Gas sensor element and gas sensor

Abstract

A gas sensor element (1) includes: a solid electrolyte body (2) having oxygen ion conductivity; a chamber (13) which faces a first surface (21) of the solid electrolyte body (2) and into which a measurement target gas is introduced; a chamber formation layer (3) which is laminated on the first surface (21) side of the solid electrolyte body (2) and which forms the chamber (13); a duct (14) which faces a second surface (22) of the solid electrolyte body (2) and into which a reference gas is introduced; and a duct formation layer (4) which is laminated on the second surface (22) side of the solid electrolyte body (2) and which forms the duct (14). At least an element tip part closer to the tip side as compared to the base end of the chamber (13) is covered with a porous protective layer (5). When the thickness of the protective layer (5) at the same X position, in the lamination direction, as the solid electrolyte body (2) is defined as L and the thickness of the solid electrolyte body (2) is defined as d, d / L≥1 is satisfied.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Application No. 2023-047901, filed on March 24, 2023, the contents of which are incorporated herein by reference. [Technical Field]

[0002] The present disclosure relates to a gas sensor element and a gas sensor. [Background technology]

[0003] Gas sensor elements for detecting the concentration of a specific gas in a measured gas include multilayer gas sensor elements formed by stacking multiple ceramic layers including a solid electrolyte. For example, as disclosed in Patent Document 1, a multilayer gas sensor element has a tip end covered with a porous protective layer. The provision of such a protective layer is intended to trap poisons in the measured gas and improve water resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-80100 Summary of the Invention

[0005] A multilayer gas sensor element having a protective layer has a problem in that measurement errors may occur, i.e., it is difficult to stably ensure high measurement accuracy with a multilayer gas sensor element having a protective layer.

[0006] The present disclosure aims to provide a gas sensor element and a gas sensor that can improve measurement accuracy.

[0007] One aspect of the present disclosure is a solid electrolyte body having oxygen ion conductivity; a chamber facing the first surface of the solid electrolyte body and into which a gas to be measured is introduced; a chamber forming layer laminated on the first surface side of the solid electrolyte body to form the chamber; a duct facing the second surface of the solid electrolyte body and through which a reference gas is introduced; a duct forming layer laminated on the second surface side of the solid electrolyte body and forming the duct, At least a distal end portion of the element that is distal to the proximal end of the chamber is covered with a porous protective layer, The gas sensor element satisfies d / L≧1, where L is the thickness of the protective layer at the same position in the stacking direction as the solid electrolyte body, and d is the thickness of the solid electrolyte body.

[0008] Another aspect of the present disclosure is a gas sensor including the gas sensor element described above, a housing that holds the gas sensor element, and an element cover that is attached to a front end side of the housing and surrounds the gas sensor element from the front end side, The element cover is provided in the gas sensor and has a vent hole at a position facing the chamber in the gas sensor element.

[0009] In the gas sensor element, the thickness L of the protective layer at the same position in the stacking direction as the solid electrolyte body and the thickness d of the solid electrolyte body satisfy d / L ≥ 1. This can improve the measurement accuracy of the gas sensor element.

[0010] The gas sensor includes the gas sensor element that satisfies d / L≧1. This allows for a gas sensor with high measurement accuracy. The element cover also has a vent hole at a position facing the chamber in the gas sensor element. This effectively improves measurement accuracy.

[0011] As described above, according to the above-described aspects, it is possible to provide a gas sensor element and a gas sensor that can improve measurement accuracy. [Brief explanation of the drawings]

[0012] The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: [Figure 1] FIG. 1 is a cross-sectional view of a gas sensor element according to a first embodiment, taken along a line II in FIG. 2, and perpendicular to an X-direction; [Figure 2] FIG. 2 is a cross-sectional view of the gas sensor element according to the first embodiment, taken along a line II-II in FIG. 1, and perpendicular to the Y direction; [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III in FIG. 1; [Figure 4] FIG. 4 is a perspective view of a tip portion of a main body of a gas sensor element according to the first embodiment; [Figure 5] FIG. 5 is a cross-sectional view taken along line VV in FIG. 2; [Figure 6] FIG. 6 is a cross-sectional view of a gas sensor element having a large thickness L, taken along a line perpendicular to the X direction. [Figure 7] FIG. 7 is a perspective view of a tip portion of a main body of a gas sensor element according to a second embodiment; [Figure 8] FIG. 8 is a cross-sectional view of the gas sensor element according to the second embodiment, taken along a line perpendicular to the Y direction; [Figure 9] FIG. 9 is a cross-sectional view of the chamber formation layer perpendicular to the Z direction in the second embodiment; [Figure 10] FIG. 10 is a cross-sectional view of the gas sensor element according to the third embodiment, taken along a line perpendicular to the X direction; [Figure 11] FIG. 11 is a cross-sectional view of the gas sensor element according to the third embodiment, taken along a line perpendicular to the Y direction; [Figure 12] FIG. 12 is a diagram showing test results in Experimental Example 1; [Figure 13] FIG. 13 is a cross-sectional view of a portion of a gas sensor perpendicular to the Z direction in accordance with a fourth embodiment; [Figure 14] FIG. 14 is an explanatory diagram of Sample 1 in Experimental Example 2; [Figure 15] FIG. 15 is an explanatory diagram of Sample 3 in Experimental Example 2; [Figure 16] FIG. 16 is a diagram showing test results in Experimental Example 2; [Figure 17] FIG. 17 is a cross-sectional view of the gas sensor element in the first modified embodiment, taken along a line perpendicular to the X direction; [Figure 18] FIG. 18 is a cross-sectional view of a gas sensor element in a second modified embodiment, taken along a line perpendicular to the X direction; [Figure 19] FIG. 19 is a cross-sectional view of the gas sensor element in the third modified embodiment, taken along a line perpendicular to the X direction. DETAILED DESCRIPTION OF THE INVENTION

[0013] (Embodiment 1) An embodiment of a gas sensor element and a gas sensor will be described with reference to FIGS. As shown in FIGS. 1 and 2, the gas sensor element 1 of this embodiment includes a solid electrolyte body 2, a chamber 13, a chamber forming layer 3, a duct 14, and a duct forming layer 4.

[0014] The solid electrolyte body 2 has oxygen ion conductivity. The chamber 13 faces the first surface 21 of the solid electrolyte body 2 and is a space into which a measurement gas is introduced. The chamber forming layer 3 is laminated on the first surface 21 side of the solid electrolyte body 2 to form the chamber 13. The duct 14 faces the second surface 22 of the solid electrolyte body 2 and is a space into which a reference gas is introduced. The duct forming layer 4 is laminated on the second surface 22 side of the solid electrolyte body 2 to form the duct 14.

[0015] At least the tip of the element, which is closer to the tip than the base end of the chamber 13, is covered with a porous protective layer 5. In the gas sensor element 1 of this embodiment, when the thickness of the protective layer 5 at the same position in the stacking direction as the solid electrolyte body 2 is L and the thickness of the solid electrolyte body 2 is d, the relationship d / L≧1 is satisfied.

[0016] The gas sensor element 1 of this embodiment is a laminated gas sensor element in which multiple ceramic layers are stacked. In the gas sensor element 1, a sensor electrode 61 and a reference gas side electrode 62 are formed on one surface (i.e., the first surface 21) and the other surface (i.e., the second surface 22) of a plate-shaped solid electrolyte body 2, respectively. The sensor electrode 61 and the reference gas side electrode 62 are arranged opposite each other with part of the solid electrolyte body 2 interposed therebetween. A sensor cell is formed by the sensor electrode 61, the reference gas side electrode 62, and a portion of the solid electrolyte body 2 between the sensor electrode 61 and the reference gas side electrode 62. The sensor electrode 61 is active with respect to a specific gas in the measurement gas. For example, the measurement gas is exhaust gas from an internal combustion engine, and the specific gas is oxygen. The sensor electrode 61 active with respect to oxygen contains, for example, platinum (Pt) and gold (Au) or rhodium (Rh).

[0017] As described above, the gas sensor element 1 is formed by laminating the chamber-forming layer 3 on the first surface 21 of the solid electrolyte body 2. The chamber-forming layer 3 is composed of buffer layers 31 and 32 and a shielding layer 33, which are sequentially laminated on the first surface 21 of the solid electrolyte body 2. The duct-forming layer 4 is laminated on the second surface 22 of the solid electrolyte body 2 via a buffer layer 41. The buffer layer 41 is also a part of the duct-forming layer 4 that forms the duct 14, but in this embodiment, the portion excluding the buffer layer 41 may also be referred to as the duct-forming layer 4. In addition, the heater layer 11 is laminated on the surface of the duct-forming layer 4 opposite to the solid electrolyte body 2. A heater wire 111 is formed between the heater layer 11 and the duct-forming layer 4. In addition, the heater layer 11 and the duct-forming layer 4 may be integrated, with no particular boundary between them.

[0018] In this specification, the stacking direction of multiple ceramic layers is referred to as the Z direction as appropriate. The gas sensor element 1 has a rod-like shape that is elongated in one direction perpendicular to the Z direction. The longitudinal direction of the gas sensor element 1 is referred to as the X direction as appropriate. The direction perpendicular to both the X direction and the Z direction is referred to as the Y direction as appropriate.

[0019] As shown in FIG. 3 , when viewed from the Z direction, the chamber 13 is surrounded by buffer layers 31 and 32, and a porous diffusion layer 15 is disposed on a portion of the outer periphery of the chamber 13. In this embodiment, the diffusion layers 15 are disposed on both sides of the chamber 13 in the Y direction. This allows the measurement gas to be introduced into the chamber 13 from both sides in the Y direction through the diffusion layers 15. That is, as shown in FIGS. 3 , 4 , and 5 , measurement gas inlets 150 to the chamber 13 are provided on the side surfaces of the main body 100 of the gas sensor element 1 facing both sides in the Y direction. Here, the main body 100 refers to the gas sensor element 1 in a state where the protective layer 5 is not formed, and FIG. 4 shows a perspective view of the tip of the main body 100. In this embodiment, the chamber 13 is longer in the X direction than in the Y direction.

[0020] In this embodiment, the solid electrolyte 2 is a ceramic layer whose main component is zirconia. The shielding layer 33, the duct forming layer 4, and the heater layer 11 are all ceramic layers whose main component is alumina. The diffusion layer 15 is also made of alumina. However, the diffusion layer 15 is made of a porous ceramic body that allows the measurement gas to pass through. On the other hand, the solid electrolyte 2, the shielding layer 33, the duct forming layer 4, the heater layer 11, and the buffer layers 31, 32, and 41 are made of a dense ceramic body that does not allow gas to pass through.

[0021] The buffer layers 31 and 32 are made of a material having a linear expansion coefficient between that of the solid electrolyte body 2 and the shielding layer 33. The buffer layer 41 is made of a material having a linear expansion coefficient between that of the solid electrolyte body 2 and the duct forming layer 4. For example, the buffer layers 31, 32, and 41 contain alumina and zirconia.

[0022] Moreover, the protective layer 5 is made of porous ceramic. In this embodiment, the porous ceramic forming the protective layer 5 is made of alumina. As shown in FIGS. 1 and 2, the protective layer 5 is formed so as to cover the entire periphery of the tip end of the element and the tip surface. As described above, when the thickness of the protective layer 5 at the same position in the stacking direction as the solid electrolyte body 2 is L and the thickness of the solid electrolyte body 2 is d, the relationship d / L≧1 is satisfied. In other words, the thickness L of the protective layer 5 is equal to or less than the thickness d of the solid electrolyte body 2.

[0023] In this embodiment, the thickness of the protective layer 5 at the portions covering the side surfaces of the gas sensor element 1 in the Y direction is substantially constant, and the thickness L is equal to or less than the thickness d of the solid electrolyte body 2. The thickness of the portion covering the front end surface of the gas sensor element 1 is also substantially constant, and the thickness L is equal to or less than the thickness d of the solid electrolyte body 2. The thicknesses L of the protective layers 5 at both sides of the gas sensor element 1 in the Y direction and the thickness L of the protective layer 5 at the front end surface of the element all satisfy d / L≧1.

[0024] In this embodiment, as described above, the inlet 150 for the measurement gas is provided on the side surface facing the Y direction. Therefore, considering the inflow route of the reference gas leaking from the duct 14 from the inlet 150 through the diffusion layer 15 into the chamber 13, it is considered more important to reduce the thickness L of each of the protective layers 5 on both sides of the gas sensor element 1 in the Y direction. Therefore, it may be possible to make the thickness L of the protective layer 5 on both sides of the gas sensor element 1 in the Y direction smaller than the thickness L of the protective layer 5 at the tip face of the element.

[0025] The thickness L of the protective layer 5 can be set to, for example, 10 to 400 μm, and the porosity of the protective layer 5 can be set to, for example, 20 to 70%.

[0026] Next, an example of a method for manufacturing the gas sensor element 1 of this embodiment will be outlined. First, prepare a ceramic green sheet to form the solid electrolyte body 2, a ceramic green sheet to form the shielding layer 33, a ceramic green sheet to form the duct forming layer 4, a ceramic green sheet to form the heater layer 11, and a ceramic green sheet to form the diffusion layer 15. The diffusion layer 15 can also be formed by printing a ceramic paste instead of using the ceramic green sheets.

[0027] The ceramic green sheets for the duct forming layer 4 and the ceramic green sheets for the heater layer 11 are laminated together. Prior to this lamination, a conductive paste for heater wiring is printed on the heater layer 11. The duct forming layer 4 can also be formed by laminating a plurality of ceramic green sheets. Hereinafter, the laminated and integrated ceramic green sheets for the duct forming layer 4 and the ceramic green sheets for the heater layer 11 will be referred to as the duct-side laminate sheet, as appropriate.

[0028] A sensor electrode 61 and a reference gas side electrode 62 are formed using conductive paste on the first surface 21 and the second surface 22 of the ceramic green sheet that will become the solid electrolyte body 2. In addition, ceramic paste that will become the buffer layer 31 is applied to predetermined positions on the first surface 21 of the ceramic green sheet that will become the solid electrolyte body 2.

[0029] Furthermore, a ceramic green sheet for the diffusion layer 15 is placed (or ceramic paste for the diffusion layer 15 is printed) at a predetermined position on the surface of the ceramic green sheet for the shielding layer 33 facing the solid electrolyte body 2, and the ceramic paste for the buffer layer 32 is applied. The ceramic green sheet for the shielding layer 33 coated with the ceramic paste is laminated and pressure-bonded to the first surface 21 of the ceramic green sheet for the solid electrolyte body 2.

[0030] Furthermore, a ceramic paste that will become the buffer layer 41 is applied to a predetermined position on the surface of the duct-side laminate sheet that faces the solid electrolyte body 2. The duct-side laminate sheet to which the ceramic paste has been applied is laminated and pressure-bonded to the second surface 22 of the ceramic green sheet for the solid electrolyte body 2. The laminate in this state is fired to form the main body 100 of the gas sensor element 1.

[0031] Next, the tip portion of the body 100 of the gas sensor element 1 is immersed in a ceramic slurry for forming the protective layer 5. As a result, the ceramic slurry adheres to the tip portion of the gas sensor element 1. The adhered ceramic slurry is dried and fired to form the protective layer 5.

[0032] The thickness L of the protective layer 5 can be controlled by adjusting, for example, the viscosity and surface tension of the ceramic slurry, the average particle size of the ceramic powder, and the like.

[0033] The gas sensor element 1 of this embodiment is incorporated into a gas sensor attached to the exhaust system of an internal combustion engine, for example. Exhaust gas as the measurement gas is introduced into the chamber 13, and air as the reference gas is introduced into the duct 14. When a predetermined voltage is applied to the sensor cell, a predetermined current flows depending on the difference in oxygen concentration between the duct 14 and the chamber 13. Within a certain applied voltage range, the output current value hardly changes even when the applied voltage changes. This current is called a limiting current. The oxygen concentration in the exhaust gas can be detected based on this limiting current value. In this way, the gas sensor element 1 of this embodiment can be used as an element of a so-called limiting current type air-fuel ratio sensor.

[0034] Furthermore, the protective layer 5 has a function of trapping, for example, poisoning substances present in exhaust gas. This can prevent the poisoning substances from entering the chamber 13. The protective layer 5 also has a function of suppressing cracking due to water. When starting an internal combustion engine, for example, it is conceivable that moisture present in the exhaust gas pipe may fly to the gas sensor element 1 along with the exhaust gas. In such a case, the presence of the protective layer 5 can prevent moisture from adhering directly to the main body 100 of the element. This can suppress element cracking (i.e., cracking due to water) caused by stress generated by the adhesion of moisture.

[0035] Next, the effects of this embodiment will be described. In the gas sensor element 1, the thickness L of the protective layer 5 at the same position in the stacking direction as the solid electrolyte body 2 and the thickness d of the solid electrolyte body 2 satisfy d / L≧1. This improves the measurement accuracy of the gas sensor element 1. The mechanism behind this improvement will be explained below. In the following explanation, the measurement gas is assumed to be exhaust gas, the reference gas is assumed to be air, and the specific gas is assumed to be oxygen.

[0036] As described above, the solid electrolyte body 2 and the duct forming layer 4 are in close contact with each other. However, due to differences in the materials used, it is difficult to achieve a complete airtight seal. Therefore, very small gaps may occur in some areas between the solid electrolyte body 2 and the duct forming layer 4. If oxygen leaks from the duct 14 through such gaps, the leaked oxygen will first reach the protective layer 5.

[0037] For example, as in the gas sensor element 9 shown in FIG. 6 , when the thickness L of the protective layer 5 is large, oxygen is likely to remain within the protective layer 5. It is conceivable that some of the oxygen remaining in the protective layer 5 will move within the protective layer 5 and enter the chamber 13. That is, it is difficult to achieve a complete hermetic seal between the solid electrolyte body 2 and the chamber forming layer 3 (i.e., the buffer layers 31, 32, and the shielding layer 33) due to differences in materials. Therefore, it is similarly difficult to avoid oxygen from entering the chamber 13 from the protective layer 5. Furthermore, if some of the oxygen reaches the inlet 150 through the protective layer 5, it is conceivable that some of the oxygen will enter the chamber 13 through the diffusion layer 15.

[0038] When oxygen enters chamber 13 in this way, the oxygen concentration in the exhaust gas inside chamber 13 fluctuates. In other words, the oxygen concentration in the exhaust gas to be measured fluctuates inside chamber 13, causing measurement errors.

[0039] For example, when an internal combustion engine is burning at stoichiometric ratio, the oxygen concentration in the exhaust gas is essentially zero, but when the above phenomenon occurs, oxygen is detected by the gas sensor. In other words, in the case of stoichiometric combustion, the output current of the gas sensor should be zero, but it is not zero and some current is output. This leads to measurement errors.

[0040] Therefore, to suppress the above-described phenomenon, in the gas sensor element 1 of the first embodiment, the thickness L of the protective layer 5 at the same position in the stacking direction as the solid electrolyte body 2 is set to satisfy the relationship d / L≧1 in relation to the thickness d of the solid electrolyte body 2. As a result, even if oxygen leaks from the duct 14, the leaked oxygen is likely to be released to the outside of the protective layer 5 and is unlikely to remain within the protective layer 5 because the thickness L of the protective layer 5 is small. In other words, when the thickness L of the protective layer 5 is small, the gradient of the oxygen concentration between the outside and inside of the protective layer 5 is large, and therefore oxygen is likely to be released to the outside of the protective layer 5. On the other hand, when the thickness L of the protective layer 5 is large, the gradient of the oxygen concentration between the outside and inside of the protective layer 5 is small, and therefore oxygen is unlikely to be released to the outside of the protective layer 5.

[0041] Furthermore, when the thickness d of the solid electrolyte body 2 is small, some of the oxygen that leaks from the duct 14 to the protective layer 5 on the second surface 22 side is likely to move toward the first surface 21 side. As a result, some of the oxygen that leaks to the protective layer 5 is likely to infiltrate into the chamber 13. On the other hand, when the thickness d of the solid electrolyte body 2 is large, some of the oxygen that leaks from the duct 14 to the protective layer 5 on the second surface 22 side is likely to move a longer distance toward the first surface 21 side. As a result, the oxygen that leaks to the protective layer 5 is less likely to infiltrate into the chamber 13.

[0042] Therefore, in order to suppress the above phenomenon, it is preferable that the thickness d is large and the thickness L is small. As a result of extensive research, the inventors of the present application have found that when d / L≧1 is satisfied, the above phenomenon can be suppressed and the detection error of the gas sensor can be sufficiently suppressed. The experimental results will be described later.

[0043] As described above, according to this embodiment, it is possible to provide a gas sensor element that can improve measurement accuracy.

[0044] (Embodiment 2) As shown in FIGS. 7 to 9, this embodiment is a gas sensor element 1 in which an exhaust gas inlet 150 is provided at the tip end face of the element. That is, the gas sensor element 1 of this embodiment is provided with a diffusion layer 15 on the tip side of the chamber 13. As a result, an inlet 150 is disposed on the tip surface of the element.

[0045] In this embodiment, the thickness L of the protective layer 5 (see FIGS. 1, 8, and 9) satisfies d / L≧1.

[0046] In this embodiment, as described above, the exhaust gas inlet 150 is provided on the front end surface. Therefore, in consideration of the inflow route of oxygen leaking from the duct 14 from the inlet 150 through the diffusion layer 15 into the chamber 13, it is considered more important to reduce the thickness L of the protective layer 5 on the front end surface. Therefore, it may be possible to make the thickness L of the protective layer 5 on the front end surface of the element smaller than the thickness L of the protective layer 5 on both sides of the gas sensor element 1 in the Y direction.

[0047] However, as described above, oxygen may enter the chamber 13 even on the side surface facing the Y direction where the diffusion layer 15 is not provided, so the thickness L of the protective layer 5 on the side surface (see Figures 1 and 9) must also satisfy d / L≧1.

[0048] Other aspects are the same as those of embodiment 1. Note that, among the symbols used in embodiment 2 and onwards, the same symbols as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified. This embodiment also has the same effects as the first embodiment.

[0049] (Embodiment 3) As shown in FIGS. 10 and 11, this embodiment is a gas sensor element 1 in which the protective layer 5 has a first protective layer 51 and a second protective layer 52. The first protective layer 51 is formed at a position including at least the boundary between the solid electrolyte body 2 and the chamber forming layer 3. The second protective layer 52 is formed at a position including at least the boundary between the solid electrolyte body 2 and the duct forming layer 4.

[0050] In this embodiment, the first protective layer 51 is formed at the tip of the element so as to cover a part of the solid electrolyte body 2 and the chamber forming layer 3 (i.e., the buffer layers 31 and 32 and the shielding layer 33). The second protective layer 52 is formed so as to cover a part of the solid electrolyte body 2, the duct forming layer 4 and the heater layer 11.

[0051] The porosity of the second protective layer 52 is higher than the porosity of the first protective layer 51. The porosity of the first protective layer 51 is 20 to 40% by volume, and the porosity of the second protective layer 52 is 40 to 70% by volume. The rest is the same as in the first embodiment.

[0052] In this embodiment, oxygen leaking from duct 14 first reaches second protective layer 52. Since second protective layer 52 has a relatively high porosity, the leaked oxygen is easily discharged to the outside of protective layer 5. On the other hand, since first protective layer 51 has a relatively low porosity, oxygen is less likely to move from second protective layer 52 to first protective layer 51. Therefore, oxygen leaking from duct 14 can be effectively prevented from entering chamber 13.

[0053] The porosity of the first protective layer 51 is 20 to 40 volume %, and the porosity of the second protective layer 52 is 40 to 70 volume %. This makes it easier to achieve the above-mentioned effects. That is, the porosity of the second protective layer 52 is higher than the porosity of the first protective layer 51, and the porosity of the first protective layer 51 is 40 volume % or less, and the porosity of the second protective layer 52 is 40 volume % or more, making it easier to achieve the above-mentioned effects. If the porosity of the first protective layer 51 is less than 20 volume %, it may be difficult to sufficiently introduce the measurement gas (exhaust gas) into the chamber 13. If the porosity of the second protective layer 52 exceeds 70 volume %, it may be difficult to sufficiently enhance the effect of the protective layer 51 in trapping poisonous substances and in suppressing cracks due to water exposure. In addition, the same effects as those of the first embodiment are achieved.

[0054] (Experimental Example 1) In this example, as shown in FIG. 12, the relationship between the thickness L of the protective layer on the side surface of the gas sensor element and the measurement accuracy of the gas sensor element was investigated.

[0055] The gas sensor element used as the sample had the same basic structure as that shown in the second embodiment. However, several types were prepared with different thicknesses L of the protective layer 5 on the side surface. The thickness L of the protective layer 5 on the front end surface was set to 100 μm in all samples. The thickness d of the solid electrolyte body 2 was set to 160 μm.

[0056] The following tests were carried out on each sample. First, a gas sensor equipped with a gas sensor element was installed in a model gas bench that simulated the flow of exhaust gas. Nitrogen gas was flowed into the model gas bench instead of exhaust gas. In other words, nitrogen gas with an oxygen concentration of virtually zero, similar to stoichiometric exhaust gas, was flowed.

[0057] The gas sensor element was heated to its activation temperature by applying a predetermined voltage to the sensor cell, and the current flowing through the sensor cell, i.e., the limiting current, was measured.

[0058] The results are shown in Figure 12. As mentioned above, in this example, the measured gas was nitrogen gas, which corresponds to a stoichiometric gas, so ideally the limiting current value should be zero. However, as shown in Figure 12, some samples output a current. A noise current was detected in samples where the thickness L of the protective layer 5 exceeded 160 μm, i.e., the thickness d of the solid electrolyte body 2 exceeded. It was also confirmed that the noise current increased with increasing thickness L. As mentioned above, this noise current is presumably due to some of the oxygen leaking from the duct 14 entering the chamber 13.

[0059] In contrast, almost no current was output in samples where the thickness L of the protective layer 5 was less than 160 μm, i.e., equal to or less than the thickness d of the solid electrolyte body 2. In particular, in samples where the thickness L of the protective layer 5 was equal to or less than half the thickness d of the solid electrolyte body 2, substantially no current was output.

[0060] From the above results, it can be seen that noise can be sufficiently reduced and detection accuracy can be improved by setting the thickness L of the protective layer to satisfy d / L ≥ 1. It can also be seen that noise can be further reduced and detection accuracy can be improved by setting d / L ≥ 2.

[0061] (Embodiment 4) As shown in FIG. 13, this embodiment is a gas sensor 10 including a gas sensor element 1.

[0062] The gas sensor 10 includes a gas sensor element 1, a housing 71, and an element cover 72. The housing 71 holds the gas sensor element 1. The element cover 72 is attached to the front end side of the housing 71 and surrounds the gas sensor element 1 from the front end side. The element cover 72 has an air hole 721 at a position facing the chamber 13 of the gas sensor element 1.

[0063] The housing 71 directly or indirectly holds the gas sensor element 1. For example, in this embodiment, the gas sensor element 1 is held via an insulator (not shown). The vent hole 721 faces the chamber 13 with the protective layer 5 and the like interposed therebetween. That is, the position of the gas sensor element 1 facing the chamber 13 means the position in the X direction that overlaps with the chamber 13.

[0064] Furthermore, in the X direction, at least a portion of the vent hole 721 overlaps with at least a portion of the chamber 13. Preferably, the center of the vent hole 721 overlaps with the chamber 13. Furthermore, it is preferable that the entire vent hole 721 is included in the formation area of ​​the chamber 13 in the X direction. Furthermore, it is preferable that a portion or the entire vent hole 721 also overlaps with the position of the diffusion layer 15 in the X direction.

[0065] 13, the exhaust gas G introduced into the element cover 72 through the vent hole 721 on the side is exhausted from the vent hole 722 at the tip. During this process, part of the exhaust gas G is introduced into the chamber 13 of the gas sensor element 1.

[0066] In this embodiment, the vent hole 721 is formed at a position facing the gas sensor element 1 in the Y direction. The configuration of the gas sensor element 1 is the same as that of embodiment 1. Therefore, the diffusion layer 15 is formed on both sides of the chamber 13 in the Y direction (see FIGS. 3 to 5).

[0067] 13, in this embodiment, the exhaust gas G flowing in through the vent hole 721 of the element cover 72 collides with the gas sensor element 1 at a position around the chamber 13. As a result, a sufficient flow rate of the exhaust gas G passes through the protective layer 5 around the chamber 13. As a result, oxygen leaking from the duct 14 is easily released from the protective layer 5 along with the flow of the exhaust gas G. This prevents oxygen from accumulating in the protective layer 5 near the chamber 13, effectively preventing oxygen from entering the chamber 13. In addition, the same effects as those of the first embodiment are achieved.

[0068] In the gas sensor 10 of this embodiment, the element cover 72 may have a double or more layer structure. In this case, the vent hole in the innermost element cover, i.e., the element cover closest to the gas sensor element 1, becomes the vent hole 721 that satisfies the above-mentioned X-direction position.

[0069] (Experimental Example 2) In this example, as shown in FIG. 16, the relationship between the position of the vent hole 721 in the X direction and the measurement accuracy of the gas sensor element was investigated.

[0070] The gas sensor elements used as samples had the same basic structure as that shown in the fourth embodiment. However, the thickness L of the protective layer 5 was 400 μm in all samples, which did not satisfy d / L≧1. Therefore, all samples were different from those of the fourth embodiment. This was because the structure of the gas sensor element itself was made disadvantageous in terms of suppressing oxygen penetration into the chamber 13, in order to make it easier to understand the effect of the position of the air vent 721. The structure of the gas sensor element was the same as that of the first embodiment, except for the thickness L of the protective layer.

[0071] Then, a plurality of types of element covers 72 were fabricated, each with a different position of the vent hole 721. Gas sensor elements having substantially the same configuration were prepared. The thickness d of the solid electrolyte body 2 was 160 μm. The vent hole 721 was circular and had a diameter of 1.6 mm.

[0072] As shown in Fig. 14, a specimen in which the center position of the vent hole 721 was 2 mm distal to the center position of the chamber 13 was produced as sample 1. As shown in Fig. 13, a specimen in which the center position of the vent hole 721 was at the same X-direction position as the center of the chamber 13 was produced as sample 2. As shown in Fig. 15, a specimen in which the center position of the vent hole 721 was 2 mm proximal to the center position of the chamber 13 was produced as sample 3. The length of the chamber 13 in the X-direction was 3.4 mm.

[0073] The same test as in Experimental Example 1 was carried out for each sample. The gas sensor was installed on the same model gas bench as in Experimental Example 1, and the limiting current value was measured in the same manner. The results are shown in FIG.

[0074] As can be seen from the figure, samples 2 and 3 are able to suppress noise current compared to sample 1. This means that providing the position of the vent hole 721 facing the center of the chamber 13 in the X direction or slightly closer to the base end can suppress oxygen intrusion into the chamber 13 more effectively than providing it closer to the tip. This is consistent with the mechanism by which oxygen leaking into the protective layer 5 near the chamber 13 is discharged by the flow of exhaust gas G introduced through the vent hole 721. Furthermore, because the noise current of sample 2 is particularly small, it is thought that a greater effect can be obtained by providing the vent hole 721 in a position facing the center of the chamber 13 in the X direction.

[0075] The gas sensor element and gas sensor of the present disclosure are not limited to those described in the above embodiments, and various configurations are conceivable.

[0076] For example, as in Modification 1 shown in FIG. 17, the thickness of the solid electrolyte body 2 may be different at the center and both ends in the Y direction. In this case, the thickness d of the solid electrolyte body 2 is defined as the thickness at the ends. As described above, the oxygen that enters the chamber 13 is oxygen that leaks from the duct 14 to the protective layer 5. Taking this into consideration, it is clear that the parameter related to the suppression of oxygen entry into the chamber 13 is not the thickness at the center of the solid electrolyte body 2, but the thickness at the ends. Therefore, in the case of the structure shown in FIG. 17, the thickness d of the solid electrolyte body 2 is defined as the thickness at the ends.

[0077] 18 , a configuration may also be adopted in which a recess 53 is provided in the protective layer 5 at a position in the Z direction overlapping the solid electrolyte body 2. This recess 53 extends in the X direction at least to an extent that covers the region where the chamber 13 is formed. This allows the thickness L of the protective layer 5 at the same Z direction position as the solid electrolyte body 2 to be small, while the thickness of the protective layer 5 at other positions can be large. Because the small thickness L of the protective layer 5 at the position in the Z direction overlapping the solid electrolyte body 2 contributes to the suppression of oxygen penetration into the chamber 13, the thickness L of this portion is set small so as to satisfy d / L≧1. On the other hand, increasing the thickness of the protective layer 5 at other positions may improve functions such as trapping poisoning substances and suppressing water-induced cracking.

[0078] 19, the solid electrolyte body 2 may be configured to protrude in the Y direction further than other portions of the main body 100 of the gas sensor element 1. In this case, too, the thickness L of the protective layer 5 at the same position in the Z direction as the solid electrolyte body 2 can be made small, while the thickness of the protective layer 5 at other positions can be made large.

[0079] In addition, in the first embodiment and the like, the diffusion layer 15 is configured not to contact the sensor electrode 61, but the present invention is not particularly limited to this configuration. For example, the chamber 13 may be filled with a porous diffusion layer, and the sensor electrode 61 and the diffusion layer may be in contact with each other.

[0080] The present disclosure is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the spirit of the present disclosure.

[0081] Although the present disclosure has been described with reference to the embodiments, it is understood that the present disclosure is not limited to the embodiments or structures. The present disclosure also encompasses various modifications and modifications within the scope of equivalents. In addition, various combinations and forms, as well as other combinations and forms including only one element, more than one element, or less than one element, are also within the scope and spirit of the present disclosure.

[0082] The features of the present disclosure are as follows: [1] A solid electrolyte body (2) having oxygen ion conductivity; a chamber (13) facing the first surface (21) of the solid electrolyte body and into which a gas to be measured is introduced; a chamber forming layer (3) laminated on the first surface side of the solid electrolyte body to form the chamber; a duct (14) facing the second surface (22) of the solid electrolyte body and through which a reference gas is introduced; a duct forming layer (4) laminated on the second surface side of the solid electrolyte body to form the duct, At least the distal end of the element, which is located distal to the base end of the chamber, is covered with a porous protective layer (5); The gas sensor element satisfies the condition d / L≧1, where L is the thickness of the protective layer at the same position in the stacking direction as the solid electrolyte body, and d is the thickness of the solid electrolyte body. [2] The gas sensor element according to [1], wherein the thickness L of the protective layer and the thickness d of the solid electrolyte body satisfy d / L≧2. [3] The gas sensor element according to [1] or [2], wherein the protective layer includes a first protective layer (51) formed at a position including at least the boundary between the solid electrolyte body and the chamber forming layer, and a second protective layer (52) formed at a position including at least the boundary between the solid electrolyte body and the duct forming layer, and the porosity of the second protective layer is higher than the porosity of the first protective layer. [4] The gas sensor element according to [3], wherein the first protective layer has a porosity of 20 to 40% by volume, and the second protective layer has a porosity of 40 to 70% by volume. [5] A gas sensor (10) comprising: the gas sensor element according to any one of [1] to [4]; a housing (71) for holding the gas sensor element; and an element cover (72) attached to a front end of the housing and surrounding the gas sensor element from the front end, The gas sensor, wherein the element cover has an air hole (721) at a position facing the chamber in the gas sensor element.

Claims

1. a solid electrolyte body (2) having oxygen ion conductivity; a chamber (13) facing the first surface (21) of the solid electrolyte body and into which a gas to be measured is introduced; a chamber forming layer (3) laminated on the first surface side of the solid electrolyte body to form the chamber; a duct (14) facing the second surface (22) of the solid electrolyte body and through which a reference gas is introduced; a duct forming layer (4) laminated on the second surface side of the solid electrolyte body to form the duct, At least the distal end of the element, which is located distal to the base end of the chamber, is covered with a porous protective layer (5); The gas sensor element satisfies the condition d / L≧1, where L is the thickness of the protective layer at the same position in the stacking direction as the solid electrolyte body, and d is the thickness of the solid electrolyte body.

2. 2. The gas sensor element according to claim 1, wherein the thickness L of said protective layer and the thickness d of said solid electrolyte body satisfy the relationship d / L≧2.

3. 3. The gas sensor element according to claim 1, wherein the protective layer comprises a first protective layer (51) formed at a position including at least a boundary between the solid electrolyte body and the chamber forming layer, and a second protective layer (52) formed at a position including at least a boundary between the solid electrolyte body and the duct forming layer, and the porosity of the second protective layer is higher than the porosity of the first protective layer.

4. 4. The gas sensor element according to claim 3, wherein the first protective layer has a porosity of 20 to 40% by volume, and the second protective layer has a porosity of 40 to 70% by volume.

5. 3. A gas sensor (10) comprising: the gas sensor element according to claim 1 or 2; a housing (71) for holding the gas sensor element; and an element cover (72) attached to a front end side of the housing and surrounding the gas sensor element from a front end side, The gas sensor, wherein the element cover has an air hole (721) at a position facing the chamber in the gas sensor element.

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