Sensor element

Abstract

To provide a sensor element with which it is possible to maintain a high NOx detection accuracy, irrespective of oxygen concentration in the gas being measured.SOLUTION: In a sensor element 101, inside oxygen pump electrodes 22, 51 having a prescribed length (L) in the longitudinal direction include a region (A) that includes an electrode edge on the side close to a gas introduction port 10 and has a prescribed length (LA) in the longitudinal direction and a region (B) that includes an electrode edge on the side far from the gas introduction port 10 and has a prescribed length (LB) in the longitudinal direction. A metal material included in the inside oxygen pump electrodes 22, 51 includes an activity reducing metal that reduces catalytic activity to decompose NOx, and the percentage content of the activity reducing metal in the metal material in the region (A) is higher than that in the region (B). The ratio (LA / L) of the longitudinal length (LA) of the region (A) to the longitudinal length (L) of the inside oxygen pump electrodes 22, 51 is 15% to 90%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sensor element using an oxygen ion conductive solid electrolyte. [Background technology]

[0002] Gas sensors are used to detect and measure the concentration of target gas components (oxygen O2, nitrogen oxides NOx, ammonia NH3, hydrocarbons HC, carbon dioxide CO2, etc.) in gases to be measured, such as automobile exhaust gases. For example, the concentration of target gas components in automobile exhaust gases is measured, and the exhaust gas purification system installed in the automobile is optimally controlled based on the measured value.

[0003] Known examples of such gas sensors include a gas sensor element that uses an oxygen ion conductive solid electrolyte such as zirconia (ZrO2).The gas sensor uses the oxygen ion conductivity of the solid electrolyte to detect an electromotive force or a current value corresponding to the concentration of a target gas component in a measured gas, thereby detecting the gas component and measuring its concentration.

[0004] For example, Japanese Patent No. 3050781 discloses a gas sensor that uses a first electrochemical pump cell and a second electrochemical pump cell to control the oxygen partial pressure to a low level that does not substantially affect the measurement of the amount of a gas component to be measured, and detects a current value corresponding to the oxygen generated by reduction or decomposition of the gas component to be measured. In other words, oxygen is removed in advance by the first electrochemical pump cell and the second electrochemical pump cell, and oxygen derived from the target gas component (e.g., nitrogen oxide NOx) is detected.

[0005] Furthermore, Japanese Patent No. 3050781 discloses that there is a linear relationship between the concentration of nitrogen oxides (NOx) and the detected current value (FIG. 5).

[0006] Japanese Patent Application Laid-Open Nos. 2014-209128 and 2014-190940 disclose NOx sensors that have a main pump cell and an auxiliary pump cell for adjusting the oxygen concentration, and disclose that the main pump cell has an inner pump electrode that is, for example, a cermet electrode made of Pt containing 1% Au and zirconia. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 3050781 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-209128 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-190940 Summary of the Invention [Problem to be solved by the invention]

[0008] In conventional gas sensors, as disclosed in, for example, Japanese Patent Application Laid-Open No. 2014-209128, a measurement gas is introduced into the space inside the sensor element through a gas inlet at one longitudinal end of the sensor element. The main pump cell and auxiliary pump cell then control the oxygen partial pressure in the measurement gas to a low level that does not substantially affect the measurement of the target gas (e.g., NOx) at the measurement electrode. In this state, the measurement pump cell detects oxygen generated by decomposition of NOx as a current value. That is, oxygen and NOx in the measurement gas are separated, and then the oxygen generated from NOx is detected.

[0009] In such gas sensors, the main pump cell and the auxiliary pump cell are required not to decompose NOx. Therefore, the pump electrodes disposed on the inner surfaces of the space inside the sensor element and constituting one electrode of the main pump cell and the auxiliary pump cell are made of a material that does not decompose NOx. A metal material in which Au is added to Pt is used as a material that does not decompose NOx (Japanese Patent Laid-Open No. 2014-209128 and Japanese Patent Laid-Open No. 2014-190940).

[0010] However, it has been found that when a high concentration of oxygen is present in the gas to be measured, NOx is decomposed at the pump electrode that constitutes the main pump cell, which can result in a decrease in the accuracy of NOx detection.

[0011] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a sensor element that can maintain high accuracy in detecting NOx regardless of the oxygen concentration in the gas to be measured. [Means for solving the problem]

[0012] The inventors of the present invention have thoroughly investigated the mechanism by which NOx detection accuracy decreases under high oxygen concentrations and have concluded the following. When a high concentration of oxygen is present in the measurement gas introduced through the gas inlet, most of the high concentration oxygen must be discharged from the internal space of the sensor element by the main pump cell. In particular, a high voltage is applied locally to the pump electrode near the gas inlet to pump out the high concentration of oxygen. When a high voltage is applied locally, NOx in the measurement gas may be decomposed in the portion of the pump electrode to which the high voltage is applied. This reduces the amount of NOx that reaches the measurement electrode that detects NOx. As a result, the NOx detection accuracy decreases.

[0013] As described above, in a gas sensor for detecting NOx in a measurement gas, an oxygen pump cell (e.g., composed of a main pump cell and an auxiliary pump cell) adjusts the oxygen partial pressure in the measurement gas introduced into the internal space of the sensor element from a gas inlet. Then, the measurement pump cell detects NOx in the measurement gas whose oxygen partial pressure has been adjusted.

[0014] In such a gas sensor, it has been found that the inner oxygen pump electrode, which is an electrode constituting the oxygen pump cell and comes into contact with the measurement gas introduced into the internal space of the sensor element, needs to be positioned closer to the gas inlet of the sensor element, particularly to further suppress the decomposition of NOx.

[0015] The inventors have discovered that by configuring the inner oxygen pump electrode so that a predetermined region closer to the gas inlet of the sensor element contains more activity-reducing metal that reduces the catalytic activity of decomposing NOx than a region farther from the gas inlet, high NOx detection accuracy can be maintained even when a high concentration of oxygen is present in the measured gas.

[0016] The present invention includes the following inventions. (1) a long, plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from a gas inlet formed at one end of the base section in the longitudinal direction and for flowing the measurement gas; an inner oxygen pump electrode disposed on an inner surface of the measurement gas flow portion; a measuring electrode disposed on an inner surface of the measurement target gas flow portion; A sensor element comprising: The inner oxygen pump electrode has a predetermined length (L) in the longitudinal direction, The electrode end portion close to the gas inlet and having a predetermined length (L A ) and a region (A) having The electrode end portion farther from the gas inlet and having a predetermined length (L B ) and region (B) having Including, the metal material contained in the inner oxygen pump electrode contains an activity-reducing metal that reduces catalytic activity for decomposing NOx, the content of the activity-reducing metal in the metal material in the region (A) is higher than the content of the activity-reducing metal in the metal material in the region (B); The ratio of the length (L) of the inner oxygen pump electrode to the length (L) of the inner oxygen pump electrode in the longitudinal direction of the region (A) A ) ratio (L A / L) is 15% to 90%.

[0017] (2) The inner oxygen pump electrode includes a plurality of electrodes disposed on the inner surface of the measurement gas flow portion, The sensor element according to (1) above, wherein the length (L) of the inner oxygen pump electrode in the longitudinal direction is the sum of the lengths of the plurality of electrodes in the longitudinal direction.

[0018] (3) The inner oxygen pump electrode is an inner main pump electrode disposed on an inner surface of the measurement gas flow portion; an auxiliary pump electrode disposed on the inner surface of the measurement gas flow portion at a position farther from the gas inlet than the inner main pump electrode; Including, The sensor element described in (1) or (2) above, wherein the longitudinal length (L) of the inner oxygen pump electrode is the sum (L1+L2) of the longitudinal length (L1) of the inner main pump electrode and the longitudinal length (L2) of the auxiliary pump electrode.

[0019] (4) The sensor element according to (3), wherein the auxiliary pump electrode and the measurement electrode are disposed in series in this order in the longitudinal direction at a position on the inner surface of the measurement gas flow portion farther from the gas inlet than the inner main pump electrode.

[0020] (5) The sensor element according to (3), wherein the auxiliary pump electrode and the measurement electrode are arranged in parallel in the longitudinal direction at a position on the inner surface of the measurement gas flow portion farther from the gas inlet than the inner main pump electrode.

[0021] (6) The ratio of the length (L) of the inner oxygen pump electrode to the length (L) of the inner oxygen pump electrode in the longitudinal direction A ) ratio (L A The sensor element according to any one of (1) to (5) above, wherein the value of the surface tension is 30% to 70%.

[0022] (7) The sensor element according to any one of (1) to (6) above, wherein the activity-reducing metal includes at least one selected from the group consisting of gold and silver.

[0023] (8) The sensor element according to any one of (1) to (7) above, wherein the content of the activity-reducing metal in the metal material in the region (A) of the inner oxygen pump electrode is 0.5% by weight to 2.0% by weight.

[0024] (9) The sensor element according to any one of (1) to (8) above, wherein the content of the activity-reducing metal in the metal material in the region (B) of the inner oxygen pump electrode is 0.1 wt % to 0.5 wt %, provided that the content is lower than the content of the activity-reducing metal in the metal material in the region (A).

[0025] (10) The content (C B ) the content (C A ) ratio (C A / C B ) is 1.5 or more and 20.0 or less.

[0026] (11) A gas sensor for detecting NOx in a gas to be measured, comprising the sensor element according to any one of (1) to (10) above. [Effects of the Invention]

[0027] According to the present invention, even when a high concentration of oxygen is present in the measurement gas, the decomposition of NOx at the inner oxygen pump electrode (e.g., the inner main pump electrode) can be significantly suppressed, thereby maintaining high NOx detection accuracy. In other words, high NOx detection accuracy can be maintained regardless of the oxygen concentration in the measurement gas. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a schematic vertical cross-sectional view of a sensor element 101 in the longitudinal direction, showing an example of the schematic configuration of a gas sensor 100. FIG. [Figure 2] 2 is a schematic cross-sectional view showing a part of a cross section taken along line II-II in Fig. 1. It is a schematic view showing a rough planar arrangement of the inner main pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44 in the sensor element 101. L1 represents the length of the inner main pump electrode 22 in the longitudinal direction of the sensor element 101, and L2 represents the length of the auxiliary pump electrode 51 in the longitudinal direction of the sensor element 101. The lower part of Fig. 2 is an image diagram of the oxygen concentration distribution in the longitudinal direction of the sensor element 101 when a measurement gas containing a high concentration of oxygen is introduced into the measurement gas flow portion 15. [Figure 3] FIG. 1 is a schematic diagram showing the relationship between the oxygen concentration and the NOx output current value Ip2 in the presence of oxygen (O2=0, 5, 10, 18%). [Figure 4] 4 is a schematic cross-sectional view showing a portion of a vertical cross section in the longitudinal direction of a sensor element 201 of an embodiment. It is a schematic diagram showing a rough arrangement of an inner main pump electrode 22 and a measurement electrode 44 in the sensor element 201. L1 represents the length of the inner main pump electrode 22 in the longitudinal direction of the sensor element 201. The lower part of FIG. 4 is an image diagram of the oxygen concentration distribution in the longitudinal direction of the sensor element 201 when a measurement gas containing a high concentration of oxygen is introduced into the measurement gas flow section. [Figure 5]3 is a cross-sectional view showing a part of a vertical cross section in the longitudinal direction of a sensor element 301 according to an embodiment of the present invention. FIG. [Figure 6] 6 is a schematic cross-sectional view showing a part of a cross section taken along line VI-VI in Fig. 5. It is a schematic view showing a rough planar arrangement of the inner main pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44 in the sensor element 301. L1 represents the length of the inner main pump electrode 22 in the longitudinal direction of the sensor element 301, and L2 represents the length of the auxiliary pump electrode 51 in the longitudinal direction of the sensor element 301. LM represents the length of the measurement electrode 44 in the longitudinal direction of the sensor element 301. The lower part of Fig. 6 is an image diagram of the oxygen concentration distribution in the longitudinal direction of the sensor element 301 when a measurement gas containing a high concentration of oxygen is introduced into the measurement gas flow section. [Figure 7] 6 of a modified sensor element 401. It is a schematic diagram showing a general planar arrangement of the inner main pump electrode 22, the auxiliary pump electrode 51, the second auxiliary pump electrode 53, and the measurement electrode 44 in the sensor element 401. L1 represents the length of the inner main pump electrode 22 in the longitudinal direction of the sensor element 401, L2 represents the length of the auxiliary pump electrode 51 in the longitudinal direction of the sensor element 401, and L3 represents the length of the second auxiliary pump electrode 53 in the longitudinal direction of the sensor element 401. [Figure 8] 1 is a graph showing the results of durability tests for Examples 1 to 9 and Comparative Examples 1 and 2. The vertical axis of the graph represents the rate of change in NOx sensitivity (%), and the horizontal axis represents the durability test time (hours: H). [Figure 9] 1 is a graph showing the results of durability tests for Examples 10 to 16 and Comparative Examples 1 and 2. The vertical axis of the graph represents the rate of change in NOx sensitivity (%), and the horizontal axis represents the durability test time (hours: H). [Figure 10] 1 is a graph showing the results of the durability test of Examples 17 to 21. The vertical axis of the graph represents the rate of change in NOx sensitivity (%), and the horizontal axis represents the durability test time (hours: H). [Figure 11] 1 is a graph showing the results of the durability test of Examples 22 to 26. The vertical axis of the graph represents the rate of change in NOx sensitivity (%), and the horizontal axis represents the durability test time (hours: H). DETAILED DESCRIPTION OF THE INVENTION

[0029] The sensor element of the present invention comprises: a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from a gas inlet formed at one end of the base section in the longitudinal direction and for flowing the measurement gas; an inner oxygen pump electrode disposed on an inner surface of the measurement gas flow portion; a measuring electrode disposed on an inner surface of the measurement target gas flow portion; A sensor element comprising: The inner oxygen pump electrode has a predetermined length (L) in the longitudinal direction, The electrode end portion close to the gas inlet and having a predetermined length (L A ) and a region (A) having The electrode end portion farther from the gas inlet and having a predetermined length (L B ) and region (B) having Including, the metal material contained in the inner oxygen pump electrode includes platinum and an activity-reducing metal that reduces catalytic activity for decomposing NOx, the content of the activity-reducing metal in the metal material in the region (A) is higher than the content of the activity-reducing metal in the metal material in the region (B); The ratio of the length (L) of the inner oxygen pump electrode to the length (L) of the inner oxygen pump electrode in the longitudinal direction of the region (A) A ) ratio (L A / L) is 15% to 90%.

[0030] At least a portion of the inner oxygen pump electrode is disposed closer to the one end in the longitudinal direction of the base portion than the measurement electrode.

[0031] By using a gas sensor including the sensor element of the present invention, NOx in a gas to be measured can be detected.

[0032] [Outline of gas sensor configuration] The sensor element of the present invention will be described below with reference to the drawings. Fig. 1 is a schematic vertical cross-sectional view in the longitudinal direction showing an example of the general configuration of a gas sensor 100 including a sensor element 101. In the following, with Fig. 1 as the reference, the upper side of Fig. 1 will be referred to as the top, the lower side as the bottom, the left side of Fig. 1 as the leading end side, and the right side as the rear end side.

[0033] In the embodiment of FIG. 1, the gas sensor 100 is an example of a limiting current type NOx sensor that detects NOx in a measurement gas by a sensor element 101 and measures its concentration.

[0034] The sensor element 101 is a long, plate-like element including a base portion 102 having a structure in which multiple oxygen-ion conductive solid electrolyte layers are stacked. The long, plate-like shape is also referred to as a long plate shape or a strip shape. The base portion 102 has a structure in which six layers are stacked in this order from bottom to top as viewed in the drawing: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is made of an oxygen-ion conductive solid electrolyte layer such as zirconia (ZrO). The solid electrolyte forming these six layers is dense and airtight. The six layers may all have the same thickness or may have different thicknesses. The layers are bonded together via adhesive layers made of solid electrolyte, and the base portion 102 includes the adhesive layers. While FIG. 1 illustrates a layer structure consisting of six layers, the layer structure of the present invention is not limited to this and any number and layer structure may be used.

[0035] The sensor element 101 is manufactured by, for example, laminating ceramic green sheets corresponding to each layer after performing predetermined processing and printing a circuit pattern on them, and then firing the sheets to integrate them.

[0036] A gas inlet 10 is formed at one longitudinal end (hereinafter referred to as the tip) 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. The measurement gas flow section 15 is formed in a manner that, in the longitudinal direction from the gas inlet 10, a first diffusion-controlling section 11, a buffer space 12, a second diffusion-controlling section 13, a first internal space 20, a third diffusion-controlling section 30, a second internal space 40, a fourth diffusion-controlling section 60, and a third internal space 61 are communicated with each other in this order.

[0037] The gas inlet 10, the buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 are spaces inside the sensor element 101, which are defined by hollowing out the spacer layer 5, with an upper portion defined by the underside of the second solid electrolyte layer 6, a lower portion defined by the upper surface of the first solid electrolyte layer 4, and sides defined by the side surfaces of the spacer layer 5.

[0038] 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 two horizontally elongated slits (the openings have their longitudinal direction perpendicular to the plane of the drawing in FIG. 1). The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, and the third diffusion rate-controlling section 30 may each have any shape that provides the desired diffusion resistance, and the shape is not limited to the slits.

[0039] The fourth diffusion rate-controlling portion 60 is provided as a single horizontally elongated slit (the opening has its longitudinal direction perpendicular to the plane of the drawing in FIG. 1 ) between the spacer layer 5 and the second solid electrolyte layer 6. The fourth diffusion rate-controlling portion 60 may have any shape that provides a desired diffusion resistance, and the shape is not limited to the slit.

[0040] Furthermore, a reference gas introduction space 43 is provided at a position farther from the tip side than the measurement gas flow section 15, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and at a position defined at its side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0041] The air introduction layer 48 is a layer made of porous alumina, and a reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is also formed so as to cover the reference electrode 42.

[0042] Reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of third substrate layer 3 and first solid electrolyte layer 4, and as described above, is surrounded by air introduction layer 48 that connects to reference gas introduction space 43. That is, reference electrode 42 is disposed so as to come into contact with the reference gas via porous air introduction layer 48 and reference gas introduction space 43. Furthermore, as will be described later, reference electrode 42 can be used to measure the oxygen concentrations (oxygen partial pressures) in first internal space 20, second internal space 40, and third internal space 61.

[0043] In the measurement gas flow section 15, the gas inlet 10 is a portion that opens to the outside space, and the measurement gas is introduced into the sensor element 101 from the outside space through the gas inlet 10.

[0044] In this embodiment, the measurement gas flow section 15 is configured such that the measurement gas is introduced through the gas inlet 10 opening at the tip end surface of the sensor element 101, but the present invention is not limited to this configuration. For example, the measurement gas flow section 15 does not need to have a recess for the gas inlet 10. In this case, the first diffusion rate-controlling section 11 essentially serves as the gas inlet. Furthermore, for example, the measurement gas flow part 15 may have an opening in a side surface along the longitudinal direction of the base part 102, the opening communicating with the buffer space 12 or a position in the first internal space 20 near the buffer space 12. In this case, the measurement gas is introduced from the side surface along the longitudinal direction of the base part 102 through the opening. Furthermore, for example, the measurement gas flow portion 15 may be configured so that the measurement gas is introduced through a porous body.

[0045] The first diffusion rate-controlling part 11 is a part that applies a predetermined diffusion resistance to the measurement gas taken in through the gas inlet 10.

[0046] The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion rate-controlling part 11 to the second diffusion rate-controlling part 13 .

[0047] The second diffusion rate-controlling portion 13 is a portion that applies a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20 .

[0048] It is sufficient that the amount of the measurement gas introduced into the first internal space 20 is within a predetermined range. That is, it is sufficient that a predetermined diffusion resistance is imparted to the entire area from the tip of the sensor element 101 to the second diffusion-controlling section 13. For example, it is also possible that the first diffusion-controlling section 11 directly communicates with the first internal space 20, that is, the buffer space 12 and the second diffusion-controlling section 13 do not exist.

[0049] The buffer space 12 is a space provided to mitigate the influence of pressure fluctuations on the detected value when the pressure of the gas to be measured fluctuates.

[0050] When the measurement gas is introduced from the outside of the sensor element 101 into the first internal space 20, the measurement gas is suddenly taken into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations if the measurement gas is automobile exhaust gas), but is not introduced directly into the first internal space 20, but is introduced into the first internal space 20 after the pressure fluctuations of the measurement gas are canceled out through the first diffusion rate-controlling section 11, buffer space 12, and second diffusion rate-controlling section 13. As a result, the pressure fluctuations of the measurement gas introduced into the first internal space 20 become almost negligible.

[0051] Fig. 2 is a schematic cross-sectional view showing a part of a cross section taken along line II-II in Fig. 1. Referring to Figs. 1 and 2, the inner oxygen pump electrode 90 is an electrode disposed on the inner surface of the measurement gas flow portion 15 and having a predetermined length (L) in the longitudinal direction of the sensor element 101. The inner oxygen pump electrode 90 is in contact with the measurement gas introduced into the measurement gas flow portion 15 and contributes to adjusting the oxygen concentration (oxygen partial pressure) in the measurement gas to a value that does not substantially affect the NOx measurement by the measuring electrode 44 described below.

[0052] In the sensor element 101 of this embodiment, the inner oxygen pump electrode 90 includes the inner main pump electrode 22 and the auxiliary pump electrode 51 .

[0053] That is, in the sensor element 101 of this embodiment, the inner oxygen pump electrode 90 is divided into the inner main pump electrode 22 and the auxiliary pump electrode 51 .

[0054] At least a portion of the inner oxygen pump electrode 90 is disposed at a position closer to the tip of the base portion 102 than the measurement electrode 44. In the sensor element 101 of this embodiment, the inner main pump electrode 22 and the auxiliary pump electrode 51 are both disposed at positions closer to the tip of the base portion 102 than the measurement electrode 44. As in Modification 2 described below, the inner main pump electrode 22 may be disposed at a position closer to the tip of the base portion 102 than the measurement electrode 44, and the auxiliary pump electrode 51 may be disposed in parallel to the measurement electrode 44 in the longitudinal direction of the base portion 102.

[0055] The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the second diffusion-controlling part 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

[0056] The main pump cell 21 is an inner main pump electrode 22 disposed on the inner surface of the measurement gas flow portion 15; The inner main pump electrode 22 and the outer pump electrode 23 are disposed on the outer surface of the base portion 102 via a second solid electrolyte layer 6. An electrochemical pump cell comprising:

[0057] That is, the main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22 having a ceiling electrode portion 22a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22a so as to be exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0058] The inner main pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20 and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that configure both side wall portions of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a tunnel-like structure at the locations where the side electrode portions are provided.

[0059] The inner main pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (electrodes in which a metal component and a ceramic component are mixed).

[0060] The main pump cell 21 is configured to adjust the oxygen concentration in the measurement gas that has flowed into the measurement gas flow section 15 to a predetermined concentration. Therefore, it is preferable that the inner main pump electrode 22 that comes into contact with the measurement gas decomposes only oxygen without reducing (decomposing) the NOx component in the measurement gas. The specific electrode configuration and constituent materials of the inner oxygen pump electrode 90 (the inner main pump electrode 22 and the auxiliary pump electrode 51 in the sensor element 101 of this embodiment) will be described later.

[0061] In the main pump cell 21, by applying a desired pump voltage Vp0 between the inner main pump electrode 22 and the outer pump electrode 23 using a variable power supply 24 and flowing a pump current Ip0 in a positive or negative direction between the inner main pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen from the first internal space 20 out to the external space or pump oxygen from the external space into the first internal space 20.

[0062] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 80 for controlling the main pump, is formed by the inner main pump electrode 22, the second solid electrolyte 6, the spacer layer 5, the first solid electrolyte 4, the third substrate layer 3, and the reference electrode 42.

[0063] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Furthermore, the pump current Ip0 is controlled by feedback controlling Vp0 so that the electromotive force V0 is constant. This allows the oxygen concentration in the first internal space 20 to be maintained at a predetermined constant value.

[0064] The third diffusion control section 30 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas to the second internal space 40.

[0065] The second internal space 40 is provided as a space for adjusting with higher precision the oxygen partial pressure in the measurement gas introduced through the third diffusion-controlling part 30. The oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50.

[0066] In the second internal space 40, the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, and then the oxygen partial pressure of the measurement gas introduced through the third diffusion-controlling section 30 is further adjusted by the auxiliary pump cell 50. This makes it possible to keep the oxygen concentration in the second internal space 40 constant with high precision, thereby enabling the gas sensor 100 to measure the NOx concentration with high precision.

[0067] The auxiliary pump cell 50 is an auxiliary pump electrode 51 disposed on the inner surface of the measurement gas flow portion 15 at a position farther from the gas inlet 10 than the inner main pump electrode 22; The electrochemical pump cell includes the auxiliary pump electrode 51 disposed on the outer surface of the base portion 102 and an outer pump electrode 23 provided with a second solid electrolyte layer 6 interposed therebetween.

[0068] That is, the auxiliary pump cell 50 is an auxiliary electrochemical pump cell that is composed of an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any appropriate electrode on the outside of the sensor element 101 will suffice), and the second solid electrolyte layer 6.

[0069] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to the inner main pump electrode 22 disposed in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40. Side electrodes (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 that provide the side walls of the second internal space 40, forming a tunnel-shaped structure.

[0070] It is preferable that the auxiliary pump electrode 51 is configured to decompose only oxygen without reducing (decomposing) the NOx component in the measurement gas, similar to the inner main pump electrode 22. The specific electrode configuration and constituent materials of the inner oxygen pump electrode 90 (the inner main pump electrode 22 and the auxiliary pump electrode 51 in the sensor element 101 of this embodiment) will be described later.

[0071] In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, it is possible to pump oxygen in the atmosphere within the second internal space 40 out to the external space or pump oxygen from the external space into the second internal space 40.

[0072] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal space 40, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, is configured by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3.

[0073] The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. This allows the oxygen partial pressure in the atmosphere within the second internal space 40 to be controlled to a low level that does not substantially affect the measurement of NOx.

[0074] In addition, the pump current Ip1 is used to control the electromotive force V0 of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and by controlling the electromotive force V0, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion-controlling part 30 into the second internal space 40 is controlled to be always constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

[0075] The fourth diffusion rate-controlling section 60 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been further controlled to a lower level by the operation of the auxiliary pump cell 50 in the second internal space 40, and guides the measurement gas to the third internal space 61.

[0076] The third internal space 61 is provided as a space for measuring the concentration of nitrogen oxides (NOx) in the measurement gas introduced through the fourth diffusion-controlling section 60. The NOx concentration is measured by the operation of the measurement pump cell 41.

[0077] The measuring pump cell 41 is a measuring electrode (44) disposed on the inner surface of the measurement target gas flow portion (15) at a position farther from the gas inlet (10) than the auxiliary pump electrode (51); The measurement electrode 44 is disposed on the outer surface of the base portion 102, and an outer pump electrode 23 is provided via the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. An electrochemical pump cell comprising:

[0078] That is, the measurement pump cell 41 measures the NOx concentration in the measurement gas in the third internal space 61. The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any appropriate electrode on the outside of the sensor element 101 will suffice), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0079] The measurement electrode 44 is a porous cermet electrode, similar to the above-described electrodes 22, 23, and 51. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the third internal space 61.

[0080] The metal material of the measuring electrode 44 is preferably a noble metal material having catalytic activity for decomposing NOx (reducing NOx). For example, platinum (Pt), rhodium (Rh), etc. may be used. For example, Pt may be used, or an alloy of Pt and Rh may be used. For example, when an alloy of Pt and Rh is used, the amount of Rh may be 10% by weight to 90% by weight of the total amount of Pt and Rh.

[0081] In the measuring pump cell 41, oxygen generated by decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 44 is pumped out, and the amount of oxygen generated can be detected as a pump current Ip2.

[0082] Furthermore, in order to detect the oxygen partial pressure around the measurement electrode 44, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42 constitute an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump. The variable power supply 46 is controlled based on the electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

[0083] The measurement gas introduced into the second internal space 40 reaches the measuring electrode 44 in the third internal space 61 through the fourth diffusion-controlling part 60 under conditions where the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measuring electrode 44 are reduced (2NO → N2 + O2) to generate oxygen. The generated oxygen is then pumped by the measuring pump cell 41, and the voltage Vp2 of the variable power supply 46 is controlled so that the electromotive force V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 remains constant. Because the amount of oxygen generated around the measuring electrode 44 is proportional to the nitrogen oxide concentration in the measurement gas, the pump current Ip2 in the measuring pump cell 41 can be used to calculate the nitrogen oxide concentration in the measurement gas.

[0084] Furthermore, by combining the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 to form an oxygen partial pressure detection means as an electrochemical sensor cell, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by reduction of the NOx components in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere, thereby making it possible to determine the concentration of the NOx components in the measured gas.

[0085] In addition, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the electromotive force Vref obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0086] In the gas sensor 100 having such a configuration, the measurement gas, in which the oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50, is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas can be determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out of the measurement pump cell 41, which is approximately proportional to the NOx concentration in the measurement gas.

[0087] Furthermore, in order to enhance the oxygen ion conductivity of the solid electrolyte, the sensor element 101 is provided with a heater section 70 that adjusts the temperature by heating and maintaining the temperature of the sensor element 101. The heater section 70 includes a heater electrode 71, a heater 72, a heater lead 76, a through-hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0088] In the sensor element 101 of this embodiment, the heater section 70 is embedded in the base section 102, but the present invention is not limited to this. It is sufficient that the sensor element 101 is heated to a degree that allows the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 to exhibit oxygen ion conductivity that enables their operation. The heater section 70 may be formed separately from the sensor element 101, or may be heated by the high-temperature measurement gas. For accurate measurement, it is preferable that the temperature of the sensor element 101 be constant regardless of the temperature of the measurement gas. Considering this point, it is preferable that the sensor element 101 includes the heater section 70, as in this embodiment.

[0089] The heater electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to a heater power supply, which is an external power supply, it is possible to supply power to the heater section 70 from outside.

[0090] The heater 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 via a heater lead 76 that is connected to the heater 72 and extends to the rear end side of the sensor element 101 in the longitudinal direction, and a through hole 73. The heater 72 generates heat when power is supplied from the outside through the heater electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 101.

[0091] The heater 72 is embedded throughout the entire area from the first internal space 20 to the third internal space 61, making it possible to adjust the entire sensor element 101 to a temperature at which the solid electrolyte is activated. The temperature needs to be adjusted so that the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 can operate. It is not necessary to adjust these entire areas to the same temperature, and the sensor element 101 may have a temperature distribution.

[0092] In the sensor element 101 of this embodiment, the heater 72 is embedded in the base portion 102, but the present invention is not limited to this. The heater 72 may be disposed so as to heat the base portion 102. That is, the heater 72 may be capable of heating the sensor element 101 to an extent that the sensor element 101 exhibits oxygen ion conductivity that enables the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 to operate. For example, the heater 72 may be embedded in the base portion 102 as in this embodiment. Alternatively, the heater portion 70 may be formed as a heater substrate separate from the base portion 102 and disposed adjacent to the base portion 102.

[0093] The heater insulating layer 74 is an insulating layer made of an insulator such as alumina and formed on the upper and lower surfaces of the heater 72 and heater lead 76. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and heater lead 76, and between the third substrate layer 3 and the heater 72 and heater lead 76.

[0094] The pressure release hole 75 penetrates the third substrate layer 3 and is formed so as to connect the heater insulating layer 74 and the reference gas introduction space 43. The pressure release hole 75 can mitigate an increase in internal pressure that accompanies an increase in temperature within the heater insulating layer 74. Note that the pressure release hole 75 may be omitted.

[0095] (inner oxygen pump electrode) As described above, the inner oxygen pump electrode 90 (the inner main pump electrode 22 and the auxiliary pump electrode 51 in the sensor element 101 of this embodiment) is preferably configured to decompose only oxygen without reducing (decomposing) the NOx component in the measurement gas. With this configuration, NOx is not decomposed at the inner oxygen pump electrode 90, and all of the NOx in the measurement gas reaches the measurement electrode 44, so that NOx can be detected accurately in the measurement pump cell 41.

[0096] The main pumping cell 21 discharges oxygen from the first internal space 20 so that the oxygen concentration in the first internal space 20 remains constant. The higher the oxygen concentration in the measurement gas, the greater the amount of oxygen to be discharged. That is, the pumping current Ip0 in the main pumping cell 21 increases. Since the applied voltage Vp0 in the main pumping cell 21 is approximately proportional to the pumping current Ip0, the higher the oxygen concentration in the measurement gas, the greater the applied voltage Vp0.

[0097] If the applied voltage Vp0 becomes too high, NOx may be decomposed at the inner main pump electrode 22. This reduces the amount of NOx that reaches the measuring electrode 44. As a result, the current value Ip2 detected by the measuring pump cell 41 becomes smaller than the value that should be detected. This reduces the accuracy of NOx detection, particularly when the oxygen concentration of the measurement gas is high.

[0098] We will explain the NOx output current value Ip2 in cases where there is no decrease in NOx detection accuracy under such high oxygen concentrations and in cases where there is. Figure 3 is a schematic diagram showing the relationship between oxygen concentration and NOx output current value Ip2 in the presence of oxygen (O2 = 0, 5, 10, 18%). The concentrations of each gas component are all listed on a volumetric basis.

[0099] As an indicator of whether high accuracy of NOx detection is maintained under high oxygen concentrations, the coefficient of determination R in the linear regression equation between multiple oxygen concentrations and the Ip2 value at each oxygen concentration is used. 2 The coefficient of determination R 2 The linearity of NOx output R 2 It is called.

[0100] In Figure 3, the "●" indicates a gas sensor that can measure with high accuracy even at high oxygen concentrations, i.e., a sensor with a linearity R of NOx output. 2 The figure shows a schematic diagram of the NOx output current value Ip2 of a gas sensor with a high NOx output linearity R. 2 10 is a schematic diagram showing the NOx output current value Ip2 in a gas sensor with a low NOx concentration.

[0101] NOx output linearity R 2 The higher the value, that is, the closer to 1, the more accurately NOx can be detected, regardless of the oxygen concentration in the gas being measured. 2 For example, it is sufficient if the linearity R of the NOx output is 0.900 or more. It is considered that the use of such a gas sensor will enable accurate measurement of NOx in practical use. More preferably, 2 is preferably 0.950 or more, and more preferably 0.975 or more.

[0102] NOx output linearity R 2can be calculated using, for example, a model gas. Four types of model gases with a constant NOx concentration of 500 ppm and oxygen concentrations of 0, 5, 10, and 18% may be used, and each model gas may be measured by the gas sensor 100. The coefficient of determination R in the linear regression equation between each oxygen concentration of the model gas and the four measured NOx output current values ​​Ip2 is 2 The model gases are not limited to these four types, and may be appropriately selected depending on the expected usage of the gas sensor 100.

[0103] The decrease in NOx detection accuracy under high oxygen concentrations will be examined in more detail. FIG. 2 is a schematic cross-sectional view showing a portion of a cross section taken along line II-II in FIG. 1. It is a schematic view showing a general planar arrangement of the inner main pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44, which are disposed on the upper surface of the first solid electrolyte layer 4. L1 represents the length of the inner main pump electrode 22 in the longitudinal direction of the sensor element 101, and L2 represents the length of the auxiliary pump electrode 51 in the longitudinal direction of the sensor element 101. Electrode leads (not shown) are disposed from each of the electrodes toward the rear end of the element, enabling connection to the outside. The spacer layer 5 forming the lower surface of the fourth diffusion-controlling section 60 is not shown.

[0104] 2 shows an image of the oxygen concentration distribution in the longitudinal direction of the sensor element 101 when a measurement gas containing a high concentration of oxygen is introduced into the measurement gas flow portion 15.

[0105] 1 and 2, the operation of the main pump cell 21 when a measurement gas with a high oxygen concentration is introduced into the first internal space 20 can be considered as follows. When the measurement gas is introduced into the first internal space 20, the main pump cell 21 discharges most of the oxygen in the measurement gas. The inner main pump electrode 22 has a predetermined length (L1) in the longitudinal direction of the sensor element 101. Referring to the image of the oxygen concentration distribution in the longitudinal direction of the sensor element 101 in FIG. 2, it is considered that more oxygen is discharged from a position of the inner main pump electrode 22 closer to the gas inlet 10. In other words, it is considered that the amount of oxygen discharged varies microscopically depending on the position within the inner main pump electrode 22. As a result, it is considered that the local pump current value Ip0(local) varies microscopically depending on the position within the inner main pump electrode 22.

[0106] It is presumed that the locally applied voltage Vp0(local) at a position close to the gas inlet 10 of the inner main pump electrode 22 is high because it is necessary to discharge more oxygen at that position. From this, it is presumed that when NOx is decomposed at the inner main pump electrode 22 under high oxygen concentration conditions, the NOx is decomposed at a position close to the gas inlet 10 of the inner main pump electrode 22.

[0107] From the above, it is considered that by using a material with reduced catalytic activity for decomposing NOx, particularly in a position close to the gas inlet 10 of the inner main pump electrode 22, the decomposition of NOx at the inner main pump electrode 22 under high oxygen concentration conditions can be effectively suppressed.

[0108] The inner oxygen pump electrode 90 (the inner main pump electrode 22 and the auxiliary pump electrode 51 in the sensor element 101 of this embodiment) will be described in detail below.

[0109] (Shape of inner oxygen pump electrode)

[0110] In the sensor element 101 of this embodiment, the inner main pump electrode 22 and the auxiliary pump electrode 51 are each substantially rectangular. The shape of the electrodes is not limited to a rectangular shape and may be determined appropriately by those skilled in the art.

[0111] The inner oxygen pump electrode 90 has a predetermined length (L) in the longitudinal direction of the base portion 102. In the sensor element 101 of this embodiment, the inner main pump electrode 22 has a predetermined length (L1) in the longitudinal direction of the sensor element 101, and the auxiliary pump electrode 51 has a predetermined length (L2) in the longitudinal direction of the sensor element 101. The length (L) of the inner oxygen pump electrode 90 is the sum of the length (L1) of the inner main pump electrode 22 and the length (L2) of the auxiliary pump electrode 51 (L=L1+L2).

[0112] The size of the inner main pump electrode 22 may be determined appropriately by those skilled in the art. It is sufficient that the main pump cell 21 is large enough to maintain the oxygen concentration in the first internal space 20 at a predetermined constant value. For example, the length (L1) of the inner main pump electrode 22 in the longitudinal direction of the sensor element 101 may be 2.0 mm to 7.0 mm. The width of the inner main pump electrode 22 perpendicular to the longitudinal direction of the sensor element 101 may be 1.0 mm to 4.0 mm. The thickness of the inner main pump electrode 22 may be 5.0 μm to 30.0 μm.

[0113] The inner main pump electrode 22 is preferably formed on the lower surface of the second solid electrolyte layer 6 facing the first internal space 20. As described above, the inner main pump electrode 22 may have a ceiling electrode portion 22a and a bottom electrode portion 22b. The ceiling electrode portion 22a and the bottom electrode portion 22b may each have the above-mentioned sizes. In the sensor element 101 of this embodiment, the ceiling electrode portion 22a and the bottom electrode portion 22b are formed to have the same shape. In the configuration having the ceiling electrode portion 22a and the bottom electrode portion 22b, the electrode area can be made larger relative to the volume of the first internal space 20, and it is therefore considered that the oxygen concentration in the first internal space 20 can be controlled with higher precision.

[0114] The size of the auxiliary pump electrode 51 may be determined appropriately by those skilled in the art. The auxiliary pump cell 50 may have any size as long as it can control the oxygen partial pressure in the atmosphere in the second internal space 40 to a low partial pressure that does not substantially affect the measurement of NOx. Typically, the auxiliary pump electrode 51 may be smaller than the inner main pump electrode 22. For example, the length (L2) of the auxiliary pump electrode 51 in the longitudinal direction of the sensor element 101 may be 1.0 mm to 2.5 mm. The width of the auxiliary pump electrode 51 perpendicular to the longitudinal direction of the sensor element 101 may be 0.3 mm to 2.5 mm. The thickness of the auxiliary pump electrode 51 may be 5.0 μm to 30.0 μm.

[0115] The auxiliary pump electrode 51 is preferably formed on the lower surface of the second solid electrolyte layer 6 facing the second internal space 40. As described above, the auxiliary pump electrode 51 may have a ceiling electrode portion 51a and a bottom electrode portion 51b. The ceiling electrode portion 51a and the bottom electrode portion 51b may each have the above-mentioned sizes. In the sensor element 101 of this embodiment, the ceiling electrode portion 51a and the bottom electrode portion 51b are formed to have the same shape. In a configuration having the ceiling electrode portion 51a and the bottom electrode portion 51b, the electrode area can be made larger relative to the volume of the second internal space 40, and it is therefore considered that the oxygen concentration in the second internal space 40 can be controlled with higher precision.

[0116] (Material of the inner oxygen pump electrode) As described above, the inner oxygen pump electrode 90 (i.e., the inner main pump electrode 22 and the auxiliary pump electrode 51) is a porous cermet electrode (an electrode in which a metal component and a ceramic component are mixed). The ceramic component is not particularly limited, but, like the base portion 102, it is preferable to use an oxygen ion conductive solid electrolyte. For example, ZrO2 can be used as the ceramic component. The metal component and ceramic component in the porous cermet electrode can be appropriately determined by those skilled in the art, but, for example, the ceramic component can be approximately 30 wt% to 50 wt% of the total of the metal component and the ceramic component. For example, when Pt is used as the metal component and ZrO2 is used as the ceramic component, the weight ratio of Pt:ZrO2 may be approximately 7.0:3.0 to 5.0:5.0.

[0117] The metal materials for the inner main pump electrode 22 and the auxiliary pump electrode 51 will be described in detail below.

[0118] (Metal material of inner oxygen pump electrode) As described above, the main pump cell 21 is configured to adjust the oxygen concentration in the measurement gas that has flowed into the measurement gas flow section 15 to a predetermined concentration. Therefore, it is preferable that the inner main pump electrode 22 that comes into contact with the measurement gas decomposes only oxygen without reducing (decomposing) the NOx components in the measurement gas.

[0119] For example, a material containing a metal having catalytic activity for decomposing oxygen as a main component and having added thereto a metal that reduces the catalytic activity for decomposing the measurement target gas (hereinafter referred to as an activity-reducing metal) can be used as the metal material for the inner main pump electrode 22. Examples of metals having catalytic activity for decomposing oxygen include platinum (Pt).

[0120] Platinum (Pt) is a material used as a catalyst not only in the field of gas sensors but also in a wide range of general applications. Pt has catalytic activity for oxygen and catalytic activity for decomposing the gas to be measured (e.g., NOx). It is believed that by adding an activity-reducing metal that reduces the catalytic activity for decomposing NOx to such Pt, it is possible to reduce the catalytic activity for decomposing NOx while maintaining catalytic activity for oxygen.

[0121] Examples of metals that reduce catalytic activity for decomposing NOx include gold (Au) and silver (Ag). These activity-reducing metals are considered to lack catalytic activity for decomposing NOx. Preferably, gold (Au) can be used.

[0122] (Composition of the metal material in the inner oxygen pump electrode) The inner oxygen pump electrode 90 includes an electrode end portion on the side closer to the gas inlet 10 (i.e., the side closer to the tip of the base portion 102) and has a predetermined length (L A ) and a region (A) having The electrode end portion on the side farther from the gas inlet 10 (i.e., the side farther from the tip of the base portion 102) and having a predetermined length (L B ) and region (B) having Including, The content of the activity-reducing metal in the metal material in the region (A) is higher than the content of the activity-reducing metal in the metal material in the region (B).

[0123] The region (B) of the inner oxygen pump electrode 90 may be the entire region other than the region (A) of the inner oxygen pump electrode 90. That is, the inner oxygen pump electrode 90 may be composed of a region (A) having a high content of the activity-reducing metal in the metal material and a region (B) having a low content.

[0124] In the sensor element 101 of this embodiment, the inner oxygen pump electrode 90 is, as described above, This embodiment includes an inner main pump electrode 22 having a predetermined length (L1) in the longitudinal direction of the sensor element 101, and an auxiliary pump electrode 51 having a predetermined length (L2) in the longitudinal direction of the sensor element 101.

[0125] In the sensor element 101 of this embodiment, the inner main pump electrode 22 and the auxiliary pump electrode 51 are The electrode end portion of the inner main pump electrode 22 near the gas inlet 10 is included, and the electrode end portion is formed to have a predetermined length (L A ) and a region (A) having The electrode end portion of the auxiliary pump electrode 51 farther from the gas inlet 10 is included, and the electrode end portion is formed by a predetermined length (L B ) and region (B) having Including, The content of the activity-reducing metal in the metal material in the region (A) is higher than the content of the activity-reducing metal in the metal material in the region (B).

[0126] The ratio of the longitudinal length (L) of the region (A) of the inner oxygen pump electrode 90 to the longitudinal length (L) of the sensor element 101 of the inner oxygen pump electrode 90 A ) ratio (L A / L) is 15% or more and 90% or less. A / L) may be 30% or more and 70% or less.

[0127] It is believed that by setting the region (A) within the above range, it is possible to effectively suppress the decomposition of NOx at the inner main pump electrode 22 under high oxygen concentration conditions.

[0128] Furthermore, by setting the region (A) within the above range, it is believed that the NOx detection sensitivity can be maintained even when the gas sensor is used for a long period of time in a high oxygen concentration and high temperature range.

[0129] Specifically, when the gas sensor is used for a long time in a high-oxygen-concentration, high-temperature range, it is thought that the deactivated metal in the inner main pump electrode 22 and the auxiliary pump electrode 51 evaporates, and the evaporated deactivated metal adheres to the measuring electrode 44. When the deactivated metal adheres to the measuring electrode 44, the NOx decomposition performance of the measuring electrode 44 deteriorates. As a result, it is thought that not all of the NOx in the measurement gas that reaches the measuring electrode 44 can be decomposed, and the NOx detection current value Ip2 becomes smaller than it should be. In other words, the NOx detection sensitivity deteriorates as the gas sensor is used.

[0130] However, by setting the region (A) within the above-mentioned range, even if the activity-decreasing metal in the inner main pump electrode 22 and the auxiliary pump electrode 51 evaporates due to long-term use of the gas sensor, it is possible to suppress the amount of activity-decreasing metal adhering to the measuring electrode 44. In other words, it is considered possible to suppress the change in NOx sensitivity over time when the gas sensor is used for a long period of time.

[0131] In the sensor element 101 of this embodiment, the length (L) of the inner oxygen pump electrode 90 is the sum (L=L1+L2) of the length (L1) of the inner main pump electrode 22 and the length (L2) of the auxiliary pump electrode 51. That is, the above-mentioned ratio (L A / L) is the L for L1+L2 A The ratio [L A / (L1+L2)].

[0132] L A is smaller than L1 (L A <L1): The length of the inner main pump electrode 22 in the longitudinal direction from the electrode end close to the gas inlet 10 to the sensor element 101 is L A In this region, there are many metals that reduce activity.

[0133] L A If L1 is equal to L A =L1): The entire inner main pump electrode 22 (length: L1 = L A) contains a large amount of metals that reduce activity.

[0134] L A is greater than L1 (L A >L1): The length of the entire inner main pump electrode 22 (length: L1) and the length of the auxiliary pump electrode 51 from the electrode end close to the gas inlet 10 to the longitudinal direction of the sensor element 101 is L A -L1 region contains a large amount of activity-reducing metals.

[0135] The content of the activity-reducing metal in the metal material in each of the region (A) and the region (B) of the inner oxygen pump electrode 90 can be appropriately set within a range that can suppress the decomposition of NOx at the inner main pump electrode 22 under high oxygen concentration conditions, provided that the content in the region (A) is higher than the content in the region (B).

[0136] For example, when platinum (Pt) is the main component and gold (Au) is added as an activity-reducing metal, the Au content (concentration) in the Au-rich region (A) may be 0.5 wt % to 2.0 wt % relative to the total amount of the metal material. Preferably, it may be 0.7 wt % to 2.0 wt %. More preferably, it may be 1.5 wt % to 2.0 wt %. It is believed that by setting the content within these ranges, it is possible to effectively suppress the decomposition of NOx at the inner main pump electrode 22 under high oxygen concentrations.

[0137] Furthermore, the Au content (concentration) in the region (B) of the inner main pump electrode 22 and the auxiliary pump electrode 51 may be 0.1 wt % or more and 0.5 wt % or less with respect to the total amount of the metal material. Preferably, it may be 0.1 wt % or more and 0.4 wt % or less. More preferably, it may be 0.1 wt % or more and 0.3 wt % or less. By setting the content within these ranges, it is possible to reduce the amount of Au evaporating from the inner main pump electrode 22 and the auxiliary pump electrode 51 even when the gas sensor is used for a long period of time, and as a result, it is thought that the amount of Au adhering to the measurement electrode 44 can be reduced. Therefore, it is thought that a decrease in NOx detection sensitivity can be suppressed.

[0138] The Au content (C) in the region (B) where the content of the deactivating metal is low B ) in the high-content region (A) relative to the Au content (C A ) ratio: Au content ratio (C A / C B ) may be 1.5 or more and 20.0 or less.

[0139] Au content ratio (C A / C B ) in this range, it is believed that it is possible to effectively suppress the decomposition of NOx under high oxygen concentrations, particularly at the tip side of the sensor element 101 of the inner main pump electrode 22. It is also believed that it is possible to suppress the amount of Au that evaporates from the inner main pump electrode 22 and the auxiliary pump electrode 51 and adheres to the measurement electrode 44.

[0140] That is, the Au content ratio (C A / C B ) in this range, it is believed that the two effects described above can be achieved at the same time. As a result, high NOx detection accuracy can be maintained regardless of the oxygen concentration in the measurement gas.

[0141] As described above, the inner oxygen pump electrode 90 (the inner main pump electrode 22 and the auxiliary pump electrode 51) may be composed of two regions with different Au contents: a region (A) with a high content of activity-reducing metal in the metal material and a region (B) with a low content of activity-reducing metal.

[0142] Alternatively, the inner oxygen pump electrode 90 may be composed of three or more regions in which the Au content decreases stepwise in the longitudinal direction from the side closer to the tip of the sensor element 101. That is, the inner oxygen pump electrode 90 may be composed of a region (A) including two or more regions with different Au concentrations and a region (B) with a constant Au concentration. That is, the content of the activity-reducing metal in the metal material may decrease stepwise in the longitudinal direction of the sensor element 101 from the part closer to the gas inlet 10 to the part farther away from the gas inlet 10.

[0143] There may also be a concentration gradient in the longitudinal direction of the sensor element 101. That is, the content of the activity-reducing metal in the metal material may decrease continuously from the portion closer to the gas inlet 10 in the region (A) to the portion farther from the region (A) in the longitudinal direction of the sensor element 101.

[0144] Even when Ag or the like is used as the activity-reducing metal, the above-mentioned content of Au and the configuration of the region (A) and the region (B) in the inner oxygen pump electrode 90 can be used as reference.

[0145] It is believed that the above-described configuration of the inner oxygen pump electrode 90 can effectively suppress the decomposition of NOx at the inner main pump electrode 22 under high oxygen concentration conditions. That is, even when the oxygen concentration in the measurement gas is high, NOx can be detected with high accuracy. In other words, high NOx detection accuracy can be maintained regardless of the oxygen concentration in the measurement gas.

[0146] Furthermore, by configuring the inner oxygen pump electrode 90 as described above, it is believed that the amount of deactivation metal that evaporates from the inner oxygen pump electrode 90 and adheres to the measurement electrode 44 can be reduced even when the gas sensor is used for a long time in a high-oxygen concentration and high-temperature range. As a result, it is possible to suppress a decrease in the NOx decomposition performance of the measurement electrode 44 due to use of the gas sensor, and therefore a decrease in NOx detection sensitivity can be suppressed. In other words, it is possible to suppress a change in NOx sensitivity over time. As a result, it is believed that durability performance is improved.

[0147] Other embodiments of the sensor element of the present invention will be described below.

[0148] (Variation 1) Fig. 4 is a schematic cross-sectional view showing a portion of a vertical cross section in the longitudinal direction of the sensor element 201 of Modification 1 used in the examples. L1 represents the length of the inner main pump electrode 22 in the longitudinal direction of the sensor element 201. The lower part of Fig. 4 shows an image of the oxygen concentration distribution in the longitudinal direction of the sensor element 201 when a measurement gas containing a high concentration of oxygen is introduced into the measurement gas flow section.

[0149] The sensor element 201 of the first modification is a sensor element having a main pump cell 21 and a measurement pump cell 41. The sensor element 201 of the first modification has two internal cavities: a first internal cavity 20 and a third internal cavity 61. The inner main pump electrode 22, which constitutes a part of the main pump cell 21, is formed on the lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20. The measurement electrode 44, which constitutes a part of the measurement pump cell 41, is formed on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61.

[0150] The sensor element 201 of the first modification adjusts the oxygen concentration in the measurement gas introduced into the first internal space 20 to a predetermined constant concentration by the main pump cell 21. Specifically, the electromotive force V0 in the main pump control oxygen partial pressure detection sensor cell 80 is controlled to a constant value corresponding to a predetermined oxygen partial pressure, thereby maintaining the oxygen concentration in the first internal space 20 at a predetermined constant value.

[0151] In the sensor element 201 of the first modification, the inner oxygen pump electrode 90 is the inner main pump electrode 22. The length (L) of the sensor element 201 of the inner oxygen pump electrode 90 in the longitudinal direction is equal to the length (L1) of the sensor element 201 of the inner main pump electrode 22 in the longitudinal direction (L=L1).

[0152] In the inner main pump electrode 22 of the sensor element 201 of the first modification, the length (L A ) occupies 15% to 90% of the length (L1) of the inner main pump electrode 22 in the longitudinal direction of the sensor element 201. A The ratio (L A / L1) is 15% to 90%. More preferably, L A The ratio (L A / L1) may be set to 30 to 70%.

[0153] For configurations other than those described above, reference can be made to the sensor element 101 of the above embodiment.

[0154] (Variation 2) FIG. 5 is a schematic cross-sectional view showing a part of a vertical cross section in the longitudinal direction of a sensor element 301 of Modified Example 2 used in the example.

[0155] 6 is a schematic cross-sectional view taken along line VI-VI in FIG. 5. This figure shows a schematic planar arrangement of the inner main pump electrode 22 disposed on the lower surface of the second solid electrolyte layer 6, and the auxiliary pump electrode 51 and the measurement electrode 44 disposed on the upper surface of the first solid electrolyte layer 4 in the sensor element 301 of the second modification. An electrode lead (not shown) is disposed from each of the electrodes toward the rear end of the element, enabling connection to the outside. The spacer layer 5 forming the diffusion rate-controlling portions 11 and 13 is not shown.

[0156] 6 shows an image of the oxygen concentration distribution in the longitudinal direction of the sensor element 301 when a measurement gas containing a high concentration of oxygen is introduced into the measurement gas flow portion 15.

[0157] The sensor element 301 of the second modification example faces one internal space 14 and has an inner main pump electrode 22 disposed on the lower surface of the second solid electrolyte layer 6 closer to the front end of the sensor element 301. An auxiliary pump electrode 51 and a measurement electrode 44 are disposed in parallel in the longitudinal direction of the sensor element 301 on the upper surface of the first solid electrolyte layer 4, closer to the rear end of the sensor element 301 than the inner main pump electrode 22.

[0158] In the sensor element 301 of the second modification, the inner oxygen pump electrode 90 is divided into the inner main pump electrode 22 and the auxiliary pump electrode 51, similarly to the sensor element 101. In the sensor element 301 of the second modification, the length (L) of the sensor element 301 of the inner oxygen pump electrode 90 in the longitudinal direction is the sum of the length (L1) of the sensor element 301 of the inner main pump electrode 22 in the longitudinal direction and the length (L2) of the sensor element 301 of the auxiliary pump electrode 51 in the longitudinal direction (L=L1+L2).

[0159] In the sensor element 301 of the second modification, the length (L2) of the auxiliary pump electrode 51 in the longitudinal direction of the sensor element 301 is equal to the length (L M ) can be as long as L2. M It can be equal to (L2=L M ), or 0.8 x L M ≦L2≦1.2×L M The length (L1) of the inner main pump electrode 22 may be about the same as the length (L M ), for example, in the range of 1 to 5 times. Preferably, it can be 2 to 4 times. Within this range, the oxygen partial pressure in the measurement gas that reaches the measuring electrode 44 can be adjusted to a sufficiently low predetermined value.

[0160] In the sensor element 301 of the second modification, the main pump cell 21 may be operated independently to adjust the oxygen partial pressure. The auxiliary pump electrode 51 may be used as an oxygen detection electrode for detecting the oxygen partial pressure near the measurement electrode 44 adjusted by the main pump cell 21. The oxygen partial pressure may be detected using the electromotive force V1 in the auxiliary pump control oxygen partial pressure detection sensor cell 81, or the current value between the auxiliary pump electrode 51 and the outer pump electrode 23 (or the reference electrode 42).

[0161] For configurations other than those described above, reference can be made to the sensor element 101 of the above embodiment.

[0162] (Variation 3) 7 is a cross-sectional schematic diagram of a sensor element 401 of Modification 3, taken along the same cross section as in FIG. 6. It is a schematic diagram illustrating a general planar arrangement of the inner main pump electrode 22 and the auxiliary pump electrode 51 disposed on the lower surface of the second solid electrolyte layer 6 in the sensor element 401, and the second auxiliary pump electrode 53 and the measurement electrode 44 disposed on the upper surface of the first solid electrolyte layer 4.

[0163] In this way, in addition to the inner main pump electrode 22 and the auxiliary pump electrode 51, a second auxiliary pump electrode 53 can be disposed in parallel with the measuring electrode 44. In this case, the inner main pump electrode 22 and the auxiliary pump electrode 51 may be used to adjust the oxygen partial pressure in the measurement gas. In that case, the second auxiliary pump electrode 53 may be used as an oxygen detection electrode for detecting the adjusted oxygen partial pressure near the measuring electrode 44. The oxygen partial pressure may be detected using the electromotive force between the second auxiliary pump electrode 53 and the reference electrode 42, or the current value between the second auxiliary pump electrode 53 and the outer pump electrode 23 (or the reference electrode 42).

[0164] In the sensor element 401 of the modified example 3, the inner oxygen pump electrode 90 is divided into the inner main pump electrode 22, the auxiliary pump electrode 51, and the second auxiliary pump electrode 53. In the sensor element 401 of the modified example 3, the length (L) of the sensor element 401 of the inner oxygen pump electrode 90 in the longitudinal direction is the sum of the length (L1) of the sensor element 401 of the inner main pump electrode 22 in the longitudinal direction, the length (L2) of the sensor element 401 of the auxiliary pump electrode 51 in the longitudinal direction, and the length (L3) of the sensor element 401 of the second auxiliary pump electrode 53 in the longitudinal direction (L=L1+L2+L3).

[0165] The total length (L1+L2) of the inner main pump electrode 22 (L1) and the auxiliary pump electrode 51 (L2) in the sensor element 401 of the modified example 3 is the same as the total length (L1+L2) of the inner main pump electrode 22 (L1) and the measurement electrode 44 (L2) in the sensor element 301 of the modified example 2. M ) can be referred to. Furthermore, for the length L3 of the second auxiliary pump electrode 53 in the sensor element 401 of the modified example 3, the relationship between the length (L2) of the auxiliary pump electrode 51 and the length (L M ) relationship can be seen.

[0166] For configurations other than those described above, reference can be made to the sensor element 101 of the above embodiment.

[0167] Although the sensor elements 101, 201, 301, and 401 have been described above as examples of embodiments of the present invention, the present invention is not limited to these embodiments. The present invention may include sensor elements including various types of inner oxygen pump electrodes 90, as long as the object of the present invention, that is, maintaining high NOx detection accuracy regardless of the oxygen concentration in the measurement gas, is achieved.

[0168] [Sensor element manufacturing method] Next, an example of a method for manufacturing the above-mentioned sensor element will be described. After performing predetermined processing and printing of a circuit pattern on a plurality of unfired sheet-shaped molded articles (so-called green sheets) containing an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component, the plurality of sheets are stacked, the stack is cut, and then fired to produce the sensor element 101.

[0169] In the following, an example will be described in which the sensor element 101 made up of six layers as shown in FIG. 1 is fabricated.

[0170] First, six green sheets containing an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component are prepared. A known forming method can be used to fabricate the green sheets. All six green sheets may be the same thickness, or the thickness may vary depending on the layer being formed. Holes and other features used for positioning during printing and lamination are formed in each of the six green sheets (blank sheets) in advance using a known method such as punching with a punching device. Through holes, such as internal voids, are also formed in the blank sheet used for the spacer layer 5 in a similar manner. Necessary through holes are also formed in the other layers in advance.

[0171] Various patterns required for each layer are printed and dried on blank sheets used for the six layers: first substrate layer 1, second substrate layer 2, third substrate layer 3, first solid electrolyte layer 4, spacer layer 5, and second solid electrolyte layer 6. Known screen printing techniques can be used to print the patterns. Known drying means can also be used for the drying process.

[0172] For example, the length L of the inner main pump electrode 22 in the longitudinal direction of the sensor element 101 from the electrode end close to the gas inlet 10 A Let us consider the case of manufacturing a sensor element 101 in which the region up to is a region (A) with a high content of activity-reducing metal. In this sensor element 101, the region other than the region (A) of the inner main pump electrode 22 and the auxiliary pump electrode 51 are regions (B) with a low content of activity-reducing metal.

[0173] When forming the inner main pump electrode 22, first, an electrode paste for a high content region (A) and an electrode paste for a low content region (B) having different Au contents in the metal material are prepared.

[0174] Next, the electrode paste for the high content region (A) is printed and dried on the second solid electrolyte layer 6 in a desired pattern to form the region (A) of the inner main pump electrode 22. Also, the electrode paste for the low content region (B) is printed and dried in a desired pattern to form the region (B) of the inner main pump electrode 22 (i.e., a region other than the high concentration region (A)). Also, the electrode paste for the low content region (B) is printed and dried in a desired pattern to form the auxiliary pump electrode 51. The order of these printing steps can be determined as appropriate.

[0175] This process is repeated until various patterns have been printed and dried on each of the six blank sheets. The six printed blank sheets are then stacked in a predetermined order while being positioned using sheet holes, etc., and then pressed under predetermined temperature and pressure conditions to form a laminate. The pressing process is carried out by applying heat and pressure using a laminator such as a known hydraulic press. The temperature, pressure, and time for heating and pressing depend on the laminator used, but can be determined appropriately to achieve good lamination.

[0176] The obtained laminate contains a plurality of sensor elements 101. The laminate is cut into individual sensor elements 101. The cut laminate is fired at a predetermined firing temperature to obtain the sensor elements 101. The firing temperature may be any temperature at which the solid electrolyte constituting the base portion 102 of the sensor element 101 is sintered to form a dense body and at which the electrodes and the like maintain a desired porosity. For example, firing is performed at a firing temperature of about 1300 to 1500°C.

[0177] The obtained sensor element 101 is incorporated into the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the gas to be measured and the rear end of the sensor element 101 contacts the reference gas. [Example]

[0178] Examples in which sensor elements were specifically fabricated and tested will be described below as examples, but the present invention is not limited to the following examples.

[0179] [Examples 1 to 16 and Comparative Examples 1 to 2] As Examples 1 to 16 and Comparative Examples 1 and 2, the sensor element 201 of Modification 1 shown in FIG. 4 was fabricated.

[0180] As described above, in the sensor element 201 of Modification 1, the inner oxygen pump electrode 90 is the inner main pump electrode 22. The length (L) of the sensor element 201 of the inner oxygen pump electrode 90 in the longitudinal direction is equal to the length (L1) of the sensor element 201 of the inner main pump electrode 22 in the longitudinal direction (L=L1).

[0181] The inner main pump electrode 22 includes an electrode end portion close to the tip end of the sensor element 201 and has a length (L A ) and a region (A) including an electrode end far from the tip of the sensor element 201 and having a length (L B ) and region (B) having L1 = L A +L B is.

[0182] The metal material of the inner main pump electrode 22 was mainly Pt with Au added. The region (A) was fabricated so that the concentration (content) of Au relative to the total amount of Pt and Au was higher than that of the region (B). Here, the region (A) is referred to as the high-concentration region (A). The region (B) is referred to as the low-concentration region (B).

[0183] As Examples 1 to 16 and Comparative Examples 1 and 2, the sensor element 201 shown in FIG. 4 was fabricated according to the manufacturing method of the sensor element 101 described above. Table 1 shows the results of the respective levels. The concentration of Au (wt%) relative to the total amount of Pt and Au in the high concentration region (A), The concentration of Au (wt%) relative to the total amount of Pt and Au in the low concentration region (B), The ratio of the longitudinal length (L) of the sensor element 201 of the high concentration region (A) to the longitudinal length (L) of the sensor element 201 of the inner oxygen pump electrode 90 (inner main pump electrode 22) A ) ratio (L A / L1)(%), and The concentration of Au in the low concentration region (B) (C B ) versus the Au concentration in the high concentration region (A) (C A ) ratio (Au concentration ratio: C A / C B ) This shows:

[0184] Specifically, electrode pastes with different concentrations of Au relative to the total amount of Pt and Au were prepared as the electrode paste for the inner main pump electrode 22. The concentrations of Au relative to the total amount of Pt and Au in each electrode paste were 0.10 wt%, 0.30 wt%, 0.50 wt%, 0.75 wt%, 0.90 wt%, 1.00 wt%, and 2.00 wt%.

[0185] The shape of the inner main pump electrode 22 was rectangular in all levels, with a length (L1) of 5.0 mm in the longitudinal direction of the sensor element 201 and a width of 2.0 mm perpendicular to the longitudinal direction of the sensor element 201. In all levels, the thickness of the inner main pump electrode 22 was 15 μm.

[0186] In each of Examples 1 to 16 and Comparative Examples 1 and 2, as shown in Table 1, the ratio (L AElectrode pastes with Au concentrations at each level were printed on the high-concentration region (A) of the inner main pump electrode 22. Electrode pastes with Au concentrations at each level were printed on the low-concentration region (B), which was the remaining region of the inner main pump electrode 22.

[0187] Other than that, the sensor elements of Examples 1 to 16 and Comparative Examples 1 and 2 were produced according to the above-mentioned method for producing the sensor element 101. Gas sensors incorporating the produced sensor elements were produced so that the determination tests described below could be performed.

[0188] [Judgment Test 1] The linearity of the NOx detection current Ip2 with respect to the oxygen concentration was determined by measurements using a model gas. Specifically, the following procedure was performed.

[0189] The gas sensor of Example 1 was measured in a model gas device. The gas sensor of Example 1 was attached to the measurement pipe of the model gas device. The gas sensor of Example 1 was driven. A model gas with NO=500 ppm and O2=0% was flowed through the measurement pipe, and the Ip2 current value (Ip2 (500,0) ) of the gas sensor of Example 1 was measured. Similarly, the Ip2 current value (Ip2 (500,5) , Ip2 (500,10) , Ip2 (500,18) ) was measured. The gas components other than NO and O2 in the model gas used for the measurement were H2O (3%) and N2 (balance).

[0190] The oxygen concentration of the model gas and the four measured Ip2 values ​​(Ip2 (500,0) , Ip2 (500,5) , Ip2 (500,10) , Ip2 (500,18) ) coefficient of determination R in the linear regression equation between 2 The coefficient of determination R 2 is referred to as the linearity of the NOx output. Similarly, for each of Examples 2 to 16 and Comparative Examples 1 and 2, the linearity R 2 was calculated.

[0191] The linearity of the NOx output R 2 The results were judged according to the following criteria (Judgment 1). A: Linearity of NOx output R 2 is 0.975 or more B: Linearity of NOx output R 2 is less than 0.975 and greater than or equal to 0.950 C: Linearity of NOx output R 2 is less than 0.950 and greater than or equal to 0.900 D: Linearity of NOx output R 2 is less than 0.900

[0192] If the result is A, B, or C, it is believed that NOx can be detected accurately even under high oxygen concentrations in actual use. In other words, it is believed that NOx can be detected and / or its concentration measured with high accuracy regardless of the oxygen concentration in the measurement gas.

[0193] [Judgment Test 2] A durability test was conducted using a diesel engine to evaluate the degree of deterioration in NOx detection sensitivity. The NOx sensitivity (Ip2 current value) of the gas sensor at an NO concentration of 500 ppm was measured before and after the durability test, and the rate of change in NOx sensitivity before and after the durability test was calculated. The rate of change in NOx sensitivity was used to evaluate and judge the degree of deterioration in NOx detection sensitivity. Specifically, the test was conducted as follows.

[0194] First, the gas sensor of Example 1 was measured in a model gas device. The gas sensor of Example 1 was attached to the measurement pipe of the model gas device. The gas sensor of Example 1 was driven. A model gas with NO=500 ppm and O2=0% was flowed through the measurement pipe, and the Ip2 current value (Ip2 fresh ) was measured. Similarly, for each of Examples 2 to 16 and Comparative Examples 1 and 2, the Ip2 current value (Ip2 fresh ) was measured. The gas components other than NO and O2 in the model gas used for the measurement were H2O (3%) and N2 (balance).

[0195] Next, a durability test was conducted using a diesel engine. The gas sensors of Examples 1 to 16 and Comparative Examples 1 and 2 were attached to the exhaust pipe of an automobile. Then, the gas sensors of Examples 1 to 16 and Comparative Examples 1 and 2 were driven. Under this condition, a 40-minute driving pattern consisting of an engine speed of 1500 to 3500 rpm and a load torque in the range of 0 to 350 N·m was repeated until 4000 hours had elapsed. The gas temperature at that time was 200°C to 600°C, and the NOx concentration was 0 to 1500 ppm.

[0196] The durability test was temporarily stopped when 1000 hours had elapsed since the start of the test, and the gas sensors of Examples 1 to 16 and Comparative Examples 1 and 2 were taken out. The Ip2 current value (Ip2 aged1000H ) was measured.

[0197] The amount of change in NOx detection sensitivity before and after the durability test was calculated for each of the gas sensors of Examples 1 to 16 and Comparative Examples 1 and 2. That is, the Ip2 current value (Ip2 fresh ) after 1000 hours of durability testing (Ip2 aged1000H The rate of change in NOx sensitivity was calculated.

[0198] NOx sensitivity change rate (%) = (Ip2 aged1000H / Ip2 fresh -1) x 100

[0199] Ip2 current value after 1000 hours of durability test (Ip2 aged1000H ), the gas sensors of Examples 1 to 16 and Comparative Examples 1 and 2 were again attached to the exhaust gas pipe. Then, the durability test using the diesel engine was resumed and continued until the cumulative elapsed time reached 2000 hours.

[0200] After 2000 hours of the durability test, the Ip2 current value (Ip2 fresh ) after 2000 hours of durability testing (Ip2 aged2000H The rate of change in NOx sensitivity was calculated.

[0201] Similarly, Ip2 current value before endurance test (Ip2 fresh ) after 3000 hours of durability testing (Ip2 aged3000H ) change rate (NOx sensitivity change rate), and Ip2 current value before endurance test (Ip2 fresh ) after 4000 hours of durability testing (Ip2 aged4000H The rate of change in NOx sensitivity was calculated.

[0202] Based on the rate of change (%) in NOx sensitivity after 3000 hours of durability testing, the test was judged according to the following criteria (Judgment 2). A: NOx sensitivity change rate is within ±10% B: NOx sensitivity change rate is greater than ±10% and within ±20% C: NOx sensitivity change rate is greater than ±20% and within ±30% D: NOx sensitivity change rate is greater than ±30%

[0203] If the result of the evaluation is A, B, or C after 3000 hours of the durability test, it is believed that NOx can be detected accurately in actual use even when used for a long period of time.

[0204] Table 1 shows the results of the evaluations (Evaluation 1 and Evaluation 2) for Examples 1 to 16 and Comparative Examples 1 and 2, as well as the NOx sensitivity change rates (%) after 1000 hours, 2000 hours, 3000 hours, and 4000 hours of the durability test in Evaluation Test 2. As described above, the Au concentration relative to the total amount of Pt and Au in the high concentration region (A) and the low concentration region (B) at each level, and the proportion of the high concentration region (A) in the inner main pump electrode 22 (L A / L1), and Au concentration ratio (C A / C B ) are also shown. Furthermore, Fig. 8 shows the durability test results for Examples 1 to 9 and Comparative Examples 1 and 2. The vertical axis of the graph shows the rate of change in NOx sensitivity (%), and the horizontal axis shows the durability test time (hours). Fig. 9 shows the durability test results for Examples 10 to 16 and Comparative Examples 1 and 2. The vertical axis of the graph shows the rate of change in NOx sensitivity (%), and the horizontal axis shows the durability test time (hours).

[0205] [Table 1]

[0206] With regard to the criteria 1 and 2, Examples 1 to 16 all obtained good results.

[0207] In this way, the total length (L = L1) of the sensor element 101 of the inner oxygen pump electrode 90 (inner main pump electrode 22) in the longitudinal direction of the sensor element 101 in the high concentration region (A) is A ) ratio (L A / L1) is in the range of 15.0% to 90.0%, the linearity R of the NOx detection current Ip2 of judgment 1 2 It was confirmed that good results were obtained for both the NOx sensitivity change rate in judgment 1 and judgment 2.

[0208] This indicates that the device can detect NOx accurately even under high oxygen concentrations, and that the NOx detection sensitivity can be maintained even after long-term use.

[0209] Comparative Example 1 can be compared with Examples 1 to 5. In Comparative Example 1, the linearity R of the NOx detection current Ip2 in Judgment 1 is 2 On the other hand, the NOx sensitivity change rate of the judgment 2 was judged as A. In the comparative example 1, the proportion of the high concentration region (A) in the inner main pump electrode 22 (L A / L1) was set to 5%. In Comparative Example 1, the high concentration region (A) was small compared to the region where the applied voltage Vp0 locally increased on the inner main pump electrode 22, which is thought to have caused NOx to decompose on the inner main pump electrode 22.

[0210] In Comparative Example 2, the Au concentration in the high concentration region (A) and the low concentration region (B) was set to the same 0.75 wt %. That is, the Au concentration in the metal material was set to 0.75 wt % over the entire area of ​​the inner main pump electrode 22 (L A / L1:100%). In Comparative Example 2, the linearity R 2 On the other hand, the NOx sensitivity change rate for judgment 2 was rated D.

[0211] In Comparative Example 2, the Au concentration was higher than in Examples 1 to 16, even at a position far from the tip of the sensor element 201 of the inner main pump electrode 22, i.e., at a position close to the measurement electrode 44. Therefore, it is presumed that a large amount of Au evaporated from the inner main pump electrode 22 during the durability test of Evaluation Test 2, and that a large amount of the evaporated Au also adhered to the measurement electrode 44. As a result, it is presumed that the NOx decomposition performance of the measurement electrode 44 was reduced after the durability test. It is presumed that, after the durability test, not all of the NOx in the measurement gas that reached the measurement electrode 44 could be decomposed, and the NOx detection current value Ip2 was smaller than it should have been. Therefore, it is presumed that the NOx sensitivity change rate in Evaluation Test 2 was large in Comparative Example 2.

[0212] [Examples 17 to 21] As Examples 17 to 21, the sensor elements 101 shown in FIGS. 1 and 2 were produced according to the method for producing the sensor element 101 described above. Table 2 shows the results of the measurements at each level. The concentration of Au (wt%) relative to the total amount of Pt and Au in the high concentration region (A), The concentration of Au (wt%) relative to the total amount of Pt and Au in the low concentration region (B), The ratio of the longitudinal length (L) of the sensor element 101 of the high concentration region (A) to the total length (L=L1+L2) of the sensor element 101 of the inner oxygen pump electrode 90 (the inner main pump electrode 22 and the auxiliary pump electrode 51) in the longitudinal direction A ) ratio [L A / (L1+L2)](%), and The concentration of Au in the low concentration region (B) (C B ) versus the Au concentration in the high concentration region (A) (C A ) ratio (Au concentration ratio: C A / C B ) This shows:

[0213] Specifically, electrode pastes with different concentrations of Au relative to the total amount of Pt and Au were prepared for the inner main pump electrode 22 and the auxiliary pump electrode 51. The concentrations of Au relative to the total amount of Pt and Au in each electrode paste were 0.40 wt%, 0.50 wt%, 0.60 wt%, 0.80 wt%, 1.00 wt%, and 2.00 wt%.

[0214] In Examples 17 to 21, the ceiling electrode portion 22a and the bottom electrode portion 22b of the inner main pump electrode 22 had the same shape. In all levels of Examples 17 to 21, the ceiling electrode portion 22a and the bottom electrode portion 22b were each rectangular, with a length (L1) of 3.5 mm in the longitudinal direction of the sensor element 101 and a width of 2.5 mm perpendicular to the longitudinal direction of the sensor element 101. In all levels, the thickness of the inner main pump electrode 22 was 15 μm.

[0215] In Examples 17 to 21, the ceiling electrode portion 51a and the bottom electrode portion 51b of the auxiliary pump electrode 51 had the same shape. In all levels of Examples 17 to 21, the ceiling electrode portion 51a and the bottom electrode portion 51b were each rectangular, with a length (L2) of 2.0 mm in the longitudinal direction of the sensor element 101 and a width of 1.5 mm perpendicular to the longitudinal direction of the sensor element 101. In all levels, the thickness of the auxiliary pump electrode 51 was 15 μm.

[0216] In each of Examples 17 to 21, as shown in Table 2, the ratio [L A The electrode paste with each level of Au concentration was printed on the high-concentration region (A) of the inner main pump electrode 22 and the auxiliary pump electrode 51. The electrode paste with each level of Au concentration was printed on the low-concentration region (B) of the inner main pump electrode 22 and the auxiliary pump electrode 51.

[0217] Other than that, the sensor elements of Examples 17 to 21 were fabricated according to the above-described manufacturing method of the sensor element 101, similarly to Examples 1 to 16 and Comparative Examples 1 and 2. Similarly to Examples 1 to 16 and Comparative Examples 1 and 2, gas sensors of Examples 17 to 21 incorporating the fabricated sensor elements were fabricated.

[0218] The gas sensors of Examples 17 to 21 were subjected to the above-described evaluation tests 1 and 2 in the same manner as in Examples 1 to 16 and Comparative Examples 1 and 2. Table 2 shows the evaluation results (e.g., evaluation 1 and evaluation 2) for Examples 17 to 21, as well as the rate of change in NOx sensitivity (%) after 1000 hours, 2000 hours, 3000 hours, and 4000 hours of the durability test in Evaluation Test 2. Furthermore, Fig. 10 shows the durability test results for Examples 17 to 21. The vertical axis of the graph represents the rate of change in NOx sensitivity (%), and the horizontal axis represents the durability test time (hours).

[0219] [Table 2]

[0220] With regard to the criteria 1 and 2, Examples 17 to 21 all obtained good results.

[0221] The sensor elements 201 of Examples 1 to 16 described above are configured to adjust the oxygen partial pressure in the measurement gas to a value that does not substantially affect the measurement of NOx at the measurement electrode 44 by operating the main pump cell 21. The inner oxygen pump electrode 90 in the sensor element 201 is the inner main pump electrode 22. On the other hand, the sensor elements 101 of Examples 17 to 21 are configured to adjust the oxygen partial pressure in the measurement gas to a value that does not substantially affect the measurement of NOx at the measurement electrode 44 by operating the main pump cell 21 and the auxiliary pump cell 50. The inner oxygen pump electrode 90 in the sensor element 101 is the inner main pump electrode 22 and the auxiliary pump electrode 51.

[0222] In the judgment 1 and the judgment 2, Examples 1 to 16 and Examples 17 to 21 all obtained good results. That is, by setting the high concentration region (A) in a predetermined range for the entire inner oxygen pump electrode 90, the linearity R 2 It was confirmed that good results could be obtained for both the NOx sensitivity change rate in judgment 1 and judgment 2.

[0223] [Examples 22 to 26] 5 and 6 were fabricated according to the manufacturing method of the sensor element 101 described above as Examples 22 to 26. The sensor element 301 faces one internal space 14 and has an inner main pump electrode 22 disposed on the lower surface of the second solid electrolyte layer 6 at a position close to the tip of the sensor element 301. An auxiliary pump electrode 51 and a measurement electrode 44 are disposed in parallel in the longitudinal direction of the sensor element 301 on the upper surface of the first solid electrolyte layer 4 at positions farther from the tip of the sensor element 301 than the inner main pump electrode 22.

[0224] Table 3 shows the results at each level. The concentration of Au (wt%) relative to the total amount of Pt and Au in the high concentration region (A), The concentration of Au (wt%) relative to the total amount of Pt and Au in the low concentration region (B), The total length (L = L1 + L2) of the sensor element 101 of the inner oxygen pump electrode 90 (the inner main pump electrode 22 and the auxiliary pump electrode 51) in the longitudinal direction of the sensor element 101 in the high concentration region (A) A ) ratio [L A / (L1+L2)](%), and The concentration of Au in the low concentration region (B) (C B ) versus the Au concentration in the high concentration region (A) (C A ) ratio (Au concentration ratio: C A / C B ) This shows:

[0225] Specifically, electrode pastes with different concentrations of Au relative to the total amount of Pt and Au were prepared for the inner main pump electrode 22 and the auxiliary pump electrode 51. The concentrations of Au relative to the total amount of Pt and Au in each electrode paste were 0.20 wt%, 0.30 wt%, 0.50 wt%, 0.60 wt%, 1.00 wt%, and 2.00 wt%.

[0226] In all levels of Examples 22 to 26, the inner main pump electrode 22 was rectangular, with a length (L1) of 5.0 mm in the longitudinal direction of the sensor element 301 and a width of 2.0 mm perpendicular to the longitudinal direction of the sensor element 301. In all levels, the thickness of the inner main pump electrode 22 was 15 μm.

[0227] In all levels of Examples 22 to 26, the auxiliary pump electrode 51 was rectangular, with a length (L2) of 1.5 mm in the longitudinal direction of the sensor element 301 and a width of 0.5 mm perpendicular to the longitudinal direction of the sensor element 301. In all levels, the thickness of the auxiliary pump electrode 51 was 15 μm.

[0228] In each of Examples 22 to 26, as shown in Table 3, the ratio [L AThe electrode paste with each level of Au concentration was printed on the high-concentration region (A) of the inner main pump electrode 22 and the auxiliary pump electrode 51. The electrode paste with each level of Au concentration was printed on the low-concentration region (B) of the inner main pump electrode 22 and the auxiliary pump electrode 51.

[0229] Other than that, the sensor elements of Examples 22 to 26 were fabricated according to the manufacturing method of the sensor element 101 described above, similarly to Examples 1 to 16 and Comparative Examples 1 and 2. Similarly to Examples 1 to 16 and Comparative Examples 1 and 2, gas sensors of Examples 22 to 26 incorporating the fabricated sensor elements were fabricated.

[0230] The gas sensors of Examples 22 to 26 were subjected to the above-described evaluation tests 1 and 2 in the same manner as in Examples 1 to 16 and Comparative Examples 1 and 2. Table 3 shows the evaluation results (e.g., evaluation 1 and evaluation 2) for Examples 22 to 26, as well as the rate of change in NOx sensitivity (%) after 1000 hours, 2000 hours, 3000 hours, and 4000 hours of the durability test in Evaluation Test 2. Furthermore, Fig. 11 shows the results of the durability test for Examples 22 to 26. The vertical axis of the graph represents the rate of change in NOx sensitivity (%), and the horizontal axis represents the durability test time (hours).

[0231] [Table 3]

[0232] With regard to the criteria 1 and 2, Examples 22 to 26 all obtained good results.

[0233] In the sensor elements 301 of Examples 22 to 26, the auxiliary pump electrode 51 and the measurement electrode 44 are arranged in parallel in the longitudinal direction of the sensor element 301 at a position farther from the tip of the sensor element 301 than the inner main pump electrode 22. On the other hand, in the sensor elements 101 of Examples 17 to 21, the auxiliary pump electrode 51 and the measurement electrode 44 are arranged in series in this order at a position farther from the tip of the sensor element 101 than the inner main pump electrode 22.

[0234] In the judgments 1 and 2, Examples 17 to 21 and Examples 22 to 26 all obtained good results. That is, even when the auxiliary pump electrode 51 is arranged in parallel with the measurement electrode 44 as in Examples 22 to 26, the linearity R of the NOx detection current Ip2 in the judgment 1 can be improved by setting the high concentration region (A) within a predetermined range for the entire inner oxygen pump electrode 90. 2 It was confirmed that good results could be obtained for both the NOx sensitivity change rate in judgment 1 and judgment 2. [Explanation of symbols]

[0235] 1 First board layer 2 Second board layer 3 Third board layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion-controlled section 12 Buffer space 13 Second diffusion-controlled section 14 Internal void 15 Measurement gas flow section 20 1st internal void 21 Main pump cell 22 Inner main pump electrode 22a (Inner main pump electrode) ceiling electrode part 22b Bottom electrode part (of inner main pump electrode) 23 Outer pump electrode 24 Variable power supply (for main pump cell) 30 Third diffusion-controlled section 40 Second internal void 41 Measuring pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measuring electrode 46 Variable power supply (for measuring pump cell) 48 Atmospheric introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51a (auxiliary pump electrode) ceiling electrode part 51b (auxiliary pump electrode) bottom electrode part 52 Variable power supply (for auxiliary pump cell) 53 Second auxiliary pump electrode 60 4th diffusion-controlled section 61 3rd internal void 70 Heater section 71 Heater electrode 72 Heater 73 through holes 74 Heater insulator 75 Pressure relief hole 76 Heater lead 80 Oxygen partial pressure detection sensor cell for main pump control 81 Oxygen partial pressure detection sensor cell for auxiliary pump control 82 Oxygen partial pressure detection sensor cell for measuring pump control 83 Sensor Cell 90 Inner oxygen pump electrode 100 Gas Sensor 101, 201, 301, 401 sensor elements 102, 202, 302 Base part

Claims

1. a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from a gas inlet formed at one end of the base section in the longitudinal direction and for flowing the measurement gas; an inner oxygen pump electrode disposed on an inner surface of the measurement gas flow portion; a measuring electrode disposed on an inner surface of the measurement target gas flow portion; A sensor element comprising: The inner oxygen pump electrode has a predetermined length (L) in the longitudinal direction, The electrode end portion near the gas inlet and the electrode portion having a predetermined length (L Ａ ) a region (A) having The electrode end portion farther from the gas inlet and having a predetermined length (L Ｂ ) and a region (B) having Including, the metal material contained in the inner oxygen pump electrode contains an activity-reducing metal that reduces catalytic activity for decomposing NOx, the content of the activity-reducing metal in the metal material in the region (A) is higher than the content of the activity-reducing metal in the metal material in the region (B); The longitudinal length (L) of the inner oxygen pump electrode relative to the longitudinal length (L) of the inner oxygen pump electrode Ａ ) ratio (L Ａ / L) is 15% to 90%, A sensor element for detecting NOx in a measurement gas, wherein the content of the activity-reducing metal in the metal material in the region (A) of the inner oxygen pump electrode is 0.5% by weight to 2.0% by weight.

2. the inner oxygen pump electrode includes a plurality of electrodes disposed on the inner surface of the measurement gas flow portion, 2. The sensor element according to claim 1, wherein the length (L) of the inner oxygen pump electrode in the longitudinal direction is the sum of the lengths of the plurality of electrodes in the longitudinal direction.

3. The inner oxygen pump electrode an inner main pump electrode disposed on an inner surface of the measurement gas flow portion; an auxiliary pump electrode disposed on the inner surface of the measurement gas flow portion at a position farther from the gas inlet than the inner main pump electrode; Including, The longitudinal length (L) of the inner oxygen pump electrode is equal to the longitudinal length (L) of the inner main pump electrode. １ ) and the longitudinal length (L ２ ) and the sum (L １ +L ２ 3. The sensor element according to claim 1, wherein

4. 4. The sensor element according to claim 3, wherein the auxiliary pump electrode and the measurement electrode are disposed in series in this order in the longitudinal direction at a position on the inner surface of the measurement gas flow portion farther from the gas inlet than the inner main pump electrode.

5. 4. The sensor element according to claim 3, wherein the auxiliary pump electrode and the measurement electrode are disposed in parallel in the longitudinal direction at a position on the inner surface of the measurement gas flow portion farther from the gas inlet than the inner main pump electrode.

6. The longitudinal length (L) of the inner oxygen pump electrode relative to the longitudinal length (L) of the inner oxygen pump electrode Ａ ) ratio (L Ａ 6. The sensor element according to claim 1, wherein the ratio of the surface area to the surface area of ​​the sensor is 30% to 70%.

7. 7. The sensor element according to claim 1, wherein the activity-reducing metal includes at least one selected from the group consisting of gold and silver.

8. The sensor element according to any one of claims 1 to 7, wherein the content of the activity-reducing metal in the metal material in the region (B) of the inner oxygen pump electrode is 0.1 wt% to 0.5 wt%, provided that the content of the activity-reducing metal in the metal material in the region (A) is lower than the content of the activity-reducing metal in the metal material in the region (B).

9. The content (C) of the activity-reducing metal in the metal material in the region (B) of the inner oxygen pump electrode Ｂ ) the content (C Ａ ) ratio (C Ａ / C Ｂ 9. The sensor element according to claim 1, wherein the value of (a) is 1.5 or more and 20.0 or less.

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