Sensor element

Abstract

To provide a sensor element with a high resistance to water.SOLUTION: A sensor element 101 includes: an element body 101a including solid electrolyte layers 1, 2, 3, 4, 5, 6 with an oxygen ion conductive property; and a protective layer 91 for covering the surface of the element body 101a at least partially. The protective layer 91 is made of a porous body with air holes in the inside. In the air holes of the protective layer 91, the ratio of the length (Lt) of the air holes in a thickness direction vertical to the surface of the element body 101a to the length (Lf) of the air holes in a plane direction vertical to the thickness direction, Lt / Lf, is in the range of 0.6 to 0.9, both inclusive. The sensor element 101 detects a measurement target gas in a gas to be measured.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a sensor element of a gas sensor for detecting a target gas in a measurement gas. [Background technology]

[0002] Gas sensors are used to detect and measure the concentration of target gas components (oxygen O2, nitrogen oxides NOx, ammonia NH3, hydrocarbons HC, carbon dioxide CO2, etc.) in gases to be measured, such as automobile exhaust gases. For example, the concentration of target gas components in automobile exhaust gases is measured, and the exhaust gas purification system installed in the automobile is optimally controlled based on the measured value.

[0003] Known examples of such gas sensors include a gas sensor having a sensor element using an oxygen ion conductive solid electrolyte such as zirconia (ZrO2), etc. It is also known that in such gas sensors, a porous protective layer is formed on the surface of the sensor element.

[0004] For example, Japanese Patent Application Laid-Open No. 2016-065852 discloses that a powdered spray material such as alumina is attached to the surface of a sensor element by plasma spraying to form a porous protective layer. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2016-065852 Summary of the Invention [Problem to be solved by the invention]

[0006] In gas sensors equipped with a sensor element using a solid electrolyte, the sensor element reaches a high temperature (for example, about 800°C) when measuring the target gas (during normal operation). If water splashes on the sensor element during normal operation of the gas sensor, only the surface of the sensor element, which is in a high temperature state, cools rapidly due to the moisture adhesion, and this thermal shock can cause cracks in the internal structure of the sensor element.

[0007] Furthermore, due to stricter automobile exhaust gas regulations, gas sensors installed in automobiles are required to measure target gases in the exhaust gas immediately after the engine is started. However, immediately after the engine is started, there is a large amount of condensed water inside the exhaust gas piping, which increases the risk of water splashing on the high-temperature sensor element.

[0008] Under these circumstances, it is necessary to further prevent cracks from occurring in the internal structure of the sensor element when the sensor element is exposed to water (water damage) at high temperatures. In other words, there is an urgent need to improve the water resistance of the sensor element.

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a sensor element that is highly water resistant. [Means for solving the problem]

[0010] As a result of extensive research, the inventors have found that the water resistance of a sensor element can be improved by forming a porous protective layer on at least a portion of the surface of the sensor element and making the pores of the protective layer thin in the thickness direction of the protective layer and wide in the surface direction (so-called flat shape).

[0011] The present invention includes the following inventions. (1) an element body including an oxygen ion conductive solid electrolyte layer; a protective layer covering at least a portion of the surface of the element body; A sensor element comprising: the protective layer is made of a porous body having pores therein, A sensor element for detecting a target gas in a measurement gas, wherein the pores in the protective layer have a ratio (Lt / Lf) of the pore length (Lt) in the thickness direction perpendicular to the surface of the element body to the pore length (Lf) in the plane direction perpendicular to the thickness direction of the element body of 0.6 to 0.9.

[0012] (2) The sensor element according to (1), wherein the protective layer has a thickness of 100 μm to 500 μm.

[0013] (3) The sensor element according to (1) or (2) above, wherein the protective layer has a porosity of 10% by volume to 40% by volume.

[0014] (4) The protective layer includes a surface layer and an inner layer formed inside the surface layer, The sensor element according to (1) above, wherein the inner layer has a higher porosity than the surface layer.

[0015] (5) The sensor element according to (4), wherein the inner layer of the protective layer has a thickness of 300 μm to 700 μm.

[0016] (6) The sensor element according to (4) or (5) above, wherein the surface layer of the protective layer has a thickness of 100 μm to 300 μm.

[0017] (7) The sensor element according to any one of (4) to (6) above, wherein the inner layer of the protective layer has a porosity of 40% by volume to 70% by volume.

[0018] (8) The element body is a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow portion formed at one end of the base portion in the longitudinal direction; at least one inner electrode disposed on an inner surface of the measurement gas flow portion; The electrodes include an outer electrode disposed so as to be in contact with at least one of the plurality of solid electrolyte layers via the outer electrode; The sensor element according to any one of (1) to (7) above, comprising:

[0019] (9) A method for manufacturing the sensor element according to any one of (1) to (8) above, a step of applying a protective layer-forming composition containing a pore-forming material to at least a portion of the surface of the element body to form a coating layer; applying pressure to the coating layer; a step of degreasing the coated layer after pressing to obtain a protective layer made of a porous material; A method for manufacturing a sensor element, comprising: [Effects of the Invention]

[0020] According to the present invention, a sensor element having high water resistance can be provided. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 2 is a perspective view showing an example of a schematic configuration of a sensor element 101. [Figure 2] 2 is a schematic vertical cross-sectional view in the longitudinal direction showing an example of the general configuration of a gas sensor 100 including a sensor element 101, the cross-sectional view including a schematic cross-sectional view of the sensor element 101 taken along line II-II in FIG. [Figure 3] Fig. 3(i) is a schematic cross-sectional view taken along line III-III in Fig. 1. It is a schematic vertical cross-sectional view perpendicular to the longitudinal direction of the sensor element 101. Fig. 3(ii) is an enlarged schematic cross-sectional view of the porous protective layer 91a in Fig. 3(i), and is a schematic view showing an example in which the shape of the pores in the cross section of the porous protective layer 91a in the XZ plane is simplified. [Figure 4] Fig. 4(A) is a schematic diagram showing an example of a simplified shape of the pore precursors H in the cross section of the porous protective layer 91a after application. Fig. 4(B) is a schematic diagram showing an example of a simplified shape of the pore precursors H in the cross section of the porous protective layer 91a after pressure application. DETAILED DESCRIPTION OF THE INVENTION

[0022] The sensor element of the present invention comprises: an element body including an oxygen ion conductive solid electrolyte layer; a protective layer covering at least a portion of the surface of the element body; Including, the protective layer is a porous body having pores therein, The pores in the protective layer have a ratio (Lt / Lf) of the pore length (Lt) in the thickness direction perpendicular to the surface of the element body to the pore length (Lf) in the plane direction perpendicular to the thickness direction of 0.6 to 0.9.

[0023] An embodiment of a gas sensor including a sensor element of the present invention will be described in detail below.

[0024] [Outline of gas sensor configuration] A gas sensor including a sensor element of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view showing an example of the general configuration of a sensor element 101. FIG. 2 is a longitudinal vertical cross-sectional view showing an example of the general configuration of a gas sensor 100 including the sensor element 101. In FIG. 2, the cross-sectional view of the sensor element 101 is a cross-sectional view taken along line II-II in FIG. 1. FIG. 3(i) is a cross-sectional view taken along line III-III in FIG. 1. In the following, with reference to FIG. 2, the upper and lower sides of FIG. 2 refer to the upper side and the lower side, respectively, and the left side of FIG. 2 refers to the leading end side and the right side to the rear end side. With reference to FIG. 3, the right side of FIG. 3 refers to the right side and the left side to the left.

[0025] In FIG. 2, a gas sensor 100 is an example of a limiting current type NOx sensor that detects NOx in a measurement gas by a sensor element 101 and measures its concentration.

[0026] The sensor element 101 includes a porous protective layer 91, which will be described in detail later. The porous protective layer 91 corresponds to the protective layer of the present invention. The portion of the sensor element 101 excluding the porous protective layer 91 will be referred to as an element body 101a below.

[0027] In the sensor element 101 of this embodiment, the inner electrodes include an inner main pump electrode 22, an auxiliary pump electrode 51, and a measurement electrode 44. The outer electrode includes an outer pump electrode 23.

[0028] The sensor element 101 is a long, plate-like element including a base portion 102 having a structure in which multiple oxygen-ion conductive solid electrolyte layers are stacked. The long, plate-like shape is also referred to as a long plate shape or a strip shape. The base portion 102 has a structure in which six layers 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, each of which is made of an oxygen-ion conductive solid electrolyte layer such as zirconia (ZrO). The solid electrolyte forming these six layers is dense and airtight. The six layers may all have the same thickness or may have different thicknesses. The layers are bonded together via adhesive layers made of solid electrolyte, and the base portion 102 includes the adhesive layers. While FIG. 2 illustrates a layer structure consisting of six layers, the layer structure of the present invention is not limited to this and any number and layer structure may be used.

[0029] The sensor element 101 is manufactured by, for example, laminating ceramic green sheets corresponding to each layer after performing predetermined processing and printing a circuit pattern on them, and then firing the sheets to integrate them.

[0030] A gas inlet 10 is formed at one longitudinal end (hereinafter referred to as the tip) of the sensor element 101, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The measurement gas flow section 15 includes, in the longitudinal direction from the gas inlet 10, a first diffusion-controlling section 11, a buffer space 12, a second diffusion-controlling section 13, a first internal space 20, a third diffusion-controlling section 30, a second internal space 40, a fourth diffusion-controlling section 60, and a third internal space 61, which are adjacently formed and communicate with each other in this order.

[0031] The gas inlet 10, the buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 are spaces inside the sensor element 101, which are defined by hollowing out the spacer layer 5, with an upper portion defined by the underside of the second solid electrolyte layer 6, a lower portion defined by the upper surface of the first solid electrolyte layer 4, and sides defined by the side surfaces of the spacer layer 5.

[0032] 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 (the openings have their longitudinal direction perpendicular to the plane of the drawing in FIG. 2). The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, and the third diffusion rate-controlling section 30 may have any shape that provides the desired diffusion resistance, and the shape is not limited to the slits.

[0033] The fourth diffusion rate-controlling portion 60 is provided as a single horizontally elongated slit (the opening has its longitudinal direction perpendicular to the plane of the drawing in FIG. 2 ) between the spacer layer 5 and the second solid electrolyte layer 6. The fourth diffusion rate-controlling portion 60 may have any shape that provides a desired diffusion resistance, and the shape is not limited to the slit.

[0034] Furthermore, a reference gas introduction space 43 is provided at a position farther from the tip side than the measurement gas flow section 15, 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 its side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0035] The air introduction layer 48 is a layer made of porous alumina, 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.

[0036] 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 is connected to reference gas introduction space 43. That is, reference electrode 42 is disposed so as to come into contact with the reference gas via porous air introduction layer 48 and reference gas introduction space 43. Furthermore, as will be described later, reference electrode 42 can be used to measure the oxygen concentrations (oxygen partial pressures) in first internal space 20, second internal space 40, and third internal space 61.

[0037] In the measurement gas flow section 15, the gas inlet 10 is open to the external space, and the measurement gas is introduced into the sensor element 101 from the external space through the gas inlet 10.

[0038] In this embodiment, the measurement gas flow section 15 is configured such that the measurement gas is introduced through the gas inlet 10 opening at the tip end surface of the sensor element 101, but the present invention is not limited to this configuration. For example, the measurement gas flow section 15 does not need to have a recess for the gas inlet 10. In this case, the first diffusion rate-controlling section 11 essentially serves as the gas inlet. Furthermore, for example, the measurement gas flow part 15 may have an opening in a side surface along the longitudinal direction of the base part 102, the opening communicating with the buffer space 12 or a position in the first internal space 20 close to the buffer space 12. In this case, the measurement gas is introduced from the side surface along the longitudinal direction of the base part 102 through the opening. Furthermore, for example, the measurement gas flow portion 15 may be configured so that the measurement gas is introduced through a porous body.

[0039] The first diffusion rate-controlling part 11 is a part that applies a predetermined diffusion resistance to the measurement gas taken in through the gas inlet 10.

[0040] The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion rate-controlling part 11 to the second diffusion rate-controlling part 13 .

[0041] The second diffusion rate-controlling portion 13 is a portion that applies a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20 .

[0042] It is sufficient that the amount of the measurement gas finally introduced into the first internal space 20 is within a predetermined range. That is, it is sufficient that a predetermined diffusion resistance is imparted to the entire area from the tip of the sensor element 101 to the second diffusion-controlling section 13. For example, it is also possible that the first diffusion-controlling section 11 directly communicates with the first internal space 20, that is, the buffer space 12 and the second diffusion-controlling section 13 do not exist.

[0043] The buffer space 12 is a space provided to mitigate the influence of pressure fluctuations on the detected value when the pressure of the gas to be measured fluctuates.

[0044] When the measurement gas is introduced from the outside of the sensor element 101 into the first internal space 20, the measurement gas is suddenly taken into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations if the measurement gas is automobile exhaust gas), but 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 rate-controlling section 11, buffer space 12, and second diffusion rate-controlling section 13. As a result, the pressure fluctuations of the measurement gas introduced into the first internal space become almost negligible.

[0045] 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 part 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

[0046] The main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22, which is an inner electrode disposed on the inner surface of the measurement gas flow portion 15, and an outer pump electrode 23, which is an outer electrode disposed so as to be in contact with the inner main pump electrode 22 via a solid electrolyte (second solid electrolyte layer 6 in FIG. 2).

[0047] That is, the main pump cell 21 is an electrochemical pump cell including an inner main 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 upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22a so as to be exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0048] The inner main 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.

[0049] The inner main 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 main pump electrode 22, which comes into contact with the measurement gas, is formed using a material with reduced ability to reduce the NOx component in the measurement gas.

[0050] In the main pump cell 21, by applying a desired pump voltage Vp0 between the inner main pump electrode 22 and the outer pump electrode 23 using a variable power supply 24 and flowing a pump current Ip0 in a positive or negative direction between the inner main 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 pump oxygen from the external space into the first internal space 20.

[0051] 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 main 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.

[0052] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Furthermore, the pump current Ip0 is controlled by feedback-controlling the voltage Vp0 of the variable power supply 24 so that the electromotive force V0 is constant. This allows the oxygen concentration in the first internal space 20 to be maintained at a predetermined constant value.

[0053] 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.

[0054] The second internal space 40 is provided as a space for adjusting with higher precision the oxygen partial pressure in the measurement gas introduced through the third diffusion-controlling section 30. The oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50. The second internal space 40 and the auxiliary pump cell 50 may be omitted. From the viewpoint of the accuracy of adjusting the oxygen partial pressure, it is more preferable to have the second internal space 40 and the auxiliary pump cell 50.

[0055] 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.

[0056] The auxiliary pump cell 50 is an electrochemical pump cell including an auxiliary pump electrode 51, which is an inner electrode, disposed on the inner surface of the measurement gas flow section 15 at a position farther from the longitudinal tip of the base section 102 than the inner main pump electrode 22, and an outer pump electrode 23, which is an outer electrode, disposed so as to be in contact with the auxiliary pump electrode 51 via a solid electrolyte (second solid electrolyte layer 6 in FIG. 2).

[0057] That is, the auxiliary pump cell 50 is an auxiliary electrochemical pump cell that is composed of 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 sensor element 101 will suffice), and the second solid electrolyte layer 6.

[0058] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to the inner main 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, and 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. 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, forming a tunnel-shaped structure.

[0059] Like the inner main pump electrode 22, the auxiliary pump electrode 51 is also formed using a material with a weakened ability to reduce the NOx component in the measurement gas.

[0060] 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.

[0061] 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.

[0062] The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force 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.

[0063] 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 its electromotive force 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.

[0064] The fourth diffusion rate-controlling section 60 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been further controlled to a lower level by the operation of the auxiliary pump cell 50 in the second internal space 40, and guides the measurement gas to the third internal space 61.

[0065] The third internal space 61 is provided as a space for measuring the concentration of nitrogen oxides (NOx) in the measurement gas introduced through the fourth diffusion-controlling section 60. The NOx concentration is measured by the operation of the measurement pump cell 41.

[0066] The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44, which is an inner electrode, disposed on the inner surface of the measurement gas flow portion 15 at a position farther from the longitudinal tip of the base portion 102 than the auxiliary pump electrode 51, and an outer pump electrode 23, which is an outer electrode, disposed so as to be in contact with the measurement electrode 44 via a solid electrolyte (in FIG. 2, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4).

[0067] That is, the measurement pump cell 41 measures the NOx concentration in the measurement gas in the third internal space 61. 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 third internal space 61, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any appropriate electrode on the outside of the sensor element 101 will suffice), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0068] 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 third internal space 61.

[0069] In the measuring pump cell 41, oxygen generated by decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 44 is pumped out, and the amount of oxygen generated can be detected as a pump current Ip2.

[0070] Furthermore, in order to detect the oxygen partial pressure around the measurement electrode 44, the second solid electrolyte layer 6, the spacer layer 5, 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 V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

[0071] The measurement gas introduced into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion-controlling part 60 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 control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 remains constant. 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.

[0072] 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.

[0073] 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 Vref obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0074] 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.

[0075] In this embodiment, there are three internal cavities: a first internal cavity 20, a second internal cavity 40, and a third internal cavity 61, and an inner electrode 22, 51, 44 is disposed in each of the internal cavities 20, 40, 61, but the number and arrangement of the internal cavities are not limited to this. There may be one or two internal cavities, or four or more internal cavities.

[0076] Furthermore, in order to enhance the oxygen ion conductivity of the solid electrolyte, the sensor element 101 is provided with a heater section 70 that adjusts the temperature by heating and maintaining the temperature of the sensor element 101. The heater section 70 includes a heater electrode 71, a heater 72, a heater lead 76, a through-hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0077] The heater electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to a heater power supply, which is an external power supply, it is possible to supply power to the heater section 70 from outside.

[0078] 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 the heater electrode 71 via a heater lead 76 that is connected to the heater 72 and extends to the rear end side of the sensor element 101 in the longitudinal direction, and a through hole 73. The heater 72 generates heat when power is supplied from the outside through the heater electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 101.

[0079] The heater 72 is embedded throughout the entire area from the first internal space 20 to the third internal space 61, making it possible to adjust the entire sensor element 101 to a temperature at which the solid electrolyte is activated. The temperature needs to be adjusted so that the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 can operate. It is not necessary to adjust these entire areas to the same temperature, and the sensor element 101 may have a temperature distribution.

[0080] In the sensor element 101 of this embodiment, the heater 72 is embedded in the base portion 102, but the present invention is not limited to this. The heater 72 may be disposed so as to heat the base portion 102. That is, the heater 72 may be capable of heating the sensor element 101 to an extent that the sensor element 101 exhibits oxygen ion conductivity that enables the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 to operate. For example, the heater 72 may be embedded in the base portion 102 as in this embodiment. Alternatively, the heater portion 70 may be formed as a heater substrate separate from the base portion 102 and disposed adjacent to the base portion 102.

[0081] 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 and heater lead 76. 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 heater lead 76, and between the third substrate layer 3 and the heater 72 and heater lead 76.

[0082] The pressure release hole 75 penetrates the third substrate layer 3 and is formed so as to connect the heater insulating layer 74 and the reference gas introduction space 43. The pressure release hole 75 can mitigate an increase in internal pressure that accompanies an increase in temperature within the heater insulating layer 74. Note that the pressure release hole 75 may be omitted.

[0083] (protective layer) The sensor element 101 includes an element body 101a and a porous protective layer 91 that covers a portion of the element body 101a. In this embodiment, the porous protective layer 91 includes porous protective layers 91a to 91e, as shown in FIG. 1. The porous protective layer 91a covers the entire upper surface of the element body 101a, covering an area that is a distance A from the tip of the element body 101a in the longitudinal direction. The porous protective layer 91b covers the entire lower surface of the element body 101a, covering an area that is a distance A from the tip of the element body 101a in the longitudinal direction. The porous protective layer 91c covers the entire right surface of the element body 101a, covering an area that is a distance A from the tip of the element body 101a in the longitudinal direction. The porous protective layer 91d covers the entire left surface of the element body 101a, covering an area that is a distance A from the tip of the element body 101a in the longitudinal direction. The porous protective layer 91e covers the entire tip surface of the element body 101a.

[0084] The porous protective layer 91e also covers the gas inlet 10. However, because the porous protective layer 91e is porous, the gas to be measured can flow through the inside of the porous protective layer 91e and reach the gas inlet 10. Therefore, the gas to be measured can be detected and measured without any problems.

[0085] The porous protective layer 91 serves to prevent cracks from occurring in the internal structure of the element body 101a, for example, when water splashes on the high-temperature sensor element 101 during normal operation of the gas sensor. Water that reaches the sensor element 101 does not adhere directly to the surface of the element body 101a, but adheres to the porous protective layer 91. The surface of the porous protective layer 91 is rapidly cooled by the adhering water, but the thermal shock applied to the element body 101a is reduced due to the insulating effect of the porous protective layer 91. As a result, cracks can be prevented from occurring in the internal structure of the element body 101a. In other words, the water resistance of the sensor element 101 is improved.

[0086] The porous protective layer 91a also covers the outer pump electrode 23. The porous protective layer 91a also serves to prevent oil components and the like contained in the gas under measurement from adhering to the outer pump electrode 23, thereby suppressing deterioration of the outer pump electrode 23.

[0087] The porous protective layer 91 in this embodiment covers the entire area (91a, 91b, 91c, 91d, 91e) of the element body 101a, including the tip surface, from the tip surface to a distance A in the longitudinal direction of the element body 101a. The distance A may be determined within the range of 0 < distance A < total length of the element body 101a in the longitudinal direction, based on the range of exposure of the element body 101a to the measurement gas in the gas sensor 100 and the position of the outer pump electrode 23. The porous protective layers 91a to 91d may have different lengths in the longitudinal direction of the element body 101a.

[0088] The porous protective layer 91 may be formed on at least one of the tip surface, top and bottom surfaces, and left and right surfaces of the element body 101a. For example, it may be formed on only the top surface, or on both the top and bottom surfaces.

[0089] The porous protective layer 91 is made of a porous body. Examples of materials constituting the porous protective layer 91 include alumina, zirconia, spinel, cordierite, mullite, titania, and magnesia. The porous protective layer 91 may be made of one of these materials, or two or more of these materials. In this embodiment, the porous protective layer 91 is made of a porous alumina body.

[0090] The pores present inside the porous protective layer 91 have a ratio (Lt / Lf) of the pore length (Lt) in the thickness direction perpendicular to the surface of the element body 101a to the pore length (Lf) in the plane direction perpendicular to the thickness direction of 0.6 to 0.9. That is, the pores present inside the porous protective layer 91 have, on average, a shape that is thin in the thickness direction of the porous protective layer 91 and spreads in the plane direction (so-called flat shape).

[0091] 3(i) is a schematic cross-sectional view taken along line III-III in FIG. 1, and is a schematic vertical cross-sectional view perpendicular to the longitudinal direction of the sensor element 101. In FIG. 3(i), the outer pump electrode 23 and the inner main pump electrode 22 are omitted. In the following, the left-right direction in FIG. 3(i) is referred to as the X-axis direction, the up-down direction as the Z-axis direction, and the direction perpendicular to the plane of the drawing as the Y-axis direction. The X-axis direction is perpendicular to the longitudinal direction of the sensor element 101 and is a direction along the surfaces of the solid electrolyte layers 1 to 6 (the width direction of the sensor element 101). The Y-axis direction is the longitudinal direction of the sensor element 101. The Z-axis direction is perpendicular to the longitudinal direction of the sensor element 101 and is a direction perpendicular to the surfaces of the solid electrolyte layers 1 to 6 (the thickness direction of the sensor element 101).

[0092] FIG. 3(ii) is an enlarged schematic cross-sectional view of the porous protective layer 91a in FIG. 3(i), showing a simplified example of the shape of the pores in the XZ plane cross section of the porous protective layer 91a. The Z-axis direction is the thickness direction of the porous protective layer 91a. The shape of the pores is not limited to the roughly elliptical shape shown in FIG. 3(ii) and can take various shapes. Furthermore, the size, number, and distribution of the pores are not limited to the example shown in FIG. 3(ii). While FIG. 3(ii) shows an example of the porous protective layer 91a, the same applies to the porous protective layers 91b to 91e.

[0093] Furthermore, most of the pores inside the porous protective layer 91 have a structure in which there is a communication part (not shown in FIG. 3(ii)) between each of one or more adjacent or nearby pores. The communication part constitutes a part of the pore. Furthermore, the pores near the surface of the porous protective layer 91 often open to the surface. The pores near the interface with the element body 101a also often open to the interface.

[0094] In the present invention, the pore length (Lt) in the thickness direction perpendicular to the surface of the element body 101a corresponds to the average value of the pore lengths in the thickness direction of all pores present inside the porous protective layer 91. The pore length (Lf) in the plane direction perpendicular to the thickness direction corresponds to the average value of the pore lengths in the plane direction of all pores present inside the porous protective layer 91. The plane direction is the X-axis direction (the width direction of the sensor element 101) in the porous protective layer 91a. Alternatively, it may be the Y-axis direction (the longitudinal direction of the sensor element 101). Pores have various shapes, and the pore length in the X-axis direction and the pore length in the Y-axis direction of each individual pore are usually different values. However, when compared as average values, they are considered to be equivalent values.

[0095] Conceptually, the ratio (Lt / Lf) is the following value. For example, consider a cross section in which n pores P1, P2, ..., Pn exist, using FIG. 3(ii) as an example. The lengths of the pores P1, P2, ..., Pn in the thickness direction are respectively z1, z2, ..., zn, and the lengths in the plane direction are respectively x1, x2, ..., xn. In this case, Pore length in the thickness direction (Lt) = [(z1 + z2 + ··· + zn) / n]; Pore length in the plane direction (Lf) = [(x1 + x2 + + xn) / n] The ratio (Lt / Lf) is the ratio of the pore length (Lt) in the thickness direction to the pore length (Lf) in the plane direction.

[0096] Pores of various shapes, including the above-mentioned communicating portions, exist in the actual cross section of the porous protective layer 91. Specifically, the ratio (Lt / Lf) in the present invention is determined as follows: The ratio is determined by performing image analysis on a CT (Computed Tomography) image of the porous protective layer 91, as follows.

[0097] 1) The microstructure of the porous protective layer 91 of the sensor element 101 is photographed by CT. 2) A cross-sectional image of the XZ plane at any point of the porous protective layer 91a is obtained. The left-right direction of the cross-sectional image is the X-axis direction, and the up-down direction is the Z-axis direction. The number of pixels in the cross-sectional image is 600 pixels horizontally by 80 pixels vertically, and one pixel is 1.5 μm square. 3) The obtained cross-sectional image is binarized using "Otsu's binarization" (also known as discriminant analysis). In the binarized cross-sectional image, the constituent material of the porous protective layer 91 (alumina in this embodiment) is displayed in white, and the pores are displayed in black. 4) In the vertical (Z-axis direction) column of one pixel width at the right end of the cross-sectional image, for each series of one or more black pixels separated by white pixels, the number of consecutive black pixels in the vertical (Z-axis direction) direction is counted and the average value is calculated. Similarly, for the second and subsequent vertical (Z-axis) columns from the right end of the cross-sectional image, the average value of the number of consecutive black pixels in the Z-axis direction is calculated for each. 5) The average number of consecutive black pixels in each column along the Z axis is further averaged to determine the average length of the pores along the Z axis. This is called the cored length along the Z axis. 6) For each horizontal row (X-axis direction) of 1 pixel width in the cross-sectional image, the average length of the pores in the X-axis direction (Cored length in the X-axis direction) is determined in the same manner as in 3) and 4) above. 7) The obtained Cored length in the Z-axis direction is taken as the pore length (Lt) in the thickness direction perpendicular to the surface of the element body 101a. The obtained Cored length in the X-axis direction is taken as the pore length (Lf) in the surface direction perpendicular to the thickness direction. Using these values, the ratio (Lt / Lf) of the pore length in the thickness direction (Lt) to the pore length in the surface direction (Lf) is calculated.

[0098] Furthermore, instead of the Coredlength in the X-axis direction, the Coredlength in the Y-axis direction may be calculated using a cross-sectional image of the YZ plane. The obtained Coredlength in the Y-axis direction may be used as the pore length (Lf) in the plane direction perpendicular to the thickness direction. This is because the Coredlength in the X-axis direction and the Coredlength in the Y-axis direction are considered to be equivalent values.

[0099] The ratio (Lt / Lf) can be calculated in the same manner for the porous protective layers 91b to 91e. However, for the porous protective layers 91c and 91d, the Cored length in the X-axis direction is taken as the pore length in the thickness direction (Lt), and either the Cored length in the Y-axis direction or the Cored length in the Z-axis direction is taken as the pore length in the surface direction (Lf). For the porous protective layer 91e, the Cored length in the Y-axis direction is taken as the pore length in the thickness direction (Lt), and either the Cored length in the X-axis direction or the Cored length in the Z-axis direction is taken as the pore length in the surface direction (Lf).

[0100] In this embodiment, the ratio (Lt / Lf) is the same in all of the porous protective layers 91a to 91e and falls within the range of 0.6 to 0.9.

[0101] It is considered that the porous protective layer 91 has a substantially identical microstructure regardless of the observation location. Therefore, as described above, the value of the ratio (Lt / Lf) calculated using any cross-sectional image may be used as the value of the ratio (Lt / Lf) in the porous protective layer 91. For example, the value of the ratio (Lt / Lf) in the porous protective layer 91a may be used as the value of the ratio (Lt / Lf) in the porous protective layer 91.

[0102] As described above, the porous protective layer 91 has a structure in which the pore ratio (Lt / Lf) is 0.6 to 0.9. That is, the pores of the porous protective layer 91 are, on average, thin in the thickness direction and flattened in the plane direction. Therefore, it is considered that a sufficient number of pores are easily arranged in the thickness direction to ensure thermal insulation, regardless of the position in the plane direction of the porous protective layer 91. This effect is easily achieved when the pore ratio (Lt / Lf) is 0.9 or less. The upper limit of the pore ratio (Lt / Lf) may be 0.85 or less, or 0.8 or less. Such a porous protective layer 91 can further suppress temperature changes in the thickness direction when water is splashed on its surface, thereby further reducing the thermal shock to the element body 101a. As a result, the water resistance of the sensor element 101 can be improved.

[0103] Furthermore, if the pore ratio (Lt / Lf) is 0.6 or more, the pores do not spread too much in the plane direction, and it is thought that the porous protective layer 91 is less likely to peel off. As a result, it is possible to maintain the strength required for the porous protective layer 91. From the viewpoint of peeling resistance, the lower limit of the pore ratio (Lt / Lf) may be 0.65 or more, or 0.7 or more.

[0104] The thickness of the porous protective layer 91 may be, for example, 100 μm or more and 1000 μm or less. It may also be 100 μm or more and 500 μm or less. The thickness is determined as follows using an image (SEM image) obtained by observation with a scanning electron microscope (SEM): The sensor element 101 is cut perpendicular to the longitudinal direction of the sensor element 101 in the region where the porous protective layer 91 is present. The cut surface is filled with resin and polished to obtain an observation sample. The magnification of the SEM is set to 80 times, and the observation surface of the observation sample is photographed to obtain an SEM image of the cross section of the porous protective layer 91a. The direction perpendicular to the surface of the element body 101a is defined as the thickness direction, and the distance from the surface of the porous protective layer 91a to the boundary surface with the element body 101a is calculated, and this distance is defined as the thickness of the porous protective layer 91a. The porous protective layer 91a is formed as a layer having a predetermined thickness. Therefore, as described above, the thickness obtained using one cross-sectional image may be used as the thickness of the porous protective layer 91a. The thicknesses of the porous protective layers 91b to 91e are similarly obtained.

[0105] In this embodiment, the porous protective layers 91a to 91e all have approximately the same thickness, but the thicknesses of the porous protective layers 91a to 91e may differ from one another.

[0106] The porosity of the porous protective layer 91 may be, for example, 10% to 70% by volume. Alternatively, it may be 10% to 40% by volume. The porosity is determined as follows using an image (SEM image) obtained by observation with a scanning electron microscope (SEM). As in the case of the thickness described above, the SEM magnification is set to 80 times to obtain an SEM image of the cross section of the porous protective layer 91a. The obtained SEM image is then binarized using "Otsu's binarization" (also known as discriminant analysis). In the binarized image, alumina is represented in white and pores are represented in black. The areas of the alumina portions (white) and pore portions (black) in the binarized image are obtained. The ratio of the area of the pore portions to the total area (the sum of the area of the alumina portions and the area of the pore portions) is calculated, and this value is defined as the porosity. The porosity is determined in the same way for the porous protective layers 91b to 91e. In this embodiment, the porous protective layers 91a to 91e all have approximately the same porosity.

[0107] It is considered that the porous protective layer 91 has substantially the same microstructure regardless of the observation location. Therefore, as described above, the porosity value obtained using one cross-sectional image may be used as the porosity value of the porous protective layer 91.

[0108] The porous protective layer 91 may be a single layer or may be composed of multiple layers. That is, the porous protective layer 91 may include a surface layer and an inner layer formed inside the surface layer. The surface layer and the inner layer may be made of different materials or may have different porosities. The porosity of the inner layer is preferably higher than that of the surface layer. The porosity of the inner layer may be, for example, 40% by volume or more and 70% by volume or less. The porosity of the surface layer may be, for example, 10% by volume or more and 40% by volume or less.

[0109] Two or more inner layers may be formed. Preferably, the porosity of at least one inner layer is higher than the porosity of the surface layer. Two or more inner layers may be formed so that the porosity increases in order from the surface layer toward the inside.

[0110] The thickness of the surface layer of the porous protective layer 91 may be 100 μm or more and 300 μm or less. The thickness of the inner layer may be 300 μm or more and 700 μm or less. When two or more inner layers are formed, the total thickness of the inner layers may be 300 μm or more and 700 μm or less.

[0111] Generally, the higher the porosity of a porous body, the better its insulating performance. However, the lower the porosity, the better the structural strength of the porous body. When the porous protective layer 91 includes a surface layer and an inner layer with a higher porosity than the surface layer, the surface layer maintains structural strength, while the inner layer with a higher porosity can further enhance the insulating effect. Therefore, the water resistance of the sensor element 101 can be improved while maintaining the strength of the porous protective layer 91. In addition, the diffusion resistance of the surface layer can also be used to adjust the measurement gas flowing into the gas inlet 10.

[0112] [Sensor element manufacturing method] Next, an example of a method for manufacturing the above-described sensor element will be described. In the method for manufacturing the sensor element 101, first, the element body 101a is manufactured, and then the porous protective layer 91 is formed on the element body 101a, thereby manufacturing the sensor element 101.

[0113] In the following, an example will be described in which the sensor element 101 made up of six layers as shown in FIG. 2 is fabricated.

[0114] (Manufacturing of the element body) First, a method for manufacturing the element body 101a will be described. First, six green sheets containing an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component are prepared. A known forming method can be used to manufacture the green sheets. All six green sheets may be the same thickness, or the thickness may vary depending on the layer to be formed. Sheet holes and the like used for positioning during printing and lamination are formed in advance in each of the six green sheets (blank sheets) using a known method such as punching with a punching device. Penetrations such as internal voids are also formed in the blank sheet used for the spacer layer 5 using a similar method. Necessary penetrations are also formed in the other layers in advance.

[0115] Various patterns required for each layer are printed and dried on blank sheets used for the six layers: first substrate layer 1, second substrate layer 2, third substrate layer 3, first solid electrolyte layer 4, spacer layer 5, and second solid electrolyte layer 6. Known screen printing techniques can be used to print the patterns. Known drying means can also be used for the drying process.

[0116] This process is repeated until various patterns have been printed and dried on each of the six blank sheets. The six printed blank sheets are then stacked in a predetermined order while being positioned using sheet holes, etc., and then pressed under predetermined temperature and pressure conditions to form a laminate. The pressing process is carried out by applying heat and pressure using a laminator such as a known hydraulic press. The temperature, pressure, and time for heating and pressing depend on the laminator used, but can be determined appropriately to achieve good lamination.

[0117] The obtained laminate contains a plurality of element bodies 101a. The laminate is cut into units of element bodies 101a. The cut laminate is fired at a predetermined firing temperature to obtain element bodies 101a. The firing temperature may be any temperature at which the solid electrolyte constituting the base portion 102 of the sensor element 101 is sintered to form a dense body and at which the electrodes and the like maintain a desired porosity. For example, firing is performed at a firing temperature of about 1300 to 1500°C.

[0118] (Protective layer manufacturing) Next, a method for forming the porous protective layer 91 on the element body 101a will be described. In this embodiment, the porous protective layer 91 is formed through the steps of coating, pressing, and degreasing. FIG. 4(A) is a simplified schematic diagram of the shape of the pore precursor H in the cross section of the porous protective layer 91a after coating, and FIG. 4(B) is a simplified schematic diagram of the shape of the pore precursor H in the cross section of the porous protective layer 91a after pressing. The pore precursor H contains a pore-forming material. After the pressing step, the pore-forming material in the pore precursor H disappears in the degreasing step, and the remaining portions become pores. As a result, a porous protective layer 91a having a cross-sectional structure as shown schematically in FIG. 3(ii) is obtained.

[0119] First, a protective layer-forming composition containing a pore-forming material for use in the coating process is prepared. In this embodiment, a porous protective layer paste is prepared as the protective layer-forming composition. The porous protective layer paste is prepared by mixing a raw material powder (alumina powder in this embodiment) made of the material of the porous protective layer 91 described above, a pore-forming material for forming pores, an organic binder, an organic solvent, and the like. The pore-forming material is an organic or inorganic material that disappears during the subsequent degreasing process. Examples of pore-forming materials that can be used include xanthine derivatives such as theobromine, organic resin materials such as acrylic resins, and inorganic materials such as carbon. The porous protective layer paste is preferably prepared so that the porosity of the porous protective layer 91 after degreasing is 10% to 40% by volume. For example, the amount of pore-forming material added can be adjusted to achieve a desired range of porosity. For example, by adding 10% to 50% by volume of a pore-forming material to the alumina powder, a porous protective layer 91 with a porosity of 10% to 40% by volume can be obtained. This can also be adjusted by the particle size of the raw material powder or the blending ratio of the organic binder.

[0120] Next, a protective layer forming composition containing a pore-forming material is applied to at least a portion of the surface of the element body 101a to form a coating layer. In this embodiment, an example of application by screen printing is shown. The above-mentioned porous protective layer paste is printed and dried in the area on the upper surface of the element body 101a where the porous protective layer 91a is to be formed, to form a coating layer of the porous protective layer 91a. A known screen printing technique can be used for printing. A known drying means can also be used for the drying process. The pore-forming material does not evaporate during the drying process, but remains in the coating layer as pore precursor H. The printing and drying process may be repeated multiple times.

[0121] The porous protective layers 91b to 91e are also subjected to the printing and drying process in the same manner. The porous protective layers 91a to 91e may be subjected to the printing and drying process in any order.

[0122] The printing thickness of the porous protective layer 91 can be appropriately set by a person skilled in the art based on the predetermined thickness (thickness after degreasing) of the porous protective layer 91 in the sensor element 101, taking into consideration the degree of compression due to pressure application in the subsequent process and shrinkage due to degreasing.

[0123] Thereafter, a step of pressurizing the coating layer is carried out. The coating layer of the porous protective layer 91 is compressed by applying pressure so that the ratio (Lt / Lf) of the degreased porous protective layer 91 is 0.6 to 0.9. The degree of pressure (the compression degree of the coating layer of the porous protective layer 91) can be appropriately set by a person skilled in the art based on the predetermined value of the ratio (Lt / Lf) of the degreased porous protective layer 91.

[0124] Pressurization may be performed using a uniaxial pressurizing device such as a known hydraulic press, with three separate pressurizing steps for the top and bottom surfaces, the left and right surfaces, and the tip surface. Alternatively, a cold isostatic pressing (CIP) device may be used. The pressure, temperature, and time during pressurization depend on the pressurizing device used, but may be appropriately set to achieve the desired degree of pressurization.

[0125] Finally, the coating layer is heat-treated to obtain the porous protective layer 91 made of a porous body. That is, the degreasing step is performed at a predetermined degreasing temperature. The degreasing temperature may be any temperature at which the organic components in the printed film of the porous protective layer 91, such as the pore-forming material, organic binder, and organic solvent, are all eliminated and the porous structure of the porous protective layer 91 is maintained. The temperature may be lower than the firing temperature of the element body 101a. For example, the degreasing is performed at a degreasing temperature of about 400 to 900°C.

[0126] In the above-described manufacturing method, the layer to be the porous protective layer 91 is applied by screen printing and then pressed and degreased to form it, but the method is not limited to this. There may be cases where the sensor element of the present invention can be manufactured without pressing, depending on the degree of shrinkage during heat treatment such as degreasing or firing.

[0127] Alternatively, the coating may be performed by dipping, followed by application of pressure. Alternatively, the layer may be formed by, for example, plasma spraying or gel casting, followed by application of pressure.

[0128] Alternatively, it may be possible to manufacture the sensor element of the present invention by optimizing the spraying conditions for plasma spraying.

[0129] The obtained sensor element 101 is incorporated into the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the gas to be measured and the rear end of the sensor element 101 contacts the reference gas.

[0130] In the above-described embodiment, the element body 101a has a flat surface and a substantially rectangular cross section, but the element body of the present invention is not limited to this. The surface of the element body 101a may be curved. The cross section may also be substantially circular or elliptical (for example, a cylindrical oxygen sensor element with a bottom as disclosed in Japanese Patent No. 3766572). Various configurations are also possible for each component of the element body. [Example]

[0131] Below, examples in which sensor elements were specifically fabricated and tested will be described as experimental examples. Experimental examples 2 to 4 correspond to examples of the present invention, and experimental examples 1 and 5 correspond to comparative examples of the present invention. Note that the present invention is not limited to the following examples.

[0132] [Experimental Examples 1-4] As Experimental Examples 1 to 4, sensor elements 101 having porous protective layers 91 with ratios (Lt / Lf) of 0.5 (Experimental Example 1), 0.7 (Experimental Example 2), 0.8 (Experimental Example 3), and 0.9 (Experimental Example 4) were fabricated according to the above-described method for manufacturing the sensor element 101. In all of Experimental Examples 1 to 4, the porous protective layer 91 had a thickness of 300 μm and a porosity of 30% by volume.

[0133] Specifically, first, an element body 101a was fabricated, the length of which was 67.5 mm in the front-rear direction, the width of which was 4.25 mm in the left-right direction, and the thickness of which was 1.45 mm in the up-down direction.

[0134] The porous protective layer paste was prepared by mixing a pore-forming material with alumina powder at a ratio of 30% by volume, and adding a solvent, binder, and dispersant.

[0135] Next, a porous protective layer 91 was formed on the surface of the element body 101a. A porous protective layer paste was applied by screen printing, followed by a pressurizing step. A degreasing step was performed to produce the sensor elements 101 of Experimental Examples 1 to 4. In the pressurizing step, the degree of pressurization was changed so that each of Experimental Examples 1 to 4 had a desired ratio (Lt / Lf).

[0136] In the pressing step, a hot press was used as a pressing device.

[0137] In each of Experimental Examples 1 to 4, the printing film thickness and the pressing pressure were adjusted so that the porous protective layer 91 had a thickness of 300 μm and a desired ratio (Lt / Lf). As the ratio (Lt / Lf) value decreased, the printing film thickness was increased and the pressing pressure was increased.

[0138] The degreasing temperature was set to 600°C.

[0139] [Experimental Example 5] As Experimental Example 5, a sensor element 101 was produced that included a porous protective layer 91 with a ratio (Lt / Lf) value of 1. As in Experimental Examples 1 to 4, the porous protective layer 91 had a thickness of 300 μm and a porosity of 30% by volume. The sensor element 101 was produced in the same manner as Experimental Examples 1 to 4, except that the pressurizing step was not performed.

[0140] [Check the ratio (Lt / Lf)] The porous protective layer 91 of the sensor element 101 of Experimental Examples 1 to 5 was photographed using a CT (Versa520, manufactured by Carl Zeiss, 140 kV, 10 W). Using the above-mentioned method, it was confirmed that the ratio (Lt / Lf) was a desired value for each of Experimental Examples 1 to 5.

[0141] [Water resistance evaluation] The performance of the porous protective layer 91 (water resistance of the sensor element 101) was evaluated for the sensor elements 101 of Experimental Examples 1 to 5. Specifically, the heater 72 was first energized to heat the sensor element 101 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 control the oxygen concentration in the first internal space 20 at a predetermined constant value. After waiting for the pump current Ip0 to stabilize, a water droplet was dropped on the upper surface (porous protective layer 91a) of the porous protective layer 91. The presence or absence of a crack in the sensor element 101 was determined based on whether the pump current Ip0 exceeded a predetermined threshold value. If a crack occurs in the sensor element 101 due to thermal shock from the water droplet, oxygen easily flows into the first internal space 20 through the crack, resulting in an increased pump current Ip0. Therefore, when the pump current Ip0 exceeded a predetermined threshold determined through experiments, it was determined that the sensor element 101 had cracked due to water droplets. Furthermore, multiple tests were conducted by gradually increasing the amount of water droplets up to 30 μL, and the maximum amount of water droplets that did not cause cracks was determined as the water-resistant capacity. Five sensor elements 101 for each of Experimental Examples 1 to 5 were prepared, and the average water-resistant capacity of the five elements was calculated for each of Experimental Examples 1 to 5. The water-resistant capacity of the sensor elements 101 for Experimental Examples 1 to 5 was evaluated based on the following criteria: an average water-resistant capacity of less than 10 μL was deemed poor, and an average water-resistant capacity of 10 μL or more was deemed good.

[0142] [Evaluation of peel resistance] The peeling resistance of the porous protective layer 91 was evaluated for the sensor elements 101 of Experimental Examples 1 to 5. Specifically, five gas sensors 100 of Experimental Examples 1 to 5 were fabricated incorporating each of the sensor elements 101 of Experimental Examples 1 to 5. A thermal vibration test was carried out under the following conditions with the gas sensors 100 of Experimental Examples 1 to 5 attached to the exhaust pipe of a propane burner set in a vibration tester.

[0143] Gas temperature: 850℃; Gas-air ratio λ: 1.05; Vibration conditions: 50Hz → 100Hz → 150Hz → 250Hz sweep for 30 minutes; Acceleration: 30G, 40G, 50G; Test duration: 150 hours.

[0144] After the thermal vibration test, the sensor elements 101 were removed from the gas sensors 100 of Experimental Examples 1 to 5. The porous protective layer 91 of each of the five sensor elements 101 of Experimental Examples 1 to 5 after the thermal vibration test was observed using a scanning electron microscope (SEM). Specifically, the sensor element 101 was cut perpendicular to the longitudinal direction of the sensor element 101 in the region where the porous protective layer 91 was present. The cut surface was embedded in resin and polished, and the magnification of the SEM was set to 80x and 500x, and the presence or absence of peeling was observed at each magnification. The peel resistance of the sensor elements 101 of Experimental Examples 1 to 5 was evaluated by rating the absence of peeling as good and the presence of peeling as poor.

[0145] Table 1 shows the evaluation results of the ratio (Lt / Lf), water resistance, and peeling resistance for the sensor elements 101 of Experimental Examples 1 to 5.

[0146] [Table 1]

[0147] As shown in Table 1, it was confirmed that good water resistance was obtained when the ratio (Lt / Lf) was 0.9 or less in the porous protective layer 91. It was also confirmed that good peel resistance was obtained when the ratio (Lt / Lf) was 0.6 or more. [Explanation of symbols]

[0148] 1 First board layer 2 Second board layer 3 Third board layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion-controlled section 12 Buffer space 13 Second diffusion-controlled section 15 Measurement gas flow section 20 1st internal void 21 Main pump cell 22 Inner main pump electrode 22a (Inner main pump electrode) ceiling electrode part 22b Bottom electrode part (of inner main pump electrode) 23 Outer pump electrode 24 Variable power supply (for main pump cell) 30 Third diffusion-controlled section 40 Second internal void 41 Measuring pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measuring electrode 46 Variable power supply (for measuring pump cell) 48 Atmospheric introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51a (auxiliary pump electrode) ceiling electrode part 51b (auxiliary pump electrode) bottom electrode part 52 Variable power supply (for auxiliary pump cell) 60 4th diffusion-controlled section 61 3rd internal void 70 Heater section 71 Heater electrode 72 Heater 73 through holes 74 Heater insulation layer 75 Pressure relief hole 76 Heater lead 80 Oxygen partial pressure detection sensor cell for main pump control 81 Oxygen partial pressure detection sensor cell for auxiliary pump control 82 Oxygen partial pressure detection sensor cell for measuring pump control 83 Sensor Cell 91, 91a~91e Porous protective layer 100 Gas Sensor 101 Sensor element 101a Element body 102 Base

Claims

1. an element body including an oxygen ion conductive solid electrolyte layer; a protective layer covering at least a portion of the surface of the element body; A sensor element comprising: the protective layer is made of a porous body having pores therein, the pores in the protective layer have a ratio (Lt / Lf) of a pore length (Lt) in a thickness direction perpendicular to the surface of the element body to a pore length (Lf) in a plane direction perpendicular to the thickness direction of the element body of 0.6 to 0.9; The sensor element for detecting a gas to be measured in a gas to be measured, wherein the protective layer has a thickness of 100 μm to 500 μm and a porosity of 10% by volume to 40% by volume.

2. The element body is a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow portion formed at one end of the base portion in the longitudinal direction; at least one inner electrode disposed on an inner surface of the measurement gas flow portion; an outer electrode disposed so as to be in contact with the inner electrode via at least one of the plurality of solid electrolyte layers; The sensor element of claim 1 , comprising:

3. A method for manufacturing the sensor element according to claim 1 or 2, comprising the steps of: a step of applying a protective layer-forming composition containing a pore-forming material to at least a portion of the surface of the element body to form a coating layer; applying pressure to the coating layer; a step of degreasing the coated layer after pressing to obtain a protective layer made of a porous body; A method for manufacturing a sensor element, comprising:

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