CN113219037A

CN113219037A - Gas sensor

Info

Publication number: CN113219037A
Application number: CN202011469033.2A
Authority: CN
Inventors: 渡边悠介; 新妻匠太郎; 幸岛康英; 青田隼实; 平川敏弘
Original assignee: NGK Insulators Ltd
Current assignee: NGK Insulators Ltd
Priority date: 2020-02-05
Filing date: 2020-12-15
Publication date: 2021-08-06
Anticipated expiration: 2040-12-15
Also published as: CN113219037B; JP2021124382A; JP7339896B2

Abstract

The invention provides a gas sensor, which is difficult to reduce the measurement accuracy of NOx concentration even if the gas sensor is used in a high-temperature region for a long time under high oxygen concentration. The gas sensor is configured to measure the concentration of a predetermined gas component in a 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. The sensor element is formed with a first internal cavity configured to introduce the gas to be measured from the external space. The sensor element includes a first pump unit and a heat generating portion. The first pump unit includes an inner pump electrode and an outer pump electrode. The inner pump electrode includes: a first electrode portion remote from the heat generating portion; and a second electrode portion adjacent to the heat generating portion. At least a part of the second electrode portion is covered with the porous body.

Description

Gas sensor

Technical Field

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.

Background

Japanese patent laid-open publication No. 2014-209128 (patent document 1) discloses a gas sensor. The gas sensor is configured to measure the NOx concentration in the gas to be measured. The gas sensor includes a sensor element whose main component is a solid electrolyte having oxygen ion conductivity.

The sensor element is formed with: a first internal cavity configured to introduce a gas to be measured from an external space; and a second internal cavity in communication with the first internal cavity. A measurement electrode for measuring the NOx concentration is formed in the second internal cavity. With this gas sensor, the oxygen concentration in the first internal cavity is adjusted by a main pump unit that includes an inner pump electrode formed inside the first internal cavity, and an outer pump electrode formed outside the first internal cavity.

That is, in this gas sensor, a gas to be measured in which the oxygen partial pressure is kept low is supplied to a measurement electrode, and the NOx concentration is measured based on the gas to be measured (see patent document 1).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2014-209128

Disclosure of Invention

With the gas sensor disclosed in patent document 1, gold (Au) is added to the inner pump electrode in order to suppress decomposition of NOx in the first inner cavity. Since the NOx amount reaching the measurement electrode is suppressed from decreasing by suppressing the decomposition of NOx in the first internal cavity, the NOx concentration can be measured with high accuracy.

However, the inventors of the present invention found that: if the gas sensor disclosed in patent document 1 is used under a high oxygen concentration and in a high temperature region for a long time, the measurement accuracy of the NOx concentration gradually decreases.

The present invention has been made in view of the above problems, and an object thereof is to provide a gas sensor in which the measurement accuracy of the NOx concentration is not easily lowered even when the gas sensor is used in a high-temperature region for a long time under a high oxygen concentration.

The gas sensor according to the present invention is configured to measure the concentration of a predetermined gas component in a 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. The sensor element is formed with a first internal cavity configured to introduce the gas to be measured from the external space. The sensor element includes a first pump unit and a heat generating portion. The heat generating portion is configured to generate heat. The first pump unit includes an inner pump electrode and an outer pump electrode. The inner pump electrode is formed in the first inner cavity and contains gold (Au). The outer pump electrode is formed in a space different from the first inner cavity. The first pump unit is configured to: oxygen within the first internal cavity is drawn out by applying a voltage between the inner pump electrode and the outer pump electrode. The inner pump electrode includes: a first electrode portion remote 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 the porous body.

It is assumed that the second electrode portion of the gas sensor is not covered with the porous body. The inventors of the present invention found that: in this case, if the gas sensor is used for a long time in a high-temperature region at a high oxygen concentration, platinum (Pt) of the inner pump electrode is oxidized to PtO₂And the Au contained in the inner pump electrode is evaporated. If the amount of Au contained in the inner pump electrode is reduced, NOx is easily decomposed in the first internal cavity. If the amount of decomposed NOx in the first internal cavity increases, the amount of NOx reaching the measurement electrode decreases. That is, in the gas sensor, the measurement accuracy of the NOx concentration decreases as the amount of Au contained in the inner pump electrode decreases (the degree of sensitivity change with respect to NOx increases) due to continuous use.

In addition, Au evaporated from the inner pump electrode may adhere to the measurement electrode. In order to measure the NOx concentration, it is necessary to reduce nitrogen oxides around the measurement electrode. If Au adheres to the measurement electrode, reduction of nitrogen oxide around the measurement electrode is suppressed, and therefore, the measurement accuracy of NOx concentration is lowered.

For example, if the amount of Au contained in the inner pump electrode is reduced, the sensitivity change against NOx is suppressed. In this case, however, NOx is more easily decomposed in the first internal cavity when the voltage applied to the first pump cell is increased at a high oxygen concentration. As a result, the measurement accuracy of the NOx concentration is lowered.

In addition, the inventors of the present invention found that: au evaporates more easily at the second electrode portion of the inner pump electrode near the heat generating portion. Therefore, for example, a scheme may be considered in which a sensitivity change with respect to NOx is suppressed by not providing the second electrode portion. However, in this case, the area of the inner pump electrode is reduced, and therefore, the voltage applied to the first pump cell needs to be increased in order to appropriately adjust the oxygen concentration in the first internal cavity. As a result, NOx is more likely to be decomposed in the first internal cavity, 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 the porous body. Therefore, according to this gas sensor, evaporation of Au in the second electrode portion is suppressed, and therefore, a decrease in the measurement accuracy of the NOx concentration can be suppressed.

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

In the above gas sensor, when a represents the maximum thickness of the porous body and B represents the porosity of the porous body, a/B may be 0.1 to 10.0.

In the gas sensor, the a/B may be 0.5 to 10.0.

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 above gas sensor, the sensor element may be formed with a second internal cavity communicating with the first internal cavity, the sensor element may further include a second pump unit, and the second pump unit may include: an auxiliary pump electrode formed within the second internal cavity; and an outer pump electrode, the second pump unit being configured to: applying a voltage between the auxiliary pump electrode and the outer pump electrode to draw out oxygen from the second internal cavity, wherein the auxiliary pump electrode comprises: a third electrode portion remote from the heat generating portion; and a fourth electrode portion adjacent to the heat generating portion, at least a part of the fourth electrode portion being covered with the porous body.

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

Effects of the invention

According to the present invention, it is possible to provide a gas sensor in which the measurement accuracy of the NOx concentration is not easily lowered even when the gas sensor is used in a high-temperature region for a long time.

Drawings

Fig. 1 is a schematic cross-sectional view schematically showing an example of the structure of the gas sensor.

Fig. 2 is a diagram for explaining a phenomenon that occurs when the bottom electrode portion is not covered with the porous layer.

Fig. 3 is an enlarged view around the first internal cavity of the gas sensor.

Fig. 4 is a schematic cross-sectional view schematically showing an example of the structure of a gas sensor including a sensor element having a three-cavity structure.

Description of the reference numerals

1 … first substrate layer, 2 … second substrate layer, 3 … third substrate layer, 4 … first solid electrolyte layer, 5 … separator, 6 … second solid electrolyte layer, 10 … gas introduction port, 11 … first diffusion rate control section, 12 … buffer space, 13 … second diffusion rate control section, 20 … first internal cavity, 21 … main pump unit, 22 … internal side pump electrode, 22a, 51aX … top electrode section, 22b, 51bX … bottom electrode section, 23 … external side pump electrode, 29 … porous layer, 30 … third diffusion rate control section, 40X … second internal cavity, 41 … measurement pump unit, 42 … reference electrode, 43 … reference gas introduction space, 44X … measurement electrode, 45 … fourth diffusion rate control section, 46, 52 … atmospheric introduction variable power supply, 48 … atmospheric introduction layer, 50 … auxiliary pump unit, 51. 51X … auxiliary pump electrode, 60 … fifth diffusion rate control section, 61 … third internal cavity, 70 … heater section, 71 … heater electrode, 72 … heater, 73 … through hole, 74 … heater insulation layer, 75 … pressure release hole, 80 … main pump control oxygen partial pressure detection sensor unit, 81 … auxiliary pump control oxygen partial pressure detection sensor unit, 82 … measurement pump control oxygen partial pressure detection sensor unit, 83 … sensor unit, 100 … gas sensor, 101 … sensor element.

Detailed Description

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

[1. gas sensor outline Structure ]

Fig. 1 is a schematic cross-sectional view schematically showing an example of the structure of a gas sensor 100. The sensor element 101 is an element having the following configuration: in the figure, a layer containing zirconium oxide (ZrO) is sequentially laminated from the lower side₂) Six layers of a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a separation layer 5, and a second solid electrolyte layer 6 of the plasma-conductive solid electrolyte. In addition, the solid electrolyte forming these six layers is a dense and airtight solid electrolyte. For example, the sensor element 101 is manufactured by performing predetermined processing, printing of a circuit pattern, and the like on ceramic green sheets corresponding to the respective layers, laminating them, and then firing them to integrate them.

At one end portion of the sensor element 101, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a gas introduction port 10, a first diffusion rate control portion 11, a buffer space 12, a second diffusion rate control portion 13, a first internal cavity 20, a third diffusion rate control portion 30, and a second internal cavity 40 are formed adjacent to each other so as to sequentially communicate with each other in this order.

The gas inlet 10, the buffer space 12, the first internal cavity 20, and the second internal cavity 40 are internal spaces of the sensor element 101 provided so as to hollow out the separator 5, wherein the upper part of the internal spaces is defined by the lower surface of the second solid electrolyte layer 6, the lower part is defined by the upper surface of the first solid electrolyte layer 4, and the side parts are defined by the side surfaces of the separator 5.

The first diffusion rate controlling section 11, the second diffusion rate controlling section 13, and the third diffusion rate controlling section 30 are each provided with 2 horizontally long (a direction perpendicular to the drawing constitutes a longitudinal direction of the opening) slits. The region from the gas inlet 10 to the second internal cavity 40 is also referred to as a gas flow portion.

Further, a reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the separator 5 at a position further from the end side than the gas flow portion, and at a position partitioned by the side surface of the first solid electrolyte layer 4 at the side portion. 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. The atmosphere introduction layer 48 is formed 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, an atmosphere introduction layer 48 communicating with the reference gas introduction space 43 is provided around the reference electrode. As will be described later, the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 and the oxygen concentration (oxygen partial pressure) in the second internal cavity 40 can be measured by the reference electrode 42.

In the gas flow portion, the gas inlet 10 is a site that is open to the outside space, and the gas to be measured is introduced from the outside space into the sensor element 101 through the gas inlet 10.

The first diffusion rate controller 11 is a part that applies a predetermined diffusion resistance to the gas to be measured introduced from the gas inlet 10.

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

The second diffusion rate controller 13 is a portion that applies a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal cavity 20.

When the gas to be measured is introduced into the first internal cavity 20 from outside the sensor element 101, the gas to be measured, which is rapidly introduced into the sensor element 101 from the gas introduction port 10 due to the pressure variation of the gas to be measured in the external space (pulsation of the exhaust pressure in the case where the gas to be measured is the exhaust gas of an automobile), is not directly introduced into the first internal cavity 20, but is introduced into the first internal cavity 20 after the concentration variation of the gas to be measured is eliminated by the first diffusion rate control unit 11, the buffer space 12, and the second diffusion rate control unit 13. Thus, the concentration of the gas to be measured introduced into the first internal space varies to a negligible extent.

The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced by the second diffusion rate control unit 13. The main pump unit 21 operates to adjust the oxygen partial pressure.

The main pump unit 21 is an electrochemical pump unit configured to include an inner pump electrode 22, an outer pump electrode 23, and a second solid electrolyte layer 6 sandwiched between the inner pump electrode 22 and the outer pump electrode 23, wherein the inner pump electrode 22 has a top electrode portion 22a provided on a substantially entire region of a lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, and the outer pump electrode 23 is provided on a region of an upper surface of the second solid electrolyte layer 6 corresponding to the top electrode portion 22a so as to be exposed to an external space.

The inner pump electrode 22 is formed such that: the solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) above and below the first internal cavity 20 and the spacers 5 constituting the sidewalls are formed so as to partition the first internal cavity. Specifically, the top electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 constituting the top surface of the first internal cavity 20, the bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 constituting the bottom surface, and the side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the separators 5 constituting the two side wall portions of the first internal cavity 20, whereby the top electrode portion 22a and the bottom electrode portion 22b are connected to each other and arranged in a tunnel shape at the arrangement portions of the side electrode portions.

The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (e.g., Pt and ZrO containing 1% Au)₂The cermet electrode of (a). The inner pump electrode 22 that is in contact with the measurement target gas is formed of a material that can reduce the reducing ability for the NOx component in the measurement target gas.

In the main pump unit 21, by applying a desired pump voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23 and flowing a pump current Ip0 between the inner pump electrode 22 and the outer pump electrode 23 in the positive or negative direction, oxygen in the first internal cavity 20 can be sucked into the external space or oxygen in the external space can be sucked into the first internal cavity 20.

The porous layer 29 is formed on the bottom electrode portion 22 b. That is, with the gas sensor 100, the bottom electrode portion 22b is covered with the porous layer (porous body) 29. The porous layer 29 is made of alumina (Al)₂O₃) A film comprising a porous body as a main component. Hereinafter, the reason why the bottom electrode portion 22b is covered with the porous layer 29 will be described in detail.

In order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, the electrochemical sensor cell, i.e., the main pump control oxygen partial pressure detection sensor cell 80 is configured to include the inner pump electrode 22, the second solid electrolyte layer 6, the separator 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42.

The oxygen concentration (oxygen partial pressure) in the first internal cavity 20 is obtained by measuring the electromotive force V0 of the main pump control oxygen partial pressure detection sensor unit 80. Further, Vp0 is feedback-controlled so that the electromotive force V0 is constant, thereby controlling the pump current Ip 0. Thereby, the oxygen concentration in the first internal cavity 20 can be maintained at a predetermined constant value.

The third diffusion rate control unit 30 is configured as follows: the gas to be measured after the oxygen concentration (oxygen partial pressure) of the first internal cavity 20 is controlled by the operation of the main pump unit 21 is guided to the second internal cavity 40 by applying a predetermined diffusion resistance to the gas.

The second internal cavity 40 is provided as a space for performing the following processes: the concentration of nitrogen oxide (NOx) in the gas to be measured introduced by the third diffusion rate control unit 30 is measured. The NOx concentration is measured mainly by the operation of the measurement pump cell 41 in the second internal cavity 40 after the oxygen concentration has been adjusted by the auxiliary pump cell 50.

In the second internal cavity 40, the oxygen partial pressure of the gas to be measured, which has been introduced by the third diffusion rate control unit 30 after the oxygen concentration (oxygen partial pressure) has been adjusted in the first internal cavity 20 in advance, is further adjusted by the auxiliary pump unit 50. This makes it possible to accurately maintain the oxygen concentration in the second internal cavity 40 constant, and therefore, with such a gas sensor 100, the NOx concentration can be accurately measured.

The auxiliary pump cell 50 is an auxiliary electrochemical pump cell configured to include an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, and sufficient if an appropriate electrode is provided on the outer side of the sensor element 101), and the second solid electrolyte layer 6, wherein the auxiliary pump electrode 51 has a top electrode portion 51a provided on the lower surface of the second solid electrolyte layer 6 so as to face substantially the entire region of the second internal cavity 40.

The auxiliary pump electrode 51 is disposed in the second internal cavity 40 in the same tunnel configuration as the inner pump electrode 22 disposed in the first internal cavity 20. That is, a top electrode portion 51a is formed on the second solid electrolyte layer 6 constituting the top surface of the second internal cavity 40, a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 constituting the bottom surface of the second internal cavity 40, and side electrode portions (not shown) connecting the top electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the separator 5 constituting the side wall of the second internal cavity 40, respectively.

The auxiliary pump electrode 51 is also formed of a material that can reduce the reducing ability for the NOx component in the measurement gas, similarly to the inner pump electrode 22.

In the auxiliary pump unit 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 cavity 40 can be sucked into the external space or oxygen can be sucked into the second internal cavity 40 from the external space.

In order to control the oxygen partial pressure in the atmosphere in the second internal cavity 40, the electrochemical sensor cell, that is, the auxiliary pump control oxygen partial pressure detection sensor cell 81 is configured to include the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the separator 5, the first solid electrolyte layer 4, and the third substrate layer 3.

The auxiliary pump unit 50 pumps by the variable power supply 52, and the variable power supply 52 controls the voltage based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor unit 81. Thereby, the oxygen partial pressure in the atmosphere inside the second internal cavity 40 is controlled to a lower partial pressure that has substantially no influence on 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 main pump control. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor unit 80, and the electromotive force V0 thereof is controlled, thereby controlling: the gradient of the oxygen partial pressure in the gas to be measured introduced from the third diffusion rate controller 30 into the second internal cavity 40 is always constant. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of about 0.001ppm by the main pump unit 21 and the auxiliary pump unit 50.

The measurement pump unit 41 measures the NOx concentration in the measurement gas in the second internal cavity 40. The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44, the outer pump electrode 23, the second solid electrolyte layer 6, the separator 5, and the first solid electrolyte layer 4, wherein the measurement electrode 44 is provided on the upper surface of the first solid electrolyte layer 4 at a position facing the second internal cavity 40 and spaced apart from the third diffusion rate controller 30.

The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as an NOx reduction catalyst that reduces NOx present in the atmosphere in the second internal cavity 40. The measurement electrode 44 is covered with a fourth diffusion rate control unit 45.

The fourth diffusion rate controlling section 45 is made of alumina (Al)₂O₃) A film comprising a porous body as a main component. The fourth diffusion rate control unit 45 plays a role of limiting the amount of NOx flowing into the measurement electrode 44, and also functions as a protective film for the measurement electrode 44.

The measurement pump unit 41 can suck out oxygen generated by decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44, and can detect the amount of generated oxygen as the pump current Ip 2.

In order to detect the oxygen partial pressure around the measurement electrode 44, the electrochemical sensor cell, i.e., the pump control oxygen partial pressure detection sensor cell 82 for measurement is configured to include the second solid electrolyte layer 6, the separation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42. The variable power supply 46 is controlled based on the electromotive force V2 detected by the measurement pump control oxygen partial pressure detection sensor unit 82.

The gas to be measured introduced into the second internal cavity 40 passes through the fourth diffusion rate controller 45 under the condition that the oxygen partial pressure is controlled, and reaches the measurement electrode 44. Nitrogen oxide in the measurement gas around the measurement electrode 44 is reduced (2NO → N)₂+O₂) Thereby generating oxygen. And the generated oxygen is measuredThe constant pump unit 41 performs pumping, and at this time, the voltage Vp2 of the variable power supply is controlled so that the control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor unit 82 becomes constant. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxide in the measurement gas, the concentration of nitrogen oxide in the measurement gas is calculated by the pump current Ip2 in the measurement pump cell 41.

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 constitute the oxygen partial pressure detection means as the electrochemical sensor cell, it is possible to detect an electromotive force corresponding to the difference between the concentration of the NOx component in the gas to be measured and the concentration of the NOx component in the gas to be measured: the difference between the amount of oxygen generated by reduction of the NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere.

The electrochemical sensor cell 83 includes the second solid electrolyte layer 6, the separator 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42, and is configured such that an electromotive force Vref can be obtained by the sensor cell 83, and the oxygen partial pressure in the measurement target gas outside the sensor can be detected by the electromotive force Vref.

In the gas sensor 100 having the above-described configuration, the measurement target gas whose oxygen partial pressure is constantly kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump unit 21 and the auxiliary pump unit 50 is supplied to the measurement pump unit 41. Therefore, the NOx concentration in the measurement target gas can be known based on the pump current Ip2 which is substantially proportional to the NOx concentration in the measurement target gas and which flows as a result of the oxygen generated by the reduction of NOx being sucked out by the measurement pump cell 41.

The sensor element 101 further includes a heater unit 70, and the heater unit 70 performs a temperature adjustment function of heating and holding the sensor element 101 so as to improve oxygen ion conductivity of the solid electrolyte. The heater section 70 includes a heater electrode 71, a heater 72, a through hole 73, a heater insulating layer 74, and a pressure release 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. The heater electrode 71 is connected to an external power supply to supply power from the outside to the heater section 70.

The heater 72 is a resistor body formed to be sandwiched vertically between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 through the through hole 73, and the heater electrode 71 is supplied with electricity from the outside to heat the heater 72, thereby heating and maintaining the temperature of the solid electrolyte forming the sensor element 101.

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

The heater insulating layer 74 is an insulating layer formed of an insulator such as alumina on the upper and lower surfaces of the heater 72. The heater insulating layer 74 is formed for the purpose of: 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 are achieved.

The pressure release 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 the purpose of forming the pressure release hole 75 is to: so that the increase in the internal pressure accompanying the temperature increase in the heater insulating layer 74 is alleviated.

[2 ] reason for covering the bottom electrode portion with the porous layer (porous body) ]

As described above, with the gas sensor 100, the bottom electrode portion 22b in the inner pump electrode 22 is covered with the porous layer 29. That is, the top electrode portion 22a of the inner pump electrode 22, which is distant from the heater portion 70, is not covered with the porous layer 29, and the bottom electrode portion 22b, which is close to the heater portion 70, is covered with the porous layer 29. The reason why the bottom electrode portion 22b is covered with the porous layer 29 will be described below.

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. Referring to fig. 2, it is assumed that the bottom electrode portion 22 of the inner pump electrode 22 of the gas sensor 100b are not covered by the porous layer 29. The inventors of the present invention found that: in this case, if such a gas sensor is used for a long time in a high-temperature region at a high oxygen concentration, platinum (Pt) of the inner pump electrode 22 is oxidized to PtO₂And the Au contained in the inner pump electrode 22 is evaporated. In particular, the inventors of the present invention found that: au evaporates more easily in the bottom electrode portion 22b of the inner pump electrode 22 near the heater portion 70 (fig. 1).

If the amount of Au contained in the inner pump electrode 22 is reduced, NOx is easily decomposed in the first internal cavity 20. If the amount of decomposed NOx in the first internal cavity 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 by continuous use, the measurement accuracy of the NOx concentration decreases (the degree of sensitivity change with respect to NOx increases).

In addition, Au evaporated from the inner pump electrode 22 may adhere to the measurement electrode 44. In order to measure the NOx concentration, it is necessary to reduce nitrogen oxides around the measurement electrode 44. If Au adheres to the measurement electrode 44, reduction of nitrogen oxide around the measurement electrode 44 is suppressed, and therefore, 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, evaporation of Au from the bottom electrode portion 22b is suppressed. Further, it is assumed that the possibility that Au evaporated in the bottom electrode portion 22b is captured by the porous layer 29 is increased. 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 can be suppressed.

[3. Structure of porous layer ]

Fig. 3 is an enlarged view around the first internal cavity 20 of the gas sensor 100. Referring to fig. 3, the porous layer 29 is formed on the entire upper surface of the bottom electrode portion 22 b. As described above, the porous layer 29 is a film made of a porous body containing alumina as a main component.

The maximum thickness a of the porous layer 29 is preferably 5 μm to 50 μm. The maximum thickness a means: the thickness of the portion of the porous layer 29 having the largest thickness.

The porosity B of the porous layer 29 is preferably 5% to 50%. The porosity B is obtained by applying a known image processing method (binarization processing or the like) to an SEM (scanning electron microscope) image of the object to be evaluated.

In this case, a/B is preferably 0.1 to 10.0, more preferably 0.5 to 10.0, and further preferably 5.0 to 10.0.

[4. characteristics ]

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, evaporation of Au in the bottom electrode portion 22b is suppressed in using the gas sensor 100. Further, it is assumed that the possibility that Au evaporated at the bottom electrode portion 22b is captured by the porous layer 29 is increased. As a result, according to the gas sensor 100, the amount of Au contained in the inner pump electrode 22 can be suppressed from decreasing, and the amount of Au adhering to the measurement electrode 44 can also be suppressed from decreasing, so that the measurement accuracy of the NOx concentration can be suppressed from decreasing.

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 cavity 20 is an example of the "first internal cavity" in the present invention, the main pump unit 21 is an example of the "first pump unit" in the present invention, and the heater portion 70 is an example of the "heat generating portion" in the present invention. 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 top 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 body" in the present invention.

The second internal cavity 40 is an example of the "second internal cavity" in the present invention, and the auxiliary pump unit 50 is an example of the "second pump unit" in the present invention. The auxiliary pump electrode 51 is an example of the "auxiliary pump electrode" in the present invention. The top 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 ]

Although the embodiments have been described above, the present invention is not limited to the above embodiments, and various modifications may be made without departing from the spirit thereof. 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 the first internal cavity 20 and the second internal cavity 40. That is, sensor element 101 is a dual cavity structure. However, sensor element 101 need not be a dual cavity structure. For example, the sensor element 101 may be a three-cavity structure.

Fig. 4 is a schematic cross-sectional view schematically showing an example of the structure of a gas sensor 100X including a sensor element 101X having a three-cavity structure. As shown in fig. 4, the second internal cavity 40 (fig. 1) may be further divided into dual cavities by a fifth diffusion rate controlling part 60 to form a second internal cavity 40X and a third internal cavity 61. In this case, the auxiliary pump electrode 51X may be disposed in the second internal cavity 40X, and the measurement electrode 44X may be disposed in the third internal cavity 61. In addition, in the case of forming the three-cavity structure, the fourth diffusion rate controlling portion 45 may be omitted.

(5－2)

In the gas sensor 100 according to the above embodiment, the porous layer 29 is formed only on the bottom electrode portion 22b, and the porous layer 29 is not formed on the top electrode portion 22 a. However, the formation position of the porous layer 29 is not limited thereto. For example, in addition to forming the porous layer 29 on the bottom electrode portion 22b, the porous layer 29 may be formed on the top electrode portion 22 a. Further, the porous layer 29 may be formed on the bottom electrode portion 51b, and the porous layer 29 may also be formed on the top electrode portion 51 a.

For example, according to the gas sensor in which the bottom electrode portion 51b is covered with the porous layer 29, evaporation of Au in the bottom electrode portion 51b is suppressed, and therefore, a 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 22 b. However, the porous layer 29 is not necessarily formed on the entire upper surface of the bottom electrode portion 22 b. For example, the porous layer 29 may be formed only on a part of the upper surface of the bottom electrode portion 22 b. In this case, at least one of the high-temperature portion and the portion with a high oxygen concentration in the upper surface of the bottom electrode portion 22b is preferably covered. That is, a portion of the upper surface of the bottom electrode portion 22b near the gas inlet 10 is preferably covered with the porous layer 29. For example, the bottom electrode portion 22b may be covered with the porous layer 29 in a region from the end portion near the gas introduction port 10 to a half of the area of the bottom electrode portion 22 b.

(5－4)

In the gas sensor 100 according to the above embodiment, the porous layer 29 contains alumina as a main component. However, the main component of the porous layer 29 is not necessarily 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)

Various gas sensors 100 of examples 1-24 were fabricated. Specifically, first, the sensor element 101 is produced by the method described below.

6 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 4 mol% of a stabilizer of yttria was added, an organic binder, and an organic solvent, and molding the mixture by tape casting. A plurality of sheet holes, required through holes, and the like are formed in advance in the green sheet for positioning in printing or stacking.

In addition, a space to be a gas flow portion is provided in advance in the green sheet to be the separator 5 by punching or the like. Then, pattern printing processing and drying processing for forming various patterns on the respective ceramic green sheets are performed in correspondence with the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the separator 5, and the second solid electrolyte layer 6, respectively.

Specifically, the patterns formed are those of the electrodes, the lead wires connected to the electrodes, the porous layer 29 (mainly containing alumina and a small amount of silica) formed on the bottom electrode portion 22b, the atmosphere introduction layer 48, the heater portion 70, and the like. Pattern printing is performed by applying a paste for pattern formation prepared in accordance with the characteristics required for each object to be formed on a green sheet by a known screen printing technique. For the drying treatment, a known drying method is also used. After the pattern printing and drying are completed, printing and drying treatment of a paste for bonding for laminating and bonding the green sheets corresponding to the respective layers is performed.

Then, the following crimping treatment was performed: the green sheets on which the adhesive paste is formed are positioned by sheet holes and stacked in a predetermined order, and are pressure-bonded under predetermined temperature and pressure conditions to form a single stacked body. The laminate thus obtained contains a plurality of sensor elements 101. The laminate is cut into a size of the sensor element 101. Then, the divided laminate is fired at a predetermined firing temperature, thereby obtaining the sensor element 101. The thus obtained sensor element 101 is assembled to obtain the gas sensor 100.

The gas sensors 100 of embodiments 1 to 24 differ only in that: the maximum thickness a and porosity B of the porous layer 29 formed on the bottom electrode portion 22B. The porosity B is adjusted by adjusting the amount of pore-forming agent added to the porous layer 29. The gas sensor of comparative example 1 is a gas sensor in which the porous layer 29 is omitted with respect to the gas sensors 100 of examples 1 to 24. The characteristics of examples 1 to 24 and comparative example 1 are shown in table 1 below.

TABLE 1

(6-2. evaluation test)

With respect to examples 1 to 24 and comparative example 1, durability tests using a diesel engine were performed, and changes in the sensitivity of each gas sensor with respect to NOx before and after the tests were evaluated. Specifically, the test was performed in the following manner. The gas sensors of examples 1 to 24 and comparative example 1 were mounted on pipes of exhaust pipes of automobiles. Then, the heater 72 is energized to reach a temperature of 800 ℃, and the sensor element 101 is heated. In this state, the 40-minute operation mode in the range of the engine speed of 1500-. The gas temperature at this time is set to 200 ℃ to 600 ℃ and the NOx concentration is set to 0 to 1500 ppm. The gas sensors before and after the durability test were mounted on a sample gas apparatus having a NOx concentration of 500ppm, and the sensitivity change rate of NOx was measured initially and after the durability test.

The determination result when the NOx sensitivity change rate is within ± 5% is "a", and the determination result when the NOx sensitivity change rate is greater than ± 5% and within ± 10% is "B". The determination result when the NOx sensitivity change rate is greater than ± 10% and within ± 15% is "C", and the determination result when the NOx sensitivity change rate is greater than ± 15% is "D".

As shown in Table 1, the results of the determinations in examples 12, 17 and 18 were "A", the results of the determinations in examples 2 to 11, 13 to 16, 23 and 24 were "B", and the results of the determinations in examples 1 and 19 to 22 were "C". On the other hand, the determination result of comparative example 1 is "D". This makes it possible to confirm that: by forming the porous layer 29 on the bottom electrode portion 22b, the sensitivity change rate of NOx of the gas sensor 100 can be suppressed.

Claims

1. A gas sensor configured to measure the concentration of a predetermined gas component in a gas to be measured,

the gas sensor is characterized in that it is,

the sensor device is provided with a sensor element,

the main component of the sensor element is a solid electrolyte having oxygen ion conductivity,

the sensor element is formed with a first internal cavity configured to introduce the gas to be measured from an external space,

the sensor element includes a first pump unit and a heat generating portion configured to generate heat,

the first pump unit includes:

an inner pump electrode formed in the first inner cavity and containing gold (Au); and

an outer pump electrode formed in a space different from the first inner cavity,

the first pump unit is configured to: drawing oxygen from within the first internal cavity by applying a voltage between the inner pump electrode and the outer pump electrode,

the inner pump electrode includes: a first electrode portion remote from the heat generating portion; and a second electrode portion adjacent to the heat generating portion,

at least a part of the second electrode portion is covered with a porous body.

2. The gas sensor according to claim 1,

the porous body is porous alumina.

3. Gas sensor according to claim 1 or 2,

when the maximum thickness of the porous body is A and the porosity of the porous body is B, A/B is 0.1-10.0.

4. The gas sensor according to claim 3,

the A/B is 0.5 to 10.0.

5. The gas sensor according to any one of claims 1 to 4,

the porosity of the porous body is 5% to 50%.

6. The gas sensor according to any one of claims 1 to 5,

the maximum thickness of the porous body is 5 [ mu ] m or more and 50 [ mu ] m or less.

7. The gas sensor according to any one of claims 1 to 6,

the sensor element is formed with a second internal cavity in communication with the first internal cavity,

the sensor element is further provided with a second pump unit,

the second pump unit includes:

an auxiliary pump electrode formed within the second internal cavity; and

the outer side of the pump electrode is provided with a pump electrode,

the second pump unit is configured to: drawing oxygen from within the second internal cavity by applying a voltage between the auxiliary pump electrode and the outer pump electrode,

the auxiliary pump electrode includes: a third electrode portion remote from the heat generating portion; and a fourth electrode portion adjacent to the heat generating portion,

at least a part of the fourth electrode portion is covered with a porous body.