JP2021124382A - Gas sensor - Google Patents

Abstract

To provide a gas sensor, with which the accuracy of measuring an NOx concentration hardly decreases even when used in a high temperature range for a long time under high oxygen concentrations.SOLUTION: The gas sensor is constituted so as to measure the concentration of a prescribed gas component in a measurement object gas. The gas sensor includes a sensor element. The main component of the sensor element is a solid electrolyte having oxygen icon conductivity. In the sensor element, a first internal void is formed that is constructed so as to introduce the measurement object gas from an external space. The sensor element includes a first pump cell and a heat generation part. The first pump cell includes an inside pump electrode and an outside pump electrode. The inside pump electrode includes a first electrode part far from the heat generation part and a second electrode part close to the heat generation part. At least one portion of the second electrode part is covered with a porous material.SELECTED DRAWING: Figure 1

Description

The present invention relates to a gas sensor, and more particularly to a gas sensor configured to measure the concentration of a predetermined gas component in a gas to be measured.

Japanese Unexamined Patent Publication No. 2014-209128 (Patent Document 1) discloses a gas sensor. This gas sensor is configured to measure the NOx concentration in the gas to be measured. This gas sensor includes a sensor element, and the main component of the sensor element is a solid electrolyte having oxygen ion conductivity.

In this sensor element, a first internal vacant space configured to introduce the gas to be measured from the external space and a second internal vacant space communicating with the first internal vacant space are formed. A measurement electrode used for measuring the NOx concentration is formed in the second internal space. In this gas sensor, the oxygen concentration in the first internal space is regulated by the main pump cell including the inner pump electrode formed in the first internal space and the outer pump electrode formed outside the first internal space. NS.

That is, in this gas sensor, a gas to be measured whose oxygen partial pressure is kept at a low value is supplied to the measurement electrode, and the NOx concentration is measured based on the gas to be measured (see Patent Document 1).

Japanese Unexamined Patent Publication No. 2014-209128

In the gas sensor disclosed in Patent Document 1, gold (Au) is added to the inner pump electrode in order to suppress the decomposition of NOx in the first internal space. By suppressing the decomposition of NOx in the first internal space, the reduction of the amount of NOx reaching the measurement electrode is suppressed, so that the NOx concentration can be measured with high accuracy.

However, the present inventors have found that when a gas sensor as disclosed in Patent Document 1 is used for a long time in a high temperature region under a high oxygen concentration, the measurement accuracy of the NOx concentration gradually decreases.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a gas sensor in which the measurement accuracy of NOx concentration does not easily decrease even when used for a long time in a high temperature region under high oxygen concentration. It is to be.

A gas sensor according to the present invention is configured to measure the concentration of a predetermined gas component in the gas to be measured. The gas sensor includes a sensor element. The main component of the sensor element is a solid electrolyte having oxygen ion conductivity. In the sensor element, a first internal space is formed so as to introduce the gas to be measured from the external space. The sensor element includes a first pump cell and a heat generating portion. The heat generating portion is configured to generate heat. The first pump cell includes an inner pump electrode and an outer pump electrode. The inner pump electrode is formed in the first internal void and contains gold (Au). The outer pump electrode is formed in a space different from the first internal space. The first pump cell is configured to pump oxygen in the first internal space by applying a voltage between the inner pump electrode and the outer pump electrode. The inner pump electrode includes a first electrode portion far from the heat generating portion and a second electrode portion close to the heat generating portion. At least a part of the second electrode portion is covered with a porous body.

It is assumed that the second electrode portion of the gas sensor is not covered with the porous body. In this case, when the gas sensor is used for a long time in a high temperature region under a high oxygen concentration, platinum (Pt) of the inner pump electrode is oxidized to PtO ₂ and evaporates, and Au contained in the inner pump electrode also evaporates. The present inventors have found that. When the amount of Au contained in the inner pump electrode is reduced, NOx is easily decomposed in the first internal space. As the amount of NOx decomposed in the first internal void increases, the amount of NOx reaching the measurement electrode decreases. That is, in such a gas sensor, as the amount of Au contained in the inner pump electrode decreases through continuous use, the measurement accuracy of the NOx concentration decreases (the change in sensitivity to NOx increases).

Further, Au evaporated from the inner pump electrode may adhere to the measurement electrode. In order to measure the NOx concentration, the nitrogen oxides around the measuring electrode need to be reduced. When Au adheres to the measuring electrode, the reduction of nitrogen oxides around the measuring electrode is suppressed, so that the measurement accuracy of the NOx concentration is lowered.

For example, reducing the amount of Au contained in the inner pump electrode suppresses the change in sensitivity to NOx. However, in this case, when the voltage applied to the first pump cell under high oxygen concentration rises, NOx is more likely to be decomposed in the first internal space. As a result, the measurement accuracy of the NOx concentration is lowered.

Further, the present inventors have found that Au is more likely to evaporate in the second electrode portion near the heat generating portion in the inner pump electrode. Therefore, for example, it is conceivable to suppress the change in sensitivity to NOx by not providing the second electrode portion. However, in this case, since the area of the inner pump electrode is reduced, it is necessary to increase the voltage applied to the first pump cell in order to appropriately adjust the oxygen concentration in the first internal space. As a result, NOx is more easily decomposed in the first internal space, and the measurement accuracy of the NOx concentration is lowered.

In the gas sensor according to the present invention, at least a part of the second electrode portion is covered with a porous body. Therefore, according to this gas sensor, since the evaporation of Au at the second electrode portion is suppressed, it is possible to suppress a decrease in the measurement accuracy of the NOx concentration.

In the gas sensor, the porous body may be porous alumina.

In the gas sensor, when the maximum thickness of the porous body is A and the porosity of the porous body is B, the A / B may be 0.1 or more and 10.0 or less.

In the gas sensor, the A / B may be 0.5 or more and 10.0 or less.

In the gas sensor, the porosity of the porous body may be 5% or more and 50% or less.

In the gas sensor, the maximum thickness of the porous body may be 5 μm or more and 50 μm or less.

In the gas sensor, in the sensor element, a second internal vacant space communicating with the first internal vacant space is formed, the sensor element further includes a second pump cell, and the second pump cell is in the second internal vacant space. The auxiliary pump electrode formed and the outer pump electrode are provided, and the second pump cell is configured to pump out oxygen in the second internal space by applying a voltage between the auxiliary pump electrode and the outer pump electrode. The auxiliary pump electrode includes a third electrode portion far from the heat generating portion and a fourth electrode portion close to the heat generating portion, and at least a part of the fourth electrode portion may be covered with a porous body.

In this gas sensor, at least a part of the fourth electrode portion is covered with a porous body. Therefore, according to this gas sensor, since the evaporation of Au at the fourth electrode portion is suppressed, it is possible to suppress a decrease in the measurement accuracy of the NOx concentration.

According to the present invention, it is possible to provide a gas sensor in which the measurement accuracy of NOx concentration does not easily decrease even if it is used for a long time in a high temperature region under a high oxygen concentration.

It is sectional drawing which showed typically an example of the structure of a gas sensor. It is a figure for demonstrating the phenomenon which occurs when the bottom electrode part is not covered with a porous layer. It is an enlarged view around the 1st internal vacancy in a gas sensor. It is sectional drawing which showed typically an example of the structure of the gas sensor including the sensor element of a three-chamber structure.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the description thereof will not be repeated.

[1. Outline configuration of gas sensor]
FIG. 1 is a schematic cross-sectional view schematically showing an example of the configuration of the gas sensor 100. The sensor element 101 includes a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, and a first solid electrolyte layer 4, each of which is composed of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO2). , The element has a structure in which six layers of the spacer layer 5 and the second solid electrolyte layer 6 are laminated in this order from the lower side in the drawing. Further, the solid electric field quality forming these six layers is dense and airtight. The sensor element 101 is manufactured, for example, by performing predetermined processing, printing of a circuit pattern, or the like on a ceramic green sheet corresponding to each layer, laminating them, and further firing and integrating them.

A gas introduction port 10, a first diffusion rate-determining portion 11, and a buffer space between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, which is one tip of the sensor element 101. 12, the second diffusion rate-determining unit 13, the first internal space 20, the third diffusion rate-determining unit 30, and the second internal space 40 are formed adjacent to each other in this order.

The gas inlet 10, the buffer space 12, the first internal space 20, and the second internal space 40 are provided with the spacer layer 5 hollowed out so that the upper portion is the lower surface of the second solid electrolyte layer 6. The lower part is the upper surface of the first solid electrolyte layer 4, and the side part is the space inside the sensor element 101 partitioned by the side surface of the spacer layer 5.

The first diffusion rate-determining section 11, the second diffusion rate-determining section 13, and the third diffusion rate-determining section 30 are all provided as two horizontally long slits (openings have a longitudinal direction in the direction perpendicular to the drawing). .. The portion from the gas introduction port 10 to the second internal vacant space 40 is also referred to as a gas distribution section.

Further, at a position far from the tip side of the gas flow portion, the side portion is partitioned between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5 by the side surface of the first solid electrolyte layer 4. A reference gas introduction space 43 is provided at such a position. For example, the atmosphere is introduced into the reference gas introduction space 43 as a reference gas for measuring the NOx concentration.

The atmosphere introduction layer 48 is a layer made of porous alumina, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. Further, the atmosphere introduction layer 48 is formed so as to cover the reference electrode 42.

The reference electrode 42 is an electrode formed so as to be sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, the reference electrode 42 is connected to the reference gas introduction space 43 around the reference electrode 42. An air introduction layer 48 is provided. Further, as will be described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40 using the reference electrode 42.

In the gas distribution section, the gas introduction port 10 is a portion that is open to the external space, and the gas to be measured is taken into the sensor element 101 from the external space through the gas introduction port 10.

The first diffusion rate-determining unit 11 is a portion 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 for guiding the gas to be measured introduced from the first diffusion rate-determining unit 11 to the second diffusion rate-determining unit 13.

The second diffusion rate-determining unit 13 is a portion that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 20.

When the gas to be measured is introduced from the outside of the sensor element 101 to the inside of the first internal space 20, the pressure fluctuation of the gas to be measured in the external space (if the gas to be measured is the exhaust gas of an automobile, the pulsation of the exhaust pressure). ) Suddenly taken into the inside of the sensor element 101 from the gas introduction port 10), the gas to be measured is not directly introduced into the first internal space 20, but the first diffusion rate-determining unit 11, the buffer space 12, and the second. After the fluctuation in the concentration of the gas to be measured is canceled through the diffusion rate-determining unit 13, the gas is introduced into the first internal space 20. As a result, the concentration fluctuation of the gas to be measured introduced into the first internal space becomes almost negligible.

The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced through the second diffusion rate-determining unit 13. The oxygen partial pressure is adjusted by operating the main pump cell 21.

The main pump cell 21 has an inner pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the lower surface of the second solid electrolyte layer 6 facing the first internal space 20, and an upper surface of the second solid electrolyte layer 6. An electrochemical pump cell composed of an outer pump electrode 23 provided in a region corresponding to the ceiling electrode portion 22a so as to be exposed to an external space, and a second solid electrolyte layer 6 sandwiched between these electrodes. be.

The inner pump electrode 22 is formed so as to straddle the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that partition the first internal space 20 and the spacer layer 5 that provides the side wall. There is. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom portion is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. A spacer layer in which electrode portions 22b are formed, and side electrode portions (not shown) form both side wall portions of the first internal space 20 so as to connect the ceiling electrode portions 22a and the bottom electrode portions 22b. It is formed on the side wall surface (inner surface) of No. 5 and is arranged in a structure in the form of a tunnel at the arrangement portion of the side electrode portion.

The inner pump electrode 22 and the outer pump electrode 23 are formed as a porous cermet electrode (for example, a cermet electrode of Pt containing 1% Au and ZrO ₂ ). The inner pump electrode 22 that comes into contact with the gas to be measured is formed by using a material having a weakened reducing ability for the NOx component in the gas to be measured.

In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23, and a pump current is applied in the positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23. By flowing Ip0, the oxygen in the first internal space 20 can be pumped into the external space, or the oxygen in the external space can be pumped into the first internal space 20.

A porous layer 29 is formed on the bottom electrode portion 22b. That is, in the gas sensor 100, the bottom electrode portion 22b is covered with the porous layer (porous body) 29. The porous layer 29 is a film composed of a porous body containing _{alumina (Al 2} O _{3) as a main component.} The reason why the bottom electrode portion 22b is covered with the porous layer 29 will be described in detail later.

Further, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4 are used. The third substrate layer 3 and the reference electrode 42 constitute an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 80 for controlling a main pump.

By measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump, the oxygen concentration (oxygen partial pressure) in the first internal space 20 can be known. Further, the pump current Ip0 is controlled by feedback-controlling Vp0 so that the electromotive force V0 becomes constant. As a result, the oxygen concentration in the first internal space 20 can be maintained at a predetermined constant value.

The third diffusion rate-determining unit 30 imparts 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 20, and applies the gas to be measured. It is a part leading to the second internal space 40.

The second internal space 40 is provided as a space for performing a process related to the measurement of the nitrogen oxide (NOx) concentration in the gas to be measured introduced through the third diffusion rate-determining unit 30. The NOx concentration is mainly measured in the second internal space 40 whose oxygen concentration is adjusted by the auxiliary pump cell 50, and further by the operation of the measurement pump cell 41.

In the second internal space 40, the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, and then the auxiliary pump cell 50 is used for the gas to be measured introduced through the third diffusion rate-determining unit. The oxygen partial pressure is adjusted. As a result, the oxygen concentration in the second internal space 40 can be kept constant with high accuracy, so that the gas sensor 100 can measure the NOx concentration with high accuracy.

The auxiliary pump cell 50 includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, and an outer pump electrode 23 (outer pump electrode 23). It is an auxiliary electrochemical pump cell composed of a sensor element 101 and an appropriate electrode on the outside) and a second solid electrolyte layer 6.

The auxiliary pump electrode 51 is arranged in the second internal space 40 in a structure having a tunnel shape similar to that of the inner pump electrode 22 provided in the first internal space 20. That is, the ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40 is formed. , The bottom electrode portion 51b is formed, and the side electrode portion (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b provides a side wall of the second internal space 40 of the spacer layer 5. It has a tunnel-like structure formed on both walls.

The auxiliary pump electrode 51 is also formed by using a material having a weakened reducing ability for the NOx component in the gas to be measured, similarly to the inner pump electrode 22.

In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, oxygen in the atmosphere in the second internal space 40 is pumped out to the external space or outside. It is possible to pump from the space into the second internal space 40.

Further, in order to control the oxygen partial pressure in the atmosphere in the second internal space 40, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte are used. The layer 4 and the third substrate layer 3 constitute an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 81 for controlling an auxiliary pump.

The auxiliary pump cell 50 pumps with the variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. As a result, the partial pressure of oxygen in the atmosphere in the second internal space 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

At the same time, 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 to the oxygen partial pressure detection sensor cell 80 for controlling the main pump as a control signal, and the electromotive force V0 is controlled, so that the third diffusion rate-determining unit 30 to the second internal space are vacant. The gradient of the oxygen partial pressure in the gas to be measured introduced into the 40 is controlled to be always constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of about 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

The measurement pump cell 41 measures the NOx concentration in the gas to be measured in the second internal space 40. The measuring pump cell 41 includes a measuring electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the second internal space 40 and at a position separated from the third diffusion rate controlling portion 30, and an outer pump electrode 23. , A second solid electrolyte layer 6, a spacer layer 5, and a first solid electrolyte layer 4.

The measuring electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx existing in the atmosphere in the second internal space 40. Further, the measurement electrode 44 is covered with the fourth diffusion rate-determining portion 45.

The fourth diffusion rate-determining unit 45 is a film composed of a porous body containing _{alumina (Al 2} O _{3) as a main component.} The fourth diffusion rate-determining unit 45 plays a role of limiting the amount of NOx flowing into the measurement electrode 44, and also functions as a protective film of the measurement electrode 44.

In the measurement pump cell 41, oxygen generated by the decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44 can be pumped out, and the amount of oxygen generated can be detected as the pump current Ip2.

Further, 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 like. The reference electrode 42 constitutes an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 82 for controlling a measuring pump. The variable power supply 46 is controlled based on the electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

The gas to be measured guided into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion rate-determining unit 45 under the condition that the oxygen partial pressure is controlled. Nitrogen oxides in the gas to be measured around the measurement electrode 44 are reduced (2NO → N ₂ + O ₂ ) to generate oxygen. Then, the generated oxygen is pumped by the measurement pump cell 41, and at that 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 becomes constant. The voltage Vp2 of 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 nitrogen oxides in the gas to be measured are used by using the pump current Ip2 in the measurement pump cell 41. The concentration will be calculated.

Further, if 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 detecting means as an electrochemical sensor cell, the measurement electrode The electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around 44 and the amount of oxygen contained in the reference atmosphere can be detected, thereby detecting the NOx component in the gas to be measured. It is also possible to determine the concentration of.

Further, the electrochemical sensor cell 83 is composed of 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 electromotive force Vref obtained by the sensor cell 83 makes it possible to detect the partial pressure of oxygen in the gas to be measured outside the sensor.

In the gas sensor 100 having such a configuration, the oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50. The gas to be measured is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the gas to be measured is determined 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 substantially proportional to the concentration of NOx in the gas to be measured. You can know it.

Further, the sensor element 101 includes a heater unit 70 that plays a role of temperature adjustment for heating and keeping the sensor element 101 warm in order to increase the oxygen ion conductivity of the solid electrolyte. The heater unit 70 includes a heater electrode 71, a heater 72, a through hole 73, a heater insulating layer 74, and a pressure dissipation hole 75.

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

The heater 72 is an electric resistor formed in a manner of being sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater electrode 71 via a through hole 73, and generates heat when power is supplied from the outside through the heater electrode 71 to heat and retain heat of the solid electrolyte forming the sensor element 101.

Further, the heater 72 is embedded in the entire area from the first internal space 20 to the second internal space 40, and the entire sensor element 101 can be adjusted to a temperature at which the solid electrolyte is activated. ing.

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

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

[2. The reason why the bottom electrode is covered with a porous layer (porous material)]
As described above, in the gas sensor 100, the bottom electrode portion 22b of the inner pump electrode 22 is covered with the porous layer 29. That is, of the inner pump electrodes 22, the ceiling electrode portion 22a far from the heater portion 70 is not covered with the porous layer 29, and the bottom electrode portion 22b close to the heater portion 70 is covered with the porous layer 29. Hereinafter, the reason why the bottom electrode portion 22b is covered with the porous layer 29 will be described.

FIG. 2 is a diagram for explaining a phenomenon that occurs when the bottom electrode portion 22b is not covered with the porous layer 29. With reference to FIG. 2, it is assumed that in the gas sensor 100, the bottom electrode portion 22b of the inner pump electrode 22 is not covered with the porous layer 29. In this case, when such a gas sensor is used for a long time in a high temperature region under a high oxygen concentration, platinum (Pt) of the inner pump electrode 22 is oxidized to PtO ₂ and evaporated, and is contained in the inner pump electrode 22. The present inventors have found that Au also evaporates. In particular, the present inventors have found that Au is more likely to evaporate at the bottom electrode portion 22b of the inner pump electrode 22 near the heater portion 70 (FIG. 1).

When the amount of Au contained in the inner pump electrode 22 is reduced, NOx is easily decomposed in the first internal space 20. As the amount of NOx decomposed in the first internal space 20 increases, the amount of NOx reaching the measurement electrode 44 decreases. That is, in such a gas sensor, as the amount of Au contained in the inner pump electrode 22 decreases through continuous use, the measurement accuracy of the NOx concentration decreases (the change in sensitivity to NOx increases).

Further, Au evaporated from the inner pump electrode 22 may adhere to the measurement electrode 44. In order to measure the NOx concentration, the nitrogen oxides around the measuring electrode 44 need to be reduced. When Au adheres to the measurement electrode 44, the reduction of nitrogen oxides around the measurement electrode 44 is suppressed, so that the measurement accuracy of the NOx concentration is lowered.

In the gas sensor 100 according to the present embodiment, the bottom electrode portion 22b of the inner pump electrode 22 is covered with the porous layer 29. Therefore, the evaporation of Au in the bottom electrode portion 22b is suppressed. Further, the Au evaporated at the bottom electrode portion 22b is more likely to be captured by the porous layer 29. As a result, according to the gas sensor 100, the decrease in the amount of Au contained in the inner pump electrode 22 is suppressed and the amount of Au adhering to the measurement electrode 44 is suppressed, so that the decrease in the measurement accuracy of the NOx concentration is suppressed. Can be done.

[3. Structure of porous layer]
FIG. 3 is an enlarged view of the periphery of the first internal space 20 in the gas sensor 100. With reference to FIG. 3, the porous layer 29 is formed on the entire upper surface of the bottom electrode portion 22b. As described above, the porous layer 29 is a film composed of a porous body containing alumina as a main component.

The maximum thickness A of the porous layer 29 is preferably 5 μm or more and 50 μm or less. The maximum thickness A refers to the thickness of the thickest portion of the porous layer 29.

The porosity B of the porous layer 29 is preferably 5% or more and 50% or less. The porosity B is obtained by applying a known image processing method (binarization treatment or the like) to the SEM (scanning electron microscope) image of the evaluation target object.

Further, when the maximum thickness of the porous layer 29 is A and the porosity of the porous layer 29 is B, the A / B is preferably 0.1 or more and 10.0 or less, more preferably. , 0.5 or more and 10.0 or less, and more preferably 5.0 or more and 10.0 or less.

[4. feature]
As described above, in the gas sensor 100 according to the present embodiment, the bottom electrode portion 22b of the inner pump electrode 22 is covered with the porous layer 29. Therefore, during the use of the gas sensor 100, the evaporation of Au at the bottom electrode portion 22b is suppressed. Further, the Au evaporated at the bottom electrode portion 22b is more likely to be captured by the porous layer 29. As a result, according to the gas sensor 100, the decrease in the amount of Au contained in the inner pump electrode 22 is suppressed and the amount of Au adhering to the measurement electrode 44 is suppressed, so that the decrease in the measurement accuracy of the NOx concentration is suppressed. Can be done.

The gas sensor 100 is an example of the "gas sensor" in the present invention, and the sensor element 101 is an example of the "sensor element" in the present invention. The first internal space 20 is an example of the "first internal space" in the present invention, the main pump cell 21 is an example of the "first pump cell" in the present invention, and the heater unit 70 is the "first internal space" in the present invention. This is an example of a "heat generating part". The inner pump electrode 22 is an example of the “inner pump electrode” in the present invention, and the outer pump electrode 23 is an example of the “outer pump electrode” in the present invention. The ceiling electrode portion 22a is an example of the "first electrode portion" in the present invention, and the bottom electrode portion 22b is an example of the "second electrode portion" in the present invention. The porous layer 29 is an example of the “porous material” in the present invention.

The second internal space 40 is an example of the "second internal space" in the present invention, and the auxiliary pump cell 50 is an example of the "second pump cell" in the present invention. The auxiliary pump electrode 51 is an example of the “auxiliary pump electrode” in the present invention. The ceiling electrode portion 51a is an example of the "third electrode portion" in the present invention, and the bottom electrode portion 51b is an example of the "fourth electrode portion" in the present invention.

[5. Modification example]
Although the embodiments have been described above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the embodiments. Hereinafter, a modified example will be described.

(5-1)
In the gas sensor 100 according to the above embodiment, the sensor element 101 is formed with a first internal space 20 and a second internal space 40. That is, the sensor element 101 had a two-chamber structure. However, the sensor element 101 does not necessarily have to have a two-chamber structure. For example, the sensor element 101 may have a three-chamber structure.

FIG. 4 is a schematic cross-sectional view schematically showing an example of the configuration of the gas sensor 100X including the sensor element 101X having a three-chamber structure. As shown in FIG. 4, the second internal space 40 (FIG. 1) is further divided into two chambers by the fifth diffusion rate-determining unit 60, and the second internal space 40X and the third internal space 61 are created. May be good. In this case, the auxiliary pump electrode 51X may be arranged in the second internal space 40X, and the measurement electrode 44X may be arranged in the third internal space 61. Further, in the case of a three-chamber structure, the fourth diffusion rate-determining unit 45 may be omitted.

(5-2)
In the gas sensor 100 according to the above embodiment, the porous layer 29 was formed only on the bottom electrode portion 22b, and the porous layer 29 was not formed on the ceiling electrode portion 22a. However, the formation position of the porous layer 29 is not limited to this. For example, the porous layer 29 may be formed on the ceiling electrode portion 22a in addition to the bottom electrode portion 22b. Further, the porous layer 29 may be formed on the bottom electrode portion 51b, or the porous layer 29 may be formed on the ceiling electrode portion 51a.

For example, according to the gas sensor in which the bottom electrode portion 51b is covered with the porous layer 29, the evaporation of Au in the bottom electrode portion 51b is suppressed, so that the decrease in the measurement accuracy of the NOx concentration can be further suppressed.

(5-3)
In the gas sensor 100 according to the above embodiment, the porous layer 29 is formed on the entire upper surface of the bottom electrode portion 22b. However, the porous layer 29 does not necessarily have to be formed on the entire upper surface of the bottom electrode portion 22b. For example, the porous layer 29 may only be formed on a part of the upper surface of the bottom electrode portion 22b. In this case, it is desirable that at least one of the high oxygen concentration portion and the high temperature portion of the upper surface of the bottom electrode portion 22b is covered. That is, it is desirable that the portion of the upper surface of the bottom electrode portion 22b near the gas introduction port 10 is covered with the porous layer 29. For example, the region of the bottom electrode portion 22b from the tip portion near the gas introduction port 10 to a portion that is half the area of the bottom electrode portion 22b may be covered with the porous layer 29.

(5-4)
In the gas sensor 100 according to the above embodiment, the main component of the porous layer 29 was alumina. However, the main component of the porous layer 29 does not necessarily have to be alumina. The main component of the porous layer 29 may be, for example, spinel, zirconia, cordierite, titania or the like.

[6. Examples, etc.]
(6-1. Examples 1-24 and Comparative Example 1)
A plurality of gas sensors 100 according to Examples 1-24 were created. Specifically, first, the sensor element 101 was created by the method described below.

Six unfired ceramic green sheets containing an oxygen ion conductive solid electrolyte such as zirconia as a ceramic component were prepared. Each ceramic green sheet was formed by mixing zirconia particles to which yttria, a stabilizer was added in an amount of 4 mol%, an organic binder, and an organic solvent, and tape molding. A plurality of sheet holes and necessary through holes used for positioning during printing and laminating are formed in advance on this green sheet.

Further, the green sheet serving as the spacer layer 5 is provided with a space serving as a gas distribution section in advance by punching or the like. Then, corresponding to each of the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6, respectively. A pattern printing process and a drying process were performed to form various patterns on the ceramic green sheet.

Specifically, the formed pattern includes the above-mentioned electrodes, lead wires connected to the electrodes, and a porous layer 29 formed on the bottom electrode portion 22b (main component is alumina, and a small amount of silica is used. Included), the atmosphere introduction layer 48, the heater section 70, and the like. The pattern printing was performed by applying a pattern forming paste prepared according to the characteristics required for each formation target onto a green sheet using a known screen printing technique. The drying treatment was also carried out using a known drying means. After the pattern printing and drying were completed, the adhesive paste for laminating and adhering the green sheets corresponding to each layer was printed and dried.

Then, the green sheets on which the adhesive paste was formed were laminated in a predetermined order while being positioned by the sheet holes, and crimped by applying predetermined temperature and pressure conditions to form a single laminated body. The laminate thus obtained included a plurality of sensor elements 101. The laminate was cut and cut into the size of the sensor element 101. Then, the cut laminate was fired at a predetermined firing temperature to obtain a sensor element 101. By incorporating the sensor element 101 thus obtained, the gas sensor 100 was obtained.

The only difference between the gas sensors 100 of Examples 1-24 was the maximum thickness A and porosity B of the porous layer 29 formed on the bottom electrode portion 22b. The porosity B was adjusted by adjusting the amount of the pore-increasing agent added to the porous layer 29. In the gas sensor in Comparative Example 1, the porous layer 29 was omitted in the gas sensor 100 in Examples 1-24. The features of Examples 1-24 and Comparative Example 1 are shown in Table 1 below.

（６−２．評価試験）
実施例１−２４及び比較例１に関し、ディーゼルエンジンを用いた耐久試験を行ない、試験前後における各ガスセンサのＮＯｘに対する感度変化を評価した。具体的には、以下のように試験を行った。実施例１−２４及び比較例１のガスセンサを自動車の排ガス管の配管に取り付けた。そして、ヒータ７２に通電して温度を８００℃とし、センサ素子１０１を加熱した。この状態で、エンジン回転数１５００−３５００ｒｐｍ、負荷トルク０−３５０Ｎ・ｍの範囲で構成した４０分間の運転パターンを、３０００時間が経過するまで繰り返した。なお、そのときのガス温度は２００℃−６００℃、ＮＯｘ濃度は０−１５００ｐｐｍとした。耐久試験前後のガスセンサをＮＯｘ濃度５００ｐｐｍのモデルガス装置に取り付け、初期と耐久後とでのＮＯｘの感度変化率を測定した。

(6-2. Evaluation test)
With respect to Examples 1-24 and Comparative Example 1, a durability test using a diesel engine was performed, and the change in sensitivity of each gas sensor to NOx before and after the test was evaluated. Specifically, the test was conducted as follows. The gas sensors of Examples 1-24 and Comparative Example 1 were attached to the piping of the exhaust gas pipe of an automobile. Then, the heater 72 was energized to set the temperature to 800 ° C., and the sensor element 101 was heated. In this state, a 40-minute operation pattern composed of an engine speed of 1500-3500 rpm and a load torque of 0-350 Nm was repeated until 3000 hours had passed. The gas temperature at that time was 200 ° C.-600 ° C., and the NOx concentration was 0-1500 ppm. A gas sensor before and after the durability test was attached to a model gas device having a NOx concentration of 500 ppm, and the rate of change in NOx sensitivity between the initial stage and after the durability test was measured.

The judgment result when the sensitivity change rate of NOx was within ± 5% was defined as “A”, and the judgment result when the sensitivity change rate of NOx was greater than ± 5% and within ± 10% was defined as “B”. .. Further, the judgment result when the sensitivity change rate of NOx is larger than ± 10% and within ± 15% is defined as “C”, and the judgment result when the sensitivity change rate of NOx is larger than ± 15% is “D”. And said.

As shown in Table 1, the determination results of Examples 12, 17, and 18 are "A", the determination results of Examples 2-11, 13-16, 23, 24 are "B", and Examples 1 and 1. The judgment result of 19-22 was "C". On the other hand, the determination result of Comparative Example 1 was "D". In this way, it was confirmed that the rate of change in the sensitivity of NOx in the gas sensor 100 was suppressed by forming the porous layer 29 on the bottom electrode portion 22b.

1 1st substrate layer, 2 2nd substrate layer, 3 3rd substrate layer, 4 1st solid electrolyte layer, 5 spacer layer, 6 2nd solid electrolyte layer, 10 gas inlet, 11 1st diffusion rate controlling part, 12 buffer Space, 13 2nd diffusion rate control part, 20 1st internal space, 21 main pump cell, 22 inner pump electrode, 22a, 51a, 51aX ceiling electrode part, 22b, 51b, 51bX bottom electrode part, 23 outer pump electrode, 29 porous Layer, 30 3rd diffusion rate control section, 40, 40X 2nd internal space, 41 measurement pump cell, 42 reference electrode, 43 reference gas introduction space, 44, 44X measurement electrode, 45 4th diffusion rate control section, 46, 52 Variable power supply, 48 atmosphere introduction layer, 50 auxiliary pump cell, 51, 51X auxiliary pump electrode, 60 5th diffusion rate control part, 61 3rd internal space, 70 heater part, 71 heater electrode, 72 heater, 73 through hole, 74 heater Insulation layer, 75 pressure dissipation holes, 80 main pump control oxygen partial pressure detection sensor cell, 81 auxiliary pump control oxygen partial pressure detection sensor cell, 82 measurement pump control oxygen partial pressure detection sensor cell, 83 sensor cell, 100 gas sensor, 101 sensor element.

Claims (7)

A gas sensor configured to measure the concentration of a predetermined gas component in the gas to be measured.
Equipped with a sensor element
The main component of the sensor element is a solid electrolyte having oxygen ion conductivity.
In the sensor element, a first internal vacant space configured to introduce the gas to be measured is formed from an external space.
The sensor element includes a first pump cell and a heat generating portion configured to generate heat.
The first pump cell is
An inner pump electrode formed in the first internal space and containing gold (Au), and
It is provided with an outer pump electrode formed in a space different from the first internal space.
The first pump cell is configured to pump out oxygen in the first internal space by applying a voltage between the inner pump electrode and the outer pump electrode.
The inner pump electrode includes a first electrode portion far from the heat generating portion and a second electrode portion close to the heat generating portion.
A gas sensor in which at least a part of the second electrode portion is covered with a porous body. The gas sensor according to claim 1, wherein the porous body is porous alumina. The first or second aspect of the present invention, wherein when the maximum thickness of the porous body is A and the porosity of the porous body is B, the A / B is 0.1 or more and 10.0 or less. Gas sensor. The gas sensor according to claim 3, wherein the A / B is 0.5 or more and 10.0 or less. The gas sensor according to any one of claims 1 to 4, wherein the porosity of the porous body is 5% or more and 50% or less. The gas sensor according to any one of claims 1 to 5, wherein the maximum thickness of the porous body is 5 μm or more and 50 μm or less. In the sensor element, a second internal vacant space communicating with the first internal vacant space is formed.
The sensor element further comprises a second pump cell.
The second pump cell is
With the auxiliary pump electrode formed in the second internal space,
With the outer pump electrode
The second pump cell is configured to pump out oxygen in the second internal space by applying a voltage between the auxiliary pump electrode and the outer pump electrode.
The auxiliary pump electrode includes a third electrode portion far from the heat generating portion and a fourth electrode portion close to the heat generating portion.
The gas sensor according to any one of claims 1 to 6, wherein at least a part of the fourth electrode portion is covered with a porous body.

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