Sensor elements for gas sensors

Abstract

This invention provides a sensor element for a gas sensor, which includes an end protection layer. The end protection layer is a porous layer disposed on the outer periphery of a predetermined range in the element substrate of the sensor element, starting from the end with the monitoring part. The end protection layer includes: a first end protection layer disposed on two main surfaces of the element substrate; a second end protection layer disposed to cover the end and four sides of the element substrate, including the two main surfaces on which the first end protection layer is formed; and a third end protection layer disposed to cover the second end protection layer, and having a porosity less than that of the second end protection layer. The first end protection layer has a porosity of 40% or more. When the extension lengths of the first end protection layer, the second end protection layer, and the third end protection layer in the length direction of the element substrate, starting from the end face of the element substrate, are set as L1, L2, and L3 respectively, L1 ≥ L2 and L1 ≥ L3.

Description

Technical Field

[0001] The present invention relates to a sensor element of a gas sensor, and in particular to a surface protection layer thereof. Background Art

[0002] Conventionally, gas sensors for detecting the concentration of a desired gas component in a measured gas, such as exhaust gas from an internal combustion engine, are well known. These gas sensors comprise a sensor element composed of a solid electrolyte having oxygen ion conductivity, such as zirconium oxide (ZrO2), and having a plurality of electrodes on the surface and within the sensor element. Such sensor elements are known to have a long, plate-like shape, with a protective layer (porous protective layer) formed of a porous body provided at the end portion thereof, where the measured gas is introduced (see, for example, Patent Document 1).

[0003] The purpose of providing a protective layer on the surface of the sensor element is to ensure the sensor element's resistance to water immersion during use. Specifically, it is to prevent water immersion cracking, which occurs when the sensor element is cracked due to thermal shock (cold source heat) caused by water droplets adhering to the sensor element surface.

[0004] However, when such a protective layer is provided to improve thermal shock resistance, the overall heat capacity of the sensor element increases, and the restraining force on the sensor element increases. This deteriorates the sensor element's ability to rapidly heat up. Furthermore, depending on the operating environment of the gas sensor, vibrations generated in that environment may cause the protective film to detach from the sensor element. Therefore, ensuring the adhesion of the protective layer is essential.

[0005] Furthermore, if water that adheres to and condenses on the protective layer during use of the gas sensor remains in the protective layer after use, the water evaporates as the temperature of the sensor element rises when it is used again, causing volume expansion in the protective layer, which may cause the protective layer to peel off.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent No. 5344375 Summary of the Invention

[0009] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a sensor element for a gas sensor that appropriately suppresses peeling of a protective layer caused by evaporation of water vapor during temperature increase.

[0010] To solve the above-mentioned problems, a first embodiment of the present invention is a sensor element for a gas sensor, characterized by comprising: an element base body, the element base body being a ceramic structure having a monitoring portion for a component of a gas to be measured; and an end protective layer, the end protective layer being a porous layer provided at an outer peripheral portion of a predetermined range starting from an end portion of the element base body on a side having the monitoring portion, the end protective layer including: a first end protective layer provided on two main surfaces of the element base body; a second end protective layer provided so as to cover the end portion and four side surfaces of the element base body including the two main surfaces on which the first end protective layer is formed; and a third end protective layer provided so as to cover the second end protective layer and having a porosity lower than that of the second end protective layer, the first end protective layer having a porosity of 40% or greater, wherein, when the extension lengths of the first end protective layer, the second end protective layer, and the third end protective layer in the longitudinal direction of the element base body starting from the end surface of the element base body are denoted as L1, L2, and L3, respectively, L1 ≥ L2 and L1 ≥ L3.

[0011] A second aspect of the present invention is the sensor element according to the first aspect, wherein L1 ≥ L2 ≥ L3 .

[0012] A third aspect of the present invention is the sensor element according to the first aspect, wherein L1 ≥ L3 ≥ L2 .

[0013] A fourth aspect of the present invention is the sensor element according to any one of the first to third aspects, wherein the porosity of the second terminal protection layer is 40% to 80%.

[0014] A fifth aspect of the present invention is the sensor element according to any one of the first to fourth aspects, wherein the porosity of the first terminal protection layer is 40% to 60%.

[0015] A sixth aspect of the present invention is the sensor element according to any one of the first to fifth aspects, wherein the third terminal protection layer surrounds at least a predetermined range of the element base of the sensor element that reaches 500° C. or higher during driving.

[0016] According to the first to sixth aspects of the present invention, it is possible to realize a gas sensor sensor element that appropriately suppresses peeling of the terminal protective layer during evaporation caused by evaporation of water that has entered the terminal protective layer as the temperature rises. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic external perspective view of the sensor element 10 .

[0018] Figure 2 1 is a schematic diagram of the structure of the gas sensor 100 including a cross-sectional view of the sensor element 10 along the longitudinal direction.

[0019] Figure 3 1 is a diagram showing a process flow when manufacturing the sensor element 10 .

[0020] Figure 4 1 is a schematic diagram of the structure of a gas sensor 100 including a cross-sectional view along the longitudinal direction of a sensor element 10 according to a modified example.

[0021] Figure 5 1 is a schematic diagram of the structure of a gas sensor 100 including a cross-sectional view along the longitudinal direction of a sensor element 10 according to a modified example. DETAILED DESCRIPTION

[0022] Overview of Sensor Elements and Gas Sensors

[0023] Figure 1 : is a perspective view of the appearance of a sensor element (gas sensor element) 10 according to an embodiment of the present invention. Figure 2 This diagram schematically illustrates the structure of gas sensor 100, including a cross-sectional view along the longitudinal direction of sensor element 10. Sensor element 10 is a ceramic structure, a key component of gas sensor 100, that monitors and measures the concentration of a specific gas component in the gas being measured. Sensor element 10 is a so-called limiting current type gas sensor element.

[0024] The gas sensor 100 mainly includes a pump unit power supply 30 , a heater power supply 40 , and a controller 50 in addition to the sensor element 10 .

[0025] like Figure 1 As shown, the sensor element 10 generally has a structure in which one end of a long, plate-shaped element substrate 1 is covered with a porous terminal protection layer 2. The terminal protection layer 2 is composed of three layers: a first terminal protection layer 21, a second terminal protection layer 22, and a third terminal protection layer 23. The terminal protection layer 2 is described in detail below.

[0026] In summary, if Figure 2As shown, the element base 1 has a long, plate-shaped ceramic body 101 as its main structure, and a main surface protective layer 170 is provided on the two main surfaces of the ceramic body 101. In addition, an end protective layer 2 is provided on the outer sides of the end surface (end surface 101e of the ceramic body 101) and the four side surfaces of the sensor element 10. It should be noted that, hereinafter, the four side surfaces of the sensor element 10 (or the element base 1, the ceramic body 101) in the longitudinal direction, excluding the two end surfaces, are simply referred to as the side surfaces of the sensor element 10 (or the element base 1, the ceramic body 101).

[0027] The ceramic body 101 is made of ceramic having zirconium oxide (yttrium-stabilized zirconium oxide) as a main component as an oxygen ion conductive solid electrolyte. In addition, various structural elements of the sensor element 10 are provided outside and inside the ceramic body 101. The ceramic body 101 having the above structure is a dense and airtight ceramic body. It should be noted that Figure 2 The structure of the sensor element 10 shown is merely an example, and the specific structure of the sensor element 10 is not limited thereto.

[0028] Figure 2 The illustrated sensor element 10 is a gas sensor element having a so-called serial three-cavity structure, comprising a first internal cavity 102, a second internal cavity 103, and a third internal cavity 104 within a ceramic body 101. In summary, in sensor element 10, first internal cavity 102 communicates with a gas inlet 105, which is open to the outside (strictly speaking, communicates with the outside via an end protective layer 2) at one end E1 of ceramic body 101, via a first diffusion rate controlling section 110 and a second diffusion rate controlling section 120. Second internal cavity 103 communicates with first internal cavity 102 via a third diffusion rate controlling section 130, and third internal cavity 104 communicates with second internal cavity 103 via a fourth diffusion rate controlling section 140. The path from gas inlet 105 to third internal cavity 104 is also referred to as a gas flow section. In the sensor element 10 according to this embodiment, the gas flow section is provided in a straight line along the length of ceramic body 101.

[0029] In the figures, the first, second, third, and fourth diffusion rate controllers 110, 120, 130, and 140 are each configured as two upper and lower slits. Each of these control sections applies a predetermined diffusion resistance to the gas being measured. A buffer space 115 is provided between the first and second diffusion rate controllers 110, 120 to mitigate pulsation of the gas being measured.

[0030] Furthermore, an external pump electrode 141 is provided on the outer surface of the ceramic body 101, and an internal pump electrode 142 is provided in the first internal cavity 102. Furthermore, an auxiliary pump electrode 143 is provided in the second internal cavity 103, and a measurement electrode 145, which serves as a direct monitoring portion for the gas component being measured, is provided in the third internal cavity 104. Furthermore, a reference gas inlet 106 is provided on the other end E2 side of the ceramic body 101, communicating with the outside and for introducing a reference gas. A reference electrode 147 is provided within this reference gas inlet 106.

[0031] For example, when the sensor element 10 is intended to measure NOx in a gas to be measured, the NOx gas concentration in the gas to be measured is calculated by the following procedure.

[0032] First, the oxygen concentration of the gas to be measured introduced into the first internal cavity 102 is adjusted to a substantially constant level by the pumping action of the main pump unit P1 (intake or extraction of oxygen), and then the gas to be measured is introduced into the second internal cavity 103. The main pump unit P1 is an electrochemical pump unit comprising an external pump electrode 141, an internal pump electrode 142, and a ceramic layer 101a that is part of the ceramic body 101 located between the two electrodes. In the second internal cavity 103, the oxygen in the gas to be measured is extracted to the outside of the element by the pumping action of the auxiliary pump unit P2, which is also an electrochemical pump unit, so that the gas to be measured is brought to a sufficiently low oxygen partial pressure state. The auxiliary pump unit P2 is comprised of an external pump electrode 141, an auxiliary pump electrode 143, and a ceramic layer 101b that is part of the ceramic body 101 located between the two electrodes.

[0033] The outer pump electrode 141, the inner pump electrode 142, and the auxiliary pump electrode 143 are formed as porous cermet electrodes (for example, a cermet electrode containing 1% Au, Pt, and ZrO2). It should be noted that the inner pump electrode 142 and the auxiliary pump electrode 143, which come into contact with the gas being measured, are formed from a material that weakens the ability to reduce NOx components in the gas being measured, or has no reducing ability.

[0034] The NOx in the gas to be measured, which is brought into a low oxygen partial pressure state by the auxiliary pump unit P2, is introduced into the third internal cavity 104 and is reduced or decomposed at the measuring electrode 145 provided in the third internal cavity 104. The measuring electrode 145 is a porous metal ceramic electrode that also functions as a NOx reduction catalyst for reducing NOx present in the atmosphere within the third internal cavity 104. During the above-mentioned reduction or decomposition, the potential difference between the measuring electrode 145 and the reference electrode 147 is kept constant. Then, the oxygen ions generated by the above-mentioned reduction or decomposition are sucked out of the element by the measuring pump unit P3. The measuring pump unit P3 is configured to include an external pump electrode 141, a measuring electrode 145, and a ceramic layer 101c that is part of the ceramic body 101 present between the two electrodes. The measuring pump unit P3 is an electrochemical pump unit that sucks out the oxygen generated by the decomposition of NOx in the atmosphere around the measuring electrode 145.

[0035] Pumping (oxygen intake or exhaust) in the main pump unit P1, auxiliary pump unit P2, and measurement pump unit P3 is achieved by applying the voltage required for pumping between the electrodes included in each pump unit using a pump unit power supply (variable power supply) 30 under the control of the controller 50. In the case of the measurement pump unit P3, a voltage is applied between the external pump electrode 141 and the measurement electrode 145 so that the potential difference between the measurement electrode 145 and the reference electrode 147 is maintained at a predetermined value. The pump unit power supply 30 is generally provided for each pump unit.

[0036] The controller 50 detects the pump current Ip2 flowing between the measuring electrode 145 and the external pump electrode 141 based on the amount of oxygen sucked out by the measuring pump unit P3, and calculates the NOx concentration in the measured gas based on the fact that there is a linear relationship between the current value (NOx signal) of the pump current Ip2 and the concentration of decomposed NOx.

[0037] Preferably, the gas sensor 100 includes a plurality of electrochemical sensor cells (not shown) that monitor the potential difference between each pump electrode and the reference electrode 147 , and each pump cell is controlled by the controller 50 based on detection signals from these sensor cells.

[0038] In addition, a heater 150 is embedded in the sensor element 10 and in the ceramic body 101. The heater 150 is embedded in the gas flow portion. Figure 2The heater 150 is provided at the bottom of the drawing in FIG. The heater 150 extends from the vicinity of one end E1 to at least the locations where the measuring electrode 145 and the reference electrode 147 are formed. The main purpose of providing the heater 150 is to heat the sensor element 10 during use in order to improve the oxygen ion conductivity of the solid electrolyte constituting the ceramic body 101. More specifically, the heater 150 is provided so as to be surrounded by an insulating layer 151.

[0039] The heater 150 is a resistance heating element made of, for example, platinum, etc. Under the control of the controller 50 , power is supplied from the heater power supply 40 to generate heat in the heater 150 .

[0040] When using the sensor element 10 according to this embodiment, the heater 150 is used to heat at least the area from the first internal cavity 102 to the second internal cavity 103 to a temperature of 500°C or higher. Furthermore, the entire gas flow area from the gas inlet 105 to the third internal cavity 104 is sometimes heated to 500°C or higher. This is to improve the oxygen ion conductivity of the solid electrolyte constituting each pump cell and to properly utilize the performance of each pump cell. In this case, the temperature near the first internal cavity 102, which is the highest, reaches approximately 700°C to 800°C. The set heating temperature of the heater 150 when driving the sensor element 10 is also referred to as the element driving temperature.

[0041] Hereinafter, among the two main surfaces of the ceramic body 101, Figure 2 The main surface of the side located at the top of the figure and mainly including the main pump unit P1, the auxiliary pump unit P2 and the measuring pump unit P3 (or the outer surface of the sensor element 10 including the main surface) is called the pump surface, and Figure 2 The principal surface located at the bottom of the figure and having heater 150 (or the outer surface of sensor element 10 having this principal surface) is referred to as the heater surface. In other words, the pump surface is the principal surface closer to gas inlet port 105, the three internal cavities, and each pump unit than heater 150, and the heater surface is the principal surface closer to gas inlet port 105, the three internal cavities, and each pump unit than heater 150.

[0042] A plurality of electrode terminals 160 are formed on the other end E2 side of each main surface of the ceramic body 101 for electrically connecting the sensor element 10 to the outside. These electrode terminals 160 electrically connect the five electrodes, the two ends of the heater 150, and a wire for detecting the heater resistance (not shown) via wires (not shown) provided within the ceramic body 101 in a prescribed correspondence. Thus, the electrode terminals 160 enable the application of voltage from the pump cell power supply 30 to each pump cell of the sensor element 10, and the heating of the heater 150 by power supply from the heater power supply 40.

[0043] Furthermore, the sensor element 10 is provided with the aforementioned main surface protective layer 170 ( 170 a , 170 b ) on the pump and heater surfaces of the ceramic body 101. Main surface protective layer 170 is formed of aluminum oxide, has a thickness of approximately 5 to 30 μm, and contains pores with a porosity of approximately 20% to 40%. Main surface protective layer 170 is provided to prevent foreign matter and toxic substances from adhering to the main surfaces (the pump and heater surfaces) of the ceramic body 101 and the external pump electrode 141 provided on the pump surface side. Therefore, main surface protective layer 170 a on the pump surface side also functions as a pump electrode protective layer, protecting the external pump electrode 141.

[0044] In addition, in this embodiment, the porosity is determined by applying a known image processing method (binarization processing, etc.) to an SEM (scanning electron microscope) image of the evaluation object.

[0045] Figure 2 In the embodiment, a main surface protection layer 170 is provided on substantially the entire surface of the pump surface and the heater surface except for exposing a portion of the electrode terminal 160. However, this is merely an example and does not Figure 2 Compared to the illustrated case, the main surface protection layer 170 may be provided closer to the vicinity of the external pump electrode 141 on the one end E1 side.

[0046] <Details of the end protection layer>

[0047] The sensor element 10 is provided with an end protection layer 2 at the outermost periphery of a predetermined range starting from one end E1 of the element base 1 having the above-described structure.

[0048] The purpose of providing the end protection layer 2 is to ensure the water resistance of the portion of the element substrate 1 which reaches a high temperature (up to about 700°C to 800°C) when the gas sensor 100 is used, and to suppress the generation of cracks (water immersion cracking) in the element substrate 1 due to thermal shock caused by direct immersion of the portion in water, thereby preventing the portion from being directly immersed in water and causing cracks.

[0049] Furthermore, the terminal protection layer 2 is provided to prevent poisoning substances such as Mg from entering the interior of the sensor element 10 , that is, to ensure poisoning resistance.

[0050] However, providing the terminal protective layer 2 increases the heat capacity of the sensor element 10 and strengthens the restraining force on the element base 1. Therefore, it is not always a beneficial measure from the perspective of rapid temperature rise. In this embodiment, the structure of the terminal protective layer 2 is also determined from the perspective of ensuring this rapid temperature rise.

[0051] Specifically, if Figure 2As shown, in the sensor element 10 according to the present embodiment, the terminal protection layer 2 is composed of three layers: a first terminal protection layer 21 , a second terminal protection layer 22 , and a third terminal protection layer 23 .

[0052] The first terminal protection layer 21 is a base layer provided to ensure adhesion (close contact) between the second terminal protection layer 22 (and third terminal protection layer 23) formed thereon and the element base 1. The first terminal protection layer 21 is provided on at least two main surfaces of the element base 1: the pump surface side and the heater surface side. Specifically, the first terminal protection layer 21 comprises a first terminal protection layer 21a on the pump surface side and a first terminal protection layer 21b on the heater surface side.

[0053] However, the first end protection layer 21 is not provided on the end surface 101e (of the element base 1) and the side surface of the ceramic body 101. This is to ensure adhesion and reduce the restraining force on the element base 1 so as not to impair the rapid temperature rise property.

[0054] The first end protection layer 21 is formed from aluminum oxide, having a porosity of 40% or greater and a thickness of 20 μm to 60 μm. It should be noted that, unlike the second end protection layer 22 and the third end protection layer 23, the first end protection layer 21 is formed together with the element substrate 1 during the fabrication process of the element substrate 1, as described later. Preferably, the porosity of the first end protection layer 21 is set to 40% to 60%.

[0055] The second and third terminal protective layers 22 and 23 are sequentially arranged from the inside to cover the terminal surface 101e and four side surfaces of the element base 1 on the one terminal portion E1 side (around the outer periphery of the element base 1 on the one terminal portion E1 side). In the second terminal protective layer 22, the portion on the terminal surface 101e side is specifically referred to as the terminal portion 221, and the portions on the pump and heater surface sides are specifically referred to as the main surface portion 222. Similarly, in the third terminal protective layer 23, the portion on the terminal surface 101e side is specifically referred to as the terminal portion 231, and the portions on the pump and heater surface sides are specifically referred to as the main surface portion 232.

[0056] The second terminal protective layer 22 is made of aluminum oxide and has a porosity of 20% or greater and a thickness of 200 μm to 800 μm. Preferably, the second terminal protective layer 22 has a porosity of 40% to 80%. Furthermore, the third terminal protective layer 23 is made of aluminum oxide and has a thickness of 100 μm to 400 μm and a porosity of 10% to 35%, which is less than the porosity of the second terminal protective layer 22. Thus, the terminal protective layer 2 is constructed such that the second terminal protective layer 22, which has the lowest thermal conductivity of the three layers, is covered by the third terminal protective layer 23, which is disposed on the outermost side and has a porosity less than that of the second terminal protective layer 22.

[0057] In other words, the second terminal protective layer 22, by being a layer with low thermal conductivity, has the function of inhibiting heat conduction from the outside to the element base 1 (a thermal insulation effect), while the third terminal protective layer 23 has the function of maintaining overall strength and inhibiting water intrusion. Due to this structure, even if water adheres to the surface (the surface of the third terminal protective layer 23) of the terminal protective layer 2 during use of the sensor element 10 at high temperatures, this intrusion is suppressed, and the cold source heat caused by the sudden cooling associated with the adhesion is unlikely to be conducted to the element base 1. In other words, the terminal protective layer 2 has excellent thermal shock resistance. As a result, the sensor element 10 is less susceptible to cracking due to water immersion and has excellent water immersion resistance.

[0058] In addition, in the sensor element 10 involved in this embodiment, by setting the second end protective layer 22 and the third end protective layer 23 to satisfy the above-mentioned range of thickness and porosity, the temperature rise performance (rapid temperature rise performance) during driving can also be ensured. That is, the sensor element 10 involved in this embodiment can take into account two different characteristics: thermal shock resistance (water immersion resistance) and ensuring temperature rise performance. It should be noted that in this embodiment, the temperature rise performance of the sensor element 10 can be evaluated using the following time: the time it takes for the sensor element 10 to reach a specified temperature (for example, the time it takes for the heater 150 to reach the element driving temperature) after the heater 150 provided inside the sensor element 10 starts heating the entire sensor element 10 including the end protective layer 2 in order to start using the gas sensor 100.

[0059] However, if the thickness of the second end protection layer 22 and the third end protection layer 23 is too large, the heat load applied to the heater 150 during temperature increase increases, potentially causing cracks in the sensor element 10. Therefore, this is not preferred. From this perspective, the thickness of the second end protection layer 22 is preferably set to 700 μm or less, and the thickness of the third end protection layer 23 is preferably set to 300 μm or less.

[0060] The second and third end protection layers 22, 23 are formed by sequentially spraying (plasma spraying) their respective constituent materials onto the element substrate 1, which has the first end protection layer 21 formed on its surface. This is to ensure an anchoring effect between the first end protection layer 21, which is pre-formed when the element substrate 1 is manufactured, and the second end protection layer 22, thereby ensuring the adhesion (close contact) of the second end protection layer 22 to the first end protection layer 21 (including the third end protection layer 23 formed on the outside). In other words, this means that the first end protection layer 21 has the function of ensuring adhesion (close contact) with the second end protection layer 22. By ensuring adhesion (close contact) in this manner, it is possible to appropriately suppress the end protection layer 2 from peeling off from the element substrate 1 due to thermal shock caused by the adhesion of water droplets.

[0061] In addition to the above, the gas sensor 100 involved in this embodiment can appropriately suppress the peeling of the end protective layer 2 (hereinafter referred to as peeling during evaporation). The peeling during evaporation is: when use is started again, the water that adheres to the end protective layer 2 and condenses and then enters its interior evaporates as the sensor element 10 heats up, thereby increasing the pressure inside the end protective layer 2 and causing the end protective layer 2 to peel off.

[0062] As described above, in the gas sensor 100 according to this embodiment, the third terminal protection layer 23 has the function of inhibiting water from entering the interior of the sensor element 10. However, since the third terminal protection layer 23 is a porous layer, it is difficult to completely block water from entering the terminal protection layer 2, and some water may enter. The aforementioned peeling during evaporation is caused by water entering the terminal protection layer 2.

[0063] In the gas sensor 100 according to this embodiment, the arrangement range of the first terminal protection layer 21 , the second terminal protection layer 22 , and the third terminal protection layer 23 constituting the terminal protection layer 2 in the device longitudinal direction is appropriately defined to appropriately suppress the above-mentioned peeling during evaporation.

[0064] Specifically, first, Figure 2 As shown, the first end protective layer 21, the second end protective layer 22 and the third end protective layer 23 are set in the following manner, that is, when the extension lengths of the first end protective layer 21, the second end protective layer 22 and the third end protective layer 23 in the element length direction starting from the end surface 101e (the end surface of the element substrate 1) are set to the first layer length L1, the second layer length L2 and the third layer length L3 respectively, they satisfy the following conditional formula.

[0065] L1≥L2 and L1≥L3····(1)

[0066] Formula (1) means that the terminal protection layer 2 is formed so that at least the end surface 21 e of the first terminal protection layer 21 opposite to the one end E1 side in the longitudinal direction of the sensor element 10 is exposed.

[0067] Furthermore, the third end protection layer 23 is provided to surround at least the area of ​​the element base 1 that reaches a temperature of 500°C or higher when the sensor element 10 is driven (when the heater 150 is heated at the element driving temperature). Typically, this area is predetermined during the design of the sensor element 10. Furthermore, this area includes at least the area extending from the end surface 101e of the ceramic body 101 to the innermost portion of the third internal cavity 104 in the element's longitudinal direction.

[0068] Because the first, second, and third terminal protective layers 21, 22, and 23 meet the aforementioned configuration ranges, sensor element 10 achieves a structure in which the highly porosity-rich portions of terminal protective layer 2 are directly connected to the outside. Consequently, even if water vapor is generated within terminal protective layer 2 due to heating, it does not remain within the layer and is readily expelled primarily from the highly porosity-rich portions. Consequently, the internal pressure of terminal protective layer 2 is less likely to rise, and the resulting flaking during evaporation is less likely to occur.

[0069] It should be noted that the first terminal protection layer 21, the second terminal protection layer 22, and the third terminal protection layer 23 may be provided to satisfy the following conditional formula:

[0070] L1≥L2≥L3····(2).

[0071] Figure 2 The example in FIG1 shows a case where the formula (2) is satisfied (more specifically, the case where L1>L2>L3). In this case, in addition to the end face 21e of the first end protection layer 21, the end face 22e of the second end protection layer 22 is also exposed, making it difficult to further peel off during evaporation.

[0072] Alternatively, the first end protection layer 21, the second end protection layer 22, and the third end protection layer 23 may be configured to satisfy the following conditional formula:

[0073] L1≥L3≥L2····(3).

[0074] In this case, although the end surface 21e of the first end protection layer 21 is exposed, the end surface 22e of the second end protection layer 22 is not exposed, so that water intrusion from the latter can be suppressed and peeling during evaporation is less likely to occur.

[0075] As described above, in the sensor element 10 according to this embodiment, the terminal protection layer 2 surrounding the portion of the element base 1 that reaches a high temperature during use of the gas sensor 100 is formed into a three-layer structure: a first terminal protection layer 21, a second terminal protection layer 22, and a third terminal protection layer 23. Each of these terminal protection layers is provided with a predetermined porosity and thickness. This ensures adhesion (close contact) between the first terminal protection layer 21 and the second terminal protection layer 22, while the second terminal protection layer 22 suppresses heat conduction from the outside to the element base 1. The third terminal protection layer 23 maintains overall strength and suppresses water intrusion. Consequently, the sensor element 10 can achieve both close contact of the terminal protection layers and thermal shock resistance and temperature rise performance.

[0076] Furthermore, by appropriately defining the arrangement range of each layer of the terminal protection layer 2 in the element length direction, it is possible to appropriately suppress peeling of the terminal protection layer 2 during evaporation caused by evaporation of water entering the sensor element 10 as the sensor element 10 is heated.

[0077] Sensor Element Manufacturing Process

[0078] Next, an example of a process for manufacturing the sensor element 10 having the above-described structure and features will be described. Figure 3 1 is a diagram showing a process flow when manufacturing the sensor element 10 .

[0079] When manufacturing the element substrate 1 , first, a plurality of unpatterned green sheets (not shown) containing an oxygen ion conductive solid electrolyte such as zirconia as a ceramic component are prepared (step S1 ).

[0080] The semi-finished sheet is provided with a plurality of sheet holes for positioning during printing and lamination. The sheet holes are formed in advance by punching out the semi-finished sheet before the pattern is formed using a punching device. It should be noted that in the case of a green sheet having an internal space formed in the corresponding portion of the ceramic body 101, a through portion corresponding to the internal space is also pre-set by the same punching process. In addition, the thickness of each semi-finished sheet does not need to be the same, and the thickness can be different according to the corresponding portions of the element substrate 1 that is finally formed.

[0081] If the semi-finished sheets corresponding to each layer are prepared, pattern printing and drying are performed on each semi-finished sheet (step S2). Specifically, patterns of various electrodes, patterns of the heater 150 and the insulating layer 151, patterns of the electrode terminals 160, patterns of the main surface protective layer 170, and patterns of internal wiring (omitted from the illustration) are formed. In addition, at the timing of the above-mentioned pattern printing, the sublimation material (disappearing material) for forming the first diffusion rate control unit 110, the second diffusion rate control unit 120, the third diffusion rate control unit 130, and the fourth diffusion rate control unit 140 is also applied or arranged. In addition, the pattern for forming the first terminal protective layer 21 (21a, 21b) is also printed on the semi-finished sheets that become the top and bottom layers after stacking (step S2a).

[0082] Each pattern is printed by applying a pattern-forming paste, prepared according to the characteristics required for each target, to the semi-finished sheet using known screen printing techniques. For example, when forming the first terminal protective layer 21, an alumina paste is used that allows the formation of the first terminal protective layer 21 with the desired porosity and thickness in the final sensor element 10. Post-printing drying can also be performed using known drying methods.

[0083] When the pattern printing of each semi-finished sheet is completed, the printing and drying of the adhesive paste for laminating and bonding the green sheets to each other are performed (step S3). For the printing of the adhesive paste, known screen printing technology can be utilized, and for the drying process after printing, known drying methods can also be utilized.

[0084] Next, the following pressing process is performed, that is, the raw sheets coated with adhesive are stacked in a prescribed order, and prescribed temperature and pressure conditions are applied to them to press them together to form a laminate (step S4). Specifically, the raw sheets to be stacked are positioned using the sheet holes and stacked and maintained in a prescribed stacking fixture not shown in the figure, and each stacking fixture is heated and pressurized using a known stacking machine such as a hydraulic press. The pressure, temperature, and time for heating and pressurizing also depend on the stacking machine used, but as long as appropriate conditions are specified, good stacking can be achieved. It should be noted that the laminate obtained by the above method can also be formed into a pattern for forming the first end protective layer 21.

[0085] When the laminated body is obtained in the above-described manner, the laminated body is then cut at a plurality of locations to be cut into unit bodies that ultimately become individual element substrates 1 (step S5 ).

[0086] Next, the obtained unit body is fired at a firing temperature of approximately 1300°C to 1500°C (step S6). Thus, an element substrate 1 having a first terminal protective layer 21 on both main surfaces is produced. Specifically, the element substrate 1 is formed by integrally firing the ceramic body 101 formed of a solid electrolyte, the electrodes, the main surface protective layer 170, and the first terminal protective layer 21. It should be noted that the electrodes of the element substrate 1 have sufficient adhesion strength by integrally firing in this manner.

[0087] After the element substrate 1 is fabricated in the above manner, the second end protective layer 22 and the third end protective layer 23 are then formed on the element substrate 1. The second end protective layer 22 is formed by spraying a pre-prepared second end protective layer forming powder (alumina powder) to a target thickness onto the element substrate 1 at the location where the second end protective layer 22 is to be formed (step S7). The element substrate 1 coated in the above manner is then fired (step S8). The alumina powder used to form the second end protective layer contains alumina powder with a predetermined particle size distribution and a pore-forming material in a ratio corresponding to the desired porosity. After spraying, the element substrate 1 is fired to thermally decompose the pore-forming material, thereby forming the second end protective layer 22 with a high porosity of 40% to 80%. Conventional techniques can be applied to the spraying and firing processes.

[0088] After the second end protective layer 22 is formed, a third end protective layer forming powder (alumina powder) containing alumina powder having a predetermined particle size distribution, similarly prepared in advance, is sprayed onto the target location of the third end protective layer 23 on the element substrate 1 (step S9), thereby forming the third end protective layer 23 with the desired porosity. The alumina powder used to form the third end protective layer does not contain a pore-forming material. This spraying can also be performed using known techniques.

[0089] The sensor element 10 can be obtained through the above-described procedures. The obtained sensor element 10 is housed in a predetermined housing and assembled to a main body (not shown) of the gas sensor 100 .

[0090] Modifications

[0091] As described above, the first end protection layer 21 , the second end protection layer 22 , and the third end protection layer 23 may be provided so as to satisfy the formula (1) or further satisfy the formula (2) or the formula (3). Figure 4 and Figure 5 1 is a schematic diagram of the structure of a gas sensor 100 including a cross-sectional view along the longitudinal direction of a sensor element 10 according to a modification example based on the above.

[0092] Specifically, Figure 4 FIG. 1 shows a configuration in which the relationship L1 = L2 = L3 satisfies equation (2). In the sensor element 10 with this configuration, the end surface 21e of the first terminal protection layer 21 and the end surface 22e of the second terminal protection layer 22 are also exposed to the outside. Therefore, even when the sensor element 10 is heated while water is present within the terminal protection layer 21, peeling during evaporation can be appropriately suppressed.

[0093] in addition, Figure 5 FIG2 shows a solution satisfying equations (2) and (3), that is, a sensor element 10 in which the relationship L1>L2=L3 holds. In the sensor element 10 with this structure, the end surface 21e of the first terminal protection layer 21 is also exposed to the outside. Therefore, even when the sensor element 10 is heated while water is present inside the terminal protection layer 21, peeling during evaporation can be appropriately suppressed.

[0094] Furthermore, in the above embodiment, a sensor element having three internal cavities is used as an object, but the sensor element does not necessarily have to have a three-cavity structure. In other words, the sensor element may have two or one internal cavity.

[0095] In addition, in the above embodiment, after the powder for forming the second terminal protective layer is sprayed in step S7, it is fired in step S8, and on this basis, the powder for forming the third terminal protective layer is sprayed in step S9. However, the order of firing in step S8 and spraying in step S9 can be reversed.

[0096] In the above embodiment, the second and third end protection layers 22 and 23 are formed using alumina, and alumina powder is used as the thermal spraying material for these two layers. However, this is not a requirement. Alternatively, the second and third end protection layers 22 and 23 may be formed using metal oxides such as zirconium oxide (ZrO2), spinel (MgAl2O4), or mullite (Al6O13Si2) instead of alumina. In this case, powders of these metal oxides may be used as the thermal spraying material.

[0097] Example

[0098] Eight sensor elements 10 (Samples No. 1 to 8) were produced with different formation conditions for the first end protection layer (hereinafter referred to as the first layer) 21, the second end protection layer (hereinafter referred to as the second layer) 22, and the third end protection layer (hereinafter referred to as the third layer) 23. Specifically, in all sensor elements 10, the first layer 21, the second layer 22, and the third layer 23 were all made of alumina. The porosity of the first and second layers 21, 22, and the combinations of the first layer length L1, the second layer length L2, and the third layer length L3 were varied. The porosity of the third layer 23 was set to 25%, and the thicknesses of the first layer 21, the second layer 22, and the third layer 23 were set to 40 μm, 500 μm, and 200 μm, respectively.

[0099] Table 1 below lists the porosity of the first layer 21 and the second layer 22 and the first layer length L1, the second layer length L2, and the third layer length L3 of each sensor element 10. In this case, samples No. 1 to No. 4 and No. 6 to No. 8 satisfy equation (1). In addition, samples No. 1 to No. 4 and No. 6 satisfy equation (2) in addition to equation (1). On the other hand, sample No. 7 satisfies equation (3) in addition to equation (1). In addition, sample No. 8 has Figure 4 The structure shown.

[0100] Furthermore, all sensor elements 10 have a minimum value of 11 mm for the first layer length L1 , the second layer length L2 , and the third layer length L3 . However, they all have in common that the range reaching 500° C. or higher during driving is surrounded by the end protection layer 2 .

[0101] The temperature rise performance of sensor elements 10, No. 1 to No. 8, was evaluated. The temperature rise performance was evaluated based on the time it took for the sensor element 10 to reach the assumed element drive temperature of 850°C (heating time) after the sensor element 10 was driven at room temperature. The temperature of the sensor element 10 was calculated based on the internal resistance of the element.

[0102] If the temperature rise time is 30 seconds or less, it can be determined that there is no problem with the temperature rise performance. Here, the temperature rise time is 30 seconds or less for all sensor elements 10. That is, all sensor elements 10 have the required temperature rise performance.

[0103] The adhesion of the terminal protective layer 2 was evaluated for sensor elements 10, samples No. 1 to No. 8. The adhesion was evaluated by stretching the element base 1 in the longitudinal direction while the terminal protective layer 2 was fixed.

[0104] If the force required to displace the element base 1 is 100 N or greater, it can be determined that there is no problem with the adhesion. Here, the force is 100 N or greater for all sensor elements 10. That is, the end protection layer 2 of all sensor elements 10 has the required adhesion.

[0105] Furthermore, the sensor elements 10 of samples No. 1 to No. 8 were evaluated for peeling of the terminal protective layer 2 during evaporation. The evaluation of peeling during evaporation was performed by removing the sensor elements 10 at room temperature from the water after being immersed for a sufficient period of time. The sensor elements 10 were then heated using a heater 150 to evaporate the water. The cross-section of the heated sensor elements 10 was then observed using a SEM. It should be noted that the immersion of the sensor elements 10 in water at this time significantly increased the likelihood of water intrusion into the terminal protective layer 2 compared to normal use of the gas sensor 100.

[0106] Furthermore, sensor elements 10 in which peeling of the end protective layer 2 was observed in the SEM image were judged to have experienced (or had) evaporation-induced peeling, while sensor elements 10 in which peeling was not observed were judged to have not experienced (or had not experienced) evaporation-induced peeling. Table 1 also shows the results of the evaporation-induced peeling determination. In Table 1, sensor elements 10 with no evaporation-induced peeling are marked with a "◯" (circle symbol), and sensor elements 10 with evaporation-induced peeling are marked with a "×" (cross symbol).

[0107] [Table 1]

[0108]

[0109] As shown in Table 1, sensor elements 10 No. 3, No. 4, and No. 6 to No. 8, which had a porosity of 40% in the first layer 21 and satisfied the formula (1), did not peel off during evaporation.

[0110] In contrast, in the sensor elements 10 of No. 1 and No. 2, in which the porosity of the first layer 21 was less than 40%, peeling occurred during evaporation, although the equations (1) and (2) were satisfied.

[0111] Furthermore, in the sensor element 10 of No. 5 that did not satisfy the equations (1) and (2), peeling occurred during evaporation.

[0112] The above results indicate that when the porosity of the first layer 21 is at least 40% and the first layer length L1, the second layer length L2, and the third layer length L3 satisfy at least the relationship of formula (1), the sensor element 10 can be realized in which peeling during evaporation is appropriately suppressed.

Claims

1. A sensor element of a gas sensor, characterized in that: have: an element substrate which is a ceramic structure having a monitoring portion for a gas component to be measured; and an end protection layer which is a porous layer provided at an outer periphery of a predetermined range from an end portion of the element substrate on a side where the monitoring portion is provided; as well as a main surface protection layer, the main surface protection layer being arranged on the two main surfaces of the element substrate, The terminal protection layer comprises: a first terminal protection layer, the first terminal protection layer being disposed on the main surface protection layer; a second terminal protection layer provided to cover the end portion and four side surfaces of the element base including the two main surfaces on which the first terminal protection layer is formed; and a third terminal protection layer, the third terminal protection layer being arranged to cover the second terminal protection layer and having a porosity smaller than that of the second terminal protection layer; The first terminal protective layer has a porosity of more than 40%, When the extension lengths of the first end protection layer, the second end protection layer, and the third end protection layer starting from the end surface of the element substrate in the length direction of the element substrate are L1, L2, and L3, respectively, L1≥L2 and L1≥L3, The porosity of the second terminal protective layer is 40% to 80%. The thickness of the second terminal protection layer is 200 μm to 800 μm.

2. The sensor element according to claim 1, characterized in that L1≥L2≥L3.

3. The sensor element according to claim 1, characterized in that L1≥L3≥L2.

4. The sensor element according to any one of claims 1 to 3, characterized in that The porosity of the first terminal protection layer is 40% to 60%.

5. The sensor element according to any one of claims 1 to 3, characterized in that The third terminal protection layer surrounds at least a predetermined range of the element base of the sensor element that reaches 500° C. or higher during driving.

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