Gas sensor element

Abstract

A gas sensor element (1) has an elongated shape and comprises: a solid electrolyte body (2) having oxygen ion conductive properties; and a heater (5) that heats the solid electrolyte body (2). When a longitudinal-direction X region, which is of a side surface (21) of the solid electrolyte body (2) on the heater (5) side and the temperature of which is at least Tmax − 300°C when the solid electrolyte body (2) is heated by the heater (5), is defined as a heated region (211), Tmax being the temperature at a position that is in the longitudinal direction X on the surface (21) and that becomes the warmest position during the heating, 70 to 1,000 pre-existing cracks (29) each having a length of 2 to 20 µm are present per 1 mm2 of the heated region (211).

Description

Gas Sensor Element CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Application No. 2024-090954, filed on June 4, 2024, the contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a gas sensor element.

[0003] Gas sensors for detecting the concentration of a specific gas in a gas to be measured are provided in the exhaust system of an internal combustion engine, etc. These gas sensors incorporate a gas sensor element as a detection element. For example, as disclosed in Patent Document 1, there is a gas sensor element in which a solid electrolyte body and a heater for heating the solid electrolyte body are integrated. The gas sensor element of Patent Document 1 has a buffer layer between the electrode and the solid electrolyte portion to mitigate the occurrence of internal cracks during firing, which is part of the manufacturing process of the gas sensor element.

[0004] JP 2017-133983 A

[0005] The technology disclosed in Patent Document 1 merely relieves stress between the electrodes and the solid electrolyte during firing, and does not prevent element cracking due to exposure to water during use (hereinafter, also referred to as "water-induced cracking" as appropriate).

[0006] Another possible method for preventing cracking due to water exposure is to slow down the rate at which the temperature of the element rises, but this method does not meet the demand for rapid activation of the gas sensor element, and other means are required to prevent cracking due to water exposure.

[0007] The present disclosure aims to provide a gas sensor element that can effectively prevent cracks due to exposure to water.

[0008] One aspect of the present disclosure is a long gas sensor element having a solid electrolyte body having oxygen ion conductivity and a heater for heating the solid electrolyte body, wherein, when a region in the longitudinal direction of the surface of the solid electrolyte body facing the heater, where the temperature Tmax is the highest when heated by the heater and the region is at least (Tmax-300°C) above the temperature Tmax, is defined as a heated region, the heated region has existing cracks having a length of 2 to 20 μm and a length of 1 mm or less. 2 There are 70 to 1000 of these per gas sensor element.

[0009] In the gas sensor element, the heated region of the solid electrolyte body has existing cracks with a length of 2 to 20 μm, and the cracks are separated by 1 mm. 2 There are 70 to 1000 per unit area. This makes it possible to suppress the growth of new cracks even if they occur in the solid electrolyte body during use of the gas sensor element. As a result, cracks due to water exposure can be effectively prevented.

[0010] As described above, according to the above-described embodiment, it is possible to provide a gas sensor element that can effectively prevent cracks due to exposure to water.

[0011] The above and other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which Fig. 1 is a cross-sectional view taken along the axial direction of a gas sensor element in a first embodiment, Fig. 2 is a cross-sectional view taken along line I-I in Fig. 2, Fig. 2 is a cross-sectional view taken along line II-II in Fig. 1, Fig. 3 is a cross-sectional view taken along line III-III in Fig. 1, Fig. 4 is an explanatory diagram of a heated region in a thermoviewer image of the surface of a solid electrolyte body in the first embodiment, with auxiliary lines drawn on it, Fig. 5 is a cross-sectional view taken along line V-V in Fig. 1, Fig. 6 is an explanatory diagram of a pre-existing crack in the first embodiment, and Fig. 7 is an SEM image of a part of a heated region in which a pre-existing crack has occurred in the first embodiment, showing the SEM image before image processing. FIG. 8 is an SEM image of a portion of a heated region in which an existing crack has occurred in embodiment 1, the SEM image being an SEM image after image processing; FIG. 9 is an explanatory diagram of a manufacturing method for a gas sensor element in embodiment 1; FIG. 10 is a schematic explanatory diagram illustrating the difference in the ease with which the element is distorted due to differences in compressive load in embodiment 1; FIG. 11 is an explanatory diagram illustrating the growth of a new crack when there are no existing cracks; and FIG. 12 is an explanatory diagram illustrating the growth of a new crack when the number density of existing cracks is high or the total length of existing cracks per unit area is long.

[0012] (Embodiment 1) An embodiment of a gas sensor element will be described with reference to Figs. 1 to 10. As shown in Figs. 1 and 2, the gas sensor element 1 of this embodiment includes a solid electrolyte body 2 and a heater 5. The solid electrolyte body 2 has oxygen ion conductivity. The heater 5 heats the solid electrolyte body 2.

[0013] In the heated region 211 of the solid electrolyte body 2, existing cracks 29 having a length of 2 to 20 μm are formed within a range of 1 mm. 2 The heated region 211 is a region in the longitudinal direction X of the surface 21 on the heater 5 side of the solid electrolyte body 2 that has a temperature equal to or higher than (Tmax-300°C) where Tmax is the highest temperature at the position in the longitudinal direction X when heated by the heater 5 (see FIG. 4).

[0014] The gas sensor element 1 of this embodiment is a laminated gas sensor element in which multiple ceramic layers are stacked. The gas sensor element 1 includes a plate-shaped solid electrolyte body 2, on one side of which a sensor electrode 31 and a reference gas-side electrode 32 are formed, respectively. The sensor electrode 31 and the reference gas-side electrode 32 are disposed opposite each other with a portion of the solid electrolyte body 2 interposed therebetween. The sensor electrode 31, the reference gas-side electrode 32, and a portion of the solid electrolyte body 2 between the sensor electrode 31 and the reference gas-side electrode 32 form a sensor cell 3. The sensor electrode 31 is active with respect to a specific gas in a measurement gas. For example, the measurement gas is exhaust gas from an internal combustion engine, and the specific gas is oxygen. The oxygen-active sensor electrode 31 contains, for example, platinum (Pt) and gold (Au).

[0015] The gas sensor element 1 has a chamber 71 in which the sensor electrode 31 is disposed, and a duct 72 in which the reference gas-side electrode 32 is arranged. A chamber-forming layer 11 and a shielding layer 12 are laminated in this order on the surface of the solid electrolyte body 2 on which the sensor electrode 31 is provided. Furthermore, a duct-forming layer 13 and a heater layer 14 are laminated in this order on the surface of the solid electrolyte body 2 on which the reference gas-side electrode 32 is provided. In this embodiment, a porous protective layer 16 is provided on a part of the tip side of the gas sensor element 1 including the sensor cell 3, covering the entire periphery and the tip side.

[0016] In this specification, the stacking direction of multiple ceramic layers will be referred to as the Z direction as appropriate. The gas sensor element 1 has a plate-like rod shape that is elongated in one direction perpendicular to the Z direction. The longitudinal direction of the gas sensor element 1 will be referred to as the X direction as appropriate. The direction perpendicular to both the X direction and the Z direction will be referred to as the Y direction as appropriate.

[0017] The chamber 71 is provided with a gas inlet 150 for introducing a gas to be measured (e.g., exhaust gas from an internal combustion engine) into the chamber 71, and a diffusion resistance member 15 is disposed in the gas inlet 150. The duct 72 opens at the base end of the gas sensor element 1, and is configured to introduce air as a reference gas from the base end side.

[0018] The solid electrolyte body 2 is a ceramic layer whose main component is zirconia. The chamber forming layer 11, the shielding layer 12, the duct forming layer 13, and the heater layer 14 are all ceramic layers whose main component is alumina. In this manner, in this embodiment, ceramic layers made of different materials are laminated on the solid electrolyte body 2. That is, ceramic layers (11, 12, 13, 14) made of materials having linear expansion coefficients different from that of the solid electrolyte body 2 are laminated on the solid electrolyte body 2.

[0019] The heater 5 is formed on the heater layer 14. That is, a heater pattern is formed on the surface of the heater layer 14 facing the duct forming layer 13. As shown in FIG. 3, the heater 5 has a heat-generating portion 51 and lead portions 52. The lead portions 52 are connected to the base end side of the heat-generating portion 51 and extend toward the base end side. The heat-generating portion 51 has a higher electrical resistance than the lead portions 52. As a result, when electricity is applied to the heater 5, the heat-generating portion 51 mainly generates heat.

[0020] The heated region 211 is defined as a region of the surface 21 on the heater 5 side of the solid electrolyte body 2 that is heated to a predetermined temperature or higher by the heat generating portion 51. As described above, the heated region 211 is a region in the longitudinal direction X of the surface 21 on the heater 5 side of the solid electrolyte body 2 that is heated to a temperature higher than the predetermined temperature by the heater 5, where Tmax is the temperature at the position in the longitudinal direction X where the temperature is highest when heated by the heater 5 (see FIG. 4 ). More specifically, the heated region 211 is defined as follows, for example. The region in the longitudinal direction X that is higher than (Tmax - 300°C) is identified based on an image obtained by measuring the surface 21 of the solid electrolyte body 2 heated by the heater 5 with a thermoviewer as shown in FIG. 4 . Both ends of the heated region 211 in the longitudinal direction X are defined as positions where the temperature along the central axis C of the solid electrolyte body 2 is (Tmax - 300°C).

[0021] For example, if the temperature Tmax of the highest temperature portion 212 on the surface 21 of the solid electrolyte body 2 heated by the heat generating portion 51 of the heater 5 is 700°C, the heated region 211 is identified as a region in the longitudinal direction X where the temperature is 400°C or higher. That is, when using the gas sensor element 1, the heater 5 heats the sensor cell 3 to a temperature equal to or higher than the activation temperature in order to activate the sensor cell 3. At this time, it is assumed that the temperature Tmax of the highest temperature portion (i.e., the highest temperature portion 212) on the surface 21 of the solid electrolyte body 2 facing the heater 5 is 700°C, for example. In this case, the heated region 211 is a region on the surface 21 of the solid electrolyte body 2 where the temperature is equal to or higher than (Tmax - 300°C), i.e., equal to or higher than 400°C. The length of the heated region 211 in the X direction is, for example, approximately ¼ to ⅛ of the overall length of the gas sensor element 1.

[0022] In the heated region 211, the existing cracks 29 having a length of 2 to 20 μm are formed. 2 That is, for example, as shown in FIG. 5, when an arbitrary portion S in the heated region 211 is observed, existing cracks 29 having a length of 2 to 20 μm are present within a range of 1 mm. 2 In this specification, the number of existing cracks 29 is 70 to 1000 per 1 mm. 2 The number of existing cracks 29 per mm was calculated as the number density (unit: pieces / mm 2 More preferably, the number density of the existing cracks 29 is 100 to 1000 pieces / mm 2 is.

[0023] 6 shows a schematic example of a pre-existing crack 29 that appears on the surface 21 of the solid electrolyte body 2. The pre-existing crack 29 is a crack that exists before the gas sensor element 1 is used, and has a length of 2 to 20 μm. Note that there are no pre-existing cracks with a length exceeding 20 μm.

[0024] Furthermore, the pre-existing crack 29 can be defined as a dark line that is clearly distinguishable from other lines (such as grain boundaries) when an image of the heated region 211 captured by a scanning electron microscope (SEM) is processed as described below. The image processing is a process that increases the brightness and contrast of the captured SEM image by 50%. An example of an SEM image before image processing is shown in FIG. 7, and an example of an image after image processing is shown in FIG. 8. The length of the pre-existing crack 29 is defined as the length measured in a straight line between both ends, as indicated by the symbol L in FIGS. 6 and 8.

[0025] In addition, in the heated region 211, existing cracks 29 having a length of 2 to 20 μm are formed within 1 mm. 2 The total length of each molecule is 100 to 3000 μm. 2 The total length of the pre-existing cracks 29 existing per 1 mm is 100 to 3000 μm. More preferably, the pre-existing cracks 29 having a length of 2 to 20 μm are present in the heated region 211 within 1 mm. 2 The total length of each of these molecules is 150 to 3000 μm.

[0026] The number density of existing cracks 29 and 1 mm 2 The total length per unit area can be controlled by adjusting the manufacturing conditions of the gas sensor element 1. When manufacturing the gas sensor element 1, for example, as shown in Fig. 9, a first laminate 101 formed by laminating a plurality of green sheets and a second laminate 102 formed by laminating a plurality of green sheets are prepared. The first laminate 101 is formed by laminating green sheets that respectively form the solid electrolyte body 2, the chamber forming layer 11, and the shielding layer 12 shown in Figs. 1 and 2. The second laminate 102 is formed by laminating green sheets that respectively form the heater layer 14 and the duct forming layer 13 shown in Figs. 1 and 2.

[0027] Here, when forming the first laminate 101, each green sheet is compressed and pressure-bonded in the stacking direction. The same process is performed when forming the second laminate 102. Thereafter, the first laminate 101 and the second laminate 102 are stacked, bonded, and fired. During firing, the binder in the green sheets is burned away, causing the volume of the laminate of the first laminate 101 and the second laminate 102 to shrink. If the degree of this volumetric shrinkage is large, it is thought that large internal stresses are likely to occur, making cracks more likely to occur. In other words, the number of existing cracks 29 is likely to increase, and they are likely to be long.

[0028] From this perspective, it can be seen that the likelihood of the occurrence of existing cracks 29 in the solid electrolyte body 2 can be adjusted by the compressive load when producing the first laminate 101. That is, when producing the first laminate 101, as described above, a plurality of green sheets are stacked and compressed together, and the magnitude of the compressive load applied at this time causes differences in the ceramic density at the stage of the first laminate 101, which in turn causes differences in the likelihood of distortion during firing. This will be explained using the schematic diagram of FIG.

[0029] As shown in the figure, when the compression load is low, the proportion of voids in the first laminate 101 after compression is relatively large, and when the compression load is high, the proportion of voids in the first laminate 101 after compression is relatively small. During firing, the binder is burned away and the voids also become smaller. Therefore, the larger the proportion of voids before firing, the greater the shrinkage rate during firing. As a result, the larger the proportion of voids before firing, the greater the internal strain during firing, which increases the number of cracks and lengthens them. In this way, by adjusting the compression load during compression of the first laminate 101, the number density of the existing cracks 29 and the crack size per mm can be controlled. 2 The total length per unit can be controlled.

[0030] Next, the effects of this embodiment will be described. In the gas sensor element 1, the heated region 211 of the solid electrolyte body 2 has existing cracks 29 with a length of 2 to 20 μm, which are formed within a range of 1 mm. 2There are 70 to 1000 of these per unit area. This makes it possible to suppress the growth of new cracks even if they occur in the solid electrolyte body 2 during use of the gas sensor element 1. As a result, cracks due to water exposure can be effectively prevented.

[0031] That is, for example, as shown in Fig. 6, assume that a pre-existing crack 29 is present in a moderate amount in the heated region 211. In this case, suppose that a crack 28 occurs on the surface 21 of the solid electrolyte body 2 due to exposure of the gas sensor element 1 to water. Once the crack 28 occurs, there is a concern that the crack 28 will grow. However, because a moderate amount of pre-existing crack 29 is present, the growth of the crack 28 will stop when it reaches the pre-existing crack 29. This makes it possible to suppress the growth of the crack 28.

[0032] 11, if a crack 28 were to occur on the surface 21 of the solid electrolyte body 2 in a situation where there are no pre-existing cracks 29, it would be difficult to stop the progression of the crack 28. As a result, as shown in the figure, the continuous crack 28 would grow significantly, which would likely lead to cracking of the element. Therefore, it is thought that if there are no pre-existing cracks 29 or if there are too few of them, it would be difficult to prevent cracks due to water exposure.

[0033] 12, if there are too many existing cracks 29, new cracks 27 are likely to occur between the existing cracks 29. There is also a concern that the new cracks 27 will connect the existing cracks 29 and develop into larger cracks. Therefore, it is thought that it is difficult to suppress element cracking when there are too many existing cracks 29.

[0034] From the above viewpoint, it can be said that cracking due to water can be easily suppressed by having an appropriate number of pre-existing cracks 29 in the heated region 211. In this embodiment, the heated region 211 of the solid electrolyte body 2 has pre-existing cracks 29 with a length of 2 to 20 μm, the pre-existing cracks 29 being 1 mm apart. 2 There are 70 to 1000 cracks per 1 mm, more preferably 100 to 1000 cracks per 1 mm. This effectively prevents cracks due to water damage. 2The total length of the pores is preferably 100 to 3000 μm, more preferably 150 to 3000 μm, per pore. This makes it possible to more effectively suppress cracking due to water exposure.

[0035] Furthermore, in the gas sensor element 1 of this embodiment, a ceramic layer made of a material having a different linear expansion coefficient from that of the solid electrolyte body 2 is laminated on the solid electrolyte body 2. Therefore, when the gas sensor element 1 is exposed to water, a large stress caused by the linear expansion coefficient is likely to occur, and cracks are likely to occur. In this way, in the gas sensor element 1 which is relatively prone to cracks, by providing an appropriate amount of pre-existing cracks 29 as described above, cracks due to water exposure can be effectively suppressed.

[0036] As described above, according to this embodiment, it is possible to provide a gas sensor element that can effectively prevent cracks due to exposure to water.

[0037] Experimental Example 1 In this example, a water exposure test was performed on the gas sensor element 1 in which the number density of the existing cracks 29 was varied. The sample used in this example had the same basic configuration as that of the first embodiment.

[0038] As described above, the number density of the pre-existing cracks 29 was controlled by adjusting the compression load when the first laminate 101 was pressed (see FIG. 9 ). That is, the samples at each level had different number densities of the pre-existing cracks 29 by varying the compression load. The error in the number density of the pre-existing cracks 29 was about 10%.

[0039] The test method was as follows. First, each sample was placed with the shielding layer 12 side in the Z direction facing vertically upward. A voltage of 14 V was applied to the heater 5 to heat the sample, and when the temperature of the outer surface of the porous protective layer 16 reached approximately 300°C, a 2 μL water droplet was dropped on the outer surface of the porous protective layer 16 on the shielding layer 12 side. 2 μL is an amount close to the maximum amount normally expected when the gas sensor element 1 is actually incorporated into a gas sensor and installed in the exhaust system of a vehicle. The position where the water droplet was dropped was the Z-direction projection area of ​​the heated region 211 onto the upper surface of the sample.

[0040] Ten samples were prepared for each level and subjected to a water exposure test. The results of the water exposure test are shown in Table 1. In the table, the "element cracking frequency" indicates the number of samples at each level that experienced element cracking out of the 10 samples. The evaluation results were rated as "Good" for an element cracking frequency of 0 / 10, "Fair" for an element cracking frequency of 1 / 10 to 9 / 10, and "Poor" for an element cracking frequency of 10 / 10 or more.

[0041]

[0042] As can be seen from the table, the number density of the existing cracks 29 is 50 / mm 2 and 1200 pieces / mm 2 In all of the above 10 samples, element cracks occurred. In addition, the number density of existing cracks 29 was 70 / mm 2 In the case of the samples of , element cracks occurred in some samples. 2 From this result, the number density of the existing cracks 29 was set to 70 to 1000 pieces / mm 2 By doing so, it is possible to suppress cracking of the element, and the number density of the existing cracks 29 is set to 100 to 1000 pieces / mm 2 This can be said to be sufficient to prevent element cracking.

[0043] In this example, a water exposure test was conducted on the gas sensor element 1 in which the total length of the existing cracks 29 per unit area was varied. The samples used in this example had the same basic structure as that of the first embodiment.

[0044] 1 mm 2 As described above, the total length of the existing cracks 29 per unit area was controlled by adjusting the compression load when the first laminate 101 was pressed (see FIG. 9). That is, the samples at each level were made to have different compression loads, so that the total length of the existing cracks 29 per unit area was controlled by adjusting the compression load when the first laminate 101 was pressed (see FIG. 9). 2 The total length of the existing cracks 29 is varied. The error in the total length is about 10%.

[0045] The test method and evaluation method were the same as those in Experimental Example 1. The results of the water exposure test are shown in Table 2. The evaluation criteria for the results, "Good", "Average", and "Poor", were the same as those in Experimental Example 1.

[0046]

[0047] As can be seen from the table, 1 mm 2 In the samples in which the total length of the existing cracks 29 per 1 mm was 3,300 μm or more, element cracks occurred in all 10 samples. 2 The total length of existing cracks 29 per 2 In the case of the samples of 1 mm, cracks occurred in the elements of some of the samples. 2 For the samples in which the total length of the existing cracks 29 per 1 mm was 150 to 3000 μm, no element cracks occurred in any of the samples. 2 By setting the total length of the existing cracks 29 per 1 mm to 100 to 3000 μm, element cracking can be suppressed. 2 It can be said that cracking of the element can be sufficiently prevented by setting the total length of the existing cracks 29 per unit area to 150 to 3000 μm. 2 ] is 1 mm 2 represents the unit of the total length of existing cracks 29 per

[0048] The present disclosure is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the spirit of the present disclosure.

[0049] Although the present disclosure has been described with reference to the embodiments, it is understood that the present disclosure is not limited to the embodiments or structures. The present disclosure also encompasses various modifications and modifications within the scope of equivalents. In addition, various combinations and forms, as well as other combinations and forms including only one element, more than one element, or less than one element, are also within the scope and spirit of the present disclosure.

[0050] The features of the present disclosure are as follows: [1] A long-shaped gas sensor element (1) having a solid electrolyte body (2) having oxygen ion conductivity and a heater (5) for heating the solid electrolyte body, wherein, when a region in the longitudinal direction of a surface (21) of the solid electrolyte body facing the heater, where the temperature Tmax is the highest when heated by the heater, is defined as a heated region (211), the heated region has a temperature Tmax of at least (Tmax-300°C) and the heated region has a pre-existing crack (29) having a length of 2 to 20 μm and a crack width of 1 mm. 2 [2] In the heated region, the existing cracks having a length of 2 to 20 μm are present at 70 to 1,000 per 1 mm. 2 [3] The gas sensor element according to [1], wherein the number of existing cracks having a length of 2 to 20 μm is 100 to 1000 per 1 mm. 2 [4] The gas sensor element according to [1] or [2], wherein the existing cracks having a length of 2 to 20 μm are present in the heated region within a range of 1 mm to 3000 μm in total length. 2 [5] The gas sensor element according to any one of [1] to [4], wherein a ceramic layer (11, 12, 13, 14) made of a material having a linear expansion coefficient different from that of the solid electrolyte body is laminated on the solid electrolyte body.

Claims

1. A long gas sensor element (1) having a solid electrolyte body (2) having oxygen ion conductivity and a heater (5) for heating the solid electrolyte body, wherein, when the heated region (211) of the surface (21) of the solid electrolyte body on the heater side is defined as a region in the longitudinal direction where the temperature Tmax at the longitudinal position where the temperature is highest when heated by the heater is equal to or higher than (Tmax - 300°C), the heated region has existing cracks (29) having a length of 2 to 20 μm and a width of 1 mm. 2 There are 70 to 1,000 gas sensor elements per unit area.

2. In the heated area, the existing cracks having a length of 2 to 20 μm are formed within 1 mm 2 2. The gas sensor element according to claim 1, wherein 100 to 1000 particles are present per one particle.

3. In the heated area, the existing cracks having a length of 2 to 20 μm are formed within 1 mm 2 3. The gas sensor element according to claim 1, wherein the total length of the plurality of electrodes is 100 to 3000 μm per electrode.

4. In the heated area, the existing cracks having a length of 2 to 20 μm are formed within 1 mm 2 4. The gas sensor element according to claim 3, wherein the total length of the plurality of electrodes is 150 to 3000 μm per electrode.

5. The gas sensor element according to claim 1 or 2, wherein ceramic layers (11, 12, 13, 14) made of a material having a linear expansion coefficient different from that of said solid electrolyte body are laminated on said solid electrolyte body.

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