Sensor element

Abstract

To provide a sensor element capable of improving shock resistance of a protective layer.SOLUTION: A sensor element 1 used in a gas sensor comprises an element body 2 and a porous protective layer 3. An end of the element body 2 on a tip side Z1 is provided with a gas detection part 4. The protective layer 3 covers an outside surface of the end of the element body 2 on the tip side Z1 in a laminar manner. The protective layer 3 includes an inner layer 31 and a plurality of outer layers 32. The outer layer 32 has greater porosity than that of the inner layer 31. When the outer layer 32 directly covering the inner layer 31 is defined as an intermediate layer 33, an outside surface 331 of the intermediate layer 33 is covered by other outer layers 32. The protective layer 3 includes a tip covering part 301 covering a tip surface 21 of the element body 2. The outside surface 331 of the intermediate layer 33 in the tip covering part 301 includes a recess 332 recessed toward the tip surface 21 of the element body 2.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] For example, as described in Patent Document 1, a gas sensor that measures the concentrations of oxygen, NOx, and the like contained in exhaust gas from an internal combustion engine of an automobile or the like is known. The gas sensor element provided in the gas sensor described in Patent Document 1 has a porous protective layer that covers the outer surface of the element body to protect the element body from water droplets and poisonous substances. In the gas sensor element described in Patent Document 1, the protective layer has an end surface portion that covers one longitudinal end surface of the element body, a side surface portion that covers the side surface of the element body, and further has corners that connect the end surface portion and the side surface portion. The gas sensor element described in Patent Document 1 aims to prevent peeling of the protective layer due to exposure to water and improve water resistance by making the protective layer thicker closer to the corners. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-39693 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the sensor element described in Patent Document 1 does not fully consider the impact resistance of the protective layer. That is, for example, if the porosity of the porous protective layer is increased to improve the responsiveness of the sensor element, the sensor element described in Patent Document 1 may not be able to ensure sufficient impact resistance of the protective layer. Therefore, it can be said that the sensor element described in Patent Document 1 has room for further improvement in terms of improving the impact resistance of the protective layer.

[0005] The present invention has been made in view of the above-mentioned problems, and aims to provide a sensor element capable of improving the impact resistance of the protective layer. [Means for solving the problem]

[0006] One aspect of the present invention is a sensor element (1) for use in a gas sensor (10), comprising: The device comprises a long plate-shaped element body (2) and a porous protective layer (3) covering the outer surface of the element body, A gas detection unit (4) is provided at an end of the element body on the tip side (Z1) which is one side in the longitudinal direction (Z), the protective layer covers the outer surface of the tip end of the element body in a layered manner, the protective layer includes an inner layer (31) that covers the outer surface of the element body, and a plurality of outer layers (32) that are provided outside the inner layer and have a higher porosity than the inner layer; When the outer layer directly covering the inner layer is defined as an intermediate layer (33), the outer surface (331) of the intermediate layer is covered by another outer layer, the protective layer has a tip covering portion (301) that covers a tip surface (21) that is an end surface on the tip side of the element body, The outer surface of the intermediate layer in the tip cover portion is in a sensor element having a recess (332) recessed toward the tip surface of the element body. [Effects of the Invention]

[0007] In the sensor element, the outer surface of the intermediate layer in the tip cover portion has a recess that is recessed toward the tip surface of the element body. Therefore, even if the porosity of the outer layer is increased, the outer layer covering the outer surface of the intermediate layer can be firmly fixed to the intermediate layer. As a result, the impact resistance of the protective layer can be improved.

[0008] As described above, according to the above aspect, it is possible to provide a sensor element that can improve the impact resistance of the protective layer. In addition, the symbols in parentheses in the claims and the means for solving the problems indicate the correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view taken along the longitudinal direction of a gas sensor including a sensor element according to a first embodiment. [Figure 2] 2 is a cross-sectional view of the tip portion of the sensor element in the first embodiment taken along the line II-II in FIG. 3, taken along both the longitudinal direction and the lamination direction. [Figure 3] 3 is a cross-sectional view perpendicular to the longitudinal direction of the tip cover in the first embodiment, taken along the line III-III in FIG. 2. FIG. [Figure 4] 4 is a cross-sectional view perpendicular to the longitudinal direction of the tip cover in the first embodiment, taken along the line IV-IV in FIG. 2. FIG. [Figure 5] 3 is a cross-sectional view showing the length of a recess in the stacking direction in the first embodiment. FIG. [Figure 6] 3 is a flow chart showing the steps of manufacturing the protective layer in the first embodiment. [Figure 7] FIG. 10 is a cross-sectional view of the tip of the sensor element in the second embodiment, taken along both the longitudinal direction and the lamination direction. [Figure 8] FIG. 11 is a cross-sectional view of the tip of the sensor element in the third embodiment, taken along both the longitudinal direction and the stacking direction. [Figure 9] FIG. 10 is a cross-sectional view of the tip of the sensor element in the fourth embodiment, taken along both the longitudinal direction and the lamination direction. DETAILED DESCRIPTION OF THE INVENTION

[0010] (Embodiment 1) An embodiment of the sensor element will be described with reference to FIGS. The sensor element 1 of this embodiment is used in a gas sensor 10, as shown in Fig. 1. As shown in Figs. 1 and 2, the sensor element 1 includes a long plate-shaped element body 2 and a porous protective layer 3 that covers the outer surface of the element body 2. A gas detection unit 4 is provided at an end of the element body 2 on the tip side Z1, which is one side in the longitudinal direction Z. The protective layer 3 covers the outer surface of the end of the element body 2 on the tip side Z1 in a layered manner.

[0011] 2 to 5, the protective layer 3 has an inner layer 31 that covers the outer surface of the element body 2, and multiple outer layers 32. The outer layers 32 are provided outside the inner layer 31 and have a higher porosity than the inner layer 31. When the outer layer 32 that directly covers the inner layer 31 is an intermediate layer 33, the outer surface 331 of the intermediate layer 33 is covered by another outer layer 32, as shown in FIGS.

[0012] The protective layer 3 also has a tip cover portion 301 that covers the tip surface 21, which is the end surface of the tip side Z1 of the element body 2. The outer surface 331 of the intermediate layer 33 in the tip cover portion 301 has a recess 332 that is recessed toward the tip surface 21 side of the element body 2.

[0013] The sensor element 1 of this embodiment is used in a gas sensor 10 that measures the concentration of a specific gas in exhaust gas from an internal combustion engine of a vehicle, for example. Specifically, the sensor element 1 can be used in, for example, a NOx sensor that measures the concentration of NOx (i.e., nitrogen oxides) contained in exhaust gas, an oxygen sensor that measures oxygen concentration, an air-fuel ratio sensor that detects the air-fuel ratio (i.e., A / F) based on the oxygen concentration, etc. The sensor element 1 of this embodiment is used in a gas sensor 10 that measures the concentration of NOx. The gas sensor 10 can measure the concentration of NOx contained in exhaust gas by being attached to, for example, an exhaust gas pipe (not shown) connected to the internal combustion engine of the vehicle. In this specification, the side opposite to the tip side Z1 in the longitudinal direction Z of the element body 2 is referred to as the base side Z2.

[0014] 1, the gas sensor 10 has a cylindrical housing H whose axial direction is the longitudinal direction Z, and a sensor element 1 is inserted and held within the housing H. An element cover S1 serving as a cover body is attached to a tip end side Z1 of the housing H, and the tip end of the sensor element 1 protrudes from the housing H and is accommodated within the element cover S1. In addition, a base end cover S2 is attached to a base end side Z2 of the housing H.

[0015] The element cover S1 has an inner cover S11 and an outer cover S12 provided on the outside of the inner cover S11. The inner cover S11 and the outer cover S12 are provided with through holes S13 and S14, respectively, which serve as exhaust gas inlet and outlet holes. When the exhaust gas passes through the through holes S13 and S14 and reaches the surface of the sensor element 1, it is taken in through the protective layer 3. The cylindrical base end cover S2 is provided with a through hole S21, which opens on the outer peripheral side and serves as an atmospheric hole, allowing atmospheric air to be taken in.

[0016] The outer periphery of the middle part of the sensor element 1 is held inside a cylindrical insulator I housed in a housing H. Sealing glass I1 is filled between the base end opening of the insulator I and the sensor element 1. Talc powder I2 is filled between the outer periphery of the insulator I and the inner periphery of the housing H, and the thin base end part of the housing H is crimped via a cylindrical insulating member I3, so that the insulator I is fixed to the housing H.

[0017] A plurality of lead wires R1, R2 connected to an external engine control unit (not shown) are insulated and held at the base end opening of the base end cover S2. Terminal portions R11, R12 are provided on the tip side Z1 of the lead wires R1, R2 and are electrically connected to the sensor element 1.

[0018] Next, the gas detection section 4 in the element body 2 will be described in detail. As shown in FIG. 2 , the gas detection unit 4 of the element body 2 includes a measurement gas chamber 101 into which a measurement gas is introduced, a sensor cell 51, a pump cell 52, and a monitor cell 53. The sensor cell 51 includes a solid electrolyte body 6 having oxygen ion conductivity and a pair of electrodes 71 and 72 provided on the solid electrolyte body 6. The pump cell 52 includes a solid electrolyte body 6 identical to the solid electrolyte body 6 constituting a part of the sensor cell 51 and a pair of electrodes 72 and 73 provided on the solid electrolyte body 6. The monitor cell 53 includes a solid electrolyte body 6 identical to the solid electrolyte body 6 constituting a part of the sensor cell 51 and a part of the pump cell 52 and a pair of electrodes 72 and 74 provided on the solid electrolyte body 6. In this specification, the stacking direction Y of the pair of electrodes 71 and 72 and the solid electrolyte body 6 in the sensor cell 51 is appropriately referred to as the Y direction. In the Y direction, the side of the solid electrolyte body 6 where the measurement gas chamber 101 is located is referred to as the upper side Y1, and the opposite side is referred to as the lower side Y2.

[0019] The sensor cell 51 detects the concentration of NOx in the measurement gas in the measurement gas chamber 101. In this embodiment, the sensor cell 51 has a sensor electrode 71 and a reference electrode 72. The sensor electrode 71 is disposed on the surface of the solid electrolyte body 6 facing the measurement gas chamber 101. The reference electrode 72 is disposed on the surface of the solid electrolyte body 6 facing the reference gas chamber 102 (described later).

[0020] The pump cell 52 adjusts the oxygen concentration of the measurement gas introduced into the measurement gas chamber 101. The pump cell 52 has a pump electrode 73 and a reference electrode 72. The pump electrode 73 is installed on the surface of the solid electrolyte body 6 facing the measurement gas chamber 101.

[0021] The monitor cell 53 measures the residual oxygen concentration of the measurement gas whose oxygen concentration has been adjusted by the pump cell 52. The monitor cell 53 has a monitor electrode 74 and a reference electrode 72. The monitor electrode 74 is installed on the surface of the solid electrolyte body 6 facing the measurement gas chamber 101. In this embodiment, the reference electrode 72 of the monitor cell 53, the reference electrode 72 of the pump cell 52, and the reference electrode 72 of the sensor cell 51 are integrated into one common electrode.

[0022] The solid electrolyte body 6 is a plate-shaped member made of a solid electrolyte material having oxygen ion conductivity. The solid electrolyte body 6 is activated by being heated by a heater 13, which will be described later. In other words, the solid electrolyte body 6 is configured to have oxygen ion conductivity at a predetermined activation temperature.

[0023] In the Y direction, a measurement gas chamber 101 is formed on the upper side Y1 of the solid electrolyte body 6, and a reference gas chamber 102 is formed on the lower side Y2 of the solid electrolyte body 6. A reference gas is introduced into the reference gas chamber 102. The reference gas is a gas that serves as a reference for oxygen concentration. In this embodiment, the reference gas is atmospheric air. The reference gas chamber 102 is configured so that atmospheric air is introduced through an opening (not shown).

[0024] A shielding layer 12 is laminated on the upper side Y1 of the solid electrolyte body 6 via an insulating layer 110 including a diffusion resistor 11. A rectangular cutout portion that forms the measurement gas chamber 101 is formed on the tip side Z1 of the insulating layer 110. The insulating layer 110 is a sheet-like member made of insulating ceramic such as alumina. A portion of the insulating layer 110 that forms the chamber wall on the tip side Z1 of the measurement gas chamber 101 is made of a diffusion resistor 11. The diffusion resistor 11 is made of gas-permeable porous ceramic. The shielding layer 12 is also a sheet-like member made of insulating ceramic, and covers the upper side Y1 of the measurement gas chamber 101 to restrict gas permeation.

[0025] A heater substrate layer 132 that constitutes part of the heater 13 is laminated on the lower side Y2 of the solid electrolyte body 6. The heater 13 has a heater substrate layer 132 made of insulating ceramic and a heater electrode 131 embedded inside the heater substrate layer 132. The heater 13 is disposed opposite the solid electrolyte body 6 in the Y direction with the reference gas chamber 102 interposed therebetween. The heater substrate layer 132 has a recess that forms the reference gas chamber 102.

[0026] The heater electrode 131 is configured to generate heat when energized. The heater 13 heats the sensor cell 51, the pump cell 52, and the monitor cell 53 so that the cells reach temperatures suitable for detection operations.

[0027] Next, measurement of the NOx concentration in the measurement gas by the sensor element 1 will be described. Measurement gas flowing through an exhaust gas pipe (not shown) of an internal combustion engine passes through the protective layer 3 and the diffusion resistor 11 and enters the measurement gas chamber 101. The pump cell 52 then discharges oxygen from the measurement gas chamber 101 to the reference gas chamber 102 to adjust the oxygen concentration of the measurement gas. Specifically, when a predetermined voltage is applied between the pump electrode 73 and the reference electrode 72, the oxygen contained in the measurement gas in the measurement gas chamber 101 is reduced and decomposed by the pump electrode 73 to form oxygen ions. The oxygen ions flow through the solid electrolyte 6 toward the reference electrode 72, where oxygen is generated and discharged from the reference gas chamber 102 to the atmosphere. At this time, the inflow of the measurement gas into the measurement gas chamber 101 is limited by the diffusion resistor 11, so the current flowing through the pump cell 52 exhibits a limiting current characteristic that depends on the oxygen concentration in the measurement gas. Therefore, by setting the applied voltage so as to be in the limiting current region of oxygen, the oxygen concentration and air-fuel ratio A / F of the measured gas can be determined from the current flowing through the pump cell 52, using the air introduced into the reference gas chamber 102 as a reference.

[0028] Next, the measurement gas from which oxygen has been discharged by the pump cell 52 reaches the sensor electrode 71, which is located closer to the base end Z2 than the pump electrode 73. When a voltage is applied between the sensor electrode 71 and the reference electrode 72, NOx in the measurement gas is decomposed into oxygen ions at the sensor electrode 71 and discharged through the solid electrolyte body 6 into the reference gas chamber 102. In this embodiment, the current flowing at this time is measured, and the NOx concentration is calculated using the measured value.

[0029] The monitor cell 53 is configured to detect the concentration of oxygen remaining in the measurement gas from which oxygen has been discharged by the pump cell 52. When a voltage is applied between the monitor electrode 74 and the reference electrode 72, the oxygen remaining in the measurement gas is ionized at the monitor electrode 74 and discharged through the solid electrolyte body 6 into the reference gas chamber 102. At this time, a current flows through the monitor cell 53. The residual oxygen concentration in the measurement gas can be calculated based on this current value. By including the monitor cell 53, the sensor element 1 can more accurately calculate the NOx concentration even when the pump cell cannot completely discharge oxygen. In other words, the sensor element 1 can calculate the NOx concentration in the measurement gas without calculating the current value due to the residual oxygen in the measurement gas detected by the sensor cell 51.

[0030] Next, the protective layer 3 of the sensor element 1 will be described in detail. The gas sensor 10 is used in an environment where it is exposed to exhaust gas, which is the gas to be measured. The protective layer 3 protects the element body 2 from condensed water and poisonous substances contained in the exhaust gas.

[0031] The porous protective layer 3 is composed of a large number of ceramic particles bonded together and pores surrounded by the ceramic particles. In this embodiment, the porosity of the inner layer 31 is 10 to 40% by volume. The porosity of the outer layer 32 is greater than 30% by volume and equal to or less than 90% by volume.

[0032] The protective layer 3 covers the tip portion of the element body 2 protruding from the insulator I (see FIG. 1). The protective layer 3 has a tip cover portion 301 and an outer peripheral cover portion 302 that covers the outer peripheral surface of the tip portion of the element body 2. The tip cover portion 301 and the outer peripheral cover portion 302 are integrally formed. The thickness of the tip cover portion 301 is greater than the thickness of the outer peripheral cover portion 302. The thickness of the tip cover portion 301 can be, for example, 1000 μm or more. The thickness of the inner layer 31 in the outer peripheral cover portion 302 is thinner than the thickness of the inner layer 31 in the tip cover portion 301. The thickness of the outer layer 32 in the outer peripheral cover portion 302 is thinner than the thickness of the outer layer 32 in the tip cover portion 301.

[0033] In this embodiment, the protective layer 3 includes a single inner layer 31. The inner layer 31 directly covers the outer surface of the element body 2. In this embodiment, the inner layer 31 directly covers the entire tip surface 21 of the element body 2. As shown in FIGS. 2 and 5 , in a cross section including the central axis C of the sensor element 1 and taken along the Y direction, the central portion of the inner layer 31 in the tip cover portion 301 is formed so that its thickness decreases toward the central axis C. The outer surface 311 of the inner layer 31 in the tip cover portion 301 has a recess 312 recessed toward the tip surface 21 of the element body 2. In this embodiment, the tip surface 21 is flat and formed so as to be perpendicular to the longitudinal direction Z.

[0034] In this embodiment, the protective layer 3 has six outer layers 32. The six outer layers 32 include one innermost intermediate layer 33 and five outer layers 32 covering the intermediate layer 33 from the outside. Here, the outer layer 32 that directly covers the outer surface 331 of the intermediate layer 33 is referred to as the second outer layer 322, and the outermost outer layer 32 is referred to as the outermost layer 323. In this embodiment, the outer surface of the second outer layer 322 in the tip cover 301 has a recess that is recessed toward the tip surface 21 of the element body 2. In addition, in the tip cover 301, each of the outer layers 32 other than the outermost layer 323 has a recess on its outer surface that is recessed toward the tip surface 21. In other words, the tip cover 301 has multiple outer layers 32 that have a recess on their outer surface that is recessed toward the tip surface 21. In addition, in a cross section including the central axis C of the sensor element 1 and along the Y direction, the outermost layer 323 of the tip cover portion 301 is thicker at the center in the Y direction than at both ends in the Y direction.

[0035] The central portion of the intermediate layer 33 in the tip covering portion 301 includes the central axis C, and is formed so that, in a cross section along the Y direction, the closer to the central axis C, the more toward the base end side Z2. In the tip covering portion 301, the thickness of the intermediate layer 33 is thinner than the thickness of the inner layer 31. In this embodiment, the thickness of the intermediate layer 33 in the tip covering portion 301 is 200 μm or less. In this embodiment, the thickness of the inner layer 31 in the tip covering portion 301 is greater than 200 μm. Furthermore, in the tip covering portion 301, each of the six outer layers 32 is thinner than the thickness of the inner layer 31.

[0036] As shown in Fig. 5, in a cross section that includes the central axis C of the sensor element 1 and is taken along the Y direction, the intermediate layer 33 in the tip cover 301 has two intermediate protrusions 333 that protrude toward the tip side Z1. In the cross section shown in Fig. 5, one intermediate protrusion 333 is formed above the central axis C in the Y1 direction, and the other intermediate protrusion 333 is formed below the central axis C in the Y2 direction. In addition, in the cross section shown in Fig. 5, the inner layer 31 in the tip cover 301 has two inner protrusions 313 that protrude toward the tip side Z1. In the cross section shown in Fig. 5, one inner protrusion 313 is formed above the central axis C in the Y1 direction, and the other inner protrusion 313 is formed below the central axis C in the Y2 direction.

[0037] In the cross section shown in Fig. 5, a tangent line VL1 is a straight line that is a tangent line to the convex curved contour line of one intermediate protrusion 333 and also a tangent line to the convex curved contour line of the other intermediate protrusion 333. In addition, in the cross section shown in Fig. 5, a tangent line P1 is the point of contact between the contour line of one intermediate protrusion 333 and the tangent line VL1, and a tangent line P2 is the point of contact between the contour line of the other intermediate protrusion 333 and the tangent line VL1. In this case, in the cross section shown in Fig. 5, the outer surface 331 of the intermediate layer 33 that connects the tangent line P1 and the tangent line P2 is also the outer surface 331 that forms the recess 332.

[0038] 5, a straight line that is a tangent to the convex curved contour line of one inner protrusion 313 and also a tangent to the convex curved contour line of the other inner protrusion 313 is designated as tangent line VL2. Also, in the cross section shown in Fig. 5, a point of contact between the contour line of one inner protrusion 313 and the tangent line VL2 is designated as tangent line P3, and a point of contact between the contour line of the other inner protrusion 313 and the tangent line VL2 is designated as tangent line P4. In this case, in the cross section shown in Fig. 5, the outer surface 311 of the inner layer 31 that connects tangent line P3 and tangent line P4 is also the outer surface 311 that forms the recess 312.

[0039] In this embodiment, as shown in FIG. 3, the recess 332 of the intermediate layer 33 has a length longer in a direction perpendicular to both the Y direction and the longitudinal direction Z than in the Y direction. Also, as shown in FIG. 5, in a cross section including the central axis C of the sensor element 1 and taken along the Y direction, the width L1 of the recess 332 of the intermediate layer 33 is at least one-third of the width L2 of the element body 2 in the Y direction. Furthermore, the width L1 is preferably at least one-half of the width L2. The width L1 is also the length from the contact point P1 to the contact point P2. Furthermore, in the cross section shown in FIG. 5, the depth L4 of the recess 332 in a direction perpendicular to the tangent line VL1 is equal to or greater than the thickness of the intermediate layer 33 in the outer peripheral cover portion 302. Furthermore, the depth L4 is preferably equal to or greater than the thickness of the intermediate layer 33 in the tip cover portion 301.

[0040] In this embodiment, as shown in FIG. 4, the recess 312 of the inner layer 31 has a length longer in a direction perpendicular to both the Y direction and the longitudinal direction Z than in the Y direction. Also, as shown in FIG. 5, in a cross section including the central axis C of the sensor element 1 and taken along the Y direction, the width L3 of the recess 312 of the inner layer 31 is at least one-third of the width L2. Furthermore, the width L3 is preferably at least one-half of the width L2. The width L3 is also the length from the contact point P3 to the contact point P4. Furthermore, in the cross section shown in FIG. 5, the depth L5 of the recess 312 in a direction perpendicular to the tangent line VL2 is equal to or greater than the thickness of the intermediate layer 33 in the outer peripheral cover portion 302. Furthermore, the depth L5 is preferably equal to or greater than the thickness of the intermediate layer 33 in the tip cover portion 301.

[0041] Next, a method for manufacturing the protective layer 3 in the sensor element 1 of this embodiment will be described. In this embodiment, the protective layer 3 is formed by applying a slurry of ceramic particles and water to the tip of the sensor element 1 by dipping, followed by drying and firing. The slurry for forming the protective layer 3 may contain a dissipating material such as carbon that dissipates during firing to form pores. The ceramic particles may be, for example, alumina particles.

[0042] The method for manufacturing the protective layer 3 will be described with reference to the flow diagram of FIG. First, in the dipping process of step St1 in FIG. 6 , the tip of a sensor element (not shown) before the protective layer 3 is formed is immersed in a slurry contained in a coating bath (not shown) and then removed from the slurry. Subsequently, in the first drying process of step St2, the slurry coating the tip of the sensor element is dried. In step St2, the slurry is dried with the longitudinal direction Z of the element body aligned vertically. Furthermore, in the dipping process of step St1, the slurry coating the outer surface of the element body 2 becomes the inner layer 31 that directly covers the outer surface of the element body 2. In this embodiment, the drying speed is adjusted by adjusting the ambient temperature, etc., in the first drying process of step St2. As a result, when drying the slurry covering the tip surface 21 of the element body 2, a recess 312 can be formed in the inner layer 31 by utilizing the so-called coffee stain phenomenon. In other words, when the slurry dries, it is easier to dry from the outer peripheral portion, which is the portion of the tip surface 21 of the element body 2 farther from the central axis C. At this time, the ceramic particles in the slurry covering the outer periphery of the tip surface 21 become less mobile as they dry. On the other hand, the slurry covering the center of the tip surface 21 contains a lot of water, so the ceramic particles are more mobile. As the slurry dries, the ceramic particles in the water-rich slurry tend to gather around the dried and solidified ceramic particles. As the drying progresses in this way, the slurry that becomes the inner layer 31 of the tip cover 301 tends to become thinner in the portion closer to the central axis C and relatively thicker in the portion on the outer periphery away from the central axis C. This allows recesses 312 to be formed on the outer surface 311 of the inner layer 31.

[0043] Next, after performing step St2, the dipping process is performed again in step St3. That is, in step St3, the tip of the sensor element 1 is again immersed in the slurry in the coating tank and then pulled out of the slurry. This causes the outer surface of the slurry coated on the outer surface of the element body 2 in step St1 to be further covered with slurry. Thereafter, in a second drying process in step St4, the slurry coating the tip of the sensor element 1 is dried. The second drying process in step St4 can be performed, for example, by air blow drying, which involves drying by blowing air, heat treatment drying, which involves drying in a high-temperature environment, or a combination of these. Furthermore, to form multiple outer layers 32, steps St3 and St4 are repeated multiple times. Thereafter, by performing a baking process in step St5, the ceramic particles are sintered together to produce a porous protective layer 3 having ceramic particles and pores. The baking process in step St5 can be performed, for example, at a temperature of approximately 900°C.

[0044] In this embodiment, the thickness of the inner layer 31 and the thickness of the outer layer 32 are different from each other. The thickness of each layer constituting the protective layer 3 can be set to a desired thickness by, for example, adjusting the viscosity of the slurry. In this embodiment, the viscosity of the slurry for the intermediate layer 33 is set lower than that of the slurry for the inner layer 31. Therefore, the thickness of the intermediate layer 33 can be made thinner than that of the inner layer 31. This also makes it easier for the intermediate layer 33 to cover the inner layer 31 by following the shape of the recesses 312 in the inner layer 31. Therefore, the intermediate layer 33 can form the recesses 332 by tracing the shape of the inner layer 31.

[0045] In this embodiment, the porosity of the inner layer 31 and the porosity of the outer layer 32 are different from each other. The porosity of each layer constituting the protective layer 3 can be set to a desired value, for example, by adjusting the particle size of the ceramic particles constituting each layer. In this embodiment, the particle size of the ceramic particles constituting the inner layer 31 is smaller than the particle size of the ceramic particles constituting the outer layer 32. Therefore, the porosity of the inner layer 31 can be made lower than the porosity of the outer layer 32. In other words, the inner layer 31 can have a denser structure than the outer layer 32. In this embodiment, the particle size of the ceramic particles constituting each of the multiple outer layers 32 is the same.

[0046] The porosity of the protective layer 3 can be measured by various methods. Specifically, the porosity of the protective layer 3 can be calculated by, for example, using a scanning electron microscope (SEM) to capture an image of a polished cross-section of the protective layer 3 at a magnification of 500 to 2000 times and then binarizing the captured image. Alternatively, for example, the porosity of the protective layer 3 can be calculated by cutting out a portion of the protective layer 3 from the sensor element 1 and measuring the apparent density or bulk density of the portion of the protective layer 3 and the theoretical density of the ceramic that constitutes the protective layer 3.

[0047] Next, the effects of this embodiment will be described. In the sensor element 1, the outer surface 331 of the intermediate layer 33 in the tip cover portion 301 has a recess 332 recessed toward the tip surface 21. Therefore, even when the porosity of the outer layer 32 is increased, the outer layer 32 covering the outer surface 331 of the intermediate layer 33 can be firmly fixed to the intermediate layer 33. As a result, the impact resistance of the protective layer 3 can be improved.

[0048] Generally, the higher the porosity of the protective layer 3, the more the responsiveness of the sensor element 1 can be improved, but the impact resistance of the protective layer 3 tends to be reduced. Therefore, in this embodiment, the outer surface 331 of the intermediate layer 33 has a recess 332. Therefore, the recess 332 of the intermediate layer 33 is configured to hook the outer layer 32 covering the outside of the intermediate layer 33, thereby firmly holding the outer layer 32 covering the intermediate layer 33 by a so-called anchor effect. Furthermore, the surface area of ​​the outer surface 331 of the intermediate layer 33 tends to be increased, which facilitates further firmly holding the outer layer 32 covering the intermediate layer 33. This improves the impact resistance of the protective layer 3. Therefore, even when the porosity of the protective layer 3 is increased, peeling of the protective layer 3 due to impacts such as vibrations that occur when the gas sensor 10 including the sensor element 1 is assembled into a vehicle or when the vehicle including the gas sensor 10 is moving can be suppressed. As a result, the impact resistance of the protective layer 3 can be improved while improving the responsiveness of the sensor element 1.

[0049] The outer surface 311 of the inner layer 31 in the tip covering portion 301 has recesses 312. Therefore, the recesses 312 of the inner layer 31 are structured to hook the intermediate layer 33, which has an anchor effect, and thus the impact resistance of the protective layer 3 is likely to be further improved. Furthermore, when the protective layer 3 is manufactured by a dipping process, the recesses 332 can be formed by tracing the shape of the recesses 312 of the inner layer 31 with the slurry that will become the intermediate layer 33. Therefore, the recesses 332 of the intermediate layer 33 can be easily formed, improving manufacturability.

[0050] In the tip covering portion 301, the thickness of the intermediate layer 33 is thinner than the thickness of the inner layer 31. Therefore, the intermediate layer 33 can be easily traced along the shape of the recess 312 of the inner layer 31. Therefore, the recess 332 in the intermediate layer 33 can be easily formed, and the recess 332 can easily be formed to have a sufficient depth. As a result, the impact resistance of the protective layer 3 can be further improved while improving manufacturability.

[0051] Furthermore, the thickness of the intermediate layer 33 in the tip cover portion 301 is 200 μm or less. Therefore, as will be explained in the experimental examples below, the intermediate layer 33 can more easily trace the shape of the recesses 312 in the inner layer 31. As a result, the recesses 332 in the intermediate layer 33 can further improve the impact resistance of the protective layer 3.

[0052] The porosity of the inner layer 31 is 10 to 40% by volume. The porosity of the outer layer 32 is greater than 30% by volume and less than 90% by volume. Therefore, the responsiveness of the sensor element 1 can be sufficiently improved while ensuring impact resistance. That is, by setting the porosity of the inner layer 31 to 10% by volume or greater and the porosity of the outer layer 32 to be greater than 30% by volume, the permeability of the measurement gas through the protective layer 3 is improved, and the responsiveness of the sensor element 1 can be sufficiently improved. The porosity of the inner layer 31 is 40% by volume or less. Therefore, the structure of the inner layer 31 can be made sufficiently dense. Therefore, the bonding strength between the outer surface of the element body 2 and the inner layer 31 can be sufficiently ensured, and impact resistance can be improved. Furthermore, by setting the porosity of the outer layer 32 to 90% by volume or less, the strength of the outer layer 32 itself can be ensured, and impact resistance can be sufficiently ensured.

[0053] The width L1 (see FIG. 5) is at least one-third of the width L2. Therefore, the anchor effect can sufficiently improve the impact resistance of the protective layer 3. Furthermore, it is preferable that the width L1 is at least one-half of the width L2. In this case, the impact resistance of the protective layer 3 can be further improved.

[0054] Furthermore, the depth L4 (see FIG. 5) is preferably equal to or greater than the thickness of the intermediate layer 33 in the tip cover portion 301. In this case, the impact resistance of the protective layer 3 can be further improved.

[0055] The width L3 (see FIG. 5) is at least one-third of the width L2. Therefore, the anchor effect can sufficiently improve the impact resistance of the protective layer 3. Furthermore, the width L3 is preferably at least one-half of the width L2. In this case, the impact resistance of the protective layer 3 can be further improved.

[0056] Furthermore, the depth L5 (see FIG. 5) is preferably equal to or greater than the thickness of the intermediate layer 33 in the tip cover portion 301. In this case, the impact resistance of the protective layer 3 can be further improved.

[0057] The tip cover portion 301 has, on its outer surface, a plurality of outer layers 32 each having a recessed portion recessed toward the tip surface 21. Therefore, the impact resistance of the protective layer 3 can be further improved.

[0058] As described above, according to this embodiment, it is possible to provide a sensor element 1 in which the impact resistance of the protective layer 3 can be improved.

[0059] In the above-mentioned embodiment 1, the protective layer 3 includes one inner layer 31. However, the protective layer may also include, for example, two or more inner layers. In other words, the protective layer may include two or more inner layers each having a lower porosity than the outer layer. Also, in the above-mentioned embodiment 1, the protective layer 3 includes six outer layers 32. However, the protective layer may also include two to five outer layers, or seven or more outer layers.

[0060] In the first embodiment, the thickness of the intermediate layer 33 in the tip covering portion 301 is thinner than the thickness of the inner layer 31. However, the thickness of the intermediate layer in the tip covering portion may be greater than or equal to the thickness of the inner layer.

[0061] In the above-described first embodiment, the recesses 332 in the intermediate layer 33 are formed in the dipping process by tracing the shape of the inner layer 31. However, it is also possible to form recesses in the intermediate layer by, for example, forming an intermediate layer without recesses and then deforming or removing a portion of the intermediate layer.

[0062] (Embodiment 2) In this embodiment, as shown in FIG. 7, a gap G is formed between the tip surface 21 of the element body 2 and the inner layer 31.

[0063] 7, the gap G is formed between the center of the distal end surface 21 and a part of the surface of the base end side Z2 of the inner layer 31 of the distal end cover portion 301. The outer peripheral portion of the surface of the base end side Z2 of the inner layer 31 of the distal end cover portion 301 directly covers the distal end surface 21. The distal end surface of the diffusion resistor 11 faces the gap G. Other aspects are the same as those of embodiment 1. Note that, among the symbols used in embodiment 2 and onwards, the same symbols as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0064] In this embodiment, too, the porosity of the inner layer 31 is lower than the porosity of the outer layer 32. This allows the inner layer 31 to have a sufficiently dense structure. Therefore, even if a gap G is formed between the tip surface 21 of the element body 2 and the inner layer 31, the bonding strength between the outer surface of the element body 2 and the inner layer 31 can be sufficiently ensured. As a result, it is possible to ensure sufficient impact resistance. In addition, the same effects as those of the first embodiment are achieved.

[0065] (Embodiment 3) In this embodiment, as shown in FIG. 8, among a plurality of outer layers 32, only the intermediate layer 33 has recesses 332. That is, in the tip covering portion 301, the outer surface of the outer layer 32 other than the intermediate layer 33 has a convex curved surface shape that protrudes toward the tip side Z1. The rest is the same as in the first embodiment.

[0066] In this embodiment, too, the outer surface 331 of the intermediate layer 33 in the tip cover portion 301 has the recess 332. Therefore, the outer layer 32 covering the outer surface 331 of the intermediate layer 33 can be firmly fixed to the intermediate layer 33. As a result, the impact resistance of the protective layer 3 can be improved. In addition, the same effects as those of the first embodiment are achieved.

[0067] (Embodiment 4) In this embodiment, as shown in FIG. 9, the inner layer 31 does not have any recesses, and only the intermediate layer 33 has recesses 332.

[0068] In the tip covering portion 301, the outer surface 311 of the inner layer 31 has a convex curved surface shape that protrudes toward the tip side Z1.

[0069] In this embodiment, the recesses 332 in the intermediate layer 33 can be formed by adjusting the drying speed by adjusting the ambient temperature, etc., when drying the slurry that will become the intermediate layer 33 in the manufacturing process. That is, similar to the case of forming the recesses 312 in the inner layer 31 in the first embodiment, the recesses 332 can be formed in the intermediate layer 33 by utilizing the coffee stain phenomenon. The rest is the same as in the third embodiment.

[0070] In this embodiment, the intermediate layer 33 also has the recess 332, so that the outer layer 32 covering the outer surface 331 of the intermediate layer 33 can be firmly fixed to the intermediate layer 33. As a result, the impact resistance of the protective layer 3 can be improved. In addition, the same effects as those of the third embodiment are obtained.

[0071] In the above-described fourth embodiment, the outer surface 311 of the inner layer 31 in the tip cover portion 301 has a convex curved shape that protrudes toward the tip side Z1. However, the outer surface of the inner layer in the tip cover portion may also be a flat surface, for example, perpendicular to the longitudinal direction Z.

[0072] (Experimental example) In this example, as shown in Table 1 below, impact tests were conducted using multiple sensor elements having the same basic structure as in Embodiment 1, but differing in the shape, porosity, and thickness of the inner and intermediate layers of the distal end cover. Specifically, each sensor element was assembled into a gas sensor as shown in FIG. 1. The gas sensor equipped with the sensor element was then dropped onto the floor at varying drop heights, applying impact to the sensor element. In Table 1, the "convex" shape of the inner layer or intermediate layer indicates that the outer surface of the inner layer or intermediate layer in the distal end cover is convex toward the distal end. Furthermore, the "concave" shape of the inner layer or intermediate layer in Table 1 indicates that the outer surface of the inner layer or intermediate layer in the distal end cover is concave toward the base end. In other words, the "concave" shape of the inner layer or intermediate layer in Table 1 indicates that the element body has a recessed recess toward the distal end surface. Furthermore, the "porosity of the outer layer" refers to the porosity of the outer layer other than the intermediate layer.

[0073] [Table 1]

[0074] The results of the impact test are shown in Table 2. In Table 2, "◯" indicates that no peeling of the protective layer occurred, and "×" indicates that peeling of the protective layer was observed.

[0075] [Table 2]

[0076] As shown in Table 2, it can be seen that, compared to Comparative Examples 1 and 2, which do not have recesses in the intermediate layer, Examples 1 to 3, which have recesses in the intermediate layer, have protective layers that are less likely to peel off and have higher impact resistance. In particular, when Examples 1 to 3 are compared with Comparative Example 2, in which the porosity of the intermediate layer and the outer layer other than the intermediate layer is 40 volume % or more, it can be seen that Examples 1 to 3 have significantly higher impact resistance than Comparative Example 2. It is believed that, because recesses are formed in the intermediate layer, the intermediate layer firmly holds the outer layer that covers the outside of the intermediate layer, and therefore the impact resistance of the protective layer is higher than that of Comparative Example 2. More specifically, it is believed that the recesses in the intermediate layer have a structure that hooks the outer layer that covers the outside of the intermediate layer, resulting in an anchor effect that improves impact resistance.

[0077] Furthermore, compared to Example 1, in which the outer surface of the inner layer is convex toward the tip, Examples 2 and 3, which have recesses on the outer surface of the inner layer, showed higher impact resistance. This is thought to be because the inner layer structure of Examples 2 and 3 hooks the intermediate layer, resulting in an anchor effect that increases the impact resistance of the protective layer. Furthermore, Example 3 showed higher impact resistance than Example 2. Here, the thickness of the intermediate layer in Example 3 was thinner than that of the intermediate layer in Example 2. Therefore, the intermediate layer in Example 3 is more likely to trace along the shape of the recesses in the inner layer than in Example 2. Therefore, it is thought that Example 3 can form deeper recesses in the intermediate layer than Example 2, thereby improving impact resistance. More specifically, in Example 3, the thickness of the intermediate layer in the tip cover portion is 200 μm or less. Therefore, it is thought that the intermediate layer is more likely to trace along the shape of the recesses in the inner layer, thereby allowing the recesses in the intermediate layer to be deeper.

[0078] The present invention is not limited to the above-described embodiments, and can be applied to various embodiments within the scope of the present invention. [Explanation of symbols]

[0079] 1...sensor element, 2...element body, 3...protective layer, 4...gas detection section, 10...gas sensor, 21...tip surface, 31...inner layer, 32...outer layer, 33...intermediate layer, 301...tip cover section, 331...outer surface, 332...recess, Z...longitudinal direction, Z1...tip side

Claims

1. A sensor element (1) for use in a gas sensor (10), The device comprises a long plate-shaped element body (2) and a porous protective layer (3) covering the outer surface of the element body, A gas detection unit (4) is provided at an end portion of the element body on the tip side (Z1) which is one side in the longitudinal direction (Z), the protective layer covers the outer surface of the tip end of the element body in a layered manner, The protective layer has an inner layer (31) that covers the outer surface of the element body, and a plurality of outer layers (32) that are provided outside the inner layer and have a higher porosity than the inner layer, When the outer layer directly covering the inner layer is defined as an intermediate layer (33), the outer surface (331) of the intermediate layer is covered by another outer layer, The protective layer has a tip covering portion (301) that covers a tip surface (21) that is an end surface on the tip side of the element body, A sensor element, wherein the outer surface of the intermediate layer in the tip cover portion has a recess (332) recessed toward the tip surface of the element body.

2. 2. The sensor element according to claim 1, wherein an outer surface (311) of the inner layer of the tip cover portion has a recess (312) recessed toward the tip surface side of the element body.

3. The sensor element according to claim 2 , wherein the thickness of the intermediate layer in the tip cover portion is smaller than the thickness of the inner layer.

4. 3. The sensor element according to claim 2, wherein the thickness of the intermediate layer in the tip cover portion is 200 [mu]m or less.

5. 5. The sensor element according to claim 1, wherein the porosity of the inner layer is 10 to 40% by volume, and the porosity of the outer layer is higher than 30% by volume and not higher than 90% by volume.

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