Gas Sensors

Abstract

To provide a gas sensor capable of more effectively decreasing the possibility of cracking in a tip detection unit of a sensor element.SOLUTION: A sensor element 2 of a gas sensor 1 includes a solid electrolyte body 31, a heater 34, and a protective layer 20. The protective layer 20 covers a detection tip 25 of the sensor element 2 and the porosity is 25% or more. In the protective layer 20, a thickness t2 of a shielding side portion 22, which is a plane portion on the side where the solid electrolyte body 31 in an opposing direction D of the solid electrolyte body 31 and the heater 34 is located is larger than a thickness t1 of a heater side portion 21 in the protective layer 20, which is a plane portion on the side where the heater 34 in the opposing direction D is located.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] A gas sensor is disposed in, for example, an exhaust pipe of an automobile and is used to detect various gases contained in exhaust gas flowing through the exhaust pipe as the detection target gas. In addition to a sensor element for detecting gas, the gas sensor includes a housing for attaching the sensor element to the exhaust pipe or the like.

[0003] The sensor element includes a solid electrolyte body provided with electrodes exposed to the gas to be detected, an insulator laminated on the solid electrolyte body, and a heater embedded in the insulator. The electrodes and the heating part of the heater are disposed opposite each other at the longitudinal detection tip of the sensor element. A porous protective layer is provided on the surface of the detection tip to protect the sensor element from cracks caused by water exposure.

[0004] In the sensor element, the thickness of the protective layer is appropriately varied at each portion around the central axis in the longitudinal direction of the sensor element depending on the purpose of preventing the occurrence of breakage (cracks) etc. For example, in the multilayer gas sensor element and gas sensor of Patent Document 1, in order to make it difficult for cracks to occur at the boundary between the heater layer having a resistance heating element inside and the detection layer having a pair of electrodes laminated on the heater layer, the thickness of the protective layer formed on the side perpendicular to the stacking direction is made thicker than the thickness of the protective layer formed on both surfaces in the stacking direction. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2009-257817 A Summary of the Invention [Problem to be solved by the invention]

[0006] Through research and development by the inventors, it has been found that there are various forms of mechanisms by which cracks occur in the sensor element. In a cross section perpendicular to the longitudinal direction of the sensor element, the temperature on the side where the heater is arranged becomes relatively high, and the temperature on the side where the heater is not arranged becomes relatively low. Considering this phenomenon, the thickness of the protective layer provided in the cross section perpendicular to the longitudinal direction of the sensor element requires a different ingenuity from that of the sensor element of the conventional gas sensor including Patent Document 1. In particular, in recent years, when even earlier activation of the sensor element is required, the temperature rise rate of the sensor element becomes even faster, and the possibility of the sensor element cracking due to thermal shock increases.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide a gas sensor capable of more effectively reducing the possibility of cracks occurring in the tip detection portion of the sensor element. [Means for solving the problem]

[0008] One aspect of the present invention is Gas to be detected Exhaust gas as A gas sensor (1) comprising a sensor element (2) for detecting a gas contained in The sensor element includes: At the detection tip portion (25) in the longitudinal direction (L) of the sensor element 、 Exposed to the detection target gas an exhaust electrode (311) and an air electrode (312) exposed to the atmosphere; a solid electrolyte body (31) provided with The above atmosphere a heater (34) having a heat generating portion (341) facing the electrode and disposed opposite the solid electrolyte body; a protective layer (20) covering the detection tip and having a porosity of 25% or more; an atmospheric duct (36) which is a space in which the atmospheric electrode is disposed and which opens to a base end side of the sensor element; having The atmospheric duct is interposed between the exhaust electrode and the heat generating portion, In the gas sensor, a thickness (t2) of a shielding side portion (22) which is a flat portion of the protective layer on the side where the solid electrolyte body is located in the opposing direction (D) between the solid electrolyte body and the heater is larger than a thickness (t1) of a heater side portion (21) which is a flat portion of the protective layer on the side where the heater is located in the opposing direction. Effect of the Invention

[0009] In the sensor element of the gas sensor of the one aspect, the porosity of the protective layer covering the detection tip of the sensor element is set to 25% or more. With this configuration, the thermal conductivity of the protective layer is appropriately reduced, and thermal sink from the element body including the solid electrolyte body and the heater to the protective layer is unlikely to occur. In addition, the thickness of the shielding side portion of the protective layer is greater than the thickness of the heater side portion of the protective layer. With this configuration, the heat retention effect of the shielding side portion of the protective layer is higher than that of the heater side portion of the protective layer, and temperature distribution in the opposing direction at the detection tip is unlikely to occur during the heating process of the detection tip by the heater. Then, the effect of hardly causing thermal sink and the effect of hardly causing temperature distribution are combined to make it possible to make it difficult for thermal stress to occur at the detection tip of the sensor element.

[0010] According to the gas sensor of the above aspect, the possibility of cracks occurring in the tip detection portion of the sensor element can be more effectively reduced.

[0011] In addition, the symbols in parentheses for each component shown in one aspect of the present invention indicate a correspondence with the symbols in the figures in the embodiment, but do not limit each component to only the contents of the embodiment. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is an explanatory diagram showing a gas sensor according to a first embodiment. [Diagram 2] FIG. 2 is an explanatory diagram showing the sensor element according to the first embodiment. [Diagram 3] FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2, showing the sensor element according to the first embodiment. [Figure 4] FIG. 4 is a cross-sectional view taken along line III-III in FIG. 2, showing another sensor element according to the first embodiment. [Diagram 5] FIG. 5 is a cross-sectional view showing a sensor element according to the second embodiment, taken along line III-III in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] A preferred embodiment of the above-mentioned gas sensor will now be described with reference to the drawings. <Embodiment 1> As shown in Figs. 1 to 3, the gas sensor 1 of this embodiment includes a sensor element 2 for detecting a gas contained in a gas to be detected. The sensor element 2 includes a solid electrolyte body 31, a heater 34, and a protective layer 20. The solid electrolyte body 31 includes electrodes 311 and 312 that are exposed to the gas to be detected at a detection tip 25 in the longitudinal direction L of the sensor element 2. The heater 34 is disposed opposite the solid electrolyte body 31, and includes a heat generating portion 341 at a position opposite the electrodes 311 and 312. The protective layer 20 covers the detection tip 25 of the sensor element 2, and has a porosity of 25% or more. A thickness t2 of a shielding side portion 22, which is a flat portion of the protective layer 20 on the side where the solid electrolyte body 31 is located in the opposing direction D between the solid electrolyte body 31 and the heater 34, is larger than a thickness t1 of a heater side portion 21, which is a flat portion of the protective layer 20 on the side where the heater 34 is located in the opposing direction D.

[0014] The gas sensor 1 of this embodiment will be described in detail below. (Gas sensor 1) 1, the gas sensor 1 is disposed at a mounting port 71 of an exhaust pipe 7 of an internal combustion engine of a vehicle, and is used to detect the concentrations of oxygen, specific gases, etc. contained in exhaust gas G flowing through the exhaust pipe 7 as the detection target gas. The gas sensor 1 may be used as an air-fuel ratio sensor (A / F sensor) that determines the air-fuel ratio in the internal combustion engine based on the concentrations of oxygen, unburned gas, etc. in the exhaust gas G.

[0015] A catalyst for purifying harmful substances in the exhaust gas G is disposed in the exhaust pipe 7, and the gas sensor 1 may be disposed either upstream or downstream of the catalyst in the flow direction of the exhaust gas G in the exhaust pipe 7. The gas sensor 1 may also be disposed in a pipe on the intake side of a supercharger that uses the exhaust gas G to increase the density of the air taken in by the internal combustion engine. The pipe in which the gas sensor 1 is disposed may also be a pipe in an exhaust gas recirculation mechanism that recirculates a portion of the exhaust gas G exhausted from the internal combustion engine to the exhaust pipe 7 to the intake pipe of the internal combustion engine.

[0016] (Sensor element 2) As shown in Figs. 2 and 3, insulators 33A and 33B are laminated on the solid electrolyte body 31, and the heater 34 is embedded in the insulator 33B. The solid electrolyte body 31 and the insulators 33A and 33B form a laminated type sensor element 2. The sensor element 2 is formed in a long rectangular shape, and is held in the housing 41 while being held by an element holding material 42 described later. The insulators 33A and 33B are composed of a first insulator 33A laminated on a first surface 301 of the solid electrolyte body 31 and a second insulator 33B laminated on a second surface 302 of the solid electrolyte body 31. The heater 34 is embedded in the second insulator 33B.

[0017] The heater 34 may be embedded in the first insulator 33A. In this case, the heater side is the side where the first insulator 33A is arranged, and the shielding side is the side where the second insulator 33B is arranged.

[0018] (Longitudinal direction L, Counter direction D, Width direction W) In this embodiment, the longitudinal direction L of the gas sensor 1 and the sensor element 2 refers to the direction in which the sensor element 2 extends in an elongated shape. A direction perpendicular to the longitudinal direction L and in which the solid electrolyte body 31 and the insulators 33A and 33B face each other (stacking direction) is referred to as a facing direction (stacking direction) D. A direction perpendicular to the longitudinal direction L and the facing direction D is referred to as a width direction W. In addition, in the longitudinal direction L of the sensor element 2, the side exposed to the exhaust gas G is referred to as a tip side L1, and the opposite side of the tip side L1 is referred to as a base side L2.

[0019] (Solid electrolyte body 31, exhaust electrode 311, and air electrode 312) As shown in FIGS. 2 and 3, the solid electrolyte body 31 converts oxygen ions (O 2- ) conductivity. An exhaust electrode 311 exposed to exhaust gas G is provided on a first surface 301 of the solid electrolyte body 31, and an air electrode 312 exposed to air A is provided on a second surface 302 of the solid electrolyte body 31. The exhaust electrode 311 and the air electrode 312 are arranged at positions overlapping each other in the opposing direction D via the solid electrolyte body 31 at a portion of the tip side L1 of the longitudinal direction L of the sensor element 2 exposed to exhaust gas G. A sensor cell is formed at a portion of the tip side L1 of the longitudinal direction L of the sensor element 2 by the exhaust electrode 311, the air electrode 312, and a portion of the solid electrolyte body 31 sandwiched between these electrodes 311, 312.

[0020] The solid electrolyte body 31 is made of a zirconia-based oxide, and is made of stabilized zirconia or partially stabilized zirconia, which is mainly composed of zirconia (zirconium oxide) (containing 50 mass% or more) and in which a rare earth metal element or an alkaline earth metal element has substituted for a part of the zirconia. A part of the zirconia constituting the solid electrolyte body 31 is substituted for by yttria, scandia, or calcia.

[0021] The exhaust electrode 311 and the atmosphere electrode 312 contain platinum as a precious metal exhibiting catalytic activity against oxygen, and zirconia-based oxide as a common material with the solid electrolyte body 31. As shown in Fig. 1 and Fig. 2, the exhaust electrode 311 and the atmosphere electrode 312 are connected to electrode lead portions for electrically connecting these electrodes 311, 312 to the outside of the gas sensor 1.

[0022] (Gas Chamber 35) 2 and 3, a gas chamber 35 surrounded by the first insulator 33A and the solid electrolyte body 31 is formed adjacent to the first surface 301 of the solid electrolyte body 31. The gas chamber 35 is formed at a position on the tip side L1 of the first insulator 33A in the longitudinal direction L, where the gas chamber 35 accommodates the exhaust electrode 311. The gas chamber 35 is formed as a space enclosed by the first insulator 33A, the diffusion resistance portion 32, and the solid electrolyte body 31. Exhaust gas G flowing in the exhaust pipe 7 passes through the diffusion resistance portion 32 and is introduced into the gas chamber 35.

[0023] (Diffusion resistor 32) 2 and 3, the diffusion resistance portion 32 (gas inlet portion) 32 in this embodiment is provided at a portion on the tip side L1 in the longitudinal direction L of the gas chamber 35. The diffusion resistance portion 32 is formed by disposing a porous body of a metal oxide such as alumina (aluminum oxide) in an inlet formed in the first insulator 33A. The diffusion speed (flow rate) of the exhaust gas G introduced into the gas chamber 35 is determined by limiting the speed at which the exhaust gas G passes through the pores of the porous body in the diffusion resistance portion 32. The diffusion resistance portion 32 may be provided on both sides of the gas chamber 35 in the width direction W.

[0024] (Atmospheric Duct 36) 2 and 3, an air duct 36 is formed adjacent to the second surface 302 of the solid electrolyte body 31, and is surrounded by the second insulator 33B and the solid electrolyte body 31, and into which the air A is introduced. The air duct 36 is formed from a portion of the second insulator 33B in the longitudinal direction L that houses the air electrode 312 to the base end position of the sensor element 2 in the longitudinal direction L.

[0025] (Insulators 33A and 33B) 2 and 3, the first insulator 33A forms the gas chamber 35, and the second insulator 33B forms the air duct 36 and embeds the heater 34. The first insulator 33A and the second insulator 33B are made of a metal oxide such as alumina (aluminum oxide). Each of the insulators 33A and 33B is formed as a dense body through which the exhaust gas G or the air A cannot pass.

[0026] (Heater 34) 2 and 3, the heater 34 is configured as a heating element, and is embedded in the second insulator 33B that forms the atmosphere duct 36. The heater 34 has a heating portion 341 that generates heat when current is applied, and a heater lead portion that is connected to a base end side L2 of the heating portion 341 in the longitudinal direction L. The heating portion 341 is disposed at a position where at least a portion of the heating portion 341 overlaps with the exhaust electrode 311 and the atmosphere electrode 312 in the opposing direction D between the solid electrolyte body 31 and the insulators 33A, 33B. The heater 34 is configured from a metallic material having electrical conductivity.

[0027] The detection tip portion 25 of the sensor element 2 is formed as a portion on the tip side in the longitudinal direction L of the sensor element 2 where the exhaust electrode 311, the air electrode 312, and the heat generating portion 341 are arranged.

[0028] (Protective layer 20) 2 and 3, the protective layer 20 is composed of a plurality of ceramic particles bonded together as a ceramic material having pores through which the exhaust gas G can pass. Particles such as alumina (aluminum oxide) and spinel (spinel) are used for the protective layer 20. The thicknesses t1 and t2 of the protective layer 20 at each portion in a cross section perpendicular to the longitudinal direction L of the detection tip 25 are appropriately different so as to prevent a temperature distribution from occurring within the cross section.

[0029] The cross section of the element body made of the solid electrolyte body 31 and the insulators 33A and 33B perpendicular to the longitudinal direction L has a quadrangular shape with four corners formed by C-faces (tapered faces) or R-faces (curved faces). The element body refers to the sensor element 2 excluding the protective layer 20. The flat portion on the side where the solid electrolyte body 31 is located in the opposing direction D between the solid electrolyte body 31 and the heater 34 is formed as the surface of the first insulator 33A. The flat portion on the side where the heater 34 is located in the opposing direction D between the solid electrolyte body 31 and the heater 34 is formed as the surface of the second insulator 33B. Each flat portion refers to a surface parallel to the width direction W excluding the C-face or R-face.

[0030] The thickness t2 of the shielding side portion 22 of the protective layer 20 refers to the thickness t2 of the flat portion on the shielding side, in other words, the portion of the protective layer 20 provided on the entire surface of the first insulator 33A in the facing direction D. The thickness t1 of the heater side portion 21 of the protective layer 20 refers to the thickness t1 of the flat portion on the heater side, in other words, the portion of the protective layer 20 provided on the entire surface of the second insulator 33B in the facing direction D.

[0031] The overall thickness t2 of the shielding side portion 22 of the protective layer 20 is greater than the overall thickness t1 of the heater side portion 21 of the protective layer 20. The shielding side portion 22 and the heater side portion 21 of the protective layer 20 are formed to be flat. The thickness t2 of the shielding side portion 22 of the protective layer 20 at any position in the width direction W is greater than the thickness t1 of the heater side portion 21 of the protective layer 20 at any position in the width direction W.

[0032] In a cross section perpendicular to the longitudinal direction L of the sensor element 2 with respect to the heat generation center position H of the longitudinal direction L of the heat generating portion 341, the overall thickness t2 of the shielding side portion 22 of the protective layer 20 is 1.3 times or more the overall thickness t1 of the heater side portion 21 of the protective layer 20. In this embodiment, in a cross section perpendicular to the longitudinal direction L of the sensor element 2 with respect to the entire position where the heat generating portion 341 is arranged, the thickness t2 at any position in the width direction W of the shielding side portion 22 of the protective layer 20 is 1.3 times or more the thickness t1 at any position in the width direction W of the heater side portion 21 of the protective layer 20. This configuration makes it possible to make it more difficult for a temperature distribution to occur in a cross section perpendicular to the longitudinal direction L of the detection tip portion 25 of the heater 34 element.

[0033] Furthermore, the thickness t2 of the shielding side portion 22 of the protective layer 20 is 498 μm or more. More specifically, the thickness t2 at any position in the width direction W of the shielding side portion 22 of the protective layer 20 is 498 μm or more. This configuration allows the heat retention effect (insulation effect) of the shielding side portion 22 of the protective layer 20 to be appropriately exhibited. The thickness t2 of the shielding side portion 22 of the protective layer 20 may be, for example, 1000 μm or less.

[0034] The thickness t1 of the heater side portion 21 of the protective layer 20 may be as thin as possible within a range that can prevent cracking of the element body of the sensor element 2 due to exposure to water. The thickness t1 of the heater side portion 21 of the protective layer 20 may be, for example, not less than 100 μm and not more than 498 μm.

[0035] Furthermore, the thickness t3 of the side portions 23 of the protective layer 20 located on both sides in the width direction W perpendicular to both the longitudinal direction L and the opposing direction D is greater than the thickness t1 of the heater side portion 21 of the protective layer 20. This configuration can improve the heat retention effect of the side portions 23 of the protective layer 20, and can make it more difficult for a temperature distribution to occur in the cross section perpendicular to the longitudinal direction L of the detection tip portion 25 of the sensor element 2.

[0036] A thickness t3 of the side portion 23 of the protective layer 20 may be smaller or larger than a thickness t2 of the shielding side portion 22 of the protective layer 20. In addition, the protective layer 20 is also formed on the tip surface of the sensor element 2 in the longitudinal direction L.

[0037] (Method of Measuring Porosity of Protective Layer 20) The porosity of the protective layer 20 can be measured by various methods. For example, a cross section of a portion of the protective layer 20 may be observed by a SEM (scanning electron microscope), and the ratio of pores in the cross section may be calculated by binarization or the like, and this may be regarded as the porosity. The magnification of the SEM may be changed according to the size of the particles and the size of the pores in the protective layer 20. For example, if the particle size and the size of the pores of the main material constituting the protective layer 20 are on the order of several microns to several tens of microns, the observation may be performed at a magnification of about 500 to 2000 times. In addition, a plurality of cross sections of the protective layer 20 may be observed, and the average value of the ratio of pores in the plurality of cross sections may be regarded as the porosity. The unit of the porosity is volume %, but it may be substantially regarded as a ratio in the cross section, that is, area %.

[0038] (Method of forming protective layer 20) The protective layer 20 may be formed by spraying a slurry of the ceramic material for forming the protective layer 20 onto the detection tip 25 of the element body of the sensor element 2 and drying it. In this case, the slurry is sprayed so that the shielding side portion 22 of the protective layer 20 is thicker than the heater side portion 21.

[0039] The protective layer 20 may also be formed by molding by placing the detection tip 25 of the element body of the sensor element 2 in a molding die and injecting a slurry of a ceramic material for forming the protective layer 20 into the molding die. In this case, a cavity is formed in the molding die so that the shielding side portion 22 of the protective layer 20 is thicker than the heater side portion 21.

[0040] The protective layer 20 may also be formed by immersing the detection tip 25 of the element body of the sensor element 2 in a slurry of a ceramic material for forming the protective layer 20, and then removing and drying it. In this case, the state after the detection tip 25 of the element body of the sensor element 2 is removed from the slurry is controlled so that the shielding side portion 22 of the protective layer 20 is thicker than the heater side portion 21.

[0041] After the protective layer 20 is formed on the detection tip portion 25 of the element body, the entire sensor element 2 including the protective layer 20 is fired.

[0042] (Configuration of another sensor element 2) As shown in FIG. 4, the sensor element 2 except for the protective layer 20 may have a composition in which the volume ratio of the zirconia material is the largest. Specifically, the insulators 33A and 33B in the sensor element 2 may be made of a zirconia material other than an alumina material. The zirconia material may be the same type of material as the zirconia material constituting the solid electrolyte body 31. When most or most of the element body including the solid electrolyte body 31 and the insulators 33A and 33B are made of a zirconia material, cracks are more likely to occur than when the insulators 33A and 33B are made of an alumina material. Therefore, in the case of the sensor element 2 having a composition in which the volume ratio of the zirconia material is the largest, the effect of making it difficult for a temperature distribution to occur in a cross section perpendicular to the longitudinal direction L of the detection tip 25 is more pronounced.

[0043] Although not shown, the sensor element 2 is not limited to having one solid electrolyte body 31, and may have a plurality of solid electrolyte bodies 31. In this case as well, the heater 34 is disposed at a position offset in the facing direction D, and in the facing direction D, the side closer to the heater 34 is the heater side, and the opposite side is the shielding side.

[0044] (Another configuration of gas sensor 1) As shown in FIG. 1, the gas sensor 1 has a housing 41, an element holding material 42, a terminal holding material 43, a contact member 431, a contact terminal 44, a tip end cover 45, a base end cover 46, a bush 47, lead wires 48, etc., for disposing the sensor element 2 in the exhaust pipe 7 and electrically wiring it to the sensor control device 5.

[0045] The housing 41 is used to fasten the gas sensor 1 to the mounting port 71 of the exhaust pipe 7. The housing 41 holds the sensor element 2 via the element holding material 42 and the like. The sensor element 2 is held by the element holding material 42 via glass 421, and the element holding material 42 is held by the housing 41 via crimping materials 422, 423, and 424. A terminal holding material 43 that holds a contact terminal 44 is connected to the base end side L2 of the element holding material 42 in the longitudinal direction L. The terminal holding material 43 is supported by the base end cover 46 by a contact member 431.

[0046] The contact terminal 44 electrically connects the electrodes 311, 312 and the heater 34 in the sensor element 2 to the lead wire 48. The contact terminal 44 is disposed in the terminal holding material 43 and is connected to the lead wire 48 via a connecting metal fitting 441.

[0047] As shown in FIG. 1, the tip cover 45 is provided on the tip side L1 of the housing 41 in the longitudinal direction L, and covers the detection tip 25 of the sensor element 2. The tip cover 45 is formed with a gas circulation hole 451 through which the exhaust gas G contacting the sensor element 2 can flow. The detection tip 25 of the sensor element 2 and the tip cover 45 are disposed in an exhaust pipe 7 of an internal combustion engine. A part of the exhaust gas G flowing in the exhaust pipe 7 flows into the tip cover 45 from the gas circulation hole 451 of the tip cover 45. Then, the exhaust gas G in the tip cover 45 passes through the protective layer 20 and the diffusion resistance portion 32 of the sensor element 2 and is guided to the exhaust electrode 311.

[0048] The base-end cover 46 is provided on the base-end side L2 of the housing 41 in the longitudinal direction L, and covers the wiring portion located on the base-end side L2 of the gas sensor 1 in the longitudinal direction L to protect the wiring portion from water and the like contained in the atmosphere A. The wiring portion is composed of a contact terminal 44, which is a portion electrically connected to the sensor element 2, a connection portion (connection metal fitting 441) between the contact terminal 44 and the lead wire 48, and the like.

[0049] A bushing 47 that holds a plurality of lead wires 48 is held on the inner periphery of the base end cover 46 at a portion of the base end side L2 in the longitudinal direction L. The base end cover 46 is formed with an air inlet hole 461 for introducing air A from the outside of the gas sensor 1. The air inlet hole 461 is covered with a water-repellent filter 462. The base end position of the air duct 36 in the sensor element 2 is open to the space within the base end cover 46, and the air A is led to the air electrode 312 within the air duct 36.

[0050] (Sensor control device 5) As shown in Fig. 1, the lead wires 48 in the gas sensor 1 are electrically connected to a sensor control device 5 that controls gas detection in the gas sensor 1. The sensor control device 5 performs electrical control of the gas sensor 1 in cooperation with an engine control device 6 that controls the combustion operation of the engine. The sensor control device 5 is configured using various control circuits, a computer, etc. The sensor control device 5 may be built in the engine control device 6. The sensor control device 5 is configured using a circuit 511 that applies a DC voltage between the exhaust electrode 311 and the atmospheric electrode 312, a circuit 512 that detects a current flowing between the exhaust electrode 311 and the atmospheric electrode 312, etc.

[0051] (Action and effect) In the sensor element 2 of the gas sensor 1 of this embodiment, the porosity of the protective layer 20 covering the detection tip 25 of the sensor element 2 is set to 25% or more. With this configuration, the thermal conductivity of the protective layer 20 is appropriately reduced, and thermal suction from the element body including the solid electrolyte body 31 and the heater 34 to the protective layer 20 is unlikely to occur. In addition, the thickness t2 of the shielding side portion 22 of the protective layer 20 is greater than the thickness t1 of the heater side portion 21 of the protective layer 20. With this configuration, the heat retention effect of the shielding side portion 22 of the protective layer 20 is higher than the heat retention effect (insulation effect) of the heater side portion 21 of the protective layer 20, and a temperature distribution (temperature difference) in the facing direction D at the detection tip 25 of the sensor element 2 is unlikely to occur during the temperature rise process of the sensor element 2 by the heater 34. The effect of hardly causing thermal suction and the effect of hardly causing a temperature distribution combine to make it possible to make it difficult for thermal stress to occur at the detection tip 25 of the sensor element 2.

[0052] In the process of heating the detection tip 25 of the sensor element 2 by the heater 34, the temperature on the heater side of the detection tip 25 becomes higher than the temperature on the shielding side of the detection tip 25, resulting in a temperature distribution in the facing direction D of the detection tip 25. At this time, knowledge gained from the inventors' research and development has revealed that a tensile stress is generated outward in the width direction W in the heater side portion of the detection tip 25, and a compressive stress is generated inward in the width direction W in the shielding side portion of the detection tip 25. The generation of this tensile stress and compressive stress creates a state in which the detection tip 25 is susceptible to cracking.

[0053] In the sensor element 2 of the gas sensor 1 of this embodiment, in order to mitigate the generation of tensile stress and compressive stress according to the temperature distribution, the porosity of the protective layer 20 is set to 25% or more, and the thickness t2 of the shielding side portion 22 of the protective layer 20 provided on the detection tip 25 is set to be greater than the thickness t1 of the heater side portion 21 of the protective layer 20. That is, since the thickness t1 of the heater side portion 21 of the protective layer 20 is relatively small, the heat retention effect of the heater side portion 21 is reduced. On the other hand, since the thickness t2 of the shielding side portion 22 of the protective layer 20 is relatively large, the heat retention effect of the shielding side portion 22 is increased. With this configuration, the temperature distribution occurring in the facing direction D of the detection tip 25 of the sensor element 2 is mitigated. Therefore, thermal stress is less likely to occur in the detection tip 25, and it is possible to make the detection tip 25 less likely to crack.

[0054] The timing when the temperature rise stress occurring in the sensor element 2 is the highest is when the sensor is started after the engine is started. When the sensor is started, main current is passed through the heater 34 to quickly activate the sensor element 2. There are also cases where control is performed to alleviate the temperature rise stress occurring in the sensor element 2 by slightly passing current through the heater 34 before main current is passed through the heater 34. However, the key to early activation is how quickly the temperature of the sensor element 2 can be raised during main current passing.

[0055] In the sensor element 2 of the gas sensor 1 of this embodiment, by improving the protective layer 20 covering the detection tip 25 of the sensor element 2, it is possible to make the detection tip 25 less likely to crack even when the temperature rise rate of the sensor element 2 is set to 20°C / s or more to quickly activate the sensor element 2.

[0056] In this way, according to the gas sensor 1 of this embodiment, the possibility of cracks occurring in the detection tip portion 25 of the sensor element 2 can be more effectively reduced.

[0057] <Embodiment 2> In this embodiment, a sensor element 2 in which a part of the protective layer 20 has a two-layer structure is shown. 52, the shielding side portion 22 of the protective layer 20 in this embodiment is composed of an inner layer 221 having a relatively large porosity and an outer layer 222 laminated on the outside of the inner layer 221 and having a relatively small porosity compared to the inner layer 221. The inner layer 221 has a higher porosity than the heater side portion 21 and the outer layer 222 of the protective layer 20 in order to enhance the heat retention effect of the portion on the shielding side in the opposing direction D of the sensor element 2.

[0058] The entire protective layer 20 except for the inner layer 221 is made of the same ceramic material, and the inner layer 221 is made of a ceramic material having a different porosity from the other parts. More specifically, the heater side portion 21, the side portion 23, and the outer layer 222 of the protective layer 20 in this embodiment are made of the same first ceramic material having a porosity of 25% or more, and the inner layer 221 is made of a second ceramic material having a porosity higher than that of the first ceramic material.

[0059] The thickness t2 of the shielding side portion 22 of the protective layer 20 refers to the combined thickness t2 of the inner layer 221 and the outer layer 222. The combined thickness t2 of the shielding side portion 22 of the inner layer 221 and the outer layer 222 is greater than the thickness t1 of the heater side portion 21.

[0060] The inner layer 221 may be formed to a thickness of, for example, 5 μm or more. The remaining portion of the protective layer 20 excluding the inner layer 221 may be determined within a range capable of protecting the element body of the sensor element 2 from cracking due to exposure to water. Since the shielding side portion 22 of the protective layer 20 is composed of the inner layer 221 and the outer layer 222, the heat retention effect of the shielding side portion 22 can be further improved, and the occurrence of a temperature distribution in the facing direction D at the detection tip portion 25 of the sensor element 2 can be further reduced.

[0061] Other configurations, functions, effects, etc. of the gas sensor 1 of this embodiment are similar to those of embodiment 1. Also in this embodiment, components denoted by the same reference numerals as those in embodiment 1 are similar to those in embodiment 1.

[0062] (Confirmation Test 1) In this confirmation test, the influence of the porosity of the protective layer 20 on the durability of the sensor element 2 was confirmed. Specifically, for a plurality of sensor element 2 samples with different porosities of the protective layer 20, the voltage applied to the heater 34 to heat the sensor element 2 was appropriately changed to confirm whether cracks occurred in each sample. The porosity of the protective layer 20 of each sample was changed within a range of 8 to 55%, and the applied voltage was changed within a range of 10 to 20 V. In the protective layer 20 of each sample, the thickness t1 of the heater side portion 21 and the thickness t2 of the shielding side portion 22 were the same, about 500 μm.

[0063] Table 1 shows the results of confirmation test 1. In Table 1, the case where no cracks occurred in any part of the sensor element 2 is indicated by a symbol O, and the case where a crack occurred in any part of the sensor element 2 is indicated by a symbol ×. [Table 1]

[0064] When the porosity of the protective layer 20 is 20% or less, the sample of the sensor element 2 cracks even when the applied voltage is 10V. On the other hand, when the porosity of the protective layer 20 is 25%, no cracks occur up to an applied voltage of 12V, and as the porosity increases from 25%, no cracks occur even when the applied voltage is increased. In particular, it was found that when the porosity of the protective layer 20 is 25% or more, the heat retention effect of the detection tip 25 of the sensor element 2 is improved and the durability of the sensor element 2 is improved. In addition, the porosity of the protective layer 20 may be set to, for example, 55% or less, taking into consideration the performance and strength of the protective layer 20 against water exposure.

[0065] (Confirmation test 2) In this confirmation test, the influence of the ratio of the thickness t2 of the shielding side portion 22 to the thickness t1 of the heater side portion 21 of the protective layer 20 on the durability of the sensor element 2 was confirmed. Specifically, for a plurality of samples of the sensor element 2 having different thickness ratios, the voltage applied to the heater 34 was appropriately changed to confirm whether cracks occurred in each sample. The thickness ratio of each sample was changed within a range of 0.85 to 3.0 times, and the applied voltage was changed within a range of 10 to 20 V. The porosity of the protective layer 20 of each sample was set to 25%.

[0066] Table 2 shows the results of confirmation test 2. In Table 2, the case where no cracks occurred in any part of the sensor element 2 is indicated by a symbol O, and the case where a crack occurred in any part of the sensor element 2 is indicated by a symbol ×. [Table 2]

[0067] It was found that the greater the ratio of the thickness t2 of the shielding side portion 22 to the thickness t1 of the heater side portion 21, the less likely the sample of the sensor element 2 to crack even if the applied voltage is increased. It was also found that this crack is likely to occur even at a low applied voltage when the ratio of the protective layer 20 is less than 1.3 times. Therefore, it was found that when the ratio of the thickness t2 of the shielding side portion 22 to the thickness t1 of the heater side portion 21 is 1.3 times or more, a temperature distribution in the facing direction D at the detection tip portion 25 of the sensor element 2 is less likely to occur, and the durability of the sensor element 2 is improved. In addition, the ratio of the thickness t2 of the shielding side portion 22 to the thickness t1 of the heater side portion 21 may be, for example, 3.0 times or less, taking into consideration the performance of the protective layer 20 against water exposure.

[0068] (Confirmation Test 3) In this confirmation test, the influence of the thickness t2 of the shielding side portion 22 of the protective layer 20 on the durability of the sensor element 2 was confirmed. Specifically, for a plurality of samples of the sensor element 2 having different thicknesses t2 of the shielding side portion 22 of the protective layer 20, the voltage applied to the heater 34 was appropriately changed to confirm whether cracks occurred in each sample. The thickness t2 of the shielding side portion 22 of the protective layer 20 of each sample was changed within a range of 112 to 730 μm, and the applied voltage was changed within a range of 10 to 20 V. In the protective layer 20 of each sample, the porosity was set to 25%, and the ratio of the thickness t2 of the shielding side portion 22 to the heater side portion 21 was set to 1.3 times.

[0069] Table 3 shows the results of confirmation test 3. In Table 3, the case where no cracks occurred in any part of the sensor element 2 is indicated by a symbol ○, and the case where a crack occurred in any part of the sensor element 2 is indicated by a symbol ×. [Table 3]

[0070] Thickness t of the shielding side portion 22 2 is It was found that the larger the thickness t2, the less likely the sample of the sensor element 2 to crack even if the applied voltage is increased. It was also found that this crack is likely to occur even at a low applied voltage when the thickness t2 of the shielding side portion 22 is less than 498 μm. Therefore, it was found that the durability of the sensor element 2 is improved by making the thickness t2 of the shielding side portion 22 498 μm or more. Also, the thickness t2 of the shielding side portion 22 may be set to, for example, 730 μm or less, taking into consideration how unlikely the temperature distribution is to occur.

[0071] The present invention is not limited to the respective embodiments, and further different embodiments can be configured without departing from the scope of the present invention. The present invention also includes various modified examples, modifications within the scope of equivalents, etc. Furthermore, the combinations and forms of various components envisioned from the present invention are also included in the technical spirit of the present invention. [Explanation of symbols]

[0072] 1 Gas sensor 2 Sensor element 20 protective layer 21 Heater side part 22 Shield side part 25 Detection tip 31 Solid electrolyte body 34 Heater

Claims

1. A gas sensor (1) including a sensor element (2) for detecting a gas contained in an exhaust gas as a detection target gas, The sensor element includes: a solid electrolyte body (31) provided with an exhaust electrode (311) exposed to the detection target gas and an air electrode (312) exposed to the air at a detection tip portion (25) in a longitudinal direction (L) of the sensor element; a heater (34) having a heat generating portion (341) at a position facing the air electrode and disposed opposite the solid electrolyte body; a protective layer (20) covering the detection tip and having a porosity of 25% or more; an atmospheric duct (36) which is a space in which the atmospheric electrode is disposed and which opens to a base end side of the sensor element; The atmospheric duct is interposed between the exhaust electrode and the heat generating portion, a thickness (t2) of a shielding side portion (22) of the protective layer which is a flat portion on the side where the solid electrolyte body is located in a facing direction (D) between the solid electrolyte body and the heater is larger than a thickness (t1) of a heater side portion (21) of the protective layer which is a flat portion on the side where the heater is located in the facing direction.

2. In a cross section perpendicular to the longitudinal direction of the sensor element with respect to a heat generation center position (H) in the longitudinal direction of the heat generating portion, 2. The gas sensor according to claim 1, wherein a total thickness of the shielding side portion of the protective layer is 1.3 times or more a total thickness of the heater side portion of the protective layer.

3. 3. The gas sensor according to claim 1, wherein the thickness of the shielding side portion of the protective layer is 498 μm or more.

4. 4. The gas sensor according to claim 1, wherein a thickness of a side portion (23) of the protective layer located on both sides of a width direction (W) perpendicular to both the longitudinal direction and the opposing direction is greater than a thickness of a heater side portion of the protective layer.

5. 5. The gas sensor according to claim 1, wherein the sensor element excluding the protective layer has a composition in which a zirconia material occupies the largest volume ratio.

6. The gas sensor according to any one of claims 1 to 5, wherein the shielding side portion of the protective layer is composed of an inner layer (221) and an outer layer (222) laminated on the outside of the inner layer and having a smaller porosity than the inner layer.

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