Sensor element and gas sensor

Abstract

To inhibit cracks from generating in a sensor element.SOLUTION: A sensor element 20 comprises an element body 60, upper and lower connector electrodes 71, 72, upper and lower porous layers 83, 84, and upper and lower dense layers 91, 92. The sensor element 20 further comprises upper and lower intermediate layers 95, 96 which have porosity lower than those of the upper and lower porous layers 83, 84 and higher than those of the upper and lower dense layers 91, 92, are in contact with the upper and lower dense layers 91, 92 at one side of a front end side and a rear end side of the element body 60 while in contact with the upper and lower porous layers 83, 84 at the other side of the front end side and the rear end side, and cover a side face of the element body 60.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

[0002] Conventionally, a sensor element for detecting the concentration of a specific gas, such as NOx, in a measurement gas, such as exhaust gas from an internal combustion engine, has been known (see, for example, Patent Document 1). The sensor element of Patent Document 1 includes an element body, a connector electrode, a porous layer, and a dense layer. The front end side of the element body is exposed to the measurement gas. The connector electrode is disposed on the rear end side of the side of the element body and is electrically connected to the contact portion of the connector. The porous layer is disposed on a portion of the side of the element body in the front-rear direction. The dense layer is disposed at a position on the side of the element body different from the porous layer in the front-rear direction, closer to the front end of the element body than the connector electrode, so as to be in contact with the porous layer in the front-rear direction. In this sensor element, when water in the measurement gas moves through the porous layer toward the rear end side of the element body due to capillary action, the water has difficulty passing through the dense layer, thereby preventing the water from reaching the connector electrode. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2022 / 209529 Summary of the Invention [Problem to be solved by the invention]

[0004] In a sensor element in which a porous layer and a dense layer are in contact, as in Patent Document 1, if the difference in porosity between the porous layer and the dense layer is relatively large, there is a concern that cracks may occur in the sensor element due to the relatively large difference in residual stress between the two.

[0005] The sensor element and gas sensor of the present invention have a main object to suppress the occurrence of cracks in the sensor element. [Means for solving the problem]

[0006] The sensor element and gas sensor of the present invention employ the following means to achieve the above-mentioned main object.

[0007] [1] The sensor element of the present invention is A sensor element for detecting the concentration of a specific gas in a measurement gas, an element body having a front end and a rear end which are both ends along a longitudinal direction and a side surface which is a surface along the longitudinal direction, the front end side being exposed to the measurement gas; a connector electrode disposed on the rear end side of the side surface for electrical connection to the outside; a porous layer covering a portion of the side surface in the front-rear direction; a dense layer having a lower porosity than the porous layer and covering the side surface at a position different from the porous layer in the front-rear direction and on the front end side of the connector electrode; an intermediate layer having a porosity lower than that of the porous layer and higher than that of the dense layer, the intermediate layer being in contact with the dense layer on one side of the front end side and the rear end side, and in contact with the porous layer on the other side of the front end side and the rear end side, and covering the side surface; The gist of the project is to provide the following:

[0008] The sensor element of the present invention includes an intermediate layer having a porosity lower than that of the porous layer but higher than that of the dense layer, contacting the dense layer at one of the front and rear ends and the porous layer at the other of the front and rear ends and covering the side surface. This reduces the difference in porosity between the two adjacent layers compared to when the dense layer and the porous layer are in direct contact without the intermediate layer, thereby reducing the difference in residual stress between the two adjacent layers (stress acting at the boundary between the two adjacent layers). As a result, the occurrence of cracks in the sensor element can be suppressed.

[0009] [2] In the sensor element of the present invention (the sensor element described in [1] above), the difference in porosity between any two adjacent layers among the dense layer, the intermediate layer, and the porous layer may be 20% or less.

[0010] [3] In this case (the sensor element according to the above [2]), the difference in porosity between the two layers in contact with each other may be 5% or less.

[0011] [4] In the sensor element of the present invention (the sensor element described in [1] above), the intermediate layers may be first to Nth (N≧2) intermediate layers arranged in descending order of porosity, and the first to Nth intermediate layers may be arranged between the dense layer and the porous layer in this order, with adjacent layers contacting each other. In this way, when the porosities of the dense layer and the porous layer are respectively set, the difference in porosity between the adjacent two layers can be made smaller, thereby further suppressing cracks in the sensor element. Furthermore, when the porosity of the dense layer and the difference in porosity between the adjacent two layers are set, the porosity of the porous layer can be increased while suppressing cracks in the sensor element.

[0012] [5] In the sensor element of the present invention (the sensor element described in [4] above), the difference in porosity between any two adjacent layers among the dense layer, the first to Nth intermediate layers, and the porous layer may be 20% or less.

[0013] [6] In the sensor element of the present invention (the sensor element described in the above [5]), the difference in porosity between the two layers in contact with each other may be 5% or less.

[0014] [7] In the sensor element of the present invention (the sensor element described in any one of [1] to [6] above), at least one of a plurality of boundaries between two adjacent layers selected from the dense layer, the intermediate layer, and the porous layer may extend toward the side surface with an inclination toward the front end or the rear end. Also, in the sensor element of the present invention (the sensor element described in any one of [4] to [6] above), at least one of a plurality of boundaries between two adjacent layers selected from the dense layer, the first to Nth intermediate layers, and the porous layer may extend toward the side surface with an inclination toward the front end or the rear end. This increases the area of ​​the boundary compared to when the boundary extends perpendicular to the side surface, thereby increasing the allowable range of residual stress difference between the two adjacent layers (the range of residual stress difference within which cracking can be suppressed). As a result, cracking in the sensor element can be further suppressed.

[0015] [8] In the sensor element of the present invention (the sensor element described in [7] above), the at least one boundary portion may extend so that the layer with the lower porosity of the two layers becomes thinner as it approaches the side surface. The inventors have confirmed through experiments and analyses that the lower the porosity of a layer and the greater the layer's thickness, the greater the layer's residual stress tends to be. Therefore, by having the layer with the lower porosity of the two layers with the boundary portion become thinner as it approaches the side surface, it is possible to prevent a relatively large stress from acting on the side surface of the element body from the layer with the lower porosity (e.g., the dense layer) at the boundary portion.

[0016] [9] The sensor element of the present invention (the sensor element described in [8] above) may further include an outer electrode disposed on the front end side of the side surface, and an outer lead portion disposed on the side surface and electrically connecting the outer electrode to the connector electrode. In this case, it is possible to prevent a relatively large stress from acting on the side surface of the element body or the outer lead portion from a layer (e.g., a dense layer) having a lower porosity than the boundary portion.

[0017]

[10] A gas sensor according to the present invention includes the sensor element according to any one of [1] to [9] above. Therefore, the gas sensor according to the present invention can achieve the same effects as those achieved by the sensor element described above, such as the effect of suppressing the occurrence of cracks in the sensor element. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 3 is a vertical cross-sectional view showing the gas sensor 10 attached to a pipe 58. [Figure 2] FIG. [Figure 3] FIG. 2 is a longitudinal cross-sectional view of the sensor element 20. [Figure 4] FIG. [Figure 5] FIG. 2 is a partially enlarged view of a main part of the sensor element 20. [Figure 6] FIG. 10 is a partially enlarged view of a main part of a sensor element 20B of a comparative example. [Figure 7] FIG. 10 is a partially enlarged view of a main part of a sensor element 120 according to a modified example. [Figure 8] FIG. 10 is a partially enlarged view of a main part of a sensor element 220 according to a modified example. [Figure 9] FIG. 10 is a partially enlarged view of a main part of a sensor element 320 according to a modified example. [Figure 10] FIG. 10 is a partially enlarged view of a main part of a sensor element 420 according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0019] Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a vertical cross-sectional view showing a gas sensor 10 according to an embodiment of the present invention attached to a pipe 58. FIG. 2 is a perspective view of a sensor element 20 included in the gas sensor 10, seen from the front right corner. FIG. 3 is a vertical cross-sectional view schematically showing a vertical cross-section of the sensor element 20. FIG. 4 is a top view of the sensor element 20. FIG. 5 is a partially enlarged view showing a main portion of the sensor element 20. In this embodiment, as shown in FIGS. 1 to 5, the longitudinal direction of the element body 60 of the sensor element 20 is defined as the front-rear direction (length direction), the stacking direction (thickness direction) of the solid electrolyte layers of the element body 60 is defined as the up-down direction, and the direction perpendicular to the front-rear direction and the up-down direction is defined as the left-right direction (width direction).

[0020] As shown in FIG. 1 , the gas sensor 10 includes an assembly 15, a nut 47, an outer cylinder 48, a connector 50, lead wires 55, and a rubber plug 57. The assembly 15 includes a sensor element 20, a protective cover 30, and an element sealing body 40. The gas sensor 10 is attached to a pipe 58, such as an exhaust gas pipe of an internal combustion engine mounted on a vehicle, and is used to detect the concentration (specific gas concentration) of a specific gas such as NOx or O2 contained in exhaust gas as a measurement gas. In this embodiment, the gas sensor 10 detects the NOx concentration as the specific gas concentration. Of both ends (front end and rear end) along the longitudinal direction of the sensor element 20, the front end side is the side exposed to the measurement gas.

[0021] 1, the protective cover 30 includes a cylindrical inner protective cover 31 with a bottom that covers the front end side of the sensor element 20, and a cylindrical outer protective cover 32 with a bottom that covers the inner protective cover 31. A plurality of holes are formed in each of the inner protective cover 31 and the outer protective cover 32 to allow the measurement gas to flow through. An element chamber 33 is formed as a space surrounded by the inner protective cover 31, and the fifth surface 60e (front end surface) of the sensor element 20 is disposed within this element chamber 33.

[0022] The element sealing body 40 is a member that seals and fixes the sensor element 20. The element sealing body 40 includes a cylindrical body 41 having a metallic shell 42 and an inner cylinder 43, insulators 44a to 44c (an example of a dense body), powder compacts 45a and 45b, and a metal ring 46. The sensor element 20 is disposed so as to extend along the central axis of the element sealing body 40 (here, an axis extending in the front-rear direction), and penetrates the element sealing body 40 in the axial direction.

[0023] The metal shell 42 is a cylindrical metal member. The metal shell 42 has a thick portion 42a at the front side, the inner diameter of which is smaller than that of the rear side. A protective cover 30 is attached to the metal shell 42 on the same side (front side) as the front end of the sensor element 20. The rear end of the metal shell 42 is welded to a flange portion 43a of the inner cylinder 43. A part of the inner circumferential surface of the thick portion 42a forms a bottom surface 42b, which is a stepped surface. This bottom surface 42b holds the insulator 44a so that it does not protrude forward. The metal shell 42 has a through hole that penetrates the metal shell 42 along the axial direction (here, the front-rear direction), and the sensor element 20 penetrates through the inside of this through hole.

[0024] The inner tube 43 is a cylindrical metal member having a flange portion 43a at its front end. The inner tube 43 and the metallic shell 42 are welded coaxially. The inner tube 43 is also formed with a reduced-diameter portion 43c for pressing the powder compact 45b in the central axial direction of the inner tube 43, and a reduced-diameter portion 43d for pressing the insulators 44a to 44c and the powder compacts 45a and 45b forward via the metal ring 46. The inner tube 43 has a through-hole that penetrates the inner tube 43 along the axial direction (here, the front-rear direction), and the sensor element 20 penetrates through this through-hole. The through-hole of the metallic shell 42 and the through-hole of the inner tube 43 are axially connected, and these constitute the through-hole of the cylindrical body 41.

[0025] The insulators 44a to 44c and the powder compacts 45a and 45b are disposed between the inner circumferential surface of the through-hole of the cylindrical body 41 and the sensor element 20. The insulators 44a to 44c serve as supporters for the powder compacts 45a and 45b. Examples of materials for the insulators 44a to 44c include ceramics such as alumina, steatite, zirconia, spinel, cordierite, and mullite, or glass. The insulators 44a to 44c are dense members, with a porosity of, for example, less than 1%. Each of the insulators 44a to 44c is a hollow columnar member having a through-hole that penetrates the insulator along the axial direction (the front-rear direction in this example), and the sensor element 20 penetrates through this through-hole. In this embodiment, the through-hole of each of the insulators 44a to 44c has a rectangular cross section perpendicular to the axial direction to match the shape of the sensor element 20. The powder compacts 45a and 45b are formed, for example, from powder and serve as sealing materials. Examples of materials for the powder compacts 45a and 45b include ceramic powders such as alumina powder and boron nitride, in addition to talc, and each of the powder compacts 45a and 45b may contain at least one of these. The average particle size of the particles constituting the powder compacts 45a and 45b may be 150 to 300 μm. The powder compact 45a is filled between the insulators 44a and 44b and is sandwiched and pressed by the insulators 44a and 44b from both sides (front and back) in the axial direction. The powder compact 45b is filled between the insulators 44b and 44c and is sandwiched and pressed by the insulators 44b and 44c from both sides (front and back) in the axial direction. The insulators 44a to 44c and the powder compacts 45a, 45b are sandwiched and pressed from both sides (front and back) in the axial direction by the bottom surface 42b of the thick-wall portion 42a of the metallic shell 42, the reduced diameter portion 43d, and the metal ring 46. The pressing forces from the reduced diameter portions 43c, 43d compress the powder compacts 45a, 45b between the cylindrical body 41 and the sensor element 20, and the powder compacts 45a, 45b seal the gap between the element chamber 33 in the protective cover 30 and a space 49 in the outer cylinder 48 and fix the sensor element 20.

[0026] Nut 47 is fixed to the outside of metallic shell 42 coaxially with metallic shell 42. A male thread portion is formed on the outer peripheral surface of nut 47. This male thread portion is threadedly engaged with a female thread portion provided on the inner peripheral surface of fixing member 59 welded to pipe 58. As a result, gas sensor 10 is fixed to pipe 58 such that the front end side of sensor element 20 and a portion of protective cover 30 protrude into pipe 58.

[0027] The outer cylinder 48 is a cylindrical metal member that covers the inner cylinder 43, the rear end of the sensor element 20, and the connector 50. The rear end of the metallic shell 42 is inserted inside the outer cylinder 48. The front end of the outer cylinder 48 is welded to the metallic shell 42. Multiple lead wires 55 connected to the connector 50 extend from the rear end of the outer cylinder 48 to the outside. The connector 50 includes housings 51a and 51b, multiple contact fittings 52, and a clamp 53. The housings 51a and 51b are made of ceramics such as alumina and are disposed above and below the rear end of the sensor element 20. The multiple contact fittings 52 are all made of metal and are disposed below the housing 51a and above the housing 51b. The clamp 53 is a member formed by bending a metal plate into a C-shape and uses elastic force to clamp the housings 51a and 51b from above and below, pressing them toward each other. The elastic force of the clamp 53 clamps and secures the rear end of the sensor element 20 between the housings 51a and 51b via the contact fittings 52. Each of the contact fittings 52 contacts and electrically connects with a corresponding one of the upper connector electrodes 71 and the lower connector electrodes 72 disposed on the rear surface of the sensor element 20, and also electrically connects with a corresponding one of the lead wires 55. Each of the lead wires 55 is electrically connected to one of the electrodes 64-68 and the heater 69 inside the sensor element 20 via the connector 50 and one of the upper connector electrodes 71 and the lower connector electrodes 72. A gap between the outer tube 48 and the lead wire 55 is sealed with a rubber plug 57. A space 49 within the outer tube 48 is filled with a reference gas. A sixth surface 60f (rear end surface) of the sensor element 20 is located in the space 49.

[0028] As shown in FIGS. 2 to 4, the sensor element 20 includes an element body 60, a detection unit 63, a heater 69, a plurality of upper connector electrodes 71, a plurality of lower connector electrodes 72, a porous layer 80, a dense layer 90, and an intermediate layer 94. The element body 60 has a laminated body in which a plurality of oxygen ion conductive solid electrolyte layers (six in FIG. 3) are stacked, such as zirconia (ZrO2). The element body 60 has a rectangular parallelepiped shape with its longitudinal direction aligned with the front-rear direction, and has first to sixth faces 60a to 60f as outer surfaces on the top, bottom, left, right, front, and rear sides, respectively. The first to fourth faces 60a to 60d are surfaces aligned with the longitudinal direction of the element body 60 and correspond to the side faces (top, bottom, left, and right faces) of the element body 60. The fifth face 60e is the front end face of the element body 60, and the sixth face 60f is the rear end face of the element body 60. The dimensions of element body 60 may be, for example, a length in the front-rear direction of 25 mm to 100 mm, a width in the left-right direction of 2 mm to 10 mm, and a thickness in the up-down direction of 0.5 mm to 5 mm. Element body 60 is formed with a measurement gas inlet 61 that opens to fifth surface 60e and introduces a measurement gas into itself, and a reference gas inlet 62 that opens to sixth surface 60f and introduces a reference gas (here, air) that serves as a reference for detecting the concentration of a specific gas into itself.

[0029] The detection unit 63 is for detecting the concentration of a specific gas in the measurement gas. The detection unit 63 has a plurality of electrodes disposed on the front end side of the element body 60. In this embodiment, the detection unit 63 includes an outer electrode 64 disposed on the first surface 60a, and an inner main pump electrode 65, an inner auxiliary pump electrode 66, a measurement electrode 67, and a reference electrode 68 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 the space inside the element body 60 and have a tunnel-like structure.

[0030] The principle by which the detection unit 63 detects the concentration of a specific gas in a measurement gas is well known, and therefore a detailed description thereof will be omitted. The detection unit 63 detects the concentration of a specific gas, for example, as follows. The detection unit 63 pumps oxygen in the measurement gas around the inner main pump electrode 65 to the outside (element chamber 33) or pumps it in based on the voltage applied between the outer electrode 64 and the inner main pump electrode 65. The detection unit 63 also pumps oxygen in the measurement gas around the inner auxiliary pump electrode 66 to the outside (element chamber 33) based on the voltage applied between the outer electrode 64 and the inner auxiliary pump electrode 66. As a result, the measurement gas, whose oxygen concentration has been adjusted to a predetermined concentration, reaches the vicinity of the measurement electrode 67. The measurement electrode 67 functions as a NOx reduction catalyst and reduces the specific gas (NOx) in the measurement gas that reaches it. The detection unit 63 generates, as an electric signal, an electromotive force generated between the measurement electrode 67 and the reference electrode 68 in accordance with the oxygen concentration after reduction, or a current flowing between the measurement electrode 67 and the outer electrode 64 based on the electromotive force. The electric signal thus generated by the detection unit 63 is a signal indicating a value in accordance with the concentration of the specific gas in the measurement gas (a value from which the concentration of the specific gas can be derived), and corresponds to the detection value detected by the detection unit 63.

[0031] The heater 69 is an electric resistor disposed inside the element body 60. When power is supplied from the outside, the heater 69 generates heat to heat the element body 60. The heater 69 heats and keeps warm the solid electrolyte layer that forms the element body 60, and can adjust the temperature to a temperature (e.g., 800°C) at which the solid electrolyte layer is activated.

[0032] Each of the plurality of upper connector electrodes 71 and the plurality of lower connector electrodes 72 is disposed on the rear end side of one of the side surfaces of the element body 60 and serves to electrically connect to the outside. The plurality of upper connector electrodes 71 and the plurality of lower connector electrodes 72 are exposed without being covered by the porous layer 80. In this embodiment, the four upper connector electrodes 71 (71a to 71d in FIG. 4) are aligned along the left-right direction and disposed on the rear end side of the first surface 60a (upper surface). The four lower connector electrodes 72 are aligned along the left-right direction and disposed on the rear end side of the second surface 60b (lower surface) opposite the first surface 60a (upper surface). Each of the four upper connector electrodes 71 (71a to 71d) and the four lower connector electrodes 72 is electrically connected to one of the plurality of electrodes 64 to 68 and the heater 69 of the detection unit 63. In this embodiment, the upper connector electrode 71a is electrically connected to the measurement electrode 67, the upper connector electrode 71b is electrically connected to the outer electrode 64, the upper connector electrode 71c is electrically connected to the inner auxiliary pump electrode 66, the upper connector electrode 71d is electrically connected to the inner main pump electrode 65, each of the three lower connector electrodes 72 is electrically connected to the heater 69, and one lower connector electrode 72 is electrically connected to the reference electrode 68. The upper connector electrode 71b and the outer electrode 64 are electrically connected via an outer lead portion 75 disposed on the first surface 60a (see FIGS. 3 and 4). The other connector electrodes are electrically connected to the corresponding electrodes or the heater 69 via lead wires or through holes disposed inside the element body 60.

[0033] The outer lead portion 75 is a conductor containing a precious metal such as platinum (Pt) or a high-melting-point metal such as tungsten (W) or molybdenum (Mo). The outer lead portion 75 is preferably a cermet conductor containing a precious metal or a high-melting-point metal and an oxygen-ion conductive solid electrolyte (zirconia in this embodiment) contained in the element body 60. In this embodiment, the outer lead portion 75 is a cermet conductor containing platinum and zirconia. Each portion of the outer lead portion 75 is covered with one of the inner porous layer 81, the upper dense layer 91, or the upper intermediate layer 95.

[0034] The porous layer 80 is a porous body that covers part of the side surfaces of the element body 60 on which the upper and lower connector electrodes 71 and 72 are disposed, i.e., the first and second surfaces 60a and 60b. In this embodiment, the porous layer 80 covers at least the front end sides of the first and second surfaces 60a and 60b. The porous layer 80 includes an inner porous layer 81 that covers the first and second surfaces 60a and 60b, respectively, and an outer porous layer 85 that is disposed on the outside of the inner porous layer 81.

[0035] The inner porous layer 81 includes an upper porous layer 83 that covers the first surface 60a (upper surface) and a lower porous layer 84 that covers the second surface 60b (lower surface). The upper porous layer 83 includes a front-end porous layer 83a and a rear-end porous layer 83b (see FIGS. 2 to 5). The front-end porous layer 83a covers the first surface 60a from the front end of the first surface 60a to the front end of the front-end intermediate layer 95a of the upper intermediate layer 95 of the intermediate layer 94. The left-to-right width of the front-end porous layer 83a is the same as the left-to-right width of the first surface 60a, and the front-end porous layer 83a covers the first surface 60a from the left end to the right end of the first surface 60a. The rear end of the front-end porous layer 83a contacts the front end of the front-end intermediate layer 95a. The rear-end porous layer 83b covers the first surface 60a from the rear end of the rear-end intermediate layer 95b of the upper intermediate layer 95 to the rear end of the first surface 60a, except for the area where the upper connector electrode 71 is located. The left-to-right width of the rear-end porous layer 83b is the same as the left-to-right width of the first surface 60a, and the rear-end porous layer 83b covers the first surface 60a from the left end to the right end of the first surface 60a. The front end of the rear-end porous layer 83b is in contact with the rear end of the rear-end intermediate layer 95b. The upper porous layer 83 covers the entire outer electrode 64 and a portion (part in the extension direction) of the outer lead 75. The upper porous layer 83 serves as a protective layer that protects the outer electrode 64 and the outer lead 75 from components such as sulfuric acid in the gas under measurement, thereby suppressing corrosion.

[0036] The lower porous layer 84 has a front-end porous layer 84a and a rear-end porous layer 84b (see FIGS. 2, 3, and 5). The front-end porous layer 84a covers the second surface 60b from the front end of the second surface 60b to the front end of the front-end intermediate layer 96a of the lower intermediate layer 96 of the intermediate layer 94. The left-to-right width of the front-end porous layer 84a is the same as the left-to-right width of the second surface 60b, and the front-end porous layer 84a covers the second surface 60b from the left end to the right end of the second surface 60b. The rear end of the front-end porous layer 84a is in contact with the front end of the front-end intermediate layer 96a. The rear-end porous layer 84b covers the second surface 60b from the rear end of the rear-end intermediate layer 96b of the lower intermediate layer 96 to the rear end of the second surface 60b, except for the area where the lower connector electrode 72 is present. The left-to-right width of the rear-end-side porous layer 84b is the same as the left-to-right width of the second surface 60b, and the rear-end-side porous layer 84b covers the second surface 60b from the left end to the right end of the second surface 60b. The front end of the rear-end-side porous layer 84b is in contact with the rear end of the rear-end-side intermediate layer 96b.

[0037] The outer porous layer 85 covers at least a portion of each of the first to fifth surfaces 60a to 60e. The outer porous layer 85 covers the first surface 60a and the second surface 60b by covering the inner porous layer 81. The outer porous layer 85 has a shorter length in the front-to-rear direction than the inner porous layer 81, and unlike the inner porous layer 81, it covers only the front end and the region near the front end of the element body 60. As a result, the outer porous layer 85 covers the portion of the element body 60 surrounding the multiple electrodes 64 to 68 of the detection unit 63—in other words, the portion of the element body 60 that is disposed in the element chamber 33 and exposed to the measured gas. As a result, the outer porous layer 85 serves as a protective layer that prevents cracks from occurring in the element body 60 due to adhesion of water or the like in the measured gas.

[0038] The porous layer 80 is made of a ceramic porous body such as porous alumina, porous zirconia, porous spinel, porous cordierite, porous titania, or porous magnesia. In this embodiment, the porous layer 80 is made of porous alumina. The thickness of each of the upper porous layer 83 and the lower porous layer 84 may be, for example, 5 μm or more and 40 μm or less. The thickness of the outer porous layer 85 may be, for example, 40 μm or more and 800 μm or less.

[0039] The porosity of the porous layer 80 is 10% or more. The porous layer 80 covers the outer electrode 64 and the measurement gas inlet 61, and as long as the porosity of the porous layer 80 is 10% or more, the measurement gas can pass through the porous layer 80. The porosity of the inner porous layer 81 may be 10% or more and 50% or less, or 10% or more and 30% or less. The porosities of the front end side porous layer 83a, the rear end side porous layer 83b, the front end side porous layer 84a, and the rear end side porous layer 84b of the inner porous layer 81 may all be the same, or at least some of them may be different. The porosity of the outer porous layer 85 may be 10% or more and 85% or less. The porosity of the outer porous layer 85 may be the same as or higher than the porosity of the inner porous layer 81.

[0040] The porosity of the inner porous layer 81 is determined as follows using an image (SEM image) obtained by observation using a scanning electron microscope (SEM). First, the sensor element 20 is cut along the thickness direction of the inner porous layer 81 so that the cross section of the inner porous layer 81 serves as the observation surface. The cut surface is then filled with resin and polished to obtain an observation sample. Next, an SEM image of the inner porous layer 81 is obtained by photographing the observation surface of the observation sample at a magnification of 1,000 to 10,000 times using the SEM. Next, the obtained image is analyzed to determine a threshold value using discriminant analysis (Otsu's binarization) based on the brightness distribution of the brightness data of the pixels in the image. Then, based on the determined threshold value, each pixel in the image is binarized into an object portion and a pore portion, and the area of ​​the object portion and the area of ​​the pore portion are calculated. The ratio of the area of ​​the pore portion to the total area (the total area of ​​the object portion and the pore portion) is then calculated as the porosity (unit: %). The porosity of the outer porous layer 85 and the porosities of the upper dense layer 91, the lower dense layer 92, the upper intermediate layer 95, and the lower intermediate layer 96, which will be described later, are also values ​​derived in the same manner.

[0041] The dense layer 90 suppresses the capillary phenomenon of water along the longitudinal direction of the element body 60. The dense layer 90 has an upper dense layer 91 and a lower dense layer 92. The upper dense layer 91 is provided on the first surface 60a at a position different from the upper porous layer 83 (front-end side porous layer 83a and rear-end side porous layer 83b) and the upper intermediate layer 95 (front-end side intermediate layer 95a and rear-end side intermediate layer 95b) in the front-to-rear direction. Specifically, the front-end side porous layer 83a, the front-end side intermediate layer 95a, the upper dense layer 91, the rear-end side intermediate layer 95b, and the rear-end side porous layer 83b are provided in this order on the first surface 60a from the front end to the rear end. The upper dense layer 91 is provided on the first surface 60a in front of the plurality of upper connector electrodes 71 and behind any of the plurality of electrodes 64 to 68 of the detection unit 63 (see FIG. 3). The upper dense layer 91 covers a region of the first surface 60a from the rear end of the front-end-side intermediate layer 95a of the upper intermediate layer 95 to the front end of the rear-end-side intermediate layer 95b. The left-to-right width of the upper dense layer 91 is the same as the left-to-right width of the first surface 60a, and the upper dense layer 91 covers the first surface 60a from the left end to the right end of the first surface 60a. The front end of the upper dense layer 91 contacts the rear end of the front-end-side intermediate layer 95a, and the rear end of the upper dense layer 91 contacts the front end of the rear-end-side intermediate layer 95b. The upper dense layer 91 is a dense layer with a porosity of less than 10%. When water in the gas to be measured moves backward in the upper porous layer 83 due to capillary action, the upper dense layer 91 serves to prevent the water from passing through the upper dense layer 91 and thereby preventing the water from reaching the upper connector electrodes 71. The upper dense layer 91 covers a part of the outer lead portion 75 (a part in the extending direction).

[0042] The lower dense layer 92 is provided on the second surface 60b at a position different from the lower porous layer 84 (the front-end porous layer 84a and the rear-end porous layer 84b) and the lower intermediate layer 96 (the front-end intermediate layer 96a and the rear-end intermediate layer 96b) in the front-to-rear direction. Specifically, the front-end porous layer 84a, the front-end intermediate layer 96a, the lower dense layer 92, the rear-end intermediate layer 96b, and the rear-end porous layer 84b are provided in this order on the second surface 60b from the front end to the rear end. The lower dense layer 92 is provided on the second surface 60b forward of the plurality of lower connector electrodes 72 and rearward of any of the plurality of electrodes 64 to 68 of the detection unit 63 (see FIG. 3). The lower dense layer 92 covers a region of the second surface 60b from the rear end of the front-end side intermediate layer 96a of the lower intermediate layer 96 to the front end of the rear-end side intermediate layer 96b. The left-right width of the lower dense layer 92 is the same as the left-right width of the second surface 60b, and the lower dense layer 92 covers the second surface 60b from the left end to the right end of the second surface 60b. The front end of the lower dense layer 92 contacts the rear end of the front-end side intermediate layer 96a, and the rear end of the lower dense layer 92 contacts the front end of the rear-end side intermediate layer 96b. The lower dense layer 92 is a dense layer with a porosity of less than 10%. When water in the gas to be measured moves backward within the lower porous layer 84 due to capillary action, the lower dense layer 92 serves to prevent the water from passing through the lower dense layer 92 and thereby preventing the water from reaching the multiple lower connector electrodes 72.

[0043] Although the upper dense layer 91 and the lower dense layer 92 have different porosities from the porous layer 80, they can be made of ceramics made of the materials exemplified for the porous layer 80. In this embodiment, both the upper dense layer 91 and the lower dense layer 92 are made of alumina ceramics. The thickness of each of the upper dense layer 91 and the lower dense layer 92 may be 5 μm or more and 40 μm or less, or 5 μm or more and less than 11 μm. The thickness of the upper dense layer 91 may be the same as or thinner than the upper porous layer 83 (the front-end porous layer 83a and / or the rear-end porous layer 83b). The thickness of the lower dense layer 92 may be the same as or thinner than the lower porous layer 84 (the front-end porous layer 84a and / or the rear-end porous layer 84b).

[0044] The lengths Ld1 and Ld2 of the upper dense layer 91 and the lower dense layer 92 in the front-to-rear direction may be 0.5 mm or more, or 5 mm or more. The lengths Ld1 and Ld2 may be 25 mm or less, or 20 mm or less. The lengths Ld1 and Ld2 may be the same as or different from each other. The porosity of each of the upper dense layer 91 and the lower dense layer 92 is preferably 8% or less, and more preferably 5% or less. The lower the porosity of each of the upper dense layer 91 and the lower dense layer 92, the more effectively the upper dense layer 91 and the lower dense layer 92 can suppress the capillary phenomenon of water along the longitudinal direction of the element body 60. The porosity of the upper dense layer 91 and the lower dense layer 92 may be the same as or different from each other.

[0045] The upper dense layer 91 and the lower dense layer 92 are each arranged so as to overlap with the inner circumferential surface of one of the plurality of insulators 44a to 44c in the front-rear direction of the element body 60. In the present embodiment, as shown in Fig. 1 , the upper dense layer 91 and the lower dense layer 92 are each arranged so as to overlap with the inner circumferential surface of the insulator 44b among the insulators 44a to 44c in the longitudinal direction of the sensor element 20. The inner circumferential surface of the insulator 44b is the surface of the insulator 44b facing the upper dense layer 91 and the lower dense layer 92, i.e., the surface exposed toward the upper dense layer 91 and the lower dense layer 92, and is the surface located on the upper and lower sides of the inner circumferential surface of the rectangular cross section of the insulator 44b.

[0046] The intermediate layer 94 has an upper intermediate layer 95 and a lower intermediate layer 96. The upper intermediate layer 95 has a front-end side intermediate layer 95a and a rear-end side intermediate layer 95b (see FIGS. 2 to 5). The front-end side intermediate layer 95a and the rear-end side intermediate layer 95b are provided on the first surface 60a at positions different from the upper porous layer 83 (the front-end side porous layer 83a and the rear-end side porous layer 83b) and the upper dense layer 91 in the front-to-rear direction. Specifically, as described above, the front-end side porous layer 83a, the front-end side intermediate layer 95a, the upper dense layer 91, the rear-end side intermediate layer 95b, and the rear-end side porous layer 83b are provided in this order from the front end to the rear end of the first surface 60a.

[0047] The front-end-side intermediate layer 95a covers an area of ​​the first surface 60a from the rear end of the front-end-side porous layer 83a to the front end of the upper dense layer 91. The left-to-right width of the front-end-side intermediate layer 95a is the same as the left-to-right width of the first surface 60a, and the front-end-side intermediate layer 95a covers the first surface 60a from the left end to the right end of the first surface 60a. The front end of the front-end-side intermediate layer 95a contacts the rear end of the front-end-side porous layer 83a, and the rear end of the front-end-side intermediate layer 95a contacts the front end of the upper dense layer 91. In this embodiment, a boundary B1 between the upper dense layer 91 and the front-end-side intermediate layer 95a and a boundary B2 between the front-end-side intermediate layer 95a and the front-end-side porous layer 83a each extend in the up-down direction (a direction perpendicular to the first surface 60a). The front-end-side intermediate layer 95a has a porosity lower than that of the front-end-side porous layer 83a and higher than that of the upper-side dense layer 91.

[0048] The rear-end-side intermediate layer 95b covers a region of the first surface 60a from the rear end of the upper dense layer 91 to the front end of the rear-end-side porous layer 83b. The left-right width of the rear-end-side intermediate layer 95b is the same as the left-right width of the first surface 60a, and the rear-end-side intermediate layer 95b covers the first surface 60a from the left end to the right end of the first surface 60a. The front end of the rear-end-side intermediate layer 95b contacts the rear end of the upper dense layer 91, and the rear end of the rear-end-side intermediate layer 95b contacts the front end of the rear-end-side porous layer 83b. In this embodiment, a boundary B3 between the upper dense layer 91 and the rear-end-side intermediate layer 95b and a boundary B4 between the rear-end-side intermediate layer 95b and the rear-end-side porous layer 83b each extend in the up-down direction (a direction perpendicular to the first surface 60a). The rear end side intermediate layer 95b has a porosity lower than that of the rear end side porous layer 83b and higher than that of the upper dense layer 91. The upper intermediate layer 95 covers a part of the outer lead portion 75 (a part in the extending direction).

[0049] The lower intermediate layer 96 has a front-end-side intermediate layer 96a and a rear-end-side intermediate layer 96b (see FIGS. 2, 3, and 5). The front-end-side intermediate layer 96a and the rear-end-side intermediate layer 96b are provided on the second surface 60b at positions different from the lower porous layer 84 (the front-end-side porous layer 84a and the rear-end-side porous layer 84b) and the lower dense layer 92 in the front-to-rear direction. Specifically, as described above, the front-end-side porous layer 84a, the front-end-side intermediate layer 96a, the lower dense layer 92, the rear-end-side intermediate layer 96b, and the rear-end-side porous layer 84b are provided in this order from the front end to the rear end of the second surface 60b.

[0050] The front-end-side intermediate layer 96a covers a region of the second surface 60b from the rear end of the front-end-side porous layer 84a to the front end of the lower dense layer 92. The left-right width of the front-end-side intermediate layer 96a is the same as the left-right width of the second surface 60b, and the front-end-side intermediate layer 96a covers the first surface 60a from the left end to the right end of the second surface 60b. The front end of the front-end-side intermediate layer 96a contacts the rear end of the front-end-side porous layer 84a, and the rear end of the front-end-side intermediate layer 96a contacts the front end of the lower dense layer 92. In this embodiment, a boundary B5 between the lower dense layer 92 and the front-end-side intermediate layer 96a and a boundary B6 between the front-end-side intermediate layer 96a and the front-end-side porous layer 84a each extend in the up-down direction (a direction perpendicular to the second surface 60b). The front-end-side intermediate layer 96 a has a lower porosity than the front-end-side porous layer 84 a and a higher porosity than the lower-side dense layer 92 .

[0051] The rear-end-side intermediate layer 96b covers a region of the second surface 60b from the rear end of the lower dense layer 92 to the front end of the rear-end-side porous layer 84b. The left-right width of the rear-end-side intermediate layer 96b is the same as the left-right width of the second surface 60b, and the rear-end-side intermediate layer 96b covers the second surface 60b from the left end to the right end of the second surface 60b. The front end of the rear-end-side intermediate layer 96b contacts the rear end of the lower dense layer 92, and the rear end of the rear-end-side intermediate layer 96b contacts the front end of the rear-end-side porous layer 84b. In this embodiment, a boundary B7 between the lower dense layer 92 and the rear-end-side intermediate layer 96b and a boundary B8 between the rear-end-side intermediate layer 96b and the rear-end-side porous layer 84b each extend in the up-down direction (a direction perpendicular to the second surface 60b). The rear end side intermediate layer 96b is a layer having a porosity lower than that of the rear end side porous layer 84b and higher than that of the lower dense layer 92.

[0052] The upper intermediate layer 95 (front-end-side intermediate layer 95a and rear-end-side intermediate layer 95b) and the lower intermediate layer 96 (front-end-side intermediate layer 96a and rear-end-side intermediate layer 96b) have different porosities from the porous layer 80, the upper dense layer 91, and the lower dense layer 92, but can be made of ceramics made of the materials exemplified for the porous layer 80. In this embodiment, the front-end-side intermediate layer 95a, the rear-end-side intermediate layer 95b, the front-end-side intermediate layer 96a, and the rear-end-side intermediate layer 96b are all made of alumina ceramics. The thickness of each of the front-end-side intermediate layer 95a, the rear-end-side intermediate layer 95b, the front-end-side intermediate layer 96a, and the rear-end-side intermediate layer 96b may be 5 μm or more and 40 μm or less, or 5 μm or more and less than 11 μm. The thickness of the front-end intermediate layer 95a may be the same as or different from the thickness of the upper dense layer 91 and / or the front-end porous layer 83a. The thickness of the rear-end intermediate layer 95b may be the same as or different from the thickness of the upper dense layer 91 and / or the rear-end porous layer 83b. The thickness of the front-end intermediate layer 96a may be the same as or different from the thickness of the lower dense layer 92 and / or the front-end porous layer 84a. The thickness of the rear-end intermediate layer 96b may be the same as or different from the thickness of the lower dense layer 92 and / or the rear-end porous layer 84b.

[0053] The lengths Lm1 to Lm4 in the front-to-rear direction of the front-to-rear intermediate layer 95a, the rear-to-rear intermediate layer 95b, the front-to-rear intermediate layer 96a, and the rear-to-rear intermediate layer 96b may be 0.1 mm or more. The lengths Lm1 to Lm4 may all be the same, or at least some may differ. The porosities of the front-to-rear intermediate layer 95a, the rear-to-rear intermediate layer 95b, the front-to-rear intermediate layer 96a, and the rear-to-rear intermediate layer 96b may all be the same, or at least some may differ. On the first surface 60a side of the element body 60, the difference in porosity between any two adjacent layers of the front-to-rear porous layer 83a, the rear-to-rear porous layer 83b, the upper dense layer 91, the front-to-rear intermediate layer 95a, and the rear-to-rear intermediate layer 95b is preferably 20% or less, and more preferably 5% or less. The porosity differences may be the same or different. On the second surface 60b side of the element body 60, the difference in porosity between any two adjacent layers among the front-end porous layer 84a, the rear-end porous layer 84b, the lower dense layer 92, the front-end intermediate layer 96a, and the rear-end intermediate layer 96b is preferably 20% or less, and more preferably 5% or less. The porosity differences may be the same or different.

[0054] Next, a method for manufacturing the gas sensor 10 thus configured will be described. After describing the method for manufacturing the sensor element 20, a method for manufacturing the gas sensor 10 incorporating the sensor element 20 will be described.

[0055] A method for manufacturing the sensor element 20 will now be described. First, a plurality of (six in this embodiment) unsintered ceramic green sheets corresponding to the element body 60 are prepared. Each green sheet is provided with cutouts, through-holes, grooves, etc., as needed, by punching or other processes, and wiring patterns, such as electrodes and outer lead portions 75, are screen-printed. Furthermore, unsintered porous layers that will become the upper porous layer 83 and the lower porous layer 84 after firing, unsintered dense layers that will become the upper dense layer 91 and the lower dense layer 92 after firing, and unsintered intermediate layers that will become the upper intermediate layer 95 and the lower intermediate layer 96 after firing, are also formed by screen printing on the surfaces of the green sheets corresponding to the first and second surfaces 60a, 60b. Then, the plurality of green sheets are stacked. The stacked plurality of green sheets constitutes the unsintered element body that will become the element body after firing, and includes unsintered porous layers, unsintered dense layers, and unsintered intermediate layers. Then, this unsintered element body is sintered to obtain the element body 60, which includes the upper porous layer 83, the lower porous layer 84, the upper dense layer 91, the lower dense layer 92, the upper intermediate layer 95, and the lower intermediate layer 96. Subsequently, the outer porous layer 85 is formed by plasma spraying, to obtain the sensor element 20.

[0056] The porosity of each layer of the upper porous layer 83, the lower porous layer 84, the upper dense layer 91, the lower dense layer 92, the upper intermediate layer 95, and the lower intermediate layer 96 can be adjusted, for example, by adjusting the particle size of the particles contained in the pattern forming paste of each layer, adjusting the particle size and content ratio of the pore-forming material, or adjusting the firing temperature and firing time when firing the laminate.

[0057] A manufacturing method for the gas sensor 10 incorporating the sensor element 20 will be described. First, the sensor element 20 is inserted axially into the through-hole of the cylindrical body 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 arranged in this order between the inner circumferential surface of the cylindrical body 41 and the sensor element 20. Next, the metal ring 46 is pressed to compress the powder compacts 45a and 45b. In this state, the reduced diameter portions 43c and 43d are formed to manufacture the element sealing body 40, sealing the gap between the inner circumferential surface of the cylindrical body 41 and the sensor element 20. Next, the protective cover 30 is welded to the element sealing body 40, and a nut 47 is attached to obtain the assembly 15. Next, a lead wire 55 passing through a rubber stopper 57 and a connector 50 connected to the lead wire 55 are prepared. The prepared connector 50 is connected to the rear end side of the sensor element 20, and the plurality of contact metal fittings 52 of the connector 50 are electrically connected to the corresponding connector electrodes among the plurality of upper connector electrodes 71 and the plurality of lower connector electrodes 72. Thereafter, the outer cylinder 48 is welded and fixed to the metallic shell 42, thereby obtaining the gas sensor 10.

[0058] Next, an example of how the gas sensor 10 configured as described above is described. When the gas sensor 10 is attached to the pipe 58 as shown in FIG. 1 and a measurement gas flows through the pipe 58, the measurement gas flows through the protective cover 30 and into the element chamber 33, exposing the front end of the sensor element 20 to the measurement gas. When the measurement gas passes through the porous layer 80 and reaches the outer electrode 64 and then the measurement gas inlet 61 and into the sensor element 20, the detection unit 63 generates an electrical signal corresponding to the NOx concentration in the measurement gas, as described above. This electrical signal is extracted via the upper and lower connector electrodes 71 and 72, and the NOx concentration is detected based on the electrical signal.

[0059] Next, a sensor element 20B of the comparative example will be described. FIG. 6 is a partially enlarged view showing a main portion of the sensor element 20B of the comparative example. The sensor element 20B of the comparative example differs from the sensor element 20 of the present embodiment in that the upper intermediate layer 95 and the lower intermediate layer 96 are replaced with an upper porous layer 83 and a lower porous layer 84. In the sensor element 20B, the upper dense layer 91 is in contact with the front end porous layer 83a and the rear end porous layer 83b of the upper porous layer 83, and the lower dense layer 92 is in contact with the front end porous layer 84a and the rear end porous layer 84b of the lower porous layer 84. The sensor element 20B can be manufactured in the same manner as the sensor element 20. In the sensor element 20B, if the difference in porosity between the upper dense layer 91 and the front-end porous layer 83a and the rear-end porous layer 83b is relatively large, the difference in residual stress due to the difference in firing shrinkage rate between the upper dense layer 91 and the front-end porous layer 83a and the rear-end porous layer 83b when firing the sensor element 20B will be relatively large, and there is a concern that cracks will occur in the sensor element 20B during manufacturing or use.

[0060] In contrast, the sensor element 20 of this embodiment includes a front-end-side intermediate layer 95a and a rear-end-side intermediate layer 95b of the upper intermediate layer 95, with the front-end-side intermediate layer 95a in contact with the upper dense layer 91 and the front-end-side porous layer 83a, and the rear-end-side intermediate layer 95b in contact with the upper dense layer 91 and the rear-end-side porous layer 83b. The lower intermediate layer 96 includes a front-end-side intermediate layer 96a and a rear-end-side intermediate layer 96b, with the front-end-side intermediate layer 96a in contact with the lower dense layer 92 and the front-end-side porous layer 84a, and the rear-end-side intermediate layer 96b in contact with the lower dense layer 92 and the rear-end-side porous layer 84b. This reduces the difference in porosity between the two layers in contact with each other, and reduces the difference in residual stress (stress acting at the boundary between the two layers in contact with each other) between the two layers in contact with each other, compared to the comparative example. As a result, it is possible to prevent cracks from occurring in the sensor element 20 during manufacturing or use of the sensor element 20.

[0061] In this case, by setting the difference in porosity between any two adjacent layers among the front-end-side porous layer 83a, the rear-end-side porous layer 83b, the upper dense layer 91, the front-end-side intermediate layer 95a, the rear-end-side intermediate layer 95b, the front-end-side porous layer 84a, the rear-end-side porous layer 84b, the lower dense layer 92, the front-end-side intermediate layer 96a, and the rear-end-side intermediate layer 96b to 20% or less, the difference in residual stress between the two adjacent layers can be further reduced, thereby further preventing cracks from occurring in the sensor element 20. Furthermore, by setting the difference in porosity between the two adjacent layers to 5% or less, the difference in residual stress between the two adjacent layers can be further reduced, thereby further preventing cracks from occurring in the sensor element 20. The inventors confirmed these facts through experiments, analyses, etc.

[0062] Here, the correspondence between the components of this embodiment and the components of the present invention will be clarified. The sensor element 20 of this embodiment corresponds to the sensor element of the present invention, the element body 60 corresponds to the element body, the upper connector electrode 71 and the lower connector electrode 72 correspond to the connector electrodes, the upper porous layer 83 and the lower porous layer 84 of the porous layer 80 correspond to the porous layers, the dense layer 90 (the upper dense layer 91 and the lower dense layer 92) correspond to the dense layers, and the intermediate layer 94 (the upper intermediate layer 95 and the lower intermediate layer 96) correspond to the intermediate layer.

[0063] The sensor element 20 included in the gas sensor 10 of this embodiment described above in detail includes a front-side intermediate layer 95a and a rear-side intermediate layer 95b of the upper intermediate layer 95, with the front-side intermediate layer 95a in contact with the upper dense layer 91 and the front-side porous layer 83a, and the rear-side intermediate layer 95b in contact with the upper dense layer 91 and the rear-side porous layer 83b. The lower intermediate layer 96 also includes a front-side intermediate layer 96a and a rear-side intermediate layer 96b, with the front-side intermediate layer 96a in contact with the lower dense layer 92 and the front-side porous layer 84a, and the rear-side intermediate layer 96b in contact with the lower dense layer 92 and the rear-side porous layer 84b. This reduces the difference in porosity between the two adjacent layers and reduces the difference in residual stress (stress acting on the boundary between the two adjacent layers) compared to when the upper dense layer 91 is in contact with the front-end porous layer 83 a and the rear-end porous layer 83 b and the lower dense layer 92 is in contact with the front-end porous layer 84 a and the rear-end porous layer 84 b, thereby preventing cracks from occurring in the sensor element 20.

[0064] It goes without saying that the present invention is not limited to the above-described embodiment, and can be embodied in various forms as long as they fall within the technical scope of the present invention.

[0065] For example, in the above-described embodiment, the boundary B1 between the upper dense layer 91 and the front-end intermediate layer 95a, the boundary B2 between the front-end intermediate layer 95a and the front-end porous layer 83a, the boundary B3 between the upper dense layer 91 and the rear-end intermediate layer 95b, the boundary B4 between the rear-end intermediate layer 95b and the rear-end porous layer 83b, the boundary B5 between the lower dense layer 92 and the front-end intermediate layer 96a, the boundary B6 between the front-end intermediate layer 96a and the front-end porous layer 84a, the boundary B7 between the lower dense layer 92 and the rear-end intermediate layer 96b, and the boundary B8 between the rear-end intermediate layer 96b and the rear-end porous layer 84b each extend in the vertical direction (perpendicular to the first surface 60a), but are not limited thereto. For example, at least some of the boundaries B1 to B8 may extend with an inclination toward the front or rear end relative to the vertical direction. 7 and 8 are partially enlarged views showing the main parts of sensor elements 120 and 220 of the modified examples.

[0066] The sensor element 120 of FIG. 7 will be described. In the sensor element 120, each of the boundaries B1 and B2 extends in a direction inclined toward the front end of the element body 60 and approaches the first surface 60a (downward). In other words, each of the boundaries B1 and B2 extends in a direction inclined toward the first surface 60a while the thicknesses of the upper dense layer 91 and the front-end-side intermediate layer 95a become thinner as the boundaries B1 and B2 extend in a direction inclined toward the rear end of the element body 60 and approaches the first surface 60a. In other words, each of the boundaries B3 and B4 extends in a direction inclined toward the rear end of the element body 60 and approaches the first surface 60a while the thicknesses of the upper dense layer 91 and the rear-end-side intermediate layer 95b become thinner as the boundaries B3 and B4 extend in a direction inclined toward the first surface 60a. In other words, each of the boundaries B5 and B6 extends in a direction inclined toward the front end of the element body 60 and approaches the second surface 60b (upward). In other words, at each of the boundaries B5 and B6, the thicknesses of the lower dense layer 92 and the front-end-side intermediate layer 96a become thinner as they extend closer to the second surface 60b. At each of the boundaries B7 and B8, the thicknesses of the lower dense layer 92 and the rear-end-side intermediate layer 96b become thinner as they extend closer to the second surface 60b, inclining toward the rear end of the element body 60. In other words, at each of the boundaries B7 and B8, the thicknesses of the lower dense layer 92 and the rear-end-side intermediate layer 96b become thinner as they extend closer to the second surface 60b. That is, in the sensor element 120, at each of the boundaries B1 to B4, the layer with the lower porosity of the two adjacent layers becomes thinner as it extends closer to the first surface 60a, and at each of the boundaries B5 to B8, the layer with the lower porosity of the two adjacent layers becomes thinner as it extends closer to the second surface 60b.

[0067] The sensor element 220 of FIG. 8 will be described. In the sensor element 220, the boundaries B1 to B8 extend in a direction obtained by inverting the boundaries B1 to B8 of the sensor element 120. Specifically, each of the boundaries B1 and B2 extends so as to incline toward the front end of the element body 60 and move away from the first surface 60a (upward). In other words, each of the boundaries B1 and B2 extends so as to move away from the first surface 60a while the thicknesses of the upper dense layer 91 and the front-end-side intermediate layer 95a become thinner. Each of the boundaries B3 and B4 extends so as to incline toward the rear end of the element body 60 and move away from the first surface 60a. In other words, each of the boundaries B3 and B4 extends so as to move away from the first surface 60a while the thicknesses of the upper dense layer 91 and the rear-end-side intermediate layer 95b become thinner. Each of the boundaries B5 and B6 extends so as to be inclined toward the front end of the element body 60 and move away from the second surface 60b (downward). In other words, each of the boundaries B5 and B6 extends so as to be away from the second surface 60b while the thicknesses of the lower dense layer 92 and the front end-side intermediate layer 96a become thinner. Each of the boundaries B7 and B8 extends so as to be inclined toward the rear end of the element body 60 and move away from the second surface 60b. In other words, each of the boundaries B7 and B8 extends so as to be away from the second surface 60b while the thicknesses of the lower dense layer 92 and the rear end-side intermediate layer 96b become thinner. That is, in the sensor element 220, the boundaries B1 to B4 extend so as to move away from the first surface 60a while the thickness of the layer with the lower porosity of the two adjacent layers becomes thinner, and the boundaries B5 to B8 extend so as to move away from the second surface 60b while the thickness of the layer with the lower porosity of the two adjacent layers becomes thinner.

[0068] In the sensor elements 120 and 220, the boundaries B1 to B8 extend at an incline in the up-down direction (the direction perpendicular to the first surface 60a or the second surface 60b). Therefore, the lengths Ld1 and Ld2 in the front-to-rear direction of the upper dense layer 91 and the lower dense layer 92, and the lengths Lm1 to Lm4 in the front-to-rear direction of the front-to-rear intermediate layer 95a, the rear-to-rear intermediate layer 95b, the front-to-rear intermediate layer 96a, and the rear-to-rear intermediate layer 96b can be defined as, for example, the length of the shortest portion of each layer in the front-to-rear direction.

[0069] In the sensor elements 120 and 220, the boundaries B1 to B8 extend at an incline in the vertical direction (perpendicular to the first surface 60a or the second surface 60b), and therefore the areas of the boundaries B1 to B8 are larger than those of the sensor element 20, in which the boundaries B1 to B8 extend in the vertical direction. This increases the allowable range of the residual stress difference between the two layers in contact with each other (the range of residual stress difference that can prevent cracks from occurring). As a result, the occurrence of cracks in the sensor elements 120 and 220 can be further prevented.

[0070] Furthermore, in the sensor element 120, the boundaries B1-B4 extend closer to the first surface 60a, with the thickness of the layer with the lower porosity of the two adjacent layers decreasing. The inventors have confirmed through experiments and analyses that the lower the porosity of a layer and the greater the layer's thickness, the greater the residual stress in the layer. Therefore, by determining the extending direction of the boundaries B1-B4 as in the sensor element 120, it is possible to suppress relatively large stresses from acting from the upper dense layer 91 and the front-end-side intermediate layer 95a on the first surface 60a of the element body 60 and the outer lead portion 75, and to suppress relatively large stresses from acting from the rear-end-side intermediate layer 95b on the second surface 60b of the element body 60, compared to the sensor element 20 in which the boundaries B1-B4 extend in the vertical direction and the sensor element 220 in which the boundaries B1-B4 extend from the first surface 60a on the lower porosity of the two adjacent layers decreasing in thickness.

[0071] In the above-described embodiment, one intermediate layer (specifically, one of the front-end intermediate layer 95a, the rear-end intermediate layer 95b, the front-end intermediate layer 96a, and the rear-end intermediate layer 96b) is provided at each of the four locations between the upper dense layer 91 and the front-end porous layer 83a, between the upper dense layer 91 and the rear-end porous layer 83b, between the lower dense layer 92 and the front-end porous layer 84a, and between the lower dense layer 92 and the rear-end porous layer 84b. However, the present invention is not limited to this. For example, multiple intermediate layers, i.e., first to Nth intermediate layers (N≧2) in order of decreasing porosity, may be provided at at least one of the four locations.

[0072] 9 is a partially enlarged view showing a main portion of a sensor element 320 according to a modified example. In the sensor element 320 shown in FIG. 9, the first surface 60a is provided with a front-end porous layer 83a, a front-end second intermediate layer 95a2, a front-end first intermediate layer 95a1, an upper dense layer 91, a rear-end first intermediate layer 95b1, a rear-end second intermediate layer 95b2, and a rear-end porous layer 83b, arranged in this order from the front end to the rear end. The upper dense layer 91 and the front-end first intermediate layer 95a1 are in contact with each other to form a boundary portion B11, the front-end first intermediate layer 95a1 and the front-end second intermediate layer 95a2 are in contact with each other to form a boundary portion B12, and the front-end second intermediate layer 95a2 and the front-end porous layer 83a are in contact with each other to form a boundary portion B13. The upper dense layer 91 and the rear-end first intermediate layer 95b1 are in contact with each other to form a boundary portion B14, the rear-end first intermediate layer 95b1 and the rear-end second intermediate layer 95b2 are in contact with each other to form a boundary portion B15, the rear-end second intermediate layer 95b2 and the rear-end porous layer 83b are in contact with each other to form a boundary portion B16, the lower dense layer 92 and the front-end first intermediate layer 96a1 are in contact with each other to form a boundary portion B17, the front-end first intermediate layer 96a1 and the front-end second intermediate layer 96a2 are in contact with each other to form a boundary portion B18, and the front-end second intermediate layer 96a2 and the front-end porous layer 84a are in contact with each other to form a boundary portion B19. The lower dense layer 92 and the rear-end-side first intermediate layer 96b1 are in contact with each other to form a boundary B20, the rear-end-side first intermediate layer 96b1 and the rear-end-side second intermediate layer 96b2 are in contact with each other to form a boundary B21, and the rear-end-side second intermediate layer 96b2 and the rear-end-side porous layer 84b are in contact with each other to form a boundary B22. Each of the boundaries B11 to B16 extends in the vertical direction (perpendicular to the first surface 60a), and each of the boundaries B17 to B22 extends in the vertical direction (perpendicular to the second surface 60b).

[0073] The front-end side first intermediate layer 95a1 and the rear-end side first intermediate layer 95b1 each have a higher porosity than the upper dense layer 91. The front-end side second intermediate layer 95a2 has a higher porosity than the front-end side first intermediate layer 95a1 and a lower porosity than the front-end side porous layer 83a. The rear-end side second intermediate layer 95b2 has a higher porosity than the rear-end side first intermediate layer 95b1 and a lower porosity than the rear-end side porous layer 83b. The front-end side first intermediate layer 96a1 and the rear-end side first intermediate layer 96b1 each have a higher porosity than the lower dense layer 92. The front-end side second intermediate layer 96a2 has a higher porosity than the front-end side first intermediate layer 96a1 and a lower porosity than the front-end side porous layer 84a. The rear end side second intermediate layer 96b2 has a porosity higher than that of the rear end side first intermediate layer 96b1 and lower than that of the rear end side porous layer 84b.

[0074] On the first surface 60a side of the element body 60, the difference in porosity between any two adjacent layers among the front-end porous layer 83a, the rear-end porous layer 83b, the upper dense layer 91, the front-end first intermediate layer 95a1, the front-end second intermediate layer 95a2, the rear-end first intermediate layer 95b1, and the rear-end second intermediate layer 95b2 is preferably 20% or less, and more preferably 5% or less. The porosity differences may all be the same, or at least some may differ. On the second surface 60b side of the element body 60, the difference in porosity between any two adjacent layers among the front-end porous layer 84a, the rear-end porous layer 84b, the lower dense layer 92, the front-end first intermediate layer 96a1, the front-end second intermediate layer 96a2, the rear-end first intermediate layer 96b1, and the rear-end second intermediate layer 96b2 is preferably 20% or less, and more preferably 5% or less. The porosity differences may all be the same, or at least some may differ.

[0075] Providing multiple intermediate layers, i.e., the first and second intermediate layers 95a1 and 95a2, between the upper dense layer 91 and the front-end-side porous layer 83a provides the following advantages over providing only one intermediate layer, i.e., the front-end-side intermediate layer 95a. When the porosity of the upper dense layer 91 is set (e.g., 5%) and the porosity of the front-end-side porous layer 83a is set (e.g., 35%), the difference in porosity between the two adjacent layers can be reduced, thereby reducing the difference in residual stress between the two adjacent layers. Furthermore, when the porosity of the upper dense layer 91 is set (e.g., 5%) and the difference in porosity between the two adjacent layers is set (e.g., 5%), the porosity of the front-end-side porous layer 83a can be increased while suppressing cracking in the sensor element 20. Increasing the porosity of the front-end-side porous layer 83a can reduce stress from the front-end-side porous layer 83a to the element body 60 and other components. Furthermore, since the porosity of the front-end-side porous layer 83 a can be increased, the degree of freedom in designing the porosity of the front-end-side porous layer 83 a can be increased. Although the description has been given with respect to the relationship between the upper dense layer 91 and the front-end-side porous layer 83 a, the same can be said for the relationship between the upper dense layer 91 and the rear-end-side porous layer 83 b, the relationship between the lower dense layer 92 and the front-end-side porous layer 84 a, and the relationship between the lower dense layer 92 and the rear-end-side porous layer 84 b.

[0076] In the sensor element 320 described above, each of the boundaries B11 to B16 extends in the vertical direction (perpendicular to the first surface 60a), and each of the boundaries B17 to B22 extends in the vertical direction (perpendicular to the second surface 60b), but this is not limiting. For example, each of the boundaries B11 to B22 may extend at an incline relative to the vertical direction, similar to the sensor elements 120 and 220 in Figures 7 and 8. Alternatively, some of the boundaries B11 to B22 may extend in the vertical direction, and the rest may extend at an incline relative to the vertical direction.

[0077] In the above-described embodiment, one intermediate layer (specifically, one of the front-end intermediate layer 95a, rear-end intermediate layer 95b, front-end intermediate layer 96a, and rear-end intermediate layer 96b) is provided at each of four locations: between the upper dense layer 91 and the front-end porous layer 83a, between the upper dense layer 91 and the rear-end porous layer 83b, between the lower dense layer 92 and the front-end porous layer 84a, and between the lower dense layer 92 and the rear-end porous layer 84b. However, this is not limiting. For example, gap regions may be provided instead of intermediate layers at some of the four locations. FIG. 10 is a partially enlarged view showing a main portion of a modified sensor element 420. The sensor element 420 in FIG. 10 includes gap regions 97 and 98 instead of the rear-end intermediate layers 95b and 96b of the sensor element 20 shown in FIGS. 1 to 5.

[0078] In the above-described embodiment, the rear-end-side porous layer 83b of the sensor element 20 covers the region of the first surface 60a from the rear end of the rear-end-side intermediate layer 95b to the rear end of the first surface 60a, excluding the region where the upper connector electrode 71 is present. However, this is not limited to this. For example, the rear-end-side porous layer 83b may cover the region from the rear end of the rear-end-side intermediate layer 95b to the front end of the upper connector electrode 71 or a predetermined position further forward. Also, the rear-end-side porous layer 83b may not be provided. In this case, the rear-end-side intermediate layer 95b may also not be provided. While the rear-end-side porous layer 83b has been described above, the same can be said for the rear-end-side porous layer 84b.

[0079] In the above-described embodiment, the sensor element 20 includes the outer porous layer 85, but this may not be included.

[0080] In the above-described embodiment, the element body 60 has a rectangular parallelepiped shape, but is not limited to this. For example, the element body 60 may have a cylindrical or columnar shape. In this case, the element body 60 has only one side surface.

[0081] In the above-described embodiment, the gas sensor 10 includes two powder compacts 45a and 45b and three insulators 44a to 44c, but the present invention is not limited to this. For example, the insulator 44b may be removed from the gas sensor 10. In this case, since the powder compacts 45a and 45b are adjacent to each other in the front-rear direction, the powder compacts 45a and 45b may be integrated.

[0082] In the above-described embodiment, the gas sensor 100 including the sensor elements 20, 120, etc. has been described. However, it goes without saying that the sensor elements 20, 120, etc. used in the gas sensor 10 may also be used. [Example]

[0083] Hereinafter, specific examples of fabricating a sensor element will be described as examples, but the present invention is not limited to the following examples.

[0084] [Comparative Example 1 and Examples 1 to 6] The sensor element 20B of the comparative example shown in FIG. 6 was fabricated by the same manufacturing method as the sensor element 20 of FIGS. 1 to 5 and was designated Comparative Example 1. The sensor element 20 of the embodiment shown in FIGS. 1 to 5 was fabricated by the same manufacturing method as Examples 1 to 4. The sensor element 320 of the modified example shown in FIG. 9 was fabricated by the same manufacturing method as Examples 5 and 6. That is, as shown in Table 1, in Comparative Example 1, similar to the sensor element 20B shown in FIG. 6, no intermediate layer was provided in each of the four positions between the upper dense layer 91 and the front-end side porous layer 83a, between the upper dense layer 91 and the rear-end side porous layer 83b, between the lower dense layer 92 and the front-end side porous layer 84a, and between the lower dense layer 92 and the rear-end side porous layer 84b. In Examples 1 to 4, one intermediate layer (such as the front-end side intermediate layer 95a) was provided in each of the four positions. In Examples 5 and 6, two intermediate layers (such as a front-end-side first intermediate layer 95a1 and a front-end-side second intermediate layer 95a2) were provided at each of the four locations. In Comparative Example 1 and Examples 1 to 6, the porosity of each of the upper dense layer 91 and the lower dense layer 92 was 5%. In Examples 1 to 6, the porosity difference between any two adjacent layers was the same, but was varied for each Example.

[0085] [Table 1]

[0086] In Comparative Example 1, the porosity of each of the upper porous layer 83 and the lower porous layer 84 was set to 35%. As a result, the difference in porosity between the two layers in contact with each other was 30%. In Example 1, the porosity of each of the upper porous layer 83 and the lower porous layer 84 was set to 15%. As a result, the difference in porosity between the two layers in contact with each other was 5%. In Example 2, the porosity of each of the upper porous layer 83 and the lower porous layer 84 was set to 25%. As a result, the difference in porosity between the two layers in contact with each other was 10%. In Example 3, the porosity of each of the upper porous layer 83 and the lower porous layer 84 was set to 35%. As a result, the difference in porosity between the two layers in contact with each other was 15%. In Example 4, the porosity of each of the upper porous layer 83 and the lower porous layer 84 was set to 45%. As a result, the difference in porosity between the two layers in contact with each other was 20%. In Example 5, the porosity of each of the upper porous layer 83 and the lower porous layer 84 was set to 20%, resulting in a porosity difference of 5% between the two layers in contact with each other. In Example 6, the porosity of each of the upper porous layer 83 and the lower porous layer 84 was set to 35%, resulting in a porosity difference of 10% between the two layers in contact with each other.

[0087] [Crack test] A test to evaluate crack resistance was conducted on the sensor elements of Comparative Example 1 and Examples 1 to 6. Specifically, 20 sensor elements of Comparative Example 1 were first prepared and placed in an autoclave and left for 5 hours under saturated steam at a temperature of 180°C. Thereafter, each of the 20 sensor elements was visually inspected using an optical microscope to determine whether or not cracks had occurred. The ratio of the number of sensor elements in which cracks had occurred to the 20 sensor elements was calculated as the crack rate. The crack rates were also calculated in the same manner for the sensor elements of Examples 1 to 6. The crack rate of Comparative Example 1 was set as a first reference value. In Examples 1 to 6, a case in which the crack rate was equal to or greater than the first reference value was judged as poor (C), a case in which the crack rate was less than the first reference value and equal to or greater than a second reference value less than the first reference value was judged as good (B), and a case in which the crack rate was less than the second reference value was judged as excellent (A).

[0088] As can be seen from Table 1, in the crack test, Examples 1 to 6, in which the difference in porosity between the two adjacent layers was 20% or less, were all evaluated as excellent (A) or good (B). This confirmed that by providing an intermediate layer between the dense layer and the porous layer and setting the difference in porosity between the two adjacent layers to 20% or less, it is possible to prevent cracks from occurring in the sensor element. Furthermore, Examples 2 to 4 and 6, in which the difference in porosity between the two adjacent layers was more than 5% and 20% or less, were evaluated as good (B), while Examples 1 and 5, in which the difference in porosity between the two adjacent layers was 5%, were evaluated as excellent (A). This confirmed that by setting the difference in porosity between the two adjacent layers to 5% or less, it is possible to further prevent cracks from occurring in the sensor element compared to when the difference is more than 5% and 20% or less.

[0089] Furthermore, as can be seen from Table 1, when comparing Example 1 and Example 5, in which the difference in porosity between the two adjacent layers is the same, it was confirmed that although the crack test evaluations were the same, Example 5, in which the number of intermediate layers is 2, can increase the porosity of the porous layer compared to Example 1, in which the number of intermediate layers is 1. [Industrial Applicability]

[0090] The present invention can be used in a gas sensor that detects the concentration of a specific gas such as NOx in a measurement gas such as exhaust gas from an internal combustion engine. [Explanation of symbols]

[0091] 10 Gas sensor, 15 Assembly, 20, 20B, 120, 220, 320, 420 Sensor element, 30 Protective cover, 31 Inner protective cover, 32 Outer protective cover, 33 Element chamber, 40 Element sealing body, 41 Cylindrical body, 42 Metal shell, 42a Thick wall portion, 42b Bottom surface, 43 Inner cylinder, 43a Flange portion, 43c, 43d Reduced diameter portion, 44a to 44c Insulator, 45a, 45b Powder compact, 46 Metal ring, 47 Nut, 48 Outer cylinder, 49 Space, 50 Connector, 51a, 51b Housing, 52 Contact metal, 53 Clamp, 55 Lead wire, 57 Rubber plug, 58 Pipe, 59 Fixing member, 60 Element body, 60a First surface, 60b Second surface, 60c Third surface, 60d Fourth surface, 60e Fifth surface, 60f Sixth surface, 61 Measurement gas inlet, 62 Reference gas inlet, 63 Detection unit, 64 Outer electrode, 65 Inner main pump electrode, 66 Inner auxiliary pump electrode, 67 Measurement electrode, 68 Reference electrode, 69 Heater, 71, 71a to 71d Upper connector electrode, 72 Lower connector electrode, 75 Outer lead portion, 80 Porous layer, 81 Inner porous layer, 83 Upper porous layer, 83a Front end porous layer, 83b Rear end porous layer, 84 Lower porous layer, 84a Front end porous layer, 84b Rear end porous layer, 85 Outer porous layer, 90 Dense layer, 91 Upper dense layer, 92 Lower dense layer, 94 Intermediate layer, 95 Upper intermediate layer, 95a Front end intermediate layer, 95a1 Front end side first intermediate layer, 95a2 front end side second intermediate layer, 95b rear end side intermediate layer, 95b rear end side first intermediate layer, 95b rear end side second intermediate layer, 96 lower intermediate layer, 96a front end side intermediate layer, 96a front end side first intermediate layer, 96a front end side second intermediate layer, 96b rear end side intermediate layer, 96b rear end side first intermediate layer, 96b rear end side second intermediate layer, 97 gap region, 100 gas sensor, B1 to B8, B11 to B22 boundary portion.

Claims

1. A sensor element for detecting the concentration of a specific gas in a measurement gas, an element body having a front end and a rear end which are both ends along a longitudinal direction and a side surface which is a surface along the longitudinal direction, the front end side being exposed to the measurement gas; a connector electrode disposed on the rear end side of the side surface for electrical connection to the outside; a porous layer covering a portion of the side surface in the front-rear direction; a dense layer having a lower porosity than the porous layer and covering the side surface at a position different from the porous layer in the front-rear direction and on the front end side of the connector electrode; an intermediate layer having a porosity lower than that of the porous layer and higher than that of the dense layer, the intermediate layer being in contact with the dense layer on one side of the front end side and the rear end side, and in contact with the porous layer on the other side of the front end side and the rear end side, and covering the side surface; A sensor element comprising:

2. 2. The sensor element according to claim 1, the difference in porosity between any two adjacent layers among the dense layer, the intermediate layer, and the porous layer is 20% or less; Sensor element.

3. 3. The sensor element according to claim 2, The difference in porosity between the two layers in contact with each other is 5% or less. Sensor element.

4. 2. The sensor element according to claim 1, The intermediate layers include first to Nth (N≧2) intermediate layers in order of decreasing porosity, the first to Nth intermediate layers are arranged between the dense layer and the porous layer in this order so that two adjacent layers are in contact with each other; Sensor element.

5. 5. The sensor element according to claim 4, a difference in porosity between any two adjacent layers among the dense layer, the first to Nth intermediate layers, and the porous layer is 20% or less; Sensor element.

6. 6. The sensor element according to claim 5, The difference in porosity between the two layers in contact with each other is 5% or less. Sensor element.

7. A sensor element according to any one of claims 1 to 6, at least one of a plurality of boundary portions, each of which is a boundary portion between two adjacent layers among the dense layer, the intermediate layer, and the porous layer, extends so as to approach the side surface while inclining toward the front end side or the rear end side; Sensor element.

8. 8. The sensor element according to claim 7, At least one of the plurality of boundary portions extends so as to approach the side surface while the thickness of the layer having a lower porosity of the two layers becomes thinner. Sensor element.

9. 9. The sensor element according to claim 8, an outer electrode disposed on the front end side of the side surface; an outer lead portion disposed on the side surface and electrically connecting the outer electrode and the connector electrode; The sensor element further comprises:

10. A gas sensor comprising the sensor element according to any one of claims 1 to 6.

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