Gas sensor element and gas sensor

Abstract

A sensor element 101 comprises: an element body 101a comprising an oxygen-ion-conducting solid-state-electrolyte layer; and an inner protective layer 92 that covers at least a portion of the element body 101a, is a porous body with a plurality of pores therein, and has a standard deviation of porosity of 2.3% or less.

Description

[Technical Field]

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

[0002] Conventionally, gas sensors equipped with a gas sensor element that detects the concentration of a specific gas, such as NOx, in a measurement gas, such as an automobile exhaust gas, have been known. It is also known that in such gas sensors, a porous protective layer is formed on the surface of the gas sensor element (see, for example, Patent Document 1). Patent Document 2 describes a porous protective layer having a two-layer structure consisting of an inner protective layer and an outer protective layer. The porous protective layer serves to prevent cracks from occurring in the sensor element due to the adhesion of moisture. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2016-065852 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-187482 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, there has been a demand for gas sensor elements that are able to detect the concentration of a specific gas even when there is a large amount of moisture around the gas sensor element, and there has been a demand for gas sensor elements that are more resistant to water damage.

[0005] The present invention has been made to solve the above problems, and has as its main object to improve the water resistance of a gas sensor element. [Means for solving the problem]

[0006] In order to achieve the above-mentioned main object, the present invention employs the following means.

[0007] The gas sensor element of the present invention comprises: an element body including an oxygen ion conductive solid electrolyte layer; a protective layer that covers at least a portion of the element body, is a porous body having a plurality of pores therein, and has a standard deviation of porosity of 2.3% or less; It is equipped with the following.

[0008] This gas sensor element has a protective layer with a standard deviation of porosity of 2.3% or less, i.e., a protective layer with small porosity variation. Since the standard deviation of porosity is 2.3% or less, there are few areas in the protective layer with locally low porosity, i.e., locally low thermal insulation, so that cooling of the element body when moisture adheres to the gas sensor element is suppressed. Therefore, the water resistance of the gas sensor element is improved.

[0009] In the gas sensor element of the present invention, the element body has a long rectangular parallelepiped shape and is provided with a measurement gas flow section therein through which a measurement gas is introduced and flows, and the protective layer is formed on the element body. Chief The protective layer may cover a closest surface, which is the surface closest to the measurement gas flow portion among the four surfaces along the longitudinal direction, and the standard deviation of the porosity of a portion of the closest surface covering a region of the measurement gas flow portion as projected onto the closest surface, may be 2.3% or less. Here, the portion of the element body between the closest surface and the measurement gas flow portion is weak and relatively vulnerable to thermal shock. Furthermore, by having the standard deviation of the porosity of the portion of the protective layer covering the region of the measurement gas flow portion as projected onto the closest surface, which is 2.3% or less, cracks can be prevented from occurring in the portion vulnerable to thermal shock, thereby improving the water resistance of the gas sensor element. In this gas sensor element, the standard deviation of the porosity of at least the above-mentioned portion of the protective layer is only required to be 2.3% or less. For example, the portion of the protective layer covering other portions (e.g., portions other than the above-mentioned region of the closest surface) may have a standard deviation exceeding 2.3%.

[0010] In the gas sensor element of the present invention, the element body has a long rectangular parallelepiped shape and is provided with a measurement gas flow portion therein through which a measurement gas is introduced and flows; Measured The gas inlet, which is the entrance of the gas flow section, may open on a longitudinal end face of the element body, and the protective layer may cover the end face of the element body, with the standard deviation of porosity in the portion covering the end face being 2.3% or less. Here, the portion of the element body surrounding the gas inlet is weak and relatively vulnerable to thermal shock. By ensuring that the standard deviation of porosity in the portion of the protective layer covering the longitudinal end face of the element body where the gas inlet opens is 2.3% or less, cracks can be suppressed in the portion vulnerable to thermal shock, thereby improving the water resistance of the gas sensor element.

[0011] In the gas sensor element of the present invention, the protective layer may have a porosity of 10% or more and 40% or less.In the gas sensor element of the present invention, the protective layer may have a thickness of 100 μm or more and 500 μm or less.

[0012] In the gas sensor element of the present invention, the protective layer may include a porous inner protective layer and a porous outer protective layer located outside the inner protective layer and having a lower porosity than the inner protective layer, and the standard deviation of the porosity of the inner protective layer may be 2.3% or less. This configuration improves the water resistance of the gas sensor element because the outer protective layer has a low porosity, making it more difficult for moisture to pass through, and the inner protective layer has a high porosity, improving its thermal insulation. Furthermore, since the standard deviation of the porosity of the inner protective layer is 2.3% or less, the number of areas in the inner protective layer with locally low porosity, i.e., areas with locally low thermal insulation, is reduced, further improving the water resistance of the gas sensor element. Note that, in the gas sensor element of this embodiment, the standard deviation of the porosity of the outer protective layer does not need to be 2.3% or less.

[0013] In the gas sensor element of the present invention, the element body has a long rectangular parallelepiped shape and is provided with a measurement gas flow portion therein through which a measurement gas is introduced and flows, and the inner protective layer is formed on the element body. ChiefThe inner protective layer may cover a closest surface, which is the surface closest to the measurement gas flow portion among the four surfaces along the longitudinal direction, and the standard deviation of the porosity of a portion of the closest surface covering a region of the measurement gas flow portion as projected onto the closest surface, may be 2.3% or less. Here, the portion of the element body between the closest surface and the measurement gas flow portion is weak and relatively vulnerable to thermal shock. Furthermore, by having the standard deviation of the porosity of the portion of the inner protective layer covering the region of the measurement gas flow portion as projected onto the closest surface, which is 2.3% or less, cracks can be prevented from occurring in the portion vulnerable to thermal shock, thereby improving the water resistance of the gas sensor element. In this gas sensor element, the standard deviation of the porosity of at least the above-mentioned portion of the inner protective layer is only required to be 2.3% or less. For example, the portion of the inner protective layer covering other portions (e.g., portions other than the above-mentioned region of the closest surface) may have a standard deviation exceeding 2.3%.

[0014] In the gas sensor element of the present invention, the element body has a long rectangular parallelepiped shape and is provided with a measurement gas flow portion therein through which a measurement gas is introduced and flows; Measured The gas inlet, which is the entrance of the gas flow section, may open on a longitudinal end face of the element body, and the inner protective layer may cover the end face of the element body, with the standard deviation of porosity in the portion covering the end face being 2.3% or less. Here, the portion of the element body surrounding the gas inlet is weak and relatively vulnerable to thermal shock. By ensuring that the standard deviation of porosity in the portion of the inner protective layer covering the longitudinal end face of the element body where the gas inlet opens is 2.3% or less, cracks can be suppressed in the portion vulnerable to thermal shock, thereby improving the water resistance of the gas sensor element.

[0015] In the gas sensor element of the present invention, in which the protective layer has an outer protective layer and an inner protective layer, the inner protective layer may have a porosity of 40% or more and 70% or less. When the porosity of the inner protective layer is 40% or more, the heat insulating effect between the outer protective layer and the element body can be prevented from becoming insufficient. When the porosity of the inner protective layer is 70% or less, the strength of the inner protective layer can be prevented from becoming insufficient.

[0016] In the gas sensor element of the present invention, in which the protective layer has an outer protective layer and an inner protective layer, the inner protective layer may have a thickness of 300 μm or more and 700 μm or less, and the outer protective layer may have a thickness of 100 μm or more and 300 μm or less.

[0017] In the gas sensor element of the present invention, the standard deviation may be 1.5% or less, which further improves the water resistance of the gas sensor element.

[0018] The gas sensor of the present invention includes the gas sensor element according to any one of the above-described embodiments, and therefore provides the same effects as those of the gas sensor element of the present invention, such as improved water resistance of the sensor element. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 2 is a perspective view schematically showing an example of the configuration of a sensor element 101. [Figure 2] 1 is a cross-sectional view schematically illustrating an example of the configuration of a gas sensor 100. FIG. [Figure 3] Cross section B-B of Figure 1. [Figure 4] Cross section CC of Figure 3. [Figure 5] FIG. 1 is an explanatory diagram of plasma spraying using a plasma gun 170. [Figure 6] FIG. 10 is a cross-sectional view of a modified porous protective layer 190. [Figure 7] DD cross section of Figure 6. [Figure 8] 1 is a graph showing the relationship between the standard deviation σ of porosity and the ratio of water-resistant capacity. DETAILED DESCRIPTION OF THE INVENTION

[0020] Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view that schematically illustrates an example of the configuration of a sensor element 101 according to one embodiment of the present invention. FIG. 2 is a cross-sectional view that schematically illustrates an example of the configuration of a gas sensor 100 that includes the sensor element 101 (an example of a gas sensor element according to the present invention). The cross section of the sensor element 101 in FIG. 2 corresponds to the cross section AA in FIG. 1. FIG. 3 is a cross-sectional view taken along the line BB in FIG. 1. FIG. 4 is a cross-sectional view taken along the line CC in FIG. 3. FIG. 4 is also a partial enlarged view of FIG. 2. In FIG. 4, only the outline of the outer protective layer 91 is shown by a dashed line. The sensor element 101 has a long rectangular parallelepiped shape. The longitudinal direction of the sensor element 101 (the left-right direction in FIG. 2) is defined as the front-rear direction, and the thickness direction of the sensor element 101 (the up-down direction in FIG. 2) is defined as the up-down direction. The width direction of the sensor element 101 (the direction perpendicular to the front-rear and up-down directions) is defined as the left-right direction.

[0021] The gas sensor 100 is attached to a pipe such as an exhaust gas pipe of a vehicle and is used to measure the concentration of a specific gas such as NOx or O2 contained in exhaust gas as a measurement gas. In this embodiment, the gas sensor 100 measures the NOx concentration as the specific gas concentration. The sensor element 101 includes an element body 101a and a porous protective layer 90 that covers the element body 101a.

[0022] As shown in FIG. 2, the sensor element 101 is an element having a laminate (element body 101a) in which six layers, each made of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO), are stacked in this order from bottom to top as viewed in the drawing: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6. The solid electrolyte forming these six layers is dense and airtight. The element body 101a is manufactured, for example, by laminating ceramic green sheets corresponding to each layer after performing predetermined processing and printing circuit patterns on them, and then firing them to integrate them.

[0023] At one tip end (front end) of the element body 101a, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a gas inlet 10, a first diffusion rate-controlling section 11, a buffer space 12, a second diffusion rate-controlling section 13, a first internal space 20, a third diffusion rate-controlling section 30, and a second internal space 40 are formed adjacent to each other and communicate with each other in this order.

[0024] The gas inlet 10, the buffer space 12, the first internal space 20, and the second internal space 40 are spaces within the element body 101a, which are defined by an upper portion cut out from the spacer layer 5 and defined by the underside of the second solid electrolyte layer 6, a lower portion cut out from the upper surface of the first solid electrolyte layer 4, and a side portion cut out from the side surface of the spacer layer 5.

[0025] The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, and the third diffusion rate-controlling section 30 are each provided as two horizontally elongated slits (with the openings extending in the direction perpendicular to the drawing). Like the first diffusion rate-controlling section 11, the gas inlet 10 is also provided as two horizontally elongated openings. The space from the gas inlet 10 to the second internal space 40 is referred to as the measurement gas flow section 9. The measurement gas flow section 9 is formed in a substantially rectangular parallelepiped shape. The longitudinal direction of the measurement gas flow section 9 is parallel to the front-rear direction.

[0026] Further, at a position farther from the tip side than the measurement gas flow section 9, a reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and at a position defined at the side by the side surface of the first solid electrolyte layer 4. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0027] The air introduction layer 48 is a layer made of porous ceramics, and a reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is also formed so as to cover the reference electrode 42.

[0028] Reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of third substrate layer 3 and first solid electrolyte layer 4, and as described above, is surrounded by air introduction layer 48 that connects to reference gas introduction space 43. Furthermore, as will be described later, reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in first internal space 20 and second internal space 40.

[0029] In the measurement gas flow section 9, the gas inlet 10 is a section that opens to the external space, and the measurement gas is introduced into the element body 101a from the external space through the gas inlet 10. The first diffusion rate-controlling section 11 is a section that imparts a predetermined diffusion resistance to the measurement gas introduced through the gas inlet 10. The buffer space 12 is a space provided for guiding the measurement gas introduced through the first diffusion rate-controlling section 11 to the second diffusion rate-controlling section 13. The second diffusion rate-controlling section 13 is a section that imparts a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20. When the measurement gas is introduced from the outside of the element body 101a into the first internal space 20, the measurement gas is suddenly taken into the element body 101a through the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (such as exhaust pressure pulsations if the measurement gas is automobile exhaust gas). The measurement gas is not introduced directly into the first internal space 20, but is introduced into the first internal space 20 after the pressure fluctuations of the measurement gas are canceled out through the first diffusion-controlling section 11, buffer space 12, and second diffusion-controlling section 13. This makes the pressure fluctuations of the measurement gas introduced into the first internal space 20 almost negligible. The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the second diffusion-controlling section 13. The oxygen partial pressure is adjusted by operating the main pump cell 21.

[0030] The main pump cell 21 is an electrochemical pump cell including an inner pump electrode 22 having a ceiling electrode portion 22a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 provided on the outside of the element body 101a in a region corresponding to the ceiling electrode portion 22a on the upper surface of the second solid electrolyte layer 6, and the second solid electrolyte layer 6 sandwiched between these electrodes. The outer pump electrode 23 is provided on the upper surface of the element body 101a.

[0031] The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20 and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that configure both side wall portions of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a tunnel-like structure at the locations where the side electrode portions are provided.

[0032] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, a cermet electrode of Pt and ZrO containing 1% Au). The inner pump electrode 22, which comes into contact with the measurement gas, is formed using a material with a weakened ability to reduce the NOx component in the measurement gas.

[0033] In the main pump cell 21, by applying a desired pump voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23 and flowing a pump current Ip0 in a positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen from the first internal space 20 out to the external space, or to pump oxygen from the external space into the first internal space 20.

[0034] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 80 for controlling the main pump, is configured by the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42.

[0035] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring the electromotive force (voltage V0) in the main pump control oxygen partial pressure detection sensor cell 80. Furthermore, the pump current Ip0 is controlled by feedback controlling the pump voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes a target value. This allows the oxygen concentration in the first internal space 20 to be maintained at a predetermined constant value.

[0036] The third diffusion control section 30 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas to the second internal space 40.

[0037] The second internal space 40 is provided as a space for carrying out processing related to measurement of the nitrogen oxide (NOx) concentration in the measurement gas introduced through the third diffusion-controlling part 30. The NOx concentration is measured mainly in the second internal space 40 where the oxygen concentration has been adjusted by the auxiliary pump cell 50, and further by the operation of the measurement pump cell 41.

[0038] In the second internal space 40, the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, and then the oxygen partial pressure of the measurement gas introduced through the third diffusion-controlling section 30 is further adjusted by the auxiliary pump cell 50. This makes it possible to keep the oxygen concentration in the second internal space 40 constant with high precision, thereby enabling the gas sensor 100 to measure the NOx concentration with high precision.

[0039] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell that includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any appropriate electrode on the outside of the element body 101a will suffice), and the second solid electrolyte layer 6.

[0040] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to the inner pump electrode 22 disposed in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40, and side electrodes (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 that provide the side walls of the second internal space 40, respectively, forming a tunnel-shaped structure. Like the inner pump electrode 22, the auxiliary pump electrode 51 is also formed using a material with a weakened ability to reduce NOx components in the measurement gas.

[0041] In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, it is possible to pump oxygen in the atmosphere within the second internal space 40 out to the external space or pump oxygen from the external space into the second internal space 40.

[0042] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal space 40, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, is configured by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3.

[0043] The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. This allows the oxygen partial pressure in the atmosphere within the second internal space 40 to be controlled to a low level that does not substantially affect the measurement of NOx.

[0044] In addition, the pump current Ip1 is used to control the electromotive force of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and the target value of the voltage V0 is controlled to always maintain a constant gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion-controlling part 30 into the second internal space 40. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

[0045] The measurement pump cell 41 measures the NOx concentration in the measurement gas in the second internal space 40. The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the second internal space 40 and spaced apart from the third diffusion-controlling part 30, the outer pump electrode 23, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0046] The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere inside the second internal space 40. Furthermore, the measurement electrode 44 is covered with a fourth diffusion-controlling part 45.

[0047] The fourth diffusion-controlling part 45 is a membrane made of a porous ceramic body. The fourth diffusion-controlling part 45 serves to limit the amount of NOx flowing into the measuring electrode 44, and also functions as a protective membrane for the measuring electrode 44. The measuring pump cell 41 pumps out oxygen produced by decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 44, and the amount of oxygen produced can be detected as a pump current Ip2.

[0048] Furthermore, in order to detect the oxygen partial pressure around the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42 constitute an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

[0049] The measurement gas introduced into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion-controlling part 45 under conditions where the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measurement electrode 44 are reduced (2NO → N2 + O2) to generate oxygen. This generated oxygen is then pumped by the measurement pump cell 41, and the voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 remains constant (target value). Because the amount of oxygen generated around the measurement electrode 44 is proportional to the nitrogen oxide concentration in the measurement gas, the pump current Ip2 in the measurement pump cell 41 is used to calculate the nitrogen oxide concentration in the measurement gas.

[0050] Furthermore, by combining the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 to form an oxygen partial pressure detection means as an electrochemical sensor cell, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by reduction of the NOx components in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere, thereby making it possible to determine the concentration of the NOx components in the measured gas.

[0051] In addition, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the electromotive force (voltage Vref) obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0052] In the gas sensor 100 having such a configuration, the measurement gas, in which the oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50, is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas can be determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out of the measurement pump cell 41, which is approximately proportional to the NOx concentration in the measurement gas.

[0053] Furthermore, the element body 101a includes a heater section 70 that adjusts the temperature by heating and maintaining the temperature of the element body 101a in order to enhance the oxygen ion conductivity of the solid electrolyte. The heater section 70 includes a heater connector electrode 71, a heater 72, a through-hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0054] The heater connector electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater connector electrode 71 to an external power supply, it is possible to supply power to the heater section 70 from the outside.

[0055] The heater 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to a heater connector electrode 71 via a through hole 73, and generates heat when power is supplied from the outside through the heater connector electrode 71, thereby heating and keeping warm the solid electrolyte that forms the element body 101a.

[0056] The heater 72 is embedded throughout the entire area from the first internal space 20 to the second internal space 40, and is capable of adjusting the temperature of the entire element body 101a to a temperature at which the solid electrolyte is activated.

[0057] The heater insulating layer 74 is an insulating layer made of an insulator such as alumina and formed on the upper and lower surfaces of the heater 72. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72, and between the third substrate layer 3 and the heater 72.

[0058] The pressure release hole 75 is a portion that penetrates the third substrate layer 3 and the atmosphere introduction layer 48 and is provided so as to communicate with the reference gas introduction space 43, and is formed for the purpose of mitigating the increase in internal pressure that accompanies a rise in temperature within the heater insulation layer 74.

[0059] 1 to 3, the element body 101a is partially covered with a porous protective layer 90. The porous protective layer 90 includes porous protective layers 90a to 90e formed on five of the six surfaces of the element body 101a. The porous protective layer 90a covers a portion of the upper surface of the element body 101a. The porous protective layer 90b covers a portion of the lower surface of the element body 101a. The porous protective layer 90c covers a portion of the left surface of the element body 101a. The porous protective layer 90d covers a portion of the right surface of the element body 101a. The porous protective layer 90e covers the entire front end surface of the element body 101a. Each of the porous protective layers 90a to 90d covers the entire surface of the element body 101a, on which it is formed, from the front end surface of the element body 101a to a distance L (see FIG. 2) rearward. The porous protective layer 90a also covers the portion where the outer pump electrode 23 is formed. The porous protective layer 90e also covers the gas inlet 10. However, because the porous protective layer 90e is porous, the measurement gas can flow through the porous protective layer 90e and reach the gas inlet 10. The porous protective layer 90 covers and protects a portion of the element body 101a (a portion from the front end face of the element body 101a to a distance L from the front end face, including the front end face). The porous protective layer 90 serves to prevent, for example, moisture in the measurement gas from adhering to the element body 101a, causing cracks. The distance L is determined within the range of 0 < distance L < the longitudinal length of the element body 101a) based on the area of ​​the element body 101a exposed to the measurement gas in the gas sensor 100 and the position of the outer pump electrode 23. In this embodiment, the distance L is longer than the longitudinal length of the measurement gas flow section 9. Therefore, the rear end of the porous protective layer 90 is located further rearward than the rear end of the measurement target gas flow portion 9 .

[0060] Each of the porous protective layers 90a to 90e has a two-layer structure. The porous protective layer 90a includes a porous outer protective layer 91a and a porous inner protective layer 92a. The inner protective layer 92a covers a portion of the upper surface of the element body 101a. The outer protective layer 91a is located outside the inner protective layer 92a (farther from the element body 101a than the inner protective layer 92a) and is stacked on top of the inner protective layer 92a. Similarly, the porous protective layer 90b includes an outer protective layer 91b and an inner protective layer 92b. The porous protective layer 90c includes an outer protective layer 91c and an inner protective layer 92c. The porous protective layer 90d includes an outer protective layer 91d and an inner protective layer 92d. The porous protective layer 90e includes an outer protective layer 91e and an inner protective layer 92e. Adjacent outer protective layers 91a to 91e are connected to each other. The outer protective layers 91a to 91e are collectively referred to as outer protective layers 91. Adjacent inner protective layers 92a to 92e are connected to each other. The inner protective layers 92a to 92e are collectively referred to as inner protective layers 92.

[0061] As shown in FIG. 3, the inner protective layers 92a to 92d cover the four longitudinal surfaces (front-rear direction) of the element body 101a, i.e., the top, bottom, left, and right surfaces. As shown in FIG. 2, the inner protective layer 92a covers the upper surface of the element body 101a from the front end to a distance L, and as shown in FIG. 3, the inner protective layer 92a covers the upper surface of the element body 101a from the left end to the right end. As described above, the distance L is longer than the length of the measurement gas flow section 9 in the front-rear direction. Therefore, as shown in FIGS. 3 and 4, the inner protective layer 92a covers the entire region 102a of the upper surface of the element body 101a, where the measurement gas flow section 9 is projected onto the upper surface. Similarly, the inner protective layer 92b covers the entire region 102b of the lower surface of the element body 101a, where the measurement gas flow section 9 is projected onto the lower surface. The inner protective layer 92c covers the entire region 102c of the left surface of the element body 101a, where the measurement gas flow section 9 is projected onto the left surface. The inner protective layer 92d covers the entire area 102d of the right surface of the element body 101a, where the measurement gas flow section 9 is projected onto the right surface.

[0062] 3, in this embodiment, of the upper, lower, left, and right surfaces of the element body 101a, the upper surface is the surface closest to the measurement gas flow section 9. That is, of the upper, lower, left, and right surfaces of the element body 101a, the upper surface is the surface closest to the measurement gas flow section 9. The inner protective layer 92a covers this surface closest to the measurement gas flow section 9.

[0063] The porous protective layer 90 (the outer protective layer 91 and the inner protective layer 92) is a porous body, and is made of, for example, ceramics containing ceramic particles as constituent particles. Examples of ceramic particles include alumina (Al2O3), zirconia (ZrO2), spinel (MgAl2O4), and mullite (Al6O 13 The porous protective layer 90 preferably contains at least one of these metal oxide particles. In this embodiment, the porous protective layer 90 is made of porous alumina.

[0064] The porous protective layer 90a includes an inner protective layer 92a and an outer protective layer 91a located outside the inner protective layer 92a and having a lower porosity than the inner protective layer 92a. The outer protective layer 91a has a low porosity P1, which makes it difficult for moisture to pass through, while the inner protective layer 92a has a high porosity P2, which provides high thermal insulation. This prevents the upper side of the element body 101a from cooling, improving the water resistance of the sensor element 101. Similarly, each of the porous protective layers 90b-90e includes a corresponding inner protective layer 92b-92e and a corresponding outer protective layer 91b-91e located outside the corresponding inner protective layer 92b-92e and having a lower porosity than the inner protective layer 92b-92e. This prevents the lower, left, right, and front sides of the element body 101a, which correspond to the corresponding porous protective layers 90b-90e, from cooling, improving the water resistance of the sensor element 101.

[0065] The porosity P1 of one or more of the outer protective layers 91a to 91e may be 10% or more. When the porosity P1 is 10% or more, the measurement gas can sufficiently pass through the outer protective layer 91. When the porosity P1 is 10% or more, the measurement gas can sufficiently pass through the outer protective layer 91. When the porosity P1 is 60% or less, moisture does not easily pass through the outer protective layer 91. When the porosity P1 is 60% or less, the moisture does not easily pass through the outer protective layer 91. When the porosity P2 is 40% or more, the porosity P2 of one or more of the inner protective layers 92a to 92e is preferably 40% or more. When the porosity P2 is 40% or more, the heat insulating effect of the inner protective layer 92 between the outer protective layer 91 and the element body 101a can be prevented from becoming insufficient. When the porosity P2 is 70% or less, the strength of the inner protective layer 92 can be prevented from becoming insufficient.

[0066] Furthermore, the standard deviation σ of the porosity P2 of the inner protective layer 92a is 2.3% or less. With the standard deviation of the porosity P2 being 2.3% or less, there are fewer areas in the inner protective layer 92a with locally low porosity, i.e., locally low heat insulating properties, and this improves the water resistance of the sensor element 101. Similarly, the standard deviation σ of the porosity P2 of each of the inner protective layers 92b to 92e is 2.3% or less. It is preferable that the standard deviation σ of one or more of the inner protective layers 92a to 92e is 1.5% or less. The lower limit of the standard deviation σ may be set to a manufacturing limit (e.g., 0.5% or more).

[0067] It is preferable that the thickness T1 of one or more of the outer protective layers 91a to 91e is 100 μm or more. If the thickness T1 is 100 μm or more, moisture is less likely to pass through the outer protective layer 91. It is preferable that the thickness T1 of one or more of the outer protective layers 91a to 91e is 300 μm or less. If the thickness T1 is 300 μm or less, the heat capacity of the outer protective layer 91 is not too large, so that an increase in the power input to the heater 72 when heating the element body 101a can be suppressed. It is preferable that the thickness T2 of one or more of the inner protective layers 92a to 92e is 300 μm or more. If the thickness T2 is 300 μm or more, the distance between the element body 101a and the outer protective layer 91 becomes relatively large, which suppresses cooling of the element body 101a and improves water resistance. It is preferable that the thickness T2 of one or more of the inner protective layers 92a to 92e is 700 μm or less. If the thickness T2 is 700 μm or less, the porous protective layer 90 can be prevented from interfering with the protective cover that covers the sensor element 101.

[0068] The porosity P2 and its standard deviation σ of the inner protective layer 92a are values ​​calculated as follows using an image (SEM image) obtained by observation using a scanning electron microscope (SEM). First, the sensor element 101 is cut along the thickness direction of the inner protective layer 92a so that the cross section of the inner protective layer 92a is used as the observation surface. The cut surface is parallel to the longitudinal direction of the sensor element 101 and passes through the center of the measurement gas flow section 9 (here, the center between the left and right of the measurement gas flow section 9). That is, the CC cross section in FIG. 3 (the cross section shown in FIG. 4) is used as the cut surface. Therefore, the cut surface is a cross section including a portion of the inner protective layer 92a directly above the measurement gas flow section 9, in other words, a cross section including a portion of the inner protective layer 92a covering the region 102a. The cut surface is then filled with resin and polished to prepare an observation sample. Next, the magnification of the SEM is set to 1000 times, and an SEM image of the inner protective layer 92a is obtained by photographing the observation surface of the observation sample. Next, as shown in FIG. 4, eight observation regions 93a are determined in the inner protective layer 92a as regions for calculating the porosity in the obtained SEM image. Each observation region 93a is a substantially rectangular region measuring 1 mm in length by the thickness of the inner protective layer 92a. The positions of the eight observation regions 93a are determined so that they are equally spaced from the front end to the rear end of the region 102a (see FIG. 4). Then, the porosity is calculated for each of the eight observation regions 93a in the SEM image by image analysis. Specifically, first, a threshold is determined for each pixel in one observation region 93a in the SEM image using discriminant analysis (Otsu's binarization) based on the brightness distribution of the pixel brightness data. Then, based on the determined threshold, each pixel in the image in one observation region 93a is binarized into an object portion and a pore portion, and the areas of the object portion and the pore portion are calculated. The ratio of the area of ​​the pores to the total area (the total area of ​​the object and pores) is then calculated as the porosity within one observation region 93a. The porosity for each of the eight observation regions 93a is calculated in a similar manner, and the average of the eight calculated porosity values ​​is defined as the porosity P2 of the inner protective layer 92a. The standard deviation of the eight porosity values ​​is defined as the standard deviation σ of the inner protective layer 92a. When the standard deviation σ was calculated for the same inner protective layer 92a while changing the number of observation regions 93a, the standard deviation σ was found to be approximately the same as long as the number of observation regions 93a was eight or more.Therefore, it is considered that if the number of observation regions 93a is set to eight, the standard deviation σ can be calculated with sufficient accuracy.

[0069] The porosity P2 and standard deviation σ of each of the inner protective layers 92b to 92d are also values ​​derived in the same manner as above. For example, the eight observation regions for calculating the porosity P2 and standard deviation σ of the inner protective layer 92b are each a substantially rectangular region of (length 1 mm) × (thickness of the inner protective layer 92b), and the positions of the eight observation regions are the same as the region 102. b The observation regions are determined so as to be equally spaced from the front end to the rear end of the inner protective layer 92e. Furthermore, since the area of ​​the inner protective layer 92e obtained by projecting the measurement gas flow section 9 onto the front surface of the element body 101a is smaller than the areas 102a to 102d, the observation regions are determined from the entire portion covering the front surface of the element body 101a in the SEM image of the cross section of the inner protective layer 92e. Specifically, the observation plane is first determined as a plane parallel to the longitudinal direction of the sensor element 101 and passing through the center of the measurement gas flow section 9. More specifically, the observation plane is determined as a cross section passing through the center of the lateral direction of the area 102a of the element body 101a closest to the inner protective layer 92a, as in the observation plane of the inner protective layer 92a shown in FIG. 4. Next, eight observation regions 93e are determined from the portion of the SEM image of the observation surface where the inner protective layer 92e covers the front surface of the element body 101a. In this embodiment, because the vertical length of the front surface of the element body 101a is less than 8 mm, the length of one observation region 93a is not set to 1 mm, and instead, the portion of the SEM image where the inner protective layer 92e covers the front surface of the element body 101a is divided into eight equal parts to determine eight observation regions 93e, as shown in Fig. 4. The value of the porosity P1 of each of the outer protective layers 91a to 91e is also a value calculated as the average value of the porosities of the eight observation regions, similar to the porosity P2 of each of the inner protective layers 92a to 92e.

[0070] The inner protective layers 92a to 92 dThe reason why the standard deviation σ is calculated based on the cross section of the portion of the inner protective layer 92 covering the regions 102a to 102d is as follows: The portions of the element body 101a between the regions 102a to 102d and the measurement gas flow portion 9 are weak and relatively vulnerable to thermal shock. Therefore, a small standard deviation σ of the portion of the inner protective layer 92 covering this weak portion contributes greatly to improving the water resistance of the sensor element 101. Therefore, the inner protective layers 92a to 92 d The standard deviation σ calculated based on the cross section of the portion covering the regions 102a to 102d of the inner protective layers 92a to 92d is d Similarly, since the portion of the element body 101a around the gas inlet 10 is weak and relatively vulnerable to thermal shock, the standard deviation σ of the inner protective layer 92e was calculated by using a cross section of the measurement gas flow portion 9 cut through the center as the observation surface of the inner protective layer 92e, as described above.

[0071] In this embodiment, the thickness T1 of the outer protective layer 91a and the thickness T2 of the inner protective layer 92a are calculated as follows: First, the cross section of the porous protective layer 90a is View An SEM image of the surface is obtained, and the boundary between the outer protective layer 91a and the inner protective layer 92a is identified using the SEM image. The direction perpendicular to the surface (here, the upper surface of the second solid electrolyte layer 6) of the element body 101a on which the porous protective layer 90a is formed is identified as the thickness direction. The distance in the thickness direction from the surface (here, the upper surface) of the porous protective layer 90a to the boundary is derived as thickness T1. The distance in the thickness direction from the surface of the element body 101a to the boundary is derived as thickness T2. The thicknesses T1 and T2 of each of the porous protective layers 90b to 90e are also derived in the same manner.

[0072] In this embodiment, the porosity P1 and thickness T1 of the outer protective layers 91a to 91e are all the same. Similarly, the porosity P2, standard deviation σ, and thickness T2 of the inner protective layers 92a to 92e are all the same.

[0073] Next, a description will be given of a method for manufacturing the gas sensor 100. In the method for manufacturing the gas sensor 100, first the element body 101a is manufactured, and then the porous protective layer 90 is formed on the element body 101a, thereby manufacturing the sensor element 101.

[0074] First, a method for manufacturing the element body 101a will be described. First, six unfired ceramic green sheets are prepared. Then, patterns of electrodes, insulating layers, resistance heating elements, etc. are printed on each ceramic green sheet corresponding to the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6, respectively. After forming various patterns in this manner, the green sheets are dried. Then, they are stacked to form a laminate. The resulting laminate contains multiple element bodies 101a. The laminate is cut to the size of the element bodies 101a and fired at a predetermined firing temperature to obtain the element bodies 101a.

[0075] Next, a method for forming the porous protective layer 90 on the element body 101a will be described. In this embodiment, the inner protective layers 92a-92e and the outer protective layers 91a-91e are formed one by one by plasma spraying. FIG. 5 is an explanatory diagram of plasma spraying using a plasma gun 170. Note that FIG. 5 shows the formation of the inner protective layer 92a as an example, and illustrates the plasma gun 170 in cross section. The plasma gun 170 includes an anode 176 and a cathode 178, which serve as electrodes for generating plasma, and a substantially cylindrical outer peripheral portion 172 that covers them. The outer peripheral portion 172 includes an insulating portion (insulator) 173 for insulating the anode 176. A powder supply portion 182 is formed at the lower end of the outer peripheral portion 172 to supply a powder spray material 184, which is the material for forming the porous protective layer 90. A water-cooled jacket 174 is provided between the outer peripheral portion 172 and the anode 176, thereby enabling the anode 176 to be cooled. The anode 176 is formed in a cylindrical shape and has a nozzle 176a that opens downward. A plasma generating gas 180 is supplied between the anode 176 and the cathode 178 from above.

[0076] When forming the inner protective layer 92a, a voltage is applied between the anode 176 and cathode 178 of the plasma gun 170, and arc discharge occurs in the presence of the supplied plasma generating gas 180, converting the plasma generating gas 180 into a high-temperature plasma state. The plasma-state gas is ejected from the nozzle 176a as a high-temperature, high-velocity plasma jet. Meanwhile, a powder spray material 184 is supplied from the powder supply unit 182 together with a carrier gas. As a result, the powder spray material 184 is heated, melted, and accelerated by the plasma, colliding with the surface (upper surface) of the element body 101a and rapidly solidifying, thereby forming the inner protective layer 92a.

[0077] An inert gas such as argon gas can be used as the plasma generating gas 180. The flow rate of the argon gas is, for example, 40 to 50 L / min, and the supply pressure is, for example, 0.5 to 0.6 MPa. The voltage applied between the anode 176 and the cathode 178 is, for example, a DC voltage of 80 to 90 V, and the current is, for example, 300 to 400 A.

[0078] The powder spray material 184 contains raw material powder, which is the raw material for the porous protective layer 90 described above. In this embodiment, the raw material powder is alumina powder. The particle size of the raw material powder is, for example, 1 μm to 50 μm, and more preferably 20 μm to 30 μm. The carrier gas used to supply the powder spray material 184 can be, for example, argon gas, which is the same as the plasma generating gas 180. The flow rate of the carrier gas is, for example, 3 to 5 L / min, and the supply pressure is, for example, 0.5 to 0.6 MPa.

[0079] When performing plasma spraying, it is preferable to set the distance W between the nozzle 176a, which is the plasma gas outlet of the plasma gun 170, and the surface of the element body 101a on which the inner protective layer 92a is to be formed (the upper surface of the element body 101a in FIG. 5) to 150 mm to 200 mm. Plasma spraying may be performed while moving the plasma gun 170 appropriately (moving it left and right in FIG. 5) depending on the area on which the porous protective layer 90 is to be formed, but even in this case, it is preferable to keep the distance W within the above-mentioned range.

[0080] The inner protective layers 92b to 92e are formed one by one in the same manner, except that the surfaces on which they are formed on the element body 101a are different. After the inner protective layer 92 (inner protective layers 92a to 92e) is formed, the outer protective layers 91a to 91e are similarly formed one by one by plasma spraying. The plasma spraying is performed, for example, in the air or at room temperature. As a result, the inner protective layers 92 (inner protective layers 92a to 92e) and the outer protective layers 91 (outer protective layers 91a to 91e) are formed on the top, bottom, left, right, and front end surfaces of the element body 101a, respectively, to form the porous protective layer 90. When the porous protective layer 90 is formed on a portion of the surface of the element body 101a (a region extending a distance L from the front end to the rear), as with the porous protective layers 90a to 90d, the region where the porous protective layer 90 is not to be formed may be covered with a mask.

[0081] When the porous protective layer 90 is formed using plasma spraying in this manner, the porosities P1 and P2 of the porous protective layer 90 can be adjusted by including a pore-forming material in the powder spray material 184 and adjusting the blending ratio of this pore-forming material. Pore-forming materials that disappear when heated can be used, such as theobromine and acrylic resin. Furthermore, the pore-forming material included in the powder spray material 184 for the inner protective layer 92 can be passed through a sieve with a predetermined mesh size to reduce the particle size variation of the pore-forming material, thereby reducing the standard deviation σ of the porosity of the inner protective layer 92. When the pore-forming material is included in the powder spray material 184, the element body 101a is subjected to heat treatment after plasma spraying. This causes the pore-forming material to disappear, thereby forming pores in the porous protective layer 90. The thicknesses T1 and T2 can be adjusted by varying the duration of plasma spraying.

[0082] By forming the porous protective layer 90 on the element body 101a in the above manner, the sensor element 101 is obtained. Thereafter, a gas sensor 100 incorporating the sensor element 101 is manufactured. For example, an element sealing body is attached to the sensor element 101 to seal and fix it, and a connector and lead wires are attached to the rear end side of the sensor element 101. A protective cover is attached to the front end side of the sensor element 101 in the element sealing body. An outer cylinder is attached to the rear end side of the sensor element 101 in the element sealing body, and lead wires are drawn out from the outer cylinder. The process of assembling the gas sensor 100 by incorporating such a sensor element 101 is well known and is described, for example, in JP 2015-178988 A.

[0083] During use of the gas sensor 100 configured as described above, the measurement gas flows into the protective cover of the gas sensor 100, reaches the sensor element 101, passes through the porous protective layer 90, and flows into the gas inlet 10. The sensor element 101 detects the NOx concentration in the measurement gas that has flowed into the gas inlet 10. At this time, moisture contained in the measurement gas may also penetrate into the protective cover and adhere to the surface of the porous protective layer 90. As described above, the element body 101a is adjusted by the heater 72 to a temperature (e.g., 800°C) at which the solid electrolyte is activated. Therefore, when moisture adheres to the sensor element 101, the temperature outside the element body 101a generally drops rapidly, creating a steep temperature gradient between the inside and outside of the element body 101a, which may result in cracks in the element body 101a. Here, in the porous protective layer 90 of this embodiment, the porosity P1 of the outer protective layer 91 is lower than the porosity P2 of the inner protective layer 92, as described above. Therefore, the outer protective layer 91 has a low porosity P1, which makes it difficult for moisture to pass through. On the other hand, the inner protective layer 92 has a high porosity P2, which improves its thermal insulation. These features improve the water resistance of the sensor element 101. Furthermore, the inner protective layer 92 of this embodiment has a standard deviation σ of the porosity P2 of 2.3% or less, which reduces the porosity variation. Since the standard deviation σ of the porosity of the inner protective layer 92 is 2.3% or less, there are fewer areas in the inner protective layer 92 with locally low porosity, i.e., locally low thermal insulation, so cooling of the element body 101a when moisture adheres to the sensor element 101 is suppressed. Therefore, the sensor element 101 of this embodiment has even better water resistance than, for example, a sensor element provided with an inner protective layer with the same porosity but a larger standard deviation σ instead of the inner protective layer 92.

[0084] According to the sensor element 101 of this embodiment described above in detail, the standard deviation of the porosity of the porous protective layer 90 (particularly the inner protective layer 92 here) is 2.3% or less, so that the water resistance of the sensor element 101 is improved.

[0085] The porous protective layer 90 includes a porous inner protective layer 92 and a porous outer protective layer 91 located outside the inner protective layer 92 and having a lower porosity than the inner protective layer 92. The standard deviation σ of the porosity of the inner protective layer 92 of the porous protective layer 90 is 2.3% or less. As a result, the outer protective layer 91 has a low porosity P1, which makes it difficult for moisture to pass through, and the inner protective layer 92 has a high porosity P2, which improves heat insulation, thereby improving the water resistance of the sensor element 101. Furthermore, since the standard deviation σ of the porosity P2 of the inner protective layer 92 is 2.3% or less, there are fewer parts in the inner protective layer 92 that have a locally low porosity, i.e., fewer parts that have locally low heat insulation, so the water resistance of the sensor element 101 is further improved.

[0086] The element body 101a has a long rectangular parallelepiped shape and includes a measurement gas flow section 9 therein, through which the measurement gas is introduced and circulated. The inner protective layer 92a of the inner protective layer 92 covers the closest surface (the upper surface, in this case) of the four longitudinal surfaces (top, bottom, left, and right surfaces) of the element body 101a. The inner protective layer 92a has a standard deviation σ of porosity of 2.3% or less in a portion of the closest surface that covers an area 102a obtained by projecting the measurement gas flow section 9 onto the closest surface. As described above, the portions of the element body 101a between the areas 102a to 102d and the measurement gas flow section 9 are weak and relatively vulnerable to thermal shock. However, the portion between the closest surface and the measurement gas flow section (the portion between the area 102a and the measurement gas flow section 9) is particularly vulnerable to thermal shock due to its thin thickness. In contrast, by setting the standard deviation σ of the porosity of the portion of the inner protective layer 92a that covers the region 102a to 2.3% or less, the occurrence of cracks in the portion that is vulnerable to thermal shock can be suppressed, thereby improving the water resistance of the sensor element 101.

[0087] Furthermore, the gas inlet 10, which is the inlet of the measurement gas flow section 9, opens at a longitudinal end face (here, the front face) of the element body 101a, and the inner protective layer 92e of the inner protective layer 92 covers the front face of the element body 101a, and the standard deviation σ of the porosity in the portion covering the front face is 2.3% or less. As described above, the portion of the element body 101a surrounding the gas inlet 10 is weak and relatively vulnerable to thermal shock. In contrast, since the standard deviation σ of the porosity of the inner protective layer 92e, which is the portion of the inner protective layer 92 covering the front face of the element body 101a where the gas inlet 10 opens, is 2.3% or less, cracks can be suppressed in the portion vulnerable to thermal shock, and the water resistance of the sensor element 101 is improved.

[0088] When the porosity P2 of the inner protective layer 92 is 40% or more, the heat insulating effect between the outer protective layer 91 and the element body 101a can be prevented from becoming insufficient.When the porosity P2 of the inner protective layer 92 is 70% or less, the strength of the inner protective layer 92 can be prevented from becoming insufficient.

[0089] Furthermore, when the standard deviation σ of the inner protective layer 92 is 1.5% or less, the water resistance of the sensor element 101 is further improved.

[0090] It goes without saying that the present invention is not limited to the above-described embodiment, and can be embodied in various forms as long as they fall within the technical scope of the present invention.

[0091] For example, in the above-described embodiment, the standard deviation σ of the porosity of each of the inner protective layers 92a-92e is 2.3% or less, but it is sufficient if the standard deviation σ of at least one of the inner protective layers 92a-92e is 2.3% or less. If the standard deviation σ of at least one of the inner protective layers 92a-92e is 2.3% or less, the above-described effects can be obtained for at least that inner protective layer. However, it is preferable that more of the inner protective layers 92a-92e have a standard deviation σ of 2.3% or less, and it is more preferable that all of the inner protective layers 92a-92e (i.e., the entire inner protective layer 92) have a standard deviation σ of 2.3% or less.

[0092] In the above-described embodiment, the standard deviation σ calculated based on the portion of the inner protective layer 92a covering the region 102a was 2.3% or less. However, it is preferable that the standard deviation σ calculated in the same manner as described above also be 2.3% or less for portions other than the portion covering the region 102a. Furthermore, the standard deviation σ of the portion of the inner protective layer 92a covering the region 102a does not need to be 2.3% or less, as long as the standard deviation σ calculated for any cross section of the inner protective layer 92a is 2.3% or less. However, as described above, a small standard deviation σ of the portion of the inner protective layer 92a covering the region 102a contributes greatly to improving water resistance, so it is preferable that the standard deviation σ of at least the portion of the inner protective layer 92a covering the region 102a be 2.3% or less. The same applies to the inner protective layers 92b to 92e.

[0093] In the above-described embodiment, each of the porous protective layers 90a to 90e has a two-layer structure, but this is not limiting. For example, the porous protective layer 90 may have another layer further outward than the outer protective layer 91, another layer between the outer protective layer 91 and the inner protective layer 92, or another layer closer to the element body 101a than the inner protective layer 92. If the porous protective layer 90 has a three-layer or more structure, the "inner protective layer" may be any layer of the porous protective layer 90 excluding the outermost layer, as long as the standard deviation σ of the porosity of that layer is 2.3% or less. That is, if the porous protective layer 90 has a three-layer or more structure, it may be any one or more layers of the porous protective layer 90 excluding the outermost layer, as long as the standard deviation σ of the porosity is 2.3% or less. In this way, a layer (inner protective layer) with a standard deviation σ of 2.3% or less is less likely to have insufficient heat insulation, thereby achieving the effect of improving the water resistance of the sensor element 101, as in the above-described embodiment. In this case, it is preferable that a layer having a lower porosity than the layer having a standard deviation σ of 2.3% or less exists outside the layer, and such a layer is referred to as an "outer protective layer."

[0094] In the above-described embodiment, the porous protective layer 90 includes the outer protective layer 91 and the inner protective layer 92. However, the porous protective layer 90 may have a single-layer structure. FIG. 6 is a cross-sectional view of a modified porous protective layer 190. FIG. 7 is a cross-sectional view taken along the line DD of FIG. 6. The porous protective layer 190 shown in FIGS. 6 and 7 includes porous protective layers 190a to 190e formed on five surfaces of the element body 101a. Each of the porous protective layers 190a to 190e has the same configuration as the porous protective layers 90a to 90e of the above-described embodiment, except for the single-layer structure. In this porous protective layer 190, if the standard deviation σ of the porosity of one or more of the porous protective layers 190a to 190e is 2.3% or less, the above-described effects can be achieved for at least that porous protective layer. The method for calculating the standard deviation σ of the porous protective layers 190a to 190e is the same as the method for calculating the standard deviation σ of the inner protective layers 92a to 92e described above. As in the above-described embodiment, it is preferable that the standard deviation σ of the porosity of the porous protective layer 190a covering the closest surface (here, the upper surface of the element body 101a) of the porous protective layer 190 in the portion covering the region 102a is 2.3% or less. As in the above-described embodiment, it is preferable that the standard deviation σ of the porosity of the porous protective layer 190e covering the front surface of the element body 101a (the surface where the gas inlet 10 is open) of the porous protective layer 190 in the portion covering the front surface of the element body 101a in the porous protective layer 190e is 2.3% or less.

[0095] In the single-layer porous protective layer 190 shown in FIGS. 6 and 7, it is preferable that at least one of the porous protective layers 190a to 190e has a porosity of 10% or more. A porosity of 10% or more can prevent the porous protective layer 190 from obstructing the flow of the gas under measurement. It is preferable that at least one of the porous protective layers 190a to 190e has a porosity of 40% or less. A porosity of 40% or less makes it difficult for moisture to pass through the porous protective layer 190. It is preferable that at least one of the porous protective layers 190a to 190e has a thickness of 100 μm or more. A thickness of 100 μm or more makes it difficult for the element body 101a to have insufficient water resistance. It is preferable that at least one of the porous protective layers 190a to 190e has a thickness of 500 μm or less. Because the porous protective layer 190 has a single-layer structure, its porosity needs to be lower than that of the inner protective layer 92 of the above-described embodiment in order to prevent moisture from passing through. Therefore, it is preferable that the porous protective layer 190, which has a single layer structure, has a smaller thickness than the inner protective layer of the above-described embodiment.

[0096] In the above-described embodiment, the porous protective layer 90 includes the porous protective layers 90a to 90e, but this is not limited thereto. The porous protective layer 90 only needs to cover at least a portion of the element body 101a. For example, the porous protective layer 90 does not need to include one or more of the porous protective layers 90a to 90e. The same applies to the porous protective layer 190 shown in FIGS. 6 and 7.

[0097] In the above-described embodiment, the outer protective layer 91 and the inner protective layer 92 are both made of the same ceramic material (alumina), but this is not limiting and the two layers may be made of different materials.

[0098] In the above-described embodiment, the porous protective layer 90 is formed by plasma spraying, but the method is not limited to this. For example, the porous protective layer 90 may be formed by screen printing, mold casting, dipping, or the like. When forming the porous protective layer 90 by these methods, the standard deviation σ of the porosity can be reduced by using a pore-forming material with small particle size variations, as in the above-described embodiment. [Example]

[0099] Specific examples of sensor elements prepared will be described below as examples. Experimental Examples 1 to 3 correspond to examples of the present invention, and Experimental Examples 4 to 6 correspond to comparative examples. Note that the present invention is not limited to the following examples.

[0100] [Experimental Example 1] As Experimental Example 1, a sensor element was fabricated by the manufacturing method of the sensor element 101 of the above-described embodiment. First, the element body 101a shown in Figures 1 and 2 was fabricated, having a length in the front-rear direction of 67.5 mm, a width in the left-right direction of 4.25 mm, and a thickness in the up-down direction of 1.45 mm. In fabricating the element body 101a, a ceramic green sheet was formed by tape casting, by mixing zirconia particles to which 4 mol% of yttria as a stabilizer had been added, an organic binder, and an organic solvent.

[0101] Next, a porous protective layer 90 was formed by plasma spraying to fabricate a sensor element 101, which was designated Experimental Example 1. The porous protective layer 90 in Experimental Example 1 was formed as follows. A SinplexPro-90 manufactured by Oerlikon Metco was used as the plasma gun 170. To form the inner protective layer 92, a mixture of alumina powder with an average particle size of 20 μm and a pore-forming material was used as the powder spray material 184. The pore-forming material was previously sieved with a predetermined mesh size to reduce the particle size variation of the pore-forming material. A mixture of argon gas (flow rate 50 L / min, supply pressure 0.5 MPa) and hydrogen (flow rate 10 L / min, supply pressure 0.5 MPa) was used as the plasma generating gas 180. A DC voltage of 70 V was applied between the anode 176 and the cathode 178. The current was 500 A. The carrier gas used to supply the powder spray material 184 was argon gas (flow rate 4 L / min, supply pressure 0.5 MPa). The distance W was 150 mm. Plasma spraying was performed in the air at room temperature. The spraying direction of the plasma gun 170 (the direction of the nozzle 176a) was perpendicular to the surface of the sensor element 101 on which the inner protective layer 92 was formed. After the inner protective layer 92 was formed under the above conditions, the outer protective layer 91 was formed. In forming the outer protective layer 91, the same alumina particles as those for the inner protective layer 92 were used as the powder spray material 184, and the powder spray material 184 did not contain a pore-forming material, so that the porosity of the outer protective layer 91 was lower than that of the inner protective layer 92. The plasma spraying conditions were the same as those for the inner protective layer 92. After forming the outer protective layer 91, the element body 101a was heat-treated to remove the pore-forming material in the inner protective layer 92, thereby obtaining the sensor element 101 of Experimental Example 1.

[0102] [Experimental Examples 2-6] The outer protective layer 91 was the same as in Experimental Example 1, and the particle size of the pore-forming material was adjusted by appropriately adjusting the mesh size of the sieve used for the pore-forming material so that the standard deviation σ of the inner protective layer 92 was a different value.The sensor element 101 was fabricated in the same manner as in Experimental Example 1, and these were designated Experimental Examples 2 to 6.

[0103] [Derivation of parameters for outer and inner protective layers] For each of Experimental Examples 1 to 6, the porosity P1 and thickness T1 of the outer protective layer 91 and the porosity P2, standard deviation σ, and thickness T2 of the inner protective layer 92 were calculated using the methods described above. In all of Experimental Examples 1 to 6, the porosity P1 was 25% and the thickness T1 was 200 μm, and the porosity P2 of the inner protective layer 92 was 55% and the thickness T2 was 550 μm. The standard deviation σ values ​​for Experimental Examples 1 to 6 were as shown in Table 1. SEM image acquisition and image analysis were performed using an SU1510 manufactured by Hitachi High-Technologies and Image-Pro Plus 7.0 manufactured by Media Cybernetics. In Experimental Example 1, the porosity P1 and thickness T1 values ​​were the same for all of the outer protective layers 91a to 91e. Similarly, in Experimental Example 1, the porosity P2, thickness T2, and standard deviation σ were all the same for all of the inner protective layers 92a to 92e. Similarly, in Experimental Examples 2 to 6, the outer protective layers 91a to 91e had the same porosity P1 and thickness T1, and the inner protective layers 92a to 92e had the same porosity P2, thickness T2, and standard deviation σ.

[0104] [Water resistance evaluation] The water resistance of the sensor element 101 of each of Experimental Examples 1 to 6 was evaluated. Specifically, the heater 72 was first energized to heat the element body 101a to 800°C. In this state, the main pump cell 21, the auxiliary pump cell 50, the oxygen partial pressure detection sensor cell 80 for controlling the main pump, and the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump were operated in the air to maintain the oxygen concentration in the first internal space 20 at a predetermined constant value. After waiting for the pump current Ip0 to stabilize, water droplets were dropped onto the porous protective layer 90. The presence or absence of cracks in the element body 101a was determined based on whether the pump current Ip0 exceeded a predetermined threshold value. If a crack occurs in the element body 101a due to thermal shock from the water droplet, oxygen easily flows into the first internal space 20 through the crack, increasing the value of the pump current Ip0. Therefore, when the pump current Ip0 exceeded a predetermined threshold determined through experiments, it was determined that cracks had occurred in the element body 101a due to water droplets. In addition, multiple tests were conducted by gradually increasing the amount of water droplets, and the amount of water droplets at which cracks first occurred was defined as the water-resistant amount [μL]. For each of the sensor elements of Experimental Examples 1 to 6, the water-resistant amount [μL] was calculated as the average value of three tests. A larger water-resistant amount means that the sensor element 101 has a higher water-resistant amount. The ratio of the water-resistant amounts of Experimental Examples 1 to 5 was calculated using Experimental Example 6, which had the smallest water-resistant amount among Experimental Examples 1 to 6, as the reference. For each experimental example, if the ratio of water-resistant amounts was 3 or more, i.e., if the water-resistant amount was three times or more the water-resistant amount of experimental example 6, it was judged as "A (excellent)", if the ratio of water-resistant amounts was 1.5 or more but less than 3, it was judged as "B (good)", and if the ratio of water-resistant amounts was less than 1.5, it was judged as "F (unacceptable)".

[0105] The standard deviation σ of the inner protective layer 92, the ratio of water-resistant amount, and the evaluation results for each of Experimental Examples 1 to 6 are summarized in Table 1. Also, Fig. 8 is a graph plotting the standard deviation σ of the porosity and the ratio of water-resistant amount for each of Experimental Examples 1 to 6.

[0106] [Table 1]

[0107] As can be seen from Table 1 and FIG. 8, Experimental Examples 1 to 4, in which the standard deviation σ was 2.3% or less, had a larger water-wet resistance amount than Experimental Examples 5 and 6, in which the standard deviation σ exceeded 2.3%, and the water-wet resistance of the sensor element 101 was improved. Specifically, Experimental Examples 1 to 4 all had a water-wet resistance ratio of 1.5 or more and were judged as A or B, whereas Experimental Examples 5 and 6 were judged as F. Furthermore, the results of Experimental Examples 1 to 6 showed a tendency for the water-wet resistance to improve as the standard deviation σ decreased. Furthermore, the results of Experimental Examples 1 to 4 confirmed a tendency for the water-wet resistance amount to change sharply when the standard deviation σ was around 1.7%. From these results, it is believed that the water-wet resistance of the sensor element 101 will be further improved if the standard deviation σ is 1.5% or less.

[0108] This application claims priority from Japanese Patent Application No. 2021-042258, filed on March 16, 2021, the entire contents of which are incorporated herein by reference. [Industrial Applicability]

[0109] The present invention can be used in gas sensor elements and gas sensors that detect the concentration of specific gases such as NOx in measurement gases such as automobile exhaust gases. [Explanation of symbols]

[0110] 1 first substrate layer, 2 second substrate layer, 3 third substrate layer, 4 first solid electrolyte layer, 5 spacer layer, 6 second solid electrolyte layer, 9 measured gas flow section, 10 gas inlet, 11 first diffusion rate-controlling section, 12 buffer space, 13 second diffusion rate-controlling section, 20 first internal space, 21 main pump cell, 22 inner pump electrode, 22a ceiling electrode section, 22b bottom electrode section, 23 outer pump electrode, 24 variable power supply, 30 third diffusion rate-controlling section, 40 second internal space, 41 measurement pump cell, 42 reference electrode, 43 reference gas introduction space, 44 measurement electrode, 45 fourth diffusion rate-controlling section, 46 variable power supply, 48 air introduction layer, 50 auxiliary pump cell, 51 auxiliary pump electrode, 51a ceiling electrode section, 51b bottom electrode section, 52 variable power supply, 70 heater section, 71 Heater connector electrode, 72 heater, 73 through hole, 74 heater insulating layer, 75 pressure relief hole, 80 oxygen partial pressure detection sensor cell for controlling main pump, 81 oxygen partial pressure detection sensor cell for controlling auxiliary pump, 82 oxygen partial pressure detection sensor cell for controlling measurement pump, 83 sensor cell, 90, 90a to 90e porous protective layer, 91, 91a to 91e outer protective layer, 92, 92a to 92e inner protective layer, 93a, 93e observation area, 100 gas sensor, 101 sensor element, 101a element body, 102a to 102d areas, 170 plasma gun, 172 outer periphery, 173 insulating part, 174 water cooling jacket, 176 anode, 176a nozzle, 178 cathode, 180 plasma generation gas, 182 powder supply part, 184 powder spray material , 190,190a~190e Porous protective layer .

Claims

1. An element body having an oxygen ion conductive solid electrolyte layer; a protective layer that covers at least a portion of the element body and is a porous body having a plurality of pores therein; Equipped with The element body has a long rectangular parallelepiped shape and includes a measurement gas flow section therein through which a measurement gas is introduced and flows, the protective layer covers a closest surface, which is the surface closest to the measurement gas flow portion among four surfaces of the element body along the longitudinal direction, and the standard deviation of porosity in a portion of the closest surface covering an area of ​​the measurement gas flow portion as projected onto the closest surface is 2.3% or less. Gas sensor element.

2. An element body having an oxygen ion conductive solid electrolyte layer; a protective layer that covers at least a portion of the element body and is a porous body having a plurality of pores therein; Equipped with The element body has a long rectangular parallelepiped shape and includes a measurement gas flow section therein through which a measurement gas is introduced and flows, a gas inlet serving as an inlet of the measurement gas flow portion is opened at an end surface of the element body in the longitudinal direction; the protective layer covers the end face of the element body, and the standard deviation of the porosity in the portion covering the end face is 2.3% or less; Gas sensor element.

3. A gas inlet, which is an inlet of the measurement gas flow portion, opens at an end surface in the longitudinal direction of the element body, the protective layer covers the end face of the element body, and the standard deviation of the porosity in the portion covering the end face is 2.3% or less; 2. The gas sensor element according to claim 1.

4. The protective layer has a porosity of 10% or more and 40% or less. The gas sensor element according to any one of claims 1 to 3.

5. An element body having an oxygen ion conductive solid electrolyte layer; a protective layer that covers at least a portion of the element body and is a porous body having a plurality of pores therein; Equipped with the protective layer includes a porous inner protective layer and a porous outer protective layer located outside the inner protective layer and having a lower porosity than the inner protective layer; The element body has a long rectangular parallelepiped shape and includes a measurement gas flow section therein through which a measurement gas is introduced and flows, the inner protective layer covers a closest surface, which is the surface closest to the measurement gas flow portion among four surfaces along the longitudinal direction of the element body, and the standard deviation of porosity in a portion of the closest surface covering an area of ​​the measurement gas flow portion as projected onto the closest surface is 2.3% or less. Gas sensor element.

6. An element body having an oxygen ion conductive solid electrolyte layer; a protective layer that covers at least a portion of the element body and is a porous body having a plurality of pores therein; Equipped with the protective layer includes a porous inner protective layer and a porous outer protective layer located outside the inner protective layer and having a lower porosity than the inner protective layer; The element body has a long rectangular parallelepiped shape and includes a measurement gas flow section therein through which a measurement gas is introduced and flows, a gas inlet serving as an inlet of the measurement gas flow portion is opened at an end surface of the element body in the longitudinal direction; the inner protective layer covers the end face of the element body, and the standard deviation of the porosity in the portion covering the end face is 2.3% or less; Gas sensor element.

7. a gas inlet serving as an inlet of the measurement gas flow portion is opened at an end surface of the element body in the longitudinal direction; the inner protective layer covers the end face of the element body, and the standard deviation of the porosity in the portion covering the end face is 2.3% or less; 6. The gas sensor element according to claim 5.

8. The inner protective layer has a porosity of 40% or more and 70% or less. The gas sensor element according to any one of claims 5 to 7.

9. The standard deviation is 1.5% or less. The gas sensor element according to any one of claims 1 to 8.

10. A gas sensor comprising the gas sensor element according to any one of claims 1 to 9.

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