CN113219037B

CN113219037B - Gas sensor

Info

Publication number: CN113219037B
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: 2024-04-05
Anticipated expiration: 2040-12-15
Also published as: JP2021124382A; JP7339896B2; CN113219037A

Abstract

The invention provides a gas sensor which is difficult to reduce the measurement accuracy of NOx concentration even under high oxygen concentration and long-time use in a high temperature area. The gas sensor is configured to measure the concentration of a predetermined gas component in a gas to be measured. The gas sensor has a sensor element. The sensor element has a solid electrolyte having oxygen ion conductivity as a main component. The sensor element is formed with a first internal cavity configured to introduce a measured gas from an 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 a concentration of a predetermined gas component in a gas to be measured.

Background

Japanese patent application laid-open 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 has 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 respect to the gas sensor, the oxygen concentration in the first internal cavity is regulated by a main pump unit including an inner pump electrode formed in the first internal cavity and an outer pump electrode formed outside the first internal cavity.

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

Prior art literature

Patent literature

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

Disclosure of Invention

Regarding the gas sensor disclosed in the above-mentioned patent document 1, gold (Au) is added to the inner pump electrode in order to suppress NOx decomposition in the first internal cavity. Since the reduction of the amount of NOx reaching the measurement electrode is suppressed 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 for a long time in a high temperature range at a high oxygen concentration, the accuracy of measuring the NOx concentration gradually decreases.

The present invention has been made in view of the above-described problems, and an object thereof is to provide a gas sensor in which the accuracy of measuring the NOx concentration is hardly lowered even when the gas sensor is used for a long period of time in a high-temperature range and at 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 has a sensor element. The sensor element has a solid electrolyte having oxygen ion conductivity as a main component. The sensor element is formed with a first internal cavity configured to introduce a measured gas from an external space. The sensor element includes a first pump unit and a heat generating portion. The heating part 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 within the first internal cavity and contains gold (Au). The outer pump electrode is formed in a different space than the first internal cavity. The first pump unit is configured to: oxygen in 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 part far from the heating part and a second electrode part near to the heating part. 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 at a high oxygen concentration and in a high temperature region, platinum (Pt) of the inner pump electrode is oxidized to PtO ₂ And the Au contained in the inner pump electrode evaporates. 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 measuring electrode decreases. That is, in the gas sensor described above, the accuracy of measuring the NOx concentration decreases (the degree of sensitivity change with respect to NOx increases.) as the amount of Au contained in the inner pump electrode decreases by continuous use.

In addition, 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 measurement electrode need to be reduced. If Au adheres to the measurement electrode, reduction of nitrogen oxides around the measurement electrode is suppressed, and thus, the accuracy of measuring the NOx concentration is lowered.

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

In addition, the inventors of the present invention found that: at the second electrode portion of the inner pump electrode near the heat generating portion, au is more easily evaporated. Therefore, for example, it is possible to suppress the sensitivity change with respect to NOx by not providing the second electrode portion. However, in this case, the area of the inner pump electrode is reduced, and therefore, in order to appropriately adjust the oxygen concentration in the first internal cavity, it is necessary to increase the voltage applied to the first pump unit. As a result, NOx is more likely to be decomposed in the first internal cavity, resulting in a decrease in the accuracy of measuring the NOx concentration.

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 from the second electrode portion is suppressed, so that a decrease in the accuracy of measuring the NOx concentration can be suppressed.

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, 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% to 50%.

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, the sensor element may be formed with a second internal cavity communicating with the first internal cavity, and the sensor element may further include a second pump unit including: an auxiliary pump electrode formed within the second interior cavity; and an outer pump electrode, the second pump unit being configured to: the auxiliary pump electrode is provided with: a third electrode portion remote from the heat generating portion; and a fourth electrode portion adjacent to the heat generating portion, at least a portion 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 from the fourth electrode portion is suppressed, so that a decrease in the accuracy of measuring the NOx concentration can be suppressed.

Effects of the invention

According to the present invention, it is possible to provide a gas sensor which is hardly degraded in accuracy of measuring the NOx concentration even when used in a high-temperature range at a high oxygen concentration for a long period of time.

Drawings

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

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

Fig. 3 is an enlarged view of the surrounding of the first interior 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-chamber structure.

Description of the reference numerals

1a first substrate layer, 2a second substrate layer, 3 a third substrate layer, 4 a first solid electrolyte layer, 5 a separator layer, 6 a second solid electrolyte layer, 10 a gas introduction port, 11 a first diffusion rate controlling portion, 12 a buffer space, 13 a second diffusion rate controlling portion, 20 a first internal cavity, 21 a main pump unit, 22 an inner pump electrode, 22a, 51aX top electrode portions, 22b, 51bX bottom electrode portions, 23 an outer pump electrode, 29 a porous layer, 30 a third diffusion rate controlling portion, 40X second internal cavities, 41 a measuring pump unit, 42 a reference electrode, 43 a reference gas introduction space, 44, 44X measuring electrodes, 45 fourth diffusion rate controlling section, 46, 52 variable power supply, 48 atmosphere introducing layer, 50 auxiliary pump unit, 51X auxiliary pump electrode, 60 fifth diffusion rate controlling section, 61 third internal cavity, 70 heater section, 71 heater electrode, 72 heater, 73 through hole, 74 heater insulating layer, 75 pressure releasing hole, 80 main pump control oxygen partial pressure detecting sensor unit, 81 auxiliary pump control oxygen partial pressure detecting sensor unit, 82 measuring pump control oxygen partial pressure detecting 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 accompanying drawings. In the drawings, the same or corresponding portions are denoted by the same reference numerals, and description thereof will not be repeated.

[1. Schematic Structure of gas sensor ]

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 drawing, zirconium oxide (ZrO-containing powder) is laminated in this order from the lower side ₂ ) A first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a separator layer 5, and a second solid electrolyte layer 6 of the plasma ion conductive solid electrolyte. In addition, the solid electrolyte forming the 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 a ceramic green sheet corresponding to each layer, laminating them, and firing them to integrate them.

At one end portion of the sensor element 101 and 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 speed control portion 11, a buffer space 12, a second diffusion speed control portion 13, a first internal cavity 20, a third diffusion speed control portion 30, and a second internal cavity 40 are formed adjacent to each other in a sequentially communicating manner.

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 dig out the separator 5, wherein an upper portion of the internal space is defined by a lower surface of the second solid electrolyte layer 6, a lower portion thereof is defined by an upper surface of the first solid electrolyte layer 4, and a side portion thereof is defined by a side surface 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 as 2 slits that are horizontally long (the direction perpendicular to the drawing forms the longitudinal direction of the opening). The portion from the gas inlet 10 to the second internal cavity 40 is also referred to as a gas flow portion.

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 farther from the distal end side than the gas flow portion, and at a position where the side portion is partitioned by the side surface of the first solid electrolyte layer 4. 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 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, 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 second internal cavity 40 can be measured by the reference electrode 42.

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

The first diffusion rate control section 11 is a portion that applies a predetermined diffusion resistance to the gas to be measured introduced 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 control unit 11 to the second diffusion rate control unit 13.

The second diffusion rate control section 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 that is rapidly introduced into the sensor element 101 from the gas introduction port 10 due to pressure fluctuation 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 the automobile) is not directly introduced into the first internal cavity 20, but is introduced into the first internal cavity 20 after the concentration fluctuation 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 variation of the gas to be measured introduced into the first internal space is almost negligible.

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

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

The inner pump electrode 22 is formed as: the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) forming the first internal cavity 20 and the spacer layer 5 constituting the side wall are formed across the partition. Specifically, a 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, a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 constituting the bottom surface, and side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the separator 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 are arranged in a tunnel-like structure at the arrangement position 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 ₂ Metal ceramic electrode of (c). Note that, with the measured gasThe inner pump electrode 22 in contact with the body is formed of a material capable of reducing the reducing ability of the NOx component in the gas to be measured.

In the main pump unit 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23, and a pump current Ip0 is caused to flow between the inner pump electrode 22 and the outer pump electrode 23 in the positive or negative direction, whereby 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.

A 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 ₂ O ₃ ) A membrane composed of a porous body as a main component. The reason why the bottom electrode portion 22b is covered with the porous layer 29 will be described in detail below.

In order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, the electrochemical sensor unit, that is, the main pump control oxygen partial pressure detection sensor unit 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 section 30 is as follows: the measured gas after the first internal cavity 20 has controlled the oxygen concentration (oxygen partial pressure) 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 measured gas.

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

In the second internal cavity 40, the oxygen partial pressure is further adjusted by the auxiliary pump unit 50 with respect to the gas to be measured, which is introduced through the third diffusion rate control section 30 after the oxygen concentration (oxygen partial pressure) is adjusted in the first internal cavity 20 in advance. Accordingly, the oxygen concentration in the second internal cavity 40 can be kept constant with high accuracy, and thus, the NOx concentration can be measured with high accuracy in the gas sensor 100.

The auxiliary pump unit 50 is an auxiliary electrochemical pump unit configured to include an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, as long as it is an appropriate electrode 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 in a substantially entire region facing the second internal cavity 40.

The auxiliary pump electrode 51 is disposed in the second internal cavity 40 in a tunnel-like structure similar to the inner pump electrode 22 previously disposed in the first internal cavity 20. That is, the top electrode portion 51a is formed with respect to the second solid electrolyte layer 6 constituting the top surface of the second internal cavity 40, and the bottom electrode portion 51b is formed in the first solid electrolyte layer 4 constituting the bottom surface of the second internal cavity 40, and the structure is such that side electrode portions (not shown) connecting the top electrode portion 51a and the bottom electrode portion 51b are formed on the two 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 capable of reducing the reducing ability of the NOx component in the measured gas, similarly to the inner pump electrode 22.

In the auxiliary pump unit 50, a desired voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23, whereby oxygen in the atmosphere in the second internal cavity 40 can be sucked into the external space or 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 unit, that is, the auxiliary pump control oxygen partial pressure detection sensor unit 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 using a 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 partial pressure of oxygen in the atmosphere within the second internal cavity 40 is controlled to a lower partial pressure that has substantially no effect on the NOx measurement.

At the same time, the pump current Ip1 is used to control the electromotive force of the main pump control oxygen partial pressure detection sensor unit 80. 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: so that the gradient of the oxygen partial pressure in the gas to be measured introduced from the third diffusion rate control section 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 kept at a constant value of about 0.001ppm by the action of the main pump unit 21 and the auxiliary pump unit 50.

The measurement pump unit 41 measures the NOx concentration in the measurement target gas in the second internal cavity 40. The measurement pump unit 41 is an electrochemical pump unit including a measurement electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, a separator 5, and a first solid electrolyte layer 4, and 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 separated from the third diffusion rate control section 30.

The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a 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 portion 45 is made of alumina (Al ₂ O ₃ ) A membrane composed of 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 plays a role of a protective film of the measurement electrode 44.

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

In order to detect the partial pressure of oxygen around the measurement electrode 44, the electrochemical sensor unit, that is, the measurement pump control oxygen partial pressure detection sensor unit 82 is configured to include the second solid electrolyte layer 6, the separator 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 control section 45 while the oxygen partial pressure is controlled, and reaches the measurement electrode 44. The nitrogen oxide in the gas to be measured around the measurement electrode 44 is reduced (2no→n ₂ +O ₂ ) And oxygen is generated. The generated oxygen is pumped by the measurement pump unit 41, 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 is constant. 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 concentration of nitrogen oxides in the gas to be measured is calculated by the pump current Ip2 in the measurement pump unit 41.

In addition, 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 an oxygen partial pressure detection mechanism as an electrochemical sensor unit, an electromotive force corresponding to a difference between the following values can be detected, and the concentration of the NOx component in the measured gas can be obtained: the difference between the amount of oxygen generated by the 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 unit 83 is configured to include 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 can obtain an electromotive force Vref by the sensor unit 83 and can detect the partial pressure of oxygen in the gas to be measured outside the sensor by the electromotive force Vref.

In the gas sensor 100 having the above-described configuration, the measured gas whose 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 unit 21 and the auxiliary pump unit 50 is supplied to the measurement pump unit 41. Therefore, the NOx concentration in the measurement gas can be obtained based on the pump current Ip2 that is approximately proportional to the NOx concentration in the measurement gas and that is caused to flow by sucking out oxygen generated by the reduction of NOx by the measurement pump unit 41.

The sensor element 101 further includes a heater portion 70, and the heater portion 70 plays a role of temperature adjustment for heating and maintaining the sensor element 101 so as to improve oxygen ion conductivity of the solid electrolyte. The heater portion 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 source, and can supply power to the heater portion 70 from the outside.

The heater 72 is a resistor formed so as to be sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 via the through hole 73, and the heater electrode 71 is supplied with power from the outside to heat the heater 72, thereby heating and insulating 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 the pressure release hole 75 is to: so that the increase in internal pressure accompanying the temperature increase in the heater insulating layer 74 is relaxed.

[2 ] reason why the bottom electrode portion is covered with a 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 occurring 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 22b of the inner pump electrode 22 of the gas sensor 100 is not covered with 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 at a high oxygen concentration and in a high temperature region, platinum (Pt) of the inner pump electrode 22 is oxidized to PtO ₂ And the Au contained in the inner pump electrode 22 evaporates. In particular, the inventors of the present invention found that: au is more easily evaporated 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, with such a gas sensor, the smaller the amount of Au contained in the inner pump electrode 22 is by continuous use, the lower the accuracy of measuring the NOx concentration (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, the nitrogen oxides around the measurement electrode 44 need to be reduced. If Au adheres to the measurement electrode 44, reduction of nitrogen oxides around the measurement electrode 44 is suppressed, and thus, the accuracy of measuring 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 of the bottom electrode portion 22b is suppressed. Further, it is assumed that the possibility that Au evaporated in the bottom electrode portion 22b is trapped by the porous layer 29 increases. 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 it is possible to suppress a decrease in the measurement accuracy of the NOx concentration.

[3 Structure of porous layer ]

Fig. 3 is an enlarged view of the surrounding of 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 or more and 50 μm or less. 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 evaluation target.

The maximum thickness of the porous layer 29 is a, and the porosity of the porous layer 29 is B, and in this case, a/B is preferably 0.1 to 10.0, more preferably 0.5 to 10.0, and even more 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, in the process of using the gas sensor 100, evaporation of Au in the bottom electrode portion 22b is suppressed. In addition, it is assumed that the possibility that Au evaporated at the bottom electrode portion 22b is trapped by the porous layer 29 increases. As a result, according to the gas sensor 100, the reduction in the amount of Au contained in the inner pump electrode 22 can be suppressed, and the amount of Au attached to the measurement electrode 44 can be suppressed, so that the reduction in the measurement accuracy of the NOx concentration can be suppressed.

The gas sensor 100 is an example of a "gas sensor" in the present invention, and the sensor element 101 is an example of a "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 "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 "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 can be made without departing from the gist thereof. The following describes modifications.

(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, the sensor element 101 is of a dual-cavity structure. However, the sensor element 101 need not be of a dual-cavity construction. For example, the sensor element 101 may be a three-chamber 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-chamber structure. As shown in fig. 4, the second internal cavity 40 (fig. 1) may be further divided into a double cavity by the fifth diffusion rate controlling portion 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-chamber structure, the fourth diffusion rate control section 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, the porous layer 29 may be formed on the top electrode portion 22a in addition to the porous layer 29 formed on the bottom electrode portion 22 b. The porous layer 29 may be formed on the bottom electrode portion 51b, or the porous layer 29 may be formed on the top electrode portion 51a.

For example, according to the gas sensor in which the bottom electrode portion 51b is covered with the porous layer 29, evaporation of Au at the bottom electrode portion 51b is suppressed, and therefore, a decrease in the accuracy of measuring 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, it is preferable that at least one of the high-temperature portion and the high-oxygen concentration portion in the upper surface of the bottom electrode portion 22b is covered. That is, the portion near the gas introduction port 10 on the upper surface of the bottom electrode portion 22b is preferably covered with the porous layer 29. For example, the region from the end portion near the gas introduction port 10 to the half of the area of the bottom electrode 22b in the bottom electrode 22b may be covered with the porous layer 29.

(5－4)

In the gas sensor 100 according to the above embodiment, the porous layer 29 has alumina as a main component. However, the porous layer 29 does not have to be alumina as a main component. The porous layer 29 may be made of spinel, zirconia, cordierite, titania, or the like as a main component.

[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 manufactured by the method described below.

6 green ceramic chips containing an oxygen ion conductive solid electrolyte such as zirconia as a ceramic component were prepared. The ceramic green sheets were molded by casting, in which zirconia particles to which 4mol% of stabilizer yttrium oxide was added, an organic binder, and an organic solvent were mixed. The green sheet is formed with a plurality of sheet holes, through holes, and the like as required for positioning at the time of printing and lamination.

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 processing or the like. Then, pattern printing treatment and drying treatment for forming various patterns on each ceramic green sheet are performed in correspondence with each of 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.

Specifically, the pattern to be formed is the pattern of each electrode, the lead wire connected to each electrode, the porous layer 29 (the main component is alumina and contains 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 pattern-forming paste prepared according to the characteristics required for each object to be formed to a green sheet by a known screen printing technique. For the drying treatment, a known drying method is also used. After pattern printing and drying are completed, a printing and drying process of an adhesive paste 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 and laminated in a predetermined order by using the sheet holes, and are pressure-bonded under predetermined temperature and pressure conditions, thereby forming a laminate. The stack thus obtained contains a plurality of sensor elements 101. The laminate is cut and divided into the 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 individual gas sensors 100 of embodiments 1-24 differ only in that: the maximum thickness a and the porosity B of the porous layer 29 formed on the bottom electrode portion 22B. The porosity B is adjusted by adjusting the amount of the 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 was omitted for 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)

For examples 1 to 24 and comparative example 1, endurance tests using a diesel engine were performed, and the sensitivity change of each gas sensor to NOx before and after the test was evaluated. Specifically, the test was performed as follows. The gas sensors of examples 1 to 24 and comparative example 1 were attached to a piping of an automobile tail gas pipe. Then, the heater 72 is energized to bring the temperature to 800 ℃, and the sensor element 101 is heated. In this state, a 40-minute operation mode in which the engine speed is 1500 to 3500rpm and the load torque is 0 to 350 N.m is repeated until 3000 hours elapses. The gas temperature at this time was set to 200℃to 600℃and the NOx concentration was set to 0 to 1500ppm. The gas sensors before and after the endurance test were mounted on a sample gas device having a NOx concentration of 500ppm, and the sensitivity change rates of NOx after the initial and endurance test were measured.

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

As shown in Table 1, the results of the determinations of examples 12, 17 and 18 were "A", the results of the determinations of examples 2 to 11, 13 to 16, 23 and 24 were "B", and the results of the determinations of examples 1 and 19 to 22 were "C". On the other hand, the result of the judgment in comparative example 1 was "D". This can 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 is configured to measure the concentration of a predetermined gas component in a gas to be measured,

the gas sensor is characterized in that,

the sensor element is provided with a sensor-element,

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

the sensor element is formed with a first internal cavity configured for introducing the measured gas 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 internal cavity and containing gold (Au); and

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

the first pump unit is configured to: the oxygen in 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 adjacent to the heat generating portion,

the region of the second electrode portion from the end portion on the gas introduction port side from which the gas to be measured is taken in from the external space to a portion reaching half of the area of the second electrode portion is covered with a porous body.

2. A gas sensor according to claim 1, wherein,

the porous body is porous alumina.

3. A gas sensor according to claim 1 or 2, wherein,

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

4. A gas sensor according to claim 3, wherein,

the A/B is 0.5 to 10.0 inclusive.

5. A gas sensor according to claim 1, wherein,

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

6. A gas sensor according to claim 1, wherein,

the maximum thickness of the porous body is 5-50 μm.

7. A gas sensor according to claim 1, wherein,

the sensor element is formed with a second internal cavity communicating 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 interior cavity; and

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

the second pump unit is configured to: the oxygen in the second internal cavity is sucked out 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; 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.