Gas sensor element and gas sensor

Abstract

Gas sensor element (1) comprising a long plate-like element body (2) and a porous protective layer (3) that protects a surface of the element body (2), wherein the element body (2) has a gas collection part (4) at its end on one side of the end surface (21) in a longitudinal direction (Z), and the protective layer (3) includes an end surface part (32) covering one end surface (21) in laminate, side surface parts (33) covering the side surfaces (22) connected to one end surface (21) in laminate, and corner parts (34) at which two adjacent parts selected from the end surface part (32) and the side surface parts (33) meet, and an outer surface (31) of one or more selected from the end surface part (32) and the side surface parts (33) has a concave shape that is continuously flowing with the corner parts (34) and is configured such that the layer thickness increases towards the corner parts (34).

Description

Cross-reference to similar registration

[0001] The present application is based on the Japanese application JP 2019 - 039 693 A, filed on August 22, 2017, the contents of which are incorporated herein by reference. Technical field

[0002] The present disclosure relates to a gas sensor element with a porous protective layer and to a gas sensor which includes a gas sensor element. State of the art

[0003] The exhaust system of an automotive engine is equipped with a gas sensor for detecting specific exhaust gases, and emission control is implemented, for example, by executing combustion control based on the gas sensor's detection result. For example, the gas sensor comprises a long, plate-like gas sensor element housed in a cover body, and the gas sensor element is protected from the ingress of water droplets and toxins by a porous protective layer provided on its outer surface, as disclosed in PTL 1. The gas sensor element is configured by incorporating a heating device component within the element body, in which a gas detection element is provided.

[0004] The porous protective layer is typically formed, for example, by immersing the gas sensor element body in a protective coating material shaped like a slurry, resulting in a certain coating thickness. Additionally, according to this coating process, the thickness of the protective layer is reduced at the corners of the element body. Therefore, in PTL 1, for example, a first protective layer is pre-formed at the corners of the element body using a distributor or dispenser method. Subsequently, a second protective layer, covering the entire outer perimeter containing the first protective layer, is formed such that the thickness of the protective layer at the corners of the element body is greater than the thickness of the corners of the element body itself.

[0005] DE 10 2012 208 282 A1 discloses a gas sensor element, a main body, and a protective layer. The main body has four flat sections and four corner sections, each formed between an adjacent pair of flat sections. The four corner sections have a pair of first corner sections formed on one side of a porous, diffusion-resistant layer in a lamination direction of the main body, and a pair of second corner sections formed on one side of a heating layer in the lamination direction. The protective layer consists of an inner protective layer covering at least the four corner sections of the main body and an outer protective layer covering the entire outer edge of the main body and the inner protective layer. The protective layer has a greater average thickness at the four corner sections than at the flat sections of the main body.

[0006] DE 10 2014 115 638 A1 discloses a gas sensor element of an air / fuel ratio sensor, with gaps formed to separate a protective layer and the element body. At least four gaps are formed to correspond to four corner points of the front end of the element body. The gas sensor element has these gaps at the corner points of the front end of the element body, where the thickness of the protective layer is likely to be minimal. Therefore, it is possible to suppress breakage of the corner points of the front end of the element body, which could otherwise result from thermal shock caused by water adhering to the element. Compared to a conventional protective layer formed by an immersion process, the thickness, and therefore the heat capacity, of the protective layer of the gas sensor element can be reduced.

[0007] DE 10 2014 113 274 A1 discloses a gas sensor element and a gas sensor. The thickness of a first protective layer on a porous layer of a gas sensor element is greater than that of the first protective layer at a position located behind the porous layer, and the thickness of a second protective layer at a position located behind the porous layer is greater than that of the second protective layer above the porous layer. That is, the first protective layer is thinner in a section located behind the porous layer than in a section located on top of the porous layer. Conversely, the second protective layer is thicker in a section located behind the porous layer than in a section located above the porous layer. Thus, a rear end section of the coating layer can be thicker compared to a conventional coating layer.Therefore, even in the event of water adhering to the rear end section of the top layer, it is unlikely that the gas sensor element will develop cracks, thus improving resistance to water adhesion. List of prior art patent literature

[0008] PTL 1: JP 2019 - 039 693 A Summary of the invention

[0009] In recent years, emissions regulations for automobiles have become stricter, requiring the gas sensor to be activated at a rapid stage to further reduce emissions. To achieve this, however, the gas sensor must be activated at the moment the engine is started, which is likely to result in water ingress. Furthermore, the diameter of the through-holes in the cover body tends to increase to allow more gas to enter. In such cases, the stress on water ingress increases, and the resistance to water ingress deteriorates because the gas sensor element is repeatedly exposed to water, causing the diameter of the water droplets reaching it to increase.

[0010] Furthermore, it was determined that the surface of the protective layer tends to detach at such a time if part of a heated element is repeatedly exposed to water. Thus, detachment can gradually progress as the number of water exposures increases, even if no problem occurs immediately after a single water exposure. It was specifically determined that cracking of the element can occur if detachment progresses near the corners of the element and the thickness of the protective layer cannot be guaranteed.

[0011] It is an objective of the present disclosure to provide a gas sensor element and a gas sensor capable of reducing the detachment of the porous protective layer covering the surface of the element body, even in an environment where repeated exposure to water occurs, and capable of achieving both rapid activation and resistance to water exposure.

[0012] One aspect of the present revelation is: A gas sensor element comprising a long, plate-like element body and a porous protective layer that protects a surface of the element body, wherein the element body has a gas collection part at its end on one side of the end surface in a longitudinal direction, and the protective layer includes an end surface part covering one end surface in laminate, side surface parts covering side surfaces connected to one end surface in laminate, and corner parts at which two adjacent parts selected from the end surface part and the side surface parts meet, and an outer surface of one or more selected from the end surface part and the side surface parts has a concave shape that is continuously flowing with the corner parts and is configured such that the layer thickness increases towards the corner parts.

[0013] Another aspect of the present revelation is: A gas sensor comprising the gas sensor element described above for detecting a specific gas component in a measured gas, comprising a cylindrical housing that supports an outer circumference of the gas sensor element, and a cover body that is attached to one end of the cylindrical housing, wherein one end of the gas sensor element, on which the protective layer is provided, is enclosed in the cover body, and the measured gas is introduced into the cover body through a through-hole provided in the cover body. Effects of the invention

[0014] For rapid activation of the gas sensor element, it is advantageous that the heat capacity of the protective layer covering the element body is low. The outer surface of the protective layer is configured to have a concave shape, thereby partially reducing the layer thickness and thus its mass and heat capacity. Additionally, it is possible to reduce the heat capacity while ensuring sufficient layer thickness at the element corners, which are relatively poorly protected against moisture, since the concave shape of the outer surface of the protective layer results in a relatively large layer thickness at these corners.

[0015] When the protective layer is exposed to water, the water droplets adhering to its surface are drawn into the interior and simultaneously evaporate, dissipating the heat capacity of the protective layer. If there is a section on the surface of the protective layer where seepage, absorption, and evaporation tend to occur, repeated exposure to water can cause localized delamination. However, the water droplets spread easily across the surface of the protective layer with a concave shape. Furthermore, it is possible to prevent thermal stress or heat buildup in a section that could cause delamination or similar issues by ensuring the surface shape is continuously smooth and seamless, including the corners.

[0016] Thus, the heat capacity can be reduced to enable rapid activation while still protecting the element corners, since the end surface part or the side surface parts of the protective layer covering the element body are configured to have a concave shape that is continuously flowing with the corner parts.

[0017] A gas sensor employing such a gas sensor element can be freed from the restrictions on the arrangement and size of the through-holes in the cover body intended to protect the gas sensor element. For example, the diameter of the through-holes in the cover body can be increased to allow for a greater volume of gas to enter. This enables the detection of the specific gas component in the measured gas with good responsiveness.

[0018] Therefore, according to the aspects described above, it is possible to provide a gas sensor element and a gas sensor that are capable of reducing the detachment of the porous protective layer covering the surface of the element, even in an environment where repeated exposure to water occurs, and that are capable of achieving both rapid activation and resistance to water exposure. Brief description of the drawings

[0019] The foregoing and other tasks, features, and advantages of the present disclosure will become clear from the following detailed description with reference to the accompanying drawings. These show: Fig. Figure 1 shows a schematic overall view of a gas sensor element and an enlarged view of its main part according to the first embodiment; Fig.2 an enlarged perspective view showing the configuration of the main part of the gas sensor element according to the first embodiment; Fig. 3 a complete cross-sectional view of a gas sensor comprising the gas sensor element according to the first embodiment; Fig. 4 a cross-sectional view showing the structure of the gas detection part of the gas sensor element according to the first embodiment; Fig. 5 an enlarged cross-sectional view of the main part to show an example of the shape of the end surface part of the protective layer of the gas sensor element according to the first embodiment; Fig. 6 an enlarged cross-sectional view of the main part to show an example of the shape of the side surface part of the protective layer of the gas sensor element according to the first embodiment; Fig.7 an enlarged view of the main part to show another example of the configuration of the main part of the gas sensor element according to the first embodiment; Fig. 8 a schematic view to explain the mechanism of surface detachment caused by water exposure of the gas sensor element according to the first embodiment; Fig. 9 a schematic view to illustrate the relationship between the surface shape of the protective layer of the gas sensor element according to the first embodiment and the heat load; Fig. 10 a view showing the manufacturing process of the gas sensor element according to the first embodiment; Fig. 11 an enlarged perspective view of the main part showing the shape of the protective layers of the gas sensor element samples used in the water exposure test carried out in Experiment 1; Fig.12 an enlarged cross-sectional view of the main part of the gas sensor element to explain the procedure of the water exposure test carried out in test example 1; Fig. 13 a graph showing the relationship between the radius of curvature of the outer surface of the protective layer and the number of water exposures in test example 1; Fig. 14 a schematic view to explain the relationship between the radius of curvature of the outer surface of the protective layer and the ease with which a water droplet spreads in experimental example 1; Fig. 15 an enlarged view of the main part, showing the configuration of the main part of the gas sensor element according to the second embodiment; Fig. 16 an enlarged view of the main part, showing another example of the configuration of the main part of the gas sensor element according to the second embodiment; Fig.17 an enlarged cross-sectional view of the main part showing an example of the shape of the end surface part of the protective layer of the gas sensor element according to the second embodiment; Fig. 18 an enlarged cross-sectional view of the main part showing an example of the shape of the side surface part of the protective layer of the gas sensor element according to the second embodiment; Fig. 19 an enlarged cross-sectional view of the main part, which schematically shows the shape of the side surface parts of the protective layer of the gas sensor element according to reference examples; Fig. 20 an enlarged cross-sectional view of the main part, which schematically shows the shape of the end surface part of the protective layer of the gas sensor element according to reference examples; Fig.21 an enlarged perspective view of the main part, showing the shape of the protective layers of the gas sensor element samples used in the water exposure test carried out in Experiment 2. Fig. 22 An enlarged cross-sectional view of the main part of the gas sensor element to illustrate the procedure of the water exposure test carried out in Experiment 2. Description of the embodiments: First embodiment

[0020] Embodiments relating to a gas sensor element and a gas sensor are described with reference to the Fig. 1 to 15 are described. The gas sensor element 1, which is located in the Fig. 1 and Fig. Figure 2 shows the main part of the gas sensor S, which is located in Fig.Figure 3 shows the gas sensor S, which is inserted into a cylindrical insulator I such that its outer circumference is supported by the cylindrical housing H. The gas sensor S can, for example, be applied to an exhaust gas purification system for an automotive engine to detect the concentration of a specific gas in the exhaust gas. More precisely, it can be used as an oxygen sensor to detect an oxygen concentration, as an air-fuel ratio sensor to detect the air-fuel ratio (i.e., A / F) based on an oxygen concentration, and so on.

[0021] In Fig.1 The gas sensor element 1 comprises a long, plate-like element body 2, which is held within the insulator I, and a porous protective layer 3, which protects the surface of the element body 2. The longitudinal direction Z of the element body 2 is the vertical direction in the drawing, and the element body 2 has at one end, on the side of the tip end face 21, which is one of its end faces in the longitudinal direction Z (that is, the lower side of the end face or end face side in the Fig. 1) a gas collection part 4. The element body 2 can, for example, have a rectangular parallelepiped shape, and the directions of the two sides of its rectangular cross-section (that is, the directions that are orthogonal to the longitudinal direction Z) are subsequently referred to as a direction X of the long side and a direction Y of the short side.

[0022] The protective layer 3 is designed to cover the outer perimeter of the end of the element body 2 on the side of the pointed end surface 21, which projects from the insulator I. The protective layer 3 comprises an end surface part 32, which covers the pointed end surface 21 of the element body 2 in laminate; a plurality of side surface parts 33, which cover the side surfaces 22 that are smoothly continuous with the pointed end surface 21 in laminate; and corner parts 34, each of which is connected to two adjacent end surface part 32 and side surface parts 33. One or more of the end surface part 32 and side surface parts 33 have a concave shape such that the outer surface 31 is smoothly continuous with the corner parts 34, and the layer thickness increases as it approaches the corner parts 34.

[0023] The gas sensor S is used in an environment exposed to the exhaust gas, which is the gas to be measured. The protective layer 3 in the gas sensor element 1, which covers the element body 2, protects the element body 2 from condensed water and toxins contained in the exhaust gas. If the outer surface 31 of the end surface part 32 and the side surface parts 33 of the protective layer 3 are formed in the desired concave shapes and are designed such that the corner parts 34 have the maximum thickness, both rapid activation and resistance to water exposure can be achieved.

[0024] The detailed configurations of the protective layer 3 and the outer surface 31 formed in the gas sensor element 1 will be described later.

[0025] In Fig.3. The gas sensor S has a cylindrical housing H, the axial direction of which is the longitudinal direction Z of the gas sensor element 1 (i.e., the vertical direction in the figure), and the gas sensor element 1 is inserted into and held in the housing H. For the gas sensor S and the gas sensor element 1, one end face, which has the gas detection part 4, is referred to as the tip end face (i.e., the lower end face in the drawing), and the opposite end face is referred to as the base end face (i.e., the upper end face in the drawing). An element cover S1, acting as a cover body, is attached to the tip end face of the housing H, and the tip end of the gas sensor element 1 projects from the housing H and is housed in the element cover S1.Similarly, a cover S2 is attached on the atmosphere side to the base end of the housing H (that is, the upper end in the drawing), and the base end of the gas sensor element 1 protrudes from the housing H and is housed in the cover S2 on the atmosphere side.

[0026] The element cover S1 is designed as inner and outer double or double-walled cylinders with a base and is arranged to surround the circumference of the tip end of the gas sensor element 1. The inner cover S11 and the outer cover S12 of the element cover S1 are provided with through-holes S13 and S14, respectively, which serve as exhaust gas inlet / outlet holes on the side and base surfaces. When the exhaust gas, having passed through the through-holes S13 and S14, reaches the surface of the gas sensor element 1, it is drawn into the interior via the protective layer 3. The cylindrical cover S2 on the atmosphere side is provided with a through-hole S21 that opens on the outer peripheral side surface to serve as an atmosphere hole and receives atmospheric air inside.

[0027] The outer circumference of the intermediate part of the gas sensor element 1 is held within the cylindrical insulator I, which is housed in the casing H, and the sealing glass I1 is inserted between the opening of the insulator I at the base end and the gas sensor element 1. The large-diameter intermediate part of the insulator I is supported on a stepped portion of the casing H, and talc powder I2 is inserted between the outer peripheral surface of the insulator I and the inner peripheral surface of the casing H. Subsequently, the thinned portion of the base end of the casing H is crimped, and a cylindrical insulating component I3 is inserted to secure the insulator.

[0028] A plurality of connecting wires R1 and R2, which are connected to an external machine control unit (not shown), are insulated and held at the base end opening of the cover S2 on the atmospheric side. Connection parts R11 and R12 are provided at the tip end of the connecting wires R1 and R2, and these are electrically connected to the electrode connection parts 41 and 42 (compare, for example, Fig. 1), which are provided at the base end of the gas sensor element 1. Furthermore, the gas sensor element 1 includes a heating device part 5, which is received in the element body 2 on the side of the tip end face 21, as shown in Fig.Figure 2 shows the heating device part 5, which includes a heating device electrode 51 and a connecting line part 52 for excitation. External excitation causes the heating device electrode 51 to generate heat, thus activating the section corresponding to the gas detection part 4 of the element body 2 at an activation temperature.

[0029] As in Fig.As shown in Figure 4, the gas detection part 4 of the gas sensor element 1 comprises, for example, an oxide-ion conductive solid electrolyte body 11, an electrode 12 on the side of the measured gas, which is provided on the surface of the solid electrolyte body 11 on the side of the measured gas and into which the gas to be measured is introduced via a porous diffusion resistance layer 14, and an electrode 13 on the side of the reference gas, which is provided on the surface of the solid electrolyte body 11 on the side of the reference gas, in order to be arranged facing a reference gas chamber 10. The porous diffusion resistance layer 14 is formed by producing a part of the diffusion resistance layer, which forms the layer 15 that is laminated over the solid electrolyte body 11, with a porous body, and this is connected to a (not shown) gas inlet.A dense shielding layer 16 is laminated over the surface of the porous diffusion resistance layer 14 opposite the solid electrolyte body 11.

[0030] A layer 17 forming the reference gas chamber 10 has a heating device substrate 53 laminated on the surface opposite the solid electrolyte body 11, and the heating device electrode 51 is embedded in the heating device substrate 53 to form the heating device part 5. The element body 2 is formed by successively laminating the heating device substrate 53, the layer 17 forming the reference gas chamber, the solid electrolyte body 11, the porous diffusion resistance layer 14, the layer 15 forming the diffusion resistance layer, and the shielding layer 16.

[0031] Although the element body 2 has a rectangular cross-sectional shape in this example, it can also have a polygonal or multi-sided cross-sectional shape. Instead of forming the two ends on the side of the gas collection part 4 and the right-angled corners on the side of the heating device part 5, as shown in the figure, these can, for example, be chamfered to form a hexagonal or octagonal cross-sectional shape. In such a case, the protective layer 3 is provided to conform to the shape of the element body 2, and the side surface parts 33 and the corner parts 34, in the form of a layer covering the side faces 22 of the polygon, are formed in laminate.

[0032] The solid electrolyte body 11 is, for example, made of a solid electrolyte based on zirconium dioxide, and the heating device substrate 53, the layer 17 forming the reference gas chamber, the porous diffusion resistance layer 14, the layer 15 forming the diffusion resistance layer and the shielding layer 16 are, for example, made of an insulating ceramic such as aluminium oxide.

[0033] This results in the sensor output of the gas detection section 4 providing a sensor output exhibiting limiting current characteristics, corresponding to the oxygen concentration, when the exhaust gas is introduced via the porous diffusion resistance layer 14 into the electrode 12 on the side of the measured gas, and a specific voltage is applied between the electrode 12 on the side of the measured gas and the electrode 13 on the side of the reference gas on the side of the reference gas chamber 10, into which atmospheric air is introduced. By utilizing this, an air-fuel ratio signal corresponding to the oxygen concentration in the exhaust gas can be obtained.

[0034] Next, the detailed structure of protective layer 3 will be described.

[0035] As in Fig.As shown in Figure 1 in an enlarged manner, the protective layer 3 covers the outer circumference of the end of the element body 2 on the side of the tip end face 21 in laminate; that is, it covers the entire tip end face 21 as well as the end portions of the side faces 22 to which it is connected. The outer shape of the protective layer 3 conforms to the outer shape of the element body 2, and the protective layer 3 is larger than the element body 2 by its own thickness. The near half has a shape generally similar to that of the element body 2, and the tip half has a tapered shape, the thickness of which increases towards the side of the tip end face 21.

[0036] As in Fig.As shown in Figure 2, the protective layer 3 has an end surface part 32 that covers the pointed end surface 21 in the laminate, and four side surface parts 33 that cover the four side surfaces 22 connected to the pointed end surface 21 in the laminate. The corner parts 34 are formed between the end surface part 32 and each of the side surface parts 32 or between two adjacent surface parts 32. The corner parts 34 are located outside element corners 23 that are formed between the pointed end surface 21 and the side surfaces 22 of the element body 2 or between two adjacent side surfaces 22.

[0037] The end surface part 32 of the protective layer 3 is formed outside the tip end surface 21 of the element body 2 in order to have a generally rectangular outer shape (compare for example Fig.1), which is larger than that of the tip end surface 21 and whose four corners project outwards. The outer surface 31 of the end surface part 32 is formed in a concave shape that is continuously connected to the corner parts 34 formed between it and the four side surface parts 33, and the layer thickness increases towards the corner parts 34 at the two ends in both the X direction of the long side and the Y direction of the short side of the rectangle. As in Fig. As shown in Figure 1, the outer shape of the end 35 of the protective layer 3 on the base end side is essentially similar to the outer shape of the end surface part 32, but the former is smaller.

[0038] Similarly, the side surface parts 33 of the protective layer 3 are provided outside the side surfaces 22, which are connected to the tip end surface 21 of the element body 2, and these have an essentially trapezoidal outer shape that is larger than that of the side surfaces 22 and tapers towards the tip end face (that is, the upper end face in Fig. 2) to widen. The outer surface 31 of each of the side surface parts 33 is also formed in a concave shape, which is continuously flowing with the corner parts 34 formed between two adjacent side surface parts 33 or between the side surface part and the end surface part 32, and the layer thickness increases towards the corner part 34 on the tip end side in the longitudinal direction Z, and the layer thickness increases towards the corner parts 34 at the two ends in the direction X of the long side or the direction Y of the short side.

[0039] Preferably, the outer surface 31, selected from the end surface part 32 and the side surface parts 33 of the protective layer 3 covering each surface of the element body 2, is formed in a concave shape that is continuously flowing with the corner parts 34. More precisely, for example, the entire surface can be a continuously curved surface, or at least a part of it can be a continuously curved surface connected with the corner parts 34. In this case, the concave shape with a continuously curved surface is sufficient if it has such a shape that the outer surface 31 of the protective layer 3 has a contour line 311 in a cross-section in the longitudinal direction Z or in a direction perpendicular to it, which includes a continuously curved part.

[0040] For example, in the longitudinal section of the end surface part 32, the protective layer 3, which is in Fig.As shown in Figure 5, the contour line 311 of the outer surface 31 is a flowing, continuous curved line extending between the corner parts 34 at the two ends, and the layer thickness t is thicker at the two corner parts 34. Similarly, in the longitudinal section of the side surface parts 33 of the protective layer 3, which is shown in Fig. As shown in Figure 6, the contour line 311 of the outer surface 31 is a flowing, continuous curved line extending from the corner part 34 on the tip end face to the base end face, and the layer thickness t is greater at the corner part 34 on the tip end face. The cross-section of the side surface parts 33 in the direction orthogonal to the longitudinal direction Z (that is, the direction X of the long side or the direction Y of the short side) resembles the cross-section of the end surface part 32, which is shown in Figure 6. Fig.5 is shown. These contour lines 311 may be partially linear, but it is desirable that they do not exhibit extreme changes in shape, such as a rounded abrupt turning point or a turning point.

[0041] It should be noted that the outer surface 31 of the end surface part 32 and the side surface parts 33 of the protective layer 3 are not limited to the shapes shown in the Fig. 1 and Fig. 2 are shown, and these can have any suitable concave shape that is continuously and smoothly connected to the corner parts 34. For example, the thickness of the side surface part 33 of the protective layer 3 in the Fig. 1 and Fig.2 towards the tip end face, and the near half of this has an essentially constant outer shape, the layer thickness of which does not change in the cross-section in the longitudinal direction Z. However, the side surface part 33 of the protective layer 3 can have a shape such as shown in the left diagram of Fig.Figure 7 shows that the layer thickness also increases towards the end 35 on the base end face in the longitudinal direction Z. In this case, the layer thickness is greatest at the two ends in the longitudinal direction Z, that is, at the end part 35 on the base end face and at the corner part 34 on the outer circumference of the end surface part 32 on the tip end face, and the thickness gradually decreases towards the middle part in the longitudinal direction Z. In the X direction of the long side or the Y direction of the short side, the layer thickness increases as it approaches the corner parts 34 at the two ends. That is, these have a concave shape, which is entirely formed from a flowing curved surface.

[0042] Alternatively, the side surface parts 33 of the protective layer 3 can have a shape as shown in the diagram on the right. Fig.Figure 7 shows that the layer thickness in the cross-section in the longitudinal direction Z is essentially constant. In the cross-section in the direction X of the long side or the direction Y of the short side, the side surface parts 33 have a layer thickness that increases towards the corner parts 34 at the two ends, as in the previous case, and these have a generally flowing concave shape. Here, the shape of the end surface part 32, which corresponds to the side surface parts 33 of the left diagram and the right diagram of Fig. 8 corresponds to a flowing concave shape similar to that of the end surface part 32, which in both cases is in the Fig. 1 and Fig. 2 is shown.

[0043] If the end surface part 32 and the side surface parts 33 of the protective layer 3 are formed into concave shapes, as described above, the layer thickness is greatest at the corner parts 34, which are located on the outer peripheral edges of each surface part. This makes it possible to protect the element corners 23, which are the parts most vulnerable to water exposure. Since the layer thickness can be reduced at parts other than the corner parts 34, the mass of the protective layer 3 is also reduced, leading to a reduced heat capacity and enabling earlier activation. Furthermore, an effect can be achieved in which the progression of delamination is prevented when water is present, as the water droplets spread out on the flowing concave outer surface 31. The delamination phenomenon and the effect of the shape of the protective layer 3 will be described next.

[0044] When the gas sensor S is activated, the gas detection part 4 is heated to an activation temperature by the heating device part 5 of the gas sensor element 1, which is located in Fig. 2 is shown, which is arousing. In Fig. 8 The surface temperature is, for example, between 400 °C and 600 °C when the protective layer 3 is in a stable, high-temperature state, and when condensation (for example, approximately 60 °C) penetrates the cover body S1 of the gas sensor S, some of it adheres as a water droplet W to the outer surface 31 of the protective layer 3 (compare, for example, Fig. 8 (1)). The water droplet W spreads out on the protective layer 3 and is absorbed into the outer surface 31 and evaporates at the same time (compare, for example, Fig. 8 (2)).

[0045] When the water droplet W is drawn in, a difference in thermal expansion occurs in the protective layer 3, and therefore, due to the difference in thermal expansion, a [missing information] occurs near the point in the protective layer 3 to which the penetrated water extends (compare, for example, Fig. 8 (3)), generates a load. This thermal load σ is expressed by the following equation 1. In equation 1, α is the coefficient of linear expansion, E is the Young's modulus or modulus of elasticity, and ΔT is the temperature change. σ=α×E×ΔT

[0046] This means that a heat load is generated between the low-temperature part 3L, into which the water droplet W has penetrated, and the high-temperature part 3H, which is in contact with the lower part of the low-temperature part, and the low-temperature part 3L undergoes thermal contraction. If this thermal contraction occurs repeatedly at the same location due to water exposure, fatigue failure occurs within the protective layer 3, resulting in delamination (compare, for example, Fig. 8 (4)).

[0047] More precisely, the protective layer 3 is a porous body, and a large number of pores exist between the ceramic material particles 3P. Due to these pores, the seepage or infiltration paths of the water droplet W, which forms within the protective layer 3, include gaps between distorted particle surfaces and particles. If the propagation of the water droplet W is prevented at such points, the stress tends to increase. It should be noted that cracking within or between particles will progress and result in delamination due to fatigue failure if repeated water exposure occurs at such stress concentration points.

[0048] Therefore, it is desirable that the shape be able to reduce the heat stress caused by water droplets W seeping into the protective layer 3, in order to prevent detachment due to repeated water exposure. More precisely, the water droplet W spreads out in essentially the same simple manner as in the case of the flat surface 36 shown in the left-hand diagram of Fig. 9 is shown, and the contact area with the water droplet W increases or enlarges, as shown in the middle diagram of Fig. Figure 9 shows that the outer surface 31 of the protective layer 3 is a flowing, curved, concave surface. On the other hand, the water droplet W does not spread out and the contact area becomes small if the radius of curvature is small, even if the outer surface 31 is curved, as in the diagram on the right. Fig.Figure 9 shows that if the infiltration and evaporation of the water droplet W proceed simultaneously as described above, the heat capacity dissipated will be the same regardless of the size of the contact area, provided the amount of water exposure is the same. Therefore, in the case of an outer surface 31 with a small radius of curvature and a small contact area, ΔT in Equation 1 tends to increase, and the heat load σ tends to increase, because the evaporation occurs locally.

[0049] Thus, the outer surface 31 of the protective layer 3 is preferably a smooth and gently concave surface to reduce heat stress upon exposure to water, and it is desirable that this surface shape does not have a section with a small radius of curvature. Preferably, the end surface part 32 or the side surface parts 33 of the protective layer 3 have a shape whose radius of curvature at the minimally rounded section of the contour line 311, which forms the outer surface 31 in the cross-section in the longitudinal direction Z or the direction X of the long side or the direction Y of the short side, which run orthogonally to it, is greater than or equal to 0.4 mm.

[0050] Next, a method for manufacturing the gas sensor element 1 will be described.

[0051] As in Fig.As shown in Figure 10, a forming process can be adopted as the process in which the protective layer 3 is provided on the surface of the element body 2. In the step shown in (1), a slurry 200 containing a ceramic material forming the protective layer 3 is first injected into a mold 100 in the shape of a container. The mold 100 can, for example, include two mold sections 101 and 102 with a split structure, and the hollow part 103 formed at the adjacent parts of the two mold sections 101 and 102 can have a shape corresponding to the outer shape of the protective layer 3. For example, the protective layer 3 formed in Fig. As shown in Figure 1, the inner surface of each of the tip halves of the two molds 101 and 102 is formed by shaping it in a tapered form that extends outwards towards the tip end.

[0052] The slurry 200 is a protective layer-forming material prepared by adding an inorganic binder, a coagulant, and / or the like to the ceramic material to form or build up the protective layer 3. In the step shown in (2), the element body 2 of the gas sensor element 1 is inserted into the mold 100, into which the slurry 200 has been injected, starting from the upper opening of the hollow part. After being positioned and held using a clamping device (not shown) or the like, the slurry 200 is temporarily cured. Then, in the step shown in (3), the two molds 101 and 102 are opened to remove the gas sensor element 1, whose element body 2 is covered with the pre-cured slurry 200, and the gas sensor element is fired to form the protective layer 3.

[0053] Apart from the heat drying process, the curing process can be carried out by adding a curing agent, such as a UV resin or a thermosetting resin, to the slurry 200 and applying UV irradiation or heat curing. As described above, the protective layer 3, which has an outer surface 31 with a desired concave shape, can be formed with high accuracy when demolding is used, since the mold 100 can be pre-formed into a shape corresponding to the protective layer 3. Alternatively, it is also possible to use the mold 100 to form the protective layer 3 and then shape the outer surface 31 of the protective layer into a concave shape or any desired shape by cutting or the like. Experiment 1

[0054] Samples of the gas sensor element 1, prepared by the procedure described above, were tested to evaluate the water resistance performance of the concave shape of the protective layer 3. As described in Fig.As shown in Figure 11, one of the side surface portions 33 of the protective layer 3 of each sample of the gas sensor element 1 has a curved concave surface, and the contour line 311 of the outer surface 31 in the cross-section in the longitudinal direction Z is a continuous curved line. The thickness of the protective layer 3 increases as it approaches the corner portion 34 on the tip end face. Near the tip end face 21 of the element body 2, the contour line 311 has a minimally rounded section 312 with the smallest radius of curvature. Here, the thickness of the side surface portion 33 is essentially constant except near the corner portion 34. Samples 1 to 8 with minimally rounded sections 312 exhibiting different radii of curvature were prepared by varying the curvature shape of the contour lines 311, and the following repeated water exposure test was performed on these.

[0055] As in Fig.As shown in Figure 12, during the repeated water exposure test, the heating device part 5 was energized so that the gas sensor element 1 reached a predetermined control temperature, and a predetermined quantity of water droplets W, whose temperature was controlled to approximately 50 °C to 70 °C, was repeatedly dripped from a dispenser D onto the minimally rounded section 312 of the protective layer 3. The distributor device or dispenser D was positioned at a predetermined height h from the dripping position, and if there was more than one minimally rounded section 312, the water droplet was dripped onto the position closest to the heat-generating center of the heating device.The dripping interval was set to a time that allowed the surface of the protective layer 3 to regain a stable temperature after the water droplet W was applied. The surface condition of the protective layer 3 was monitored by images or video during the dripping process, and the number of water exposures required to cause the protective layer 3 to detach was counted. The control temperature of the gas sensor element 1 and the layer thickness of each section of the protective layer 3 were as follows, and the surface temperature of the protective layer 3 was measured with an infrared thermometer. Controlled temperature: 750 °C Water exposure amount: 2 µL Layer thickness at corner piece 34: approximately 250 µm Layer thickness of a side surface part 33 except near a corner part 34: approximately 200 µm Height h of dispenser D: 30 mm

[0056] As shown in Table 1, the results of the repeated water exposure test demonstrate that it is possible to withstand thousands of repeated water exposures, even when 2 µL water droplets W are repeatedly applied to the same spot, provided that the outer surface 31 of the protective layer 3 has a curved concave shape and the layer thickness at the corner part 34 is increased. Furthermore, the number of water exposures required to cause delamination increases rapidly in the region b where the radius of curvature is greater than or equal to 0.4 mm, as shown in Fig.Figure 13 shows the relationship between the radius of curvature of the minimally rounded section 312 and the number of water impacts, and the number of water impacts increases as the radius of curvature increases until it converges to approximately 2 mm. It can be considered that this is because, in the region a where the radius of curvature is less than 0.4 mm, gravity and surface tension acting on the water droplet W prevent the water droplet W from spreading, as shown in the left-hand figure (a) of Fig. Figure 14 shows that the shape of the water droplet W does not change significantly. On the other hand, in the region b, where the radius of curvature is larger, the force of the water droplet W to spread outwards is greater than gravity and surface tension, as shown in the right-hand figure (b) of Fig.14 is shown. As a result, the shape of the dropped water droplet W can change significantly, and the contact area between the dropped water droplet W and the outer surface 31 increases, reducing ΔT shown in Equation 1 above, and it becomes possible to improve the detachment resistance performance against repeated water exposure.

[0057] Therefore, it is preferable that the end surface part 32 and the side surface parts 33 of the protective layer 3 are configured such that the radius of curvature at the minimally rounded section 312 of the outer surface 31 is greater than or equal to 0.4 mm, and it is desirable that these have a concave shape with a contour line 311 which is continuously flowing with the corner parts 34. Table 1 Sample No. Radius of curvature of a minimally rounded section (mm) Number of water impacts 1 0,13 6926 2 0,28 11281 3 0,38 56447 4 0,48 78975 5 0,75 98799 6 0,99 121456 7 1,64 134567 8 2,31 142397 Second embodiment

[0058] The second embodiment according to a gas sensor element and a gas sensor is described with reference to the Fig. The gas sensor element 1 according to the first embodiment is configured such that the end surface part 32 or the side surface part 33 of the protective layer 3 has a concave shape, which is formed entirely from a flowing curved surface, and the contour line 311 of the outer surface 31 is a curved line. However, this does not necessarily have to be formed from a curved surface, as long as it has a concave shape overall and is continuously flowing with the corner parts 34.

[0059] The basic structures of the gas sensor element 1 and the gas sensor S of this embodiment are the same as those of the first embodiment, and their description will be omitted.

[0060] It should be noted that the reference numerals used in the second and subsequent embodiments are the same as those used in the earlier embodiment(s) and denote components or the like similar to those of the earlier embodiment(s), unless otherwise specified.

[0061] The gas sensor element 1, which is in Fig.As shown in Figure 15, the protective layer 3 is configured such that the end surface part 32 and the side surface parts 33 are each formed in a concave shape created by a combination of inclined surfaces. More precisely, with regard to the side surface parts 33, each of these can be formed, for example, by a combination of two inclined surfaces in such a way that the layer thickness is greatest at the corner part 34 on the tip end side in the longitudinal direction Z and at the end 35 on the base end side, and the layer thickness is smallest at the middle part in the longitudinal direction Z.In this case, a downwardly inclined surface, the layer thickness of which gradually decreases from the corner part 34 on the tip end side, and a downwardly inclined surface, the layer thickness of which gradually decreases from the end 35 on the base end side, are symmetrically placed, and a bending part 314 is formed at a position where the two intersect in the middle part.

[0062] Similarly, the side surface section 33 can be configured in the X direction of the long side or the Y direction of the short side, which are perpendicular to the longitudinal direction Z, such that the layer thickness gradually decreases from the corner sections 34 at the two ends towards the middle section, for example by combining two downwardly inclined surfaces. The bending section 314 is formed at a position where the two downwardly inclined surfaces also intersect in these directions. As a result, the downwardly inclined surfaces are combined such that the layer thickness is thickest at the four corners of the essentially rectangular side surface section 33 and thinnest at the middle section, resulting in an overall concave shape that is continuously and smoothly connected to the corner sections 34.

[0063] The end surface section 32 can be configured similarly. In the X direction of the long side and the Y direction of the short side, two downwardly inclined surfaces can be combined, for example, such that the layer thickness is greatest at the corner sections 34 at the two ends and gradually decreases towards the middle section. The bending section 314 is formed at a position where the two downwardly inclined surfaces also intersect in these directions. As a result, the downwardly inclined surfaces are combined such that the layer thickness is greatest at the four corners of the essentially rectangular end surface section 32 and thinnest at the middle section, resulting in an overall concave shape that is continuously smooth with the corner sections 34.

[0064] Preferably, the end surface part 32 or the side surface part 33 also has a flowing concave shape in the case where the protective layer 3 is configured by combining a plurality of inclined surfaces. To achieve this, for example, the number of inclined surfaces forming the end surface part 32 or the side surface part 33 can be increased so that a plurality of bent parts 314 are present at different locations.

[0065] More precisely, the gas sensor element 1, which is located in Fig.As shown in Figure 16, each side surface part 33 of the protective layer 3 has a plurality of downwardly inclined surfaces with different angles of inclination in the longitudinal direction Z between the corner part 34 on the tip end side or the end 35 on the base end side (for example, two in each section), where the layer thickness is greatest, and the middle part, where the layer thickness is thinnest. In this case, a plurality of bending parts 314 (in this case, for example, three) are formed at positions where two adjacent inclined surfaces intersect.

[0066] Similarly, the side surface section 33 can be configured in the X direction of the long side or the Y direction of the short side, perpendicular to the longitudinal direction Z, such that the layer thickness gradually decreases from the corner sections 34 at the two ends towards the middle section by combining a plurality of downwardly inclined surfaces (for example, a total of 4). A plurality of bending sections 314 (in this case, for example, three) are formed at positions where the two adjacent inclined surfaces also intersect in these directions. As a result, the downwardly inclined surfaces are combined such that the layer thickness is thickest at the four corners of the essentially rectangular side surface section 33 and thinnest at the middle section, resulting in a more flowing concave shape overall.

[0067] The end surface section 32 can be configured similarly. In the X direction of the long side and the Y direction of the short side, a plurality of downwardly inclined surfaces (for example, a total of 4) can be combined such that the layer thickness is greatest at the corner sections 34 at the two ends and gradually decreases towards the middle section. A plurality of bending sections 314 (in this case, for example, three) are formed at positions where the downwardly inclined surfaces also intersect in these directions. As a result, the downwardly inclined surfaces are combined such that the layer thickness is greatest at the four corners of the essentially rectangular end surface section 32 and thinnest at the middle section, resulting in a more flowing concave shape overall.

[0068] If the end surface part 32 or the side surface parts 33 of the protective layer 3 have a concave shape formed by the continuously inclined surfaces, as in the preceding examples, the layer thickness is greatest at the corner parts 34 located on the outer peripheral edges of each surface part, and it is possible to protect the element corners 23, which are the parts least protected against water exposure. Since the layer thickness can be reduced on parts other than the corner parts 34, the mass of the protective layer 3 is also reduced, leading to a reduced heat capacity and enabling faster activation. Furthermore, an effect can be achieved in which the progression of delamination is prevented when water exposure occurs, as the water droplets spread out on the concave outer surface 31.

[0069] Preferably, the outer surface 31, each selected from the end surface part 32 and the side surface parts 33 of the protective layer 3 covering each surface of the element body 2, is formed into a flowing concave shape. Here, a flowing concave shape indicates that, with respect to the cross-section of the end surface part 32 of the protective layer 3, the contour line 313 of the outer surface 31 is a combination of straight segments that are continuously flowing between the corner parts 34 at the two ends, as for example in Fig. Figure 17 shows that it is desirable for the angles of the bending parts 314 formed by two adjacent straight segments to be large. Similarly, in the cross-section of the side surface part 33 of the protective layer 3, which is shown in Fig.As shown in Figure 18, the contour line 313 of the outer surface 31 is a combination of straight segments which flow continuously towards each other from the corner part 34 on the tip end side to the base end side, and it is desirable that the angles of the bending parts 314 formed by two adjacent straight segments are large.

[0070] Furthermore, it is preferably desirable that the minimum angle of the bending elements 314, formed by two adjacent straight segments, in the cross-section of the end surface part 32 of the protective layer 3 or in the cross-section of the side surface part 33, be 150° or greater. As a result, the contact area with the water droplet W increases when exposed to water. Thus, the water droplet spreads easily on the flowing concave outer surface 31, and the effect of preventing the progression of the detachment of the protective layer 2 is further improved.

[0071] As a schematic example in the Fig. 19 and Fig.As shown in Figure 20, the shape of the outer surface 31 of the end surface part 32 or the side surface parts 33 can include a step as an example of a shape of the outer surface 31 of the protective layer 3 that is not continuously flowing with the corner parts 34. In the middle diagram of Fig. In the direction X of the long side, a stepped recessed part 37 is formed in the middle part of the outer surface 31 of the end surface part 32, and the outer surface is not continuously flowing with the corner parts 34 at the two ends. In the right diagram of Fig. 19 The outer surface 31 of each side surface part 33 on the tip end side is formed in the longitudinal direction Z with a stepped part 38 that is bent in a stepped manner near the corner part 34 and is not continuously connected to the corner part 34 on the tip end side. In the left diagram of Fig.19 the recessed part 37 and the stepped parts 38 are each formed in the end surface part 32 and the side surface parts 33.

[0072] Furthermore, a stepped recessed portion 37 is formed in the central part of the outer surface 31 of the side surface portion 33 in the direction X of the long side or the direction Y of the short side, as shown in the left diagram of Fig. 20 is shown, and the outer surface is not continuously flowing with the corner pieces 34 at the two ends. As in the right-hand diagram of Fig. As shown in Figure 20, even in a case where each of the corner parts 34 protrudes at the two ends in the design form or shape of an arc, a step 39 is formed on the outer surface 31 of the side surface part 33 and is not continuously flowing with the corner parts 34.

[0073] If the outer surface 31 of the protective layer 3 has a shape that is not continuously flowing with the corner parts 34, as in the examples above, the propagation of the water droplet W is inhibited and the heat load σ tends to increase. Experiment 2

[0074] Similar to experimental example 1, samples of the gas sensor element 1 with the shape shown in Fig. 21 is shown, prepared and tested to evaluate the performance of the resistance to water exposure provided by the concave shape of the protective layer 3. In Fig.In 21, the gas sensor element 1 sample is configured such that one selected from the side surface parts 33 of the protective layer 3 is a concave surface incorporating an inclined surface, and the contour line 313 in the cross-section in the longitudinal direction Z includes a bend 314 at which the straight segments meet. The thickness of the protective layer 3 increases as it approaches the corner part 34 on the tip end face of the element body 2, and the thickness of the side surface part 33 is essentially constant except near the corner part 34. Samples 9 to 17 were prepared by changing the shape of the inclined surface leading to the corner part 34 so that the angle θ at the bend 314 varies, and a repeated water exposure test was performed in the same manner as in Experiment 1.

[0075] In the same way as in experimental example 1, a repeated water exposure test was carried out, and the number of water exposures required to cause detachment was evaluated.

[0076] As shown in Table 2, the results of the repeated water exposure test demonstrate that the protective layer was also able to withstand thousands of repeated water exposures when 2 µL water droplets W were repeatedly applied to the same spot, since the protective layer 3, which is formed from a concave surface incorporating an inclined surface, exhibited a greater layer thickness at the corner portion 34. Furthermore, the number of water exposures required to cause delamination increases rapidly in the area where the angle θ is 150° or greater, as shown in Fig.Figure 22 shows the relationship between the angle θ of the bending element 314 and the number of water exposures. This indicates that the shape of a dropped water droplet W can change significantly on a concave surface with a large angle θ of the bending element 314, such that it is continuously smooth with the corner element 34. Subsequently, the contact area between the water droplet W and the outer surface 31 increases, reducing ΔT, as shown in Equation 1 above, and it becomes possible to improve the detachment resistance performance against repeated water exposure.

[0077] Thus, the end surface part 32 and the side surface parts 33 of the protective layer 3 are preferably configured such that the minimum angle of the bending parts / bend part 314 formed on the outer surface 31 is 150° or greater to form a concave shape with a contour line 313 that is continuously flowing with the corner parts 34. Table 2 Sample No. Angle of bent parts (°) Number of water impacts 9 92 6874 10 103 9784 11 111 15478 12 124 21457 13 133 25415 14 146 51356 15 152 84567 16 160 112456 17 168 134578

[0078] In the first and second embodiments described above, the end surface part 32 and the side surface parts 33 of the protective layer 3 have a concave shape with a flowing curved surface, or a concave shape with a plurality of inclined surfaces that are continuously flowing relative to each other. However, it is also possible to combine these to form a concave shape.

[0079] It should be noted that the present disclosure is not limited to the embodiments shown, but other embodiments can also be implemented without deviating from their basic principles. Furthermore, the structures of the gas sensor element 1 and the gas sensor S are not limited to those shown in the preceding embodiments, and, for example, the configuration of the element cover and other parts can be suitably modified according to the application. Moreover, the gas to be measured is not limited to exhaust gas from the automotive engine, and the specific gas component can also be any gas component.

Claims

[1] Gas sensor element (1) comprising a long plate-like element body (2) and a porous protective layer (3) that protects a surface of the element body (2), wherein the element body (2) has a gas collection part (4) at its end on one side of the end surface (21) in a longitudinal direction (Z), and the protective layer (3) includes an end surface part (32) covering one end surface (21) in laminate, side surface parts (33) covering the side surfaces (22) connected to one end surface (21) in laminate, and corner parts (34) at which two adjacent parts selected from the end surface part (32) and the side surface parts (33) meet, and an outer surface (31) of one or more selected from the end surface part (32) and the side surface parts (33) has a concave shape that is continuously flowing with the corner parts (34) and is configured such that the layer thickness increases towards the corner parts (34). [2] Gas sensor element (1) according to claim 1, wherein the outer surface (31) of the protective layer (3) has a concave shape which is continuously flowing with the corner parts (34) on each selected from the end surface part (32) and the side surface parts (33). [3] Gas sensor element (1) according to claim 1 or 2, wherein the outer surface (31) has a concave shape with a flowing curved surface. [4] Gas sensor element (1) according to claim 1 or 2, wherein in a cross-section in the longitudinal direction (Z) or in a direction orthogonal to the longitudinal direction (Z) the outer surface (31) has a contour line (311) which includes a smoothly continuous curved line. [5] Gas sensor element (1) according to claim 4, wherein the outer surface (31) is configured such that a radius of curvature at a minimally rounded section (312) of the contour line (311) is greater than or equal to 0.4 mm. [6] Gas sensor element (1) according to claim 1 or 2, wherein the outer surface (31) has a concave shape with a plurality of inclined surfaces which are continuously flowing towards each other. [7] Gas sensor element (1) according to claim 1 or 2, wherein in a cross-section in the longitudinal direction (Z) or in a direction orthogonal to the longitudinal direction (Z) the outer surface (31) has a contour line (313) comprising a combination of a plurality of straight segments which are continuously flowing towards each other. [8] Gas sensor element (1) according to claim 7, wherein the outer surface (31) is configured such that a bending part (314) formed in the contour line (313) has a minimum angle of 150° or more. [9] Gas sensor (S) comprising the gas sensor element (1) according to any one of claims 1 to 8 for detecting a specific gas component in a gas to be measured, wherein the gas sensor (S) further comprises: a cylindrical housing (H) which supports an outer circumference of the gas sensor element (1), and a cover body (S1) which is attached to one end of the cylindrical housing (H), wherein one end of the gas sensor element (1) on which the protective layer (3) is provided is enclosed in the cover body (S1), and the measured gas is introduced into the cover body (S1) through a through-hole (S13, S14) which is provided in the cover body (S1).

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