Gas sensor element and gas sensor

Abstract

The sensor element 101 includes an element main body 101a including a solid electrolyte layer having oxygen ion conductivity, and an inner protective layer 92 that is a porous body that covers at least a portion of the element main body 101a and has a plurality of pores inside, and that has a standard deviation of a porosity of 2.3% or less.

Description

Technical Field

[0001] This invention relates to gas sensor elements and gas sensors. Background Technology

[0002] To date, gas sensors are known that include a gas sensor element for detecting the concentration of specific gases such as NOx in a measured gas, such as automobile exhaust. Furthermore, it is known that such gas sensors have a porous protective layer formed on the surface of the gas sensor element (e.g., Patent Document 1). Patent Document 2 describes a porous protective layer with a two-layer structure consisting of an inner protective layer and an outer protective layer. This porous protective layer helps to suppress cracking of the sensor element caused by moisture adhesion.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2016-065852

[0006] Patent Document 2: Japanese Patent Application Publication No. 2017-187482 Summary of the Invention

[0007] However, in recent years, there has been a demand for gas sensor elements that can detect specific gas concentrations even when there is more moisture around them, and there is a desire for gas sensor elements with higher water resistance.

[0008] The present invention was implemented to solve the above-mentioned problems, and its main objective is to improve the water resistance of gas sensor elements.

[0009] The present invention employs the following means to achieve the aforementioned main objectives.

[0010] The gas sensor element of the present invention comprises:

[0011] The component body includes an oxygen ion-conducting solid electrolyte layer; and

[0012] A protective layer, wherein the protective layer is a porous material that covers at least a portion of the main body of the component and has a plurality of pores inside, and the standard deviation of the porosity is less than 2.3%.

[0013] This gas sensor element features a protective layer with a standard deviation of porosity of less than 2.3%, meaning a small deviation in porosity. By maintaining a standard deviation of less than 2.3% in porosity, the protective layer contains fewer areas with locally low porosity, i.e., areas with locally low thermal insulation. Therefore, the cooling of the element body when moisture adheres to it is suppressed. Consequently, the water resistance of the gas sensor element is improved.

[0014] In the gas sensor element of the present invention, the main body of the element is a long rectangular parallelepiped shape, and an internal gas flow section is provided for introducing and circulating the gas to be measured. The protective layer covers the surface of the main body along its length that is closest to the gas flow section, i.e., the closest surface. The standard deviation of the porosity of the portion of the closest surface covering the area where the gas flow section is projected onto the closest surface can be 2.3% or less. Here, the portion between the closest surface and the gas flow section in the main body is a weaker part and less resistant to thermal shock. Furthermore, by ensuring that the standard deviation of the porosity of the portion of the protective layer covering the area where the gas flow section is projected onto the closest surface is 2.3% or less, cracking in the thermally sensitive portion can be suppressed, thus improving the water resistance of the gas sensor element. It should be noted that in the gas sensor element of this scheme, the standard deviation of the porosity of at least the above-mentioned portion in the protective layer can be less than 2.3%. For example, the portion of the protective layer that covers other portions (e.g., the portion in the closest surface other than the above-mentioned area) may have a standard deviation exceeding 2.3%.

[0015] In the gas sensor element of the present invention, the element body is a long rectangular parallelepiped shape, and a gas flow section is provided inside for introducing and circulating the gas to be measured. The gas inlet, serving as the entrance to the gas flow section, is open at the end face of the element body along its length. The protective layer covers the end face of the element body, and the standard deviation of the porosity of the portion covering this end face can be 2.3% or less. Here, the peripheral portion of the gas inlet in the element body is a weaker part and less resistant to thermal shock. Furthermore, by ensuring that the standard deviation of the porosity of the portion of the element body covering the open gas inlet along its length is 2.3% or less, cracking in the thermally sensitive portion can be suppressed, thus improving the water resistance of the gas sensor element.

[0016] In the gas sensor element of the present invention, the porosity of the protective layer can be 10% or more and 40% or less. In the gas sensor element of the present invention, the thickness of the protective layer can be 100 μm or more and 500 μm or less.

[0017] In the gas sensor element of the present invention, the protective layer comprises: an inner protective layer of porous material and an outer protective layer of porous material located further outward than the inner protective layer and having a lower porosity than the inner protective layer. The standard deviation of the porosity of the inner protective layer can be 2.3% or less. Accordingly, the outer protective layer, due to its lower porosity, makes it difficult for moisture to pass through, while the inner protective layer, due to its higher porosity, increases its thermal insulation properties, thus improving the water resistance of the gas sensor element. Furthermore, since the standard deviation of the porosity of the inner protective layer is 2.3% or less, there are fewer locally low porosity portions within the inner protective layer, i.e., portions with locally low thermal insulation properties, further improving the water resistance of the gas sensor element. It should be noted that in this gas sensor element, a standard deviation of 2.3% or less of the porosity of the outer protective layer is not required.

[0018] In the gas sensor element of the present invention, the main body of the element is a long rectangular parallelepiped shape, and a gas flow section is provided inside for introducing and circulating the gas to be measured. The inner protective layer covers the surface of the main body along its length that is closest to the gas flow section, i.e., the closest surface. The standard deviation of the porosity of the portion of the closest surface covering the area where the gas flow section is projected onto the closest surface can be 2.3% or less. Here, the portion between the closest surface and the gas flow section in the main body is a weaker part and less resistant to thermal shock. Furthermore, by ensuring that the standard deviation of the porosity of the portion of the inner protective layer covering the area where the gas flow section is projected onto the closest surface is 2.3% or less, cracking in the thermally sensitive portion can be suppressed, thus improving the water resistance of the gas sensor element. It should be noted that in the gas sensor element of this scheme, the standard deviation of the porosity of at least the above-mentioned portion in the inner protective layer is less than 2.3%. For example, the portion of the inner protective layer that covers other portions (e.g., the portion in the closest surface other than the above-mentioned area) may have a standard deviation exceeding 2.3%.

[0019] In the gas sensor element of the present invention, the element body is a long rectangular parallelepiped shape, and a gas flow section is provided inside for introducing and circulating the gas to be measured. The gas inlet, serving as the entrance to the gas flow section, is open at the end face of the element body along its length. An inner protective layer covers the end face of the element body, and the standard deviation of the porosity of the portion covering this end face can be 2.3% or less. Here, the peripheral portion of the gas inlet in the element body is a weaker part and less resistant to thermal shock. Furthermore, by ensuring that the standard deviation of the porosity of the portion of the element body covering the open gas inlet along its length is 2.3% or less, cracking in the thermally sensitive portion can be suppressed, thus improving the water resistance of the gas sensor element.

[0020] In the gas sensor element of the present invention, which has an outer protective layer and an inner protective layer, the porosity of the inner protective layer can be 40% or more and 70% or less. If the porosity of the inner protective layer is 40% or more, insufficient heat insulation between the outer protective layer and the element body can be suppressed. If the porosity of the inner protective layer is 70% or less, insufficient strength of the inner protective layer can be suppressed.

[0021] In the gas sensor element of the present invention, which has an outer protective layer and an inner protective layer, the thickness of the inner protective layer can be 300 μm or more and 700 μm or less. Conversely, the thickness of the outer protective layer can be 100 μm or more and 300 μm or less.

[0022] In the gas sensor element of the present invention, the standard deviation can be 1.5% or less. Accordingly, the water resistance of the gas sensor element is further improved.

[0023] The gas sensor of the present invention comprises a gas sensor element according to any of the above-described embodiments. Therefore, this gas sensor achieves the same effects as the gas sensor element of the present invention described above, such as improved water resistance of the sensor element. Attached Figure Description

[0024] Figure 1 This is a perspective view showing an example of the configuration of sensor element 101.

[0025] Figure 2 This is a simplified cross-sectional view illustrating an example of the configuration of the gas sensor 100.

[0026] Figure 3 yes Figure 1 BB cross-sectional view.

[0027] Figure 4 yes Figure 3CC section view.

[0028] Figure 5 This is an explanatory diagram of plasma spraying using a plasma gun 170.

[0029] Figure 6 This is a cross-sectional view of the porous protective layer 190 in the modified example.

[0030] Figure 7 yes Figure 6 DD cross-sectional view.

[0031] Figure 8 It is a graph showing the relationship between the standard deviation σ of porosity and the water resistance. Detailed Implementation

[0032] The embodiments of the present invention will now be described with reference to the accompanying drawings. Figure 1 This is a perspective view that schematically illustrates an example of the configuration of a sensor element 101 as an embodiment of the present invention.

[0033] Figure 2 This is a simplified cross-sectional view illustrating an example of the configuration of a gas sensor 100 equipped with a sensor element 101 (an example of the gas sensor element of the present invention). It should be noted that... Figure 2 The cross-section of sensor element 101 in the middle is equivalent to Figure 1 AA section. Figure 3 yes Figure 1 BB cross-sectional view. Figure 4 yes Figure 3 CC section view. Figure 4 still Figure 2 A magnified view of a portion of the image. Figure 4 In the diagram, the outer protective layer 91 is outlined with dashed lines. It should be noted that the sensor element 101 is in the shape of a long rectangular parallelepiped, with its length direction (…) Figure 2 The left and right directions are set as the front and back directions, and the thickness direction of the sensor element 101 is set as the front and back directions. Figure 2 The vertical direction is set as vertical. In addition, the width direction of the sensor element 101 (the direction perpendicular to the front-back direction and the vertical direction) is set as horizontal.

[0034] The gas sensor 100 is installed in a pipe such as the exhaust pipe of a vehicle to measure the concentration of specific gases such as NOx and O2 contained in the exhaust gas, which is the gas to be measured. In this embodiment, the gas sensor 100 measures the NOx concentration as the concentration of the specific gas. The sensor element 101 includes: an element body 101a, and a porous protective layer 90 covering the element body 101a.

[0035] like Figure 2As shown, sensor element 101 is a laminate (element body 101a) consisting of six layers stacked in the following order from bottom to top in the attached figure. These six layers are a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, an insulating layer 5, and a second solid electrolyte layer 6, each comprising an oxygen ion-conducting solid electrolyte such as zirconium oxide (ZrO2). Furthermore, the solid electrolyte forming these six layers is a dense, gas-tight solid electrolyte. The element body 101a is manufactured as follows: for example, the ceramic green sheets corresponding to each layer undergo a prescribed processing and circuit pattern printing, then they are stacked, and subsequently fired to integrate them.

[0036] At one end (front end) of the element body 101a, and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, the following components are connected in sequence and adjacent to each other: a gas inlet 10, a first diffusion rate control unit 11, a buffer space 12, a second diffusion rate control unit 13, a first internal cavity 20, a third diffusion rate control unit 30, and a second internal cavity 40.

[0037] The gas inlet 10, buffer space 12, first internal cavity 20 and second internal cavity 40 are spaces inside the component body 101a provided by hollowing out the isolation layer 5. The upper part of the space is separated by the lower surface of the second solid electrolyte layer 6, the lower part of the space is separated by the upper surface of the first solid electrolyte layer 4, and the side part of the space is separated by the side of the isolation layer 5.

[0038] The first diffusion rate control unit 11, the second diffusion rate control unit 13, and the third diffusion rate control unit 30 are all configured as two horizontally elongated slits (the openings have a length direction perpendicular to the drawing). The gas inlet 10, like the first diffusion rate control unit 11, is configured as two horizontally elongated openings. It should be noted that the space from the gas inlet 10 to the second internal cavity 40 is called the measured gas flow section 9. The measured gas flow section 9 is formed in a generally cuboid shape. The length direction of the measured gas flow section 9 is parallel to the front-back direction.

[0039] Furthermore, a reference gas introduction space 43 is provided at a position farther from the end portion than the gas flow section 9 being measured. This reference gas introduction space 43 is located between the upper surface of the third substrate layer 3 and the lower surface of the isolation layer 5, and the side portion of the reference gas introduction space 43 is separated by the side portion of the first solid electrolyte layer 4. For example, atmospheric air is introduced into the reference gas introduction space 43 as a reference gas for NOx concentration measurement.

[0040] The atmosphere introduction layer 48 is a layer made of porous ceramic, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. In addition, the atmosphere introduction layer 48 is formed such that the reference electrode 42 is covered.

[0041] The reference electrode 42 is an electrode formed in a shape that is sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, an atmospheric inlet layer 48 connected to the reference gas inlet space 43 is provided around it. In addition, as will be described later, the reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 and the second internal cavity 40.

[0042] In the gas flow section 9, the gas inlet 10 is an open portion relative to the external space, through which the gas to be measured enters the component body 101a from the external space. The first diffusion rate control section 11 is a portion that imparts a predetermined diffusion resistance to the gas to be measured entering through the gas inlet 10. The buffer space 12 is a space provided for introducing the gas to be measured from the first diffusion rate control section 11 to the second diffusion rate control section 13. The second diffusion rate control section 13 is a portion that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal cavity 20. When the gas to be measured is introduced from outside the element body 101a into the first internal cavity 20, the gas that is rapidly entering the element body 101a from the gas inlet 10 due to pressure changes in the external space (in the case of exhaust gas from a car, the pressure fluctuations) is not directly introduced into the first internal cavity 20. Instead, it passes through the first diffusion rate control unit 11, the buffer space 12, and the second diffusion rate control unit 13 to eliminate the pressure change before being introduced into the first internal cavity 20. Therefore, the pressure change of the gas introduced into the first internal cavity 20 is negligible. The first internal cavity 20 is configured as a space for adjusting the oxygen partial pressure in the gas introduced through the second diffusion rate control unit 13. This oxygen partial pressure is adjusted by operating the main pump unit 21.

[0043] The main pump unit 21 is an electrochemical pump unit consisting of an inner pump electrode 22, an outer pump electrode 23, and a second solid electrolyte layer 6 sandwiched between these electrodes. The inner pump electrode 22 has a top electrode portion 22a disposed on approximately the entire lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20. The outer pump electrode 23 is disposed on the outer side of the element body 101a in a region corresponding to the top electrode portion 22a on the upper surface of the second solid electrolyte layer 6. The outer pump electrode 23 is disposed on the upper surface of the element body 101a.

[0044] The inner pump electrode 22 is formed as follows: a solid electrolyte layer (second solid electrolyte layer 6 and first solid electrolyte layer 4) spanning the upper and lower parts that divide the first internal cavity 20, and an isolation layer 5 forming the sidewall. Specifically, a top electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 forming the top surface of the first internal cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 forming the bottom surface. Side electrode portions (not shown) are formed on the sidewall surfaces (inner surfaces) of the isolation layer 5 forming the two sidewall portions of the first internal cavity 20 in such a way that the top electrode portion 22a and the bottom electrode portion 22b are connected. Thus, the side electrode portions are arranged in a tunnel-shaped structure.

[0045] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous metal-ceramic electrodes (e.g., metal-ceramic electrodes of Pt and ZrO2 containing 1% Au). It should be noted that the inner pump electrode 22, which is in contact with the gas being measured, is formed using a material whose reducing ability for NOx components in the gas being measured is weakened.

[0046] In the main pump unit 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23, so that the pump current Ip0 flows between the inner pump electrode 22 and the outer pump electrode 23 in either the positive or negative direction. This allows oxygen in the first internal cavity 20 to be drawn out to the external space, or oxygen in the external space to be drawn into the first internal cavity 20.

[0047] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, an electrochemical sensor unit, namely the main pump control oxygen partial pressure detection sensor unit 80, is constructed from the inner pump electrode 22, the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42.

[0048] The oxygen concentration (oxygen partial pressure) within the first internal cavity 20 can be determined by measuring the electromotive force (voltage V0) in the oxygen partial pressure detection sensor unit 80 for main pump control. Furthermore, the pump current Ip0 is controlled by feedback control of the pump voltage Vp0 of the variable power supply 24 to ensure that the voltage V0 reaches a target value. Thus, the oxygen concentration within the first internal cavity 20 can be maintained at a predetermined constant value.

[0049] The third diffusion rate control unit 30 is a part that, after controlling the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 using the operation of the main pump unit 21, imparts a predetermined diffusion resistance to the gas to be measured and introduces the gas to be measured into the second internal cavity 40.

[0050] The second internal cavity 40 is configured as a space for performing the following process: measuring the concentration of nitrogen oxides (NOx) in the gas to be measured introduced through the third diffusion rate control unit 30. The NOx concentration is measured mainly as follows: within the second internal cavity 40, where the oxygen concentration has been adjusted using the auxiliary pump unit 50, the NOx concentration is further measured by operating the measuring pump unit 41.

[0051] In the second internal cavity 40, the oxygen partial pressure of the gas to be measured, which has been pre-adjusted in the first internal cavity 20 and then introduced through the third diffusion rate control unit 30, is adjusted again using the auxiliary pump unit 50. This allows the oxygen concentration in the second internal cavity 40 to be maintained at a high precision, thus enabling the gas sensor 100 to measure NOx concentration with high accuracy.

[0052] The auxiliary pump unit 50 is an auxiliary electrochemical pump unit consisting of an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode on the outside of the element body 101a), and a second solid electrolyte layer 6. The auxiliary pump electrode 51 has a top electrode portion 51a that is generally integrally disposed on the lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40.

[0053] The auxiliary pump electrode 51 is arranged within the second internal cavity 40 in a tunnel-like structure, similar to the inner pump electrode 22 located in the first internal cavity 20. Specifically, a top electrode portion 51a is formed on the second solid electrolyte layer 6 constituting the top surface of the second internal cavity 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 constituting the bottom surface of the second internal cavity 40. Side electrode portions (not shown) connecting the top electrode portion 51a and the bottom electrode portion 51b are formed on the two walls of the insulating layer 5 constituting the sidewall of the second internal cavity 40, thus creating a tunnel-like structure. It should be noted that, like the inner pump electrode 22, the auxiliary pump electrode 51 is formed using a material that reduces the reducing power of NOx components in the measured gas.

[0054] In the auxiliary pump unit 50, a desired voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23, thereby enabling oxygen in the atmosphere inside the second internal cavity 40 to be drawn out to the external space, or drawn from the external space into the second internal cavity 40.

[0055] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal cavity 40, an electrochemical sensor unit, namely an auxiliary pump control oxygen partial pressure detection sensor unit 81, is constructed from an auxiliary pump electrode 51, a reference electrode 42, a second solid electrolyte layer 6, an isolation layer 5, a first solid electrolyte layer 4, and a third substrate layer 3.

[0056] Furthermore, the auxiliary pump unit 50 utilizes a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the oxygen partial pressure detection sensor unit 81 for auxiliary pump control to perform pumping. As a result, the oxygen partial pressure in the atmosphere within the second internal cavity 40 is controlled to a low partial pressure that substantially has no effect on the determination of NOx.

[0057] Additionally, the pump current Ip1 is simultaneously used to control the electromotive force of the oxygen partial pressure detection sensor unit 80 for main pump control. Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor unit 80 for main pump control, controlling its voltage V0 to the aforementioned target value. This ensures that the gradient of oxygen partial pressure in the gas being measured, introduced from the third diffusion rate control unit 30 into the second internal cavity 40, remains constant. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of approximately 0.001 ppm through the operation of the main pump unit 21 and the auxiliary pump unit 50.

[0058] The measuring pump unit 41 measures the NOx concentration in the gas to be measured within the second internal cavity 40. The measuring pump unit 41 is an electrochemical pump unit composed of a measuring electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, an isolation layer 5, and a first solid electrolyte layer 4. The measuring electrode 44 is positioned on the upper surface of the first solid electrolyte layer 4 facing the second internal cavity 40 and is separated from the third diffusion rate control unit 30.

[0059] The measuring electrode 44 is a porous cermet electrode. The measuring electrode 44 also functions as a NOx reduction catalyst for reducing NOx present in the atmosphere within the second internal cavity 40. Furthermore, the measuring electrode 44 is covered by a fourth diffusion rate control unit 45.

[0060] The fourth diffusion rate control unit 45 is a membrane made of a porous ceramic body. The fourth diffusion rate control unit 45 limits the amount of NOx flowing into the measuring electrode 44 and also functions as a protective film for the measuring electrode 44. In the measuring pump unit 41, oxygen generated from the decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 44 is drawn out, and its generation amount is detected in the form of pump current Ip2.

[0061] In addition, to detect the oxygen partial pressure around the measuring electrode 44, an electrochemical sensor unit, namely an oxygen partial pressure detection sensor unit 82 for measuring pump control, is constructed from the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor unit 82 for measuring pump control.

[0062] The gas to be measured, introduced into the second internal cavity 40, passes through the fourth diffusion rate control unit 45 and reaches the measuring electrode 44 under controlled oxygen partial pressure. Nitrogen oxides in the gas to be measured around the measuring electrode 44 are reduced (2NO→N2+O2) to generate oxygen. This generated oxygen is then pumped by the measuring pump unit 41. At this time, the voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the oxygen partial pressure detection sensor unit 82 for measuring pump control is constant (target value). The amount of oxygen generated around the measuring electrode 44 is directly proportional to the concentration of nitrogen oxides in the gas to be measured; therefore, the concentration of nitrogen oxides in the gas to be measured is calculated using the pump current Ip2 in the measuring pump unit 41.

[0063] Furthermore, if the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form an oxygen partial pressure detection mechanism in the form of an electrochemical sensor unit, the electromotive force corresponding to the following difference can be detected, thereby allowing the determination of the concentration of NOx in the measured gas, wherein the difference is the difference between the amount of oxygen generated by the reduction of NOx in the atmosphere surrounding the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere.

[0064] In addition, the electrochemical sensor unit 83 is composed of the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42. The electromotive force (voltage Vref) obtained from the sensor unit 83 can be used to detect the oxygen partial pressure in the gas to be measured outside the sensor.

[0065] In the gas sensor 100 with such a structure, the gas to be measured, whose oxygen partial pressure is always maintained at a constant low value (a value that substantially has no effect on the determination of NOx) by operating the main pump unit 21 and the auxiliary pump unit 50, is supplied to the measuring pump unit 41. Therefore, based on the pump current Ip2 flowing through the measuring pump unit 41, which draws oxygen out, the NOx concentration in the gas to be measured can be determined, wherein the oxygen is approximately proportional to the NOx concentration in the gas to be measured and is generated by the reduction of NOx.

[0066] Furthermore, the component body 101a includes a heater section 70, which performs temperature regulation functions to heat and maintain the temperature of the component body 101a in order to improve 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 insulation layer 74, and a pressure relief hole 75.

[0067] The heater connector electrode 71 is an electrode formed in a configuration that is in contact with the lower surface of the first substrate layer 1. By connecting the heater connector electrode 71 to an external power source, power can be supplied to the heater section 70 from the outside.

[0068] The heater 72 is a resistive element formed in a configuration where it is sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater connector electrode 71 via a through-hole 73, through which it is powered from the outside to generate heat and heat and maintain the temperature of the solid electrolyte forming the element body 101a.

[0069] In addition, the heater 72 is embedded in the entire area from the first internal cavity 20 to the second internal cavity 40, which can adjust the entire element body 101a to the temperature at which the solid electrolyte is activated.

[0070] The heater insulation layer 74 is an insulation layer formed on the upper and lower surfaces of the heater 72 by means of an insulator such as alumina. The purpose of forming the heater insulation layer 74 is to obtain electrical insulation between the second substrate layer 2 and the heater 72, and electrical insulation between the third substrate layer 3 and the heater 72.

[0071] The pressure relief hole 75 is a portion that penetrates the third substrate layer 3 and the atmospheric inlet layer 48 and is connected to the reference gas inlet space 43. The purpose of forming the pressure relief hole 75 is to mitigate the increase in internal pressure that accompanies the temperature rise in the heater insulation layer 74.

[0072] In addition, such as Figures 1-3As shown, a portion of the component body 101a is covered by a porous protective layer 90. The porous protective layer 90 comprises porous protective layers 90a to 90e formed on five of the six surfaces of the component body 101a. Porous protective layer 90a covers a portion of the upper surface of the component body 101a. Porous protective layer 90b covers a portion of the lower surface of the component body 101a. Porous protective layer 90c covers a portion of the left surface of the component body 101a. Porous protective layer 90d covers a portion of the right surface of the component body 101a. Porous protective layer 90e covers the entire front end surface of the component body 101a. It should be noted that the porous protective layers 90a to 90d cover the surface of the component body 101a on which they are formed, extending a distance L (refer to...) from the front end surface of the component body 101a toward the rear. Figure 2 The entire area up to the point L is covered. Additionally, 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, since the porous protective layer 90e is porous, the gas to be measured can flow through the interior of the porous protective layer 90e and reach the gas inlet 10. The porous protective layer 90 covers a portion of the element body 101a (including the front end face of the element body 101a, extending from the front end face to a distance L), thereby protecting that portion. The porous protective layer 90 serves to suppress the adhesion of substances such as moisture in the gas to be measured, which could cause cracks in the element body 101a. It should be noted that the distance L is determined by the range of the element body 101a exposed to the gas to be measured in the gas sensor 100, or by a range obtained based on the position of the outer pump electrode 23 (0 < distance L < length in the longitudinal direction of the element body 101a). In this embodiment, the distance L is longer than the length of the gas flow section 9 in the longitudinal direction. Therefore, the rear end of the porous protective layer 90 is located further back than the rear end of the gas flow section 9 being measured.

[0073] The porous protective layers 90a to 90e each have a two-layer structure. Porous protective layer 90a comprises 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 component body 101a. The outer protective layer 91a is located further outward than the inner protective layer 92a (from the perspective of the component body 101a, on the side furthest from the inner protective layer 92a) and is stacked on top of the inner protective layer 92a. Similarly, porous protective layer 90b comprises an outer protective layer 91b and an inner protective layer 92b. Porous protective layer 90c comprises an outer protective layer 91c and an inner protective layer 92c. Porous protective layer 90d comprises an outer protective layer 91d and an inner protective layer 92d. Porous protective layer 90e comprises an outer protective layer 91e and an inner protective layer 92e. The adjacent layers of the outer protective layers 91a to 91e are interconnected. The outer protective layers 91a to 91e are collectively referred to as outer protective layers 91. The adjacent layers of the inner protective layers 92a to 92e are interconnected. The inner protective layers 92a to 92e are collectively referred to as inner protective layers 92.

[0074] like Figure 3 As shown, the inner protective layers 92a to 92d respectively cover the four surfaces (top, bottom, left, and right) along the length direction (here, the front-to-back direction) of the component body 101a. Inner protective layer 92a is as follows... Figure 2 The portion of the upper surface of the component body 101a from the front end to a distance L is covered, as shown. Figure 3 The upper surface of the component body 101a is covered from the left end to the right end. The distance L, as described above, is longer than the length of the gas flow section 9 in the front-rear direction. Therefore, as... Figure 3 , 4 As shown, the inner protective layer 92a completely covers the area 102a on the upper surface of the component body 101a, which is the area on which the gas to be measured flows 9 is projected. Similarly, the inner protective layer 92b completely covers the area 102b on the lower surface of the component body 101a, which is the area on which the gas to be measured flows 9 is projected. The inner protective layer 92c completely covers the area 102c on the left surface of the component body 101a, which is the area on the left surface of the component body 101a. The inner protective layer 92d completely covers the area 102d on the right surface of the component body 101a, which is the area on the right surface of the component body 101a.

[0075] It should be explained that, for example Figure 3As shown, in this embodiment, among the upper, lower, left, and right surfaces of the component body 101a, the upper surface is the surface closest to the gas flow section 9 to be measured. That is, the upper surface of the component body 101a is the closest surface to the gas flow section 9 to be measured. The inner protective layer 92a covers this closest surface.

[0076] The porous protective layer 90 (outer protective layer 91 and inner protective layer 92) is a porous material, such as ceramics whose constituent particles include ceramic particles. Examples of ceramic particles include: alumina (Al2O3), zirconium oxide (ZrO2), spinel (MgAl2O4), and andalusite (Al6O4). 13 The porous protective layer 90 preferably contains at least one of the aforementioned ceramic particles, such as Si2 metal oxide particles. In this embodiment, the porous protective layer 90 is formed of alumina porous body.

[0077] The porous protective layer 90a includes an inner protective layer 92a and an outer protective layer 91a located further outward than the inner protective layer 92a and having a lower porosity than the inner protective layer 92a. Accordingly, the outer protective layer 91a, due to its lower porosity P1, makes it difficult for moisture to pass through, while the inner protective layer 92a, due to its higher porosity P2, increases its thermal insulation properties. Therefore, cooling of the upper side of the element body 101a is suppressed, thereby improving the water resistance of the sensor element 101. Similarly, the porous protective layer 90b has an inner protective layer 92b and an outer protective layer 91b located on the outer side of the inner protective layer 92b and having a smaller porosity than the inner protective layer 92b; the porous protective layer 90c has an inner protective layer 92c and an outer protective layer 91c located on the outer side of the inner protective layer 92c and having a smaller porosity than the inner protective layer 92c; the porous protective layer 90d has an inner protective layer 92d and an outer protective layer 91d located on the outer side of the inner protective layer 92d and having a smaller porosity than the inner protective layer 92d; and the porous protective layer 90e has an inner protective layer 92e and an outer protective layer 91e located on the outer side of the inner protective layer 92e and having a smaller porosity than the inner protective layer 92e. Accordingly, cooling of the lower, left, right and front sides of the element body 101a corresponding to the porous protective layers 90b to 90e is suppressed, thereby improving the water resistance of the sensor element 101.

[0078] The porosity P1 of any one or more of the outer protective layers 91a to 91e can be 10% or more. If the porosity P1 is 10% or more, the gas being measured can pass sufficiently through the outer protective layer 91. The porosity P1 of any one or more of the outer protective layers 91a to 91e is preferably 60% or less. If the porosity P1 is 60% or less, moisture does not easily pass through the outer protective layer 91. The porosity P2 of any one or more of the inner protective layers 92a to 92e is preferably 40% or more. If the porosity P2 is 40% or more, it is possible to prevent the insulation effect of the inner protective layer 92 between the outer protective layer 91 and the component body 101a from becoming insufficient. The porosity P2 of any one or more of the inner protective layers 92a to 92e is preferably 70% or less. If the porosity P2 is 70% or less, it is possible to prevent the strength of the inner protective layer 92 from becoming insufficient.

[0079] Furthermore, the standard deviation σ of the porosity P2 of the inner protective layer 92a is 2.3% or less. With a standard deviation of P2 of 2.3% or less, there are fewer locally low porosity areas within the inner protective layer 92a, i.e., areas with locally low thermal insulation, thus improving 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. The standard deviation σ of any one or more of the inner protective layers 92a to 92e is preferably 1.5% or less. The lower limit of the standard deviation σ can be a manufacturing limit value (e.g., 0.5% or more).

[0080] The thickness T1 of any one or more of the outer protective layers 91a to 91e is preferably 100 μm or more. If the thickness T1 is 100 μm or more, moisture cannot easily pass through the outer protective layer 91. The thickness T1 of any one or more of the outer protective layers 91a to 91e is preferably 300 μm or less. If the thickness T1 is 300 μm or less, the heat capacity of the outer protective layer 91 will not be too large, thus suppressing the increase in the power input to the heater 72 when heating the component body 101a. The thickness T2 of any one or more of the inner protective layers 92a to 92e is preferably 300 μm or more. If the thickness T2 is 300 μm or more, the distance between the component body 101a and the outer protective layer 91 is relatively large, suppressing the cooling of the component body 101a, thereby improving water resistance. The thickness T2 of any one or more of the inner protective layers 92a to 92e is preferably 700 μm or less. If the thickness T2 is less than 700 μm, interference between the porous protective layer 90 and the protective cover that covers the sensor element 101 can be suppressed.

[0081] The porosity P2 and its standard deviation σ of the inner protective layer 92a are derived from the image (SEM image) obtained by observation using a scanning electron microscope (SEM) as follows. First, the sensor element 101 is cut along the thickness direction of the inner protective layer 92a, with the cross-section of the inner protective layer 92a as the observation plane. The cutting plane at this time is a plane parallel to the length direction of the sensor element 101 and passes through the center of the gas flow section 9 to be measured (here, the center of the left and right sides of the gas flow section 9 to be measured). That is, the... Figure 3 CC section ( Figure 4 The cross-section shown is designated as the cutting plane. Therefore, the cutting plane is the cross-section of the inner protective layer 92a including the portion directly above the gas flow section 9 being measured; in other words, the cross-section of the inner protective layer 92a including the portion covering region 102a. Then, resin filling and grinding are performed on the cutting plane to prepare an observation sample. Next, the SEM magnification is set to 1000x, and the observation surface of the observation sample is photographed to obtain an SEM image of the inner protective layer 92a. Next, the area in the obtained SEM image from which the porosity is calculated is... Figure 4 As shown, eight observation areas 93a are defined within the inner protective layer 92a. Each observation area 93a is defined as an approximately rectangular region (length 1 mm) × (thickness of the inner protective layer 92a). The positions of the eight observation areas 93a are determined by arranging them at equal intervals from the front end to the rear end of region 102a (refer to...). Figure 4 Then, for each of the eight observation regions 93a in the SEM image, the porosity was calculated using image analysis. Specifically, firstly, for each pixel within one observation region 93a in the SEM image, a threshold was determined 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 within one observation region 93a was binarized into an object portion and a pore portion, and the area of ​​the object portion and the area of ​​the pore portion were calculated. Then, the ratio of the pore portion area to the overall area (the combined area of ​​the object portion and the pore portion) was derived as the porosity within one observation region 93a. The same method was used to calculate the porosity for each of the eight observation regions 93a. The average value of the eight calculated porosity values ​​was set as the porosity P2 of the inner protective layer 92a, and the standard deviation of the eight porosity values ​​was set as the standard deviation σ of the inner protective layer 92a. It should be noted that when calculating the standard deviation σ by changing the number of observation areas 93a for the same inner protective layer 92a, if the number of observation areas 93a is 8 or more, the standard deviation σ will be approximately the same. Therefore, it is considered that if the number of observation areas 93a is 8, the standard deviation σ can be calculated with sufficient accuracy.

[0082] The porosity P2 and standard deviation σ of the inner protective layers 92b to 92d are values ​​derived using the same method as described above. For example, regarding the eight observation areas used to calculate the porosity P2 and standard deviation σ of the inner protective layer 92b, each observation area is set as an approximately rectangular area (length 1 mm) × (thickness of the inner protective layer 92b), and the positions of the eight observation areas are determined by arranging them at equal intervals from the front end to the rear end of region 102a. Furthermore, regarding the inner protective layer 92e, since the area obtained by projecting the measured gas flow section 9 onto the front surface of the element body 101a is smaller than regions 102a to 102d, the observation area is determined entirely based on the portion of the front surface of the element body 101a covered by the SEM image of the cross-section of the inner protective layer 92e. Specifically, firstly, a surface parallel to the length direction of the sensor element 101 and passing through the center of the measured gas flow section 9, more specifically, a surface parallel to the length direction of the sensor element 101 and passing through the center of the measured gas flow section 9, is defined as... Figure 4 The observation surface of the inner protective layer 92a shown is similarly defined as the cross-section passing through the center of the region 102a closest to the surface of the component body 101a. Next, eight observation areas 93e are determined from the portion of the front surface of the component body 101a covered by the inner protective layer 92e in the SEM image of the observation surface. In this embodiment, since the vertical length of the front surface of the component body 101a is less than 8 mm, the length of one observation area 93a is less than 1 mm. Figure 4 As shown, the portion of the inner protective layer 92e covering the front surface of the component body 101a in the SEM image is divided into eight equal parts to determine eight observation areas 93e. Similarly, the porosity P1 values ​​of the outer protective layers 91a to 91e and the porosity P2 values ​​of the inner protective layers 92a to 92e are set as values ​​calculated in the form of the average porosity of the eight observation areas.

[0083] It should be explained that the reason for calculating the standard deviation σ based on the cross-section of the portion of the inner protective layers 92a-92e that covers regions 102a-102d is as follows: The portion of the element body 101a between regions 102a-102d and the gas flow section 9 being measured is weaker and less resistant to thermal shock. Therefore, a smaller standard deviation σ for the portion of the inner protective layer 92 that covers this weaker portion greatly contributes to improving the water resistance of the sensor element 101. Therefore, the standard deviation σ calculated based on the cross-section of the portion of the inner protective layers 92a-92e that covers regions 102a-102d is set as the standard deviation σ of each of the inner protective layers 92a-92e. Similarly, the peripheral portion of the gas inlet 10 in the element body 101a is weaker and less resistant to thermal shock. Therefore, as described above, the standard deviation σ of the inner protective layer 92e is calculated by taking a cross-section cut off at the center of the gas flow section 9 being measured as the observation surface of the inner protective layer 92e.

[0084] In this embodiment, the thickness T1 of the outer protective layer 91a and the thickness T2 of the inner protective layer 92a are derived as follows: First, as described above, an SEM image with the cross-section of the porous protective layer 90a as the observation plane is obtained, and the boundary between the outer protective layer 91a and the inner protective layer 92a is determined using the SEM image. Furthermore, the direction perpendicular to the surface of the element body 101a where the porous protective layer 90a is formed (here, the upper surface of the second solid electrolyte layer 6) is defined as the thickness direction. Then, 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 the thickness T1. Similarly, the distance in the thickness direction from the surface of the element body 101a to the boundary is derived as the thickness T2. The thicknesses T1 and T2 of each of the porous protective layers 90b to 90e are set to the same derived values.

[0085] It should be noted that in this embodiment, the values ​​of porosity P1 and thickness T1 are set to the same value in any of the outer protective layers 91a to 91e. Similarly, the values ​​of porosity P2, standard deviation σ, and thickness T2 are set to the same value in any of the inner protective layers 92a to 92e.

[0086] Next, the manufacturing method of such a gas sensor 100 will be described. In the manufacturing method of the gas sensor 100, firstly, an element body 101a is manufactured, and secondly, a porous protective layer 90 is formed on the element body 101a to manufacture the sensor element 101.

[0087] First, the method for manufacturing the element body 101a will be described. First, six unfired ceramic green sheets are prepared. Then, patterns of electrodes, insulating layers, resistive 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 insulating layer 5, and the second solid electrolyte layer 6, respectively. After forming various patterns in this way, 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.

[0088] Next, the method for forming a porous protective layer 90 on the component body 101a will be described. In this embodiment, plasma sputtering is used to form one layer each of the inner protective layers 92a-92e and the outer protective layers 91a-91e. Figure 5 This is an explanatory diagram of plasma spraying using a plasma gun 170. It should be noted that... Figure 5 The plasma gun 170 is shown in cross-section, illustrating the formation of the inner protective layer 92a. The plasma gun 170 includes an anode 176 and a cathode 178, which serve as electrodes for generating plasma, and a generally cylindrical outer peripheral portion 172 covering the anode 176 and cathode 178. The outer peripheral portion 172 includes an insulating portion (insulator) 173 for insulating the anode 176. A powder supply portion 182 for supplying powder-sprayed material 184, which is the forming material of the porous protective layer 90, is formed at the lower end of the outer peripheral portion 172. A water-cooling jacket 174 is provided between the outer peripheral portion 172 and the anode 176, thereby cooling the anode 176. The anode 176 has a nozzle 176a that is cylindrical and opens downwards. Plasma-generating gas 180 is supplied from above between the anode 176 and the cathode 178.

[0089] During the formation of the inner protective layer 92a, an external voltage is applied between the anode 176 and cathode 178 of the plasma gun 170, and an arc discharge is performed in the presence of the supplied plasma generating gas 180, thereby making the plasma generating gas 180 a high-temperature plasma state. The plasma-state gas is ejected from the nozzle 176a in the form of a high-temperature and high-speed plasma jet. On the other hand, powder coating material 184 is supplied from the powder supply section 182 along with the carrier gas. Accordingly, the powder coating material 184 is heated, melted, and accelerated by the plasma, impacts the surface (upper surface) of the component body 101a, and rapidly solidifies, thereby forming the inner protective layer 92a.

[0090] As the plasma generating gas 180, an inert gas such as argon can be used. The flow rate of argon 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.

[0091] The powder coating material 184 comprises raw material powder, which serves as the raw material for the porous protective layer 90. 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, more preferably 20 μm to 30 μm. Argon gas, the same as that used for plasma generation gas 180, can be used as the carrier gas supplied to the powder coating material 184. 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.

[0092] During plasma spraying, it is preferable to place the nozzle 176a of the plasma gun 170, which serves as the outlet for the plasma gas, and the surface of the component body 101a where the inner protective layer 92a is to be formed. Figure 5 The distance W between the upper surface of the component body 101a and the plasma gun 170 is set to 150mm to 200mm. The plasma gun 170 can be moved appropriately according to the area of ​​the porous protective layer 90 to be formed. Figure 5 While moving in the left and right direction, plasma spraying is performed. However, in this case, the distance W is preferably kept within the above range.

[0093] Regarding the inner protective layers 92b to 92e, each is formed once, except that they are formed on different surfaces of the component body 101a. Furthermore, after forming the inner protective layers 92 (inner protective layers 92a to 92e), the outer protective layers 91a to 91e are also formed once using plasma spraying. For example, plasma spraying is performed in an atmospheric or room temperature environment. Through the above operations, inner protective layers 92 (inner protective layers 92a to 92e) and outer protective layers 91 (outer protective layers 91a to 91e) are formed on the top, bottom, left, right, and front surfaces of the component body 101a, respectively, thus forming a porous protective layer 90. When the porous protective layer 90 is formed on a portion of the surface of the component body 101a (the area from the front end towards the rear to a distance L), as with the porous protective layers 90a to 90d, the area where the porous protective layer 90 is not formed can be covered with a mask.

[0094] When plasma spraying is used to form a porous protective layer 90, a pore-forming material is included in the powder spraying material 184 beforehand. By adjusting the proportion of this pore-forming material, the porosity P1 and P2 of the porous protective layer 90 can be adjusted. As the pore-forming material, materials that disappear upon heating can be used, such as theobromine and acrylic resin. Furthermore, regarding the pore-forming material included in the powder spraying material 184 used for the inner protective layer 92, by passing it through a sieve with a specified aperture beforehand to reduce the particle size deviation, the standard deviation σ of the porosity of the inner protective layer 92 can be reduced. When the powder spraying material 184 contains the pore-forming material, the plasma-sprayed component body 101a is heat-treated. Accordingly, pores are formed within the porous protective layer 90 by eliminating the pore-forming material. The thicknesses T1 and T2 can be adjusted according to the duration of the plasma spraying.

[0095] The sensor element 101 is obtained by forming a porous protective layer 90 on the element body 101a as described above. Then, a gas sensor 100 with the sensor element 101 embedded is manufactured. For example, an element sealing body is installed on the sensor element 101 for sealing and fixation, and a connector and lead wire are installed on the rear end side of the sensor element 101. Additionally, a protective cover is installed on the front end side of the sensor element 101 within the element sealing body. Furthermore, an outer cylinder is installed on the rear end side of the sensor element 101 within the element sealing body, and the lead wire is led out from the outer cylinder to the outside. It should be noted that this process of assembling the gas sensor 100 by embedding the sensor element 101 is known, for example, as described in Japanese Patent Application Publication No. 2015-178988.

[0096] When using the gas sensor 100 configured as described above, the gas to be measured flows into the protective cover of the gas sensor 100 and reaches the sensor element 101, passing through the porous protective layer 90 and flowing into the gas inlet 10. The sensor element 101 then detects the NOx concentration in the gas flowing into the gas inlet 10. At this time, moisture contained in the gas to be measured also intrudes into the protective cover, sometimes adhering to the surface of the porous protective layer 90. The element body 101a is adjusted to the solid electrolyte activation temperature (e.g., 800°C) by the heater 72 as described above. Therefore, normally, if moisture adheres to the sensor element 101, the temperature on the outside of the element body 101a drops sharply, causing a steep temperature gradient between the inside and outside of the element body 101a, sometimes resulting in cracks in the element body 101a. Here, in the porous protective layer 90 of this embodiment, as described above, the porosity P1 of the outer protective layer 91 is lower than the porosity P2 of the inner protective layer 92. Therefore, the outer protective layer 91 has a lower porosity P1, making it difficult for moisture to pass through. On the other hand, the inner protective layer 92 has a higher porosity P2, resulting in increased heat insulation. Accordingly, the water resistance of the sensor element 101 is improved. Furthermore, in the inner protective layer 92 of this embodiment, the standard deviation σ of the porosity P2 is less than 2.3%, indicating a small deviation in porosity. Because the standard deviation σ of the porosity of the inner protective layer 92 is less than 2.3%, there are fewer portions of locally low porosity within the inner protective layer 92, i.e., portions of locally low heat insulation. Therefore, the cooling of the element body 101a when moisture adheres to the sensor element 101 is suppressed. Therefore, for example, compared to a sensor element that has an inner protective layer with the same porosity but a larger standard deviation σ instead of an inner protective layer 92, the water resistance of the sensor element 101 of this embodiment is further improved.

[0097] According to the detailed description above, the sensor element 101 of this embodiment has improved water resistance by having a standard deviation of porosity of the porous protective layer 90 (here, especially the inner protective layer 92) of 2.3% or less.

[0098] Furthermore, the porous protective layer 90 comprises: an inner porous protective layer 92 and an outer porous protective layer 91 located further outward than the inner protective layer 92 and having a lower porosity than the inner protective layer 92. In addition, the standard deviation σ of the porosity of the inner protective layer 92 in the porous protective layer 90 is 2.3% or less. Accordingly, the outer protective layer 91, due to its lower porosity P1, makes it difficult for moisture to pass through, while the inner protective layer 92, due to its higher porosity P2, increases its heat insulation properties, thus improving the water resistance of the sensor element 101. Furthermore, because the standard deviation σ of the porosity P2 of the inner protective layer 92 is 2.3% or less, there are fewer locally lower porosity portions within the inner protective layer 92, i.e., portions with locally lower heat insulation properties, thus further improving the water resistance of the sensor element 101.

[0099] Furthermore, the main body 101a is a long rectangular parallelepiped shape, and a gas flow section 9 for introducing and circulating the gas to be measured is provided inside. Additionally, the inner protective layer 92a of the inner protective layer 92 covers the surface closest to the gas flow section 9 (here, the upper surface) of the four surfaces (top, bottom, left, and right) of the main body 101a along its length. Moreover, the standard deviation σ of the porosity of the portion of the inner protective layer 92a covering the region 102a on which the gas flow section 9 is projected is less than 2.3%. Here, as described above, the portion of the main body 101a between regions 102a to 102d and the gas flow section 9 is relatively weak and less resistant to thermal shock. In particular, the portion between the closest surface and the gas flow section (here, the portion between region 102a and the gas flow section 9) is relatively thin, and therefore particularly susceptible to thermal shock. In contrast, since the standard deviation σ of the porosity of the portion of the inner protective layer 92a covering region 102a is less than 2.3%, cracks can be suppressed in the portion that is not resistant to thermal shock, thus improving the water resistance of sensor element 101.

[0100] Furthermore, the gas inlet 10, which serves as the inlet for the gas flow section 9, is open at its end face (here, the front surface) along the length of the element body 101a. The inner protective layer 92e of the inner protective layer 92 covers the front surface of the element body 101a, and the standard deviation σ of the porosity of the portion covering the front surface is 2.3% or less. Here, as described above, the peripheral portion of the gas inlet 10 in the element body 101a is a weaker and less resistant to thermal shock. In contrast, because the standard deviation σ of the porosity of the portion of the element body 101a covered by the inner protective layer 92 that opens the gas inlet 10 is 2.3% or less, cracking in the less resistant to thermal shock can be suppressed. Therefore, the water resistance of the sensor element 101 is improved.

[0101] Furthermore, if the porosity P2 of the inner protective layer 92 is 40% or more, insufficient heat insulation between the outer protective layer 91 and the component body 101a can be suppressed. If the porosity P2 of the inner protective layer 92 is 70% or less, insufficient strength of the inner protective layer 92 can be suppressed.

[0102] Furthermore, if the standard deviation σ of the inner protective layer 92 is less than 1.5%, the water resistance of the sensor element 101 is further improved.

[0103] It should be noted that the present invention is not limited to any of the above embodiments. Of course, as long as it falls within the technical scope of the present invention, it can be implemented in various ways.

[0104] For example, in the above embodiment, the standard deviation σ of the porosity of any one of the inner protective layers 92a to 92e is 2.3% or less. However, it is sufficient if the standard deviation σ of one or more of the inner protective layers 92a to 92e is 2.3% or less. If the standard deviation σ of one of the inner protective layers 92a to 92e is 2.3% or less, then at least that inner protective layer achieves the above-mentioned effect. However, it is preferable that more inner protective layers have a standard deviation σ of 2.3% or less among the inner protective layers 92a to 92e, and more preferably that the standard deviation σ of any one of the inner protective layers 92a to 92e (i.e., the entire inner protective layer 92) is 2.3% or less.

[0105] In the above embodiment, the standard deviation σ derived from the portion of the inner protective layer 92a covering region 102a is 2.3% or less. However, it is preferable that the standard deviation σ derived from the portion other than the portion covering region 102a is 2.3% or less, similarly to the method described above. Furthermore, even if the standard deviation σ of the portion of the inner protective layer 92a covering region 102a is not 2.3% or less, it is acceptable as long as the standard deviation σ calculated at any cross-section of the inner protective layer 92a is 2.3% or less. However, as mentioned above, a smaller standard deviation σ of the portion of the inner protective layer 92a covering region 102a is more helpful for improving water resistance; therefore, it is preferable that the standard deviation σ of at least the portion of the inner protective layer 92a covering region 102a is 2.3% or less. The same applies to the inner protective layers 92b to 92e.

[0106] In the above embodiments, the porous protective layers 90a to 90e are each a two-layer structure, but are not limited to this. For example, the porous protective layer 90 may have other layers located outside the outer protective layer 91, or between the outer protective layer 91 and the inner protective layer 92, or closer to the element body 101a than the inner protective layer 92. It should be noted that when the porous protective layer 90 has a structure of three or more layers, the "inner protective layer" is defined as any one of the layers in the porous protective layer 90 except for the outermost layer, and the standard deviation σ of the porosity of this layer only needs to be 2.3% or less. That is, when the porous protective layer 90 has a structure of three or more layers, the standard deviation σ of the porosity of any one or more of the layers in the porous protective layer 90 except for the outermost layer only needs to be 2.3% or less. Accordingly, the insulation effect of the layer (inner protective layer) with a standard deviation σ of 2.3% or less is less likely to become insufficient, and thus, the effect of improving the water resistance of the sensor element 101 is obtained in the same way as in the above embodiment. In this case, it is preferable to have a layer with a lower porosity than the layer with a standard deviation σ of 2.3% or less located on the outer side, and such a layer is designated as the "outer protective layer".

[0107] In the above embodiments, the porous protective layer 90 has an outer protective layer 91 and an inner protective layer 92; however, the porous protective layer 90 can be a single-layer structure. Figure 6 This is a cross-sectional view of the porous protective layer 190 in the modified example. Figure 7 yes Figure 6 DD cross-sectional view. Figure 6 , 7The porous protective layer 190 shown 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 is a single-layer structure, and otherwise exhibits the same configuration as the porous protective layers 90a to 90e in the above embodiment. Regarding 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, then at least the porous protective layer achieves the aforementioned effect. It should be noted that the method for calculating the standard deviation σ of the porous protective layers 190a to 190e is the same as the method described above for calculating the standard deviation σ of the inner protective layers 92a to 92e. Furthermore, similarly to the above embodiment, regarding the porous protective layer 190a that covers the closest surface (here, the upper surface of the element body 101a), it is preferable that the standard deviation σ of the porosity of the portion of the porous protective layer 190a covering region 102a is 2.3% or less. Similarly, similarly to the above embodiment, regarding the porous protective layer 190e that covers the front surface of the element body 101a (the surface where the gas inlet 10 is open), it is preferable that the standard deviation σ of the porosity of the portion of the porous protective layer 190e covering the front surface of the element body 101a is 2.3% or less.

[0108] Figure 6 , 7 In the porous protective layer 190 with a single-layer structure shown, the porosity of any one or more of the porous protective layers 190a to 190e is preferably 10% or more. If the porosity is 10% or more, it can suppress the flow of the measured gas by the porous protective layer 190. Furthermore, the porosity of any one or more of the porous protective layers 190a to 190e is preferably 40% or less. If the porosity is 40% or less, moisture does not easily pass through the porous protective layer 190. Additionally, the thickness of any one or more of the porous protective layers 190a to 190e is preferably 100 μm or more. If the thickness is 100 μm or more, the water resistance of the element body 101a is less likely to be insufficient. The thickness of any one or more of the porous protective layers 190a to 190e is preferably 500 μm or less. Since the porous protective layer 190 has a single-layer structure, for preventing moisture passage, the porosity needs to be lower than that of the inner protective layer 92 in the above embodiment. Therefore, for the porous protective layer 190 with a single-layer structure, it is preferable to make its thickness thinner than that of the inner protective layer in the above embodiment.

[0109] In the above embodiments, the porous protective layer 90 includes porous protective layers 90a to 90e, but is not limited thereto. It is sufficient that the porous protective layer 90 covers at least a portion of the element body 101a. For example, the porous protective layer 90 may not include one or more of the porous protective layers 90a to 90e. Figure 6 , 7 The porous protective layer 190 shown is the same.

[0110] In the above embodiments, both the outer protective layer 91 and the inner protective layer 92 are made of the same material (alumina) ceramic, but are not limited thereto, and the two materials may be different.

[0111] In the above embodiment, a porous protective layer 90 is formed using plasma spraying, but this is not a limitation. For example, the porous protective layer 90 can be formed using screen printing, molding, or impregnation. When the porous protective layer 90 is formed using the above method, similarly to the above embodiment, by using a pore-forming material with a smaller particle size deviation, the standard deviation σ of the porosity can be reduced.

[0112] Example

[0113] The following description uses specific examples of sensor element fabrication as embodiments. Examples 1-3 correspond to embodiments of the present invention, and Examples 4-6 correspond to comparative examples. It should be noted that the present invention is not limited to the following embodiments.

[0114] [Experimental Example 1]

[0115] As an experimental example 1, a sensor element was fabricated using the manufacturing method of the sensor element 101 described in the above embodiment. First, a sensor element with a length of 67.5 mm in the front-to-back direction, a width of 4.25 mm in the left-to-right direction, and a thickness of 1.45 mm in the top-to-bottom direction was fabricated. Figure 1 , 2 The component body 101a is shown. It should be noted that in the production of the component body 101a, zirconium oxide particles with 4 mol% yttrium oxide stabilizer, organic binder and organic solvent are mixed and formed by casting to obtain a ceramic green sheet.

[0116] Next, a porous protective layer 90 was formed by plasma spraying to fabricate a sensor element 101, as Experimental Example 1. The formation of the porous protective layer 90 in Experimental Example 1 was performed as follows: An Oerlikon Metco Sinplex Pro-90 was used as the plasma gun 170. When forming the inner protective layer 92, a powder spraying material 184 was used, which was a mixture of alumina powder with an average particle size of 20 μm and a pore-forming material. The pore-forming material was a material whose particle size deviation was minimized by passing it through a sieve with a specified pore size. The plasma generating gas 180 was a gas obtained by mixing argon (flow rate 50 L / min, supply pressure 0.5 MPa) and hydrogen (flow rate 10 L / min, supply pressure 0.5 MPa). The voltage applied between the anode 176 and the cathode 178 was set to a DC voltage of 70 V. The current was 500 A. Argon gas (flow rate 4 L / min, supply pressure 0.5 MPa) was used as the carrier gas for supplying the powder-coated material 184. The distance W was set to 150 mm. Plasma spraying was performed in an atmospheric and room temperature environment. The spraying direction of the plasma gun 170 (the orientation of the nozzle 176a) was perpendicular to the formation surface of the inner protective layer 92 in the sensor element 101. After forming the inner protective layer 92 under the above conditions, the outer protective layer 91 was formed. When forming the outer protective layer 91, the same alumina particles as those used in the inner protective layer 92 were used as the powder-coated material 184. By ensuring that the powder-coated material 184 did not contain pore-forming material, 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, thereby eliminating the pore-forming material in the inner protective layer 92, resulting in the sensor element 101 of Experimental Example 1.

[0117] [Experimental Examples 2-6]

[0118] The outer protective layer 91 is the same as in Experimental Example 1. Regarding the inner protective layer 92, the particle size of the sieve used for the pore-forming material is adjusted by appropriately adjusting the pore size, so that the standard deviation σ has different values. Otherwise, the sensor element 101 is fabricated in the same way as in Experimental Example 1 as in Experimental Examples 2 to 6.

[0119] [Derivation of parameters for the outer and inner protective layers]

[0120] For Examples 1-6, 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 derived using the method described above. In Examples 1-6, the porosity P1 was 25% and the thickness T1 was 200 μm, while the porosity P2 of the inner protective layer 92 was 55% and the thickness T2 was 550 μm. The standard deviation σ of Examples 1-6 is shown in Table 1. It should be noted that a Hitachi SU1510 and a MediaCybernetics Image-Pro Plus 7.0 were used for SEM image acquisition and image analysis. Furthermore, in Example 1, the values ​​of porosity P1 and thickness T1 were the same for any of the outer protective layers 91a-91e. Similarly, in Example 1, the values ​​of porosity P2, thickness T2, and standard deviation σ were also the same for any of the inner protective layers 92a-92e. In each of the experimental examples 2 to 6, similarly, the values ​​of porosity P1 and thickness T1 of the outer protective layers 91a to 91e were the same, and the values ​​of porosity P2, thickness T2 and standard deviation σ of the inner protective layers 92a to 92e were the same.

[0121] [Evaluation of water resistance]

[0122] For the sensor elements in Experiments 1-6, the water resistance of sensor element 101 was evaluated. Specifically, firstly, the heater 72 was energized to raise the temperature to 800°C, and the element body 101a was heated. In this state, the main pump unit 21, auxiliary pump unit 50, main pump control oxygen partial pressure detection sensor unit 80, and auxiliary pump control oxygen partial pressure detection sensor unit 81 were operated in an atmospheric atmosphere to control the oxygen concentration in the first internal cavity 20 to maintain a predetermined constant value. Then, after the pump current Ip0 stabilized, water droplets were dropped onto the porous protective layer 90, and the presence of cracks in the element body 101a was determined based on whether the pump current Ip0 exceeded a predetermined threshold value. It should be noted that if cracks are generated in the element body 101a due to thermal shock from the water droplets, oxygen can easily flow into the first internal cavity 20 through the cracks, thus increasing the value of the pump current Ip0. Therefore, if the pump current Ip0 exceeds the experimentally determined threshold, it is determined that a crack has occurred in the component body 101a due to water droplets. Furthermore, the amount of water droplets was gradually increased, and multiple tests were conducted. The amount of water droplets at the time of the first crack occurrence was defined as the water resistance [μL]. It should be noted that for the sensor elements of Experimental Examples 1-6, the average value of three tests was used as the water resistance [μL]. A higher water resistance indicates higher water resistance of the sensor element 101. Using Experimental Example 6, which had the lowest water resistance among Experimental Examples 1-6, as a benchmark, the water resistance ratios of Experimental Examples 1-5 were calculated. For each experimental example, a water resistance ratio of 3 or higher (i.e., a water resistance ratio more than three times that of Experimental Example 6) was classified as "A (Excellent)"; a water resistance ratio of 1.5 or higher but less than 3 was classified as "B (Good)"; and a water resistance ratio less than 1.5 was classified as "F (Poor)".

[0123] The standard deviation σ, water resistance ratio, and judgment results of the inner protective layer 92 for each of Experiments 1-6 are summarized in Table 1. Additionally, Figure 8 The graph is obtained by plotting the standard deviation σ of the porosity of each of the experimental examples 1 to 6 against the ratio of water resistance.

[0124] [Table 1]

[0125]

[0126] From Table 1 and Figure 8It can be seen that, compared with Experiments 5 and 6, where the standard deviation σ is less than 2.3%, the water resistance of Experiments 1-4 is higher, indicating improved water resistance of sensor element 101. Specifically, in Experiments 1-4, the water resistance ratio is greater than 1.5 and the judgment result is A or B, while in Experiments 5 and 6, the judgment result is F. Furthermore, the results of Experiments 1-6 show the following trend: the smaller the standard deviation σ, the higher the water resistance. Additionally, the results of Experiments 1-4 confirm the following trend: around 1.7% of the standard deviation σ, the water resistance changes drastically. Based on these results, it is concluded that if the standard deviation σ is less than 1.5%, the water resistance of sensor element 101 is further improved.

[0127] This application is based on Japanese Patent Application No. 2021-042258, filed on March 16, 2021, the entire contents of which are incorporated herein by reference.

[0128] Industrial availability

[0129] This invention relates to a gas sensor element and a gas sensor for detecting the concentration of specific gases such as NOx in measured gases such as automobile exhaust.

[0130] Explanation of reference numerals in the attached figures

[0131] 1 First substrate layer, 2 Second substrate layer, 3 Third substrate layer, 4 First solid electrolyte layer, 5 Isolation layer, 6 Second solid electrolyte layer, 9 Measured gas flow section, 10 Gas inlet, 11 First diffusion rate control section, 12 Buffer space, 13 Second diffusion rate control section, 20 First internal cavity, 21 Main pump unit, 22 Inner pump electrode, 22a Top electrode section, 22b Bottom electrode section, 23 Outer pump electrode, 24 Variable power supply, 30 Third diffusion rate control section, 40 Second internal cavity, 41 Measurement pump unit, 42 Reference electrode, 43 Reference gas inlet space, 44 Measurement electrode, 45 Fourth diffusion rate control section, 46 Variable power supply, 48 Atmosphere inlet layer, 50 Auxiliary pump unit, 51 Auxiliary pump electrode, 51a Top electrode section, 51b Bottom electrode section, 52 Variable power supply, 70 Heater section, 71 Heater connector electrode, 72 heater, 73 through hole, 74 heater insulation layer, 75 pressure relief hole, 80 oxygen partial pressure detection sensor unit for main pump control, 81 oxygen partial pressure detection sensor unit for auxiliary pump control, 82 oxygen partial pressure detection sensor unit for measuring pump control, 83 sensor unit, 90, 90a-90e porous protective layer, 91, 91a-91e outer protective layer, 92, 92a-92e inner protective layer, 93a, 93e observation area, 100 gas sensor, 101 sensor element, 101a element body, 102a-102d area, 170 plasma gun, 172 outer periphery, 173 insulation part, 174 water cooling jacket, 176 anode, 176a nozzle, 178 cathode, 180 plasma generating gas, 182 powder supply part, 184 powder spraying material.

Claims

1. A gas sensor element, comprising: The component body includes an oxygen ion-conducting solid electrolyte layer; and A protective layer, wherein the protective layer is a porous material that covers at least a portion of the main body of the component and has multiple pores inside. The main body of the component is a long rectangular parallelepiped, and its interior is provided with a gas flow section for introducing and circulating the gas to be measured. The protective layer covers the surface closest to the gas flow section being measured, one of the four surfaces of the main body of the component along the length direction. The standard deviation of the porosity of the portion of the closest surface that covers the area onto which the gas flow section being measured is projected is less than 2.3%.

2. A gas sensor element, comprising: The component body includes an oxygen ion-conducting solid electrolyte layer; and A protective layer, wherein the protective layer is a porous material that covers at least a portion of the main body of the component and has multiple pores inside. The main body of the component is a long rectangular parallelepiped, and it has an internal gas flow section for introducing and circulating the gas to be measured. The gas inlet, which serves as the inlet for the gas flow section being measured, is open at the end face of the main body of the component along its length. The protective layer covers the end face of the component body, and the standard deviation of the porosity of the portion of the end face covered is less than 2.3%.

3. The gas sensor element according to claim 1, wherein, The gas inlet, which serves as the inlet for the gas flow section being measured, is open at the end face of the main body of the component along its length. The protective layer covers the end face of the component body, and the standard deviation of the porosity of the portion of the end face covered is less than 2.3%.

4. The gas sensor element according to claim 1, wherein, The porosity of the protective layer is more than 10% and less than 40%.

5. A gas sensor element, comprising: The component body includes an oxygen ion-conducting solid electrolyte layer; and A protective layer, wherein the protective layer is a porous material that covers at least a portion of the main body of the component and has multiple pores inside. The protective layer comprises: a porous inner protective layer and a porous outer protective layer located further outward than the inner protective layer and having a lower porosity than the inner protective layer. The main body of the component is a long rectangular parallelepiped, and it has an internal gas flow section for introducing and circulating the gas to be measured. The inner protective layer covers the surface closest to the gas flow section being measured, one of the four surfaces of the main body of the component along the length direction. The standard deviation of the porosity of the portion of the closest surface that covers the area onto which the gas flow section being measured is projected is less than 2.3%.

6. A gas sensor element, comprising: The component body includes an oxygen ion-conducting solid electrolyte layer; and A protective layer, wherein the protective layer is a porous material that covers at least a portion of the main body of the component and has multiple pores inside. The protective layer comprises: a porous inner protective layer and a porous outer protective layer located further outward than the inner protective layer and having a lower porosity than the inner protective layer. The main body of the component is a long rectangular parallelepiped, and it has an internal gas flow section for introducing and circulating the gas to be measured. The gas inlet, which serves as the inlet for the gas flow section being measured, is open at the end face of the main body of the component along its length. The inner protective layer covers the end face of the component body, and the standard deviation of the porosity of the portion of the end face covered is less than 2.3%.

7. The gas sensor element according to claim 5, wherein, The gas inlet, which serves as the inlet for the gas flow section being measured, is open at the end face of the main body of the component along its length. The inner protective layer covers the end face of the component body, and the standard deviation of the porosity of the portion of the end face covered is less than 2.3%.

8. The gas sensor element according to claim 5, wherein, The porosity of the inner protective layer is above 40% and below 70%.

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

10. A gas sensor, wherein, It has a gas sensor element according to any one of claims 1 to 9.

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