Gas Sensors

Abstract

To improve the responsiveness to first particular gas and second particular gas.SOLUTION: A sensor element 110 of a gas sensor 100 includes a detection unit that detects the concentration of first particular gas among measurement gas flowing from a measurement gas inlet 21a, and a mixed potential cell that generates electromotive force according to the concentration of second particular gas in the measurement gas. A detection electrode 56 of the mixed potential cell is disposed outside an element main body 20 and in the rear of the measurement gas inlet 21a. The measurement gas inlet 21a and the detection electrode 56 are disposed in a sensor element chamber 124 inside an inner protection cover 130. The inner protection cover 130 includes first entrances 125 and second entrances 127 as element chamber entrances 123. An opening 129 of the second entrance 127 on the sensor element chamber 124 side is disposed ahead of the detection electrode 56 and the first entrance 125 and opens forward.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a gas sensor. [Background technology]

[0002] Conventionally, gas sensors that detect a predetermined gas concentration such as NOx or oxygen in a measurement gas such as an automobile exhaust gas are known. For example, Patent Document 1 describes a gas sensor that includes a sensor element capable of detecting a predetermined gas concentration of a measurement gas flowing into the inside from a gas inlet, an outer protective cover covering the tip of the sensor element, and an inner protective cover arranged between the outer protective cover and the sensor element. The inner protective cover of Patent Document 1 forms a gas flow path that runs from the rear end side of the sensor element to the tip side and opens into the space where the gas inlet is arranged, in the path of the measurement gas from the outer gas hole of the outer protective cover to the gas inlet of the sensor element. This makes it possible to achieve both the response of the detection of a predetermined gas concentration and the heat retention of the sensor element. Also, for example, Patent Document 2 describes a gas sensor that includes a NOx sensor portion that measures the NOx concentration of the measurement gas introduced into the internal space, as well as an NH 3 NH with sensing electrode 3 A gas sensor having a gas sensor portion is described. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2014 / 192945 Brochure [Patent Document 2] JP 2018-40746 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, when the sensor element of Patent Document 2 is used in the gas sensor of Patent Document 1, in other words, when a detection electrode for detecting a gas (hereinafter also referred to as a second specified gas) different from a predetermined gas (hereinafter also referred to as a first specified gas) is added to the surface of the sensor element in the gas sensor of Patent Document 1, the responsiveness to the first specified gas is good, but there is the following problem. That is, it takes a long time for the measured gas to reach the detection electrode, and the responsiveness to the second specified gas is sometimes low. Therefore, a gas sensor with good responsiveness to the first specified gas and the second specified gas has been desired.

[0005] The present invention has been made to solve the above problems, and has as its main object to improve the responsiveness of a gas sensor to a first specified gas and a second specified gas. [Means for solving the problem]

[0006] In order to achieve the above-mentioned main object, the present invention adopts the following means.

[0007] The gas sensor of the present invention comprises: a sensor element including an element body having a front end and a rear end opposite to the front end, the element body having a measurement gas flow section therein through which a measurement gas is introduced and flows, the sensor element including a detection section for detecting a concentration of a first specified gas in the measurement gas that has flowed into the measurement gas flow section from a gas inlet which is an inlet of the measurement gas flow section; a cylindrical inner protective cover having a sensor element chamber therein in which the tip of the sensor element and the gas inlet are disposed, the sensor element chamber having one or more element chamber inlets which are inlets to the sensor element chamber and one or more element chamber outlets which are outlets from the sensor element chamber; a cylindrical outer protective cover having one or more outer inlets which are inlets for the measurement gas from the outside and one or more outer outlets which are outlets for the measurement gas to the outside, the outer protective cover being disposed outside the inner protective cover; Equipped with the sensor element includes a mixed potential cell that generates an electromotive force corresponding to a concentration of a second specified gas in the measurement gas that is different from the first specified gas, the mixed potential cell includes a detection electrode disposed outside the element body and behind the gas inlet, the detection electrode is disposed inside the sensor element chamber, the inner protective cover has a cylindrical first member surrounding the periphery of the sensor element and a cylindrical second member surrounding the periphery of the first member, and has, as the element chamber inlets, one or more first inlets disposed in the first member and one or more second inlets formed as gaps between the first member and the second member, an opening of the second inlet on the sensor element chamber side is disposed forward of the detection electrode and the first inlet and opens forward; It is something.

[0008] In this gas sensor, the measurement gas flowing around the gas sensor flows into the outer protective cover from the outer inlet of the outer protective cover, flows into the sensor element chamber from the element chamber inlet of the inner protective cover, and reaches the gas inlet and the detection electrode. Here, the first inlet of the element chamber inlet is disposed in a cylindrical first member surrounding the sensor element. The second inlet of the element chamber inlet is formed as a gap between the cylindrical first member and the cylindrical second member surrounding the periphery of the first member. Furthermore, the opening of the second inlet on the sensor element chamber side is disposed forward of the detection electrode and the first inlet, and opens toward the front. In the element chamber inlet formed in this way, the measurement gas flowing into the sensor element chamber from the second inlet flows from the rear end side to the front end side of the sensor element, in other words, from the rear to the front, and flows into the sensor element chamber from the front of the detection electrode. Therefore, the measurement gas flowing into the sensor element chamber from the second inlet quickly reaches the gas inlet located forward of the detection electrode. Also, the measurement gas flowing into the sensor element chamber from the first inlet quickly reaches the detection electrode arranged behind the gas inlet. As a result, this gas sensor has good responsiveness to the first and second specified gases. With respect to the opening of the second inlet on the sensor element chamber side, "opening toward the front" does not mean opening toward the rear or straight side (direction perpendicular to the front). For this reason, the opening of the second inlet on the sensor element chamber side may be opened, for example, parallel to the front-rear direction, or may be opened at an angle from the front-rear direction so as to approach the sensor element as it moves toward the front.

[0009] In the gas sensor of the present invention, the sensor element chamber may have a ratio A / B of a total opening area A of the one or more first inlets to a total opening area B of the one or more second inlets of 0.02 or more and 19.76 or less. When the ratio A / B is 0.02 or more and 19.76 or less, a good balance is achieved between the flow rate of the measurement gas flowing into the sensor element chamber from the first inlet and the flow rate of the measurement gas flowing into the sensor element chamber from the second inlet, resulting in better responsiveness to the first specified gas and the second specified gas. In this gas sensor, the ratio A / B may be 0.05 or more and 7.29 or less. This further improves the responsiveness to the first specified gas and the second specified gas.

[0010] In the gas sensor of the present invention, the detection section may include an outer electrode disposed outside the element body, and the detection electrode may be disposed rearward of the outer electrode. When the detection electrode is disposed rearward of the outer electrode, the distance from the second inlet to the detection electrode is relatively long, and it takes time for the measurement gas flowing into the sensor element chamber from the second inlet to reach the detection electrode, so that the provision of the first inlet is significant.

[0011] In the gas sensor of the present invention, at least one of the one or more first inlets may be disposed at a position where a shortest path length L1 from the first inlet to the detection electrode is shorter than a shortest path length L2 from the second inlet to the detection electrode. Since the first inlet is disposed at a position closer to the detection electrode, the response to the second specified gas is improved.

[0012] In the gas sensor of the present invention, all of the first inlets may be arranged at positions where the detection electrode does not overlap with a projection area of ​​the first inlets projected onto the surface of the element body parallel to the axial direction of the first inlets. In this way, the measurement gas that flows straight into the sensor element chamber parallel to the axial direction of the first inlets and has a relatively high flow rate does not directly hit the detection electrode, thereby suppressing deterioration of the detection electrode.

[0013] In the gas sensor of the present invention, the first member may have a cylindrical side portion, and the first inlet may be a hole disposed in the side portion of the first member.

[0014] The gas sensor of the present invention may include a porous protective layer covering the detection electrode, and the sensor element chamber may have a ratio A / B of the total opening area A of the one or more first inlets to the total opening area B of the one or more second inlets, which is 0.02 to 19.76. At least one of the one or more first inlets may be arranged such that, in a virtual projection of the first member and the sensor element vertically projected onto a virtual plane perpendicular to the axial direction of the first member, an angle θ between a first straight line perpendicular to the arrangement surface on which the detection electrode of the sensor element is arranged and drawn from the center of the arrangement surface to the opposite side to the element body, and a second straight line drawn from the center of the arrangement surface to the center of the first inlet is 73.6° or less. When the gas sensor of the present invention is used to detect the concentration of a first specified gas in a gas to be measured, the element body is usually heated so that the detection unit reaches a predetermined high temperature suitable for detecting the concentration of the first specified gas. Here, when the gas sensor includes a protective layer covering the detection electrode, the protective layer also reaches a high temperature as the element body becomes hot. For this reason, in a gas sensor having such a protective layer, the measurement gas flowing into the sensor element chamber from the first inlet may come into contact with the high-temperature protective layer before reaching the detection electrode, causing the second specified gas in the measurement gas to burn, and the sensitivity to the second specified gas may decrease. However, in a first inlet having an angle θ of 73.6° or less, the distance over which the measurement gas flowing into the sensor element chamber from the first inlet comes into contact with the protective layer before reaching the detection electrode is relatively short. For this reason, in a gas sensor having a first inlet having an angle θ of 73.6° or less, even if the gas sensor has a protective layer covering the detection electrode, the decrease in sensitivity to the second specified gas in the measurement gas is suppressed.

[0015] In the gas sensor of the present invention having a first inlet with the angle θ of 73.6° or less, a plurality of the first inlets may be disposed in the first member, and all of the first inlets may be disposed such that, in the virtual projection view, a line obtained by rotating the first line by 73.6° around the center of the disposition surface as a rotation center is defined as a third line, a line obtained by rotating the first line by 73.6° in the opposite direction to the third line is defined as a fourth line, an intersection point between the third line and an inner circumferential surface of the first member is defined as a first intersection point, and an intersection point between the fourth line and the inner circumferential surface of the first member is defined as a second intersection point. If the first inlets are disposed in this manner, the angle θ is 73.6° or less in at least one of the first inlets, regardless of the mounting orientation of the first member with respect to the sensor element. For this reason, it is easy to manufacture a gas sensor having a first inlet with an angle θ of 73.6° or less.

[0016] In the gas sensor of the present invention having the first inlets with the angle θ of 73.6° or less, all of the first inlets may be arranged so that the angle θ is 23.5° or more. In a first inlet with an angle θ of less than 23.5°, the detection electrode may be easily deteriorated by the measurement gas that flows straight into the sensor element chamber parallel to the axial direction of the first inlet and has a relatively high flow rate, and hits the detection electrode. However, since there is no such first inlet, deterioration of the detection electrode is suppressed.

[0017] In the gas sensor of the present invention, at least one of the one or more first inlets may have a shortest path length L1 from the first inlet to the detection electrode of 12.0 mm or less, and all of the first inlets may have a shortest path length L1 from the first inlet to the detection electrode of 0.2 mm or more. If the shortest path length L1 is 0.2 mm or more in all of the first inlets, the flow rate of the measurement gas flowing from the first inlet into the sensor element chamber is appropriately suppressed until it reaches the detection electrode, so that deterioration of the detection electrode due to the measurement gas hitting the detection electrode is suppressed. In addition, if there is a first inlet with a shortest path length L1 of 12.0 mm or less, the measurement gas flowing from the first inlet into the sensor element chamber reaches the detection electrode while maintaining a relatively high flow rate, so that the response to the second specified gas is better. [Brief description of the drawings]

[0018] [Figure 1] FIG. 2 is a schematic explanatory diagram of a state in which the gas sensor 100 is attached to a pipe 10. [Diagram 2] FIG. 1 is a schematic cross-sectional view of a gas sensor 100. [Diagram 3] FIG. 2 is a schematic cross-sectional view of a sensor element 110. [Figure 4] A partially enlarged view of Figure 2. [Diagram 5] Cross section AA of Figure 2. [Figure 6] Cross-sectional view of FIG. 2 taken along line B-B. [Figure 7] View C of Figure 2. [Figure 8] An explanatory diagram of shortest path lengths L1 and L2. [Figure 9] 1 is a virtual projection view of the gas sensor 100. [Figure 10] 13 is a virtual projection view of a gas sensor in which the mounting direction of the protective cover 120 relative to the sensor element 110 is changed. [Figure 11] 13 is a virtual projection view of the gas sensor 100 when the detection electrode 56 is moved to the right. [Figure 12] FIG. 4 is a schematic cross-sectional view of a gas sensor 200 according to a modified example. [Figure 13] FIG. 13 is a schematic diagram showing a second inlet 327 of a modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic explanatory diagram of a state in which a gas sensor 100 is attached to a pipe 10. FIG. 2 is a schematic cross-sectional view showing an example of the configuration of a gas sensor 100 according to an embodiment of the present invention. FIG. 3 is a schematic cross-sectional view of a sensor element 110 included in the gas sensor 100. FIG. 4 is a partially enlarged view of FIG. 2. FIG. 5 is a cross-sectional view taken along line AA in FIG. 2. FIG. 6 is a cross-sectional view taken along line BB in FIG. 2. FIG. 7 is a view taken from the perspective of FIG. 2. FIG. 8 is an explanatory diagram of the shortest path lengths L1 and L2, showing the shortest paths as viewed from the front. As shown in FIG. 2, the longitudinal direction of the element body 20 included in the sensor element 110 is defined as the front-rear direction, and the thickness direction of the element body 20 is defined as the up-down direction. Also, as shown in FIG. 5, the width direction of the element body 20 (the direction perpendicular to the longitudinal direction and the thickness direction) is defined as the left-right direction. Note that FIG. 2 corresponds to the cross-sectional view taken along line DD in FIG. 5. The internal structure of the sensor element 110 is shown in detail in FIG. 3, and is not shown in FIGS.

[0020] As shown in Fig. 1, the gas sensor 100 is attached to a pipe 10 which is an exhaust path from an engine of a vehicle. The gas sensor 100 detects the concentration of a first specified gas in the exhaust gas discharged from the engine as a measurement gas and the concentration of a second specified gas in the measurement gas. In this embodiment, a case will be described in which the first specified gas is NOx and the second specified gas is ammonia.

[0021] As shown in FIG. 2, the gas sensor 100 includes a sensor element 110 and a protective cover 120 that protects the sensor element 110. The gas sensor 100 also includes an element encapsulation body 101 that encapsulates and fixes the sensor element 110, and a bolt 103 attached to the element encapsulation body 101. The element encapsulation body 101 includes a cylindrical metal housing 102, a ceramic supporter 104 that is filled in a through hole inside the housing 102, and a powder compact 105 that is formed by molding ceramic powder such as talc and that is filled in the through hole inside the housing 102. The sensor element 110 is located on the central axis of the element encapsulation body 101 and penetrates the element encapsulation body 101 in the front-rear direction. The powder compact 105 is compressed between the housing 102 and the sensor element 110. As a result, the powder compact 105 seals the through hole in the housing 102 and fixes the sensor element 110. The bolt 103 is a cylindrical member made of metal and has a male thread on its outer circumferential surface. The housing 102 of the element sealing body 101 is welded to the pipe 10 and inserted into a fixing member 12 having a female thread on its inner circumferential surface. The bolt 103 is further screwed into the fixing member 12, thereby fixing the housing 102 in the fixing member 12. In this way, the gas sensor 100 is fixed in the pipe 10.

[0022] The sensor element 110 is an element having a long, thin plate shape, and has a tip end and a rear end opposite to the tip end. As shown in Fig. 3, the sensor element 110 includes an element body 20, a detection unit 23, and a mixed potential cell 55. The tip end of the sensor element 110 is the end disposed in a sensor element chamber 124 (described later) among both ends in the longitudinal direction of the sensor element 110, that is, the front end, and will hereinafter also be referred to as the front end.

[0023] As shown in FIG. 3, the element body 20 is made of zirconia (ZrO 2The element body 20 has a laminate in which a plurality of oxygen ion conductive solid electrolyte layers (six in FIG. 3) such as oxygen ion conductive solid electrolyte layers are stacked in the vertical direction. Since the element body 20 has a rectangular parallelepiped shape, as shown in FIGS. 3 and 5, the element body 20 has an upper surface 20a, a lower surface 20b, a left surface 20c, a right surface 20d, a front end surface 20e, and a rear end surface 20f as outer surfaces. The upper surface 20a is a surface along the longitudinal direction (axial direction) of the element body 20. The dimensions of the element body 20 may be, for example, a front-rear length of 25 mm to 100 mm, a left-right length (width) of 2 mm to 10 mm, and a top-bottom length (thickness) of 0.5 mm to 3 mm. The element body 20 is also formed with a measurement gas inlet 21a (corresponding to the gas inlet of the present invention) that opens at the front end face 20e and introduces a measurement gas into the element body 20, and a reference gas inlet 22 that opens at the rear end face 20f and introduces a reference gas (here, air) that is a reference for detecting the NOx concentration into the element body 20. The element body 20 is provided with a measurement gas flow section 21 that introduces and flows the measurement gas. The measurement gas flow section 21 is a space from the measurement gas inlet 21a, which is the inlet of the measurement gas flow section 21, to the measurement electrode 27. The left-right length (width) of the measurement gas inlet 21a is, for example, 1 mm or more and 9 mm or less. The vertical length from the top face 20a to the measurement gas inlet 21a, in other words, the vertical length from the top end of the front end face 20e to the measurement gas inlet 21a, is, for example, 0.1 mm or more and 2.4 mm or less (see FIGS. 3 and 4).

[0024] The detection unit 23 detects the NOx concentration in the measurement gas flowing from the measurement gas inlet 21a to the measurement gas flow section 21. The detection unit 23 has a plurality of electrodes arranged on the front end side of the element body 20. In this embodiment, the detection unit 23 includes the plurality of electrodes, namely, an outer electrode 24, an inner main pump electrode 25, an inner auxiliary pump electrode 26, a measurement electrode 27, and a reference electrode 28. The outer electrode 24 is arranged on the outside of the element body 20, more specifically, on the upper surface 20a of the element body 20. The inner main pump electrode 25, the inner auxiliary pump electrode 26, and the measurement electrode 27 are arranged inside the element body 20, and are arranged in this order from the measurement gas inlet 21a to the rear in the measurement gas flow section 21. The reference electrode 28 is arranged inside the element body 20, and the reference gas reaches the reference electrode 28 through the reference gas inlet 22. The inner main pump electrode 25 and the inner auxiliary pump electrode 26 are disposed on the inner peripheral surface of the space inside the element body 20 and may have a tunnel-shaped structure. The outer electrode 24 is, for example, a porous cermet electrode (for example, Au, Pt, and ZrO 2 The other electrodes 25 to 28 of the detection unit 23 may also be formed as porous cermet electrodes in the same manner.

[0025] The principle of detecting the NOx concentration in the measurement gas using the detection unit 23 is well known and will not be described in detail. However, the detection unit 23 is controlled by a controller (not shown) connected to the sensor element 110 (particularly the detection unit 23 and the mixed potential cell 55) and operates as follows. The controller applies a voltage Vp0 between the outer electrode 24 and the inner main pump electrode 25 so that the voltage V0 between the outer electrode 24 and the reference electrode 28 becomes a target value. This voltage Vp0 causes the detection unit 23 to pump oxygen in the measurement gas around the inner main pump electrode 25 to the outside (the sensor element chamber 124 in FIG. 2). At this time, a pump current Ip0 flows between the outer electrode 24 and the inner main pump electrode 25. This pump current Ip0 becomes a value corresponding to the oxygen concentration in the measurement gas (a value from which the oxygen concentration can be derived). The controller also applies a voltage Vp1 between the outer electrode 24 and the inner auxiliary pump electrode 26 so that the voltage V1 between the inner auxiliary pump electrode 26 and the reference electrode 28 becomes a target value. This voltage Vp1 causes the detection unit 23 to pump oxygen in the measurement gas around the inner auxiliary pump electrode 26 to the outside (sensor element chamber 124). As a result, the measurement gas after the oxygen concentration has been adjusted to a predetermined value reaches the vicinity of the measurement electrode 27. The measurement electrode 27 functions as a reduction catalyst for NOx, and reduces NOx in the measurement gas that has arrived there. Then, the controller applies a voltage Vp2 between the outer electrode 24 and the measurement electrode 27 so that the voltage V2 between the measurement electrode 27 and the reference electrode 28 becomes a target value. This voltage Vp2 causes the detection unit 23 to pump oxygen in the measurement gas around the measurement electrode 27 to the outside (sensor element chamber 124). As a result, the detection unit 23 pumps out oxygen around the measurement electrode 27 so that the amount of oxygen generated by the reduction of NOx in the measurement gas becomes substantially zero. At this time, a pump current Ip2 flows between the outer electrode 24 and the measurement electrode 27. This pump current Ip2 has a value corresponding to the NOx concentration in the measurement gas (a value from which the NOx concentration can be derived).

[0026] Further, the element body 20 is provided with a detection electrode 56. The detection electrode 56 is disposed outside the element body 20 and behind the measurement gas inlet 21a so as to come into contact with the measurement gas. More specifically, the detection electrode 56 is disposed on the upper surface 20a of the element body 20. The detection electrode 56 is disposed behind the outer electrode 24 of the element body 20. In this embodiment, the detection electrode 56 is disposed at the same size and position as the measurement electrode 27 when viewed from above, and is disposed in the center of the width direction of the sensor element 110. The part of the upper surface 20a where the detection electrode 56 is disposed (the part in contact with the lower surface of the detection electrode 56) is referred to as the disposition surface 56a. The detection electrode 56, the reference electrode 28 also serving as the detection unit 23, and the solid electrolyte layer between the two electrodes constitute a mixed potential cell 55. In the mixed potential cell 55, a mixed potential (electromotive force EMF) corresponding to the ammonia concentration in the measured gas is generated in the detection electrode 56. The value of the electromotive force EMF between the detection electrode 56 and the reference electrode 28 is used to detect the concentration of ammonia in the measured gas. The detection electrode 56 generates a mixed potential corresponding to the ammonia concentration and is mainly composed of a material having detection sensitivity to the ammonia concentration. The detection electrode 56 may mainly be composed of a precious metal such as gold (Au), or a conductive oxide. When a precious metal is mainly composed, it is preferable that the detection electrode 56 mainly be composed of an Au-Pt alloy. Here, the main component refers to a component that is present in the largest amount (atm%, atomic weight ratio) among all the components contained. In this embodiment, the detection electrode 56 is a porous cermet electrode of an Au-Pt alloy and zirconia. The detection electrode 56 has, for example, a front-rear length of 0.1 mm to 3.0 mm, a left-right length (width) of 1.0 mm to 9.0 mm, and a top-bottom length (thickness) of 5 μm to 40 μm. The detection electrode 56 is disposed at a position such that a distance K1 from the front end surface 20e of the element body 20 to the front end of the detection electrode 56 is 7.0 mm to 14.0 mm.

[0027] The sensor element 110 includes a heater 29. The heater 29 is an electric resistor disposed inside the element body 20. The heater 29 generates heat by receiving power from the outside, and heats the element body 20. The heater 29 heats and keeps the solid electrolyte layer of the element body 20 warm, and can adjust the temperature to a temperature (e.g., 800° C.) at which the solid electrolyte layer of the detection unit 23 is activated. This also adjusts the temperature of the detection electrode 56 of the mixed potential cell 55 to a temperature (e.g., a temperature lower than 800° C.) suitable for detecting ammonia. The temperatures of the detection unit 23 and the detection electrode 56 can be adjusted, for example, by adjusting the power supplied to the heater 29 from the outside and by adjusting the positional relationship between the heater 29, the detection unit 23, and the detection electrode 56.

[0028] The sensor element 110 also includes a buffer layer 84 and a protective layer 85. The buffer layer 84 is a porous body and is disposed on the surface of the element body 20 to bond the protective layer 85 to the element body 20. The buffer layer 84 includes an upper buffer layer 84a that covers at least a part of the upper surface 20a of the element body 20, and a lower buffer layer 84b that covers at least a part of the lower surface 20b of the element body 20. The upper buffer layer 84a also covers the detection electrode 56 and the outer electrode 24. Each of the upper buffer layer 84a and the lower buffer layer 84b is present in a region from the front end of the element body 20 to the rear of the electrodes 24, 25, 26, 27, 28, and 56 included in the detection unit 23 and the mixed potential cell 55. Each of the upper buffer layer 84a and the lower buffer layer 84b is present to the rear of the rear end of the measurement gas flow unit 21 and the rear end of the protective layer 85. The protective layer 85 is a porous body, and covers the periphery of the front end of the element body 20, more specifically, the periphery of the region where the detection unit 23 and the mixed potential cell 55 are present. The protective layer 85 covers the entire front end face of the element body 20 on which the buffer layer 84 is formed, and covers parts of the upper, lower, left, and right faces. The protective layer 85 exists beyond the rear end of the measurement gas flow section 21. The protective layer 85 plays a role in suppressing the occurrence of cracks in the element body 20 due to the adhesion of moisture and the like in the measurement gas. The protective layer 85 also plays a role in suppressing the adhesion of oil components and the like contained in the measurement gas to the outer electrode 24 and the detection electrode 56 disposed outside the element body 20. The buffer layer 84 also plays a role similar to that of the protective layer 85. Therefore, the buffer layer 84 and the protective layer 85 correspond to the protective layer of the present invention. The buffer layer 84 and the protective layer 85 are made of porous ceramics such as alumina, zirconia, spinel, cordierite, and magnesia. The buffer layer 84 and the protective layer 85 preferably have the same main component. In this embodiment, the buffer layer 84 and the protective layer 85 are porous ceramics made of alumina. Since the protective layer 85 is porous, the measured gas can flow through the inside of the protective layer 85 and reach the measured gas inlet 21a. Also, since the protective layer 85 and the buffer layer 84 are porous, the measured gas can flow through the inside of the protective layer 85 and the buffer layer 84 and reach the outer electrode 24 and the detection electrode 56.The sensor element 110 does not necessarily have to include at least one of the buffer layer 84 and the protective layer 85. As long as the buffer layer 84 and the protective layer 85 cover at least the detection electrode 56, they do not necessarily have to cover other portions. The thickness of the protective layer covering the detection electrode 56 (the total thickness of the buffer layer 84 and the protective layer 85 in this embodiment) is, for example, 200 μm or more and 900 μm or less.

[0029] As shown in Figs. 2, 4 to 7, the protective cover 120 is disposed so as to surround the sensor element 110. The protective cover 120 has a cylindrical inner protective cover 130 with a bottom that covers the tip of the sensor element 110, and a cylindrical outer protective cover 140 with a bottom that covers the inner protective cover 130. A sensor element chamber 124 is formed as a space surrounded by the inner protective cover 130. A first gas chamber 122 and a second gas chamber 126 are formed as spaces surrounded by the inner protective cover 130 and the outer protective cover 140. The central axes of the gas sensor 100, the sensor element 110, the inner protective cover 130, and the outer protective cover 140 are coaxial. The protective cover 120 is formed of metal (for example, stainless steel such as SUS310S).

[0030] The inner protective cover 130 includes a first member 131 and a second member 135. The first member 131 includes a cylindrical large diameter portion 132, a cylindrical first cylindrical portion 134 having a smaller diameter than the large diameter portion 132, and a step portion 133 connecting the large diameter portion 132 and the first cylindrical portion 134. The first member 131 surrounds the periphery of the sensor element 110. The second member 135 includes a second cylindrical portion 136 having a larger diameter than the first cylindrical portion 134, a tip portion 138 located forward of the second cylindrical portion 136, a step portion 139 connected to the rear end of the tip portion 138 and protruding outward from the outer circumferential surface of the tip portion 138, and a connection portion 137 connecting the front end of the second cylindrical portion 136 and the step portion 139. The second member 135 surrounds the periphery of the first member 131 (particularly the first cylindrical portion 134). The tip portion 138 has a side portion 138d and a bottom portion 138e. The tip portion 138 is formed with one or more element chamber outlets 138a which communicate with the sensor element chamber 124 and the second gas chamber 126 and serve as outlets for the measurement gas from the sensor element chamber 124. In this embodiment, the element chamber outlets 138a have a plurality (four) of circular holes 138b formed at equal intervals in the circumferential direction in the side portion 138d. The element chamber outlets 138a are not provided in the bottom portion 138e of the tip portion 138. The element chamber outlets 138a are disposed forward of the measurement gas inlet 21a. The radius R1 (see FIG. 5) of the outer diameter of the first cylindrical portion 134 is, for example, 3 mm or more and 6 mm or less. The inner radius R2 (see FIG. 5) of the second cylindrical portion 136 is, for example, 3.2 mm or more and 7 mm or less. The thickness of the inner protective cover 130 including the first cylindrical portion 134 and the second cylindrical portion 136 is, for example, 0.2 mm or more and 0.5 mm or less.

[0031] The large diameter portion 132, the first cylindrical portion 134, the second cylindrical portion 136, and the tip portion 138 share the same central axis. The large diameter portion 132 has an inner circumferential surface that abuts against the housing 102, thereby fixing the first member 131 to the housing 102. The second member 135 has an outer circumferential surface of a connection portion 137 that abuts against an inner circumferential surface of the outer protective cover 140, and is fixed by welding or the like. The outer diameter of the tip side (front end side) of the connection portion 137 may be formed slightly larger than the inner diameter of the tip portion 146 of the outer protective cover 140, and the tip portion of the connection portion 137 may be press-fitted into the tip portion 146 to fix the second member 135.

[0032] A plurality of protruding portions 136a are formed on the inner peripheral surface of the second cylindrical portion 136, protruding toward the outer peripheral surface of the first cylindrical portion 134 and in contact with this outer peripheral surface. As shown in Figs. 5 and 6, three protruding portions 136a are provided and are evenly arranged along the circumferential direction of the inner peripheral surface of the second cylindrical portion 136. The protruding portions 136a are formed in a substantially hemispherical shape. By providing such protruding portions 136a, the positional relationship between the first cylindrical portion 134 and the second cylindrical portion 136 is easily fixed by the protruding portions 136a. It is preferable that the protruding portions 136a press the outer peripheral surface of the first cylindrical portion 134 toward the inside in the radial direction. In this way, the positional relationship between the first cylindrical portion 134 and the second cylindrical portion 136 can be more reliably fixed by the protruding portions 136a. It is preferable that the number of the protruding portions 136a is not limited to three, but may be two, four or more. In order to facilitate stable fixation between the first cylindrical portion 134 and the second cylindrical portion 136, it is preferable to provide three or more protruding portions 136a.

[0033] As described above, the inner protective cover 130 has the sensor element chamber 124 inside. The sensor element chamber 124 is formed as a space inside the inner protective cover 130, and the rear end of the sensor element chamber 124 is blocked by the element sealing body 101. More specifically, the sensor element chamber 124 is a space surrounded by the tip portion 138, the step portion 139, the connection portion 137, the second cylindrical portion 136, the first cylindrical portion 134, the housing 102, and the supporter 104. The tip of the sensor element 110 and the measurement gas introduction port 21a are arranged inside the sensor element chamber 124. In addition, the detection electrode 56 is arranged inside the sensor element chamber 124. The inner protective cover 130 has one or more first inlets 125 and one or more second inlets 127 as the element chamber inlet 123, which is an inlet to the sensor element chamber 124.

[0034] The first inlet 125 is disposed in the first member 131. In this embodiment, the first inlet 125 has a plurality (six) of circular holes 125a formed at equal intervals in the circumferential direction in the cylindrical side portion (first cylindrical portion 134) of the first member 131 (see FIG. 5). As shown in FIG. 4, the holes 125a are formed as flow paths perpendicular to the front-rear direction and in a direction (radial direction) toward the central axis of the first cylindrical portion 134. The hole 125a has a constant hole diameter from the first gas chamber 122 side to the sensor element chamber 124 side, so that the inner circumferential surface of the hole 125a is formed parallel to the central axis of the hole 125a. The hole 125a has an element side opening 125b which is an opening on the sensor element chamber 124 side. The element side opening 125b is open to the inner circumferential surface 134a of the first cylindrical portion 134 among the holes 125a. The diameter D1 (see FIG. 4) of each hole 125a of the first inlet 125 is, for example, 1.00 mm or more and 3.53 mm or less. In this embodiment, the diameters of the holes 125a are all the same value. The first inlet 125 is disposed at a position where, for example, a distance K4 (see FIG. 4) in the front-rear direction from the front end face of the first cylindrical portion 134 to the rear end of each hole 125a is 4 mm or more and 8 mm or less.

[0035] The second inlet 127 is formed as a gap between the first member 131 and the second member 135. In this embodiment, the second inlet 127 is formed as a cylindrical gap (gas flow path) between the outer peripheral surface of the first cylindrical portion 134 and the inner peripheral surface of the second cylindrical portion 136. The second inlet 127 is a space from the rear end of the second member 135 (here, the rear end of the second cylindrical portion 136) to the front end of the first member 131 (here, the front end of the first cylindrical portion 134). The second inlet 127 has a rear opening 128 which is an opening on the first gas chamber 122 side, which is the space where the outer inlet 144a is arranged, and a front opening 129 which is an opening on the sensor element chamber 124 side, which is the space where the measurement gas introduction port 21a is arranged. The rear opening 128 is a ring-shaped gap between the rear end of the inner peripheral surface of the second cylindrical portion 136 and the outer peripheral surface of the first cylindrical portion 134. The front opening 129 is a ring-shaped gap between the inner circumferential surface of the second cylindrical portion 136 and the front end of the outer circumferential surface of the first cylindrical portion 134. The rear opening 128 is formed rearward of the front opening 129. Therefore, in the path of the measurement gas from the outer inlet 144a to the measurement gas introduction port 21a, the second inlet 127 forms a flow path from the rear end side to the tip side of the sensor element 110, in other words, from the rear to the front. In addition, the second inlet 127 forms a flow path parallel to the rear end-to-tip direction of the sensor element 110 (a flow path parallel to the front-rear direction).

[0036] The front opening 129 is disposed forward of the detection electrode 56 and the first inlet 125, and opens in the direction from the rear end to the front end of the sensor element 110 (forward). The front opening 129 opens parallel to the rear end-to-front direction (front-rear direction) of the sensor element 110. Therefore, the sensor element 110 is disposed at a position other than the region where the second inlet 127 is virtually extended from the front opening 129 (the region directly in front of the front opening 129). The front opening 129 is preferably formed at a position where the distance K2 (see FIG. 4) from the measurement gas inlet 21a is −5 mm or more and 2 mm or less. Note that the distance K2 is the distance along the axial direction of the sensor element 110 (the distance in the front-rear direction), and the distance K2 is a positive value when the front opening 129 is located rearward of the measurement gas inlet 21a. The distance K2 is the distance between the end of the opening of the measurement gas inlet 21a that is closest to the front opening 129 and the end of the front opening 129 that is closest to the measurement gas inlet 21a in the front-rear direction. For example, in FIG. 4, when the measurement gas inlet is a hole that opens on the side surface (e.g., the top surface 20a) of the element body 20, when the front opening 129 is located behind the rear end of the opening of the measurement gas inlet, the distance between the rear end of the opening of the measurement gas inlet and the front opening 129 is the distance K2, and the distance K2 is a positive value. Similarly, in FIG. 4, when the measurement gas inlet is a hole that opens on the side surface of the element body 20, when the front opening 129 is located forward of the front end of the opening of the measurement gas inlet, the distance between the front end of the opening of the measurement gas inlet and the front opening 129 is the distance K2, and the distance K2 is a negative value. In addition, in FIG. 4, when the measurement gas inlet is a hole opening on the side surface of the element body 20, when the front opening 129 is located between the rear end and the front end of the opening of the measurement gas inlet, the distance K2 has a value of 0. In this embodiment, the front opening 129 is formed at a position where the distance K2 is a positive value. That is, the front opening 129 is formed behind the measurement gas inlet 21a. The front opening 129 may be formed at a position where the distance K2 is a negative value. That is, the front opening 129 may be located forward of the measurement gas inlet 21a.

[0037] The first inlet 125 and the second inlet 127 are disposed in the sensor element chamber 124 such that the ratio A / B of the total opening area A of the first inlet 125 to the total opening area B of the second inlet 127 is 0.02 or more and 19.76 or less. The ratio A / B is preferably 0.05 or more and 7.29 or less, more preferably 0.10 or more and 4.45 or less. The total opening area A of the first inlet 125 is the sum of the opening areas a of the holes 125a on the sensor element chamber 124 side (A=6×a in this embodiment). The opening area a is the area in the direction perpendicular to the axial direction of the holes 125a. In this embodiment, since all the holes 125a are circular holes, the area of ​​this circle is the opening area a. The total opening area B of the second inlet 127 is the sum of the opening areas b of the second inlet 127 on the sensor element chamber 124 side (B=b in this embodiment). The opening area b is the area in a direction perpendicular to the axial direction of the front opening 129. In this embodiment, the outer peripheral surface of the front end of the first cylindrical portion 134 and the inner peripheral surface of the second cylindrical portion 136 are both circular. Therefore, the opening area b is the area of ​​the front opening 129, which is the area of ​​the circle whose diameter is the outer diameter of the front end of the first cylindrical portion 134 minus the area of ​​the circle whose diameter is the inner diameter of the second cylindrical portion 136. The total opening area A is, for example, 3.14 mm 2 Over 76.97mm 2 The opening area a of each hole 125a is, for example, 0.79 mm 2 Over 9.62mm 2 The total opening area B is, for example, 3.90 mm 2 Over 125.66mm 2 The following is the result.

[0038] At least one of the first inlets 125 (holes 125a in this example) is disposed at a position where the shortest path length L1 from the holes 125a to the detection electrode 56 is shorter than the shortest path length L2 from the second inlet 127 to the detection electrode 56. The shortest path lengths L1 and L2 will be described with reference to Figs. 4 and 8. The shortest path length L1 is the length of the shortest path in the flow path of the measurement gas from each hole 125a to the detection electrode 56. In the case of the hole 125a shown in Fig. 4, the length of the straight line from the rear end of the element-side opening 125b to the center of the front end of the upper surface of the detection electrode 56 (the length of the path shown as the thick dashed line L1 in Figs. 4 and 8) is the shortest path length L1. The shortest path length L2 is the length of the shortest path in the flow path of the measurement gas from the second inlet 127 to the detection electrode 56. In this embodiment, the shortest path length L2 is the total length (length of the path that appears as a thick dashed line L2 in FIG. 8) of a straight line from the inner circumference of the front opening 129 of the second inlet 127 (the outer circumference of the front end surface of the first member 131) to the inner circumference of the front end surface of the first member 131 and a straight line from the inner circumference of the front end surface of the first member 131 to the right end (or the left end) of the front end of the upper surface of the detection electrode 56. Note that the thick dashed line L2 in FIG. 8 is an extension of a straight line that connects the center axis O of the first cylindrical portion 134 and the right end of the front end of the upper surface of the detection electrode 56 when viewed from the front. Also, the thick dashed line (L2) in FIG. 4 is shown for the sake of convenience to help understand the starting point and the bending point of the path that is the shortest path length L2, and the path that is the shortest path length L2 does not strictly match the position in the up-down direction of the thick dashed line (L2) in FIG. 4. Among the first inlets 125, the first inlet 125 (hole 125a) having the shortest shortest path length L1 is also referred to as the closest inlet. In this embodiment, the hole 125a located in the 0 o'clock direction (upward direction) of the sensor element 110 among the six holes 125a in FIG. 5 is the closest inlet. Note that, in cases where the length of the detection electrode 56 in the left-right direction is longer than that in FIG. 8, the holes 125a located in the 2 o'clock and 10 o'clock directions of the sensor element 110 among the six holes 125a may be the closest inlets. In the closest inlet, the shortest path length L1 is, for example, 12.0 mm or less. In addition, in all the first inlets 125, the shortest path length L1 is, for example, 0.2 mm or more, and the shortest path length L2 is, for example, 1.5 mm or more and 19.9 mm or less.

[0039] Moreover, all the first inlets 125 (holes 125a in this case) are disposed at positions where the detection electrode 56 does not overlap the projection area of ​​the holes 125a projected toward the surface of the element body 20 in parallel with the axial direction of the holes 125a. The projection area will be described with reference to FIG. 4. In the case of the hole 125a shown in FIG. 4, the projection area is a cylindrical area that is coaxial with and has the same diameter as the hole 125a, and extends from the element side opening 125b to the upper surface 20a of the element body 20, and is the area PA surrounded by a two-dot chain line in FIG. 4. In this embodiment, the detection electrode 56 is disposed behind the projection areas of all the holes 125a, so that the detection electrode 56 does not overlap the projection area in all the holes 125a. The projection area is an area through which the gas to be measured can pass, and is blocked by an object (for example, the element body 20) through which the gas to be measured cannot pass. For this reason, even if the detection electrode 56 is arranged so as to overlap the area PA in FIG. 4, the detection electrode 56 does not overlap the projection area of ​​the hole 125a on the back side of the element body 20 (the hole 125a in the 6 o'clock direction in FIG. 5).

[0040] At least one of the first inlets 125 (holes 125a in this case) is disposed such that, in a virtual projection diagram in which the first member 131 and the sensor element 110 are vertically projected onto a virtual plane perpendicular to the axial direction of the first member 131, an angle θ between a first line perpendicular to the arrangement surface 56a of the sensor element 110 on which the detection electrode 56 is arranged and drawn from the center of the arrangement surface 56a toward the opposite side to the element body 20, and a second line drawn from the center of the arrangement surface 56a toward the center of the hole 125a is 73.6° or less. The center of the hole 125a is the center of the element-side opening 125b. In the gas sensor 100 of this embodiment, this angle θ is 0° in the hole 125a shown in FIG. 4. The angle θ will be described with reference to FIG. 9. FIG. 9 is a virtual projection diagram of the gas sensor 100. 9, the angle θ of the hole 125a in the 2 o'clock direction is the angle θ1 between a first line S1 perpendicular to the mounting surface 56a and drawn from the center X of the mounting surface 56a toward the opposite side to the element body 20, and a second line S2 drawn from the center X of the mounting surface 56a toward the center X0 of the hole 125a. The angle θ of the other holes 125a can be calculated in a similar manner. In this embodiment, the angle θ of the holes 125a in the 0 o'clock direction, the 2 o'clock direction, and the 10 o'clock direction among the six holes 125a is 73.6° or less.

[0041] 9, in the first member 131, when the first line S1 is rotated 73.6° around the center X of the arrangement surface 56a as the center of rotation, it is defined as a third line S3, the first line S1 is rotated 73.6° in the opposite direction to the third line S3 as a fourth line S4, the intersection of the third line S3 and the inner circumferential surface 134a of the first member 131 as a first intersection X1, and the intersection of the fourth line S4 and the inner circumferential surface 134a of the first member 131 as a second intersection X2, all of the holes 125a are arranged in the circumferential direction of the first member 131 such that the distance between the centers of adjacent holes 125a is equal to or smaller than the distance P between the first intersection X1 and the second intersection X2 (the length of the thick dashed line P in FIG. 9). In other words, in the inner peripheral surface 134a of the first member 131, when the central angle of the arc connecting the first intersection point X1 and the second intersection point X2 is the angle φ, all the holes 125a are arranged so that the central angles of the arc connecting the centers of the adjacent holes 125a (for example, the central angle α in FIG. 9) are all equal to or less than the angle φ. If the first member 131 in which all the holes 125a are arranged in this manner is used, the center of at least one of the holes 125a will be located on the thick dashed line P in FIG. 9, that is, at a position equal to or less than θ=73.6°, regardless of the mounting orientation of the first member 131 relative to the sensor element 110. Therefore, the above-mentioned angle θ of at least one of the holes 125a is equal to or less than 73.6°. In the gas sensor 100 of this embodiment, six holes 125a are formed in the first member 131 at equal intervals in the circumferential direction, so that the central angles of the arc connecting the centers of the adjacent holes 125a are all 60°, which is clearly smaller than the angle φ. Therefore, regardless of the mounting orientation of the first member 131 relative to the sensor element 110, at least one of the holes 125a can be disposed so that the angle θ is 73.6° or less.

[0042] As shown in FIG. 2, the outer protective cover 140 has a cylindrical body 143 and a tip end 146 that is cylindrical with a bottom and has a smaller inner diameter than the body 143. The body 143 has a side portion 143a having a side surface along the central axis direction (front-rear direction) of the outer protective cover 140, and a step portion 143b that is the bottom of the body 143 and connects the side portion 143a and the tip end 146. The central axes of the body 143 and the tip end 146 are both the same as the central axis of the inner protective cover 130. The inner peripheral surface of the portion around the rear end of the body 143 abuts against the housing 102 and the large diameter portion 132, thereby fixing the outer protective cover 140 to the housing 102. The body 143 is positioned so as to cover the outer peripheries of the large diameter portion 132, the first cylindrical portion 134, and the second cylindrical portion 136. The tip portion 146 is positioned so as to cover the tip portion 138, and the inner peripheral surface abuts against the outer peripheral surface of the connection portion 137. The tip portion 146 has a side portion 146a having a side surface along the central axis direction (front-rear direction) of the outer protective cover 140 and an outer diameter smaller than the inner diameter of the side portion 143a, a bottom portion 146b which is the bottom of the outer protective cover 140, and a tapered portion 146c which connects the side portion 146a and the bottom portion 146b and reduces in diameter from the side portion 146a to the bottom portion 146b. The tip portion 146 is positioned forward of the body portion 143. This outer protective cover 140 has one or more (multiple in this embodiment, specifically 12) outer inlets 144a which are formed in the body portion 143 and serve as inlets from the outside of the measurement gas, and one or more outer outlets 147a which are formed in the tip portion 146 and serve as outlets for the measurement gas to the outside.

[0043] The outer inlet 144a is a hole that communicates with the outside (outside) of the outer protective cover 140 and the first gas chamber 122. The outer inlet 144a has a plurality of holes 144b (six in this embodiment) formed at equal intervals in the circumferential direction in the side portion 143a, and a plurality of holes 144c (six in this embodiment) formed at equal intervals in the circumferential direction in the step portion 143b. The outer inlet 144a (holes 144b and 144c) is a circular hole. As shown in Figs. 5 to 7, the outer inlet 144a is formed such that the holes 144b and the holes 144c are alternately positioned at equal intervals along the circumferential direction of the outer protective cover 140. All of the outer inlets 144a are positioned below the rear end of the second member 135 (here, the rear end of the second cylindrical portion 136).

[0044] The outer outlet 147a is a hole that communicates with the outside (outside) of the outer protective cover 140 and the second gas chamber 126. The outer outlet 147a has one or more holes 147c (one in this embodiment) formed in the center of the bottom 146b of the tip 146 (see Figs. 2 and 7). Unlike the outer inlet 144a, the outer outlet 147a is not disposed on the side of the outer protective cover 140 (side 146a of the tip 146 in this embodiment). The outer outlet 147a (hole 147c in this embodiment) is a circular hole.

[0045] The outer protective cover 140 and the inner protective cover 130 form the first gas chamber 122 and the second gas chamber 126 as described above. The first gas chamber 122 is formed as a space between the body portion 143 and the inner protective cover 130. More specifically, the first gas chamber 122 is a space surrounded by the step portion 133, the first cylindrical portion 134, the second cylindrical portion 136, the side portion 143a, and the step portion 143b. The second gas chamber 126 is formed as a space between the tip portion 146 and the inner protective cover 130. More specifically, the second gas chamber 126 is a space surrounded by the tip portion 138 and the tip portion 146. Note that since the inner peripheral surface of the tip portion 146 abuts against the outer peripheral surface of the connection portion 137, the first gas chamber 122 and the second gas chamber 126 are not directly connected to each other.

[0046] Next, the flow of the measurement gas in the protective cover 120 when the gas sensor 100 detects the NOx concentration and the ammonia concentration will be described. The measurement gas flowing in the pipe 10 first flows into the first gas chamber 122 through at least one of the multiple outer inlets 144a (holes 144b and 144c in this case). A part of the measurement gas that has flowed into the first gas chamber 122 flows into the sensor element chamber 124 through the first inlet 125 of the element chamber inlet 123. At least a part of the measurement gas that has flowed into the sensor element chamber 124 from the first inlet 125 reaches the detection electrode 56 of the sensor element 110. When the measurement gas reaches the detection electrode 56, a mixed potential (electromotive force EMF) corresponding to the ammonia concentration in the measurement gas is generated in the mixed potential cell 55 of the sensor element 110. The controller detects the ammonia concentration based on this electromotive force EMF. Also, a remaining part of the measurement gas that has flowed into the first gas chamber 122 flows into the sensor element chamber 124 through the second inlet 127 of the element chamber inlet 123. At least a part of the measurement gas that has flowed into the sensor element chamber 124 from the second inlet 127 reaches the measurement gas inlet 21a of the sensor element 110. When the measurement gas reaches the measurement gas inlet 21a and flows into the inside of the sensor element 110, a pump current Ip2 corresponding to the NOx concentration in the measurement gas flows in the detection unit 23 under the control of the controller described above. The controller detects the NOx concentration based on this pump current Ip2. Also, the measurement gas in the sensor element chamber 124 flows into the second gas chamber 126 through at least one of the element chamber outlets 138a (hole 138b in this case), and then flows out to the outside through the outer outlet 147a.

[0047] Here, the second inlet 127 is formed as a gap between the first member 131 and the second member 135, and the front opening 129 is disposed forward of the detection electrode 56 and the first inlet 125 and opens forward. In the second inlet 127 thus formed, the measurement gas flowing from the second inlet 127 into the sensor element chamber 124 flows from the rear to the front, from the front of the detection electrode 56 and the first inlet 125 into the sensor element chamber 124. According to a simulation, the flow rate of the measurement gas flowing from the inlet formed as the gap between the first member 131 and the second member 135 into the sensor element chamber 124 is faster than that of the measurement gas flowing from a hole such as the first inlet 125. As a result, the measurement gas flowing from the second inlet 127 into the sensor element chamber 124 quickly reaches the measurement gas inlet 21a located forward of the detection electrode 56. The measurement gas flowing into the measurement gas flow portion 21 from the measurement gas inlet 21a quickly reaches the measurement electrode 27 as oxygen is pumped from the periphery of the measurement electrode 27 to the periphery of the outer electrode 24, and is used to detect the NOx concentration. Therefore, the gas sensor 100 has good responsiveness in detecting the NOx concentration.

[0048] The measurement gas that has flowed into the sensor element chamber 124 from the first inlet 125 quickly reaches the detection electrode 56, which is disposed rearward of the front opening 129 of the second inlet 127 and rearward of the measurement gas inlet 21a, and is used for detecting the ammonia concentration, etc. Therefore, the gas sensor 100 has good responsiveness in detecting the concentration of ammonia.

[0049] The measurement gas flowing into the sensor element chamber 124 from the first inlet 125 passes through the protective layer 85 and the buffer layer 84 before reaching the detection electrode 56. The protective layer 85 and the buffer layer 84 are heated to a high temperature due to the heating of the element body 20, so that ammonia in the measurement gas may burn while the measurement gas passes through the protective layer 85 and the buffer layer 84, and the sensitivity to ammonia may decrease. Here, the smaller the angle θ of the hole 125a, the shorter the distance over which the measurement gas flowing into the sensor element chamber 124 from the hole 125a comes into contact with the protective layer 85 and the buffer layer 84 before reaching the detection electrode 56. For this reason, it is preferable that the smaller the angle θ of the hole 125a is, since the sensitivity to ammonia is less likely to decrease. Specifically, it is preferable that the hole 125a has an angle θ of 73.6° or less. As described above, the gas sensor 100 has holes 125a with an angle θ of 73.6° or less, including holes 125a arranged so that the angle θ is 0° (holes 125a in the 0 o'clock direction in FIG. 9) and holes 125a in the 2 o'clock direction and 10 o'clock direction in FIG. 9, so that the sensitivity to ammonia in the measured gas is less likely to decrease.

[0050] The gas sensor 100 of this embodiment described above in detail has the first inlet 125 and the second inlet 127 formed as described above as the element chamber inlet 123, and therefore has good responsiveness in detecting the concentrations of NOx and ammonia.

[0051] In the sensor element chamber 124, the ratio A / B of the total opening area A of the one or more first inlets 125 to the total opening area B of the one or more second inlets 127 is 0.02 or more and 19.76 or less. This provides a good balance between the flow rate of the measurement gas flowing into the sensor element chamber 124 from the first inlet 125 and the flow rate of the measurement gas flowing into the sensor element chamber 124 from the second inlet 127, resulting in better responsiveness in concentration detection for NOx and ammonia. The ratio A / B is preferably 0.05 or more and 7.29 or less, more preferably 0.10 or more and 4.45 or less.

[0052] Furthermore, the detection electrode 56 is disposed rearward of the outer electrode 24. More specifically, the front end of the detection electrode 56 is disposed rearward of the rear end of the outer electrode 24. In this case, the shortest path length L2 from the second inlet 127 to the detection electrode 56 is relatively long, and it takes time for the measurement gas flowing into the sensor element chamber 124 from the second inlet 127 to reach the detection electrode 56, so that providing the first inlet 125 is very meaningful.

[0053] Furthermore, at least one of the one or more first inlets 125 is disposed at a position where the shortest path length L1 from the first inlet 125 to the detection electrode 56 is shorter than the shortest path length L2 from the second inlet 127 to the detection electrode 56. In other words, the first inlet 125 is disposed at a position closer to the detection electrode 56. This results in better responsiveness in detecting the concentration of ammonia.

[0054] In addition, the shortest path length L1 is 0.2 mm or more in all the first inlets 125. As a result, the flow rate of the measurement gas flowing from the first inlet 125 into the sensor element chamber 124 is appropriately suppressed until it reaches the detection electrode 56, so that deterioration of the detection electrode 56 due to the measurement gas hitting the detection electrode 56 is suppressed. In addition, since there is a first inlet 125 with the shortest path length L1 of 12.0 mm or less, the measurement gas flowing from the first inlet 125 into the sensor element chamber 124 reaches the detection electrode 56 while maintaining a relatively high flow rate. This improves the responsiveness of the concentration detection for ammonia.

[0055] Furthermore, all of the first inlets 125 are arranged at positions where the detection electrode 56 does not overlap with a projection area of ​​the first inlets 125 projected onto the surface of the element body 20 parallel to the axial direction of the first inlets 125. This prevents the measurement gas, which flows straight into the sensor element chamber 124 parallel to the axial direction of the first inlets 125 and has a relatively high flow rate, from directly hitting the detection electrode 56, thereby suppressing deterioration of the detection electrode 56.

[0056] Furthermore, at least one of the one or more first inlets 125 is arranged so that the angle θ is 73.6° or less. Therefore, even if the gas sensor 100 includes the buffer layer 84 and the protective layer 85 that cover the detection electrode 56, the decrease in sensitivity to ammonia in the measured gas is suppressed. In particular, when the angle θ of the closest inlet among the first inlets 125 is 73.6° or less, the decrease in sensitivity to ammonia in the measured gas is more suppressed. Since the decrease in sensitivity to ammonia in the measured gas is suppressed more as the angle θ is smaller, at least one of the one or more first inlets 125 may be arranged so that the angle θ is, for example, 53.8° or less, or 39.1° or less.

[0057] Furthermore, all of the first inlets 125 are arranged such that the distance between the centers of adjacent first inlets 125 in the circumferential direction of the first member 131 is equal to or smaller than the distance between the first intersection point X1 and the second intersection point X2 described above. In other words, in the first member 131, the central angles of the arcs connecting the centers of adjacent holes 125a are all smaller than the angle φ described above. For this reason, it is easy to manufacture the gas sensor 100 in which the angle θ is 73.6° or less.

[0058] It goes without saying that the present invention is not limited to the above-described embodiment, and can be embodied in various forms within the technical scope of the present invention.

[0059] For example, in the above embodiment, the first inlet 125 with an angle θ of 0° exists, but all the first inlets 125 may be arranged so that the angle θ is 23.5° or more. For example, when using the sensor element 110 and the protective cover 120 used in the gas sensor 100, the mounting orientation of the protective cover 120 (particularly the first member 131) with respect to the sensor element 110 may be adjusted so that the first inlet 125 is not arranged in the non-arrangement area N shown in FIG. 10. FIG. 10 is a virtual projection view of a gas sensor in which the mounting orientation of the protective cover 120 with respect to the sensor element 110 is changed. The non-arrangement area N is set, for example, as follows. In the virtual projection view of FIG. 10, the straight line obtained by rotating the first straight line S1 by 23.5° around the center X of the arrangement surface 56a as the rotation center is set as the fifth straight line S5, and the straight line obtained by rotating the first straight line S1 by 23.5° in the opposite direction to the fifth straight line S5 is set as the S6. The intersection of the fifth straight line S5 and the inner circumferential surface 134a of the first member 131 is the third intersection X3, and the intersection of the sixth straight line S6 and the inner circumferential surface 134a of the first member 131 is the fourth intersection X4. The arc-shaped region (the region indicated by the thick dashed line in FIG. 10) between the third intersection X3 and the fourth intersection X4 on the inner circumferential surface 134a of the first member 131 is the non-arrangement region N. In the first inlet 125 arranged in the non-arrangement region N, that is, the first inlet 125 with an angle θ of less than 23.5°, the measurement gas having a relatively high flow rate that flows straight into the sensor element chamber 124 parallel to the axial direction of the first inlet 125 may hit the detection electrode 56, which may cause the detection electrode 56 to deteriorate. In contrast, when there is no first inlet 125 with an angle θ of less than 23.5° as shown in FIG. 10, deterioration of the detection electrode 56 is suppressed. The angle θ may be greater than or equal to 31.2°.

[0060] In the above-described embodiment, in the gas sensor 100, a controller (not shown) connected to the sensor element 110 acquires the electromotive force EMF of the mixed potential cell 55 and detects the ammonia concentration in the measured gas based on the electromotive force EMF. At this time, since the electromotive force EMF is also affected by oxygen in the measured gas, it is preferable that the controller detects the ammonia concentration in the measured gas using the electromotive force EMF and the oxygen concentration in the measured gas derived based on the pump current Ip0. For example, a relational expression or a map expressing the correspondence between the oxygen concentration in the measured gas (or the pump current Ip0 as a value representing the oxygen concentration), the electromotive force EMF, and the ammonia concentration is created in advance by experiment and stored in a memory. Then, the controller detects the ammonia concentration in the measured gas using the oxygen concentration in the measured gas, the electromotive force EMF, and this correspondence. In this way, the gas sensor 100 can derive the ammonia concentration in which the error in the electromotive force EMF due to the oxygen concentration is corrected.

[0061] In the above-described embodiment, in the gas sensor 100, the controller connected to the sensor element 110 may obtain the electromotive force EMF of the mixed potential cell 55 and correct the error of the NOx concentration in the measured gas based on the electromotive force EMF. The pump current Ip2 may also be affected by ammonia in the measured gas. This is for the following reason. When ammonia is contained in the measured gas, the ammonia reacts with oxygen in the measured gas flow section 21 to generate NO, and oxygen derived from the NO is generated in the space around the measuring electrode 27. Therefore, the oxygen pumped out from the space around the measuring electrode 27 to the periphery of the outer electrode 24 contains oxygen derived from such ammonia. Therefore, the value of the pump current Ip2 also changes depending on the concentration of ammonia in the measured gas. Therefore, in the gas sensor 100, it is preferable that the controller detects the NOx concentration in the measured gas using the pump current Ip2 and the ammonia concentration in the measured gas derived based on the above-described electromotive force EMF. For example, a relational expression or a map expressing the correspondence relationship between the pump current Ip2, the ammonia concentration, and the NOx concentration is created in advance by experiment and stored in a memory. The controller derives the NOx concentration in the measurement gas using the pump current Ip2, the ammonia concentration in the measurement gas, and this correspondence. This allows the gas sensor 100 to derive the NOx concentration with the error of the pump current Ip2 due to ammonia corrected. Note that when the electromotive force EMF is used to correct the NOx concentration in this manner, the electromotive force EMF may be used directly to correct the NOx concentration without detecting (deriving) the ammonia concentration.

[0062] In the above embodiment, the inner protective cover 130 has the first inlet 125 whose shortest path length L1 is shorter than the shortest path length L2, but it is sufficient to have the first inlet 125, and it is not necessary to have a first inlet 125 whose shortest path length L1 is shorter than the shortest path length L2. In addition, the inner protective cover 130 has the first inlet 125 whose shortest path length L1 is 12.0 mm or less, but it is sufficient to have the first inlet 125, and it is not necessary to have the first inlet 125 whose shortest path length L1 is 12.0 mm or less. In addition, the shortest path length L1 is set to 0.2 mm or more for all the first inlets 125, but it is also possible to have a first inlet 125 whose shortest path length L1 is less than 0.2 mm.

[0063] In the above-described embodiment, the front end of the detection electrode 56 is disposed rearward of the rear end of the outer electrode 24, but it may be disposed rearward of the front end of the outer electrode 24. Also, the rear end of the detection electrode 56 may be disposed forward of the front end of the outer electrode 24.

[0064] In the above-described embodiment, the ratio A / B is set to 0.02 or more and 19.76 or less, but the ratio A / B may be less than 0.02 or more than 19.76.

[0065] In the above embodiment, all the first inlets 125 are arranged at positions where the detection electrode 56 does not overlap the projection area, but at least one of the first inlets 125 may be arranged at a position where the detection electrode 56 overlaps the projection area. In this case, the measurement gas having a relatively high flow rate that flows straight parallel to the axial direction of the first inlets 125 into the sensor element chamber 124 quickly reaches the detection electrode 56, so that the responsiveness to ammonia is improved. In addition, the distance over which the measurement gas that flows from the first inlet 125 into the sensor element chamber 124 comes into contact with the buffer layer 84 and the protective layer 85 before reaching the detection electrode 56 is relatively short, so that even if the gas sensor 100 includes the buffer layer 84 and the protective layer 85 that cover the detection electrode 56, the decrease in sensitivity to ammonia in the measurement gas is suppressed.

[0066] In the embodiment described above, the inner protective cover 130 had a first inlet 125 with an angle θ of 73.6° or less, but it is sufficient to have a first inlet 125, and the first inlet 125 with an angle θ of 73.6° or less does not necessarily have to be present.

[0067] In the above embodiment, the first member 131 has a plurality of (six) holes 125a formed at equal intervals in the circumferential direction as one or more first inlets 125, but the number of holes 125a is not particularly limited and may be one, and at least some of the holes 125a may not be formed at equal intervals. If a first member 131 having four or more holes 125a formed at equal intervals as the first inlets 125 is used, the central angles of the arcs connecting the centers of the adjacent holes 125a are likely to be equal to or less than the angle φ. This makes it easy to manufacture a gas sensor 100 with an angle θ of 73.6° or less in at least one of the first inlets 125, regardless of the mounting direction of the first member 131 relative to the sensor element 110. In addition, the number of holes 125a may be six or less. In addition, the positions of the holes 125a in the front-rear direction are all the same, but they do not have to be the same. Also, although the holes 125a are circular, they do not have to be circular.

[0068] In the above-described embodiment, hole 125a is formed as a flow path perpendicular to the front-rear direction and toward the central axis of first cylindrical portion 134, but it may be inclined from a direction perpendicular to the front-rear direction, or may be inclined from a direction toward the central axis of first cylindrical portion 134. Also, the inner circumferential surface of hole 125a is parallel to the axial direction of hole 125a, but the inner circumferential surface of hole 125a may be inclined from the axial direction of hole 125a so that hole 125a becomes larger or smaller in diameter as it approaches element-side opening 125b.

[0069] In the above-described embodiment, the rear end of the first inlet 125 (particularly, the element-side opening 125b) is disposed forward of the front end of the detection electrode 56, but it may be disposed rearward of the front end of the detection electrode 56. In addition, the front end of the first inlet 125 (particularly, the element-side opening 125b) may be disposed rearward of the rear end of the detection electrode 56.

[0070] In the above embodiment, the detection electrode 56 is disposed at the center in the width direction of the sensor element 110, but may be shifted from the center in the width direction of the sensor element 110. FIG. 11 is a virtual projection diagram of the gas sensor 100 when the detection electrode 56 is moved to the right. In this case, it is preferable that there is at least one hole 125a with an angle θ of 73.6° or less in the virtual projection diagram. It is also preferable that all holes 125a are disposed so that the central angles of the arcs connecting the centers of the adjacent holes 125a are all angle φ or less. In FIG. 11, among the six holes 125a, the holes 125a located at the 0 o'clock and 2 o'clock directions have an angle θ of 73.6° or less. As described above, in the gas sensor 100, the six holes 125a are formed in the first member 131 at equal intervals in the circumferential direction, so that the central angles of the arcs connecting the centers of the adjacent holes 125a are all 60°, which is clearly smaller than the angle φ.

[0071] In the above-described embodiment, one reference electrode 28 is provided in the element body 20, and the reference electrode 28 serves both as the reference electrode for the detection unit 23 and the reference electrode for the mixed potential cell 55. However, this is not limited to this. For example, the reference electrode 28 may be a reference electrode for the detection unit 23, and a reference electrode provided in the mixed potential cell 55 may be provided separately from the reference electrode 28.

[0072] In the above-described embodiment, the second inlet 127 is a flow path parallel to the rear end-front end direction of the sensor element 110 (a flow path parallel to the front-rear direction), but the second inlet 127 may be a flow path inclined from the front-rear direction so as to approach the sensor element 110 as it moves forward. FIG. 12 is a longitudinal sectional view of a gas sensor 200 of a modified example in this case. In FIG. 12, the same components as those in the gas sensor 100 are given the same reference numerals, and detailed description is omitted. As shown in FIG. 12, the protective cover 220 of the gas sensor 200 includes an inner protective cover 230 instead of the inner protective cover 130. The inner protective cover 230 includes a first member 231 and a second member 235. Compared to the first member 131, the first member 231 does not include the first cylindrical portion 134, but instead includes a cylindrical body portion 234a and a cylindrical first cylindrical portion 234b whose diameter decreases as it moves forward. The inner protective cover 230 has one or more first inlets 225 and one or more second inlets 227 as the element chamber inlet 223, which is an inlet to the sensor element chamber 124. Each hole 225a of the first inlet 225 is arranged in the body 234a. The first cylindrical portion 234b is connected to the body 234a at the rear end. Compared to the second member 135, the second member 235 does not have the second cylindrical portion 136, but instead has a cylindrical second cylindrical portion 236 that is narrowed toward the front. The outer peripheral surface of the first cylindrical portion 234b and the inner peripheral surface of the second cylindrical portion 236 are not in contact with each other, and the gap formed by the two is the second inlet 227. The second inlet 227 has a rear opening 228 and a front opening 229. Due to the shapes of the first cylindrical portion 234b and the second cylindrical portion 236, the second inlet 227 is a flow path that is inclined from the front-rear direction so as to approach the sensor element 110 (to approach the central axis of the inner protective cover 230) as it moves forward. Similarly, the front opening 229 opens at an incline from the front-rear direction so as to approach the sensor element 110 as it moves forward (see the enlarged view of FIG. 12). The direction of the front opening 229 is the axial direction of the front opening 229, which is determined based on the outer circumferential surface of the first cylindrical portion 234b and the inner circumferential surface of the second cylindrical portion 236 around the front opening 229. The opening area b of the front opening 229 is the area of ​​a surface perpendicular to the axial direction of the front opening 229.In the gas sensor 200 of FIG. 12, the flow path width of the second inlet 227 narrows toward the front. Therefore, the opening area of ​​the front opening 229 is smaller than the opening area of ​​the rear opening 228. As a result, the flow rate of the measurement gas when it flows from the second inlet 227 into the sensor element chamber 124 is higher than when the measurement gas flows from the first gas chamber 222 into the second inlet 227. Therefore, the response of the NOx concentration detection can be improved. In FIG. 12, the second inlet 227 has a flow path inclined from the front-rear direction, the front opening 229 opens at an incline from the front-rear direction, and the opening area of ​​the front opening 229 is smaller than the opening area of ​​the rear opening 228. However, one or more of these three features may be omitted, or the gas sensor may have one or more of these three features.

[0073] In the above-described embodiment, the second inlet 127 is a cylindrical gap between the outer circumferential surface of the first cylindrical portion 134 of the first member 131 and the inner circumferential surface of the second cylindrical portion 136 of the second member 135, but is not limited thereto. For example, a recess (groove) may be formed on at least one of the outer circumferential surface of the first cylindrical portion and the inner circumferential surface of the second cylindrical portion, and the second inlet may be a gap between the first cylindrical portion and the second cylindrical portion formed by the recess. FIG. 13 is a schematic diagram showing a second inlet 327 of a modified example in this case, and is a cross-sectional view corresponding to the cross section BB in FIG. 2. As shown in FIG. 13, the gas sensor 300 has an inner protective cover 330 instead of the inner protective cover 130. The inner protective cover 330 has a first member 331 instead of the first member 131. The first cylindrical portion 334 of the first member 331 is the same as that of the gas sensor 100 in a cross section corresponding to the AA cross section in FIG. 2, but the portion surrounded by the second cylindrical portion 136 of the second member 135 is the thick-walled portion 334a. The outer peripheral surface of the thick-walled portion 334a is in contact with the inner peripheral surface of the second cylindrical portion 136, and a plurality of (four in FIG. 13) recesses 334b ​​are formed at equal intervals on the outer peripheral surface of the thick-walled portion 334a. The gap between the recesses 334b ​​and the inner peripheral surface of the second cylindrical portion 136 is the second inlet 327. The first inlet 125 is disposed in a portion of the first cylindrical portion 334 that is rearward of the thick-walled portion 334a. In this way, the inner protective cover 330 has one or more first inlets 125 and one or more second inlets 327 as the element chamber inlet 323 that is the inlet to the sensor element chamber.

[0074] In the above embodiment, the measurement gas inlet 21a is open to the tip end surface (the front end surface 20e of the element body 20) of the sensor element 110, but is not limited thereto. For example, the measurement gas inlet 21a may be open to any one of the upper surface 20a, the lower surface 20b, the left surface 20c, and the right surface 20d of the sensor element 110, so long as it is open to the front of the detection electrode 56.

[0075] In the above-described embodiment, the element body 20 is made of zirconia (ZrO 2 In the above embodiment, the solid electrolyte layer 55 is formed by stacking a plurality of oxygen ion conductive solid electrolyte layers such as oxygen ion conductive solid electrolyte layers formed in the thickness direction. However, in the stack, the portions other than the solid electrolyte layers forming the detection unit 23 and the mixed potential cell 55 do not have to be made of a solid electrolyte.

[0076] In the above embodiment, the first specified gas is NOx, but it may be an oxide gas other than NOx or oxygen. Also, the second specified gas is ammonia, but it may be any gas different from the first specified gas, such as a flammable gas such as a hydrocarbon gas. EXAMPLES

[0077] A specific example of a gas sensor will be described below as an embodiment, however, the present invention is not limited to the following embodiment.

[0078] [Example A1] (Fabrication of gas sensors) The above-mentioned gas sensor 100 was fabricated. In Example A1, the detection electrode 56 was disposed at a position where the distance K1 from the front end surface 20e of the element body 20 to the front end of the detection electrode 56 was 7.5 mm, the measurement gas inlet 21a was disposed at a position where the distance K3, which is the length in the vertical direction from the upper surface 20a of the element body 20, was 0.1 mm, and the outer electrode 24 was disposed forward of the detection electrode 56. In addition, the radius R1 of the outer diameter of the first cylindrical portion 134 was 3.0 mm, and the radius R2 of the inner diameter of the second cylindrical portion 136 was 8.5 mm. Four holes 125a with a diameter D1 of 0.8 mm were formed in the first cylindrical portion 134 at equal intervals in the circumferential direction (four equal intervals). In Example A1, the distance K2 in the front-rear direction from the front end face 20e of the element body 20 to the second inlet 127 is 1.7 mm, the detection electrode 56 is disposed rearward of the rear ends of the holes 125a of the first inlet 125, and the arrangement of the sensor element 110 and the inner protective cover 130 is adjusted so that the minimum value of the shortest path length L1 from the first inlet 125 to the detection electrode 56 is shorter than the shortest path length L2 from the second inlet 127 to the detection electrode 56. In Example A1, the total opening area A of the first inlet 125 is 2.0 mm 2 The total opening area B of the second inlet 127 is 200 mm 2 and the ratio A / B was 0.01.

[0079] (Evaluation of output characteristics) The output characteristics were confirmed using the obtained gas sensor 100. Specifically, the gas sensor 100 was first fixed to a pipe in the same manner as in FIG. 2 Concentration 10%, H 2 A measurement gas with an O concentration of 5% (ammonia concentration of 0 ppm, base gas is nitrogen gas) was flowed through the pipe. The ammonia concentration of the measurement gas flowing through the pipe was changed from 0 ppm to 500 ppm, and the output value (electromotive force EMF and pump current Ip2) of the gas sensor 100 was measured. The output value of the gas sensor 100 immediately before the ammonia concentration was changed was set to 0%, and the output value when the output of the gas sensor 100 was stabilized with the ammonia concentration set to 500 ppm was set to 100%, and the output value when the output value was 100% was evaluated as the sensitivity, and the elapsed time from when the output value exceeded 10% to when it exceeded 90% was evaluated as the response time to a specific gas. The larger the electromotive force EMF, the better the sensitivity to ammonia, and the shorter the response time of the electromotive force EMF, the better the responsiveness to ammonia. Also, the shorter the response time of the pump current Ip2, the better the responsiveness to NOx. In addition, the pump current Ip2 used to measure the NOx concentration varies slightly in output depending on the ammonia, so the responsiveness to NOx can be determined by examining the response time of Ip2 when the ammonia concentration is changed. The results are shown in Table 1.

[0080] [Examples A2 to A8] Example A2 was the same as Example A1, except that the diameter D1 of the hole 125a was changed to 1.0 mm, and the radius R2 of the inner diameter of the second cylindrical portion 136 was changed to 7.0 mm. In Example A2, the total opening area A of the first inlet 125 was 3.1 mm 2 The total opening area B of the second inlet 127 is 125.7 mm 2 The ratio A / B was 0.03. Example A3 was the same as Example A2, except that the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 4.0 mm, and the radius R2 of the inner diameter of the second cylindrical portion 136 was changed to 6.0 mm. In Example A3, the total opening area A of the first inlet 125 was 3.1 mm 2 The total opening area B of the second inlet 127 is 62.8 mm 2and the ratio A / B was 0.05. Example A4 was the same as Example A2, except that the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 4.5 mm, and the radius R2 of the inner diameter of the second cylindrical portion 136 was changed to 5.3 mm. In Example A4, the total opening area A of the first inlet 125 was 3.1 mm 2 The total opening area B of the second inlet 127 is 25.8 mm 2 and the ratio A / B was 0.10. Example A5 was the same as Example A1, except that the diameter D1 of the hole 125a was changed to 3.5 mm, the number of the holes 125a was changed to 8 (equally spaced apart), the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 5.0 mm, and the radius R2 of the inner diameter of the second cylindrical portion 136 was changed to 5.5 mm. In Example A5, the total opening area A of the first inlet 125 was 77.0 mm 2 The total opening area B of the second inlet 127 is 17.4 mm 2 The ratio A / B was 4.45. Example A6 was the same as Example A5, except that the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 5.5 mm, and the radius R2 of the inner diameter of the second cylindrical portion 136 was changed to 5.8 mm. In Example A6, the total opening area A of the first inlet 125 was 77.0 mm 2 The total opening area B of the second inlet 127 is 10.6 mm 2 The ratio A / B was 7.29. Example A7 was the same as Example A5, except that the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 6.0 mm, and the radius R2 of the inner diameter of the second cylindrical portion 136 was changed to 6.1 mm. In Example A7, the total opening area A of the first inlet 125 was 77.0 mm 2 The total opening area B of the second inlet 127 is 3.9 mm 2 and the ratio A / B was 19.76. Example A8 was the same as Example A5, except that the diameter D1 of the hole 125a was changed to 4.0 mm, the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 6.5 mm, and the radius R2 of the inner diameter of the second cylindrical portion 136 was changed to 6.6 mm. In Example A8, the total opening area A of the first inlet 125 was 100.5 mm 2 The total opening area B of the second inlet 127 is 4.8 mm 2 and the ratio A / B was 21.00.

[0081] [Considerations of Examples A1 to A8] Table 1 shows the sensitivity to ammonia (value of electromotive force EMF), the response time to ammonia (response time of electromotive force EMF), and the response time to NOx (response time of pump current Ip2) for each of Examples A1 to A8. As shown in Table 1, by providing the first inlet 125 in the first cylindrical portion 134, it was possible to shorten the response time to ammonia (1.0 s or less) while keeping the response time to NOx relatively short (1.3 s or less). Furthermore, among the gas sensors having a first inlet 125 (Examples A1 to A8), when the ratio A / B was 0.02 or more and 19.76 or less, the response time to ammonia could be shortened (0.7 s or less) while maintaining the response time to NOx shorter (1.2 s or less); when the ratio A / B was 0.05 or more and 7.29 or less, the response time to ammonia could be shortened even further (0.6 s or less) while maintaining the response time to NOx even shorter (1.1 s or less); and when the ratio A / B was 0.10 or more and 4.45 or less, the response time to ammonia could be shortened even further (0.5 s or less) while maintaining the response time to NOx even shorter (1.0 s or less). When the gas sensors of Examples A1 to A8 were repeatedly evaluated for output characteristics as described above, the sensitivity of Example A6 with an A / B ratio of 7.29 and Example A7 with an A / B ratio of 19.76 decreased faster than that of Examples A1 to A5, and the sensitivity of Example A8 with an A / B ratio of 21.00 decreased faster than that of Examples A6 and A7. From this, it was found that from the viewpoint of suppressing deterioration of the detection electrode 56, it is preferable that the A / B ratio is 19.76 or less, and more preferably 4.45 or less.

[0082] [Table 1]

[0083] [Examples B1 to B7] Example B1 was the same as Example A4, except that the number of holes 125a was changed to 6 (six equally spaced), the thickness of the element body 20 was changed to 1.4 mm, and the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 4.7 mm. In Example 1, the arrangement of the sensor element 110 and the inner protective cover 130 was adjusted so that the angle θ of the hole 125a with the smallest angle θ (the minimum value of the angle θ) was 0°. Example B2 was the same as Example B1, except that the arrangement of the sensor element 110 and the inner protective cover 130 was adjusted to set the minimum value of the angle θ to 23.5°. Example B3 was the same as Example B1, except that the arrangement of the sensor element 110 and the inner protective cover 130 was adjusted to set the minimum value of the angle θ to 31.2°. Example B4 was the same as Example B1, except that the thickness of the element body 20 was changed to 1.5 mm, the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 5.0 mm, and the arrangement of the sensor element 110 and the inner protective cover 130 was adjusted so that the minimum value of the angle θ was 34.7°. Example B5 was the same as Example B1, except that the thickness of the element body 20 was changed to 1.5 mm, the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 3.0 mm, and the arrangement of the sensor element 110 and the inner protective cover 130 was adjusted so that the minimum value of the angle θ was 39.1°. Example B6 was the same as Example B1, except that the thickness of the element body 20 was changed to 3.0 mm, the radius R1 of the outer diameter of the first cylindrical portion 134 was changed to 3.0 mm, and the arrangement of the sensor element 110 and the inner protective cover 130 was adjusted so that the minimum value of the angle θ was 53.8°. Example B7 was the same as Example B6, except that the number of holes 125a was four (equally spaced apart by four), and the arrangement of the sensor element 110 and the inner protective cover 130 was adjusted so that the minimum value of the angle θ was 73.6°. In Examples B1 to B7, the plate thickness (thickness) of the inner protective cover 130 was constant.

[0084] [Consideration of Examples B1 to B7 and Comparative Examples] Table 2 shows the sensitivity to ammonia (value of electromotive force EMF), response time to ammonia (response time of electromotive force EMF), and response time to NOx (response time of pump current Ip2) for each of Examples B1 to B7. As shown in Table 2, by providing the first inlet 125 to the first cylindrical portion 134, it was possible to increase the sensitivity to ammonia (output value 160 mV or more) and shorten the response time to ammonia (0.9 s or less). In addition, among the gas sensors (Examples B1 to B7) having the first inlet 125, when the minimum value of the angle θ is 73.6° or less, the sensitivity to ammonia can be further increased (output value 160 mV or more) and the response time to ammonia can be further shortened (0.9 s or less), when the minimum value of the angle θ is 39.1° or less, the sensitivity to ammonia can be further increased (output value 180 mV or more) and the response time to ammonia can be further shortened (0.8 s or less), and when the minimum value of the angle θ is 34.7° or less, the sensitivity to ammonia can be further increased (output value 190 mV or more) and the response time to ammonia can be further shortened (0.7 s or less). When the gas sensors of Examples B1 to B7 were repeatedly evaluated for output characteristics as described above, it was found that the sensitivity of Example B2, which has a small minimum value of the angle θ of 23.5°, decreased faster than Examples B3 to B7, and the sensitivity of Example B1, which has an angle θ of 0°, decreased faster than Example B2. From this, it was found that, from the viewpoint of suppressing deterioration of the detection electrode 56, the minimum value of the angle θ is preferably equal to or greater than 23.5°, and more preferably equal to or greater than 31.2°.

[0085] [Table 2] [Explanation of symbols]

[0086] 10 piping, 12 fixing member, 20 element body, 20a upper surface, 20b lower surface, 20c left surface, 20d right surface, 20e front end surface, 20f rear end surface, 21 measured gas flow section, 21a measured gas inlet, 22 reference gas inlet, 23 detection section, 24 outer electrode, 25 inner main pump electrode, 26 inner auxiliary pump electrode, 27 measurement electrode, 28 reference electrode, 29 heater, 55 mixed potential cell, 56 detection electrode, 56a arrangement surface, 84 buffer layer, 84a upper buffer layer, 84b lower buffer layer, 85 protective layer, 100, 200 gas sensor, 101 element sealing body, 102 housing, 103 bolt, 104 supporter, 105 powder compact, 110 sensor element, 120, 220 Protective cover, 122, 222 First gas chamber, 123, 223, 323 Element chamber inlet, 124 Sensor element chamber, 125, 225 First inlet, 125a, 225a Hole, 125b Element side opening, 126 Second gas chamber, 127, 227, 327 Second inlet, 128, 228 Rear opening, 129, 229 Front opening, 130, 230, 330 Inner protective cover, 131, 231, 331 First member, 132 Large diameter portion, 133 Step portion, 134, 334 First cylindrical portion, 134a Inner peripheral surface, 135, 235 Second member, 136, 236 Second cylindrical portion, 136a Protruding portion, 137 Connection portion, 138 Tip portion, 138a Element chamber outlet, 138b Hole, 138d side, 138e bottom, 139 step, 140 outer protective cover, 143 body, 143a side, 143b step, 144a outer inlet, 144b hole, 144c hole, 146 tip, 146a side, 146b bottom, 146c taper, 147a Outer outlet, 147c hole, 234a body, 234b first cylindrical part, 334a thick wall part, 334b ​​recess.

Claims

1. a sensor element including an element body having a front end and a rear end opposite to the front end, the element body having a measurement gas flow section therein through which a measurement gas is introduced and flows, the sensor element including a detection section for detecting a concentration of a first specified gas in the measurement gas that has flowed into the measurement gas flow section from a gas inlet which is an inlet of the measurement gas flow section; a cylindrical inner protective cover having a sensor element chamber therein in which the tip of the sensor element and the gas inlet are disposed, the sensor element chamber having one or more element chamber inlets which are inlets to the sensor element chamber and one or more element chamber outlets which are outlets from the sensor element chamber; a cylindrical outer protective cover having one or more outer inlets which are inlets for the measurement gas from the outside and one or more outer outlets which are outlets for the measurement gas to the outside, the outer protective cover being disposed outside the inner protective cover; Equipped with the sensor element includes a mixed potential cell that generates an electromotive force corresponding to a concentration of a second specified gas in the measurement gas that is different from the first specified gas, the mixed potential cell includes a detection electrode disposed outside the element body and behind the gas inlet, the detection electrode is disposed inside the sensor element chamber, the inner protective cover has a cylindrical first member surrounding the periphery of the sensor element, and a cylindrical second member surrounding the periphery of the first member, and has, as the element chamber inlets, one or more first inlets disposed in the first member and one or more second inlets formed as gaps between the first member and the second member, an opening of the second inlet on the sensor element chamber side is disposed forward of the detection electrode and the first inlet and opens forward, At least one of the one or more first inlets is disposed at a position where a shortest path length L1 from the first inlet to the detection electrode is shorter than a shortest path length L2 from the second inlet to the detection electrode. Gas sensor.

2. In the sensor element chamber, a ratio A / B of a total opening area A of the one or more first inlets to a total opening area B of the one or more second inlets is 0.02 or more and 19.76 or less.

2. The gas sensor according to claim 1.

3. The ratio A / B is 0.05 or more and 7.29 or less; 3. The gas sensor according to claim 2.

4. The detection unit includes an outer electrode disposed outside the element body, The detection electrode is disposed rearward of the outer electrode. The gas sensor according to any one of claims 1 to 3.

5. All of the first inlets are arranged at positions where the detection electrodes do not overlap with projected areas of the first inlets projected toward the surface of the element body in a direction parallel to the axial direction of the first inlets. The gas sensor according to any one of claims 1 to 4.

6. the first member has a cylindrical side, and the first inlet is a hole disposed in the side of the first member. The gas sensor according to any one of claims 1 to 5.

7. The gas sensor according to any one of claims 1 to 6, a porous protective layer covering the detection electrode; In the sensor element chamber, a ratio A / B of a total opening area A of the one or more first inlets to a total opening area B of the one or more second inlets is 0.02 or more and 19.76 or less, At least one of the one or more first inlets is arranged such that, in a virtual projection view in which the first member and the sensor element are vertically projected onto a virtual plane perpendicular to the axial direction of the first member, an angle θ formed by a first line perpendicular to an arrangement surface on which the detection electrode of the sensor element is arranged and drawn from the center of the arrangement surface to an opposite side to the element body, and a second line drawn from the center of the arrangement surface toward the center of the first inlet is 73.6° or less. Gas sensor.

8. a plurality of the first inlets are arranged in the first member, and in the virtual projection view, a line obtained by rotating the first line by 73.6° around the center of the arrangement surface as a rotation center is defined as a third line, a line obtained by rotating the first line by 73.6° in the opposite direction to the third line is defined as a fourth line, an intersection point between the third line and an inner circumferential surface of the first member is defined as a first intersection point, and an intersection point between the fourth line and the inner circumferential surface of the first member is defined as a second intersection point, all of the first inlets are arranged such that a distance between the centers of adjacent first inlets in the circumferential direction of the first member is equal to or smaller than a distance between the first intersection point and the second intersection point.

8. The gas sensor according to claim 7.

9. 9. The gas sensor according to claim 7, wherein all of the first inlets are disposed so that the angle θ is equal to or greater than 23.5°.

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