US-12618798-B2 - Gas sensor element and gas sensor

US12618798B2US 12618798 B2US12618798 B2US 12618798B2US-12618798-B2

Abstract

Provided is a gas sensor element or the like in which a diffusion mode of NO x reaching a measurement electrode is changed from molecular diffusion to a mode of diffusing while repeatedly colliding with a wall face of a sufficiently narrow flow path. In a gas sensor element according to one aspect of the present invention, a porous diffusion layer covering a measurement electrode has a porosity that is lower than the porosity of a leading end protection layer covering at least a face of an element substrate in which a gas inlet is open, and that is 5% or more and 25% or less.

Inventors

Yusuke Watanabe
Daichi Ichikawa

Assignees

NGK INSULATORS, LTD.

Dates

Publication Date: 20260505
Application Date: 20230718
Priority Date: 20220720

Claims (19)

1 . A gas sensor element comprising: an element substrate having a surface in which a gas inlet is open, and including an internal space into which a measurement target gas is introduced from the gas inlet; a leading end protection layer covering at least a face of the element substrate in which the gas inlet is open; a measurement electrode provided in the internal space and containing at least either silica or alumina; and a porous diffusion layer covering the measurement electrode and having a porosity that is 5% or more and 25% or less and is lower than a porosity of the leading end protection layer, wherein the porous diffusion layer has two faces in a thickness direction that are an internal face opposing the measurement electrode and an external face, and the internal face has a porosity higher than a porosity of the external face.
2 . The gas sensor element according to claim 1 , further comprising: a diffusion control portion configured to apply a predetermined diffusion resistance to the measurement target gas in the internal space, wherein the measurement electrode is disposed in an internal cavity that is demarcated by the diffusion control portion on an upstream side in a flow direction of the measurement target gas.
3 . The gas sensor element according to claim 1 , wherein a distance from an outermost face of the leading end protection layer to the gas inlet is 0.2 mm or more.
4 . The gas sensor element according to claim 1 , wherein the leading end protection layer includes at least: an internal leading end protection layer in contact with the face of the element substrate in which the gas inlet is open; and an external leading end protection layer constituting an outermost face of the leading end protection layer, wherein the internal leading end protection layer has a porosity larger than a porosity of the external leading end protection layer, and wherein the internal leading end protection layer has a thickness that is 30% or more and 90% or less of a thickness of the leading end protection layer.
5 . A gas sensor comprising the gas sensor element according to claim 1 and configured to measure an amount of a specific gas component in the measurement target gas.
6 . The gas sensor element according to claim 1 , wherein the measurement electrode and the porous diffusion layer are not in contact with each other, and wherein a distance between the measurement electrode and the porous diffusion layer is 0.15 mm or less.
7 . The gas sensor element according to claim 1 , wherein the porous diffusion layer has two faces in a thickness direction that are an internal face opposing the measurement electrode and an external face, and the internal face has a porosity that is at least 10% higher than a porosity of the external face.
8 . A gas sensor element comprising: an element substrate having a surface in which a gas inlet is open, and including an internal space into which a measurement target gas is introduced from the gas inlet; a leading end protection layer covering at least a face of the element substrate in which the gas inlet is open; a measurement electrode provided in the internal space and containing at least either silica or alumina; and a porous diffusion layer covering the measurement electrode and having a porosity that is 5% or more and 25% or less and is lower than a porosity of the leading end protection layer, wherein the measurement electrode and the porous diffusion layer are not in contact with each other, and wherein a distance between the measurement electrode and the porous diffusion layer is 0.15 mm or less.
9 . The gas sensor element according to claim 8 , further comprising: a diffusion control portion configured to apply a predetermined diffusion resistance to the measurement target gas in the internal space, wherein the measurement electrode is disposed in an internal cavity that is demarcated by the diffusion control portion on an upstream side in a flow direction of the measurement target gas.
10 . The gas sensor element according to claim 8 , wherein a distance from an outermost face of the leading end protection layer to the gas inlet is 0.2 mm or more.
11 . The gas sensor element according to claim 8 , wherein the leading end protection layer includes at least: an internal leading end protection layer in contact with the face of the element substrate in which the gas inlet is open; and an external leading end protection layer constituting an outermost face of the leading end protection layer, wherein the internal leading end protection layer has a porosity larger than a porosity of the external leading end protection layer, and wherein the internal leading end protection layer has a thickness that is 30% or more and 90% or less of a thickness of the leading end protection layer.
12 . A gas sensor comprising the gas sensor element according to claim 8 and configured to measure an amount of a specific gas component in the measurement target gas.
13 . The gas sensor element according to claim 8 , wherein the porous diffusion layer has two faces in a thickness direction that are an internal face opposing the measurement electrode and an external face, and the internal face has a porosity that is at least 10% higher than a porosity of the external face.
14 . A gas sensor element comprising: an element substrate having a surface in which a gas inlet is open, and including an internal space into which a measurement target gas is introduced from the gas inlet; a leading end protection layer covering at least a face of the element substrate in which the gas inlet is open; a measurement electrode provided in the internal space and containing at least either silica or alumina; and a porous diffusion layer covering the measurement electrode and having a porosity that is 5% or more and 25% or less and is lower than a porosity of the leading end protection layer, wherein the porous diffusion layer has two faces in a thickness direction that are an internal face opposing the measurement electrode and an external face, and the internal face has a porosity that is at least 10% higher than a porosity of the external face.
15 . The gas sensor element according to claim 14 , further comprising: a diffusion control portion configured to apply a predetermined diffusion resistance to the measurement target gas in the internal space, wherein the measurement electrode is disposed in an internal cavity that is demarcated by the diffusion control portion on an upstream side in a flow direction of the measurement target gas.
16 . The gas sensor element according to claim 14 , wherein the measurement electrode and the porous diffusion layer are not in contact with each other, and wherein a distance between the measurement electrode and the porous diffusion layer is 0.15 mm or less.
17 . The gas sensor element according to claim 14 , wherein a distance from an outermost face of the leading end protection layer to the gas inlet is 0.2 mm or more.
18 . The gas sensor element according to claim 14 , wherein the leading end protection layer includes at least: an internal leading end protection layer in contact with the face of the element substrate in which the gas inlet is open; and an external leading end protection layer constituting an outermost face of the leading end protection layer, wherein the internal leading end protection layer has a porosity larger than a porosity of the external leading end protection layer, and wherein the internal leading end protection layer has a thickness that is 30% or more and 90% or less of a thickness of the leading end protection layer.
19 . A gas sensor comprising the gas sensor element according to claim 14 and configured to measure an amount of a specific gas component in the measurement target gas.

Description

CROSS-REFERENCE TO RELATED APPLICATION The present application claims priority from Japanese application JP 2022-115399, filed on Jul. 20, 2022, and JP 2023-094953, filed on Jun. 8, 2023, the contents of which are hereby incorporated by reference into this application. TECHNICAL FIELD The invention relates to a gas sensor element and a gas sensor. BACKGROUND ART Various attempts at applying predetermined diffusion resistance to a measurement target gas introduced into an internal space in a gas sensor element, which is used to measure the concentration of a specific gas component in the measurement target gas are conventionally known. For example, JP 2011-102793A discloses a gas sensor element that includes a diffusion control portion that applies predetermined diffusion resistance to the measurement target gas introduced into the internal space. JP 2011-102793A is an example of related art. SUMMARY OF THE INVENTION The inventors found the following problem with the conventional gas sensor element that includes a diffusion control portion such as the aforementioned one. Specifically, the concentration of H2O in exhaust gases is higher in gasoline vehicles than in diesel vehicles. In addition, hydrogen engine vehicles are expected to be used under highly lean conditions for environmental reasons, and the concentration of H2O in exhaust gases is also expected to be high. H2O has a smaller molecular weight than NOx and O2. The inventors found that the following problem will occur in such an environment with high H2O concentration. FIG. 11 shows an example of molecular diffusion in which one molecule is diffused in response to collision with another molecule. The inventors conceived that the following event will occur in a region where molecular diffusion of such a type as that illustrated in FIG. 11 is dominant. That is, since diffusion of molecules proceeds as a result of one molecule colliding with another molecule in molecular diffusion, as illustrated in FIG. 11, the diffusion coefficient changes due to the other molecule with which one molecule collides, i.e. the diffusion coefficient changes depending on the gas composition of a measurement target gas. Thus, the presence of H2O, which has a smaller molecular weight, in the measurement target gas allows NOx and O2 molecules to diffuse easily between H2O molecules, and it is conceivable that the amount of NOx and O2 gases reaching a measurement electrode for measuring the concentration of a specific gas component in the measurement target gas will increase. The inventors conceived that, consequently, NOx output may vary and the measurement electrode may be more susceptible to deterioration depending on the H2O concentration (e.g. as the H2O concentration increases). The inventors confirmed through experiments that NOx output is more likely to vary and the deterioration of the measurement electrode is accelerated at higher H2O concentration than at lower concentration. FIG. 12 shows an example of Knudsen diffusion, which is a diffusion mode different from molecular diffusion. In Knudsen diffusion, diffusion of a certain molecule is promoted as a result of the molecule colliding with a porous wall (a wall face of a flow path), as illustrated in FIG. 12. Since the pore size in the wall face is determined during burning, the diffusion coefficient does not change even if the gas composition in the measurement target gas changes. The inventors then found the following method useful as a solution to the aforementioned problem that is considered to be caused by molecular diffusion of NOx under high H2O concentration. That is, the inventors found it useful to adopt a method of changing the diffusion mode of NOx that reaches the measurement electrode from molecular diffusion to a mode of diffusing while repeatedly colliding with a wall face of a sufficiently narrow channel, as in Knudsen diffusion illustrated in FIG. 12. In one aspect, the present invention has been made in view of these circumstances, and an object of the invention is to provide a gas sensor element or the like in which the diffusion mode of NOx that reaches the measurement electrode is changed from molecular diffusion to a mode of diffusing while repeatedly colliding with a wall face of a sufficiently narrow flow path. The present invention adopts the following configurations in order to solve the aforementioned problem. A gas sensor element according to a first aspect includes: an element substrate having a surface in which a gas inlet is open, and including an internal space into which a measurement target gas is introduced from the gas inlet; a leading end protection layer covering at least a face of the element substrate in which the gas inlet is open; a measurement electrode provided in the internal space and containing at least either silica or alumina; and a porous diffusion layer covering the measurement electrode and having a porosity that is 5% or more and 25% or less and is lower than a