JP2024014733A - 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 covering a measurement electrode, the porous diffusion layer having a porosity in a range of 5% to 25%, inclusive, which is lower than that of a tip protection layer covering at least a surface of an element substrate having an opening of a gas inlet port.

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 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 sensor element. Various attempts to do so are known. For example, Patent Document 1 listed below discloses a sensor element that includes a diffusion-limiting 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. 11 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. 11. In other words, as illustrated in Figure 11, 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 exists 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. 12 is a diagram showing an example of Knudsen diffusion, which is a diffusion form different from molecular diffusion. As illustrated in Fig. 12, 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 that reaches the measurement electrode from molecular diffusion to a form in which it diffuses while repeatedly colliding with the walls of a sufficiently narrow channel, such as the Knudsen diffusion illustrated in FIG. 12. 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 a gas to be measured is introduced into the internal space from a gas inlet opening 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 containing at least one of silica and alumina, covering the measurement electrode and having a porosity of 5% or more and 25% or less, and pores of the tip protection layer; a porous diffusion layer having a porosity lower than the porosity. 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, and the porous The average porosity of the diffusion layer may be lower than the porosity of the tip protection layer.

In this configuration, the measurement electrode is covered by the porous diffusion layer, which has a porosity of 5% or more and 25% or less, which is lower than the porosity of the tip protection layer. The porous diffusion layer covering the measurement electrode can change the diffusion form around the measurement electrode to a form such as Knudsen diffusion, in which the material diffuses while repeatedly colliding with the wall surface of a sufficiently narrow channel. can. Therefore, in the gas sensor element, even if H ₂ O gas is present in the gas to be measured, the porous diffusion layer covering the measurement electrode separates H ₂ O gas, NO _x gas (and O ₂ gas) can be reduced. Specifically, in the gas sensor element, the porous diffusion layer covering the measurement electrode suppresses fluctuations in NO _x output, which are considered to be caused by molecular diffusion of NO _x under high H ₂ O concentration, and changes in the measurement electrode. Deterioration can be suppressed.

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 with the tip protection layer, that is, can capture (capture) poisonous substances and the like with the tip protection layer.

Particularly, in the gas sensor element, the porosity of the tip protection layer is higher (larger) than the porosity of the porous diffusion layer surrounding the measurement electrode. In the gas sensor element, by making the porosity of the tip protection layer higher than the porosity of the porous diffusion layer, "the tip protection layer becomes clogged with poisonous substances, etc., and NO _x output decreases." It is possible to avoid situations such as "doing".

Moreover, in the gas sensor element, the measurement electrode includes at least one of silica and alumina. Here, when measuring NO _x at high temperatures (eg, 700 to 800 degrees Celsius), the measurement electrode constantly undergoes repeated expansion and contraction. Even under such an environment, the gas sensor element can achieve the following effects because the measurement electrode contains at least one of silica and alumina. That is, the gas sensor element suppresses expansion and contraction at the measurement electrode, thereby preventing cracks, cracks, etc. from forming in the porous diffusion layer covering the measurement electrode, and also suppresses expansion and contraction at the measurement electrode. can be prevented from peeling off from the element substrate.

In the gas sensor element according to the second aspect, of the two surfaces in the thickness direction of the porous diffusion layer of the gas sensor element according to the first aspect, the porosity of the inner surface facing the measurement electrode is higher than the porosity of the inner surface facing the measurement electrode. The porosity of the surface may be higher than that of the surface.

In this configuration, of the two surfaces in the thickness direction of the porous diffusion layer, the porosity of the inner surface facing the measurement electrode (that is, the side closer to the measurement electrode) is higher than that of the outer surface (that is, the side closer to the measurement electrode). higher than the porosity of the surface (the far side).

Here, if H ₂ O adhering to the surface of the measurement electrode is decomposed and H ₂ is generated immediately after the sensor is activated, the H ₂ around the measurement electrode causes a steady state after the gas sensor is activated. The light-off time required to enter the operating state may become longer.

Therefore, in the gas sensor element, of the two surfaces in the thickness direction of the porous diffusion layer, the porosity of the inner surface facing the measurement electrode is made higher than the porosity of the outer surface. Quickly diffuses H ₂ generated near the surface of the measurement electrode. As a result, even if H ₂ O adhering to the surface of the measurement electrode is decomposed and H ₂ is generated, the gas sensor element can suppress the influence of H ₂ and increase the light-off time. You can avoid putting it away.

The gas sensor element according to the third aspect further includes a diffusion-limiting part that imparts a predetermined diffusion resistance to the gas to be measured in the internal space of the gas sensor element according to the first or second aspect, and the measurement electrode may be arranged in an internal space whose upstream side in the flow direction of the gas to be measured is defined by the diffusion-limiting section.

In this configuration, the gas sensor element includes the diffusion-limiting section, and the measurement electrode is arranged in an internal space defined by the diffusion-limiting section on the upstream side in the direction in which the gas to be measured flows. With such a configuration, the gas sensor element changes the diffusion mode of _NO By repeating this, we can get closer to a form of diffusion.

A gas sensor element according to a fourth aspect is a gas sensor element according to any one of the first to third aspects, wherein the measurement electrode and the porous diffusion layer are not in contact with each other, and the measurement electrode and the porous diffusion layer are not in contact with each other. The distance between the diffusion layer and the diffusion layer may be 0.15 mm or less.

In this configuration, the measurement electrode and the porous diffusion layer are not in contact with each other, and the distance between the measurement electrode and the porous diffusion layer is 0.15 mm or less. As mentioned above, if H ₂ O attached to the surface of the measurement electrode is decomposed and H ₂ is generated immediately after the sensor is activated, the H ₂ around the measurement electrode will cause The light-off time required to reach a steady operating state may become longer. Therefore, in the gas sensor element, by providing a space (gap) between the measurement electrode and the porous diffusion layer, H ₂ generated near the surface of the measurement electrode is quickly diffused. As a result, even if H ₂ O adhering to the surface of the measurement electrode is decomposed and H ₂ is generated, the gas sensor element can suppress the influence of H ₂ and increase the light-off time. You can avoid putting it away.

However, if the distance between the measurement electrode and the porous diffusion layer is made too wide, the diffusion form around the measurement electrode will change due to repeated collisions with the walls of a sufficiently narrow channel, such as Knudsen diffusion. The effectiveness of the porous diffusion layer in changing the shape into a shape that decreases. Therefore, in the gas sensor element, the distance between the measurement electrode and the porous diffusion layer is set to 0.15 mm or less. By setting the distance between the two to 0.15 mm or less, the inventors of the present invention have made the diffusion form around the measurement electrode preferable by the porous diffusion layer, as in the case where the two are in contact with each other. I confirmed that it can be done. That is, by using the porous diffusion layer whose distance to the measurement electrode is 0.15 mm or less, the diffusion form around the measurement electrode is caused to repeatedly collide with the wall surface of a sufficiently narrow channel, such as Knudsen diffusion. It was confirmed that it is possible to create a form that diffuses while Therefore, in the gas sensor element, due to the porous diffusion layer whose distance to the measurement electrode is 0.15 mm or less, fluctuations in NO _x output that are considered to be caused by molecular diffusion of NO _x under high H ₂ O concentration can be prevented. And deterioration of the measurement electrode can be suppressed.

In the gas sensor element according to the fifth aspect, the inner surface facing the measurement electrode is selected from among the two surfaces in the thickness direction of the porous diffusion layer of the gas sensor element according to any one of the first to fourth aspects. The porosity of the outer surface may be 10% or more higher than the porosity of the outer surface.

In this configuration, of the two surfaces in the thickness direction of the porous diffusion layer, the porosity of the inner surface facing the measurement electrode is 10% or more higher than the porosity of the outer surface. As mentioned above, by making the porosity of the inner surface facing the measurement electrode higher than the porosity of the outer surface among the two surfaces in the thickness direction of the porous diffusion layer, the following effects can be achieved. It can be realized. That is, by making the porosity of the inner surface higher than the porosity of the outer surface, the H ₂ generated near the surface of the measurement electrode can be quickly diffused, and the increase in light-off time can be prevented. Can be suppressed. The inventors of the present invention have found that when the porosity of the inner surface of the porous diffusion layer is 10% or more higher than the porosity of the outer surface, the light-off time is longer than when the porosity is less than 10%. was confirmed to be shorter. Therefore, in the gas sensor element, of the two surfaces in the thickness direction of the porous diffusion layer, the porosity of the inner surface facing the measurement electrode is 10% or more higher than the porosity of the outer surface. By doing so, the light-off time can be made shorter than when it is less than 10%.

The gas sensor element according to a sixth aspect is the gas sensor element according to any one of the first to fifth aspects, wherein the distance from the outermost surface of the tip protective layer to the gas inlet is 0.2 mm or more. Good too.

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. In other words, the gas sensor element can reliably capture poisonous substances in the tip protective layer even in a harsh environment where there are many poisonous substances, and prevent the poisonous substances from entering the vicinity of the gas inlet. By suppressing clogging, it is possible to avoid a decrease in NO _x sensitivity.

A gas sensor element according to a seventh aspect is a gas sensor element according to any one of the first to sixth aspects, wherein the tip protective layer is arranged at least on a 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 greater 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. and 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 of the thickness of the tip protection layer, It is 90% or less.

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. Specifically, 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 It is possible to avoid clogging caused by poisonous 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 covering the measurement electrode 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. be able to.

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 the relationship between the measurement electrode and the porous diffusion layer in the element substrate of FIG. 2. FIG. 5 is a diagram showing an example of a porous diffusion layer according to a modification. FIG. 6 is a diagram illustrating an example of the relationship between a measurement electrode and a porous diffusion layer according to a modification. FIG. 7 is an enlarged view for explaining the main parts of the element base according to the modification. FIG. 8 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. 9 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 covering the measurement electrode. FIG. 10 is a graph showing the difference in H ₂ O dependence of NO _x output depending on the presence or absence of a porous diffusion layer covering the measurement electrode. FIG. 11 is a diagram showing an example of molecular diffusion. FIG. 12 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-mentioned problem could be solved by doing so.

Therefore, in the gas sensor element 101 according to the present embodiment, the measurement electrode 44 is covered with a porous diffusion layer 91 having a porosity of 5% or more and 25% or less. Specifically, the measurement electrode 44 is covered by the porous diffusion layer 91, which has a porosity of 5% or more and 25% or less, and is disposed within a distance d2 of 0.15 mm to the measurement electrode 44. There is. The gas sensor element 101 changes the diffusion form around the measurement electrode 44 by the porous diffusion layer 91 covering the measurement electrode 44 . 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 can suppress 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. Further, the gas sensor element 101 suppresses fluctuations in NO _x output under high H ₂ O concentration, for example, suppresses the H ₂ O dependence of NO _x output when NO _x gas is flowing, and suppresses NO _x Improve accuracy of concentration measurements. 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 tip protection layer 200 traps (captures) poisonous substances and the like that cause clogging of the porous diffusion layer 91 covering 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, the gas sensor element 101 can prevent the porous diffusion layer 91 covering the measurement electrode 44 from being clogged with poisonous substances, resulting in a decrease in NO _x output and a decrease in measurement accuracy. . 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, at one end 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, a gas to be measured is received from an external space. An internal space is provided which is configured to allow gas to be introduced. 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 voids 17 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 two-chamber structure (first internal space 15 and second internal space 17).

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 (a flow path through which the gas to be measured flows) is formed (the opening has a long side direction). 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 through which the gas to be measured flows. Similarly, the second diffusion-limiting section 13 and the third diffusion-limiting section 16 are members that impart a predetermined diffusion resistance to the gas to be measured, and in the example shown in FIG. Each of the rate-limiting parts 16 forms a hole (a flow path through which the gas to be measured flows) whose length extending in the direction perpendicular to the drawing is shorter than each of the first internal space 15 and the second internal space 17. There is.

As illustrated in FIG. 2, the second diffusion-limiting section 13 and the third diffusion-limiting section 16 both 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 having a side direction) may be formed. 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 between the second diffusion-limiting section 13 and the layer 6 and the second The space between the diffusion rate controlling portion 13 and the layer 4 becomes a slit, that is, a flow path 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 portion 16 and the layer 4 becomes a slit, that is, a flow path through which the gas to be measured flows. Each of the second diffusion controlling section 13 and the third diffusion controlling section 16 will be explained in detail later. A region (internal space) extending from the gas inlet 10 to the second internal space 17 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 need 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 part 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 space 15 and the second internal space 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 take in a gas to be measured (introduce the gas to be measured) from an external space into the interior 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. In other words, 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 of the element substrate 100 or on the left side. You may also do so.

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 introduction port 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 used 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 formed by the second diffusion-limiting section 13). It is set up as a space. 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 section 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 cavity 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 that constitutes 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 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 applies a desired pump voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23 in a positive direction or a negative direction. By flowing the pump current Ip0, 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 for controlling the main pump (ie, an electrochemical sensor cell).

The element substrate 100 is configured to be able to specify the oxygen concentration (oxygen partial pressure) in the first internal space 15 by measuring the electromotive force V0 in the main pump control oxygen partial pressure detection sensor cell 80. 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 is for further adjusting the oxygen partial pressure in the gas to be measured introduced through the third diffusion-limiting section 16 (that is, through the flow path 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 substrate 100 pumps oxygen in the atmosphere in the second internal space 17 to the external space by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 in the auxiliary pump cell 50, Alternatively, it is configured to be able to be 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. Thereby, 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 NO _x .

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 space 17 at a constant value of about 0.001 ppm.

That is, in the gas sensor element 101, 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 _NOx ).

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, in the second internal space 17, for the gas to be measured introduced through the third diffusion rate controlling part, The oxygen partial pressure is further adjusted by the auxiliary pump cell 50. Thereby, the oxygen concentration of the gas to be measured in the second internal space 17 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 within the second internal space 17 . That is, the NO _x concentration of the gas to be measured whose oxygen concentration has been adjusted by the auxiliary pump cell 50 in the second internal space 17 is measured by the operation of the measuring pump cell 41 . 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 upper surface of the first solid electrolyte layer 4 adjacent to (facing) the second internal cavity 17. Further, the measurement electrode 44 is surrounded by a porous diffusion layer 91.

The measurement electrode 44 is a porous cermet electrode and contains 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 porous diffusion layer 91 covering the measurement electrode 44 is prevented from cracking, or the measurement electrode 44 is peeled from the first solid electrolyte layer 4. This phenomenon will no longer occur.

The measurement electrode 44 also functions as a NO _x reduction catalyst that reduces NO _x present in the atmosphere within the second internal space 17 . The measurement electrode 44 is surrounded by a porous diffusion layer 91, that is, is coated with the porous diffusion layer 91.

The porous diffusion layer 91 is a porous body having a porosity of 5% or more and 25% or less, and has a lower porosity than the tip protection layer 200. When the porous diffusion layer 91 includes a plurality of surfaces (layers) with different porosities, the porous diffusion layer 91 has an average porosity of 5% or more and 25% or less, and the average porosity is equal to or higher than that of the tip protection layer 200. porosity is lower than that of . For example, if the porous diffusion layer 91 has a different porosity between the two surfaces in the thickness direction of the porous diffusion layer 91, the inner surface facing the measurement electrode 44 and the outer surface not facing the measurement electrode 44 have different porosity. The average porosity of satisfies the following conditions. That is, 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. In other words, of the two surfaces in the thickness direction of the porous diffusion layer 91 that covers the measurement electrode 44, the side closer to the measurement electrode 44 (inner side) and the opposite side farther from the measurement electrode 44 (outer side) When the porosity is different between the surfaces of , the average porosity of the porous diffusion layer 91 satisfies the following conditions. That is, the average porosity of the porous diffusion layer 91 is 5% or more and 25% or less, and the average porosity of the porous diffusion layer 91 is lower than the porosity of the tip protection layer 200. The surface far from the measurement electrode 44 (outer surface) may also be referred to as the surface facing (opposing) the gas flow section 7 to be measured, and in the example shown in FIG. 2 (and FIG. 3 described later), It may also be referred to as the surface facing the second internal space 17.

By covering the measurement electrode 44, the porous diffusion layer 91 changes the diffusion form of the gas to be measured (particularly _NOx gas) around the measurement electrode 44 to the wall surface of a sufficiently narrow channel, such as Knudsen diffusion. The structure is such that it spreads through repeated collisions. Further, the porous diffusion layer 91 also acts as a protective film for the measurement electrode 44. The porous diffusion layer 91 may be composed of, for example, a porous film containing alumina (Al ₂ O ₃ ) as a main component.

The NO _x concentration is measured by the operation of the measurement pump cell 41 . That is, 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 detect the amount of oxygen generated as the 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 source 46 is controlled based on the voltage (electromotive force) V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measuring pump.

The gas to be measured guided into the second internal space 17 through the flow path formed by the third diffusion rate controlling section 16 reaches the measurement electrode 44 after the oxygen partial pressure is controlled by the auxiliary pump cell 50. I will do it. In particular, the gas to be measured has a diffusion form suitable for measuring the nitrogen oxide (NO _x ) concentration through the porous diffusion layer 91, in other words, a predetermined diffusion resistance is provided. 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.

Furthermore, 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 arranged 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. There is.

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, which is a porous layer covering the measurement electrode 44 disposed in the second internal cavity 17. By providing a porous diffusion layer 91 with a porosity of 5% or more and 25% or less, which is lower than the porosity of the tip protection layer 200, as a porous layer covering the measurement electrode 44, the porous diffusion layer 91 is supplied to the measurement electrode 44. Diffusion resistance of the gas to be measured is adjusted. In particular, due to the porous diffusion layer 91, the diffusion form of the gas to be measured (particularly NO _x gas) around the measurement electrode 44 is controlled by repeatedly colliding with the wall surface of a sufficiently narrow channel, such as Knudsen diffusion. It becomes a form of diffusion. Thereby, even when the H ₂ O concentration is high, it is possible to suppress fluctuations in the NO _x output and deterioration of the measurement electrode that would 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 upper surface (upper surface) of the lower surface of the second solid electrolyte layer 6 and the lower surface (lower surface) of the upper surface of the first solid electrolyte layer 4 . The gas flow section 7 includes a first internal space 15 and a second internal space 17 .

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 (ceiling electrode section 51a and bottom electrode section 51b), an outer pump electrode 23 and a second solid electrolyte layer 6, which are not shown in FIG.

In the second internal space 17 , the nitrogen oxide concentration of the gas to be measured whose oxygen partial pressure has been adjusted by the auxiliary pump cell 50 is measured by the measuring pump cell 41 . 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.

The measurement electrode 44 is surrounded by a porous diffusion layer 91 having a porosity of 5% or more and 25% or less and lower than that of the tip protection layer 200. 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

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 the porosity 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. .

Among the two surfaces in the thickness direction of the porous diffusion layer 91, if the inner surface facing the measurement electrode 44 and the outer surface have different porosity, the porous diffusion layer 91 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. In other words, if the porosity of the two surfaces of the porous diffusion layer 91 in the thickness direction is different between the surface closer to the measurement electrode 44 and the surface farther from the measurement electrode 44, the average porosity of the porous diffusion layer 91 is 5. % or more and 25% or less, and the average porosity is lower than the porosity of the tip protection layer 200. For example, when the porous diffusion layer 91 includes a plurality of surfaces (layers) having different porosities, the average porosity of the porous diffusion layer 91 calculated from the porosity of each of the plurality of surfaces (layers) is 5. % or more and 25% or less, which is lower than the porosity of the tip protection layer 200. For details, see FIG. 5 for an example of a porous diffusion layer that covers the measurement electrode 44 and has a different porosity between the surface closer to the measurement electrode 44 (inner side) and the surface farther away from the measurement electrode 44 (outer side). will be described later using

(Necessity of contact between porous diffusion layer and measurement electrode)
The porous diffusion layer 91 has a porosity of 5% or more and 25% or less and is lower than the porosity of the tip protection layer 200, and is provided in order to make the diffusion form around the measurement electrode 44 suitable. Specifically, it is provided so that the distance d2 to the measurement electrode 44 is 0.15 mm or less. The porous diffusion layer 91 may cover the measurement electrode 44 while in contact with the measurement electrode 44, or it may cover the measurement electrode 44 without contacting the measurement electrode 44 so that the distance d2 to the measurement electrode 44 is 0.15 mm or less. Additionally, the measurement electrode 44 may be covered. Below, using FIGS. 4 to 6, a porous diffusion layer having a porosity of 5% or more and 25% or less, lower than the porosity of the tip protection layer 200, and covering the measurement electrode 44 will be described. 44 and an example in which the measurement electrode 44 is not contacted will be explained in detail.

(Example where the porous diffusion layer and measurement electrode are in contact)
FIG. 4 is a diagram showing an example of the relationship between the measurement electrode 44 and the porous diffusion layer 91 in the element substrate 100. Specifically, FIG. 4 is a diagram showing an example of a porous diffusion layer 91 that covers (covers) the measurement electrode 44 while being in contact with the measurement electrode 44. The porous diffusion layer 91 illustrated in FIG. 4 is a porous layer with a constant porosity of 5% or more and 25% or less throughout, and is in contact with the upper surface, front surface, and rear surface of the measurement electrode 44. and covers the measurement electrode 44. The porous diffusion layer 91 that covers the measurement electrode 44 in contact with the measurement electrode 44 allows the diffusion form of the gas to be measured (especially NO _x gas) to reach the measurement electrode 44 to be sufficiently narrow such as Knudsen diffusion. It can be configured to diffuse while repeatedly colliding with the wall surface of the channel. As illustrated in FIG. 4, in the gas sensor element 101, the porous diffusion layer that covers the measurement electrode 44 has a porosity of 5% or more and 25% or less, which is lower than the porosity of the tip protection layer 200, and that covers the measurement electrode 44. It may be in contact with the electrode 44. However, for the gas sensor element 101, it is not essential that the porous diffusion layer be in contact with the measurement electrode 44. In the gas sensor element 101, the porous diffusion layer has a porosity of 5% or more and 25% or less, lower than the porosity of the tip protection layer 200, and covers the measurement electrode 44, even if it is not in contact with the measurement electrode 44. good. An example of a porous diffusion layer that is not in contact with the measurement electrode 44 will be described later using FIG. 6.

FIG. 5 is a diagram showing an example of a porous diffusion layer 91A according to a modification. Specifically, FIG. 5 is a diagram showing an example of a porous diffusion layer 91A in which the porosity is not constant throughout, that is, includes a plurality of surfaces (layers) with different porosity. The porous diffusion layer 91 illustrated in FIG. 4 had a constant porosity of 5% or more and 25% or less throughout. However, in the gas sensor element 101 (element substrate 100), it is not essential that the porosity of the porous diffusion layer surrounding the measurement electrode 44 be constant throughout the porous diffusion layer. Like the porous diffusion layer 91A illustrated in FIG. The porosity may be different between the surfaces. That is, of the two surfaces in the thickness direction of the porous diffusion layer covering the measurement electrode 44, the inner surface facing the measurement electrode 44 and the outer surface not facing the measurement electrode 44 have different porosity. You can leave it there. The gas sensor element 101 may include a porous diffusion layer 91A instead of the porous diffusion layer 91.

The porous diffusion layer 91A illustrated in FIG. A second porous diffusion layer 912 (inner surface) that is a porous layer facing the electrode 44 is included. The first porous diffusion layer 911 and the second porous diffusion layer 912 have different porosity from each other, and the porosity of the second porous diffusion layer 912 is higher than that of the first porous diffusion layer 911. That is, of the two surfaces in the thickness direction of the porous diffusion layer 91A, the porosity of the inner surface (second porous diffusion layer 912) facing the measurement electrode 44 is the same as that of the outer surface (first porous diffusion layer 912). 911). In the example shown in FIG. 5, of the two surfaces in the thickness direction of the porous diffusion layer 91A, the inner surface (second porous diffusion layer 912) faces the measurement electrode 44, and the outer surface (first porous diffusion layer 912) faces the measurement electrode 44. The diffusion layer 911) faces (opposes) the gas flow section 7 to be measured.

Of the two surfaces in the thickness direction of the porous diffusion layer 91A, the porosity of the inner surface facing the measurement electrode 44 is greater than the porosity of the outer surface (for example, the surface facing the gas flow section 7 to be measured). By increasing the value, 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, like the porous diffusion layer 91A, the porosity of the inner surface facing the measurement electrode 44 (the side closer to the measurement electrode 44; the second porous diffusion layer 912) is different from the porosity of the outer surface (the first porous diffusion layer 912). By making the porosity higher than that of the layer 911), the gas sensor element 101 achieves the following effects. That is, by making the porosity of the inner surface facing the measurement electrode 44 higher than that of the outer surface, the gas sensor element 101 can quickly diffuse H ₂ generated near the surface of the measurement electrode 44. can. 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 second porous diffusion layer 912 which has a higher porosity than the first porous diffusion layer 911 (outer surface). (the inner surface facing the measurement electrode 44) allows quick 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, the porosity of the second porous diffusion layer 912 is set to the porosity of the first porous diffusion layer 911. It is desirable to make it higher than that. In other words, when the porous diffusion layer 91A and the measurement electrode 44 are in contact with each other, the porosity of the inner surface of the porous diffusion layer 91A that is in contact with the measurement electrode 44 is the same as that of the outer surface (the surface that is not in contact with the measurement electrode 44. For example, it is desirable that the porosity be higher than that of the surface facing the gas flow section 7 to be measured. In the porous diffusion layer 91A, by making the porosity of the inner surface in contact with the measurement electrode 44 higher than the porosity of the outer surface, even when the porous diffusion layer 91A and the measurement electrode 44 are in contact with each other, , 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 side of the porous diffusion layer 91A closer to the measurement electrode 44 (second porous diffusion layer 912)" was derived, for example, as follows. That is, first, near the center of the measurement electrode 44 when viewed in the length direction (axial direction of the sensor element) and from the interface between the measurement electrode 44 and the porous diffusion layer 91A (second porous diffusion layer 912). SEM images were acquired in the 10-15 μm range. Next, by applying a known image processing method such as binarization processing to the acquired SEM image, "the side of the porous diffusion layer 91A near the measurement electrode 44 (the side of the second porous diffusion layer 912 )” was determined. The porosity of "the surface side of the porous diffusion layer 91A (the side facing the gas flow section 7 to be measured; specifically, the first porous diffusion layer 911)" was also determined by applying the same method. . That is, a SEM image is obtained in a range of 10 to 15 μm from the surface (for example, the top surface) of the porous diffusion layer 91A (first porous diffusion layer 911), and a known image processing method is applied to the SEM image. Thus, the porosity of the surface side of the porous diffusion layer 91A was derived.

As described above, in the porous diffusion layer 91A, the porosity of the second porous diffusion layer 912 is higher than that of the first porous diffusion layer 911, for example, higher than the porosity of the first porous diffusion layer 911. is also more than 10% higher. That is, of the two surfaces in the thickness direction of the porous diffusion layer 91A, the porosity of the inner surface (second porous diffusion layer 912) facing the measurement electrode 44 is higher than that of the outer surface (first porous diffusion layer 912). The porosity is 10% or more higher than that of 911).

As described above, the porosity of the second porous diffusion layer 912 (the porosity of the inner surface facing the measurement electrode 44) of the porous diffusion layer 91A is compared with the porosity of the first porous diffusion layer 911 (the outer surface). By setting the ratio higher than the ratio, the following effects can be achieved. That is, with such a configuration, the porous diffusion layer 91A can quickly diffuse H ₂ generated near the surface of the measurement electrode 44, and can suppress an increase in the light-off time. The inventors of the present invention propose that when the porosity of the inner surface facing the measurement electrode 44 is 10% or more higher than the porosity of the outer surface among the two surfaces in the thickness direction of the porous diffusion layer 91A. , it was confirmed that the light-off time was shorter than that in the case of less than 10%. Therefore, in the porous diffusion layer 91A, the porosity of the inner surface facing the measurement electrode 44 (the surface closer to the measurement electrode 44; the second porous diffusion layer 912) and the porosity of the outer surface (the first porous diffusion layer 912) The difference in porosity from the diffusion layer 911) is desirably 10% or more. Specifically, in the porous diffusion layer 91A, the porosity of the inner surface facing the measurement electrode 44 is preferably 10% or more higher than the porosity of the outer surface. In the gas sensor element 101, the porosity of the second porous diffusion layer 912 of the porous diffusion layer 91A is made higher than the porosity of the first porous diffusion layer 911 by 10% or more, compared to a case where the porosity is less than 10%. Light off time can be shortened. That is, in the gas sensor element 101, of the two surfaces in the thickness direction of the porous diffusion layer 91A, the porosity of the inner surface facing the measurement electrode 44 is determined by the porosity of the outer surface (the surface facing the gas flow section 7 to be measured). ) By increasing the porosity by 10% or more, the light-off time can be further shortened.

As explained above, of the two surfaces in the thickness direction of the porous diffusion layer 91A, the porosity of the inner surface facing the measurement electrode 44 is different from that of the outer surface (for example, the gas distribution section 7 to be measured). The porosity is higher than the porosity of the surface (the surface facing the surface), particularly by 10% or more. Here, the porosity of the porous diffusion layer 91A is changed from the inner surface facing the measurement electrode 44 to the outer surface not facing the measurement electrode 44 (for example, the surface facing the gas flow section 7 to be measured). The mode of change is not particularly limited.

That is, as illustrated in FIG. 5, the porosity of the porous diffusion layer 91A may change stepwise (discontinuously) from the inner surface facing the measurement electrode 44 to the outer surface. . The porous diffusion layer 91A illustrated in FIG. ) The inner surface (for example, the lower surface) is constituted by a second porous diffusion layer 912. The first porous diffusion layer 911 and the second porous diffusion layer 912 have different porosity from each other. Specifically, the porosity of the first porous diffusion layer 911 is different from that of the second porous diffusion layer 912. The porosity is lower than that of the second porous diffusion layer 912, for example, 10% or more lower than the porosity of the second porous diffusion layer 912. 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 gradually changes from the inner surface facing the measurement electrode 44 to the outer surface. change in a targeted (discontinuous) manner.

In addition, in the porous diffusion layer 91A, the porosity is continuously increased from the inner surface facing (facing) the measurement electrode 44 to the outer surface (for example, the surface facing the gas flow section 7 to be measured). May change. For example, the porosity gradually decreases from the inner surface facing (facing) the measurement electrode 44 to the outer surface (for example, the surface facing the gas flow section 7 to be measured), and as a result, both The porous diffusion layer 91A may be configured such that the difference in porosity is 10% or more.

As explained above, of the two surfaces in the thickness direction of the porous diffusion layer 91A, the porosity of the inner surface facing the measurement electrode 44 is higher than the porosity of the outer surface. More than 10% higher. The manner in which the porosity changes from the inner surface facing the measurement electrode 44 to the outer surface in the porous diffusion layer 91A is not particularly limited, and may be, for example, stepwise (discontinuously It may change continuously (target) or it may change continuously.

In the gas sensor element 101, when the porous diffusion layer surrounding the measurement electrode 44 includes a plurality of surfaces (layers) having different porosities, like the porous diffusion layer 91A, the porous diffusion layer The average porosity satisfies the following conditions. That is, when the porous diffusion layer surrounding the measurement electrode 44 has a plurality of surfaces with different porosities, 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. For example, as illustrated in FIG. 5, in a porous diffusion layer 91A in which the porosity is different between the surface closer to the measurement electrode 44 (inner side) and the surface farther away from the measurement electrode 44 (outer side), the average pore size of the porous diffusion layer 91A is The porosity 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 911 and the second porous diffusion layer 912 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.

In FIG. 5, a porous diffusion layer 91A having a different porosity between an inner surface facing the measurement electrode 44 and an outer surface not facing the measurement electrode 44 is shown in a state in which the measurement electrode 44 is in contact with the measurement electrode 44. An example of covering is shown. However, it is not essential for the porous diffusion layer 91A to cover the measurement electrode 44 while being in contact with the measurement electrode 44. A space (gap) may be provided between the porous diffusion layer 91A and the measurement electrode 44. Similarly, a space (gap) may be provided between the porous diffusion layer 91 and the measurement electrode 44. That is, in the gas sensor element 101 , the porous diffusion layer that covers the measurement electrode 44 and has a porosity of 5% or more and 25% or less and that is lower than the porosity of the tip protection layer 200 covers the measurement electrode 44 . They do not need to be in contact with each other, and a space may be provided between them. Hereinafter, with reference to FIG. 6, an example will be described in which the porous diffusion layer covering the measurement electrode 44 is not in contact with the measurement electrode 44 in the gas sensor element 101.

(Example where the porous diffusion layer and measurement electrode are not in contact)
FIG. 6 is a diagram showing an example of the relationship between the measurement electrode 44 and the porous diffusion layer 91 (porous diffusion layer 91B) according to a modification. Specifically, FIG. 6 is a diagram showing an example of a porous diffusion layer 91B that covers the measurement electrode 44 without contacting the measurement electrode 44 and has a distance d2 to the measurement electrode 44 of 0.15 mm or less. . The porous diffusion layers 91 and 91A illustrated in FIGS. 4 and 5 covered the measurement electrode 44 while being in contact with the measurement electrode 44. However, in the gas sensor element 101 (element substrate 100), it is not essential that the porous diffusion layer surrounding the measurement electrode 44 be in contact with the measurement electrode 44. Like the porous diffusion layer 91B illustrated in FIG. 6, the porous diffusion layer in the gas sensor element 101 may cover the measurement electrode 44 without being in contact with the measurement electrode 44. That is, there may be a space (gap) between the porous diffusion layer and the measurement electrode 44. Instead of the porous diffusion layer 91, the gas sensor element 101 covers the measurement electrode 44 without contacting the measurement electrode 44, and has a porosity of 5% or more and 25% or less, which is lower than the porosity of the tip protection layer 200. A porous diffusion layer 91B with low porosity may be included.

The porous diffusion layer 91B illustrated in FIG. 6 is not in contact with the measurement electrode 44. That is, a space (gap) is provided between the porous diffusion layer 91B provided around the measurement electrode 44 and the measurement electrode 44. However, the distance d2 between the porous diffusion layer 91B surrounding the measurement electrode 44 and the measurement electrode 44 is 0.15 mm or less. The distance d2 between the porous diffusion layer 91B and the measurement electrode 44 is, for example, from the surface of the porous diffusion layer 91B facing (opposed to) the measurement electrode 44 to the measurement electrode 44 (particularly its surface. This refers to the distance to the surface facing (opposing) the diffusion layer 91B.

The porous diffusion layer 91B is not in contact with the measurement electrode 44, but the distance d2 to the measurement electrode 44 is 0.15 mm or less, that is, the distance d2 between the porous diffusion layer 91B and the measurement electrode 44 is 0.15 mm or less. As mentioned above, if H ₂ O adhering to the surface of the measurement electrode 44 is decomposed and H ₂ is generated immediately after the sensor is activated, the H ₂ around the measurement electrode 44 will cause The light-off time required to reach a steady operating state may become longer. Therefore, the gas sensor element 101 quickly diffuses H ₂ generated near the surface of the measurement electrode 44 by providing a space (gap) between the measurement electrode 44 and the porous diffusion layer 91B. As a result, 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 increases the light-off time. You can avoid putting it away. In other words, by providing a space between the porous diffusion layer 91 and the measurement electrode 44, the gas sensor element 101 quickly diffuses H ₂ generated when H ₂ O attached to the surface of the measurement electrode 44 is decomposed. let Therefore, during constant control, the potential difference between the measurement electrode 44 and the reference electrode 42 does not become too large, and the gas sensor element 101 can shorten the light-off time.

However, if the distance between the measurement electrode 44 and the porous diffusion layer 91B is made too wide, the diffusion form around the measurement electrode 44 will change due to repeated collisions with the walls of a sufficiently narrow channel, such as Knudsen diffusion. The effect of the porous diffusion layer 91B in changing the shape to the shape of the porous diffusion layer 91B is reduced. Therefore, in the gas sensor element 101, the distance between the measurement electrode 44 and the porous diffusion layer 91B is set to 0.15 mm or less. By setting the distance between the two to 0.15 mm or less, the inventors of the present invention have made the diffusion form around the measurement electrode 44 favorable by the porous diffusion layer 91B, as in the case where the two are in contact. I confirmed that it can be done. That is, by using the porous diffusion layer 91B whose distance to the measurement electrode 44 is 0.15 mm or less, the diffusion form around the measurement electrode 44 is changed by repeatedly colliding with the wall surface of a sufficiently narrow channel, such as Knudsen diffusion. It was confirmed that it is possible to create a form that diffuses while Therefore, in the gas sensor element 101, due to the porous diffusion layer 91B whose distance to the measurement electrode 44 is 0.15 mm or less, fluctuations in NO _x output, which are considered to be caused by molecular diffusion of NO _x under high H ₂ O concentration, are prevented. And deterioration of the measurement electrode 44 can be suppressed. In other words, in the gas sensor element 101, the influence of H ₂ O gas on NO _x gas (and O ₂ gas) can be suppressed by the porous diffusion layer 91B whose distance to the measurement electrode 44 is 0.15 mm or less. can.

Note that for a porous diffusion layer such as the porous diffusion layer 91B that covers the measurement electrode 44 without contacting the measurement electrode 44, the porosity of the inner surface facing the measurement electrode 44 is the same as the porosity of the outer surface. It may be higher than that. For example, of the two surfaces in the thickness direction of the porous diffusion layer 91B, the porosity of the inner surface facing the measurement electrode 44 may be higher than the porosity of the outer surface. When the distance d2 to the measurement electrode 44 is 0.15 mm or less, the porous diffusion layer that covers the measurement electrode 44 without contacting the measurement electrode 44 has a porosity of the inner surface facing the measurement electrode 44, and a porosity of the outer surface. By making it higher than the porosity, H ₂ can be diffused more quickly. Therefore, the gas sensor element 101 including such a porous diffusion layer can further shorten the light-off time.

(About the placement position of the measurement electrode)
Up to now, an example in which the measurement electrode 44 is arranged in the second internal cavity 17 has been described using FIGS. 2 to 6. However, in the gas sensor element 101 , it is not essential that the measurement electrode 44 be arranged in the second internal space 17 . The measuring electrode 44 may be arranged in the first internal cavity 15 . Furthermore, it is not essential that the element substrate of the gas sensor element 101 has a two-chamber structure, that is, the first internal space 15 and the second internal space 17. For example, the element substrate included in the gas sensor element 101 may have a one-chamber structure, that is, may have a structure that does not include a diffusion-limiting section. Further, the element substrate of the gas sensor element 101 may have three or more internal cavities, for example, it may have a three-chamber structure (a structure with three internal cavities), or it may have a four-chamber structure. A structure including the above internal space may be used. An example in which the element base of the gas sensor element 101 has a three-chamber structure will be described below with reference to FIG.

(Example where the measurement electrode is placed in the third internal space)
FIG. 7 is an enlarged view for explaining main parts of an element base 100C according to a modification. Specifically, FIG. 7 illustrates the main part of the element substrate 100C including three internal cavities (first internal cavity 15, second internal cavity 17, and third internal cavity 19). This is an enlarged view for Instead of the element base 100, the gas sensor element 101 may include an element base 100C, which will be described in detail below.

Like the element substrate 100, the element substrate 100C has 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. It includes a laminate formed by stacking layers in order from the sides. Similarly to the element substrate 100, in the element substrate 100C, there is a portion to be measured between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4 at one tip of the element substrate 100C. An internal space (measured gas flow section 7C) configured to introduce gas is provided.

The configuration of the measured gas distribution section 7C is the same as the configuration of the internal space (measured gas distribution section 7) provided in the element substrate 100 from the gas inlet 10 to the second internal space 17. However, the element substrate 100C further includes a fourth diffusion-limiting section 18 and a third internal space 19. Specifically, the measured gas distribution section 7C 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, The second internal space 17, the fourth diffusion-limiting part 18, and the third internal space 19 are configured to be formed adjacent to each other in this order so as to communicate with each other. That is, the measured gas flow section 7C has a three-chamber structure (a first internal cavity 15, a second internal cavity 17, and a third internal cavity 19), and has a three-chamber structure (a first internal cavity 15, a second internal cavity 17, and a third internal cavity 19). This is the part (internal space) leading to 19. The measured gas flow section 7C is provided by hollowing out the spacer layer 5. The upper part of the gas flow section 7C to be measured is defined by the lower surface of the second solid electrolyte layer 6. The lower part of the gas flow section 7C to be measured is defined by the upper surface of the first solid electrolyte layer 4. The side portions of the gas flow portion 7C to be measured are defined by the side surfaces of the spacer layer 5.

The gas inlet 10, the first diffusion-limiting section 11, the buffer space 12, the second diffusion-limiting section 13, the first internal space 15, the third diffusion-limiting section 16, and the second internal section provided in the gas distribution section 7C to be measured The void space 17'' is the same as that provided in the gas distribution section 7 to be measured, and therefore a description thereof will be omitted.

The fourth diffusion rate controlling section 18 is a member (part) that imparts a predetermined diffusion resistance to the gas to be measured. In the example shown in FIG. 7, the fourth diffusion-limiting section 18 forms a hole (flow path through which the gas to be measured flows) whose length extending in the direction perpendicular to the drawing is shorter than the third internal space 19. . Specifically, the fourth diffusion rate-controlling part 18 forms one horizontally long slit (the opening has a longitudinal direction in the direction perpendicular to the drawing) formed as a gap between the second solid electrolyte layer 6 and the lower surface of the second solid electrolyte layer 6. are doing. That is, the fourth diffusion rate-limiting section 18 is in contact with the upper surface of the first solid electrolyte layer 4. 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 through which the gas to be measured flows. However, similarly to the first diffusion controlling section 11, the second diffusion controlling section 13, and the third diffusion controlling section 16, the fourth diffusion controlling section 18 has two horizontally elongated sections (the opening in the direction perpendicular to the drawing is on the long side). A slit having a direction may also be formed. That is, the fourth diffusion rate-limiting section 18 does not need to be in contact with the upper surface of the first solid electrolyte layer 4. The fourth diffusion rate controlling section 18 imparts a predetermined diffusion resistance to the gas to be measured flowing from the second internal space 17 to the third internal space 19, and also provides a predetermined diffusion resistance to the gas flowing from the second internal space 17 to the third internal space 19. It is only necessary to form a flow path for the gas to be measured to 19.

As described above, the fourth diffusion rate controlling section 18 applies a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump cell 50 in the second internal space 17. This is a part that guides the gas to be measured into the third internal space 19.

The third internal space 19 is provided as a space for performing processing related to measurement of the nitrogen oxide (NO _x ) concentration in the gas to be measured introduced through the fourth diffusion rate controlling section 18 . The NO _x concentration is measured by the operation of the measurement pump cell 41 . In the element substrate 100C, the oxygen concentration (oxygen partial pressure) in the first internal space 15 is adjusted in advance. Thereafter, in the second internal space 17, the oxygen partial pressure is further adjusted by the auxiliary pump cell 50 with respect to the gas to be measured that has been introduced through the flow path formed by the third diffusion rate controlling section 16. 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 precision. Therefore, the element substrate 100C enables highly accurate measurement of NO _x concentration.

The measurement pump cell 41 in the element substrate 100C measures the nitrogen oxide concentration in the gas to be measured not in the second internal cavity 17 but in the third internal cavity 19. This is similar to the measurement pump cell 41 in . That is, the _NOx concentration of the gas to be measured whose oxygen concentration has been adjusted in the second internal space 17 is measured by operating the measurement pump cell 41. 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. 7 , the measurement electrode 44 is provided on the upper surface of the first solid electrolyte layer 4 adjacent to (facing) the third internal space 19 . Furthermore, the measurement electrode 44 is surrounded by a porous diffusion layer 91 .

The measurement electrode 44 on the element substrate 100C is similar to the measurement electrode 44 on the element substrate 100. However, the measurement electrode 44 in the element substrate 100C also functions as a NO _x reduction catalyst that reduces NO _x present in the atmosphere within the third internal space 19 .

As described above with reference to FIG. 7, the gas sensor element 101 has a predetermined diffusion resistance for the gas to be measured in the internal space (the gas distribution section 7C) into which the gas to be measured is introduced from the gas inlet 10. It may be provided with a diffusion control section (fourth diffusion control section 18) that provides the following. In this case, the measurement electrode 44 is arranged in an internal space (third internal space 19) whose upstream side in the flow direction of the gas to be measured is partitioned by the fourth diffusion rate controlling section 18 . The third internal space 19 is a space into which the gas to be measured is introduced from the second internal space 17 through a flow path (slit) formed by the fourth diffusion rate limiting section 18 . The second internal space 17 is a space in which an auxiliary pump cell 50 capable of adjusting the oxygen partial pressure in the gas to be measured is arranged, that is, a space where oxygen is pumped into or pumped into the external space. It is. In other words, the measurement electrode 44 is connected to the third interior space into which the gas to be measured is introduced from the second interior space 17 where the auxiliary pump cell 50 is arranged through the flow path (slit) formed by the fourth diffusion control section 18. It is placed in the empty space 19. By having such a configuration, the gas sensor element 101 can achieve the following effects. In other words, the gas sensor element 101 changes the diffusion mode of the _NO It is possible to get closer to a form of diffusion through repeated collisions.

(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, and may include, for example, a plurality of layers having different porosity. 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. 8.

FIG. 8 is a schematic cross-sectional view schematically showing an example of the configuration of a gas sensor element 101 including a tip protection layer 200D according to a modification. Specifically, FIG. 8 is a diagram showing an example of a tip protection layer 200D 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 200D illustrated in FIG. 8, 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 200D 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. 8, the tip protection layer 200D 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 200D 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 200D. 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 200D. That is, the tip protection layer 200D 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 porosity of the inner tip protection layer 201 is The thickness is 30% to 90% of the thickness of the tip protection layer 200D.

In this configuration, the tip protection layer 200D 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 200, 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 200 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 200D 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 200D to the gas inlet 10 is 0.2 mm or more. It is. That is, in the example shown in FIG. 8, 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 200D to the gas inlet 10 sufficiently long (specifically, 0.2 mm or more), that is, the thickness of the tip protection layer 200D 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 are reliably trapped (captured) in the tip protective layer 200D, and the vicinity of the gas inlet 10 is prevented from being clogged by poisonous substances. can be suppressed to avoid a decrease in NO _x sensitivity.

The tip protection layer 200D illustrated in FIG. 8 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 200D has a two-layer structure, and the tip protection layer 200D may include three or more layers. That is, the tip protection layer 200D 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 200D may have a three-layer structure, or may have a four or more layer structure. It may have a multilayer structure. The tip protection layer 200D 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 200D. Further, another layer may be included between the two. In the tip protection layer 200D, 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 90% or more of the thickness of the tip protection layer 200D. % or less is sufficient.

[Features]
As described above, the gas sensor element 101 according to the present embodiment includes the element base 100 (or 100C), the tip protection layer 200 (or 200D), the measurement electrode 44, and the porous diffusion layer 91 (or 91A and 91B). (any). 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 and contains at least one of silica and alumina. For example, the porous diffusion layer 91 covers the measurement electrode 44 and has a porosity of 5% or more and 25% or less, which is lower than that of the tip protection layer 200. In the porous diffusion layer 91A including a plurality of surfaces (layers) with different porosity, the average porosity of the porous diffusion layer 91A is 5% or more and 25% or less, and the average porosity of the porous diffusion layer 91A is 5% or more and 25% or less. The porosity is lower than the porosity of the tip protection layer 200.

In this configuration, a porous diffusion layer 91 is provided around the measurement electrode 44. Specifically, the measurement electrode 44 is covered with a porous diffusion layer 91 having a porosity of 5% or more and 25% or less and lower than that of the tip protection layer 200. The porous diffusion layer 91 covering the measurement electrode 44 can change the diffusion mode around the measurement electrode 44 to a mode such as Knudsen diffusion, in which the diffusion occurs while repeatedly colliding with the wall of a sufficiently narrow channel. can. 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 covering the measurement electrode 44 separates the H ₂ O gas, NO _x gas (and O ₂ gas). ) can be reduced. Specifically, the gas sensor element 101 uses the porous diffusion layer 91 that covers the measurement electrode 44 to suppress fluctuations in NO _x output that are considered to be caused by molecular diffusion of NO _x under high H ₂ O concentration and Deterioration can be suppressed.

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 (the tip surface). 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 higher (larger) than the porosity of the porous diffusion layer 91 (91A, 91B) that covers the measurement electrode 44. By making the porosity of the tip protection layer 200 higher than that 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".

Furthermore, in the gas sensor element 101, the measurement electrode 44 contains 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 gas sensor element 101 can achieve the following effects because the measurement electrode 44 contains at least one of silica and alumina. That is, the gas sensor element 101 can suppress expansion and contraction at the measurement electrode 44. Therefore, the gas sensor element 101 can prevent cracks, cracks, etc. from entering the porous diffusion layer 91 covering the measurement electrode 44, and can also prevent the measurement electrode 44 from peeling off from the element base 100.

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, in this gas sensor, the porous diffusion layer 91 (91A, 91B) covering the measurement electrode 44 prevents fluctuations in NO _x output, which are considered to be caused by molecular diffusion of NO _x under high H ₂ O concentration, and deterioration 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 shown in which the measurement electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4, but in the gas sensor element 101, it is essential that the measurement electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4. isn't it. The measurement electrode 44 may be arranged on the lower surface of the second solid electrolyte layer 6, for example.

(Modification 2)
Until now, the measurement electrode 44 has been configured so that an internal cavity (for example, the second internal cavity 17 or the third internal cavity) is provided with a diffusion-limiting section (for example, the third diffusion-limiting section 16 or the fourth diffusion-limiting section 18) on the upstream side. We have shown an example in which it is placed in location 19). However, it is not essential that the measuring electrode 44 be arranged in an internal cavity with a diffusion-limiting section on the upstream side. Furthermore, it is not essential that the gas sensor element 101 has a plurality of internal cavities (for example, two or three chambers). 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 (either porous diffusion layer 91, 91A, or 91B) that covers the measurement electrode 44, and a tip protection layer that covers at least the surface of the element base 100 where the gas inlet 10 is open. (either the tip protection layer 200 or 200D). Where to arrange the measurement electrode 44 in the gas sensor element 101 can be selected as appropriate depending on the usage situation.

[Example]
As described above, the gas sensor element 101 includes a porous diffusion layer (any one of 91, 91A, or 91B) that covers the measurement electrode 44 and a tip protection layer (that covers at least the surface of the element base 100 where the gas inlet 10 is open). 200 or 200D), 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. Furthermore, 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 5 for gas sensors equipped with gas sensor elements according to Examples 1 to 13 and Comparative Examples 1 to 3. In the following description, a gas sensor including a gas sensor element according to each of Examples 1 to 13 and Comparative Examples 1 to 3 will be referred to as a gas sensor (NO _x sensor) according to each of Examples 1 to 13 and Comparative Examples 1 to 3. ) is sometimes abbreviated as

(Details of each of Examples 1 to 13 and Comparative Examples 1 to 3)
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 91B illustrated in FIG. 6. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91B, and the distance d2 between the two is 0.1 mm, which is 0.15 mm or less. In the gas sensor according to the first embodiment, the porous diffusion layer 91B is a porous layer with a constant porosity of 5% or more and 25% or less over the whole. The porosity is 12%. In addition, the porosity of the surface of the porous diffusion layer 91B (the outer surface facing the measured gas flow section 7) and the electrode-side surface (the inner surface facing the measurement electrode 44, the inner surface facing the measurement electrode 44) The porosity of both surfaces is 12%, and the difference in porosity between them is 0%. That is, both the outer surface and the inner surface in the thickness direction of the porous diffusion layer 91B have a porosity of 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 tip protection layer 200 is 20%, which is higher than the porosity of the porous diffusion layer 91B. That is, in the gas sensor according to Example 1, the porosity of the porous diffusion layer 91B (12%) is lower than the porosity of the tip protection layer 200 (20%). 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 200 illustrated in FIG. 1 and a porous diffusion layer 91A illustrated in FIG. 5. Specifically, in Example 2, unlike Example 1, there is no space (gap) between the measurement electrode 44 and the porous diffusion layer 91A, and the distance d2 between them is 0 mm. However, similarly to Example 1, in Example 2, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91A is 0.15 mm or less. In the gas sensor according to the second embodiment, the porous diffusion layer 91A has a surface (outer surface facing the gas flow section 7 to be measured) and an electrode side surface (inner surface facing the measurement electrode 44, The porosity differs depending on the facing (contacting) surface. Specifically, in the gas sensor according to Example 2, the porosity of the surface of the porous diffusion layer 91A is 6%, and the porosity of the surface on the electrode side is 10%. That is, in the gas sensor of Example 2, the porosity (10%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is equal to the porosity (6%) of the outer surface (surface). %). However, the difference between the two is less than 10% (specifically, 4%), that is, the porosity (10%) of the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A is lower than that of the outer surface. 4% higher than the porosity (6%) of In Example 2, of the two surfaces in the thickness direction of the porous diffusion layer 91A, the porosity of the inner surface facing the measurement electrode 44 (10%) is higher than the porosity of the outer surface (6%). High, specifically 4% higher. Further, the average porosity of the porous diffusion layer 91A is 8%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 2, 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 tip protection layer 200 is 25%, which is higher than the porosity (average porosity) of the porous diffusion layer 91A. That is, in the gas sensor according to Example 2, the porosity (average porosity) (8%) of the porous diffusion layer 91A is lower than the porosity (25%) of the 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 280 μm, which is 200 μm (0.2 mm). ) That's it. As mentioned above, in the gas sensor according to the second 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 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 "-".

Example 3 is a gas sensor including a gas sensor element 101 including a tip protection layer 200D illustrated in FIG. 8 and a porous diffusion layer 91A illustrated in FIG. 5. Specifically, there is no space (gap) between the measurement electrode 44 and the porous diffusion layer 91A, and the distance d2 between them is 0 mm. However, similarly to Examples 1 and 2, in Example 3, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91A is 0.15 mm or less. In the gas sensor according to the third embodiment, the porous diffusion layer 91A has a surface (outer surface facing the gas flow section 7 to be measured) and an electrode side surface (inner surface facing the measurement electrode 44, The porosity differs depending on the facing (contacting) surface. Specifically, in the gas sensor according to Example 3, the porosity of the surface of the porous diffusion layer 91A is 8%, and the porosity of the surface on the electrode side is 18%. That is, in the gas sensor of Example 3, the porosity (18%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is equal to the porosity (8%) of the outer surface (surface). %). Unlike Example 2, the difference between the porosity (18%) of the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A and the porosity (8%) of the outer surface is 10% or more ( Specifically, it is 10%). Further, the average porosity of the porous diffusion layer 91A is 12%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 3, the presence or absence of the tip protection layer 200D is "yes", and unlike Examples 1 and 2, the tip protection layer 200D includes an inner tip protection layer 201. Specifically, the porosity (average porosity) of the tip protection layer 200D is 20%, which is higher than the porosity (average porosity) of the porous diffusion layer 91A. That is, in the gas sensor according to Example 3, the porosity (average porosity) (12%) of the porous diffusion layer 91A is lower than the porosity (average porosity) (20%) of the tip protection layer 200D. Further, the porosity of the inner tip protection layer 201 included in the tip protection layer 200D is 60%. The shortest distance (d1) between the tip protective layer 200D and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200D 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 200D is 280 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200D 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 200D is 73%, which is 30% or more and 90% or less.

Example 4 is a gas sensor including a gas sensor element 101 including a tip protection layer 200D illustrated in FIG. 8 and a porous diffusion layer 91A illustrated in FIG. 5. Specifically, there is no space (gap) between the measurement electrode 44 and the porous diffusion layer 91A, and the distance d2 between them is 0 mm. However, similarly to Example 1-3, in Example 4, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91A is 0.15 mm or less. In the gas sensor according to the fourth embodiment, the porous diffusion layer 91A has a surface (outer surface facing the gas flow section 7 to be measured) and an electrode side surface (inner surface facing the measurement electrode 44, The porosity differs depending on the facing (contacting) surface. Specifically, in the gas sensor according to Example 4, the porosity of the surface of the porous diffusion layer 91A is 20%, and the porosity of the surface on the electrode side is 30%. That is, in the gas sensor of Example 4, the porosity (30%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is the same as the porosity (20%) of the outer surface (surface). %). Unlike Example 2, the difference between the porosity (30%) of the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A and the porosity (20%) of the outer surface is 10% or more ( Specifically, it is 10%). Further, the average porosity of the porous diffusion layer 91A is 25%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 4, the presence or absence of the tip protection layer 200D is "yes", and unlike Examples 1 and 2, the tip protection layer 200D includes an inner tip protection layer 201. Specifically, the porosity (average porosity) of the tip protection layer 200D is 20%, which, unlike Example 3, is lower than the porosity (average porosity) of the porous diffusion layer 91A. That is, unlike Example 3, in the gas sensor according to Example 4, the porosity (average porosity) (25%) of the porous diffusion layer 91A is different from the porosity (average porosity) (20%) of the tip protection layer 200D. ) higher than However, the porosity of the inner tip protection layer 201 included in the tip protection layer 200D is 60%, which is higher than the porosity (average porosity) (25%) of the porous diffusion layer 91A. The shortest distance (d1) between the tip protection layer 200D and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protection layer 200D to the gas inlet 10 is 1000 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 200D is 300 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200D is 700 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 200D is 70%, 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 200D illustrated in FIG. 8 and a porous diffusion layer 91A illustrated in FIG. 5. Specifically, there is no space (gap) between the measurement electrode 44 and the porous diffusion layer 91A, and the distance d2 between them is 0 mm. However, similarly to Example 1-4, in Example 5, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91A is 0.15 mm or less. In the gas sensor according to Example 5, the porous diffusion layer 91A has a surface (outer surface facing the gas flow section 7 to be measured) and an electrode side surface (inner surface facing the measurement electrode 44, The porosity differs depending on the facing (contacting) surface. Specifically, in the gas sensor according to Example 5, the porosity of the surface of the porous diffusion layer 91A is 10%, and the porosity of the surface on the electrode side is 20%. That is, in the gas sensor of Example 5, the porosity (20%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is equal to the porosity (10%) of the outer surface (surface). %). Unlike Example 2, the difference between the porosity (20%) of the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A and the porosity (10%) of the outer surface is 10% or more ( Specifically, it is 10%). Further, the average porosity of the porous diffusion layer 91A is 15%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 5, the presence or absence of the tip protection layer 200D is "yes", and unlike Examples 1 and 2, the tip protection layer 200D includes an inner tip protection layer 201. Specifically, the porosity (average porosity) of the tip protection layer 200D is 25%, which is higher than the porosity (average porosity) of the porous diffusion layer 91A. That is, in the gas sensor according to Example 5, the porosity (average porosity) (15%) of the porous diffusion layer 91A is lower than the porosity (average porosity) (25%) of the tip protection layer 200D. Further, the porosity of the inner tip protection layer 201 included in the tip protection layer 200D is 65%. The shortest distance (d1) between the tip protective layer 200D and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200D 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 200D is 360 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200D 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 200D is 64%, 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 200D illustrated in FIG. 8 and a porous diffusion layer 91A illustrated in FIG. 5. Specifically, there is no space (gap) between the measurement electrode 44 and the porous diffusion layer 91A, and the distance d2 between them is 0 mm. However, similarly to Examples 1-5, in Example 6, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91A is 0.15 mm or less. In the gas sensor according to the sixth embodiment, the porous diffusion layer 91A has a surface (outer surface facing the gas flow section 7 to be measured) and an electrode side surface (inner surface facing the measurement electrode 44, The porosity differs depending on the facing (contacting) surface. Specifically, in the gas sensor according to Example 6, the porosity of the surface of the porous diffusion layer 91A is 7%, and the porosity of the surface on the electrode side is 15%. That is, in the gas sensor of Example 6, the porosity (15%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is equal to the porosity (7%) of the outer surface (surface). %). However, unlike Examples 3, 4, and 5, the difference between the two is less than 10% (specifically, 8%), that is, the pores on the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A The porosity (15%) is 8% higher than the porosity of the outer surface (7%). Further, the average porosity of the porous diffusion layer 91A is 10%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 6, the presence or absence of the tip protection layer 200D is "yes", and unlike Examples 1 and 2, the tip protection layer 200D includes an inner tip protection layer 201. Specifically, the porosity (average porosity) of the tip protection layer 200D is 15%, which is higher than the porosity (average porosity) of the porous diffusion layer 91A. That is, in the gas sensor according to Example 6, the porosity (average porosity) (10%) of the porous diffusion layer 91A is lower than the porosity (average porosity) (15%) of the tip protection layer 200D. Further, the porosity of the inner tip protection layer 201 included in the tip protection layer 200D is 50%. The shortest distance (d1) between the tip protection layer 200D and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protection layer 200D 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 200D is 200 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200D 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 200D is 81%, which is 30% or more and 90% or less.

Example 7 includes a gas sensor element 101 including a tip protection layer 200D illustrated in FIG. 8 and a porous diffusion layer 91B illustrated in FIG. 6 (however, the porosity is different between the surface and the electrode side surface). It is a gas sensor. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91B, and the distance d2 between the two is 0.1 mm. However, similarly to Examples 1-6, in Example 7, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91B is 0.15 mm or less. In addition, in the gas sensor according to the seventh embodiment, the porous diffusion layer 91B differs from the first embodiment in that the surface (the outer surface facing the gas distribution section 7 to be measured) and the electrode side surface (the outer surface facing the measurement electrode 44) are different from those in the first embodiment. The porosity differs between the inner surface and the surface facing the measurement electrode 44. Specifically, in the gas sensor according to Example 7, the porosity of the surface of the porous diffusion layer 91B is 8%, and the porosity of the surface on the electrode side is 18%. That is, in the gas sensor of Example 7, the porosity (18%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91B is equal to the porosity (8%) of the outer surface (surface). %), and the difference between the two is 10% or more (specifically, 10%). Further, the average porosity of the porous diffusion layer 91B is 15%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 7, the presence or absence of the tip protection layer 200D is "yes", and unlike Examples 1 and 2, the tip protection layer 200D includes an inner tip protection layer 201. Specifically, the porosity (average porosity) of the tip protection layer 200D is 23%, which is higher than the porosity (average porosity) of the porous diffusion layer 91B. That is, in the gas sensor according to Example 7, the porosity (average porosity) (15%) of the porous diffusion layer 91B is lower than the porosity (average porosity) (23%) of the tip protection layer 200D. Further, the porosity of the inner tip protection layer 201 included in the tip protection layer 200D is 55%. The shortest distance (d1) between the tip protective layer 200D and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200D 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 200D is 350 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200D is 150 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 200D is 30%, which is 30% or more and 90% or less.

Example 8 includes a gas sensor element 101 including a tip protection layer 200 illustrated in FIG. 1 and a porous diffusion layer 91B illustrated in FIG. 6 (the porosity is different between the surface and the electrode side surface). It is a gas sensor. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91B, and the distance d2 between the two is 0.13 mm. However, similarly to Examples 1-7, in Example 8, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91B is 0.15 mm or less. In addition, in the gas sensor according to the eighth embodiment, the porous diffusion layer 91B differs from the first embodiment in that the surface (the outer surface facing the gas distribution section 7 to be measured) and the electrode side surface (the outer surface facing the measurement electrode 44) are different from those in the first embodiment. The porosity differs between the inner surface and the surface facing the measurement electrode 44. Specifically, in the gas sensor according to Example 8, the porosity of the surface of the porous diffusion layer 91B is 17%, and the porosity of the surface on the electrode side is 22%. That is, in the gas sensor of Example 8, the porosity (22%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91B is different from the porosity (17%) of the outer surface (surface). %). However, unlike Examples 3, 4, 5, and 7, the difference between the two is less than 10% (specifically, 5%). The porosity of the face (22%) is 5% higher than the porosity of the outer face (17%). Further, the average porosity of the porous diffusion layer 91B is 20%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 8, the presence or absence of the tip protection layer 200 is "yes", and unlike Example 3-7, the tip protection layer 200 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 tip protection layer 200 is 30%, which is higher than the porosity (average porosity) of the porous diffusion layer 91B. That is, in the gas sensor according to Example 8, the porosity (average porosity) (20%) of the porous diffusion layer 91B is lower than the porosity (30%) of the 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 8, 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 9 is a gas sensor including a gas sensor element 101 including a tip protection layer 200D illustrated in FIG. 8 and a porous diffusion layer 91B illustrated in FIG. 6. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91B, and the distance d2 between the two is 0.1 mm. However, similarly to Examples 1-8, in Example 9, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91B is 0.15 mm or less. Furthermore, in the gas sensor according to Example 9, the porous diffusion layer 91B is a porous layer with a constant porosity of 5% or more and 25% or less throughout, unlike in Example 2-8. Specifically, the porosity of the porous diffusion layer 91B is 25%. In addition, the porosity of the surface of the porous diffusion layer 91B (the outer surface facing the measured gas flow section 7) and the electrode-side surface (the inner surface facing the measurement electrode 44, the inner surface facing the measurement electrode 44) The porosity of both surfaces is 25%, and the difference in porosity between them is 0%. Regarding the gas sensor according to Example 9, the presence or absence of the tip protection layer 200D is "yes", and unlike Examples 1 and 2, the tip protection layer 200D includes an inner tip protection layer 201. Specifically, the porosity (average porosity) of the tip protection layer 200D is 15%, which is lower than the porosity (average porosity) of the porous diffusion layer 91B. That is, unlike Examples 1-3 and 5-8, in the gas sensor according to Example 9, the porosity (25%) of the porous diffusion layer 91B is different from the porosity (average porosity) (15%) of the tip protection layer 200D. %). However, the porosity of the inner tip protection layer 201 included in the tip protection layer 200D is 45%, which is higher than the porosity (25%) of the porous diffusion layer 91B. The shortest distance (d1) between the tip protective layer 200D and the gas inlet 10, in other words, the distance d1 from the outermost surface of the tip protective layer 200D 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 200D is 300 μm, and the thickness of the inner tip protection layer 201 included in the tip protection layer 200D 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 200D is 67%, which is 30% or more and 90% or less.

Example 10 includes a gas sensor element 101 including a tip protection layer 200 illustrated in FIG. 1 and a porous diffusion layer 91B illustrated in FIG. 6 (however, the porosity is different between the surface and the electrode side surface). It is a gas sensor. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91B, and the distance d2 between the two is 0.15 mm. However, similarly to Example 1-9, in Example 10, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91B is 0.15 mm or less. In the gas sensor according to the tenth embodiment, the porous diffusion layer 91B differs from the first embodiment in that the porous diffusion layer 91B has a surface (the outer surface facing the gas flow section 7 to be measured) and an electrode side surface (the inner surface facing the measurement electrode 44). The porosity differs depending on the surface (the surface facing the measurement electrode 44). Specifically, in the gas sensor according to Example 10, the porosity of the surface of the porous diffusion layer 91B is 12%, and the porosity of the surface on the electrode side is 10%. That is, unlike Examples 3, 4, 5, and 7, in the gas sensor of Example 10, the porosity (10%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91B is lower than the porosity of the outer surface (12%), specifically 2% lower. Further, the average porosity of the porous diffusion layer 91B is 10%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 10, the presence or absence of the tip protection layer 200 is "yes", and unlike Example 3-7, the tip protection layer 200 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 tip protection layer 200 is 20%, which is higher than the porosity (average porosity) of the porous diffusion layer 91B. That is, in the gas sensor according to Example 10, the porosity (average porosity) (10%) of the porous diffusion layer 91B is lower than the porosity (20%) of the 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 10, 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 "-".

Example 11 includes a gas sensor element 101 including a tip protection layer 200 illustrated in FIG. 1 and a porous diffusion layer 91B illustrated in FIG. 6 (however, the porosity is different between the surface and the electrode side surface). It is a gas sensor. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91B, and the distance d2 between the two is 0.1 mm. However, similarly to Example 1-10, in Example 11, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91B is 0.15 mm or less. In the gas sensor according to Example 11, the porous diffusion layer 91B differs from Example 1 in that the porous diffusion layer 91B has a surface (the outer surface facing the gas flow section 7 to be measured) and an electrode side surface (the inner surface facing the measurement electrode 44). The porosity differs depending on the surface (the surface facing the measurement electrode 44). Specifically, in the gas sensor according to Example 11, the porosity of the surface of the porous diffusion layer 91B is 7%, and the porosity of the surface on the electrode side is 12%. That is, in the gas sensor of Example 11, the porosity (12%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91B is equal to the porosity (7%) of the outer surface (surface). %). However, unlike Examples 3, 4, 5, and 7, the difference between them is less than 10% (specifically, 5%). Further, the average porosity of the porous diffusion layer 91B is 10%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 11, the presence or absence of the tip protection layer 200 is "yes", and unlike Example 3-7, the tip protection layer 200 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 tip protection layer 200 is 20%, which is higher than the porosity (average porosity) of the porous diffusion layer 91B. That is, in the gas sensor according to Example 11, the porosity (average porosity) (10%) of the porous diffusion layer 91B is lower than the porosity (20%) of the tip protection layer 200. Further, in the gas sensor according to Example 11, 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 100 μm. It is. That is, unlike Examples 1-10, in Example 11, the distance d1 from the outermost surface of the tip protection layer 200 to the gas inlet 10 is less than 200 μm (0.2 mm). As mentioned above, in the gas sensor according to Example 11, 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. 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 "-".

Example 12 includes a gas sensor element 101 including a tip protection layer 200 illustrated in FIG. 1 and a porous diffusion layer 91B illustrated in FIG. 6 (however, the porosity is different between the surface and the electrode side surface). It is a gas sensor. Specifically, there is a space (gap) between the measurement electrode 44 and the porous diffusion layer 91B, and the distance d2 between the two is 0.2 mm. That is, unlike Example 1-11, in Example 12, the distance d2 from the porous diffusion layer 91B to the measurement electrode 44 is 0.2 mm, which is larger than 0.15 mm. In the gas sensor according to the twelfth embodiment, the porous diffusion layer 91B differs from the first embodiment in that the porous diffusion layer 91B has a surface (the outer surface facing the gas distribution section 7 to be measured) and an electrode side surface (the inner surface facing the measurement electrode 44). The porosity differs depending on the surface (the surface facing the measurement electrode 44). Specifically, in the gas sensor according to Example 12, the porosity of the surface of the porous diffusion layer 91B is 12%, and the porosity of the surface on the electrode side is 14%. That is, in the gas sensor of Example 12, the porosity (14%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91B is equal to the porosity (12%) of the outer surface (surface). %). However, unlike Examples 3, 4, 5, and 7, the difference between them is less than 10% (specifically, 2%). Further, the average porosity of the porous diffusion layer 91B is 15%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 12, the presence or absence of the tip protection layer 200 is "yes", and unlike Example 3-7, the tip protection layer 200 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 tip protection layer 200 is 25%, which is higher than the porosity (average porosity) of the porous diffusion layer 91B. That is, in the gas sensor according to Example 12, the porosity (average porosity) (15%) of the porous diffusion layer 91B is lower than the porosity (25%) of the 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 12, 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 "-".

Example 13 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 91A illustrated in FIG. 5. Specifically, in Example 13, unlike Example 1, there is no space (gap) between the measurement electrode 44 and the porous diffusion layer 91A, and the distance d2 between them is 0 mm. However, similarly to Example 1-11, in Example 13, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91A is 0.15 mm or less. In the gas sensor according to Example 13, the porous diffusion layer 91A has a surface (outer surface facing the gas flow section 7 to be measured) and an electrode side surface (inner surface facing the measurement electrode 44, The porosity differs depending on the facing (contacting) surface. Specifically, in the gas sensor according to Example 13, the porosity of the surface of the porous diffusion layer 91A is 15%, and the porosity of the surface on the electrode side is 6%. That is, unlike Examples 1-9, 11, and 12, in the gas sensor of Example 13, the porosity (6%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is lower than the porosity of the outer surface (15%). Specifically, the porosity (6%) of the inner surface of the porous diffusion layer 91A facing the measurement electrode 44 is 9% lower than the porosity (15%) of the outer surface. Further, the average porosity of the porous diffusion layer 91A is 11%, and the average porosity is 5% or more and 25% or less. Regarding the gas sensor according to Example 13, the presence or absence of the tip protection layer 200 is "yes", and unlike Example 3-7, the tip protection layer 200 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 tip protection layer 200 is 25%, which is higher than the porosity (average porosity) of the porous diffusion layer 91A. That is, in the gas sensor according to Example 13, the porosity (average porosity) (11%) of the porous diffusion layer 91A is lower than the porosity (25%) of the 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 280 μm, which is 200 μm (0.2 mm). ) That's it. As mentioned above, in the gas sensor according to Example 13, 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 1 is a gas sensor including a sensor element having the same structure as Example 2, except that it does not include the tip protection layer 200. Specifically, in Comparative Example 1, unlike Example 1, there is no space (gap) between the measurement electrode 44 and the porous diffusion layer 91A, and the distance d2 between them is 0 mm. However, similarly to Examples 1-11 and 1-13, in Comparative Example 1, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91A is 0.15 mm or less. In the gas sensor according to Comparative Example 1, the porous diffusion layer 91A has a surface (outer surface facing the gas flow section 7 to be measured) and an electrode side surface (inner surface facing the measurement electrode 44, The porosity differs depending on the facing (contacting) surface. Specifically, in the gas sensor according to Comparative Example 1, the porosity of the surface of the porous diffusion layer 91A is 6%, and the porosity of the surface on the electrode side is 12%. That is, in the gas sensor of Comparative Example 1, the porosity (12%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is the same as the porosity (6%) of the outer surface (surface). %). However, unlike Example 3-7, the difference between the two is less than 10% (specifically, 6%), that is, the porosity of the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A ( 12%) is 6% higher than the porosity of the outer surface (6%). Further, the average porosity of the porous diffusion layer 91A is 10%, and the average porosity is 5% or more and 25% or less. 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 tip protection layer 200 and the inner tip protection layer 201 are The porosity is "-" in all cases. 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 is a gas sensor that does not include a porous diffusion layer (porous diffusion layers 91, 91A, 91B) and includes a sensor element that includes only the tip protection layer 200 illustrated in FIG. 1. Comparative Example 2 does not have a porous diffusion layer, so the distance between the measurement electrode 44 and the porous diffusion layer, the average porosity of the porous diffusion layer, the surface of the porous diffusion layer (the gas distribution part 7 to be measured) The porosity of the facing outer surface is "-" in all cases. In addition, the porosity of the surface of the porous diffusion layer on the electrode side (the inner surface facing the measurement electrode 44, the surface facing the measurement electrode 44), and the porosity of the surface of the porous diffusion layer (the surface and the surface of the measurement electrode 44 side) The difference in porosity (between the surfaces) is also "-" in both cases. Regarding the gas sensor according to Comparative Example 2, the presence or absence of the tip protection layer 200 is "yes", and unlike Example 3-7, the tip protection layer 200 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 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 has the same structure as Example 2, but includes a gas sensor element 101 in which the average porosity of the porous diffusion layer 91A is larger (higher) than 25% and higher than the porosity of the tip protection layer 200. It is a gas sensor that includes. Specifically, in Comparative Example 3, unlike Example 1, there is no space (gap) between the measurement electrode 44 and the porous diffusion layer 91A, and the distance d2 between them is 0 mm. However, similarly to Examples 1-11 and 1-13, in Comparative Example 3, the distance d2 between the measurement electrode 44 and the porous diffusion layer 91A is 0.15 mm or less. In the gas sensor according to Comparative Example 3, the porous diffusion layer 91A has a surface (outer surface facing the gas flow section 7 to be measured) and an electrode side surface (inner surface facing the measurement electrode 44, The porosity differs depending on the facing (contacting) surface. Specifically, in the gas sensor according to Comparative Example 3, the porosity of the surface of the porous diffusion layer 91A is 30%, and the porosity of the surface on the electrode side is 40%. That is, in the gas sensor of Comparative Example 3, the porosity (40%) of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is the same as the porosity (30%) of the outer surface (surface). %), and the difference between the two is 10% or more (specifically, 10%). Further, unlike Examples 1-3, 5-8, and 10-13, the average porosity of the porous diffusion layer 91A is 35%, which is larger (higher) than 25%. Regarding the gas sensor according to Comparative Example 3, the presence or absence of the tip protection layer 200 is "yes", and unlike Example 3-7, the tip protection layer 200 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 tip protection layer 200 is 30%, which is lower than the porosity (average porosity) of the porous diffusion layer 91A. That is, unlike Examples 1-3, 5-8, and 10-13, in the gas sensor according to Comparative Example 3, the porosity (average porosity) (35%) of the porous diffusion layer 91A is the same as that of the tip protection layer 200. Higher than porosity (30%). 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 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 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 "-".

(Details of each judgment 1 to 5)
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 13 and Comparative Examples 1 to 3. The inventors of the present invention have discovered that when the NO _x sensors according to Examples 1 to 13 and Comparative Examples 1 to 3 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 13 and Comparative Examples 1 to 3, NO _x concentration = 500 ppm and H _{2 O} concentration = 3% as bases. The H ₂ O concentration was changed to 500 ppm and 15%. Then, the degree of change in NO _x output during such changes was investigated. "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 13. The following Mg poisoning test was conducted on the NO _x sensors according to each of Comparative Examples 1 to 3. 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 13 and Comparative Examples 1 to 3, 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%."

Judgment 5 is to verify (measure) the light-off time (the time required from when the NO _x sensor is activated until it enters a steady operating state). Specifically, a mixed gas environment was created in which the NO _x concentration = 100 ppm, the H ₂ O concentration = 9%, and the remainder was N ₂ , and the NO _x sensors according to each of Examples 1 to 13 and Comparative Examples 1 to 3 were was attached to the chamber, the above-mentioned mixed gas was flowed, and the light-off time was measured. The light-off time was determined as the time during which the NO _x concentration value fell within the range of 90 ppm to 110 ppm after the start of energization to the gas sensor element. "A" indicates that "the light-off time was within 100 seconds.""B" indicates that "the light-off time was greater than 100 seconds and within 130 seconds.""C" indicates that "the light-off time was greater than 130 seconds and within 200 seconds."

(Summary of items that can be confirmed from Table 1)
Below, we summarize the matters that can be confirmed from Table 1 showing the test results for Judgments 1 to 5 for gas sensors equipped with sensor elements according to Examples 1 to 13 and Comparative Examples 1 to 3. do.

As shown in the comparison results between Example 1-13 and Comparative Example 1 in Judgment 3, by providing the tip protection layer 200 (or tip protection layer 200D), the gas sensor can achieve the following effects. confirmed. That is, the results of determination 3 (“A” or “B”) of Examples 1-13, which include the tip protection layer 200 or the tip protection layer 200D, are the same as those of the comparison which does not include the tip protection layer 200 or the tip protection layer 200D. This is better than the result of Judgment 3 in Example 1 ("F"). Therefore, by including the tip protection layer 200 (or tip protection layer 200D), 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 1-13 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 1-13, which includes the porous diffusion layer 91 etc., is 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 result of Judgment 2 (“A” or “B”) of Example 1-13, which includes the porous diffusion layer 91, etc., is the same as that 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).

Regarding the porous diffusion layer 91A that is in contact with the measurement electrode 44 and covers the measurement electrode 44, the porosity is set to be 5% or more and 25% or less, and further lower than the porosity of the tip protection layer. The results of determinations 1 and 2 are significantly different between Example 2 and Comparative Example 3. Specifically, the results of determinations 1 and 2 of Example 2, which includes the porous diffusion layer 91A with "a porosity of 5% or more and 25% or less and lower than the porosity of the tip protective layer", are as follows: All are "A". On the other hand, the results of Judgments 1 and 2 of Comparative Example 3, which includes the porous diffusion layer 91A whose porosity is greater than 25% and higher than the porosity of the tip protective layer, are both ``F ”. Therefore, by making the porosity of the porous diffusion layer 91A "5% or more and 25% or less, and further lower than the porosity of the tip protection layer," the gas sensor can achieve the following effects. was confirmed. In other words, the gas sensor prevents deterioration of the measurement electrode due to high H <sub>2</sub> O concentration by setting the porosity of the porous diffusion layer 91A to ``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 addition, in Examples 4 and 9, the porosity (average porosity) of the porous diffusion layers 91A and 91B is 5% or more and 25% or less, but in Examples 1-3, 5-8, and 10-13. Differently, the porosity (average porosity) is higher than that of the tip protection layer 200D. However, in Examples 4 and 9, the porosity (average porosity) of the porous diffusion layers 91A and 91B is lower than the porosity of the inner tip protection layer 201, that is, the gas inlet 10 of the element substrate 100 is open. The porosity is lower than the porosity of the inner tip protection layer 201 that is in contact with the surface. The result of determination 2 in Example 4 is "A", and the result of determination 2 in Example 9 is "B". On the other hand, in Comparative Example 3, 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 3 is "F". Therefore, the porosity of the porous diffusion layers 91A and 91B is set to be 5% or more and 25% or less, and further, at least "pores of the inner tip protection layer 201 in contact with the surface where the gas inlet 10 of the element substrate 100 is opened" 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."

In Examples 2 and 13, of the two surfaces in the thickness direction of the porous diffusion layer 91A, the porosity of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is , have a similar configuration except that the porosity is higher than that of the outer surface. The result of determination 5 in Example 2 is "B", whereas the result of determination 5 in Example 13 is "C". Therefore, it has been confirmed that by making the porosity of the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A higher than the porosity of the outer surface, the gas sensor can achieve the following effects. Ta. In other words, it was confirmed that the light-off time of the gas sensor can be shortened by making the porosity of the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A higher than the porosity of the outer surface (determination). 5).

Example 8 and Example 12 have the same configuration except that the distance d2 from the porous diffusion layer 91B to the measurement electrode 44 is 0.15 mm or less. The results of determination 1 and determination 2 in Example 8 are both "A", whereas the results of determination 1 and determination 2 in Example 12 are both "B". Therefore, it has been confirmed that by setting the distance d2 from the porous diffusion layer 91B to the measurement electrode 44 to 0.15 mm or less, the gas sensor can achieve the following effects. In other words, it was confirmed that the gas sensor can "suppress deterioration of the measurement electrode due to high H ₂ O concentration" by setting the distance d2 from the porous diffusion layer 91B to the measurement electrode 44 to 0.15 mm or less. (Judgment 1). Furthermore, by setting the distance d2 from the porous diffusion layer 91B to the measurement electrode 44 to 0.15 mm or less, the gas sensor suppresses the H ₂ O dependence of NO _x sensitivity (NO _x output) and improves measurement accuracy. It was confirmed that it was possible to "improve" (determination 2).

In Examples 5 and 6, of the two surfaces in the thickness direction of the porous diffusion layer 91A, the porosity of the inner surface (electrode side surface) facing the measurement electrode 44 of the porous diffusion layer 91A is , have the same configuration except that the porosity is 10% or more higher than the porosity of the outer surface. The result of determination 5 in Example 5 is "A", whereas the result of determination 5 in Example 6 is "B". Therefore, by making the porosity of the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A 10% or more higher than the porosity of the outer surface, the gas sensor can achieve the following effects. was confirmed. That is, it has been confirmed that the light-off time of the gas sensor can be shortened by making the porosity of the inner surface facing the measurement electrode 44 of the porous diffusion layer 91A higher than the porosity of the outer surface by 10% or more. (Judgment 5).

Example 8 and Example 11 have the same configuration except that the distance d1 from the outermost surface of the tip protection layer 200 to the gas introduction port 10 is 0.2 mm (200 μm) or more. The result of determination 4 in Example 8 is "B", whereas the result of determination 4 in Example 11 is "F". Therefore, it has been confirmed that by setting the distance d1 from the outermost surface of the tip protection layer 200 to the gas introduction port 10 to 0.2 mm or more, the gas sensor can achieve the following effects. In other words, by setting the distance d1 to 0.2 mm or more, the gas sensor can trap poisonous substances and perform measurements even in harsh environments where there are many poisonous substances that can clog the tip protective layer itself. It was confirmed that clogging around the electrode could be suppressed (Judgment 4).

Example 2 and Example 6 have the same configuration except for whether the tip protection layer 200 or the tip protection layer 200D including the inner tip protection layer 201 and the outer tip protection layer 202 is provided. That is, Example 6 includes the tip protection layer 200D, 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 greater than that of the tip protection layer 200D. The thickness is 30% or more and 90% or less of the thickness. The result of determination 4 in Example 2 is "B", whereas the result of determination 4 in Example 6 is "A". Therefore, by providing such a tip protection layer 200D 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 200D, the gas sensor can at least have the following effects related to determination 4. It was confirmed that this can be achieved. In other words, by including the tip protection layer 200D with the above-described configuration, the gas sensor can trap poisonous substances even in harsh environments where there are many poisonous substances that can cause clogging of the tip protection layer itself, and the measurement electrode can be protected. It was confirmed that clogging in the surrounding area could be suppressed (Judgment 4).

(About the rate of change in NO _x sensitivity)
FIG. 9 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 covering the measurement electrode. Specifically, FIG. 9 shows a NO _x sensor having a similar structure except for the presence or absence of a porous diffusion layer (any one of 91, 91A, or 91B) covering the measurement electrode 44 under a high H ₂ O concentration. It shows the change over time in the NO _x output of each NO _x sensor at (for example, H ₂ O concentration = 25%). In the graph of FIG. 9, the horizontal axis is time (driving time), and the vertical axis is the rate of change in NO _x sensitivity. The solid black line indicates the change over time in the NO _x output of the “NO _x sensor equipped with a porous diffusion layer covering the measurement electrode 44”. In addition, the gray dotted line indicates the NO x output of a NO _x sensor that does not include a porous diffusion layer covering the measurement electrode 44 (specifically, a conventional NO _x sensor that only has a slit structure with a diffusion control part) _. It shows the change over time.

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. 9 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. 9, 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) has a high H ₂ O concentration. 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. 10 is a graph showing the difference in H ₂ O dependence of NO _x output depending on the presence or absence of a porous diffusion layer covering the measurement electrode. Specifically, FIG. 10 shows the H ₂ of NO _x output for a NO _x sensor having a similar structure except for the presence or absence of a porous diffusion layer (any one of 91, 91A, or 91B) covering the measurement electrode 44. This shows the difference in O dependence. In the graph of FIG. 10, 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 of the “NO _x sensor equipped with a porous diffusion layer covering the measurement electrode 44” over time. In addition, the gray dotted line indicates the NO x output of a NO _x sensor that does not include a porous diffusion layer covering the measurement electrode 44 (specifically, a conventional NO _x sensor that only has a slit structure with a diffusion control part) _. The graph shows the H ₂ O-dependent changes over time.

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. 10 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. 10, 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. 9 and 10 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, the measurement electrode 44 is covered with a porous diffusion layer (for example, the porous diffusion layer 91, etc.) having a porosity of 5% or more and 25% or less, and in particular, the distance d2 from the porous diffusion layer to the measurement electrode 44 is 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, 200D) 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.). It is possible to suppress clogging and suppress a decrease in NO _x sensitivity. 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, 100C...Element base, 101...Gas sensor element, 10...Gas inlet,
7, 7C... Gas distribution part to be measured (internal space), 44... Measurement electrode,
18... Fourth diffusion controlling part (diffusion controlling part), 91, 91A, 91B... Porous diffusion layer,
19... Third internal cavity (internal cavity), 911... First porous diffusion layer (outer surface),
912... second porous diffusion layer (inner surface facing the measurement electrode),
200, 200D...Tip protection layer, 201...Inner tip protection layer, 202...Outer tip protection layer

Claims (8)

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 and containing at least one of silica and alumina;
a porous diffusion layer that covers the measurement electrode and has a porosity of 5% or more and 25% or less and that is lower than the porosity of the tip protective layer;
A gas sensor element including: Of the two surfaces in the thickness direction of the porous diffusion layer, the porosity of the inner surface facing the measurement electrode is higher than the porosity of the outer surface.
The gas sensor element according to claim 1. further comprising a diffusion-limiting section that imparts a predetermined diffusion resistance to the gas to be measured in the internal space,
The measurement electrode is arranged in an internal space whose upstream side in the flow direction of the gas to be measured is defined by the diffusion-limiting section.
The gas sensor element according to claim 1 or 2. The measurement electrode and the porous diffusion layer are not in contact with each other,
The distance between the measurement electrode and the porous diffusion layer is 0.15 mm or less,
The gas sensor element according to claim 1 or 2. Of the two surfaces in the thickness direction of the porous diffusion layer, the porosity of the inner surface facing the measurement electrode is 10% or more higher than the porosity of the outer surface.
The gas sensor element according to claim 1 or 2. 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 claim 1 or 2. 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 claim 1 or 2. 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 claim 1 or 2.