH(C) of the gas to be measured that reaches the measurement electrode 44 is defined by the fourth diffusion-limiting section 18C and the second solid electrolyte layer 6. Specifically, in the example shown in FIG. 7, the flow path CH(C) (that is, the third internal space 19C) has an upper surface formed by the second solid electrolyte layer 6 and a lower surface formed by the fourth diffusion-limiting section 18C. The side and rear surfaces are defined (divided) by a spacer layer 5. Then, the porous diffusion layer 91C completely blocks the flow path CH(C) (that is, the third internal space 19C) defined by the fourth diffusion rate controlling part 18C and the second solid electrolyte layer 6.

Like the channels CH and CH(B), at least one surface of the channel CH(C) is defined (divided) by the fourth diffusion rate controlling section 18C, and in the example shown in FIG. The lower surface of (C) is defined by the fourth diffusion-limiting section 18C. The gas sensor element 101 can guide the gas to be measured to the measurement electrode 44 using the flow path CH(C) in which at least one surface is defined by the fourth diffusion rate-limiting part 18C. Therefore, the gas sensor element 101 allows the diffusion form of the gas to be measured to reach the measurement electrode 44 to be controlled by the Knudsen diffusion type by the porous diffusion layer 91 that closes a predetermined range of the flow path CH (C) through which the gas to be measured is guided. It is possible to get closer to a suitable diffusion form.

Further, a surface of the flow path CH(C) different from the surface defined by the fourth diffusion-limiting portion 18C (in the example shown in FIG. 7, the upper surface) is defined (divided) by the second solid electrolyte layer 6. ing). The porous diffusion layer 91C is arranged at the entrance of the channel CH(C), that is, upstream of the measurement electrode 44, and extends downstream to contact the measurement electrode 44. By the porous diffusion layer 91, a predetermined range of the surface of the flow path CH(C) (that is, the third internal space 19C) that is orthogonal to the flow direction DR of the gas to be measured is blocked, and specifically, More than 70% is blocked.

In particular, in the example shown in FIG. 7, the porous diffusion layer 91C has a surface (in the example shown in FIG. The second solid electrolyte layer 6 (particularly the lower surface of the second solid electrolyte layer 6) is in contact with the second solid electrolyte layer 6. That is, there is no space (gap) between the porous diffusion layer 91C and the fourth diffusion-limiting section 18C, and between the porous diffusion layer 91C and the surface defining the gas distribution section 7 to be measured. Therefore, in the gas sensor element 101, the gas to be measured flows through the gap between the porous diffusion layer 91C and the fourth diffusion controlling section 18C, and between the porous diffusion layer 91C and the surface defining the gas distribution section 7. It is possible to prevent at least one of the gaps from reaching the measurement electrode 44. Therefore, the gas sensor element 101 allows the diffusion form of the gas to be measured to reach the measurement electrode 44 to be controlled by the Knudsen diffusion type by the porous diffusion layer 91C that closes a predetermined range of the flow path CH(C) through which the gas to be measured is guided. It is possible to get closer to a suitable diffusion form.

(Flow path according to modified example)
The channels CH, CH(B), and CH(C) that have been explained so far are all connected to one surface (for example, , lower surface) was defined (divided). However, the flow path that is blocked in a predetermined area by a porous diffusion layer (any one of the porous diffusion layers 91, 91A, and 91C) provided to optimize the diffusion form around the measurement electrode 44 is difficult to diffuse. It is not essential that one surface be defined by the rate-limiting section. A flow path whose predetermined area is closed by a porous diffusion layer provided to optimize the diffusion form around the measurement electrode 44 may have a plurality of surfaces defined by diffusion-limiting parts, for example, At least two surfaces may be defined by diffusion-limiting parts. Furthermore, the flow path whose predetermined area is closed by the porous diffusion layer provided to make the diffusion form around the measurement electrode 44 suitable does not have to be defined by the diffusion-limiting section. Hereinafter, with reference to FIGS. 8 to 10, an example will be described in which a flow path whose predetermined area is blocked by a porous diffusion layer has a plurality of surfaces defined by diffusion-limiting parts. Further, with reference to FIG. 11, an example will be described in which a flow path whose predetermined area is blocked by a porous diffusion layer is not defined (divided) by a diffusion-limiting section.

(Example 1 of a flow path where multiple surfaces are defined by a diffusion-limiting section)
FIG. 8 is an enlarged view for explaining essential parts of an element substrate 100 (gas sensor element 101) including a flow path CH(D) according to a modification. Specifically, FIG. 8 shows a flow path CH(D) whose upper surface and lower surface are defined (divided) by the fourth diffusion-limiting section 18D(1) and the fourth diffusion-limiting section 18D(2). It is a figure showing an example. In addition, in the following description, when the fourth diffusion-limiting part 18D(1) and the fourth diffusion-limiting part 18D(2) are not particularly distinguished, the fourth diffusion-limiting part 18D(1) and the fourth diffusion-limiting part Together with 18D(2), it is referred to as a "fourth diffusion controlling section 18D." Furthermore, the first solid electrolyte layer 4, spacer layer 5, second solid electrolyte layer 6, etc. in FIG. 8 are the same as those described using FIGS. 2, 3, etc., so the description will not be repeated. The gas sensor element 101 may include a channel CH (D) and a porous diffusion layer 91D, which will be described in detail below, instead of the channel CH and the porous diffusion layer 91 described using FIG. 2 and the like. .

Each of the flow channels CH, CH(B), and CH(C) described so far has one surface formed by a diffusion-limiting section (any of the fourth diffusion-limiting section 18, 18B, or 18C). It was defined (divided). Specifically, the lower surface of each of the channels CH etc. was defined by a diffusion-limiting section (any of the fourth diffusion-limiting sections 18, 18B, 18C), and the upper surface was defined by the second solid electrolyte layer 6 ( sectioned). On the other hand, the flow path CH(D) has at least two surfaces defined by a diffusion-limiting section (specifically, the fourth diffusion-limiting section 18D). That is, the flow path CH(D) is defined by the fourth diffusion-limiting section 18D(1) on the upper surface and by the fourth diffusion-limiting section 18D(2) on the lower surface.

The flow path CH(D) whose upper and lower surfaces are defined by the fourth diffusion-limiting part 18D(1) and the fourth diffusion-limiting part 18D(2) is integrated with the third internal space 19D in which the measurement electrode 44D is arranged. It becomes. Specifically, in the example shown in FIG. 8, the flow path CH(D) (that is, the third internal space 19D) has an upper surface and a lower surface formed by the fourth diffusion-limiting part 18D, and a side surface and a rear surface formed by the spacer layer 5. , defined (divided). Then, the inlet of the flow path CH(D) (i.e., the third internal space 19D) defined by the fourth diffusion-limiting section 18D(1) and the fourth diffusion-limiting section 18D(2) is connected to the measuring electrode 44D. It is closed by a porous diffusion layer 91D with a distance d2 of 0.15 mm or less.

The basic configuration of the fourth diffusion control section 18D is the same as the fourth diffusion control section 18 described above. That is, the fourth diffusion rate-limiting section 18D imparts a predetermined diffusion resistance to the gas to be measured in the gas to be measured distribution section 7 (internal space). Further, the fourth diffusion rate controlling portion 18D has a lower porosity than the porous diffusion layer 91D, that is, it is denser than the porous diffusion layer 91D.

However, the fourth diffusion rate controlling part 18D is constituted by a solid electrolyte layer having oxygen ion conductivity such as zirconia (ZrO ₂ ). The measurement electrode 44D illustrated in FIG. 8 is provided on the surface of the fourth diffusion-limiting section 18D. Therefore, the fourth diffusion-limiting section 18D is configured of a solid electrolyte layer having oxygen ion conductivity so that oxygen can be pumped out and pumped in through the fourth diffusion-limiting section 18D.

The fourth diffusion-limiting section 18D, like the fourth diffusion-limiting sections 18B and 18C, is provided upstream of the measurement electrode 44D and extends downstream (that is, toward the measurement electrode 44D). In particular, the fourth diffusion-limiting section 18D illustrated in FIG. 8 extends to the downstream side of the measurement electrode 44D and is in contact with the spacer layer 5, similarly to the fourth diffusion-limiting sections 18B and 18C.

The measurement electrode 44D is provided on the lower surface of the fourth diffusion-limiting section 18D (1) instead of on the lower surface of the second solid electrolyte layer 6 adjacent to (facing) the third internal cavity 19. This is similar to the measurement electrode 44 described above. The measurement pump cell 41 includes a measurement electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, and a fourth diffusion-limiting portion 18D having oxygen ion conductivity. Ru. Note that it is not essential to provide the measurement electrode 44D on the lower surface of the fourth diffusion-limiting section 18D(1), and the measurement electrode 44D may be provided on the upper surface of the fourth diffusion-limiting section 18D(2).

Except that the porous diffusion layer 91D is disposed on the channel CH(D) in which at least two surfaces (in the example shown in FIG. 8, the upper surface and the lower surface) are defined by the fourth diffusion-limiting section 18D. , is similar to the porous diffusion layer 91 described above. That is, the porosity of the porous diffusion layer 91D is lower than that of the tip protection layer 200, and is 5% or more and 25% or less. Further, the porous diffusion layer 91D is provided upstream of the measurement electrode 44D in the direction DR in which the gas to be measured flows, at a position where the distance to the measurement electrode 44D is 0.15 mm or less. A predetermined range of the entrance of the channel CH(D) is blocked by the porous diffusion layer 91D. Specifically, as will be described later with reference to FIG. 9, the area of the surface of the porous diffusion layer 91D perpendicular to the flow direction DR of the gas to be measured is It occupies (occupies) 70% or more of the area of the surface perpendicular to the gas flow direction DR.

FIG. 9 is a diagram showing an example of a cross section of the element substrate 100 shown in FIG. 8 taken along the line II-II. The area of the surface of the porous diffusion layer 91D perpendicular to the flow direction DR of the gas to be measured (in the example shown in FIG. It occupies 70% or more of the area of the surface perpendicular to the flow direction DR. In the example shown in FIG. 9, the flow path CH(D) of the gas to be measured from the second internal space 17 to the third internal space 19 is formed by the fourth diffusion rate controlling part 18D (defined as ). Specifically, a flow path CH(D) is formed as a slit (gap) between the fourth diffusion rate limiting part 18D(1) and the fourth diffusion rate limiting part 18D(2). That is, the flow path CH illustrated using the two-dot chain line in FIG. are divided (defined) by the side surfaces of the spacer layer 5. 70% or more of the area of the surface of the flow path CH(D) perpendicular to the flow direction DR of the gas to be measured is occupied by the surface of the porous diffusion layer 91D perpendicular to the flow direction DR of the gas to be measured. It is being In other words, the porous diffusion layer 91D provided in contact with the upper surface of the fourth diffusion rate controlling portion 18D(2) occupies (blocks) 70% or more of the channel CH(D).

Note that in the example shown in FIG. 9, there is no space (gap) between the upper surface of the porous diffusion layer 91D and the lower surface of the fourth diffusion rate-controlling section 18D(1). That is, in the example shown in FIG. 9, the area of the surface of the flow path CH(D) perpendicular to the flow direction DR of the gas to be measured is compared to the area of the surface of the porous diffusion layer 91D perpendicular to the flow direction DR of the gas to be measured. The area occupied by the surface is 100%. However, the ratio of the area of the surface of the porous diffusion layer 91D perpendicular to the flow direction DR of the gas to be measured to the area of the surface of the flow path CH(D) perpendicular to the flow direction DR of the gas to be measured. may be 70% or more, and may not be 100%. In other words, the surface of the porous diffusion layer 91D that is perpendicular to the flow direction DR of the gas to be measured does not have to occupy the entire surface of the flow path CH(D) that is perpendicular to the flow direction DR of the gas to be measured. . In accordance with the example shown in FIG. 9, the porous diffusion layer 91D and the fourth diffusion-limiting section 18D(1) do not need to be in contact with each other. Specifically, the upper surface of the porous diffusion layer 91D and the fourth It does not need to be in contact with the lower surface of the diffusion-limiting portion 18D(1).

As described above, at least two surfaces (in the example shown in FIG. 8, the upper surface and the lower surface) of the flow path CH(D) are defined by the fourth diffusion-limiting section 18D. As described above, the fourth diffusion rate-limiting portion 18D is denser (has a lower porosity) than the porous diffusion layer 91D. In other words, the gas sensor element 101 utilizes the fourth diffusion-limiting portion 18D, which is denser than the porous diffusion layer 91D, to control at least two surfaces of the flow path CH(D) that guides the gas to be measured to the measurement electrode 44D. define (divide). Therefore, the gas sensor element 101 transfers the gas to be measured to the measurement electrode 44D by using the flow path CH(D) in which at least two surfaces are defined by the fourth diffusion-limiting part 18D, which is denser than the porous diffusion layer 91D. can lead to. In the gas sensor element 101, for example, a situation occurs in which the gas to be measured leaks out in the middle of the flow path CH(D) and reaches the measurement electrode 44D without having its diffusion form changed by the porous diffusion layer 91D. Possibilities can be further reduced. Therefore, the gas sensor element 101 allows the diffusion form of the gas to be measured to reach the measurement electrode 44D to be similar to Knudsen diffusion by the porous diffusion layer 91D that closes a predetermined range of the flow path CH(D) through which the gas to be measured is guided. It is possible to get closer to a suitable diffusion form.

(Example 2 of a flow path where multiple surfaces are defined by a diffusion-limiting section)
FIG. 10 shows an element substrate 100 including a channel CH(E) similar to the channel CH(D) illustrated in FIG. 8 and a porous diffusion layer 91E different from the porous diffusion layer 91D illustrated in FIG. (Gas sensor element 101) It is an enlarged view for explaining the main part. Specifically, FIG. 10 shows a flow path CH(E) whose upper surface and lower surface are defined (divided) by the fourth diffusion-limiting section 18E(1) and the fourth diffusion-limiting section 18E(2); FIG. 12 is a diagram showing an example of a porous diffusion layer 91E extending downstream. In addition, in the following description, when the fourth diffusion-limiting part 18E(1) and the fourth diffusion-limiting part 18E(2) are not particularly distinguished, the fourth diffusion-limiting part 18E(1) and the fourth diffusion-limiting part Together with 18E(2), it is referred to as a "fourth diffusion controlling section 18E." Furthermore, the first solid electrolyte layer 4, spacer layer 5, second solid electrolyte layer 6, etc. in FIG. 10 are the same as those described using FIGS. 2, 3, etc., so the description will not be repeated.

Like the flow path CH(D), the flow path CH(E) has at least two surfaces (in the example shown in FIG. ). That is, the flow path CH(E) is defined by the fourth diffusion-limiting part 18E(1) on the upper surface and the fourth diffusion-limiting part 18E(2) on the lower surface, and is defined by the third internal space in which the measurement electrode 44 is arranged. It is integrated with Tokoro 19E. Specifically, in the example shown in FIG. 10, the flow path CH(E) (that is, the third internal cavity 19E) has an upper surface and a lower surface formed by the fourth diffusion-limiting part 18E, and a side surface and a rear surface formed by the spacer layer 5. , defined (divided). The entire channel CH(E) (that is, the third internal space 19E) is closed by the porous diffusion layer 91E.

The configuration of the fourth diffusion control section 18E is similar to that of the fourth diffusion control section 18D. That is, the fourth diffusion rate controlling section 18E imparts a predetermined diffusion resistance to the gas to be measured in the gas to be measured distribution section 7 (internal space). The fourth diffusion rate controlling part 18E has a lower porosity than the porous diffusion layer 91E, that is, it is denser than the porous diffusion layer 91E. The fourth diffusion rate-limiting section 18E is constituted by a solid electrolyte layer having oxygen ion conductivity such as zirconia (ZrO ₂ ). The fourth diffusion-limiting section 18E is provided upstream of the measurement electrode 44E and extends downstream (that is, toward the measurement electrode 44E). In particular, the fourth diffusion-limiting section 18E illustrated in FIG. It extends to the downstream side of the electrode 44E. Specifically, the fourth diffusion rate-limiting section 18E, which is arranged upstream of the measurement electrode 44E, extends downstream and is in contact with the spacer layer 5.

As described above, at least two surfaces (in the example shown in FIG. 10, the upper surface and the lower surface) of the flow path CH(E) are defined by the fourth diffusion-limiting section 18E. As described above, the fourth diffusion-limiting portion 18E is denser (has a lower porosity) than the porous diffusion layer 91E. In other words, the gas sensor element 101 utilizes the fourth diffusion-limiting portion 18E, which is denser than the porous diffusion layer 91E, to control at least two surfaces of the flow path CH(E) that guides the gas to be measured to the measurement electrode 44E. define (divide). Therefore, the gas sensor element 101 transfers the gas to be measured to the measurement electrode 44E by using the flow path CH(E) in which at least two surfaces are defined by the fourth diffusion-limiting portion 18E, which is denser than the porous diffusion layer 91E. can lead to. In the gas sensor element 101, for example, a situation occurs in which the gas to be measured leaks out in the middle of the flow path CH(E) and reaches the measurement electrode 44E without having its diffusion form changed by the porous diffusion layer 91E. Possibilities can be further reduced. Therefore, the gas sensor element 101 allows the diffusion form of the gas to be measured to reach the measurement electrode 44E to be controlled by the Knudsen diffusion type by the porous diffusion layer 91E that closes a predetermined range of the flow path CH(E) through which the gas to be measured is guided. It is possible to get closer to a suitable diffusion form.

The measurement electrode 44E, like the measurement electrode 44D, is provided on the lower surface of the fourth diffusion-limiting section 18E(1). The measurement pump cell 41 includes a measurement electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, and a fourth diffusion-limiting portion 18E having oxygen ion conductivity. Ru. Note that it is not essential to provide the measurement electrode 44E on the lower surface of the fourth diffusion-limiting section 18E(1), and the measurement electrode 44E may be provided on the upper surface of the fourth diffusion-limiting section 18E(2).

Porous diffusion layer 91E is similar to porous diffusion layer 91D except that it is in contact with measurement electrode 44E. That is, like the porous diffusion layers 91A and 91C, the porous diffusion layer 91E is provided upstream of the measurement electrode 44E and extends downstream. Specifically, the porous diffusion layer 91E extends toward the measurement electrode 44E. and is in contact with the measurement electrode 44E. In particular, the porous diffusion layer 91E illustrated in FIG. 10 extends to the downstream side of the measurement electrode 44E and covers the measurement electrode 44E. Further, the porous diffusion layer 91E has a porosity lower than that of the tip protection layer 200, and is 5% or more and 25% or less, similar to the porous diffusion layer 91 and the like described above. . The area of the surface of the porous diffusion layer 91E perpendicular to the flow direction DR of the gas to be measured is 70% of the area of the surface perpendicular to the flow direction DR of the gas to be measured of the flow path CH(E) of the gas to be measured. occupies (occupies) more than

(Example of flow path not defined by diffusion-limiting part)
FIG. 11 is an enlarged view for explaining the main parts of the element substrate 100 (gas sensor element 101) that includes a flow path CH(F) that is not defined by a diffusion regulating section (fourth diffusion regulating section 18, etc.). Specifically, FIG. 11 shows an example of a porous diffusion layer 91F that does not include the fourth diffusion rate controlling part 18 and closes a predetermined range of the flow path CH(F) of the gas to be measured leading to the measurement electrode 44. It is a diagram. The gas sensor element 101 may include a channel CH (F) and a porous diffusion layer 91F, which will be described in detail below, instead of the channel CH and the porous diffusion layer 91 described using FIG. 2 and the like. .

Each of the channels CH, CH(B), CH(C), CH(D), and CH(E) described so far has at least one It was defined (divided) into two sides (e.g., the lower side). In other words, the gas sensor element 101 described so far provides a predetermined diffusion resistance to the gas to be measured in the gas to be measured flow section 7, has a porosity lower than that of the porous diffusion layer, A "diffusion-limiting section" provided upstream of the electrode 44 was provided. Then, by the diffusion-limiting portion, at least one surface of the flow path (such as the above-mentioned flow path CH) whose predetermined range (specifically, 70% or more) is blocked by the porous diffusion layer 91 or the like is defined. was.

On the other hand, none of the surfaces of the channel CH(F) illustrated in FIG. 11 are defined by such a diffusion-limiting section (fourth diffusion-limiting section 18, etc.). Specifically, the flow channel CH(F) illustrated in FIG. , the side surfaces are defined (divided) by the side surfaces of the spacer layer 5.

More than 70% of the surface of the flow path CH(F) perpendicular to the flow direction DR of the gas to be measured (from the left side to the right side of the page in the example shown in FIG. 11) is covered by the gas to be measured in the porous diffusion layer 91F. is occupied by a plane perpendicular to the flow direction DR. That is, the area of the surface of the porous diffusion layer 91F perpendicular to the flow direction DR of the gas to be measured is equal to the area of the surface perpendicular to the flow direction DR of the gas to be measured in the flow path CH(F) (that is, the gas flow section 7). It occupies (occupies) more than 70% of the area of the orthogonal planes.

The porous diffusion layer 91F has a point that blocks a predetermined range (specifically, 70% or more) of the flow path CH(F) that is not defined by the diffusion-limiting section (fourth diffusion-limiting section 18, etc.) on any surface. Except for this, it is the same as the porous diffusion layer 91 and the like that have been explained so far. That is, the porous diffusion layer 91F has a porosity lower than that of the tip protection layer 200, and is 5% or more and 25% or less. Further, the porous diffusion layer 91F is provided upstream of the measurement electrode 44 in the flow direction DR of the gas to be measured at a position where the distance to the measurement electrode 44 is 0.15 mm or less. Furthermore, the area of the surface of the porous diffusion layer 91F perpendicular to the flow direction DR of the gas to be measured accounts for 70% or more of the area of the surface of the flow path CH(F) perpendicular to the flow direction DR of the gas to be measured. occupy (occupy) Due to the porous diffusion layer 91F, the gas sensor element 101 adopts a diffusion form around the measurement electrode 44, such as Knudsen diffusion, in which the gas is diffused while repeatedly colliding with the wall surface of a sufficiently narrow channel.

The measurement electrode 44 is arranged in the third internal space 19F (internal space) defined (divided) on the upstream side by the porous diffusion layer 91F in the gas flow section 7 (internal space) to be measured. The gas to be measured is introduced into the third internal space 19F through the channel CH(F) whose predetermined range (70% or more) is blocked by the porous diffusion layer 91F. The operation measures nitrogen oxide (NO _x ) concentration. Except that the third internal space 19F is defined on the upstream side by a porous diffusion layer 91F that can impart a predetermined diffusion resistance to the gas to be measured, instead of the fourth diffusion rate limiting section 18. This is similar to the third internal space 19.

As described using FIG. 11, in the gas sensor element 101, the flow path of the gas to be measured leading to the measurement electrode 44 is defined by a diffusion-limiting section (a fourth diffusion-limiting section) disposed upstream of the measurement electrode 44. 18 etc.). In the gas sensor element 101, the flow path whose predetermined range is blocked by the "porous diffusion layer for optimizing the diffusion form around the measurement electrode 44" is "arranged upstream of the measurement electrode 44". The invention is not limited to one in which at least one surface is defined by a "diffusion-limiting part". In the gas sensor element 101, in the flow path of the gas to be measured leading to the measurement electrode 44, 70% or more of the surface perpendicular to the flow direction DR of the gas to be measured is the above-mentioned porous diffusion layer in the direction in which the gas to be measured flows. It suffices if it is occupied by a plane orthogonal to DR. A porous diffusion layer that provides a suitable diffusion form around the measurement electrode 44 is provided upstream of the measurement electrode 44 at a position where the distance to the measurement electrode 44 is 0.15 mm or less. It is only necessary to close a predetermined range (specifically, 70% or more) of the flow path leading to. It is not essential for the gas sensor element 101 that the flow path whose predetermined range is blocked by the porous diffusion layer is defined on at least one surface by a diffusion-limiting section disposed upstream of the measurement electrode 44.

Note that FIG. 11 shows an example in which the longitudinal direction of the gas to be measured distribution section 7 (the longitudinal direction of the gas sensor element 101) matches the flow direction DR of the gas to be measured. However, for the gas sensor element 101, it is not essential that the longitudinal direction of the gas to be measured distribution section 7 (the longitudinal direction of the gas sensor element 101) coincides with the direction DR in which the gas to be measured flows. In the gas sensor element 101, the flow direction DR of the gas to be measured in the flow path CH(F) may be inclined with respect to the longitudinal direction of the gas to be measured portion 7; The direction may be perpendicular to the direction.

(Example where the tip protective layer has a multilayer structure)
The tip protection layer 200 illustrated in FIG. 1 has a constant porosity throughout. However, it is not essential that the tip protection layer included in the gas sensor element 101 has a constant porosity throughout. The tip protection layer included in the gas sensor element 101 may have a multilayer structure. An example in which the tip protection layer included in the gas sensor element 101 has a multilayer structure will be described below with reference to FIG. 12.

FIG. 12 is a schematic cross-sectional view schematically showing an example of the configuration of a gas sensor element 101 including a tip protection layer 200G according to a modification. Specifically, FIG. 12 is a diagram showing an example of a tip protection layer 200G including an inner tip protection layer 201 and an outer tip protection layer 202 having different porosities. The gas sensor element 101 may include a tip protection layer 200G illustrated in FIG. 12, which includes an inner tip protection layer 201 and an outer tip protection layer 202, instead of the tip protection layer 200 illustrated in FIG.

The tip protection layer 200G covers at least the surface of the element substrate 100 where the gas inlet 10 is open (the tip surface of the element substrate 100). In the example shown in FIG. 12, the tip protection layer 200G is provided to cover the tip surface of the element substrate 100 and four side surfaces of the element substrate 100 that are continuous with the tip surface.

The tip protection layer 200G includes at least an inner tip protection layer 201 and an outer tip protection layer 202. The inner tip protection layer 201 is in contact with the surface of the element base 100 where the gas introduction port 10 is open. The outer tip protection layer 202 constitutes the outermost surface of the tip protection layer 200G. The porosity of the inner tip protection layer 201 is greater than the porosity of the outer tip protection layer 202, and the thickness of the inner tip protection layer 201 is 30% or more and 90% or less of the thickness of the tip protection layer 200G. That is, the tip protection layer 200G includes an inner tip protection layer 201 and an outer tip protection layer 202, and the porosity of the inner tip protection layer 201 is larger than that of the outer tip protection layer 202, and The thickness is 30% to 90% of the thickness of the tip protection layer 200G.

In this configuration, the tip protection layer 200G includes at least two or more layers, and the inner layer (for example, the inner tip protection layer 201) has more pores than the outer layer (for example, the outer tip protection layer 202). The rate is large. By making the porosity of the inner layer (inner tip protection layer 201) larger than the porosity of the outer layer (outer tip protection layer 202), the gas sensor element 101 can prevent poisonous substances near the gas inlet 10. It is possible to suppress clogging due to the like and avoid a decrease in NO _x sensitivity.

In particular, by increasing the thickness of the inner tip protection layer 201 having a higher porosity than the outer tip protection layer 202, that is, by increasing the ratio of the thickness of the inner tip protection layer 201 to the thickness of the tip protection layer 200G, The gas sensor element 101 achieves the following effects. That is, by ensuring a sufficient thickness of the inner tip protective layer 201 with a large porosity, clogging caused by poisonous substances etc. near the gas inlet 10 can be suppressed, and in particular, the layer near the gas inlet 10 (i.e. , inner tip protection layer 201) is suppressed from clogging. Specifically, by setting the ratio of the thickness of the inner tip protection layer 201 having a large porosity to the thickness of the tip protection layer 200G from 30% to 90%, the inner tip protection layer 201 in contact with the gas inlet 10 is It is possible to avoid clogging caused by poisonous substances, etc.

Further, the tip protection layer 200G has a predetermined thickness similarly to the tip protection layer 200, and specifically, the distance d1 from the outermost surface of the tip protection layer 200G to the gas inlet 10 is 0.2 mm or more. It is. That is, in the example shown in FIG. 12, the distance d1 from the outermost surface of the outer tip protection layer 202 to the gas introduction port 10 is 0.2 mm or more. By making the distance d1 from the outermost surface of the tip protection layer 200G to the gas inlet 10 sufficiently long (specifically, 0.2 mm or more), that is, the thickness of the tip protection layer 200G is made sufficiently thick. By doing so, the following effects can be achieved. In other words, even in a harsh environment where there are many poisonous substances, such poisonous substances can be reliably trapped (captured) in the tip protective layer 200G, and the vicinity of the gas inlet 10 can be prevented from being clogged by poisonous substances. can be suppressed to avoid a decrease in NO _x sensitivity.

The tip protection layer 200G illustrated in FIG. 12 includes an inner tip protection layer 201 and an outer tip protection layer 202, that is, has a two-layer structure. However, it is not essential that the tip protection layer 200G has a two-layer structure, and the tip protection layer 200G may include three or more layers. That is, the tip protection layer 200G may include another layer in addition to the inner tip protection layer 201 and the outer tip protection layer 202. For example, the tip protection layer 200G may have a three-layer structure, or may have four or more layers. It may have a multilayer structure. The tip protection layer 200G includes at least an inner tip protection layer 201 that is in contact with the surface of the element substrate 100 where the gas inlet 10 is open, and an outer tip protection layer 202 that constitutes the outermost surface of the tip protection layer 200G. Further, another layer may be included between the two. In the tip protection layer 200G, the porosity of the inner tip protection layer 201 is larger than the porosity of the outer tip protection layer 202, and the thickness of the inner tip protection layer 201 is 30% or more of the thickness of the tip protection layer 200G. % or less is sufficient.

[Features]
As described above, the gas sensor element 101 according to the present embodiment includes the element base 100, the tip protection layer 200, the measurement electrode 44, and the porous diffusion layer 91. The gas sensor element 101 may include a tip protection layer 200G instead of the tip protection layer 200. Further, the gas sensor element 101 may include either a measurement electrode 44D or a measurement electrode 44E instead of the measurement electrode 44. Furthermore, the gas sensor element 101 may include any one of porous diffusion layers 91A, 91C, 91D, 91E, and 91F instead of the porous diffusion layer 91. For example, the element substrate 100 includes a gas to be measured distribution section 7 as an internal space, and a gas to be measured is introduced into the gas distribution section 7 from a gas inlet 10 opened on the surface of the element substrate 100. . For example, the tip protection layer 200 covers at least the surface of the element substrate 100 where the gas inlet 10 is open. The measurement electrode 44 is provided in the gas distribution section 7 to be measured. For example, the porous diffusion layer 91 has a porosity lower than that of the tip protection layer 200, and is 5% or more and 25% or less. When the porous diffusion layer 91 includes a plurality of surfaces (layers) with different porosities, the average porosity of the porous diffusion layer 91 is 5% or more and 25% or less, and is lower than the porosity of the tip protection layer 200. low. The porous diffusion layer 91 is provided upstream of the measurement electrode 44 in the flow direction DR of the gas to be measured at a position where the distance to the measurement electrode 44 is 0.15 mm or less. Further, the surface (area of the surface) of the porous diffusion layer 91 perpendicular to the flow direction DR of the gas to be measured is the same as the surface (area of the surface) perpendicular to the flow direction DR of the gas to be measured in the flow path CH of the gas to be measured. Occupies (occupies) more than 70% of the area (area).

In this configuration, the porous diffusion layer 91, which has a porosity of 5% or more and 25% or less and has a lower porosity than the tip protection layer 200, is located upstream of the measurement electrode 44 in the direction DR in which the gas to be measured flows. , is provided at a position where the distance to the measurement electrode 44 is 0.15 mm or less. The surface (area of the surface) of the porous diffusion layer 91 perpendicular to the flow direction DR of the gas to be measured is the same as the surface (area of the surface) perpendicular to the flow direction DR of the gas to be measured in the flow path CH of the gas to be measured. occupies more than 70% of the total area (area). That is, the porous diffusion layer 91 is provided upstream of the measurement electrode 44 at a position where the distance to the measurement electrode 44 is 0.15 mm or less, and in the direction DR in which the gas to be measured flows, It occupies (occupies) the range (more than 70%).

The porous diffusion layer 91 allows the diffusion pattern around the measurement electrode 44, that is, the diffusion pattern of the gas to be measured that reaches the measurement electrode 44 through the channel CH, to a sufficiently narrow flow such as Knudsen diffusion. It can be configured to diffuse while repeatedly colliding with the road wall. Therefore, in the gas sensor element 101, even if H ₂ O gas is present in the gas to be measured, the porous diffusion layer 91 suppresses the influence of H ₂ O gas on NO _x gas (and O ₂ gas). Can be made smaller. Specifically, the gas sensor element 101 uses the porous diffusion layer 91 to suppress fluctuations in NO _x output and deterioration of the measurement electrode 44 that are thought to be caused by molecular diffusion of NO _x under high H ₂ O concentration. I can do it.

Here, if the porous diffusion layer 91 having a large diffusion resistance is provided around the measurement electrode 44, the porous diffusion layer 91 may become clogged with poisonous substances or the like. Therefore, the gas sensor element 101 includes a tip protection layer 200 that covers at least the surface of the element base 100 where the gas inlet 10 is open. Therefore, the gas sensor element 101 can trap poisonous substances and the like with the tip protection layer 200, that is, can capture (capture) poisonous substances and the like with the tip protection layer 200.

In particular, in the gas sensor element 101, the porosity of the tip protection layer 200 is larger than the porosity of the porous diffusion layer 91 that closes a predetermined range of the channel CH at a position where the distance to the measurement electrode 44 is 0.15 mm or less. (expensive). By making the porosity of the tip protection layer 200 larger than the porosity of the porous diffusion layer 91, the gas sensor element 101 prevents the tip protection layer 200 from becoming clogged with poisonous substances and reducing _NOx output. It is possible to avoid situations such as "doing".

As described above, the gas sensor element 101 includes a porous diffusion layer (for example, a porous A quality diffusion layer 91) is provided. The area of the surface of the porous diffusion layer perpendicular to the flow direction DR of the gas to be measured is 70% or more of the area of the surface perpendicular to the flow direction DR of the gas to be measured of the flow path CH of the gas to be measured. occupies . With such a porous diffusion layer, the gas to be measured (particularly NO _x gas) reaching the measurement electrode 44 is diffused in a form such as Knudsen diffusion, in which it repeatedly collides with the wall of a sufficiently narrow channel. By doing so, the gas sensor element 101 achieves the following effects. That is, in the gas sensor element 101, even if H ₂ O gas is present in the gas to be measured, the porous diffusion layer 91 suppresses the influence of H ₂ O gas on NO _x gas (and O ₂ gas). Can be made smaller.

Here, the position where the porous diffusion layer is provided is desirably closer to the measurement electrode 44, and is provided at a position where the distance d2 to the measurement electrode 44 is at least 0.15 mm or less. A porous diffusion layer that provides a suitable diffusion form around the measurement electrode 44 may be in contact with the measurement electrode 44 . A porous diffusion layer provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less allows the gas sensor element 101 to diffuse H ₂ O gas, NO _x gas (and O ₂ gas) can be suppressed.

At least one side of the channel CH whose predetermined range is blocked by such a porous diffusion layer may be defined by the fourth diffusion-limiting section 18 . The flow path CH may be configured, for example, as a slit between the second solid electrolyte layer 6 (for example, the lower surface thereof) and the fourth diffusion control section 18 (for example, the upper surface thereof). However, it is not essential that the flow path whose predetermined range is blocked by the porous diffusion layer has at least one surface defined by the fourth diffusion-limiting section 18 . The flow path whose predetermined range (specifically, 70% or more) of the surface perpendicular to the flow direction DR of the gas to be measured is blocked by the porous diffusion layer may be the gas to be measured flow section 7 .

A porous diffusion layer provided upstream of the measurement electrode 44 at a position where the distance d2 to the measurement electrode 44 is 0.15 mm or less may be extended downstream. It may be extended to an internal cavity. For example, the measurement electrode 44 is disposed in the third internal space 19, and a porous diffusion layer is disposed at the entrance of the channel CH (in the example shown in FIG. 5, the channel CH constituted by the fourth diffusion rate controlling section 18). (In the example shown in FIG. 5, the porous diffusion layer 91A) may be extended to the third internal cavity 19.

Furthermore, the gas sensor according to one aspect of the present invention is configured to use the gas sensor element 101 to measure the amount of a specific gas component in the gas to be measured, in other words, the concentration of the specific gas component. Good too. Such a gas sensor changes the diffusion form of NO _x that reaches the measurement electrode 44 from molecular diffusion to a form in which it diffuses while repeatedly colliding with the wall surface of a sufficiently narrow channel. Therefore, such a gas sensor uses any one of the porous diffusion layers 91, 91A, 91C, 91D, _91E , and 91F to prevent fluctuations in _{NO x output} and Deterioration of the measurement electrode 44 can be suppressed.

[Modified example]
Although the embodiments of the present invention have been described above, the description of the embodiments thus far is merely an illustration of the present invention in all respects. Various improvements and modifications may be made to the embodiments described above. Regarding each component of the above embodiment, the component may be omitted, replaced, or added as appropriate. Further, the shape and dimensions of each component in the above embodiments may be changed as appropriate depending on the embodiment. For example, the following changes are possible. In addition, below, the same code|symbol is used regarding the same component as the said embodiment, and description is abbreviate|omitted suitably about the same point as the said embodiment. The following modified examples can be combined as appropriate.

(Modification 1)
Up to now, an example has been described in which the measurement electrode 44 is provided on the lower surface of the second solid electrolyte layer 6, but in the gas sensor element 101, it is not essential to provide the measurement electrode 44 on the lower surface of the second solid electrolyte layer 6. In addition, in the example described so far, the porous diffusion layer 91 is connected to the fourth diffusion rate controlling section 18 so that the distance d2 to the measurement electrode 44 arranged on the lower surface of the second solid electrolyte layer 6 is 0.15 mm or less. (especially its upper surface) and the second solid electrolyte layer 6 (especially its lower surface). However, in the gas sensor element 101, the position of the porous diffusion layer 91 is not limited to between the fourth diffusion-limiting section 18 (especially its upper surface) and the second solid electrolyte layer 6 (especially its lower surface). do not have.

The measurement electrode 44 may be provided on the upper surface of the first solid electrolyte layer 4, for example. In that case, the porous diffusion layer 91 is connected to the fourth diffusion-limiting portion 18 (especially its lower surface) and the first solid electrolyte layer 4 (especially its lower surface) so that the distance d2 to the measurement electrode 44 is 0.15 mm or less. (top surface). Further, as explained using FIGS. 8, 10, etc., the measurement electrode 44 is provided on the surface (upper surface or lower surface) of the diffusion-limiting section (for example, the fourth diffusion-limiting section 18D or the fourth diffusion-limiting section 18E). It's okay to be hit. In the gas sensor element 101, the arrangement position of the measurement electrode 44 can be selected as appropriate depending on the usage status of the gas sensor element 101. Further, a porous diffusion layer (porous diffusion layer 91, etc.) that provides a suitable diffusion form around the measurement electrode 44 may be placed upstream of the measurement electrode 44 depending on the arrangement position of the measurement electrode 44. It may be provided at a position where the distance to the electrode 44 is 0.15 mm or less.

(Modification 2)
Up to now, an example has been shown in which the measurement electrode 44 is arranged in the third internal cavity 19. However, it is not essential for the gas sensor element 101 that the measuring electrode 44 be arranged in the third internal cavity 19 . Furthermore, it is not essential that the gas sensor element 101 has a plurality of internal cavities (for example, two or three chambers), and the gas sensor element 101 may have, for example, a one-chamber structure. That is, it is essential for the gas sensor element 101 to include a diffusion-limiting section (at least one of the first diffusion-limiting section 11, the second diffusion-limiting section 13, the third diffusion-limiting section 16, and the fourth diffusion-limiting section 18). do not have. The gas sensor element 101 includes a porous diffusion layer (porous diffusion layer 91, etc.) that closes a predetermined range of the flow path CH around the measurement electrode 44, and covers at least the surface of the element base 100 where the gas inlet 10 is open. It is only necessary to include a tip protection layer (either tip protection layer 200 or 200G). Whether the gas sensor element 101 has a one-chamber structure or a multi-chamber structure (a structure with two or more internal spaces) can be appropriately selected depending on the usage status of the gas sensor element 101. Similarly, in the gas sensor element 101, where the measurement electrode 44 is arranged can be appropriately selected depending on the usage status of the gas sensor element 101 and the like.

[Example]
As described above, the gas sensor element 101 includes a porous diffusion layer 91 and the like that allows the desired diffusion form of the gas to be measured to reach the measurement electrode 44, and at least the surface of the element substrate 100 where the gas inlet 10 is open. By providing the covering tip protection layer 200 and the like, the following effects are achieved. That is, in the gas sensor element 101, the porous diffusion layer 91 can suppress deterioration of the measurement electrode 44 in, for example, a high H ₂ O concentration environment, and can also improve durability. The gas sensor element 101 can suppress clogging of the porous diffusion layer 91 due to poisonous substances, for example, by the tip protection layer 200, and can maintain measurement accuracy over a long period of time.

The present inventors manufactured gas sensors according to the following Examples and Comparative Examples, etc., and conducted various tests to verify the above-mentioned effects. However, the present invention is not limited to the following examples.

Table 1 shows the configuration of each gas sensor element and the test results for each of Judgments 1 to 4 for gas sensors equipped with gas sensor elements according to Examples 1 to 9 and Comparative Examples 1 to 5. In the following description, a gas sensor including a gas sensor element according to each of Examples 1 to 9 and Comparative Examples 1 to 5 will be referred to as a gas sensor (NO _x sensor) according to each of Examples 1 to 9 and Comparative Examples 1 to 5. ) is sometimes abbreviated as

(Details of each of Examples 1 to 9 and Comparative Examples 1 to 5)
Example 1 is a gas sensor including a gas sensor element 101 including a tip protection layer 200 illustrated in FIG. 1 and a porous diffusion layer 91 illustrated in FIG. 3. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.1 mm, which is 0.15 mm or less. In the gas sensor according to the first embodiment, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout the porous diffusion layer 91. The porosity is 12%. Regarding the gas sensor according to Example 1, the presence or absence of the tip protection layer 200 is "yes", and the tip protection layer 200 does not include the inner tip protection layer 201, that is, the tip protection layer 200 is entirely The porosity is constant. Specifically, the porosity of the outer tip protection layer 202 (tip protection layer 200) is 20%, which is higher than the porosity of the porous diffusion layer 91. That is, in the gas sensor according to Example 1, the porosity (12%) of the porous diffusion layer 91 is lower than the porosity (20%) of the outer tip protection layer 202 (tip protection layer 200). In addition, the shortest distance (d1) between the tip protection layer 200 and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protection layer 200 to the gas inlet 10 is 300 μm, which is 200 μm (0.2 mm). ) That's it. As mentioned above, in the gas sensor according to the first embodiment, the tip protection layer 200 does not include the inner tip protection layer 201, so the thickness of the outer tip protection layer 202 is equal to the thickness from the outermost surface of the tip protection layer 200 to the gas inlet 10. It is the same as the distance d1, which is 300 μm. Further, the porosity of the inner tip protection layer 201, the thickness of the inner tip protection layer 201, and the thickness ratio of the inner tip protection layer 201 are all "-".

Example 2 is a gas sensor including a gas sensor element 101 including a tip protection layer 200G illustrated in FIG. 12 and a porous diffusion layer 91 illustrated in FIG. 3. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.05 mm, which is 0.15 mm or less. In the gas sensor according to the second embodiment, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout. The porosity is 8%. Regarding the gas sensor according to the second embodiment, the presence or absence of the tip protection layer 200G is "yes", and unlike the first embodiment, the tip protection layer 200G includes an inner tip protection layer 201. Specifically, the porosity of the outer tip protection layer 202 is 20%, which is higher than the porosity of the porous diffusion layer 91 (8%). That is, in the gas sensor according to Example 2, the porosity of the porous diffusion layer 91 (8%) is lower than the porosity of the outer tip protection layer 202 (20%). Further, the porosity of the inner tip protection layer 201 included in the tip protection layer 200G is 60%, which is higher than the porosity (8%) of the porous diffusion layer 91. That is, in the gas sensor according to Example 2, the porosity of the porous diffusion layer 91 (8%) is lower than the porosity of the inner tip protection layer 201 (60%). The shortest distance (d1) between the tip protective layer 200G and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200G to the gas inlet 10 is 1020 μm, which is 200 μm (0.2 mm) or more. It is. The thickness of the outer tip protection layer 202 included in the tip protection layer 200G is 280 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200G is 740 μm. Therefore, the thickness ratio of the inner tip protection layer 201, that is, the ratio of the thickness of the inner tip protection layer 201 to the thickness of the tip protection layer 200G is 73%, which is 30% or more and 90% or less.

Example 3 is a gas sensor including a gas sensor element 101 including a tip protection layer 200G illustrated in FIG. 12 and a porous diffusion layer 91 illustrated in FIG. 3. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.02 mm, which is 0.15 mm or less. In the gas sensor according to the third embodiment, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout the porous diffusion layer 91. The porosity is 13%. Regarding the gas sensor according to the third embodiment, the presence or absence of the tip protection layer 200G is "yes", and unlike the first embodiment, the tip protection layer 200G includes an inner tip protection layer 201. Specifically, the porosity of the outer tip protection layer 202 is 17%, which is higher than the porosity of the porous diffusion layer 91. That is, in the gas sensor according to Example 3, the porosity of the porous diffusion layer 91 (13%) is lower than the porosity of the outer tip protection layer 202 (17%). Further, the porosity of the inner tip protection layer 201 included in the tip protection layer 200G is 62%, which is higher than the porosity of the porous diffusion layer 91 (13%). That is, in the gas sensor according to Example 3, the porosity of the porous diffusion layer 91 (13%) is lower than the porosity of the inner tip protection layer 201 (62%). The shortest distance (d1) between the tip protective layer 200G and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200G to the gas inlet 10 is 480 μm, which is 200 μm (0.2 mm) or more. It is. The thickness of the outer tip protection layer 202 included in the tip protection layer 200G is 400 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200G is 80 μm. Therefore, the thickness ratio of the inner tip protection layer 201, that is, the ratio of the thickness of the inner tip protection layer 201 to the thickness of the tip protection layer 200G is 17%, which is different from Example 2 and is less than 30%. .

Example 4 is a gas sensor including a gas sensor element 101 including a tip protection layer 200G illustrated in FIG. 12 and a porous diffusion layer 91 illustrated in FIG. 3. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.05 mm, which is 0.15 mm or less. In the gas sensor according to the fourth embodiment, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout. The porosity is 10%. Regarding the gas sensor according to the fourth embodiment, the presence or absence of the tip protection layer 200G is "yes", and unlike the first embodiment, the tip protection layer 200G includes an inner tip protection layer 201. Specifically, the porosity of the outer tip protection layer 202 is 25%, which is higher than the porosity of the porous diffusion layer 91. That is, in the gas sensor according to Example 4, the porosity of the porous diffusion layer 91 (10%) is lower than the porosity of the outer tip protection layer 202 (25%). Further, the porosity of the inner tip protection layer 201 included in the tip protection layer 200G is 65%, which is higher than the porosity (10%) of the porous diffusion layer 91. That is, in the gas sensor according to Example 4, the porosity of the porous diffusion layer 91 (10%) is lower than the porosity of the inner tip protection layer 201 (65%). The shortest distance (d1) between the tip protective layer 200G and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200G to the gas inlet 10 is 990 μm, which is 200 μm (0.2 mm) or more. It is. The thickness of the outer tip protection layer 202 included in the tip protection layer 200G is 360 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200G is 630 μm. Therefore, the thickness ratio of the inner tip protection layer 201, that is, the ratio of the thickness of the inner tip protection layer 201 to the thickness of the tip protection layer 200G is 64%, which is 30% or more and 90% or less.

Example 5 is a gas sensor including a gas sensor element 101 including a tip protection layer 200G illustrated in FIG. 12 and a porous diffusion layer 91 illustrated in FIG. 3. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.05 mm, which is 0.15 mm or less. In the gas sensor according to the fifth embodiment, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout the porous diffusion layer 91. The porosity is 10%. Regarding the gas sensor according to the fifth embodiment, the presence or absence of the tip protection layer 200G is "yes", and unlike the first embodiment, the tip protection layer 200G includes an inner tip protection layer 201. Specifically, the porosity of the outer tip protection layer 202 is 15%, which is higher than the porosity of the porous diffusion layer 91. That is, in the gas sensor according to Example 5, the porosity of the porous diffusion layer 91 (10%) is lower than the porosity of the outer tip protection layer 202 (15%). Further, the porosity of the inner tip protection layer 201 included in the tip protection layer 200G is 50%, which is higher than the porosity (10%) of the porous diffusion layer 91. That is, in the gas sensor according to Example 5, the porosity of the porous diffusion layer 91 (10%) is lower than the porosity of the inner tip protection layer 201 (50%). The shortest distance (d1) between the tip protective layer 200G and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200G to the gas inlet 10 is 1050 μm, which is 200 μm (0.2 mm) or more. It is. The thickness of the outer tip protection layer 202 included in the tip protection layer 200G is 200 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200G is 850 μm. Therefore, the thickness ratio of the inner tip protection layer 201, that is, the ratio of the thickness of the inner tip protection layer 201 to the thickness of the tip protection layer 200G is 81%, which is 30% or more and 90% or less.

Example 6 is a gas sensor including a gas sensor element 101 including a tip protection layer 200G illustrated in FIG. 12 and a porous diffusion layer 91 illustrated in FIG. 3. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.1 mm, which is 0.15 mm or less. In the gas sensor according to the sixth embodiment, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout. The porosity is 15%. Regarding the gas sensor according to the sixth embodiment, the presence or absence of the tip protection layer 200G is "yes", and unlike the first embodiment, the tip protection layer 200G includes an inner tip protection layer 201. Specifically, the porosity of the outer tip protection layer 202 is 23%, which is higher than the porosity of the porous diffusion layer 91. That is, in the gas sensor according to Example 6, the porosity of the porous diffusion layer 91 (15%) is lower than the porosity of the outer tip protection layer 202 (23%). Further, the porosity of the inner tip protection layer 201 included in the tip protection layer 200G is 55%, which is higher than the porosity (15%) of the porous diffusion layer 91. That is, in the gas sensor according to Example 6, the porosity of the porous diffusion layer 91 (15%) is lower than the porosity of the inner tip protection layer 201 (55%). The shortest distance (d1) between the tip protective layer 200G and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200G to the gas inlet 10 is 500 μm, which is 200 μm (0.2 mm) or more. It is. The thickness of the outer tip protection layer 202 included in the tip protection layer 200G is 350 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200G is 150 μm. That is, unlike Examples 2, 4, and 5, in Example 6, the thickness of the inner tip protection layer 201 (150 μm) is thinner than the thickness of the outer tip protection layer 202 (350 μm). However, the thickness ratio of the inner tip protection layer 201 in Example 6, that is, the ratio of the thickness of the inner tip protection layer 201 to the thickness of the tip protection layer 200G is 30% as in Examples 2, 4, and 5. and is 30% or more and 90% or less.

Example 7 is a gas sensor including a gas sensor element 101 including a tip protection layer 200 illustrated in FIG. 1 and a porous diffusion layer 91 illustrated in FIG. 3. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.13 mm, which is 0.15 mm or less. In the gas sensor according to the seventh embodiment, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout the porous diffusion layer 91. The porosity is 20%. Regarding the gas sensor according to Example 7, the presence or absence of the tip protection layer 200 is "presence". However, unlike Example 2-6, the tip protection layer 200 in Example 7 does not include the inner tip protection layer 201, that is, the tip protection layer 200 has a constant porosity throughout. Specifically, the porosity of the outer tip protection layer 202 (tip protection layer 200) is 30%, which is higher than the porosity of the porous diffusion layer 91. That is, in the gas sensor according to Example 7, the porosity (20%) of the porous diffusion layer 91 is lower than the porosity (30%) of the outer tip protection layer 202 (tip protection layer 200). In addition, the shortest distance (d1) between the tip protection layer 200 and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protection layer 200 to the gas inlet 10 is 200 μm, which is 200 μm (0.2 mm). ) That's it. As mentioned above, in the gas sensor according to Example 7, the tip protection layer 200 does not include the inner tip protection layer 201, so the thickness of the outer tip protection layer 202 is the same from the outermost surface of the tip protection layer 200 to the gas inlet 10. It is the same as the distance d1, which is 200 μm. Further, the porosity of the inner tip protection layer 201, the thickness of the inner tip protection layer 201, and the thickness ratio of the inner tip protection layer 201 are all "-".

Example 8 is a gas sensor including a gas sensor element 101 including a tip protection layer 200G illustrated in FIG. 12 and a porous diffusion layer 91 illustrated in FIG. 3. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.1 mm, which is 0.15 mm or less. In the gas sensor according to the eighth embodiment, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout the porous diffusion layer 91. The porosity is 25%. Regarding the gas sensor according to the eighth embodiment, the presence or absence of the tip protection layer 200G is "yes", and unlike the first embodiment, the tip protection layer 200G includes the inner tip protection layer 201. Specifically, the porosity of the outer tip protection layer 202 is 15%, which is lower than the porosity of the porous diffusion layer 91. That is, unlike Examples 1-7, in the gas sensor according to Example 8, the porosity of the porous diffusion layer 91 (25%) is higher than the porosity of the outer tip protection layer 202 (15%). However, the porosity of the inner tip protection layer 201 included in the tip protection layer 200G is 45%, which is higher than the porosity (25%) of the porous diffusion layer 91. That is, in the gas sensor according to Example 8, the porosity of the porous diffusion layer 91 (25%) is lower than the porosity of the inner tip protection layer 201 (45%). The shortest distance (d1) between the tip protective layer 200G and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200G to the gas inlet 10 is 900 μm, which is 200 μm (0.2 mm) or more. It is. The thickness of the outer tip protection layer 202 included in the tip protection layer 200G is 300 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200G is 600 μm. Therefore, the thickness ratio of the inner tip protection layer 201, that is, the ratio of the thickness of the inner tip protection layer 201 to the thickness of the tip protection layer 200G is 67%, which is 30% or more and 90% or less.

Example 9 is a gas sensor that includes a gas sensor element 101 that includes the tip protection layer 200 illustrated in FIG. 1 and the porous diffusion layer 91 illustrated in FIG. 3. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.15 mm, which is 0.15 mm or less. In the gas sensor according to Example 9, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout. The porosity is 10%. Regarding the gas sensor according to Example 9, the presence or absence of the tip protection layer 200 is "presence". However, unlike Examples 2-6 and 8, the tip protection layer 200 in Example 9 does not include the inner tip protection layer 201. In other words, the tip protection layer 200 has a constant porosity throughout. be. Specifically, the porosity of the outer tip protection layer 202 (tip protection layer 200) is 20%, which is higher than the porosity of the porous diffusion layer 91. That is, in the gas sensor according to Example 9, the porosity (10%) of the porous diffusion layer 91 is lower than the porosity (20%) of the outer tip protection layer 202 (tip protection layer 200). In addition, the shortest distance (d1) between the tip protection layer 200 and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protection layer 200 to the gas inlet 10 is 300 μm, which is 200 μm (0.2 mm). ) That's it. As mentioned above, in the gas sensor according to Example 9, the tip protection layer 200 does not include the inner tip protection layer 201, so the thickness of the outer tip protection layer 202 is the same from the outermost surface of the tip protection layer 200 to the gas inlet 10. It is the same as the distance d1, which is 300 μm. Further, the porosity of the inner tip protection layer 201, the thickness of the inner tip protection layer 201, and the thickness ratio of the inner tip protection layer 201 are all "-".

Comparative Example 1 is a gas sensor including a sensor element having the same structure as Example 1, except that it does not include the tip protection layer 200. Specifically, in Comparative Example 1, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.1 mm, which is 0.15 mm or less. . In the gas sensor according to Comparative Example 1, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout. The porosity is 10%. As mentioned above, since the gas sensor according to Comparative Example 1 does not include the tip protection layer 200, the presence or absence of the tip protection layer 200 is "absent", and the porosity of the outer tip protection layer 202 and the inner tip protection layer 201 The porosity of each is "-". Furthermore, the shortest distance between the tip protection layer 200 and the gas inlet 10, the thickness of the outer tip protection layer 202, the thickness of the inner tip protection layer 201, and the thickness ratio of the inner tip protection layer 201 are all "-". .

Comparative Example 2 uses a sensor element that does not include a porous diffusion layer (such as the porous diffusion layer 91) that makes the diffusion form around the measurement electrode 44 suitable, but includes only the tip protection layer 200 illustrated in FIG. This is a gas sensor that includes. Since Comparative Example 2 does not include a porous diffusion layer, the distance between the measurement electrode 44 and the porous diffusion layer and the porosity of the porous diffusion layer are both "-". Regarding the gas sensor according to Comparative Example 2, the presence or absence of the tip protection layer 200 is "presence". However, unlike Examples 2-6 and 8, the tip protection layer 200 in Comparative Example 2 does not include the inner tip protection layer 201. In other words, the tip protection layer 200 has a constant porosity throughout. be. Specifically, the porosity of the outer tip protection layer 202 (tip protection layer 200) is 15%. Further, the shortest distance (d1) between the tip protection layer 200 and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protection layer 200 to the gas inlet 10 is 250 μm, which is 200 μm (0.2 mm). ) That's it. As mentioned above, in the gas sensor according to Comparative Example 2, since the tip protection layer 200 does not include the inner tip protection layer 201, the thickness of the outer tip protection layer 202 is the same from the outermost surface of the tip protection layer 200 to the gas inlet 10. It is the same as the distance d1, which is 250 μm. Further, the porosity of the inner tip protection layer 201, the thickness of the inner tip protection layer 201, and the thickness ratio of the inner tip protection layer 201 are all "-".

Comparative Example 3 is a gas sensor including the same sensor element as in Example 1, except that the distance d2 between the measurement electrode 44 and the porous diffusion layer 91 is greater than 0.15 mm. Specifically, in Comparative Example 3, unlike Example 1, there was a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them was 0.2 mm. , larger than 0.15 mm. In the gas sensor according to Comparative Example 3, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout. The porosity is 15%. Regarding the gas sensor according to Comparative Example 3, the presence or absence of the tip protection layer 200 is "presence". However, unlike Examples 2-6 and 8, the tip protection layer 200 in Comparative Example 3 does not include the inner tip protection layer 201. In other words, the tip protection layer 200 has a constant porosity throughout. be. Specifically, the porosity of the outer tip protection layer 202 (tip protection layer 200) is 25%, which is higher than the porosity of the porous diffusion layer 91. That is, in the gas sensor according to Comparative Example 3, the porosity (15%) of the porous diffusion layer 91 is lower than the porosity (25%) of the outer tip protection layer 202 (tip protection layer 200). In addition, the shortest distance (d1) between the tip protection layer 200 and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protection layer 200 to the gas inlet 10 is 300 μm, which is 200 μm (0.2 mm). ) That's it. As mentioned above, in the gas sensor according to Comparative Example 3, since the tip protection layer 200 does not include the inner tip protection layer 201, the thickness of the outer tip protection layer 202 is the same from the outermost surface of the tip protection layer 200 to the gas inlet 10. It is the same as the distance d1, which is 300 μm. Further, the porosity of the inner tip protection layer 201, the thickness of the inner tip protection layer 201, and the thickness ratio of the inner tip protection layer 201 are all "-".

In Comparative Example 4, the porosity of the porous diffusion layer 91 is higher than 25%, and the porosity of the porous diffusion layer 91 is higher than the porosity of the outer tip protection layer 202 (tip protection layer 200). Except for this, the structure is a gas sensor including a sensor element similar to that of Example 1. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.1 mm, which is 0.15 mm or less. In the gas sensor according to Comparative Example 4, the porous diffusion layer 91 is a porous layer with a constant porosity throughout; however, unlike in Example 1, the pores of the porous diffusion layer 91 in Comparative Example 4 are The rate is 35%, which is greater than 25%. Regarding the gas sensor according to Comparative Example 4, the presence or absence of the tip protection layer 200 is "presence". However, unlike Examples 2-6 and 8, the tip protection layer 200 in Comparative Example 4 does not include the inner tip protection layer 201. In other words, the tip protection layer 200 has a constant porosity throughout. be. Specifically, the porosity of the outer tip protection layer 202 (tip protection layer 200) is 30%, which is lower than the porosity of the porous diffusion layer 91. That is, unlike Example 1, in the gas sensor according to Comparative Example 4, the porosity (35%) of the porous diffusion layer 91 is higher than the porosity (30%) of the outer tip protection layer 202 (tip protection layer 200). It's also expensive. In addition, the shortest distance (d1) between the tip protection layer 200 and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protection layer 200 to the gas inlet 10 is 280 μm, which is 200 μm (0.2 mm). ) That's it. As mentioned above, in the gas sensor according to Comparative Example 4, the tip protection layer 200 does not include the inner tip protection layer 201, so the thickness of the outer tip protection layer 202 is the same from the outermost surface of the tip protection layer 200 to the gas inlet 10. It is the same as the distance d1, which is 280 μm. Further, the porosity of the inner tip protection layer 201, the thickness of the inner tip protection layer 201, and the thickness ratio of the inner tip protection layer 201 are all "-".

Comparative Example 5 is a gas sensor including the same sensor element as in Example 1, except that the shortest distance (d1) between the tip protection layer 200 and the gas inlet 10 is less than 0.2 mm (200 μm). . Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91, and the distance d2 between them is 0.1 mm, which is 0.15 mm or less. In the gas sensor according to Comparative Example 5, the porous diffusion layer 91 is a porous layer with a constant porosity of 5% or more and 25% or less throughout. The porosity is 10%. Regarding the gas sensor according to Comparative Example 5, the presence or absence of the tip protection layer 200 is "presence", and the tip protection layer 200 does not include the inner tip protection layer 201, that is, the tip protection layer 200 has no inner tip protection layer 201 throughout. The porosity is constant. Specifically, the porosity of the outer tip protection layer 202 (tip protection layer 200) is 20%, which is higher than the porosity of the porous diffusion layer 91. That is, in the gas sensor according to Comparative Example 5, the porosity (10%) of the porous diffusion layer 91 is lower than the porosity (20%) of the outer tip protection layer 202 (tip protection layer 200). However, unlike Example 1, in the gas sensor according to Comparative Example 5, the shortest distance (d1) between the tip protective layer 200 and the gas inlet 10 is from the outermost surface of the tip protective layer 200 to the gas inlet 10. The distance d1 is less than 0.2 mm (200 μm). Specifically, the shortest distance (d1) between the tip protection layer 200 and the gas inlet 10 is 100 μm, which is less than 0.2 mm (200 μm). As mentioned above, in the gas sensor according to Comparative Example 5, the tip protection layer 200 does not include the inner tip protection layer 201, so the thickness of the outer tip protection layer 202 is the same from the outermost surface of the tip protection layer 200 to the gas inlet 10. It is the same as the distance d1, which is 100 μm. Further, the porosity of the inner tip protection layer 201, the thickness of the inner tip protection layer 201, and the thickness ratio of the inner tip protection layer 201 are all "-".

(Details of each judgment 1 to 4)
Judgment 1 verifies the effect of suppressing deterioration of the measurement electrode due to high H ₂ O concentration. Specifically, first, an environment with a H ₂ O concentration of 25% and an O ₂ concentration of 20.5% was prepared. Then, under such an environment, a long-term durability test of 2000 hours was conducted on the NO _x sensors according to each of Examples 1 to 9 and Comparative Examples 1 to 5. The inventors of the present invention have discovered that when the NO _x sensors according to Examples 1 to 9 and Comparative Examples 1 to 5 are used continuously for a long period of time, the characteristics deteriorate (the measurement electrodes deteriorate due to high H ₂ O concentration). In order to understand the extent of deterioration), this long-term durability test was conducted under the following accelerated deterioration test conditions. That is, the present inventors conducted a long-term test under accelerated deterioration test conditions in which the heating temperature by the heat generating part 72 was higher than the sensor element driving temperature by a predetermined temperature (100 degrees Celsius in this long-term durability test). A durability test was conducted. The sensor element driving temperature is the heating temperature by the heat generating unit 72 when the NO _x sensor is used (actually used), and can be regarded as the heating temperature when the gas sensor element 101 is driven. Before and after the test, evaluation was performed using a model gas, and the degree of change in NO _x output when NO _x =500 ppm was flowed was investigated. "A" indicates that "the NO _x sensitivity change rate was within plus or minus 10%.""B" indicates that "the NO _x sensitivity change rate was greater than plus or minus 10% and within 20%.""F" indicates that "the rate of change in _NOx sensitivity was greater than plus or minus 20%."

Judgment 2 was to verify the effect of suppressing H ₂ O dependence when NO _x gas is flowing and improving measurement accuracy. Specifically, the following verification (investigation) was performed. That is, after conducting the test for Judgment 1, for the NO _x sensors according to each of Examples 1 to 9 and Comparative Examples 1 to 5, NO _x concentration = 500 ppm and H _{2 O} concentration = 3% as bases. The degree of change in NO _x output was investigated when changing the H ₂ O concentration to 500 ppm and 15%. "A" means that "the rate of change (degree of change) in NO _x sensitivity from when H ₂ O concentration = 3% to when H ₂ O concentration = 15% was within plus or minus 5%." show. "B" indicates that "the rate of change in NO _x sensitivity from when the H ₂ O concentration was 3% to when the H ₂ O concentration was 15% was within plus or minus 10%.""F" indicates that "the rate of change in NO _x sensitivity from when the H ₂ O concentration was 3% to when the H ₂ O concentration was 15% was greater than plus or minus 10%."

Judgment 3 is to verify the "effect of trapping poisonous substances and suppressing clogging around the measurement electrode (e.g., porous diffusion layer)" of the tip protection layer, and is based on Examples 1 to 9. The following Mg poisoning test was conducted on the NO _x sensors according to each of Comparative Examples 1 to 5. That is, 10 μL of an Mg ion solution having an Mg ion concentration of 5 mmol/L was dropped, and after being allowed to stand for 1 minute, the cycle of driving the gas sensor at 800 degrees Celsius for 10 minutes was repeated 10 times, and a total of 100 μL of the Mg ion solution was dropped. . Then, the degree of change (rate of change) in NO _x output before and after the test was investigated. Specifically, first, using the NO _x sensors according to Examples 1 to 9 and Comparative Examples 1 to 5, NO _x sensitivity was measured in a NO _x model gas with an NO _x concentration of 500 ppm. The sensitivity was defined as the initial NO _x sensitivity. Then, 10 μL of the above-mentioned Mg ion solution was dropped into the gas inlet of each NO _x sensor, and after leaving it for 1 minute, each NO _x sensor was driven at 800 degrees Celsius for 10 minutes. This cycle was repeated 10 times, resulting in a total of 100 μL. Mg ion solution was added dropwise. Then, using each NO _x sensor, the NO _x sensitivity was measured again in the NO _x model gas described above, and the measured NO _x sensitivity was compared with the initial NO _x sensitivity to calculate the sensitivity reduction rate. . "A" indicates that "the rate of change in NO _x sensitivity was within 10%.""B" indicates that "the rate of change in NO _x sensitivity was greater than 20% and within 30%.""F" indicates that "the rate of change in NO _x sensitivity was greater than 30%."

Judgment 4, like Judgment 3, verifies the "effect of suppressing clogging around the measurement electrode" of the tip protective layer, but verifies this effect under conditions that are more severe than the method of Judgment 3. Specifically, this increased the possibility that the tip protective layer would become clogged. That is, in Judgment 4, the same Mg poisoning test as in Judgment 3 was conducted, except that the amount of Mg ion solution to be dropped was 500 μL in total. Then, the degree of change (rate of change) in the NO _x output before and after the test was investigated, that is, the degree of change in the NO _x output when a NO _x model gas with an NO _x concentration of 500 ppm was flowed. "A" indicates that "the rate of change in NO _x sensitivity was within 10%.""B" indicates that "the rate of change in NO _x sensitivity was greater than 20% and within 30%.""F" indicates that "the rate of change in NO _x sensitivity was greater than 30%."

(Summary of items that can be confirmed from Table 1)
The following summarizes the matters that can be confirmed from Table 1 showing the test results for Judgments 1 to 4 for gas sensors equipped with sensor elements according to Examples 1 to 9 and Comparative Examples 1 to 5. do.

As shown in the comparison results between Example 1-9 and Comparative Example 1 in Judgment 3 (and Judgment 4), by providing the tip protection layer 200 (or tip protection layer 200G), the gas sensor achieves the following effects. It has been confirmed that it can be done. That is, the results of determination 3 (“A” or “B”) of Examples 1-9, which include the tip protection layer 200 or the tip protection layer 200G, are the same as those of the comparison which does not include the tip protection layer 200 or the tip protection layer 200G. This is better than the result of Judgment 3 in Example 1 (“F”). Therefore, by including the tip protection layer 200 (or the tip protection layer 200G), the gas sensor traps poisonous substances and suppresses clogging around the measurement electrode 44 (for example, the porous diffusion layer 91). It was confirmed that it is possible to do this.

As shown in the comparison results between Example 1-9 and Comparative Example 2 in Judgment 1 and Judgment 2, the porous diffusion layer (porous diffusion layer 91 etc.) that makes the diffusion form around the measurement electrode 44 suitable It has been confirmed that by including the gas sensor, the following effects can be achieved. In other words, the result of Judgment 1 (“A” or “B”) of Example 1-9, which includes the porous diffusion layer 91, etc., is the same as the result of Judgment 1 of Comparative Example 2, which does not include the porous diffusion layer 91, etc. This is good compared to the result (“F”). Therefore, the gas sensor is equipped with a porous diffusion layer (porous diffusion layer 91, etc.) that makes the diffusion form around the measurement electrode ₄₄ suitable. It was confirmed that it was possible to "suppress the deterioration of the measurement electrode" (Judgment 1). Furthermore, the results of Judgment 2 (“A” or “B”) of Example 1-9, which includes the porous diffusion layer 91, etc., are the same as those of Judgment 2 of Comparative Example 2, which does not include the porous diffusion layer 91, etc. This is good compared to the result (“F”). Therefore, the gas sensor is equipped with a porous diffusion layer (such as the porous diffusion layer ₉₁ ) that makes the diffusion form around _the measurement electrode 44 _suitable . It was confirmed that it was possible to "suppress the risk of damage and improve measurement accuracy" (Judgment 2).

The difference between Example 1 and The results of Judgment 2 are significantly different from Comparative Example 3. Specifically, the result of determination 2 in Example 1 where the distance d2 is 0.1 mm (0.15 mm or less) is "B", whereas the result is "B" when the distance d2 is 0.2 mm (greater than 0.15 mm). The result of Judgment 2 of Comparative Example 3 is "F". Therefore, by setting the distance d2 from the porous diffusion layer (porous diffusion layer 91, etc.) to the measurement electrode 44, which makes the diffusion form around the measurement electrode 44 suitable, to 0.15 mm or less, the gas sensor can be configured as follows. It was confirmed that this effect can be achieved. In other words, by setting the distance d2 from the porous diffusion layer to the measurement electrode 44 to 0.15 mm or less, the gas sensor can suppress the H ₂ O dependence of NO _x sensitivity (NO _x output) and improve measurement accuracy. It was confirmed that it is possible to do this.

Regarding the porous diffusion layer (porous diffusion layer 91, etc.) in which the distance d2 to the measurement electrode 44 is 0.15 mm or less, the porosity is set to be 5% or more and 25% or less, and the porosity of the tip protection layer is The results of Judgments 1 and 2 differ greatly between Example 1 and Comparative Example 4, depending on whether or not "the value should be lower than ." Specifically, the results of Judgments 1 and 2 of Example 1, which includes a porous diffusion layer 91 with a "porosity of 5% or more and 25% or less and lower than the porosity of the tip protective layer", are both It is "B". On the other hand, the results of Judgments 1 and 2 of Comparative Example 4, which includes a porous diffusion layer 91 whose porosity is greater than 25% and higher than the porosity of the tip protective layer, are both ``F ”. Therefore, the porosity of the porous diffusion layer (porous diffusion layer 91, etc.) whose distance d2 to the measurement electrode 44 is 0.15 mm or less is set to 5% or more and 25% or less, and the porosity of the tip protection layer is set to 5% or more and 25% or less. It was confirmed that the gas sensor can achieve the following effects by lowering the In other words, the gas sensor prevents deterioration of the measurement electrode due to high H <sub>2</sub> O concentration by making the porosity of the porous diffusion layer ``5% or more and 25% or less, and lower than the porosity of the tip protective layer.'' It was confirmed that it was possible to suppress (determination 1). In addition, the gas sensor can improve NO _x sensitivity ( _NO It was confirmed that "H ₂ O dependence can be suppressed and measurement accuracy can be improved" (Judgment 2).

In Example 8, the porosity of the porous diffusion layer 91 is 5% or more and 25% or less, but unlike Examples 1-7, it is higher than the porosity of the outer tip protection layer 202. The results of determination 2 in Examples 2-7 are all "A", whereas the results of determination 2 in Example 8 are "B". However, in Example 8, the porosity of the porous diffusion layer 91 is lower than the porosity of the inner tip protection layer 201, that is, the inner tip protection layer in contact with the surface where the gas inlet 10 of the element substrate 100 is opened. The porosity is lower than that of layer 201. On the other hand, in Comparative Example 4, the tip protection layer 200 does not include the inner tip protection layer 201, and the porosity of the porous diffusion layer 91 is greater than 25%, and the tip protection layer 200 does not include the inner tip protection layer 201. higher than the porosity of The result of determination 2 of comparative example 4 is "F". Therefore, the porosity of the porous diffusion layer 91 is set to be 5% or more and 25% or less, and further, the porosity of the inner tip protection layer 201 that is in contact with the surface of the element substrate 100 where the gas inlet 10 is opened is set to be at least 5% or more and 25% or less. It is thought that the following effects can be achieved by lowering the In other words, it is considered possible to "suppress the H ₂ O dependence of NO _x sensitivity (NO _x output) and improve measurement accuracy."

Judgment 3 is determined between Example 1 and Comparative Example 5 depending on whether the distance d1 from the outermost surface of the tip protection layer 200 (or tip protection layer 200G) to the gas inlet 10 is 0.2 mm (200 μm) or more. , 4 have different results. Specifically, the results of determinations 3 and 4 of Example 1, which includes the tip protection layer 200 where "the distance d1 from the outermost surface to the gas inlet 10 is 300 μm and 0.2 mm or more", are as follows: A" and "B". On the other hand, the results of determinations 3 and 4 of Comparative Example 5, which includes a tip protection layer 200 in which "the distance d1 from the outermost surface to the gas inlet 10 is 100 μm and less than 0.2 mm", are as follows: B" and "F". Therefore, it was confirmed that the gas sensor can achieve the following effects by setting the distance d1 to 0.2 mm or more. In other words, by setting the distance d1 to 0.2 mm or more, the gas sensor can trap poisonous substances and suppress clogging around the measurement electrode (e.g., porous diffusion layer). Confirmed (determination 3). In addition, by setting the distance d1 to 0.2 mm or more, the gas sensor can trap poisonous substances and It was confirmed that clogging around the measurement electrode could be suppressed (Judgment 4).

Comparing Example 1 with Examples 2 and 4-6, the results of determinations 1, 2, and 4 are better in Examples 2 and 4-6 than in Example 1. Further, when comparing Example 7 with Examples 2 and 4-6, the results of determinations 3 and 4 are better in Examples 2 and 4-6 than in Example 7. Further, while the results of determination 4 in Examples 2 and 4-6 are all "A," the result of determination 4 in Example 3 is "B." Here, as described above, Examples 2 and 4-6 differ from Examples 1 and 7 in that they have the following configuration. That is, Examples 2 and 4-6 include the tip protection layer 200G including the outer tip protection layer 202 and the inner tip protection layer 201 having a larger porosity than the outer tip protection layer 202, and the inner tip protection layer 200G. The thickness of 201 is 30% or more and 90% or less of the thickness of the tip protection layer 200G. Further, unlike Examples 2 and 4-6, in Example 3, the thickness ratio of the inner tip protection layer 201 is less than 30%. Therefore, by providing such a tip protection layer 200G and setting the thickness of the inner tip protection layer 201 to 30% or more and 90% or less of the thickness of the tip protection layer 200G, the gas sensor can at least meet the following conditions according to determination 4. It was confirmed that the effect can be achieved. In other words, gas sensors are capable of trapping poisonous substances and removing them from the surroundings of the measurement electrode (e.g. porous It was confirmed that clogging in the diffusion layer can be suppressed.

(About the rate of change in NO _x sensitivity)
FIG. 13 is a graph showing the difference in the change in NO _x output over time depending on the presence or absence of a porous diffusion layer that closes a predetermined range of the channel CH around the measurement electrode. Specifically, FIG. 13 shows the same structure except for the presence or absence of a porous diffusion layer (any one of 91, 91A, 91C, 91D, 91E, and 91F) that closes a predetermined range of the flow path CH around the measurement electrode 44. 3 shows the change over time in the NO _x output of each NO _x sensor under a high H ₂ O concentration for the NO _x sensor having the structure. "Under high H ₂ O concentration" means, for example, H ₂ O concentration = 25%. In the graph of FIG. 13, the horizontal axis is time (driving time), and the vertical axis is the rate of change in NO _x sensitivity. The solid black line shows the change over time in the NO _x output of "the NO _x sensor equipped with a porous diffusion layer that closes a predetermined range of the flow path CH around the measurement electrode 44." In addition, the gray dotted line indicates "a _NO It shows the change over time in the NO _x output of the NO _x sensor).

Specifically, the NO _{x current (pump current Ip2) was measured for each of the above-mentioned NO x sensors} using a model gas device under a model gas atmosphere in which the NO _x concentration was 500 ppm and the remainder was nitrogen. The graph shown in FIG. 13 was created by plotting the NO _x sensitivity (rate of change in NO _x sensitivity) calculated from the measurement results at each drive time.

As shown in FIG. 13, in a NO _x sensor that does not have a porous diffusion layer around the measurement electrode ₄₄ (a conventional NO _x sensor that has only a slit structure with a diffusion control part), the In long-term driving tests, there were large fluctuations in NO _x sensitivity. This is considered to be because in the conventional "slit structure using a diffusion-limiting section", the diffusion form around the measurement electrode 44 is dominated by molecular diffusion. On the other hand, the NO _x sensor equipped with a porous diffusion layer uses a suitable diffusion form such as Knudsen diffusion around the measurement electrode 44 to prevent fluctuations in NO _x sensitivity even under high H ₂ O concentration. (changes over time) can be suppressed.

(About H ₂ O dependence of NO _x output)
FIG. 14 is a graph showing the difference in the H ₂ O dependence of the NO _x output depending on the presence or absence of a porous diffusion layer that closes a predetermined range of the channel CH around the measurement electrode. Specifically, FIG. 14 shows the same structure except for the presence or absence of a porous diffusion layer (any one of 91, 91A, 91C, 91D, 91E, and 91F) that closes a predetermined range of the flow path CH around the measurement electrode 44. This shows the difference in the H ₂ O dependence of the NO _x output for NO _x sensors having the following structure. In the graph of FIG. 14, the horizontal axis is time (driving time), and the vertical axis is the H ₂ O dependence of NO _x sensitivity. The solid black line shows the H ₂ O dependence of the NO _x output over time of the “NO _x sensor equipped with a porous diffusion layer that closes a predetermined range of the flow path CH around the measurement electrode 44”. In addition, the gray dotted line indicates "a _NO 3 shows the H ₂ O dependence of the NO _x output of the NO _x sensor).

The H ₂ O dependence of the NO _x output was determined from the degree of change (rate of change) in the NO _x current (pump current Ip2) measured under the following conditions. In other words, based on the NO _x concentration ₌ 500 ppm and H ₂ O concentration = 3 _% , the NO _x output is calculated from the rate of change in the NO _x current when changing the NO H ₂ O dependence was calculated. A graph shown in FIG. 14 was created by plotting the H ₂ O dependence of NO _x output (rate of change in NO _x current) for each drive time.

As shown in FIG. 14, in a NO _x sensor without a porous diffusion layer around the measurement electrode 44 (a conventional NO _x sensor with only a slit structure using a diffusion-limiting part), under a high H ₂ O concentration, In a long-term driving test, the H ₂ O dependence of NO _x output fluctuated significantly. This is considered to be because in the conventional "slit structure using a diffusion-limiting section", the diffusion form around the measurement electrode 44 is dominated by molecular diffusion. On the other hand, the NO _x sensor equipped with a porous diffusion layer uses a diffusion form around the measurement electrode 44 that is similar to Knudsen diffusion, in which it diffuses while repeatedly colliding with the walls of a sufficiently narrow channel, and has a high Even under H ₂ O concentration, the H ₂ O-dependent change in NO _x output over time is suppressed. On the other hand, the NO _x sensor equipped with a porous diffusion layer uses a suitable diffusion form such as Knudsen diffusion around the measurement electrode 44, so that even under a high H ₂ O concentration, the H of NO _x output is reduced. ₂ O-dependent fluctuations (changes over time) can be suppressed.

(Matters confirmed through various verifications)
Some of the test results (verification results) in Table 1, FIGS. 13 and 14 that have been explained so far can be organized as follows. That is, in a gas sensor (conventional NO _x sensor) that does not provide a porous diffusion layer around the measurement electrode 44 and has only _a slit structure using a diffusion rate controlling part, NO _x Large variations in sensitivity and NO _x output for high H ₂ O concentrations. This is considered to be because in the conventional "slit structure using a diffusion-limiting section", the diffusion form around the measurement electrode 44 is dominated by molecular diffusion.

Therefore, a porous diffusion layer (for example, porous diffusion layer 91, etc.) having a porosity of 5% or more and 25% or less is provided upstream of the measurement electrode 44, and in particular, from such a porous diffusion layer to the measurement electrode 44. The distance d2 was made sufficiently small (specifically, 0.15 mm or less). The following effects were confirmed in a gas sensor employing such a configuration. That is, such a gas sensor is able to suppress changes in NO _x sensitivity. When Knudsen diffusion becomes dominant around the measurement electrode 44, even if H ₂ O gas with a small molecular weight exists, the ease of diffusion of NO _x and O ₂ gases does not change easily and they reach the measurement electrode 44. This is thought to be due to the small increase in NO _x and O ₂ gases.

As shown in the results in Table 1, the shortest distance (distance d1) from the outermost surface of the tip protection layer (200, 200G) to the gas inlet 10 is preferably 0.2 mm or more. By increasing the distance d1 from the outermost surface of the tip protective layer to the gas inlet 10, the gas sensor can be easily operated near the gas inlet 10 even when exposed to a harsh environment with many clogging substances (poisonous substances, etc.). clogging, and a decrease in _NOx sensitivity can be suppressed. That is, by setting the distance d1 from the outermost surface of the tip protective layer to the gas inlet 10 to be 0.2 mm or more, the gas sensor can maintain the gas inlet 10 even if exposed to a harsh environment with many poisonous substances. It is possible to suppress clogging in the vicinity and suppress a decrease in NO _x sensitivity.

The tip protection layer further includes at least two or more layers, and the porosity of the inner layer (inner tip protection layer 201) is larger (higher) than the porosity of the outer layer (outer tip protection layer 202). It is desirable to do so. In particular, in the tip protection layer, it is desirable that the ratio of the thickness of the inner layer to the total thickness of the tip protection layer be 30% or more and 90% or less. By increasing the ratio of the thickness of the inner layer, which has a higher porosity than the outer layer, to the total thickness of the tip protective layer, the layer near the gas inlet 10 (in other words, the inner layer) is protected from poisonous substances, etc. The possibility of clogging can be suppressed.

100...Element base, 101...Sensor element, 200, 200G...Tip protective layer,
201...Inner tip protection layer, 202...Outer tip protection layer, 10...Gas inlet,
7... Gas distribution part to be measured (internal space), 44, 44D, 44E... Measuring electrode,
91, 91A, 91C, 91D, 91E, 91F... Porous diffusion layer,
18, 18B, 18C, 18D, 18E...fourth diffusion controlling section (diffusion controlling section),
19, 19B, 19C, 19D, 19E, 19F...Third internal space (internal space),
CH, CH (B), CH (C), CH (D), CH (E), CH (F)...flow path

Claims (7)

an element substrate into which a gas to be measured is introduced into the internal space from a gas inlet opening on the surface;
a tip protective layer that covers at least a surface of the element substrate where the gas inlet is open;
a measurement electrode provided in the internal space;
a porous diffusion layer provided upstream of the measurement electrode in the flow direction of the gas to be measured at a position where the distance to the measurement electrode is 0.15 mm or less,
The porous diffusion layer is
The porosity is 5% or more and 25% or less, and the porosity is lower than the porosity of the tip protective layer,
The surface perpendicular to the flow direction of the gas to be measured occupies 70% or more of the surface of the flow path of the gas to be measured perpendicular to the flow direction of the gas to be measured,
Gas sensor element. further comprising a diffusion-limiting section that imparts a predetermined diffusion resistance to the gas to be measured in the internal space,
The diffusion-limiting part is
the porosity is lower than the porosity of the porous diffusion layer;
provided upstream of the measurement electrode in the flow direction of the gas to be measured,
The flow path has at least one surface defined by the diffusion-limiting section,
The gas sensor element according to claim 1. The porous diffusion layer is
the diffusion controlling section;
in contact with a surface defining the internal space;
The gas sensor element according to claim 2. At least two surfaces of the flow path are defined by the diffusion-limiting section,
The gas sensor element according to claim 2 or 3. The distance from the outermost surface of the tip protective layer to the gas inlet is 0.2 mm or more,
The gas sensor element according to any one of claims 1 to 3. The tip protective layer includes at least
an inner tip protection layer in contact with the surface of the element base where the gas inlet is open;
an outer tip protection layer constituting the outermost surface of the tip protection layer;
including;
The porosity of the inner tip protection layer is greater than the porosity of the outer tip protection layer,
The thickness of the inner tip protection layer is 30% or more and 90% or less of the thickness of the tip protection layer.
The gas sensor element according to any one of claims 1 to 3. A gas sensor configured to measure the amount of a specific gas component in the gas to be measured using the gas sensor element according to any one of claims 1 to 3.