Gas sensor element and gas sensor

Abstract

Gas sensor element (1) for detecting a specific gas component in a measured gas, wherein the gas sensor element (1) is enclosed in a cover body (S) and has the following features: an element body (2) in the form of a long plate, which has a gas collection part (20) at its end on one side of the end surface (21) in a longitudinal direction (X); and a porous protective layer (3) which covers an outer circumference of the end on this side of the end surface (21) of the element body (2), wherein in a cross-section comprising two adjacent surfaces selected from the end surface (21) and the side surfaces (22) connected to the end surface (21), an outer surface (31) of the protective layer (3) facing an element corner (23) at which the two surfaces (21, 22) meet, has a shape with a corner part (4), the corner part (4) is configured such that the ratio (D / L) of an assumed diameter (D) of a water droplet (W) contained in the measured gas in an application environment to an effective length (L) of the corner part (4) in the cross-section including the two surfaces (21, 22) is greater than or equal to 1.5, wherein the cover body (S) has an inner cover (S11) and an outer cover (S12), and the assumed diameter (D) is determined as a function of the size of a hole diameter of through holes (S13, S14) of the cover body (S) and a space between the inner cover (S11) and the outer cover (S12) of the cover body (S), and in the cross-section which includes these two surfaces (21, 22), an angle formed by two surfaces (32, 33, 321, 331) of the outer surface (31) which meet at the corner part (4) is an acute angle.

Description

Cross-reference to similar registration

[0001] The present application is based on the Japanese application JP 2017 - 159 692 A, filed on August 22, 2017. Technical field

[0002] The present disclosure relates to a gas sensor element covered 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 readings. The gas sensor has a long, plate-like gas sensing element housed in a cover, and the gas sensing element is protected from the ingress of water droplets and toxins by a porous protective layer on its outer surface. The gas sensing element is configured by incorporating a heating device within the element body, which houses a gas detection element.

[0004] As described in PTL 1, the porous protective layer is usually formed, for example, by immersing the element body of the gas sensor element in a slurry state using the immersion method or similar. The protective layer formed in this way surrounds the entire outer circumference of the element body with a generally circular or elliptical outer shape and has a curved outer surface.

[0005] Furthermore, PTL 1 proposes specifying the thermal conductivity of the protective layer or a physical property that indicates the relationship of thermal conductivity, density and specific heat to a certain numerical range, in order to increase the water-repellent ability of the protective layer and to prevent penetration of water droplets.

[0006] JP 2016 - 161 414 A discloses the following: The method for forming a gas sensor element serves to obtain a gas sensor element having an element body part and a protective film, and includes a main body preparation process, a film formation process, and a calcination process. In the film processing, the upper end of the element body part is partially immersed in a slurry of a protective film-forming material comprising a ceramic material, an inorganic binder, and a coagulant, in a mold, and the protective film-forming material applied to the element body part is temporarily cured in the mold.

[0007] US 2013 / 0233708A1 discloses the following: A gas sensor element having a chamfered section. The chamfered section has a front chamfered section formed in a front end section of the gas sensor element, a rear chamfered section formed in a rear end section of the gas sensor element, and a middle chamfered section connecting the front chamfered section and the rear chamfered section. The chamfer angle of the rear chamfered section is such that it is greater than the chamfer angle of the front chamfered section.

[0008] US 2016 / 0018357A1 discloses the following: a gas sensor element with a porous protective layer exhibiting excellent water repellency. The gas sensor element comprises a sensing section containing a stack of a solid electrolyte body with a pair of electrodes on opposite sides and a heat-generating body with a heat-generating source, as well as a porous protective layer formed around the sensing section.

[0009] JP 2012 - 247 293 A discloses the following: A gas sensor element comprises an element body part having: a solid electrolyte; a measuring gas side electrode and a reference gas side electrode; a diffusion resistance layer; and a heating layer with a heating device. The element body part, which has a nearly square shape, comprises: two side sections oriented in the direction Y of the lamination; two side sections oriented in a direction that intersects the direction Y of the lamination orthogonally; and four corner sections arranged between the mutual side sections.The element body section contains an element protection layer consisting of a first protective layer covering two diffusion-side corner sections, each featuring a gas inlet opening in the diffusion resistance layer, and a second protective layer covering the entire outer circumference of the element body section, including the first protective layer. The element protection layer is configured such that its thickness in the diffusion-side corner sections is greater than its thickness in the side sections. List of prior art patent literature PTL 1: JP 2016-29360 A PTL2: JP 2016 - 161 414 A PTL 3: US 2013 / 0 233 708 A1 PTL 4: US 2016 / 0 018 357 A1 PTL 5: JP 2012 - 247 293 A Summary of the invention

[0010] On the other hand, there is a need for improved responsiveness to meet increasingly stringent emissions regulations and the demand for better fuel economy. For example, it is necessary to modify the mounting position of the gas sensor and the diameter of the cover's through-hole to improve responsiveness and sensitivity. The diameter of water droplets entering the cover also increases if the diameter of the through-hole is enlarged to more easily accommodate gas flow. In this case, from a water resistance perspective, a thicker protective layer would be desirable. However, a thicker protective layer impedes gas flow, and consequently, reduces responsiveness.

[0011] Additionally, it is difficult to create a protective layer like the one described in PTL 1 that is completely uniform, and the protective layer tends to be thin, especially at element corners where cracking is a concern. Furthermore, the contact area with water droplets increases because the outer surface is curved, thus increasing water droplet absorption. Therefore, thickening the protective layer corresponding to the element corners further increases the overall layer thickness, and improving the response sensitivity was not straightforward.

[0012] This creates a contradictory relationship, where the protective layer covering the element body is preferably thin to increase response sensitivity, but preferably thick to increase water resistance. It is desirable to achieve both response sensitivity and water resistance.

[0013] It is an objective of the present disclosure to provide a gas sensor element and a gas sensor which includes a protective layer capable of achieving both a response sensitivity and a water resistance even in an environment where water exposure is more likely to occur.

[0014] This problem is solved by the gas sensor element with the features of claim 1 and the gas sensor with the features of claim 10. Further embodiments and developments according to the invention are the subject of the subsequent claims.

[0015] One aspect of the present revelation is: A gas sensor element (1) for detecting a specific gas component in a measured gas, comprising: an element body in the form of a long plate, which has a gas collection part at its end on one side of the end surface in a longitudinal direction; and a porous protective layer that covers an outer circumference of the end on this side of the end surface of the element body, wherein in a cross-section that includes two adjacent selected from the end surface and the side surfaces connected to the end surface, an outer surface of the protective layer facing an element corner where the two surfaces meet, has a shape with a corner part, and the corner part is configured such that the ratio D / L of an assumed diameter D of a water droplet contained in the measured gas in an application environment to an effective length L of the corner part in the cross-section that includes the two surfaces is greater than or equal to 1.5.

[0016] Another aspect of the present revelation is: A gas sensor comprising a cylindrical housing that supports an outer circumference of the gas sensor element described above, and a cover body 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

[0017] According to the gas sensor element with the aforementioned configuration, the protective layer covering the element body has a corner piece corresponding to one of the element's corners, and the ratio of the assumed diameter of the water droplets to the effective length (which represents the size of the corner piece) in a given cross-section is less than or equal to a certain value. More precisely, it was determined that water droplets reaching the surface of the gas sensor element split upon contact with the corner pieces when this ratio is greater than or equal to 1.5. Thus, it is possible to reduce the number of water droplets reaching the element corners located within the corner pieces, as the split water droplets move away from the corners, significantly reducing the amount of water droplets absorbed by the corner pieces.

[0018] Thus, the water exposure of the protective layer is reduced by providing specific corner pieces that correspond to the element corners, and water resistance can be improved without increasing the thickness of the protective layer. A gas sensor using such a gas sensor element can be freed from the restrictions on the arrangement and size of the through-holes in the cover body used to protect the gas sensor element. For example, the diameter of the through-holes in the cover body can be increased to allow more gas to pass through. Therefore, the specific gas component in the measured gas can be detected with good response characteristics and high sensitivity.

[0019] Therefore, in accordance with the above aspects, it is possible to provide a gas sensor element and a gas sensor that includes a protective layer capable of achieving both response sensitivity and water resistance, even in an environment where water exposure is likely to occur. Brief description of the drawings

[0020] 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 view of a gas sensor element and an enlarged view of its main part according to the first embodiment; Fig. 2 a schematic overall view 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 according to the first embodiment; Fig. 4 an enlarged perspective view showing the configuration of the main part of the gas sensor element according to the first embodiment; Fig. 5 a schematic view to explain the operation and effect of the gas sensor element according to the first embodiment compared to the conventional configuration; Fig. 6 a cross-sectional view showing the structure of a gas detection part of the gas sensor element according to the first embodiment; Fig. 7 a schematic view showing a state in which the protective layer and a water droplet are in contact with each other, according to a conventional gas sensor element; Fig.8 an enlarged cross-sectional view of the main part, showing the configuration of the protective layer of the gas sensor element according to the first embodiment; Fig. 9 an enlarged cross-sectional view of the main part to explain the operation and effect of the protective layer of the gas sensor element according to the first embodiment; Fig. 10 an enlarged cross-sectional view of the main part, showing another example of the shape of the protective layer of the gas sensor element according to the first embodiment; Fig. 11 an enlarged cross-sectional view of the main part, showing another example of the shape of the protective layer of the gas sensor element according to the first embodiment; Fig. 12 a view showing the manufacturing process of the gas sensor element according to the first embodiment; Fig.13 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 test example 1; Fig. 14 an enlarged view of the main part to explain the configuration of the protective layer of the gas sensor element according to test example 1; Fig. 15 a graph showing the relationship between L / t and the amount of water drops in test example 1; Fig. 16 an enlarged cross-sectional view of the main part showing the configuration of the protective layer of the gas sensor element according to the second embodiment; Fig. 17 an enlarged cross-sectional view of the main part, showing another example of the shape 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 another example of the shape 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 showing another example of the shape of the protective layer of the gas sensor element according to the second embodiment; Fig. 20 a view showing the manufacturing process of the gas sensor element according to the second embodiment; and Fig. 21 an enlarged cross-sectional view of the main part showing the configuration of the protective layer of the gas sensor element according to the third embodiment. Description of the embodiments: First embodiment

[0021] Embodiments according to a gas sensor element and a gas sensor are described with reference to the Fig. 1 to 6 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 that the gas sensor S 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, and the gas sensor element 1 detects a specific gas concentration in the exhaust gas, which is the gas to be measured. 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.

[0022] In the Fig. 1 and Fig.2 The gas sensor element 1 comprises a long, plate-like element body 2 and a porous protective layer 3, which protects the surface of the element body 2. The element body 2 is held inside the insulator I and its longitudinal direction (i.e., the vertical direction in Fig. 2) X corresponds to the axial direction of the insulator I. This includes at its end on the side of the distal end surface 21 (compare for example Fig. 1), which is one of its end faces in the longitudinal direction X, the gas collection part 20. The element body 2 can, for example, have a rectangular parallelepiped shape with a rectangular cross-section, as in Fig. 4 is shown.

[0023] The protective layer 3 is designed to cover the outer circumference of the end of the element body 2 on the side of the distal end surface 21, which projects from the insulator I. In a cross-section encompassing two adjacent surfaces of the distal end surface 21 of the element body 2 and of its side surfaces 22 connected to the distal end surface 21, the outer surface 31 of the protective layer 3, facing the element corner 23 where the two surfaces intersect, has a shape with a corner part 4. An enlarged vertical cross-section, which in Fig.Figure 1 shows, for example, an element corner 23 at which the distal end surface 21 and a side surface 22 connected to it intersect. Outside this, a corner part 4 with an effective length L is provided on the outer surface 31 at which an end surface 32 and a side surface 33 of the protective layer 3 intersect. Preferably, each corner part 4 of the protective layer 3 is located on an extension of a line A that bisects the element corner 23 of the element body 2.

[0024] The gas sensor S is, for example, attached to an exhaust pipe and is used in an environment exposed to the exhaust gas, which is the gas to be measured. The protective layer 3, which covers the element body 2 of the gas sensor element 1, protects the element body 2 from condensed water and toxins contained in the exhaust gas. This protection is achieved by appropriately adjusting the shape and size of the corner pieces 4 of the protective layer 3, in particular by adjusting them such that the ratio D / L of the assumed diameter D of the water droplets W (compare, for example, Fig. 5) that are contained in the exhaust gas in the application environment, and where the effective length L of the corner pieces 4 is greater than or equal to 1.5, it is possible to split the water droplets W that are in contact and improve the water resistance. In this case, the effective length LL > 0.

[0025] More precisely, each corner piece 4 has a water droplet contact surface 41 on the surface located outside the element corner 23. The water droplet contact surface 41 is a surface which, in a cross-section comprising two adjacent faces of the element body 2, includes an intersection point c between a line extension of line A, which bisects the element corner 23, and the outer surface 31 of the protective layer 3, and this splits a water droplet W, which is in contact with the area near the intersection point c. The effective length L of the corner pieces 4 can be the distance between the two ends of the water droplet contact surface 41 in a cross-section comprising two faces.

[0026] The detailed configurations of the protective layer 3 and the corner part 4, which are formed in the gas sensor element 1, will be described later.

[0027] In Fig.In Figure 3, the gas sensor S has a cylindrical housing H, the axial direction of which is the longitudinal direction X 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. In the gas sensor S, one end face of the gas sensor element 1, which has the gas detection part 20, is referred to as the distal 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 distal end face of the housing H, and the distal end of the gas sensor element 1 projects from the housing H and is housed within 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.

[0028] The element cover S1 is designed as inner and outer double cylinders with a base and is arranged to surround the circumference of the distal 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 bottom 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 draws in atmospheric air.

[0029] 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.

[0030] 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. Connecting parts R11 and R12 are provided on the distal end of the connecting wires R1 and R2 and are electrically connected to the electrode connecting parts (not shown) provided at the base end of the gas sensor element 1. Furthermore, the gas sensor element 1 includes a heating element 5, which is received in the element body 2 on the distal end face 21, as shown in Fig.Figure 4 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 20 of the element body 2 at an activation temperature.

[0031] In Fig.6 The gas detection part 20 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 includes a dense shielding layer 15, which is laminated onto its surface opposite the solid electrolyte body 11, and a gas inlet connection 16 is formed, which is exposed on the lateral outer surface.

[0032] A layer 17 forming the reference gas chamber, which forms 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.

[0033] 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 and the shielding layer 15.

[0034] Although the element body 2 has a rectangular cross-sectional shape in this example, it can also have a polygonal or polygonal cross-sectional shape. Instead of producing the side edge parts on the side of the gas collection part 20 or the right-angled corners on the side of the heating device part 5, as shown in the figure, these can, for example, have inclined surfaces that are arranged at an angle with respect to the lamination direction in order to form a hexagonal or octagonal cross-sectional shape, or the like. In such a case, the corner parts 4 of the protective layer 3 are formed, which correspond to the element corners 23.

[0035] The protective layer 3 is made of a porous material in which a large number of pores exist between the ceramic particles and is adapted to exhibit a desired porosity. The ceramic particles are made, for example, of an insulating ceramic such as aluminum oxide. The solid electrolyte body 11 is made, for example, of a zirconium dioxide-based solid electrolyte, and the heating device substrate 18, the layer 17 forming the reference gas chamber, the solid electrolyte body 11, the porous diffusion resistance layer 14, and the shielding layer 15 are made, for example, of an insulating ceramic such as aluminum oxide or spinel.

[0036] Thus, the sensor output of the gas detection section 20 provides 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.

[0037] Next, the detailed structure of protective layer 3 will be described.

[0038] In the left diagram of Fig.1. The protective layer 3 has a substantially uniform thickness and covers the outer circumference of the end part of the element body 2 on the side of its distal end surface 21. That is, the protective layer 3 covers the entire distal end surface 21 and also covers the end parts of the side surfaces 22 to which it is connected. The corner parts 4 are formed at the points where the end surface 32, which is positioned distal to the distal end surface 21, meets the side surfaces 33, which are positioned next to the side surfaces 22. The outer shape of the protective layer 3 generally resembles the outer shape of the element body 2, and the protective layer 3 is larger than the element body 2 by its own thickness.

[0039] More precisely, in Fig.4 With respect to the end surface 32 of the protective layer 3, which corresponds to the distal end surface 21 of the element body 2, and the side surfaces 33 connected to the end surface 32, a corner element 4 is formed between each pair of adjacent surfaces selected from the end surface 32 and the side surfaces 33. For example, in the rectangular end surface 32 of the protective layer 3, corner elements 4 are formed along the connecting elements between the four sides of the outer peripheral edges and the edges of the four side surfaces 33 connected to the four sides, extending linearly in directions orthogonal to the longitudinal direction X. Similarly, in each rectangular side surface 33 of the protective layer 3, a corner element 4 is formed along each connecting element between lateral edges of two adjacent side surfaces, extending linearly in the longitudinal direction X.

[0040] As shown in the left diagram of Fig.As schematically shown in Figure 5, a water droplet W can be detached from the corner piece 4 by sufficiently reducing the water droplet contact surface 41 of the corner piece 4 with respect to the water droplet W reaching the surface of the gas sensor element 1. More precisely, this effect can be achieved if the ratio D / L between the assumed diameter D of the water droplet W and the effective length L of the corner piece 4 (for example, the distance between the two ends of the water droplet contact surface 41) is 1.5 or greater. As a result, the water droplets W1 detaching from the water droplet W move away from the water droplet contact surface 41, and absorption of the entire amount of water droplet W at the corner piece 4 can be prevented.Accordingly, it is possible to significantly reduce the water exposure load and to protect the element corners 23, which are least protected against water exposure, thereby achieving an effect that prevents cracking of the element.

[0041] On the other hand, the outer surface of the protective layer 30 in the gas sensor element 1, which is provided with the conventional protective layer 30, is completely curved, as shown in comparison with the right-hand diagram of Fig.Figure 5 shows that in this case, the contact area with a water droplet W reaching the outer surface of the protective layer 30 increases, and the water droplets W are not only absorbed in a simple manner, but the layer thickness is also reduced at sections corresponding to the element corners 23. Therefore, if the diameter of the water droplet W adhering to the protective layer 30 is small, it will not reach the element body 2, even if the entire amount is absorbed, as shown in the diagram on the left. Fig. Figure 7 shows that the absorbed water droplet W tends to reach element corner 23, where the layer is thin, if the diameter of the water droplet W increases, as shown in the right-hand diagram of Figure 7. Fig. 7 is shown.

[0042] When the gas sensor S is in operation, the heating device 5 of the gas sensor element 1, which is located in Fig.As shown in Figure 4, the gas detection element 20 is excited to raise its temperature to the activation temperature, and the surface temperature of the protective layer 3 is, for example, 400 °C to 600 °C. If a large water droplet W adheres to it and penetrates into the interior from the outer surface 31, a difference in thermal expansion occurs in the protective layer 3, and a stress is concentrated on the element corner 23, which can cause cracking of the element.

[0043] To solve this problem, the outer surface of the element corners 23 of the gas sensor element 1 is covered with the protective layer 3 with the corner parts 4, and the water droplet contact surface 41 of each corner part 4 is preferably located on the extension line of line A, which bisects the corresponding element corner 23 in order to split the water droplets W. Thus, the water droplet contact surface 41 is sufficient if it has a size corresponding to the assumed water droplets W, and the element corners 23 can be reliably protected by arranging them to correspond to the element corners 23 of the element body 2.

[0044] Such a protective layer 3 can be produced by a molding process, as will be described later.

[0045] With reference to the Fig. Sections 8 to 10 will describe specific examples of the shape of these corner pieces 4.

[0046] As in Fig.As shown in Figure 8, the corner parts 4 of the protective layer 3 can, for example, have a C-surface shape. The water droplet contact surface 41 of each corner part 4 is a generally flat surface located on the extension line of line A, which bisects the corresponding element corner 23 located within it, and which includes an intersection point c between the extension line of line A and the outer surface 31 of the protective layer 3. The water droplet contact surface 41 has a deformation point a, which bends in a direction towards the side surface 22 of the element body 2 with respect to the intersection point c, and it also has a deformation point b, which bends in a direction towards the side of the distal end surface 21.

[0047] The deformation points a and b are the two endpoints of the water droplet contact surface 41, as shown in the vertical cross-section in Fig.Figure 8 shows that the linear distance between deformation points a and b is the effective length L when the water droplet W contacts corner piece 4. As deformation point a or deformation point b approaches intersection point c, the effective length L decreases, and thus corner piece 4 becomes smaller.

[0048] The intersection point c can coincide with the deformation point a or the deformation point b, or it can be located at a position that essentially coincides with both deformation points a and b. In this case, the effective length L of the corner piece 4 is extremely small.

[0049] The outer surface 31 of the protective layer 3 can have a shape that features a plurality of deformation points extending from the intersection point c towards the side of the end face 32 or the side of the side face 33 of the protective layer 3. In this case, the water droplet contact surface 41 is also defined by the deformation points a and b that are located closest to the intersection point c. It should be noted that bending in a direction towards the distal end face 21 or the side face 22 preferably means that the bending angle at the bent portion where flat surfaces meet is approximately 150° or less, as shown in Fig. Figure 8 shows that the bent part can be defined as a deformation point. Furthermore, the outer surface 31 of the end face 32 or the side faces 33 of the protective layer 3 need not be formed solely by a flat surface.

[0050] Fig.Figure 8 shows a cross-section of the corner section 4 where the end surface 32 and a side surface 33 of the protective layer 3 intersect. However, a similar water droplet contact surface 41 is formed in the other cross-sections of the continuous corner section 4. Furthermore, a water droplet contact surface 41, having an effective length L, is also formed at the corner sections 4 where the end surface 32 and the other side surfaces 33 meet, or where the side surfaces 33 meet each other. Preferably, the effective length L of the water droplet contact surface 41 for each corner section 4 is set such that the ratio D / L with respect to the assumed diameter D of the water droplets W, which are expected to reach the corner section 4 together with the exhaust gas in the application environment, is 1.5 or greater. The effective lengths L of the corner sections 4 of the protective layer 3 can generally be the same or different from each other.Although the cross-sectional shape and the effective length L of a continuous corner part 4 are preferably generally constant, they may differ in part.

[0051] As in Fig.As shown in Figure 9, the effect of water droplet splitting W can be obtained when the ratio D / L is 1.5 or greater, when a water droplet W with an assumed diameter D collides with the water droplet contact surface 41 of the corner 4. The more the ratio D / L exceeds 1.5, for example, when a specific assumed diameter D is set, the smaller the effective length L of the corner part 4, the smaller the contact area with the water droplet W, and the greater the effect of water droplet splitting W. Additionally, the amount of water droplet absorbed by the corner part 4 is reduced because the water droplet contact surface 41 is smaller compared to the size of the water droplet W, and split water droplets W1 with smaller diameters leave the water droplet contact surface 41 of the corner part 4 rapidly.This allows the amount of water exposure to be greatly reduced.

[0052] In the application environment where the gas sensor S is located, the size of the water droplets W reaching the gas sensor element 1 is normally limited by the through-holes S13 and S14 in the element cover S1, which houses the gas sensor element 1. That is, the assumed diameter D of the water droplets W reaching the gas sensor element 1 is determined by the size of the hole diameters of S13 and S14 in the element cover S1 and the space between the inner cover S11 and the outer cover S12. This diameter D is typically larger than the minimum diameter of the through-holes S13 and S14. If a good response from the gas sensor S is required, the diameters of the through-holes S13 and S14 tend to increase, and the assumed diameter D also increases.Therefore, it is preferable to set the assumed diameter D for each gas sensor S, for example, based on predicted and test values ​​of the average or maximum diameter of the water droplets W passing through the element cover S1, as well as according to the application environment. Furthermore, based on the assumed diameter D, the shape of the water droplet contact surface 41 of the corner parts 4 can be adjusted to achieve a certain D / L ratio.

[0053] The D / L ratio is preferably 2.0 or greater, and the effective length L of the corner part 4 is preferably less than 1.0 mm. For example, in the case of a general gas sensor S used in an exhaust gas purification system, the D / L ratio would be 2.0 or greater if the corner part 4 is designed such that it has an effective length L of less than 1.0 mm, and the assumed water droplets W can be cleaved off at the water droplet contact surface 41. Thus, the penetration of the water droplets W into the interior of the protective layer 3 is prevented, the effect of protecting the element corners 23 is increased, and cracking of the element can be prevented.

[0054] Additionally, the effective length L or the layer thickness t of the protective layer 3 is preferably adjusted such that the ratio L / t between the effective length L of the corner part 4 and the layer thickness t of the protective layer 3 at the corner part 4 is less than or equal to 6. The layer thickness t at the corner part 4 is represented by the shortest distance between the corresponding element corner 23 and the outer surface 31 of the protective layer 3, that is, the distance between the intersection point c, obtained by extending the line A, which bisects the element corner 23, and the element corner 23 itself. The further the ratio L / t falls below 6, the greater the layer thickness t becomes relative to the effective length L of the corner part 4, and the effect of reducing the number of water droplets W absorbed by the corner part 4 and reaching the element corner 23 can be increased.Furthermore, the effect of the splitting of the water droplet W at corner 4 is increased because the effective length L of the corner part 4 becomes smaller relative to the layer thickness t.

[0055] As in Fig.As shown in Figure 10 as a modification example, the corner parts 4 of the protective layer 3 can, for example, have an R-surface shape. In this case, the water droplet contact surface 41 of each corner part 4 is also a curved surface located on the extension line of line A, which bisects the corresponding element corner 23 located within it, and which includes an intersection point c between the extension line of line A and the outer surface 31 of the protective layer 3. The water droplet contact surface 41 has a shape change point a, at which the shape changes abruptly with respect to the intersection point c in a direction towards the side surface 22 of the element body 2, and it also has a shape change point b, at which the shape changes abruptly in a direction towards the side of the distal end surface 21.Here, the deformation points a and b are, for example, connection points between the curved water droplet contact surface 41 and the flat end surface 32 or the flat side surface 33 on the outer surface 31 of the protective layer 3, as illustrated.

[0056] The deformation points a and b are the two endpoints of the water droplet contact surface 41, as shown in the vertical cross-section in Fig.Figure 10 shows that the linear distance between deformation points a and b is the effective length L when the water droplet W contacts the corner part 4. As in the case of the C surface shape, the effective length L decreases in the case of the R surface shape as deformation point a or deformation point b approaches the intersection point c, and thus corner part 4 becomes smaller. Furthermore, the intersection point c may coincide with deformation point a or deformation point b, or it may be located at a position that substantially coincides with both deformation points a and b.In the case that the outer surface 31 of the protective layer 3 has a shape which has a plurality of deformation points between the intersection point c and the side of the end surface 32 or the side of the side surface 33 of the protective layer 3, the water droplet contact surface 41 is similarly defined by the deformation points a and b which are located closest to the intersection point c.

[0057] As in Fig.As shown in Figure 11 as another modification example, the corner part 4 of the protective layer 3 can alternatively have a protruding shape that extends outwards from the end surface 32 or the side surface 33. In this case, the corner part 4 also lies on the extension line of line A, which bisects the element corner 23 located within it, and can be defined in a similar way. More precisely, the flat surface that includes the intersection point c between the extended line of line A and the outer surface 31 of the protective layer 3 is the water droplet contact surface 41. The water droplet contact surface 41 has a deformation point a that bends in a direction towards the side surface 22 of the element body 2 with respect to the intersection point c, and this side surface also has a deformation point b that bends in a direction towards the side of the distal end surface 21.

[0058] The deformation points a and b are the two endpoints of the water droplet contact surface 41, as shown in the vertical cross-section in Fig. Figure 11 shows that the linear distance between deformation points a and b is the effective length L when the water droplet W contacts corner piece 4. As deformation point a or deformation point b approaches intersection point c, the effective length L decreases, and thus corner piece 4 becomes smaller. Furthermore, intersection point c may coincide with deformation point a or deformation point b, or it may be located at a position that substantially coincides with both deformation points a and b.

[0059] The outer surface 31 of the protective layer 3 can have a shape which, extending from the intersection point c towards the side of the end surface 32 or the side of the side surface 33 of the protective layer 3, exhibits a plurality of deformation points. The corner part 4 with a protruding shape, as shown in the figure, further exhibits, for example, a deformation point d that bends inwards in a direction extending from the deformation point a towards the side surface 33. In this case, the water droplet contact surface 41 is similarly defined by the deformation points a and b that are located closest to the intersection point c.

[0060] The water droplet contact surface 41 of the corner parts 4, which are located in the Fig. 10 and Fig.Figure 11 shows that the structures are designed such that the ratio D / L between the assumed diameter D of water droplets W and the effective length L is 1.5 or greater. Advantageously, the same effects can be obtained by adjusting the effective length L and the layer thickness t with respect to the assumed diameter D, such that the ratio L / t to the layer thickness t of the protective layer 3 is 6 or less.

[0061] Next, a method for manufacturing the gas sensor element 1 will be described.

[0062] As in Fig.As shown in Figure 12, a demolding process can be adopted in which the protective layer 3 is applied to 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 includes, for example, 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 has a shape corresponding to the outer shape of the protective layer 3. Here, the inner peripheral edges of the bottoms of the two molds 101 and 102 are designed in such a way that they form a C-surface shape, so that they form the shape of the corner parts 104, which are arranged inclined inwards from the inner side surfaces towards the bottom surface.

[0063] 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 is inserted into the mold 100, into which the slurry 200 has been injected, starting from the upper opening of the hollow part, and the slurry 200 is temporarily cured after the element body is positioned and held by a (not shown) clamping device or the like. Then, in the step shown in (3), the two molds 101 and 102 are opened to remove the element body 2, which is covered with the temporarily cured slurry 200, and the element body is fired (for example, at 1000 °C) to form the protective layer 3.

[0064] Apart from heat drying, curing 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 heating. As described above, a protective layer 3, which has corner pieces 4 with a desired shape, can be formed with high accuracy when demolding is used, since the corner pieces 104, which have a shape corresponding to the corner pieces 4 of the protective layer 3, can be pre-formed in the mold 100. Alternatively, it is also possible to use a mold 100 that does not have corner pieces 104 with a C-surface shape, and the corner pieces 4 are formed by cutting the protective layer 3 after it has been formed into any desired shape. Test example 1

[0065] Samples of the gas sensor element 1, produced by the method described above, were tested to evaluate the influence of the shape of the corner parts 4 of the protective layer 3 on the water resistance. Samples of the gas sensor element 1 were prepared such that the corner parts 4 of the protective layer 3 have a C-surface shape (compare, for example, Fig. 9), and so that these have different effective lengths L, and the following water exposure test was carried out. The ratio D / L between the effective length L of the corner piece 4 and the assumed diameter D of the water droplet W was changed by combining the effective length with two types of water droplet quantities corresponding to the assumed diameters D of the water droplet W.

[0066] As in Fig.As shown in Figure 13, the gas sensor element 1 was fixed in the water exposure test using a clamping device (not shown) such that the corner part 4 of the protective layer 3 was located at the upper end. After this was heated to a specific controlled temperature by energizing the heating device part 5, a predetermined quantity of water droplets W was dripped onto the corner part 4 from above. The controlled temperature provided by the heating device 5, the thickness of the protective layer 3 of the gas sensor element 1, the longitudinal length of the protective layer 3, the bending angles at the two ends of the water droplet contact surface 41 of the corner part 4, and the dimensions of the element body 2 were as follows. Layer thickness of a protective layer 3: 0.15 mm Longitudinal length of a protective layer 3: 10 mm Bending angle of corner piece 4: 150 °C Controlled temperature: 750 °C Longitudinal length of element body 2: approximately 50 mm

[0067] The evaluation was carried out using a high-speed camera (for example, a high-speed camera setting of 10000 fps (frames per second)) to visually determine whether the droplet W has split, with the amount of water droplet W (i.e., the drop quantity) being set to 2 µL or 3 µL and the effective length L of the corner part 4 being varied from 0.82 mm to 1.22 mm.

[0068] The droplet diameter, corresponding to the droplet quantity, was considered the assumed diameter D for calculating the D / L ratio. The relationship of this ratio to the water droplet splitting W was investigated, and the results are shown in Table 1 (i.e., Examples 1 to 3, Comparative Examples 1 and 2). If the entire water droplet W was absorbed by the protective layer 3, it was judged that it did not split; conversely, if part of the water droplet W was not absorbed by the protective layer 3, it was judged that it did split.

[0069] Furthermore, for each of examples 1 to 3 and comparison examples 1 and 2, the IL change ratio of the gas sensor element 1 was investigated before and after the water exposure test, and the results are also shown in Table 1. The IL change ratio was obtained by measuring the sensor current IL output from the gas detection part 20 of the gas sensor element 1 using a test gas with a specific gas composition, and calculating the change ratio of the sensor current IL before and after the water exposure test. The evaluation was defined as follows. IL change ratio greater than 10%: Unacceptable IL change ratio greater than 5% and less than or equal to 10%: Acceptable IL change ratio less than or equal to 5%: Excellent

[0070] The IL change ratio is a parameter used to determine the degree of cracking on the element due to moisture. When cracking occurs on the element, the amount of gas flowing into the gas detection part 20 increases, and the sensor current IL increases. Since it can be assumed that the sensor current IL exhibits a measurement variation of up to approximately 10%, cases where the IL change ratio exceeds 10% were considered unacceptable. Furthermore, it was assumed that there was almost no increase in the sensor current IL if the IL change ratio was less than or equal to 5%, and the sample was evaluated as excellent. If the increase was within the range of measurement variation, the sample was evaluated as acceptable. Table 1 Example, comparison example no. Drop volume [µL] Droplet diameter (D) [mm] L[mm] D / L IL change ratio Droplet splitting Comparative example 1 2 1,57 1,21 1,30 Acceptable No Comparative example 2 3 1,79 1,22 1,47 Unacceptable No Example 1 3 1,79 1,01 1,77 Acceptable Yes Example 2 3 1,79 0,91 1,97 Excellent Yes Example 3 3 1,79 0,82 2,18 Excellent Yes

[0071] As can be seen from Table 1 regarding comparison examples 1 and 2, which show D / L ratios less than or below 1.5 and effective lengths L of the corner piece 4 greater than or equal to 1.2 mm, which are relatively large in relation to the water droplet W, the water droplet W does not split. Furthermore, the IL change ratio in comparison example 1, which has a small droplet quantity of 2 µL (i.e., it corresponds to D = 1.57 mm), is acceptable and within the permissible range, but in comparison example 2, which has a larger droplet quantity of 3 µL (i.e., it corresponds to D = 1.79 mm), the IL change ratio is unacceptable, and cracking of the element has occurred. On the other hand, the D / L ratio in examples 1 to 3 is greater than or equal to 1.5, and splitting of the water droplet W was observed in all of these.The IL change ratio of Example 1, which has an effective length L of the corner piece 4 that is greater than or equal to 1.0 mm, is acceptable, and the IL change ratios of Examples 2 and 3, which have effective lengths L of the corner piece 4 that are less than 1.0 mm, are excellent.

[0072] The preceding results indicate that the water droplet W can be split at the corner piece 4 if the D / L ratio is greater than or equal to 1.5, and even if the droplet quantity is relatively large, absorption of the water droplet W can be prevented and cracking of the element can be avoided. Furthermore, the D / L ratio can preferably be set to approximately 2.0 or greater by making the effective length L of the corner piece 4 less than 1.0 mm, and this promotes splitting of the water droplet W, reduces water exposure stress, and maintains an excellent IL change ratio. Test example 2

[0073] Next, samples of the gas sensor element 1, manufactured in a similar manner to test example 1, were tested to evaluate the influence on crack formation on the element, varying the ratio L / t between the effective length L of the corner part 4 and the layer thickness t of the protective layer 3 at the corner part 4. As in Fig. As shown in Figure 14, the samples of the gas sensor element 1 were designed such that they have a relatively small effective length L of the corner part 4 (for example, approximately 0.9 mm), and the layer thickness t of the protective layer 3 was varied to vary the ratio L / t of this within a range of less than or equal to 10.

[0074] The water exposure test was performed similarly to Test Example 1, and the IL change ratio was used to determine whether cracking had occurred on the element, with the water droplet quantity W set at 3 µL. Furthermore, the water exposure was repeated, gradually increasing the droplet quantity until cracking on the element was confirmed, i.e., until the IL change ratio became unacceptable, indicating that no cracking had occurred. The results are presented in Fig. 15 shown.

[0075] As from Fig.As can be seen in Figure 15, none of the samples with an L / t ratio of 10 or less showed any cracking of the element when the drop volume was 3 µL. Even when the drop volume was increased to 4 µL, no cracking of the element was observed in the region where the L / t ratio was less than or equal to 6, and sufficient water resistance could be obtained. When the drop volume exceeds 4 µL, the lower the L / t ratio, the greater the drop volume that causes cracking of the element.

[0076] Thus, the effective length L of the corner piece 4 and the thickness t of the protective layer 3 at the corner piece 4 are adjusted such that the ratio L / t is less than or equal to 6. The smaller the effective length L of the corner piece 4, the smaller the amount of water droplet W absorbed into the protective layer 3. The greater the layer thickness t, the lower the transfer of cold from the water droplet W in contact with the corner piece 4 to the element corner 23 within it. Therefore, the lower the ratio L / t, the lower the stress on the element corner 23, thus preventing cracking of the element and improving its durability. Second embodiment

[0077] 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 has a protective layer 3 with an outer shape of a rectangular parallelepiped, which is essentially similar to the rectangular parallelepiped element main body 2, but the two opposing surfaces need not be arranged such that they are parallel. The protective layer 3 can, for example, be configured such that the end surface 32 or the side surface 33 is inclined with respect to the distal end surface 21 and a side surface 22 of the element body 2, and the angle formed between the end surface 32 and a side surface 33 is not a right angle.

[0078] 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.

[0079] 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.

[0080] For example, in Fig.As shown in Figure 16, both the end surface 32 and the side surface 33 of the protective layer 3 can be inclined relative to the distal end surface 21 and the side surface 22 of the element body 2. Here, the end surface 32 and the side surface 33 of the protective layer 3 are inclined to widen as they move away from the corner part 4, and the angle θ formed by the end surface 32 and the side surface 33 is an obtuse angle. In this case, the same effects can also be obtained by appropriately adjusting the effective length L of the corner part 4 relative to the assumed diameter D of the water droplet W.

[0081] As in Fig.Figure 17, as shown as a modification example, can be selected from the end surface 32 and the side surface 33 parallel to the opposite distal end surface 21 or the side surface 22. Here, the end surface 32 of the protective layer 3 is arranged parallel to the distal end surface 21 of the element body 2, and the side surface 33 of the protective layer 3 is inclined with respect to the side surface 22 of the element body 2. Preferably, the side surface 33 of the protective layer 3 is inclined to widen as it approaches the end surface 32, and the angle θ formed by the end surface 32 and the side surface 33 is an acute angle, as shown in the drawing.

[0082] Such a configuration facilitates the detachment or splitting of water droplets W1 at the corner part 4, exiting it from the end face 32 or the side face 33, and also reduces the probability that the detached droplets will re-contact the outer surface 31 of the protective layer 3. This is also advantageous when increasing the layer thickness t at the corner part 4 and reducing the ratio L / t, since the layer thickness of the protective layer 3 increases towards the side of the end face 32. Furthermore, it is possible to configure both the end face 32 and the side face 33 to slope and widen as they approach the corner part 4.

[0083] As in Fig.As shown in Figure 18 as a modification example, the shape of the end surface 32 or the side surface 33 is not limited to a flat surface or an inclined surface, and can be a curved surface or a shape obtained by combining them. In the left diagram of Fig.For example, the protective layer 3 has a curved concave end surface 32. The side surface 33 has a portion where the outer surface 31 is a smooth, curved concave surface at its end part (i.e., the lower end part in the drawing) 331 on the side of the end surface 32. Since the corner part 4 is formed at the junction between the curved end surface 32 and the end part 331 of the side surface 33, and the corner part is arranged to project outwards with respect to the end surface 32 and the side surface 33, the water droplets W1 that detach from the corner part 4 tend to move away, and it is also less likely that they will contact the corner part again.

[0084] As shown in the middle diagram of Fig.As shown in Figure 18, the end surface 32 can be a concave surface combining a flat surface and inclined surfaces. In this case, its end parts 321 on the sides of the side surface 33 are inclined surfaces extending towards the outside, and the corner parts 4 are formed at their connections to the side surfaces 33, which are flat surfaces. Furthermore, the end surface 32 can be a flat surface, and the corner parts 4 can be formed at their connections with the end parts 331 of curved side surfaces 33, as shown in the diagram on the right. Fig. 18 is shown.

[0085] Furthermore, the corner part 4 at the connection of the two side surfaces 33 can be formed by two curved concave side surfaces 33, as in the left diagram of Fig. Figure 19 is shown as a modification example. As in the diagram on the right of Fig.As shown in Figure 19, each of the two side surfaces 33 can be a concave surface, combining a flat surface and inclined surfaces. In this case, the corner part 4 is formed at the junction between the end parts 331 of the two side surfaces 33, which are inclined surfaces. In each case, the water droplets W1 that detach from the corner part 4 tend to move away, and it is unlikely that they will contact the corner part, since the corner part 4 formed between two side surfaces 33, or the corner part 4 formed between two end parts 331, projects outwards from the two side surfaces 33.

[0086] Thus, the protective layer 3 exhibits the same effects as long as the two surfaces connected to the corner part 4 form an acute angle, even if the entire end surface 32 or the side surface 33 is neither an inclined nor a concave surface. That is, it is sufficient if a part of the end surface 32 or the side surface 33, which forms part of the outer surface 31 of the protective layer 3, forms an inclined or curved surface that extends outwards as it approaches the corner part 4.

[0087] In a similar manner to the first embodiment, the gas sensor element 1, which has such a shape, can also be produced, for example, by molding.

[0088] As in the step shown in (1), in Fig.As shown in Figure 20, the container-like mold 100 comprises two molds 101 and 102 with a split structure, and the hollow part 103, which is formed on the adjacent parts of the two molds 101 and 102, has a shape corresponding to the outer shape of the protective layer 3. Here, the inner side surfaces 105 of the two molds 101 and 102 are designed as inclined surfaces that expand as they approach the base surface, and the inner peripheral edges of the bases of the molds are designed such that they form a C-shaped surface, thus forming the shape of the corner parts 104, which are inclined inwards from the inner side surfaces towards the base surface.

[0089] A slurry 200 containing a ceramic material for forming the protective layer 3 is injected into a mold 100. Subsequently, in step (2), the element body 2 is inserted into the mold 100 in which the slurry 200 has been injected, and the slurry 200 is temporarily cured after the element body is positioned and held by a clamping device (not shown) or the like. Then, in step (3), the two molds 101 and 102 are opened to remove the element body 2, which is covered with the temporarily cured slurry 200, and the element body is fired (for example, at 1000 °C) to form the protective layer 3. Test example 3

[0090] Next, samples of the gas sensor element 1, produced by the procedure described above and exhibiting different values ​​for the angle θ formed between the end face 32 and the side face 33 of the protective layer 3, were tested to evaluate the influence of cracking on the element. The gas sensor element 1 samples were configured such that the corner part 4 had an effective length L of approximately 0.9 mm and the angle θ was either obtuse or acute, as one of the end face 32 and the side face 33 of the protective layer 3, or both, were inclined (i.e., Examples 4 and 5). The thickness t of the protective layer 3 at the corner part 4 was 0.15 mm.

[0091] The water exposure test was performed in the same manner as in test example 2. The IL change ratio was calculated as the amount of water droplet W was gradually increased from 3 µL, and the maximum amount of water droplet at which no cracking of the element was deemed to occur (i.e., the IL change ratio was 5% or less) was determined. The results are shown in Table 2. Table 2 Example No. L [mm] angle θ [°] Drop volume [µL] Example 4 0,918 110 3 Example 5 0,920 85 8

[0092] As can be seen from Table 2, even in Example 4, where the angle θ formed by two surfaces of the protective layer 3 is an obtuse angle of 110°, if the effective length L of the corner part 4 is approximately 0.9 mm, the IL change ratio could be reduced to less than or equal to 5%, even when the droplet volume was 3 µL (i.e., corresponding to a water droplet diameter of 1.79 mm). Furthermore, in Example 5, where the angle θ is an acute angle of 85°, even when the droplet volume was 8 µL, which is larger than in the previous case (i.e., corresponding to a water droplet diameter of 2.48 mm), the IL change ratio could be reduced to less than or equal to 5%, and it is evident that the effect of water droplet W being cleaved and the amount of absorption being reduced can be further enhanced.If the droplet diameters in examples 3 and 4 are considered to be the assumed diameters D, the ratios D / L are 1.95 and 2.70 respectively. Third embodiment

[0093] The third embodiment according to a gas sensor element and a gas sensor is described with reference to Fig. 21. The gas sensor element 1 shown in the preceding embodiments can be modified by applying a water-repellent property to the protective layer 3 to further improve water resistance. To apply or impart a water-repellent property to the protective layer 3, the protective layer 3 can, for example, have a configuration such that the Leidenfrost phenomenon or Leidenfrost effect can occur, or it can be provided with a hydrophobic film on its surface, as described in the PTL 1 specified above.

[0094] 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.

[0095] More precisely, the configuration resembles the protective layer 3 found in Fig. 21 shows the second embodiment, which is shown in Fig.Figure 16 shows that the end face 32 and the side face 33 of the protective layer 3 are arranged at an angle to extend as they move away from the corner part 4, and the angle θ formed by the end face 32 and the side face 33 is an obtuse angle. With such a configuration, for example, a hydrophobic film can be formed on the surface of the protective layer 3 by applying a heat-resistant fluorinated resin or the like to create a water-repellent protective layer 3. Alternatively, a water-repellent property can be achieved by increasing the thermal conductivity of the ceramic particle layer forming the surface layer of the protective layer 3 by making this layer relatively dense. The thermal conductivity is preferably, for example, in the range of 0.2 to 5 W / mK.Increased thermal conductivity increases the heat flow with respect to the water droplets W, and thermal boiling can form a film which causes the Leidenfrost phenomenon.

[0096] Thus, the force of adhesion to the water droplet contact surface 41 weakens, and the speed of the water droplet W is maintained because the protective layer 3 exhibits a water-repellent property when the water droplet W collides with the corner part 4. This assists the water in leaving the protective layer. Since the water droplets W are prevented from being absorbed within the corner parts 4 of the protective layer 3, and the detached water droplets W1 are unlikely to be absorbed even if they come into contact with the outer surface 31 again, the water resistance is further increased. Test example 4

[0097] Next, a sample of the gas sensor element 1 was fabricated in a manner similar to that used in Test Example 3, and a water-repellent property was applied to the protective layer 3 to evaluate its effect on crack formation in the element. The sample of the gas sensor element 1 was formed in the same shape as that of Example 4 in Test Example 3 described above, and the effective length L of the corner part 4 was approximately 0.9 mm, and the angle θ formed between the end face 32 and the side face 33 of the protective layer 3 was an obtuse angle of 110°. The thickness t of the protective layer 3 at the corner part 4 was 0.15 mm, and a hydrophobic film was further formed on the surface of the protective layer 3 by applying a heat-resistant fluorinated resin, thus yielding Example 6.

[0098] The water exposure test was performed in the same manner as in test example 3. The IL change ratio was calculated as the amount of water droplet W was gradually increased from 3 µL, and the maximum amount of water droplet at which no cracking of the element was deemed to occur (i.e., the IL change ratio was 5% or less) was determined. The results are shown in Table 3.

[0099] The effective length L of the corner part 4 was 0.9 mm, and the angle θ formed between the end face 32 and the side face 33 of the protective layer 3 was an obtuse angle of 110°. The layer thickness t of the protective layer 3 at the corner part 4 was 0.15 mm. Table 3 Example No. L [mm] Water-repellent capability Drop volume [µL] Example 4 0,918 No 3 Example 6 0,925 Yes 10

[0100] As can be seen from Table 3, Example 6, obtained by further applying a water-repellent property to the protective layer 3 of Example 4, whose angle θ formed by the two surfaces of protective layer 3 is an obtuse angle, was able to suppress the IL change ratio, which is less than or equal to 5%, in the range from a droplet quantity of 3 µL (i.e., corresponding to a droplet diameter of 1.79 mm) to a larger droplet quantity of 10 µL (i.e., corresponding to a droplet diameter of 2.68 mm). This indicates that the effect of water droplet W being shed and the amount of absorption being reduced can be further enhanced if the protective layer 3 has a water-repellent property. If the droplet diameter is considered to be the assumed diameter D, the ratio D / L is 2.90.

[0101] 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) for detecting a specific gas component in a measured gas, wherein the gas sensor element (1) is enclosed in a cover body (S) and has the following features: an element body (2) in the form of a long plate, which has a gas collection part (20) at its end on one side of the end surface (21) in a longitudinal direction (X); and a porous protective layer (3) which covers an outer circumference of the end on this side of the end surface (21) of the element body (2), wherein in a cross-section comprising two adjacent surfaces selected from the end surface (21) and the side surfaces (22) connected to the end surface (21), an outer surface (31) of the protective layer (3) facing an element corner (23) at which the two surfaces (21, 22) meet, has a shape with a corner part (4), the corner part (4) is configured such that the ratio (D / L) of an assumed diameter (D) of a water droplet (W) contained in the measured gas in an application environment to an effective length (L) of the corner part (4) in the cross-section including the two surfaces (21, 22) is greater than or equal to 1.5, wherein the cover body (S) has an inner cover (S11) and an outer cover (S12), and the assumed diameter (D) is determined as a function of the size of a hole diameter of through holes (S13, S14) of the cover body (S) and a space between the inner cover (S11) and the outer cover (S12) of the cover body (S), and in the cross-section which includes these two surfaces (21, 22), an angle formed by two surfaces (32, 33, 321, 331) of the outer surface (31) which meet at the corner part (4) is an acute angle. [2] Gas sensor element (1) according to claim 1, wherein the corner part (4) is located on an extension line of a line (A) which bisects the element corner (23), the corner part (4) has a water droplet contact surface (41) which includes an intersection point (c) between the extension line of a line (A) and the outer surface (31) of the protective layer (3), and the effective length (L) of the corner part (4) is a distance between two ends of the water droplet contact surface (41) in the cross-section which includes two surfaces (21, 22). [3] Gas sensor element (1) according to claim 2, wherein the water droplet contact surface (41) has deformation points (a, b) which are located in directions from the intersection point (c) towards the two surfaces (21, 22), and the two ends of the water droplet contact surface (41) are defined at the deformation points (a, b). [4] Gas sensor element (1) according to claim 2 or 3, wherein the water droplet contact surface (41) is flat or curved. [5] Gas sensor element (1) according to any one of claims 1 to 4, wherein the effective length (L) of the corner part (4) is less than 1.2 mm. [6] Gas sensor element (1) according to any one of claims 1 to 5, wherein a ratio (L / t) of the effective length (L) of the corner part (4) to a layer thickness (t) of the protective layer (3) on the corner part (4) is less than or equal to 6. [7] Gas sensor element (1) according to one of claims 1 to 6, wherein the corner part (4) projects outwards from at least one of the two surfaces (32, 33) of the protective layer (3) which faces these two surfaces (32, 33). [8] Gas sensor element (1) according to any one of claims 1 to 7, wherein the effective length (L) of the corner part (4) is greater than or equal to 0.8 mm and less than 1.0 mm. [9] Gas sensor element (1) according to any one of claims 1 to 8, wherein the protective layer (3) has a water-repellent capability. [10] Gas sensor (S) comprising the gas sensor element (1) according to any one of claims 1 to 9, further comprising a cylindrical housing (H) which supports an outer circumference of the gas sensor element (1), wherein an end of the gas sensor element (1) on which the protective layer (3) is provided is enclosed in the cover body (S) which is attached to an end of the cylindrical housing (H), and the measured gas is introduced into the cover body (S) through a through-hole (S13, S14) provided in the cover body (S).

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