Patent Application
20230152269
The present invention relates to a sensor element and a gas sensor.
A sensor element that detects the concentration of a specific gas such as NOx in a measurement-object gas such as an exhaust gas of an automobile is a known art (for example, Patent Literature 1). The sensor element in Patent Literature 1 includes: an elongate element body; an outer electrode disposed on the upper surface of the element body; an outer lead portion; a connector electrode; and a porous layer that covers the outer electrode and the outer lead portion. The outer electrode, the outer lead portion, and the connector electrode are connected in this order and are electrically continuous with each other, and the connector electrode is electrically connected to the outside. The sensor element in Patent Literature 1 further includes a dense layer disposed so as to divide the porous layer in the longitudinal direction of the element body. The dense layer covers the outer lead portion. The dense layer does not easily allow moisture to pass therethrough. Therefore, even when moisture in the measurement-object gas moves through the porous layer by capillary action, the presence of the dense layer prevents the moisture from reaching the connector electrode. A method for producing the above sensor element that is described in Patent Literature 1 includes: forming electrodes, green porous layers, and green dense layers on a plurality of green ceramic sheets corresponding to the element body by screen printing; stacking the plurality of green ceramic sheets; and firing the stacked green ceramic sheets.
As for the sensor element including the dense layer as described in Patent Literature 1, cracking may occur in the sensor element. It is therefore desirable to reduce the occurrence of cracking in the sensor element.
The present invention has been made to solve the foregoing problem, and it is a main object to further reduce the occurrence of cracking in a sensor element.
To achieve the above main object, the present invention employs the following means.
The sensor element of the present invention is a sensor element for detecting a specific gas concentration in a measurement-object gas, the sensor element including: an elongate element body that includes a solid electrolyte layer and has a shape including at least one side surface extending in a longitudinal direction and forward and rear ends that are ends opposite to each other in the longitudinal direction; at least one connector electrode that is disposed on a rear end side of any of the at least one side surface and provided for electrical continuity with the outside of the sensor element; a porous layer that has a porosity of 10% or more and covers at least a forward end side of the side surface on which the connector electrode is disposed; a dense layer that is disposed on the side surface so as to divide the porous layer in the longitudinal direction or to be located rearward of the porous layer, is located forward of the connector electrode, and has a porosity of less than 10%; and an intermediate layer disposed at least between the dense layer and the element body, wherein, when thermal expansion coefficients of the solid electrolyte layer, the dense layer, and the intermediate layer in a temperature range of from 20° C. to 1360° C. are denoted by thermal expansion coefficients Ea, Eb, and Ec, respectively, the ratio Ea/Eb is more than 1.0 and 5.0 or less, and Ea>Ec>Eb is satisfied.
The sensor element includes the solid electrolyte layer, the dense layer, and the intermediate layer. The ratio Ea/Eb of the thermal expansion coefficient Ea of the solid electrolyte layer to the thermal expansion coefficient Eb of the dense layer is more than 1.0 and 5.0 or less, and the thermal expansion coefficient of the solid electrolyte layer is relatively close to the thermal expansion coefficient of the dense layer. Moreover, the intermediate layer is present at least between the dense layer and the solid electrolyte layer, and the thermal expansion coefficient Ec of the intermediate layer satisfies Ea>Ec>Eb. Specifically, the intermediate layer whose thermal expansion coefficient Ec is between the thermal expansion coefficient of the solid electrolyte layer and the thermal expansion coefficient of the dense layer is present between them. Since the solid electrolyte layer, the dense layer, and the intermediate layer satisfy the above positional relation and the relations between the thermal expansion coefficients Ea to Ec, the intermediate layer reduces stress caused by the difference between the thermal expansion coefficient Ea of the solid electrolyte layer and the thermal expansion coefficient Eb of the dense layer when the sensor element is heated during use. When the stress is generated in the sensor element, cracking is likely to occur. However, in the above sensor element, since the stress is reduced, the occurrence of cracking is reduced.
In the sensor element of the present invention, when the mean value of the thermal expansion coefficient Ea and the thermal expansion coefficient Eb is denoted by Ed (=(Ea+Eb)/2), formula (1) below may be satisfied. In this case, the thermal expansion coefficient Ec is relatively close to the median Ed of the thermal expansion coefficients Ea and Eb. Specifically, the thermal expansion coefficient Ec is not excessively close to the thermal expansion coefficient Ea and not excessively close to the thermal expansion coefficient Eb. Therefore, the stress generated when the sensor element is heated is further reduced, and the occurrence of cracking is further reduced.
Ed−0.8×(Ed−Eb)<Ec<Ed+0.8×(Ea−Ed) (1)
In the sensor element of the present invention, the ratio Ea/Eb may be 3.0 or less. In this case, the thermal expansion coefficient Ea of the solid electrolyte layer and the thermal expansion coefficient Eb of the dense layer are closer to each other, so that the occurrence of cracking in the sensor element is further reduced.
In the sensor element of the present invention, the intermediate layer may have a thickness T of 1 μm or more. In this case, the effect of the presence of the intermediate layer in reducing the occurrence of cracking in the sensor element is obtained more reliably. The thickness T of the intermediate layer may be 10 μm or less.
In the sensor element of the present invention, the solid electrolyte layer may contain zirconia as a main component, and the dense layer may contain alumina as a main component. The intermediate layer may contain zirconia and alumina. The main component as used herein means a component with the highest content and is specifically a component with the highest volume ratio.
In the sensor element of the present invention, the sensor element may include: a detection portion including a plurality of electrodes disposed on a forward end side of the element body and used to detect the specific gas concentration in the measurement-object gas; and an outer lead portion that is disposed on the side surface on which the connector electrode is disposed and provides electrical continuity between any of the plurality of electrodes and the connector electrode. The porous layer may cover at least part of the outer lead portion. In this case, the porous layer may fully cover a portion of the outer lead portion that is not covered with the dense layer. The sensor element of the present invention may include an outer electrode that is one of the plurality of electrodes included in the detection portion and that is electrically continuous with the connector electrode through the outer lead portion and disposed on the side surface on which the connector electrode is disposed. In this case, the porous layer may cover the outer electrode.
The gas sensor of the present invention includes the sensor element in any of the above modes. Therefore, the gas sensor has the same effect as the effect of the above-described sensor element of the present invention, e.g., the effect of reducing the occurrence of cracking in the sensor element.
Next, embodiments of the present invention will be described using the drawings.
As shown in
As shown in
The element-sealing member 40 is a member for sealing and fixing the sensor element 20. The element-sealing member 40 includes: a cylindrical member 41 including a main metal fitting 42 and an inner cylinder 43; insulators 44a to 44c; powder compacts 45a and 45b; and a metal ring 46. The sensor element 20 is located on the center axis of the element-sealing member 40 and pierces through the element-sealing member 40 in the forward-rearward direction.
The main metal fitting 42 is a cylindrical metallic member. The main metal fitting 42 has a thick-walled portion 42a located on the forward side and having an inner diameter smaller than that of the rear side. The protective cover 30 is attached to a portion of the main metal fitting 42 that is on the same side as the forward end of the sensor element 20 (i.e., the forward side). The rear end of the main metal fitting 42 is welded to a flange portion 43a of the inner cylinder 43. A part of the inner circumferential surface of the thick-walled portion 42a is formed as a bottom surface 42b that is a step surface. The bottom surface 42b bears the insulator 44a such that the insulator 44a does not protrude forward.
The inner cylinder 43 is a cylindrical metallic member and has the flange portion 43a at its forward end. The inner cylinder 43 and the main metal fitting 42 are welded and fixed to each other so as to be coaxial with each other. The inner cylinder 43 has a reduced diameter portion 43c for pressing the powder compact 45b in a direction toward the center axis of the inner cylinder 43 and a reduced diameter portion 43d for pressing the insulators 44a to 44c and the powder compacts 45a and 45b in a downward direction in
The insulators 44a to 44c and the powder compacts 45a and 45b are disposed between the inner circumferential surface of the cylindrical member 41 and the sensor element 20. The insulators 44a to 44c serve as supporters for the powder compacts 45a and 45b. Examples of the material of the insulators 44a to 44c include ceramics such as alumina, steatite, zirconia, spinel, cordierite, and mullite and glass. The powder compacts 45a and 45b are formed, for example, by molding a powder and each serve as a sealing medium. Examples of the material of the powder compacts 45a and 45b include talc and ceramic powders such as alumina powder and boron nitride powder, and the powder compacts 45a and 45b may each contain at least one of these materials. The powder compact 45a is filled between the insulators 44a and 44b, sandwiched therebetween from opposite sides (forward and rear sides) in the axial direction, and pressed by the insulators 44a and 44b. The powder compact 45b is filled between the insulators 44b and 44c, sandwiched therebetween from opposite sides (forward and rear sides) in the axial direction, and pressed by the insulators 44b and 44c. The insulators 44a to 44c and the powder compacts 45a and 45b are sandwiched between the bottom surface 42b of the thick-walled portion 42a of the main metal fitting 42 and both the reduced diameter portion 43d and the metal ring 46 and pressed from the forward and rear sides. The pressing force applied by the reduced diameter portions 43c and 43d causes the powder compacts 45a and 45b to be compressed between the cylindrical member 41 and the sensor element 20, and the powder compacts 45a and 45b close the communication between the element chamber 33 in the protective cover 30 and a space 49 in the external cylinder 48 and fix the sensor element 20.
The bolt 47 is fixed to the outer side of the main metal fitting 42 so as to be coaxial with the main metal fitting 42. The bolt 47 has a male thread portion formed on the outer circumferential surface of the bolt 47. The male thread portion is inserted into a fixing member 59 that is welded to the pipe 58 and has a female thread portion formed on the inner circumferential surface of the fixing member 59. In this manner, the gas sensor 10 can be fixed to the pipe 58 with the forward end side of the sensor element 20 of the gas sensor 10 and the protective cover 30 protruding into the pipe 58.
The external cylinder 48 is a cylindrical metallic member and covers the inner cylinder 43, the rear end side of the sensor element 20, and the connector 50. An upper portion of the main metal fitting 42 is inserted into the external cylinder 48. The lower end of the external cylinder 48 is welded to the main metal fitting 42. The plurality of lead wires 55 connected to the connector 50 are drawn from the upper end of the external cylinder 48 to the outside. The connector 50 is in contact with and electrically connected to upper connector electrodes 71 and lower connector electrodes 72 that are disposed on rear end portions of respective surfaces of the sensor element 20. The lead wires 55 are electrically continuous with electrodes 64 to 68 and a heater 69 of the sensor element 20 through the connector 50. The gap between the external cylinder 48 and the lead wires 55 is sealed by the rubber stopper 57. The space 49 inside the external cylinder 48 is filled with a reference gas. A sixth surface 60f (rear end surface) of the sensor element 20 is disposed inside the space 49.
As shown in
The detection portion 63 is used to detect the specific gas concentration in the measurement-object gas. The detection portion 63 includes a plurality of electrodes disposed on a forward end side of the element body 60. In the present embodiment, the detection portion 63 includes an outer electrode 64 disposed on the first surface 60a and further includes an inner main pump electrode 65, an inner auxiliary pump electrode 66, a measurement electrode 67, and a reference electrode 68 that are disposed inside the element body 60. The inner main pump electrode 65 and the inner auxiliary pump electrode 66 are disposed on the inner circumferential surface of an internal space of the element body 60 and each have a tunnel-like structure.
The principle of the detection of the specific gas concentration in the measurement-object gas by the detection portion 63 is well known, and its detailed description will be omitted. The detection portion 63 detects the specific gas concentration, for example, in the following manner. The detection portion 63 pumps oxygen in the measurement-object gas around the inner main pump electrode 65 to the outside (the element chamber 33) or pumps oxygen from the outside according to a voltage applied between the outer electrode 64 and the inner main pump electrode 65. Moreover, the detection portion 63 pumps oxygen in the measurement-object gas around the inner auxiliary pump electrode 66 to the outside (the element chamber 33) or pumps oxygen from the outside according to a voltage applied between the outer electrode 64 and the inner auxiliary pump electrode 66. This allows the measurement-object gas whose oxygen concentration has been adjusted to a prescribed value to reach the measurement electrode 67. The measurement electrode 67 functions as a NOx reduction catalyst and reduces the specific gas (NOx) in the measurement-object gas that has reached the measurement electrode 67. Then the detection portion 63 generates an electric signal corresponding to an electromotive force generated between the measurement electrode 67 and the reference electrode 68 according to the oxygen concentration in the reduced gas or corresponding to a current flowing between the measurement electrode 67 and the outer electrode 64 according to the electromotive force. The electric signal generated by the detection portion 63 is a signal indicating a value corresponding to the specific gas concentration in the measurement-object gas (a value from which the specific gas concentration can be derived) and corresponds to the detection value detected by the detection portion 63.
The heater 69 is an electric resistor disposed inside the element body 60. When electric power is supplied to the heater 69 from the outside, the heater 69 generates heat and heats the element body 60. The heater 69 can heat the solid electrolyte layers 78a to 78f included in the element body 60, can keep them hot, and can adjust their temperature to the temperature at which the solid electrolyte layers 78a to 78f are activated (e.g., 800° C.)
The upper connector electrodes 71 and the lower connector electrodes 72 are disposed on rear end-side portions of side surfaces of the element body 60 and are electrodes that allow electrical continuity between the element body 60 and the outside. The upper and lower connector electrodes 71 and 72 are not covered with the protective layer 80 and are exposed. In the present embodiment, the upper connector electrodes 71 include four upper connector electrodes 71a to 71d arranged in the left-right direction and disposed on the rear end side of the first surface 60a. The lower connector electrodes 72 include four lower connector electrodes 72a to 72d arranged in the left-right direction and disposed on the rear end side of the second surface 60b (lower surface) opposite to the first surface 60a (upper surface). Each of the connector electrodes 71a to 71d and 72a to 72d is electrically continuous with a corresponding one of the heater 69 and the plurality of electrodes 64 to 68 of the detection portion 63. In the present embodiment, the upper connector electrode 71a is electrically continuous with the measurement electrode 67, and the upper connector electrode 71b is electrically continuous with the outer electrode 64. The upper connector electrode 71c is electrically continuous with the inner auxiliary pump electrode 66, and the upper connector electrode 71d is electrically continuous with the inner main pump electrode 65. The lower connector electrodes 72a to 72c are electrically continuous with the heater 69, and the lower connector electrode 72d is electrically continuous with the reference electrode 68. The upper connector electrode 71b is electrically continuous with the outer electrode 64 through an outer lead wire 75 disposed on the first surface 60a (see
The outer lead wire 75 is a conductor containing a noble metal such as platinum (Pt) or a high-melting point metal such as tungsten (W) or molybdenum (Mo). Preferably, the outer lead wire 75 is a cermet conductor containing a noble metal or a high-melting point metal and the oxygen-ion-conductive solid electrolyte contained in the element body 60 (zirconia in the present embodiment). In the present embodiment, the outer lead wire 75 is a cermet conductor containing platinum and zirconia. The porosity of the outer lead wire 75 may be, for example, from 5% to 40% inclusive. The line width of the outer lead wire 75 (its thickness, i.e., the width in the left-right direction) is, for example, from 0.1 mm to 1.0 mm inclusive. An unillustrated insulating layer for insulation between the outer lead wire 75 and the solid electrolyte layer 78a of the element body 60 may be disposed between the outer lead wire 75 and the first surface 60a of the element body 60.
The protective layer 80 includes inner porous layers 81 and an outer porous layer 85. The inner porous layers 81 are porous bodies that cover at least the forward end side of the side surfaces of the element body 60 on which the upper and lower connector electrodes 71 and 72 are disposed, i.e., of the first and second surfaces 60a and 60b. In the present embodiment, the inner porous layers 81 cover the first and second surfaces 60a and 60b. The outer porous layer 85 is a porous body that covers the forward end side of the element body 60. The outer porous layer 85 is disposed on the outer side of the inner porous layers 81.
The inner porous layers 81 include a first inner porous layer 83 that covers the first surface 60a and a second inner porous layer 84 that covers the second surface 60b. The first inner porous layer 83 covers the entire region, from the forward end to the rear end, of the first surface 60a on which the upper connector electrodes 71a to 71d are disposed, except for the regions in which the first dense layer 92 and the upper connector electrodes 71 are present (see
The second inner porous layer 84 covers the entire region, from the forward end to the rear end, of the second surface 60b on which the lower connector electrodes 72a to 72d are disposed, except for the regions in which the second dense layer 95 and the lower connector electrodes 72 are present (see
The outer porous layer 85 covers the first to fifth surfaces 60a to 60e. The outer porous layer 85 covers the inner porous layers 81 to thereby cover the first surface 60a and the second surface 60b. The length of the outer porous layer 85 in the forward-rearward direction is shorter than that of the inner porous layers 81. Unlike the inner porous layers 81, the outer porous layer 85 covers only the forward end of the element body 60 and a region around the forward end. In this case, the outer porous layer 85 covers a portion of the element body 60 that surrounds the electrodes 64 to 68 of the detection portion 63, i.e., a portion of the element body 60 that is disposed inside the element chamber 33 and exposed to the measurement-object gas. In this manner, the outer porous layer 85 plays a role in preventing the occurrence of cracking in the element body 60 that are caused by adhesion of, for example, moisture etc. in the measurement-object gas.
The protective layer 80 is formed of, for example, a ceramic porous material such as an alumina porous material, a zirconia porous material, a spinel porous material, a cordierite porous material, a titania porous material, or a magnesia porous material. In the present embodiment, the protective layer 80 is formed of an alumina porous material. The thickness of the first inner porous layer 83 and the thickness of the second inner porous layer 84 may be, for example, 5 μm or more and may be 14 μm or more. The thickness of the first inner porous layer 83 and the thickness of the second inner porous layer 84 may be 40 μm or less and may be 23 μm or less. The thickness of the outer porous layer 85 is, for example, from 40 μm to 800 μm inclusive. The porosity of the protective layer 80 is 10% or more. The protective layer 80 covers the outer electrode 64 and the measurement-object gas inlet 61. However, when the porosity is 10% or more, the measurement-object gas can pass through the protective layer 80. The porosity of the inner porous layers 81 may be from 10% to 50% inclusive. The porosity of the outer porous layer 85 may be from 10% to 85% inclusive. The porosity of the outer porous layer 85 may be higher than the porosity of the inner porous layers 81.
The porosity of the inner porous layers 81 is a value derived as follows using an image (SEM image) obtained by observation using a scanning electron microscope (SEM). First, the sensor element 20 is cut in the thickness direction of the inner porous layers 81, and a cross section of one of the inner porous layers 81 is used as an observation surface. The cross-section is embedded in a resin and polished to obtain an observation sample. Next, the magnification of the SEM is set to 1000× to 10000×, and an image of the observation surface of the observation sample is captured to thereby obtain an SEM image of the inner porous layer 81. Next, the image obtained is subjected to image analysis, and a threshold value is determined by a discriminant analysis method (Otsu's binarization) using a brightness distribution obtained from the brightness data of pixels in the image. Using the determined threshold value, the pixels in the image are binarized and classified into object portions and pore portions, and the area of the object portions and the area of the pore portions are computed. Then the ratio of the area of the pore portions to the total area (the total area of the object portions and the pore portions) is computed as a porosity (unit: %). The porosity of the outer porous layer 85 and the porosities of the first dense layer 92, the second dense layer 95, and the intermediate layer 98 described later are computed in the same manner as described above.
The first dense layer 92 and the second dense layer 95 serve as water intrusion preventing portions that prevent capillary action of water in the longitudinal direction of the element body 60. The first dense layer 92 is disposed on the first surface 60a on which the upper connector electrodes 71 and the first inner porous layer 83 are disposed. The first dense layer 92 is disposed on the first surface 60a so as to divide the first inner porous layer 83 into forward and rear portions in the longitudinal direction as described above. The first dense layer 92 is disposed closer to the forward end of the element body 60 than the upper connector electrodes 71, i.e., disposed forward of the upper connector electrodes 71. The first dense layer 92 is disposed rearward of the outer electrode 64. The first dense layer 92 is disposed rearward of all the plurality of electrodes 64 to 68, including the outer electrode 64, included in the detection portion 63 (see
The second dense layer 95 is disposed on the second surface 60b on which the lower connector electrodes 72 and the second inner porous layer 84 are disposed. The second dense layer 95 is disposed on the second surface 60b so as to divide the second inner porous layer 84 into forward and rear portions in the longitudinal direction as described above. The second dense layer 95 is disposed closer to the forward end of the element body 60 than the lower connector electrodes 72, i.e., disposed forward of the lower connector electrodes 72. The second dense layer 95 is disposed rearward of the outer electrode 64. The second dense layer 95 is disposed rearward of all the plurality of electrodes 64 to 68, including the outer electrode 64, included in the detection portion 63 (see
The length Le of each of the first dense layer 92 and the second dense layer 95 in the longitudinal direction (see
The first dense layer 92 and the second dense layer 95 differ from the protective layer 80 in that their porosity is less than 10%. However, a ceramic composed of any of the materials exemplified for the protective layer 80 described above can be used. Specifically, the first dense layer 92 may be a ceramic porous body containing, as a main component, at least one type of ceramic particles selected from alumina particles, zirconia particles, spinel particles, cordierite particles, titania particles, and magnesia particles. In the present embodiment, the first dense layer 92 and the second dense layer 95 are both formed of a ceramic containing alumina as a main component. The thickness of the first dense layer 92 and the thickness of the second dense layer 95 may each be, for example, from 1 μm to 40 μm inclusive. The thickness of the first dense layer 92 and the thickness of the second dense layer 95 may each be 20 μm or less, may be 9 μm or less, and may be 3 μm or less. The porosity of the first dense layer 92 and the porosity of the second dense layer 95 are each preferably 8% or less and more preferably 5% or less. The smaller the porosity, the further the first dense layer 92 and the second dense layer 95 can reduce the capillary action of water in the longitudinal direction of the element body 60.
As shown in
The thickness T of the intermediate layer 98 may be the thickness T1 of the thinnest portion of the intermediate layer 98 (for example, a portion located directly above the outer lead wire 75 as shown in
The intermediate layer 98 may be formed, for example, of a ceramic containing, as a main component, at least one type of ceramic particles selected from alumina particles, zirconia particles, spinel particles, cordierite particles, titania particles, and magnesia particles. The intermediate layer 98 may be formed of a noble metal such as platinum. The intermediate layer 98 may be formed of cermet containing the above-described ceramic particles and noble metal particles. Preferably, the intermediate layer 98 contains, as main components, the main component of the solid electrolyte layers 78a to 78f and the main component of the first dense layer 92. In the present embodiment, the intermediate layer 98 is a ceramic containing, as main components, zirconia used as the main component of the solid electrolyte layers 78a to 78f and alumina used as the main component of the first dense layer 92.
A method for producing the gas sensor 10 having the above-described structure will be described below. First a method for producing the sensor element 20 will be described. The method for producing the sensor element 20 includes a production step of producing a green sensor element that is the sensor element 20 before firing and a firing step of firing the green sensor element. In the present embodiment, the outer porous layer 85 is formed by plasma spraying after the firing step. Therefore, the green sensor element produced in the production step does not include a green outer porous layer 85, and the sensor element 20 after the firing step does not include the outer porous layer 85.
[Production Step]
In the production step, the green sensor element that is the sensor element 20 before firing is produced. In the production step, first, six ceramic green sheets (green solid electrolyte layers) corresponding to the solid electrolyte layers 78a to 78f included in the element body 60 are prepared. The ceramic green sheets are produced, for example, by mixing a solvent, a binder, etc. with a raw material powder containing the material of the solid electrolyte layers 78a to 78f (a zirconia powder in the present embodiment) to obtain a paste containing the material of the raw material powder as a main component and then forming the paste into a sheet shape. If necessary, through holes, grooves, etc. are punched in the ceramic green sheets to form portions that later become inner spaces of the element body 60 through firing. Next, patterns for green electrodes, green lead wires, green connector electrodes, a green heater, etc. are formed by screen printing on the ceramic green sheets to be used as the solid electrolyte layers 78a to 78f. The green electrodes later become the above-described electrodes 64 to 68 of the detection portion 63 through firing. The green lead wires later become, through firing, the lead wires that connect the electrodes to the upper connector electrodes 71 and the lower connector electrodes 72. The green lead wires include a lead wire that later becomes the outer lead wire 75 through firing. The green connector electrodes later become the upper connector electrodes 71 and the lower connector electrodes 72 through firing. The green heater later becomes the heater 69 through firing. Moreover, patterns for a green intermediate layer that later becomes the intermediate layer 98 through firing, a green first dense layer that later becomes the first dense layer 92 through firing, and a green first inner porous layer that later becomes the first inner porous layer 83 through firing are formed by screen printing on a surface of the ceramic green sheet that later becomes the solid electrolyte layer 78a through firing (a surface that later becomes the first surface 60a of the element body 60). Similarly, patterns for a green second dense layer that later becomes the second dense layer 95 through firing and a green second inner porous layer that later becomes the second inner porous layer 84 through firing are formed by screen printing on a surface of the ceramic green sheet that later becomes the solid electrolyte layer 78f through firing (a surface that later becomes the second surface 60b of the element body 60). Next, the six ceramic green sheets with the patterns formed thereon are stacked to form a layered body. The layered body is cut into small layer bodies having the same size as the size of the sensor element 20. These small layered bodies are green sensor elements. The patterns for the green first inner porous layer, the green second inner porous layer, the green intermediate layer, the green first dense layer, and the green second dense layer may be printed after the production of the layered body described above.
The paste used to form the green first inner porous layer is, for example, a paste that is prepared by mixing a raw material powder composed of the material of the above-described first inner porous layer 83 (an aluminum powder in the present embodiment), a binder, a solvent, a pore-forming material, etc. and that contains, as a main component, the material of the raw material powder. The paste forming the green second inner porous layer is prepared in the same manner as described above. The paste used to form the green first dense layer is, for example, a paste that is prepared by mixing a raw material powder composed of the material of the above-described first dense layer 92 (an aluminum powder in the present embodiment), a binder, a solvent, etc. and that contains, as a main component, the material of the raw material powder. To control the porosity of the first dense layer 92, a pore-forming material may be added to the paste. The paste for forming the green second dense layer is prepared in the same manner as above. The paste used to form the green intermediate layer is, for example, a paste that is prepared by mixing a raw material powder composed of the materials of the above-described intermediate layer 98 (an aluminum powder and a zirconia powder in the present embodiment), a binder, a solvent, etc. and that contains, as main components, the materials of the raw material powder. To control the porosity of the intermediate layer 98, a pore-forming material may be added to the paste.
The green first inner porous layer and the green second inner porous layer may be formed using the same paste or using pastes prepared using different raw material powders. The green first dense layer and the green second dense layer may also be formed using the same paste or using pastes prepared using different raw material powders.
[Firing Step]
Next, the firing step of firing the green sensor element obtained in the production step is performed. In the firing step, the green sensor element is fired at a prescribed firing temperature (e.g., 1360° C.±50° C.), and then the temperature is lowered to room temperature (e.g., 20° C.) after firing. In this manner, the six ceramic green sheets become the solid electrolyte layers 78a to 78f, and the green electrodes become the electrodes 64 to 68. The green lead wires become the plurality of wires including the outer lead wire 75, and the green connector electrodes become the upper connector electrodes 71 and the lower connector electrodes 72. Moreover, the green heater becomes the heater 69. The green intermediate layer becomes the intermediate layer 98, and the green first dense layer becomes the first dense layer 92. The green first inner porous layer becomes the first inner porous layer 83, and the green second dense layer becomes the second dense layer 95. The green second inner porous layer becomes the second inner porous layer 84. The sensor element 20 is obtained through the firing step.
In the present embodiment, after the firing step has been performed to produce the sensor element 20, the outer porous layer 85 is formed by plasma spraying. The plasma spraying can be performed, for example, in the same manner as in plasma spraying described in Japanese Unexamined Patent Application Publication No. 2016-109685. Then the gas sensor 10 equipped with the sensor element 20 is produced. First, the sensor element 20 is caused to pierce axially through the cylindrical member 41, and the insulator 44a, the powder compact 45a, the insulator 44b, the powder compact 45b, the insulator 44c, and the metal ring 46 are placed in this order between the inner circumferential surface of the cylindrical member 41 and the sensor element 20. Next, the metal ring 46 is pressed to compress the powder compacts 45a and 45b. With this state maintained, the reduced diameter portions 43c and 43d are formed to thereby produce the element-sealing member 40, and the gap between the inner circumferential surface of the cylindrical member 41 and the sensor element 20 is thereby sealed. Then the protective cover 30 is welded to the element-sealing member 40, and the bolt 47 is attached to obtain the assembly 15. Then the lead wires 55 piercing through the rubber stopper 57 and the connector 50 connected to the lead wires 55 are prepared, and the connector 50 is connected to the rear end side of the sensor element 20. Then the external cylinder 48 is welded and fixed to the main metal fitting 42 to thereby obtain the gas sensor 10.
In the sensor element 20 in the present embodiment, when the thermal expansion coefficients of the solid electrolyte layers 78a to 78f, the first dense layer 92, and the intermediate layer 98 in the temperature range of from 20° C. to 1360° C. are denoted by thermal expansion coefficients Ea, Eb, and Ec, respectively, the ratio Ea/Eb is more than 1.0 and 5.0 or less, and the relation Ea>Ec>Eb is satisfied. The thermal expansion coefficients Ea to Ec are not volume expansion coefficients but are linear expansion coefficients. Let the median of the thermal expansion coefficient Ea and the thermal expansion coefficient Eb be Ed (=(Ea+Eb)/2). Then it is preferable that the thermal expansion coefficient Ec satisfies formula (1) below. The thermal expansion coefficients Ea, Eb, and Ec are referred to also as thermal expansion coefficients A′, B′, and C′. The median Ed is referred to also as the median D′.
Ed−0.8×(Ed−Eb)<Ec<Ed+0.8×(Ea−Ed) (1)
The thermal expansion coefficient Ea of the solid electrolyte layers 78a to 78f is measured by thermomechanical analysis (TMA) as follows. First, the sensor element 20 is cut such that a portion including the solid electrolyte layers 78a to 78f of the element body 60 is cut out to thereby obtain a measurement piece. Next, the measurement piece is placed in a container, and the expansion coefficient of the measurement piece when the temperature is changed from 20° C. to 1360° C. is measured under an applied load of 1 g. Specifically, a dimension a1′ of the measurement piece at 20° C. is measured. Next, a dimension a2′ of the measurement piece heated to 1360° C. while a load of 1 g is applied is measured. Then the thermal expansion coefficient Ea is computed using the formula: the thermal expansion coefficient Ea[%]=(a2′−a1′)/a1′×100. The dimensions a1′ and a2′ of the measurement piece are measured as dimensions in the forward-rearward direction, i.e., the longitudinal direction, of the sensor element 20. Similarly, the thermal expansion coefficient Eb is computed using a measurement piece that is cut from the sensor element 20 so as to include part of the first dense layer 92, and the thermal expansion coefficient Ec is computed using a measurement piece that is cut from the sensor element 20 so as to include part of the intermediate layer 98. When thermal expansion coefficients of the solid electrolyte layers 78a to 78f are not the same, e.g., when the materials of the solid electrolyte layers 78a to 78f are not the same, the thermal expansion coefficient of a layer closest to the first dense layer 92 and the intermediate layer 98 (the solid electrolyte layer 78a in the present embodiment) is used as the thermal expansion coefficient Ea.
The thermal expansion coefficients Ea to Ec of the sensor element 20 can be controlled as follows. For example, the thermal expansion coefficient Ea of the solid electrolyte layers 78a to 78f can be controlled by changing the material of the raw material powder contained in the paste for forming the ceramic green sheets. The thermal expansion coefficient Eb of the first dense layer 92 can be controlled by chaining the material of the raw material powder contained in the paste for forming the green first dense layer. The thermal expansion coefficient Ec of the intermediate layer 98 can be controlled by changing the material of the raw material powder contained in the paste for forming the green intermediate layer. Therefore, the ratio Ea/Eb can be set to be more than 1.0 and 5.0 or less by selecting an appropriate combination of the material of the raw material powder of the solid electrolyte layers 78a to 78f and the material of the raw material powder of the first dense layer 92. For example, the thermal expansion coefficient of zirconia at 40° C. to 400° C. is 10.5×10−6/° C., and the thermal expansion coefficient of alumina at 40° C. to 400° C. is 7.2×10−6/° C. Therefore, the thermal expansion coefficient of zirconia is larger than the thermal expansion coefficient of alumina. When the raw material powder of the solid electrolyte layers 78a to 78f is zirconia and the raw material powder of the first dense layer 92 is alumina, the thermal expansion coefficient Ea is larger than the thermal expansion coefficient Eb, and the ratio Ea/Eb can be set to be more than 1.0 and 5.0 or less. When cordierite (thermal expansion coefficient: less than 0.1×10−6/° C.) or silicon nitride (thermal expansion coefficient: 2.8×10−6/° C.), which are materials having a smaller thermal expansion coefficient than alumina, is used as the raw material powder of the first dense layer 92, the value of the ratio Ea/Eb can be larger than that when alumina is used (for example, a value of about 5.0) while the value of the ratio Ea/Eb is set to be more than 1.0 and 5.0 or less. When a material whose thermal expansion coefficient is between the thermal expansion coefficient of the material of the raw material powder of the solid electrolyte layers 78a to 78f and the thermal expansion coefficient of the material of the raw material powder of the first dense layer 92 is appropriately selected as the material of the raw material powder of the intermediate layer 98, Ea>Ec>Eb can be satisfied, and formula (1) can be satisfied. Alternatively, the raw material powder of the intermediate layer 98 may contain both the material of the raw material powder of the solid electrolyte layers 78a to 78f and the material of the raw material powder of the first dense layer 92. In this case also, Ea>Ec>Eb can be satisfied, and formula (1) can be satisfied. By appropriately controlling the volume ratio of the material of the raw material powder of the solid electrolyte layers 78a to 78f and the volume ratio of the material of the raw material powder of the first dense layer 92 in the raw material powder of the intermediate layer 98, the thermal expansion coefficient Ec can be controlled while Ea>Ec>Eb is satisfied, and this allows formula (1) to be satisfied.
Next, an example of the use of the thus-produced gas sensor 10 will be described below. When the measurement-object gas flows through the pipe 58 with the gas sensor 10 attached to the pipe 58 as shown in
In this case, the measurement-object gas may contain moisture, and the moisture may move through the protective layer 80 by capillary action. When the moisture reaches the exposed upper and lower connector electrodes 71 and 72, rust or corrosion may occur in the upper and lower connector electrodes 71 and 72 due to components such as water and sulfuric acid dissolved in water, or a short circuit may occur between adjacent ones of the upper and lower connector electrodes 71 and 72. However, in the present embodiment, even when moisture in the measurement-object gas moves through the protective layer 80 (in particular, the first inner porous layer 83 and the second inner porous layer 84) toward the rear end of the element body 60 by capillary action, the moisture reaches the first dense layer 92 or the second dense layer 95 before it reaches the upper and lower connector electrodes 71 and 72. Since the porosity of the first dense layer 92 is less than 10%, the capillary action of water in the longitudinal direction of the element body 60 is unlikely to occur. In this case, the first dense layer 92 can prevent moisture from passing through the first dense layer 92 from the forward end-side portion 83a side and reaching the upper connector electrodes 71 (the upper connector electrodes 71a to 71d). Therefore, in the sensor element 20, the occurrence of the above-described problem caused by water adhering to the upper connector electrodes 71 can be reduced. Similarly, the second dense layer 95 can prevent moisture from passing through the second dense layer 95 from the forward end-side portion 84a side and reaching the lower connector electrodes 72 (the lower connector electrodes 72a to 72d). Therefore, in the sensor element 20, the occurrence of the above-described problem caused by water adhering to the lower connector electrodes 72 is reduced. Preferably, the length Le of the first dense layer 92 in the longitudinal direction is 0.5 mm or more because the passage of moisture through the first dense layer 92 can be reduced sufficiently. Similarly, the length Le of the second dense layer 95 is 0.5 mm or more.
Moreover, the sensor element 20 has a ratio Ea/Eb of more than 1.0 and 5.0 or less and satisfies Ea>Ec>Eb as described above. Therefore, the intermediate layer 98 reduces the stress caused by the difference between the thermal expansion coefficient Ea of the solid electrolyte layers 78a to 78f and the thermal expansion coefficient Eb of the first dense layer 92 when the sensor element 20 is heated by the heater 69 during use. When stress is generated in the sensor element 20, cracking tends to occur. However, in the sensor element 20 in the present embodiment, the stress generated during heating is reduced, so that the occurrence of cracking in the sensor element 20 is reduced. Moreover, since formula (1) above is satisfied, the stress during heating of the sensor element 20 is further reduced, and the occurrence of cracking during heating of the sensor element 20 is further reduced. The ratio Ea/Eb is preferably 3.0 or less. When the ratio Ea/Eb is 3.0 or less, the thermal expansion coefficient Ea of the solid electrolyte layers 78a to 78f is closer to the thermal expansion coefficient Eb of the first dense layer 92, so that the occurrence of cracking in the sensor element 20 is further reduced.
When cracking occurs in the sensor element 20, particularly in the first dense layer 92 or the second dense layer 95, the function of the first dense layer 92 or the second dense layer 95 as the water intrusion preventing portion described above may deteriorate. However, since the occurrence of cracking in the sensor element 20 in the present embodiment is reduced, the function of the first dense layer 92 and the second dense layer 95 as the water intrusion preventing portions is unlikely to deteriorate, and thus the occurrence of the above-described problem caused by water adhering to the upper connector electrodes 71 and the lower connector electrodes 72 is reduced.
The correspondences between the components in the present embodiment and the components in the present invention will be clarified. The solid electrolyte layers 78a to 78f in the present embodiment correspond to the solid electrolyte layer in the present invention, and the element body 60 corresponds to the element body. The upper connector electrodes 71a to 71d correspond to the connector electrode, and the first surface 60a corresponds to the side surface on which the connector electrode is disposed. The first inner porous layer 83 corresponds to the porous layer, and the first dense layer 92 corresponds to the dense layer. The intermediate layer 98 corresponds to the intermediate layer. The detection portion 63 corresponds to the detection portion, and the outer lead wire 75 corresponds to the outer lead portion. The outer electrode 64 corresponds to the outer electrode.
In the gas sensor 10 in the present embodiment described above in detail, as for the thermal expansion coefficients Ea, Eb, and Ec of the solid electrolyte layers 78a to 78f, the first dense layer 92, and the intermediate layer 98 of the sensor element 20 in the temperature range of from 20° C. to 1360° C., the ratio Ea/Eb is more than 1.0 and 5.0 or less, and Ea>Ec>Eb is satisfied. In this case, the intermediate layer 98 reduces the stress caused by the difference between the thermal expansion coefficient Ea of the solid electrolyte layers 78a to 78f and the thermal expansion coefficient Eb of the first dense layer 92 when the sensor element 20 is heated during use. Therefore, the occurrence of cracking in the sensor element 20 during heating is reduced. Moreover, since the thermal expansion coefficients Ea to Ec in the sensor element 20 satisfy formula (1) above, the stress generated when the sensor element 20 is heated is reduced, and the occurrence of cracking is further reduced. Moreover, since the ratio Ea/Eb in the sensor element 20 is 3.0 or less, the occurrence of cracking is further reduced. In the sensor element 20, since the thickness T of the intermediate layer 98 is 1 μm or more, the above-described effect of the presence of the intermediate layer 98 in reducing the occurrence of cracking in the sensor element 20 is obtained more reliably.
The present invention is not limited to the embodiment described above. It will be appreciated that the present invention can be implemented in various forms so long as they fall within the technical scope of the invention.
For example, in the above embodiment, the length of the intermediate layer 98 in the forward-rearward direction is the same as the length Le of the first dense layer 92 as shown in
In the embodiment described above, as shown in
In the embodiment described above, the intermediate layer 98 is present between the element body 60 and the first dense layer 92 disposed on the first surface 60a side (the upper side) of the element body 60, but this is not a limitation. As shown in
In the embodiment described above, the second surface 60b of the element body 60 may have an exposed portion on which the second inner porous layer 84 and the second dense layer 95 are not present.
In the embodiment described above, the outer porous layer 85 is formed by plasma spraying after the firing step, but this is not a limitation. For example, a green outer porous layer that later becomes the outer porous layer 85 through the firing step may be formed by, for example, dipping in the production step. In this case, the green outer porous layer becomes the outer porous layer 85 through the firing step. In the embodiment described above, the protective layer 80 may not include the outer porous layer 85.
In the embodiment described above, the first dense layer 92 divides the first inner porous layer 83 in the longitudinal direction into the forward end-side portion 83a and the rear end-side portion 83b, but this is not a limitation. The first dense layer 92 may be located rearward of the protective layer 80. For example, in the embodiment described above, the first inner porous layer 83 may not include the rear end-side portion 83b. Similarly, the second dense layer 95 may not divide the second inner porous layer 84 and may be located rearward of the protective layer 80. However, when the first inner porous layer 83 does not include the rear end-side portion 83b, part of the outer lead wire 75 is exposed. It is therefore preferable that the first inner porous layer 83 includes the rear end-side portion 83b.
In the embodiment described above, the first dense layer 92 and the second dense layer 95 are disposed at respective positions overlapping the insulator 44b in the forward-rearward direction, but this is not a limitation. For example, the first dense layer 92 and the second dense layer 95 may be disposed at positions overlapping the insulator 44a or the insulator 44c in the forward-rearward direction or may be disposed rearward of the metal ring 46. In the embodiment described above, the first dense layer 92 and the second dense layer 95 are disposed at positions that are not exposed to the element chamber 33. However, at least one of the first dense layer 92 and the second dense layer 95 may be disposed at a position exposed to the element chamber 33, i.e., a position exposed to the measurement-object gas. For example, at least one of the first dense layer 92 and the second dense layer 95 may be disposed at a position located rearward of the outer porous layer 85 and exposed to the element chamber 33.
In the embodiment described above, the sensor element 20 may not include the second inner porous layer 84, and the second surface 60b may not be covered with the second inner porous layer 84. In this case, the sensor element 20 may not include the second dense layer 95. It is only necessary that a dense layer be disposed on at least one side surface on which connector electrodes and a porous layer are disposed (the first and second surfaces 60a and 60b in the embodiment described above) among the side surface of the element body (the first to fourth surfaces 60a to 60d in the embodiment described above). In this case, moisture is prevented from reaching the connector electrodes at least on the side surfaces on which the dense layer is disposed. Moreover, it is only necessary that the intermediate layer be disposed between the element body and the dense layer.
In the embodiment described above, the first inner porous layer 83 covers a region of the first surface 60a that extends from the forward end of the first surface 60a to its rear end except for the regions in which the first dense layer 92 and the upper connector electrodes 71 are present, but this is not a limitation. For example, the first inner porous layer 83 may cover a region extending from forward end of the first surface 60a to the forward ends of the upper connector electrodes 71a to 71d except for the region in which the first dense layer 92 is present. Alternatively, the first inner porous layer 83 may cover at least a region extending from the forward end of the first surface 60a to a position rearward of the first dense layer 92 except for the region in which the first dense layer 92 is present. The same applies to the second inner porous layer 84.
In the embodiment described above, the element body 60 has a rectangular parallelepiped shape, but this is not a limitation. For example, the element body 60 may be cylindrical or columnar. In this case, the element body 60 has one side surface.
In the embodiment described above, the gas sensor 10 detects the NOx concentration as the specific gas concentration, but this is not a limitation. The concentration of a different oxide may be used as the specific gas concentration. In the case where the specific gas is an oxide, when the specific gas itself is reduced near the measurement electrode 67, oxygen is generated, as in the embodiment described above. Therefore, the specific gas concentration can be detected based on the value that is detected by the detection portion 63 and corresponds to the oxygen. The specific gas may be a non-oxide such as ammonia. When the specific gas is a non-oxide, the specific gas is converted to an oxide, for example, near the inner main pump electrode 65 (for example, ammonia is oxidized and converted to NO). When the oxide produced by conversion is reduced near the measurement electrode 67, oxygen is generated, and the specific gas concentration can be detected based on the value that is detected by the detection portion 63 and corresponds to the oxygen. As described above, even when the specific gas is an oxide or a non-oxide, the gas sensor 10 can detect the specific gas concentration based on oxygen derived from the specific gas and generated near the measurement electrode 67.
Examples will next be described. In each Example, sensor elements were actually produced. However, the present invention is not limited to the following Examples.
A sensor element that was the same as the sensor element 20 shown in
Sensor elements 20 with the relations between the thermal expansion coefficients Ea, Eb, and Ec different from those in Example 1 were produced and used as Examples 2 to 10 and Comparative Examples 1 to 3. In Examples 2 to 10 and Comparative Examples 1 to 3, the solid electrolyte layers 78a to 78f were the same as those in Example 1.
Specifically, the value of the thermal expansion coefficient Ea was the same for Examples 1 to 10 and Comparative Examples 1 to 3. In Examples 2 to 10 and Comparative Examples 1 to 3, the thermal expansion coefficient Eb of the first dense layer 92 was changed by selecting the raw material powder of the green dense layer from alumina, cordierite, and silicon nitride. In Examples 2 to 10 and Comparative Examples 1 to 3, the thermal expansion coefficient Ec of the intermediate layer 98 was changed by adjusting the volume ratio of the material of the raw material powder of the solid electrolyte layers 78a to 78f contained in the green intermediate layer and the volume ratio of the material of the raw material powder of the first dense layer 92 contained in the green intermediate layer. In Examples 2 to 8 and Comparative Examples 1 to 3, the thickness T of the intermediate layer 98 was set to 5 μm, which was the same as the thickness in Example 1. In Example 9, the thickness T of the intermediate layer 98 was set to 10 μm. In Example 10, the thickness T of the intermediate layer 98 was set to 1 μm. The thickness T of the intermediate layer 98 was controlled by changing the amount of the solvent contained in the paste for the green intermediate layer to thereby adjust the viscosity or adjusting the number of screen printing operations for printing the green intermediate layer.
[Measurement of Thermal Expansion Coefficients Ea to Ec]
The thermal expansion coefficients Ea to Ec in each of Examples 1 to 10 and Comparative Examples 1 to 3 were measured using the method described above. A thermomechanical analysis apparatus (type: TMA4000SA) manufactured by NETZSCH was used for the measurement.
[Evaluation of Cracking Resistance]
For each of Examples 1 to 10 and Comparative Examples 1 to 3, ten sensor elements 20 were produced, and each sensor element 20 was subjected to a test in which the sensor element 20 was exposed to high-temperature high-pressure vapor for a prescribed time to thereby evaluate cracking resistance. This test was performed using a method according to JIS A 1509-8:2014. First, the sensor element 20 was placed in an autoclave. Then the pressure inside the autoclave was gradually increased so as to reach 1 MPa or higher over about 1 hour, and the increased pressure was maintained for 1 hour or longer. Then the pressure was reduced to normal pressure as fast as possible, and the sensor element 20 was left to cool. The cooled sensor element 20 was visually checked to determine whether or not cracking occurred in the first dense layer 92. When the number of cracked sensor elements 20 out of the ten sensor element 20 was zero, the cracking resistance was rated “excellent (A).” When the number of cracked sensor elements 20 was one, the cracking resistance was rated “good (B).” When the number of cracked sensor elements 20 was two or more, the cracking resistance was rated “fail (F).” The high-temperature high-pressure state in the autoclave during the test is severer than a normal use environment of the sensor element 20 attached to a vehicle.
Table 1 summarizes the thermal expansion coefficients Ea to Ec in each of Examples 1 to 10 and Comparative Examples 1 to 3 and the results of the evaluation of the cracking resistance of the sensor elements 20. In Table 1, the values of the thermal expansion coefficients Ea to Ec are the ratios with respect to the thermal expansion coefficient Ea with the value of the thermal expansion coefficient Ea used as a reference (value: 1). Table 1 also shows the ratio Ea/Eb, the magnitude relation between Ea to Ec, the median Ed of the thermal expansion coefficient Ea and the thermal expansion coefficient Eb, the value of the left-hand side of formula (1), i.e., (Ed−0.8×(Ed−Eb)), the value of the right-hand side of formula (1), i.e., (Ed+0.8×(Ea−Ed)), whether the thermal expansion coefficients Ea to Ec satisfy formula (1), and the thickness T of the intermediate layer 98.
As can be seen from Table 1, in Examples 1 to 10 in which the ratio Ea/Eb was more than 1.0 and 5.0 or less and Ea>Ec>Eb was satisfied, the evaluation of the cracking resistance of the sensor elements 20 was “excellent (A)” or “good (B),” and the occurrence of cracking was reduced. However, in Comparative Examples 1 and 2 in which Ea>Ec>Eb was not satisfied and in Comparative Example 3 in which the ratio Ea/Eb was more than 5.0, the evaluation of the cracking resistance of the sensor elements 20 was “fail (F).” This confirms that, when the ratio Ea/Eb is more than 1.0 and 5.0 or less and Ea>Ec>Eb is satisfied, the occurrence of cracking in the sensor element 20 can be reduced. In Examples 2 and 3 in which formula (1) was not satisfied, the evaluation of the cracking resistance was “good (B).” However, in Examples 1, 4 to 6, and 8 to 10 in which formula (1) was satisfied, the evaluation of the cracking resistance was “excellent (A).” This confirms that, when formula (1) is satisfied, the occurrence of cracking in the sensor element 20 can be further reduced. In Example 7 in which the ratio Ea/Eb was more than 3.0, the evaluation of the cracking resistance was “good (B).” However, in Examples 1, 4 to 6, and 8 to 10 in which the ratio Ea/Eb was 3.0 or less, the evaluation of the cracking resistance was “excellent (A).” This confirms that, when the ratio Ea/Eb is 3.0 or less, the occurrence of cracking in the sensor element 20 can be further reduced. The results in Example 10 confirm that the effect of reducing the occurrence of cracking is obtained when the thickness T of the intermediate layer 98 is in the range of 1 μm or more. The results in Example 9 confirm that the effect of reducing the occurrence of cracking is obtained when the thickness T of the intermediate layer 98 is in the range of 10 μm or less.
The present application claims priority based on U.S. Patent Application No. 63/211,665 filed on Jun. 17, 2021, and the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63211665 | Jun 2021 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2022/012184 | Mar 2022 | US |
| Child | 18157853 | US |
