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 gas to be measured in a gas to be measured.

Background Art

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

[0003] As such a gas sensor, a gas sensor including a sensor element using an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) is known. Further, in such a gas sensor, it is known to form a porous protective layer on the surface of the sensor element.

[0004] For example, Japanese Patent Application Laid-Open No. 2016-065852 discloses that a powder spraying material such as alumina is adhered 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

Summary of the Invention

Problems to be Solved by the Invention

[0006] When a gas sensor equipped with a sensor element using a solid electrolyte measures a gas to be measured (during normal operation), the sensor element is at a high temperature (for example, about 800 °C). When water is applied to the sensor element during normal operation of the gas sensor, only the surface of the sensor element in the high-temperature state rapidly cools due to the adhesion of moisture, and there is a problem that cracks occur in the internal structure of the sensor element due to the thermal shock.

[0007] In addition, due to the strengthening of exhaust gas regulations for automobiles, gas sensors installed in automobiles are required to measure the gas to be measured in the exhaust gas immediately after the engine is started. However, immediately after the engine is started, there is more condensed water inside the exhaust gas pipe. Therefore, the risk of water being applied to the sensor element in the high-temperature state increases.

[0008] Under such circumstances, it is required to further suppress the occurrence of cracks in the internal structure of the sensor element when water is applied to the sensor element in the high-temperature state (water-affected). That is, it is an urgent task to improve the water resistance of the sensor element.

[0009] Therefore, an object of the present invention is to provide a sensor element having high water resistance.

Means for Solving the Problems

[0010] As a result of intensive studies, the present inventor has found that by forming a porous protective layer on at least a part of the surface of the sensor element and making the pores of the protective layer have a shape that is thin in the thickness direction of the protective layer and spreads in the plane direction (so-called flat shape), the water resistance of the sensor element is improved.

[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 part of the surface of the element body, A sensor element including, The protective layer is made of a porous body having pores inside, The pores in the protective layer are such that the 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 is 0.6 to 0.9, and it is a sensor element for detecting a gas to be measured in the gas to be measured.

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

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

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

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

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

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

[0018] (8) The element body includes a long plate-shaped substrate portion including a plurality of laminated oxygen ion conductive solid electrolyte layers, a gas to be measured flow portion formed from one end in the longitudinal direction of the substrate portion, at least one inner electrode disposed on the inner surface of the gas to be measured flow portion, and an outer electrode disposed so as to be in contact with the electrode through at least one of the plurality of solid electrolyte layers, and it is the sensor element according to any one of (1) to (7) above.

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

Effect of the Invention

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

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0022] The sensor element of the present invention is an element body including a solid electrolyte layer having oxygen ion conductivity, A protective layer covering at least a part of the surface of the element body; including; The protective layer is a porous body having pores inside; For the pores of the protective layer, the 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 is 0.6 to 0.9.

[0023] Hereinafter, an example of an embodiment of a gas sensor including the sensor element of the present invention will be described in detail.

[0024] [Schematic Configuration of Gas Sensor] The gas sensor including the 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 schematic configuration of the sensor element 101. FIG. 2 is a longitudinal vertical cross-sectional schematic view showing an example of the schematic configuration of the gas sensor 100 including the sensor element 101. In FIG. 2, the cross-sectional schematic view of the sensor element 101 is a cross-sectional schematic view along line II-II of FIG. 1. FIG. 3(i) is a cross-sectional schematic view along line III-III of FIG. 1. Hereinafter, with reference to FIG. 2, up and down means the upper side of FIG. 2 is up and the lower side is down, the left side of FIG. 2 is the front end side, and the right side is the rear end side. Also, with reference to FIG. 3, left and right mean the right side of FIG. 3 is right and the left side is left.

[0025] In FIG. 2, the gas sensor 100 shows an example of a limit current type NOx sensor that detects NOx in the gas to be measured by the 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 part of the sensor element 101 excluding the porous protective layer 91 is hereinafter referred to as the element body 101a.

[0027] Also, in the sensor element 101 of the present embodiment, an inner main pump electrode 22, an auxiliary pump electrode 51, and a measurement electrode 44 are provided as inner electrodes. An outer pump electrode 23 is provided as an outer electrode.

[0028] The sensor element 101 is an elongated plate-shaped element including a substrate portion 102 having a structure in which a plurality of oxygen ion conductive solid electrolyte layers are laminated. The elongated plate shape also means a long plate shape or a strip shape. The substrate portion 102 includes 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 (ZrO2). These six layers are laminated in this order from the lower side in a plan view. 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 for each layer. The spaces between the layers are adhered via an adhesive layer made of a solid electrolyte, and the substrate portion 102 includes the adhesive layer. In FIG. 2, the layer configuration consisting of the six layers is illustrated, but the layer configuration in the present invention is not limited to this, and any number of layers and layer configurations may be used.

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

[0030] At one end in the longitudinal direction of the sensor element 101 (hereinafter referred to as the tip portion), a gas inlet 10 is formed between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The gas flow portion 15 to be measured is adjacent in a manner such that a first diffusion rate-limiting portion 11, a buffer space 12, a second diffusion rate-limiting portion 13, a first internal cavity 20, a third diffusion rate-limiting portion 30, a second internal cavity 40, a fourth diffusion rate-limiting portion 60, and a third internal cavity 61 communicate in this order in the longitudinal direction from the gas inlet 10.

[0031] The gas inlet 10, the buffer space 12, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 are spaces inside the sensor element 101 defined by the lower surface of the second solid electrolyte layer 6 at the upper part, the upper surface of the first solid electrolyte layer 4 at the lower part, and the side surface of the spacer layer 5 at the side part in a manner of punching out the spacer layer 5.

[0032] The first diffusion rate-determining part 11, the second diffusion rate-determining part 13, and the third diffusion rate-determining part 30 are all provided as two horizontally long slits (with the opening having the longitudinal direction in the direction perpendicular to the drawing in FIG. 2). The first diffusion rate-determining part 11, the second diffusion rate-determining part 13, and the third diffusion rate-determining part 30 may be in any form that imparts a desired diffusion resistance, and the form is not limited to the slit.

[0033] The fourth diffusion rate-determining part 60 is provided between the spacer layer 5 and the second solid electrolyte layer 6 as a single horizontally long slit (with the opening having the longitudinal direction in the direction perpendicular to the drawing in FIG. 2). The fourth diffusion rate-determining part 60 may be in any form that imparts a desired diffusion resistance, and the form is not limited to the slit.

[0034] Also, at a position farther from the tip side than the measured gas flow part 15, a reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and the side part is partitioned 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 the reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. Also, the air introduction layer 48 is formed so as to cover the reference electrode 42.

[0036] The reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, an air introduction layer 48 connected to the reference gas introduction space 43 is provided around it. That is, the reference electrode 42 is disposed so as to be in contact with the reference gas via the porous air introduction layer 48 and the reference gas introduction space 43. Also, as will be described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 using the reference electrode 42.

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

[0038] In the present embodiment, the measured gas flow portion 15 is in a form in which the measured gas is introduced from the gas inlet 10 opened at the tip surface of the sensor element 101, but the present invention is not limited to this form. For example, in the measured gas flow portion 15, there may be no recess in the gas inlet 10. In this case, the first diffusion rate-limiting portion 11 substantially serves as the gas inlet. Also, for example, the measured gas flow portion 15 may be in a form having an opening communicating with a position close to the buffer space 12 or the buffer space 12 of the first internal cavity 20 on the side surface along the longitudinal direction of the base body portion 102. In this case, the measured gas is introduced from the side surface along the longitudinal direction of the base body portion 102 through the opening. Also, for example, the measured gas flow portion 15 may be configured such that the measured gas is introduced through a porous body.

[0039] The first diffusion rate-limiting portion 11 is a portion that imparts a predetermined diffusion resistance to the measured gas taken in from the gas inlet 10.

[0040] The buffer space 12 is a space provided to guide the measured gas introduced from the first diffusion rate-limiting portion 11 to the second diffusion rate-limiting portion 13.

[0041] The second diffusion rate-limiting section 13 is a section that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal cavity 20.

[0042] Ultimately, it is only necessary that the amount of the gas to be measured introduced into the first internal cavity 20 is within a predetermined range. That is, it is only necessary that a predetermined diffusion resistance is imparted from the tip of the sensor element 101 to the entire second diffusion rate-limiting section 13. For example, a mode in which the first diffusion rate-limiting section 11 communicates directly with the first internal cavity 20, that is, a mode in which the buffer space 12 and the second diffusion rate-limiting section 13 do not exist may also be adopted.

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

[0044] When the gas to be measured is introduced from outside the sensor element 101 into the first internal cavity 20, the gas to be measured that has been rapidly taken into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the gas to be measured in the external space (if the gas to be measured is the exhaust gas of an automobile, the pulsation of the exhaust pressure) is not directly introduced into the first internal cavity 20, but rather after the pressure fluctuations of the gas to be measured are canceled through the first diffusion rate-limiting section 11, the buffer space 12, and the second diffusion rate-limiting section 13, it is introduced into the first internal cavity 20. As a result, the pressure fluctuations of the gas to be measured introduced into the first internal space become negligible.

[0045] The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced through the second diffusion rate-limiting section 13. Such oxygen partial pressure is adjusted by operating 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 gas to be measured flow-through section 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 (in FIG. 2, the second solid electrolyte layer 6).

[0047] That is, the main pump cell 21 is an electrochemical pump cell composed of an inner main pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, an outer pump electrode 23 provided in a manner exposed to the external space in a region corresponding to the ceiling electrode portion 22a on the upper surface of the second solid electrolyte layer 6, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[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) partitioning the first internal cavity 20 and the spacer layer 5 providing side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 providing the ceiling surface of the first internal cavity 20, a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 providing the bottom surface, and side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 constituting both side wall portions of the first internal cavity 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a structure in a tunnel form at the arrangement sites of the side electrode portions.

[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 containing 1% Au and ZrO2). Note that the inner main pump electrode 22 in contact with the gas to be measured is formed using a material with reduced reduction ability for NOx components in the gas to be measured.

[0050] In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner main pump electrode 22 and the outer pump electrode 23 by a variable power supply 24, and a pump current Ip0 is caused to flow in the positive or negative direction between the inner main pump electrode 22 and the outer pump electrode 23, so that oxygen in the first internal cavity 20 can be pumped out to the external space, or oxygen in the external space can be pumped into the first internal cavity 20.

[0051] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal cavity 20, an inner main pump electrode 22, a second solid electrolyte layer 6, a spacer layer 5, and a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42 constitute an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 80 for main pump control.

[0052] By measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for main pump control, the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 can be known. Further, the pump current Ip0 is controlled by feedback-controlling the voltage Vp0 of the variable power supply 24 so that the electromotive force V0 becomes constant. As a result, the oxygen concentration in the first internal cavity location 2 0 can be maintained at a predetermined constant value.

[0053] The third diffusion rate-limiting part 30 is a part that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal cavity 20 and guides the measurement gas to the second internal cavity 40.

[0054] The second internal cavity 40 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the third diffusion rate-limiting part 30 with higher precision. Such an oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50. It is also possible to adopt a configuration without the second internal cavity 40 and the auxiliary pump cell 50. From the viewpoint of the accuracy of oxygen partial pressure adjustment, it is more preferable to have the second internal cavity 40 and the auxiliary pump cell 50.

[0055] In the second internal cavity 40, after the oxygen concentration (oxygen partial pressure) is adjusted in the first internal cavity 20 in advance, the oxygen partial pressure of the measurement gas introduced through the third diffusion rate-limiting part 30 is further adjusted by the auxiliary pump cell 50. As a result, the oxygen concentration in the second internal cavity 40 can be maintained at a constant value with high precision, so that highly accurate NOx concentration measurement is possible in such a gas sensor 100.

[0056] The auxiliary pump cell 50 is an electrochemical pump cell including an auxiliary pump electrode 51 which is an inner electrode disposed at a position on the inner surface of the measured gas flow portion 15 farther from the tip end portion in the longitudinal direction of the base portion 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 (in FIG. 2, the second solid electrolyte layer 6).

[0057] That is, the auxiliary pump cell 50 is an auxiliary electrochemical pump cell 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 cavity 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, and any appropriate electrode outside the sensor element 101 is sufficient), and the second solid electrolyte layer 6.

[0058] The auxiliary pump electrode 51 is disposed in the second internal cavity 40 in a structure having a tunnel form similar to that of the inner main pump electrode 22 provided in the previous first internal cavity 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 providing the ceiling surface of the second internal cavity 40, a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 providing the bottom surface of the second internal cavity 40, and side electrode portions (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 providing the side walls of the second internal cavity 40 in a tunnel form structure.

[0059] Note that the auxiliary pump electrode 51 is also formed using a material with a reduced reduction ability for NOx components in the measured gas, similar to the inner main pump electrode 22.

[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 out oxygen in the atmosphere in the second internal cavity 40 to the external space, or to pump it into the second internal cavity 40 from the external space.

[0061] Further, in order to control the oxygen partial pressure in the atmosphere within the second internal cavity 40, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for auxiliary pump control, is constituted by an auxiliary pump electrode 51, a reference electrode 42, a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, and a third substrate layer 3.

[0062] Note that the auxiliary pump cell 50 performs pumping by a variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by the oxygen partial pressure detection sensor cell 81 for auxiliary pump control. As a result, the oxygen partial pressure in the atmosphere within the second internal cavity 40 is controlled to a low partial pressure that has substantially no influence on the measurement of NOx.

[0063] Also, together with this, the pump current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for main pump control. Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor cell 80 for main pump control, and by controlling its electromotive force V0, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate-limiting section 30 into the second internal cavity 40 is always made constant. When used as a NOx sensor, due to the functions of the main pump cell 21 and the auxiliary pump cell 50, the oxygen concentration within the second internal cavity 40 is maintained at a constant value of about 0.001 ppm.

[0064] The fourth diffusion rate-limiting section 60 is a site that imparts a predetermined diffusion resistance to the measurement gas in which the oxygen concentration (oxygen partial pressure) is further reduced by the operation of the auxiliary pump cell 50 in the second internal cavity 40, and guides the measurement gas to the third internal cavity 61.

[0065] The third internal cavity 61 is provided as a space for measuring the concentration of nitrogen oxides (NOx) in the measurement gas introduced through the fourth diffusion rate-limiting 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 at a position on the inner surface of the measured gas flow portion 15 farther from the tip end portion in the longitudinal direction 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 measured gas in the third internal cavity 61. The measurement pump cell 41 includes a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61, an outer pump electrode 23 (not limited to the outer pump electrode 23, and any appropriate electrode outside the sensor element 101 is sufficient), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4, and is an electrochemical pump cell constituted thereby.

[0068] The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst for reducing NOx present in the atmosphere in the third internal cavity 61.

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

[0070] Also, in order to detect the oxygen partial pressure around the measurement electrode 44, an electrochemical sensor cell, that is, a measurement pump control oxygen partial pressure detection sensor cell 82 is constituted by 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. The variable power supply 46 is controlled based on the electromotive force V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82. solution

[0071] 60 The measured gas introduced into the second internal cavity 40 is under the condition that the oxygen partial pressure is controlled and at the fourth diffusion rate limiting portion 60It will reach the measurement electrode 44 through [it]. Nitrogen oxides in the gas to be measured around the measurement electrode 44 are reduced (2NO → N2 + O2) to generate oxygen. Then, the generated oxygen will be pumped by the measurement pump cell 41. At this time, the voltage Vp2 of the variable power supply 46 is controlled so that the control voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump becomes constant. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured, the concentration of nitrogen oxides in the gas to be measured is calculated using the pump current Ip2 in the measurement pump cell 41.

[0072] Also, if the measurement 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 means as an electrochemical sensor cell, the electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere can be detected, and thereby the concentration of the NOx component in the gas to be measured can also be obtained.

[0073] Also, an electrochemical sensor cell 83 is composed of 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, and the oxygen partial pressure in the gas to be measured outside the sensor can be detected by the electromotive force Vref obtained by this sensor cell 83.

[0074] In the gas sensor 100 having such a configuration, by operating the main pump cell 21 and the auxiliary pump cell 50, the gas to be measured in which the oxygen partial pressure is always maintained at a constant low value (a value that has substantially no influence on the measurement of NOx) is supplied to the measurement pump cell 41. Therefore, based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out by the measurement pump cell 41 in proportion to the concentration of NOx in the gas to be measured, the concentration of NOx in the gas to be measured can be known.

[0075] In this embodiment, there are three internal cavities, namely a first internal cavity 20, a second internal cavity 40, and a third internal cavity 61, and inner electrodes 22, 51, and 44 are disposed in the respective internal cavities 20, 40, and 61. However, the number and arrangement form 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 includes a heater unit 70 that plays a role in temperature adjustment for heating and maintaining the temperature of the sensor element 101. The heater unit 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 relief hole 75.

[0077] The heater electrode 71 is an electrode formed in a manner of contacting 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 source, power can be supplied to the heater unit 70 from the outside.

[0078] The heater 72 is an electrical resistor formed in a manner of being sandwiched from above and below by 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 in the longitudinal direction of the sensor element 101 and a through hole 73, and generates heat when powered from the outside through the heater electrode 71, thereby heating and maintaining the temperature of the solid electrolyte forming the sensor element 101.

[0079] Also, the heater 72 is embedded across the entire region from the first internal cavity 20 to the third internal cavity 61, making it possible to adjust the temperature of the entire sensor element 101 to a temperature at which the solid electrolyte is activated. It is only necessary that the temperature 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 for these entire regions to be adjusted to the same temperature, and there may be a temperature distribution in the sensor element 101.

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

[0081] The heater insulating layer 74 is an insulating layer formed of an insulator such as alumina on the upper and lower surfaces of the heater 72 and the 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 the heater lead 76, and electrical insulation between the third substrate layer 3 and the heater 72 and the heater lead 76.

[0082] The pressure release hole 75 penetrates the third substrate layer 3 and is formed so that the heater insulating layer 74 communicates with the reference gas introduction space 43. The pressure release hole 75 can relieve the increase in internal pressure accompanying the temperature rise in the heater insulating layer 74. Note that a configuration without the pressure release hole 75 may also be employed.

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

[0084] The porous protective layer 91e also covers the gas inlet 10. However, since the porous protective layer 91e is a porous body, the gas to be measured can flow through the inside of the porous protective layer 91e and reach the gas inlet 10. Therefore, the detection and measurement of the gas to be measured can be performed without problems.

[0085] The porous protective layer 91, for example, when water is splashed on the high-temperature sensor element 101 during normal operation of the gas sensor, plays a role in suppressing the occurrence of cracks in the internal structure of the element main body 101a. The water that reaches the sensor element 101 does not directly adhere to the surface of the element main 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 due to the heat insulation effect of the porous protective layer 91, the thermal shock applied to the element main body 101a is reduced. As a result, the occurrence of cracks in the internal structure of the element main body 101a can be suppressed. That is, the water resistance of the sensor element 101 is improved.

[0086] Also, the porous protective layer 91a covers the outer pump electrode 23. The porous protective layer 91a also plays a role in suppressing the adhesion of oil components and the like contained in the gas to be measured to the outer pump electrode 23 and suppressing the deterioration of the outer pump electrode 23.

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

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

[0089] The porous protective layer 91 is made of a porous body. Examples of the constituent material of the porous protective layer 91 include alumina, zirconia, spinel, cordierite, mullite, titania, magnesia, etc. It may be any one of these, or two or more of them. In this embodiment, the porous protective layer 91 is made of an alumina porous body.

[0090] The pores existing 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, on average, the pores existing inside the porous protective layer 91 have a shape that is thin in the thickness direction and spreads in the plane direction of the porous protective layer 91 (so-called flat shape).

[0091] FIG. 3(i) is a schematic cross-sectional view taken along line III-III of FIG. 1 and is a schematic vertical cross-sectional view orthogonal 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 from the description. In the following, the left-right direction in FIG. 3(i) is defined as the X-axis direction, the up-down direction is defined as the Z-axis direction, and the direction perpendicular to the plane of the drawing is defined 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 plane 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 plane of the solid electrolyte layers 1 to 6 (the thickness direction of the sensor element 101).

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

[0093] Moreover, most of the pores inside the porous protective layer 91 have a structure in which each has a communication part (not shown in FIG. 3(ii)) with one or two or more adjacent or nearby pores. Note that the communication part constitutes a part of the pores. Also, 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] The pore length (Lt) in the thickness direction perpendicular to the surface of the element body 101a in the present invention corresponds to the average value of the pore lengths in the thickness direction of all the pores existing 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 the pores existing 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). The shapes of the pores are various, and in each individual pore, the pore length in the X-axis direction and the pore length in the Y-axis direction are usually different values. However, when compared as average values, it is considered that they all become equivalent values.

[0095] The ratio (Lt / Lf) conceptually has the following values. For example, taking Fig. 3(ii) as an example, consider a cross-section where n pores P1, P2, ···, Pn exist. Let the lengths in the thickness direction of each pore P1, P2, ···, Pn be z1, z2, ···, zn respectively, and the lengths in the plane direction be x1, x2, ···, xn respectively. In this case, The pore length (Lt) in the thickness direction = [(z1 + z2 + ··· + zn) / n]; The pore length (Lf) in the plane direction = [(x1 + x2 + ··· + xn) / n] is. The ratio (Lt / Lf) is the ratio of the pore length (Lt) in the above thickness direction to the pore length (Lf) in the above plane direction.

[0096] In the actual cross-section of the porous protective layer 91, pores of various shapes including the above-mentioned communication parts exist. Specifically, the ratio (Lt / Lf) in the present invention is determined as follows. By performing image analysis on the CT (Computed Tomography) image of the porous protective layer 91, it is determined by the following procedure.

[0097] 1) Photograph the microstructure of the porous protective layer 91 of the sensor element 101 by CT. 2) Obtain a cross-sectional image of the XZ plane at any location of the porous protective layer 91a. Let the left-right direction of the cross-sectional image be the X-axis direction and the up-down direction be the Z-axis direction. The number of pixels of the cross-sectional image is 600 pixels horizontally × 80 pixels vertically, and 1 pixel is 1.5 μm square. 3) Binarize the obtained cross-sectional image using "Otsu's binarization" (also called discriminant analysis method). In the binarized cross-sectional image, the constituent material of the porous protective layer 91 (alumina in this embodiment) is represented in white, and the pores are represented in black. 4) For each continuous series of one or more black pixels separated by white pixels in the vertical (Z-axis direction) column with a width of 1 pixel at the right end of the cross-sectional image, count the number of vertically (Z-axis direction) continuous black pixels and calculate their average value. Similarly, for the second and subsequent columns from the right end of the cross-sectional image, calculate the average value of the number of vertically (Z-axis direction) continuous black pixels for all vertical (Z-axis direction) columns with a width of 1 pixel. 5) Take the value obtained by further averaging the average values of the number of vertically (Z-axis direction) continuous black pixels in each column as the average length of the pores in the Z-axis direction. This is referred to as the Coredlength in the Z-axis direction. 6) For the horizontal (X-axis direction) column with a width of 1 pixel of the cross-sectional image, obtain the average length of the pores in the X-axis direction (Coredlength in the X-axis direction) in the same manner as in 3) and 4) above. 7) Take the obtained Coredlength in the Z-axis direction as the pore length (Lt) in the thickness direction perpendicular to the surface of the element body 101a. Take the obtained Coredlength in the X-axis direction as the pore length (Lf) in the plane direction perpendicular to the thickness direction. Using these values, calculate the ratio (Lt / Lf) of the pore length (Lt) in the thickness direction to the pore length (Lf) in the plane direction.

[0098] Alternatively, instead of the Coredlength in the X-axis direction, the Coredlength in the Y-axis direction may be calculated using the 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 have equivalent values.

[0099] Similarly, for the porous protective layers 91b to 91e, the ratio (Lt / Lf) can be calculated. However, in the porous protective layers 91c and 91d, the Coredlength in the X-axis direction is used as the pore length (Lt) in the thickness direction, and either the Coredlength in the Y-axis direction or the Coredlength in the Z-axis direction is used as the pore length (Lf) in the plane direction. Further, in the porous protective layer 91e, the Coredlength in the Y-axis direction is used as the pore length (Lt) in the thickness direction, and either the Coredlength in the X-axis direction or the Coredlength in the Z-axis direction is used as the pore length (Lf) in the plane direction.

[0100] In this embodiment, for any of the porous protective layers 91a to 91e, it is assumed that the ratio (Lt / Lf) has the same value and is within the range of 0.6 to 0.9.

[0101] Note that the porous protective layer 91 is considered to have substantially equivalent microstructures regardless of the observation location. Therefore, as described above, the value of the ratio (Lt / Lf) obtained using an arbitrary 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 ratio of pore lengths (Lt / Lf) is 0.6 to 0.9. That is, the pores of the porous protective layer 91 are, on average, flat in shape, being thin in the thickness direction and spreading in the plane direction. Therefore, regardless of the position in the plane direction of the porous protective layer 91, it is considered that an adequate number of pores are likely to be arranged to ensure heat insulation in the thickness direction. If the ratio of pore lengths (Lt / Lf) is 0.9 or less, this effect is easily obtained. The upper limit value of the ratio of pore lengths (Lt / Lf) may be 0.85 or less, or 0.8 or less. When water is applied to the surface of such a porous protective layer 91, the temperature change in the thickness direction can be more suppressed, and thus it is considered that the thermal shock applied to the element body 101a is further reduced. As a result, the water resistance of the sensor element 101 can be improved.

[0103] Also, if the ratio of pore lengths (Lt / Lf) is 0.6 or more, since the pores do not spread too much in the plane direction, it is considered that the porous protective layer 91 is difficult to peel off. As a result, the required strength of the porous protective layer 91 can be maintained. From the viewpoint of peel resistance, the lower limit value of the ratio of pore lengths (Lt / Lf) may be 0.65 or more, or 0.7 or more.

[0104] Further, 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 in a region where the porous protective layer 91 exists so as to be orthogonal to the longitudinal direction of the sensor element 101. The cut surface is resin-embedded and polished to obtain an observation sample. The observation surface of the observation sample is photographed with the SEM magnification set to 80 times 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 interface with the element body 101a is derived, and this distance is taken as the thickness of the porous protective layer 91a. Note that the porous protective layer 91a is formed as a layer having a predetermined thickness. Therefore, as described above, the thickness obtained using a single cross-sectional image may be used as the thickness in the porous protective layer 91a. The thicknesses of the porous protective layers 91b to 91e are determined in the same manner.

[0105] In this embodiment, the porous protective layers 91a to 91e all have substantially the same thickness, but the thicknesses of the porous protective layers 91a to 91e may be different from each other.

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

[0107] Note that the porous protective layer 91 is considered to have substantially the same microstructure regardless of the observation location. Therefore, as described above, the value of the porosity obtained using a certain cross-sectional image may be used as the value of the porosity in the porous protective layer 91.

[0108] Also, the porous protective layer 91 may be a single layer or may be composed of a plurality of 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 have different constituent materials or different porosities. It is preferable that the porosity of the inner layer is higher than the porosity 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. Also, the porosity of the surface layer may be, for example, 10% by volume or more and 40% by volume or less.

[0109] Also, two or more inner layers may be formed. It is preferable that the porosity of at least one inner layer is higher than the porosity of the surface layer. Also, two or more inner layers may be formed such 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 of their thicknesses may also be 300 μm or more and 700 μm or less.

[0111] Generally, the higher the porosity of a porous body, the better its heat insulation performance. However, the lower the porosity, the higher the structural strength of the porous body. When the porous protective layer 91 includes a surface layer and an inner layer having a higher porosity than the surface layer, the structural strength can be maintained by the surface layer, and the heat insulation effect can be further enhanced by the inner layer having a high porosity. Therefore, the water resistance of the sensor element 101 can be improved while maintaining the strength of the porous protective layer 91. Also, it is possible to adjust the gas to be measured flowing into the gas inlet 10 by the diffusion resistance of the surface layer.

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

[0113] Hereinafter, the case of manufacturing the sensor element 101 composed of the six layers shown in FIG. 2 will be described as an example.

[0114] (Manufacture 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 for the production of the green sheets. The six green sheets may all have the same thickness, or the thickness may vary depending on the layer to be formed. For each of the six green sheets, sheet holes or the like used for positioning during printing or lamination are pre-formed by a known method such as punching with a punching device (blank sheet). For the blank sheet used for the spacer layer 5, through portions such as internal cavities are also formed by the same method. Through portions necessary for the other layers are also pre-formed.

[0115] For the blank sheets used for the six layers of the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6, printing and drying processes of various patterns necessary for each layer are performed. For the printing of the patterns, a known screen printing technique can be used. For the drying process as well, known drying means can be used.

[0116] By repeating such steps, when the printing and drying of various patterns on each of the six blank sheets are completed, the six printed blank sheets are stacked in a predetermined order while being positioned by sheet holes or the like, and a laminating process is performed in which they are pressure-bonded under predetermined temperature and pressure conditions to form a laminate. The laminating process is performed by heating and pressurizing with a laminator such as a known hydraulic press. The temperature, pressure, and time for heating and pressurizing depend on the laminator used, but can be appropriately determined so that good lamination can be achieved.

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

[0118] (Manufacture of the protective layer) Next, a method of forming the porous protective layer 91 on the element body 101a will be described. In the present embodiment, the porous protective layer 91 is formed through steps of coating, pressurizing, and degreasing. FIG. 4(A) is a schematic diagram simplifying 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 schematic diagram simplifying the shape of the pore precursor H in the cross section of the porous protective layer 91a after pressurizing. A pore-forming material exists in the pore precursor H. After the pressurizing step, in the degreasing step, the pore-forming material in the pore precursor H disappears, and that portion becomes pores. As a result, a porous protective layer 91a having a cross-sectional structure schematically shown in FIG. 3(ii) is obtained.

[0119] First, a composition for forming a protective layer containing a pore-forming material for use in the coating step is prepared. In the present embodiment, a porous protective layer paste is prepared as the composition for forming a protective layer. The porous protective layer paste is prepared by mixing a raw material powder (alumina powder in the present 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 material or an inorganic material that disappears by degreasing in a subsequent step. As the pore-forming material, for example, xanthine derivatives such as theobromine, organic resin materials such as acrylic resins, inorganic materials such as carbon, and the like can be used. 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 porosity of the porous protective layer 91 may be set within a desired range by adjusting the addition amount of the pore-forming material. For example, a porous protective layer 91 having a porosity of 10% to 40% by volume may be obtained by adding 10% to 50% by volume of the pore-forming material to the alumina powder. Further, it may be adjusted according to the particle size of the raw material powder and the blending ratio of the organic binder.

[0120] Next, a step of applying a composition for forming a protective layer containing a pore-forming material to at least a part of the surface of the element body 101a to form a coating layer is performed. In the present embodiment, an example of applying by screen printing is shown. The above-described porous protective layer paste is subjected to printing and drying treatment in the range where the porous protective layer 91a on the upper surface of the element body 101a is to be formed, and a coating layer of the porous protective layer 91a is formed. Known screen printing techniques can be used for printing. Known drying means can also be used for the drying treatment. In the drying treatment, the pore-forming material does not evaporate and remains in the coating layer as the pore precursor H. The printing and drying treatment may be repeated a plurality of times.

[0121] Similarly, the printing and drying treatment is performed for the porous protective layers 91b to 91e. The porous protective layers 91a to 91e may be subjected to the printing and drying treatment in any order.

[0122] The printed film thickness of the porous protective layer 91 can be appropriately set by those skilled in the art in consideration of the degree of compression due to the subsequent pressurization and the shrinkage due to degreasing, starting from the predetermined thickness (thickness after degreasing) of the porous protective layer 91 in the sensor element 101.

[0123] Thereafter, a step of pressurizing the coating layer is performed. In the porous protective layer 91 after degreasing, the coating layer of the porous protective layer 91 is pressurized and compressed so that the ratio (Lt / Lf) becomes 0.6 to 0.9. The degree of pressurization (the degree of compression of the coating layer of the porous protective layer 91) can be appropriately set by those skilled in the art from the value of the predetermined ratio (Lt / Lf) in the porous protective layer 91 after degreasing.

[0124] Pressurization may be performed in three steps on the upper and lower surfaces, the left and right surfaces, and the front end surface using a uniaxial pressurizing device such as a known hydraulic press. Alternatively, a cold isostatic pressing (CIP) device or the like may be used. The pressure, temperature, and time during pressurization depend on the pressurizing device used, but may be appropriately set so as to achieve the desired degree of pressurization.

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

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

[0127] Alternatively, it may be applied using the dipping method and then pressed. Further, for example, layer formation may be performed using plasma spraying or the gel casting method, and then pressed.

[0128] Alternatively, there may be a case where the sensor element of the present invention can be manufactured by optimizing the spraying conditions of plasma spraying.

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

[0130] In the above-described embodiment, the surface of the element body 101a is flat and the cross section is substantially rectangular, but the element body of the present invention is not limited to this. The surface of the element body 101a may be curved. Further, the cross section may be substantially circular or substantially elliptical (for example, a bottomed cylindrical oxygen sensor element as disclosed in Japanese Patent No. 3766572). Each component of the element body can also take various forms.

Examples

[0131] Hereinafter, an example in which a sensor element was specifically manufactured and tested will be described as an experimental example. Experimental Examples 2 to 4 correspond to the examples of the present invention, and Experimental Examples 1 and 5 correspond to the comparative examples of the present invention. Note that the present invention is not limited to the following examples.

[0132] [Experimental Examples 1 to 4] As Experimental Examples 1 to 4, sensor elements 101 provided with a porous protective layer 91 having a ratio (Lt / Lf) value of 0.5 (Experimental Example 1), 0.7 (Experimental Example 2), 0.8 (Experimental Example 3), and 0.9 (Experimental Example 4), respectively, were manufactured according to the manufacturing method of the sensor element 101 described above. In any 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 having a length of 67.5 mm in the front-rear direction, a width of 4.25 mm in the left-right direction, and a thickness of 1.45 mm in the up-down direction was manufactured.

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

[0135] Next, a porous protective layer 91 was formed on the surface of the element body 101a. The porous protective layer paste was applied by screen printing, and then a pressing process was performed. A degreasing process was carried out to fabricate the sensor elements 101 of Experimental Examples 1 to 4. In the pressing process, by changing the degree of pressing, Experimental Examples 1 to 4 were each made to have a desired value of the ratio (Lt / Lf).

[0136] In the pressing process, a hot press was used as the 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 value of the ratio (Lt / Lf). The smaller the value of the ratio (Lt / Lf), the thicker the printing film thickness and the greater the pressing pressure.

[0138] Also, the degreasing temperature was set at 600°C.

[0139] [Experimental Example 5] As Experimental Example 5, a sensor element 101 provided with a porous protective layer 91 having a ratio (Lt / Lf) value of 1 was fabricated. Similar to Experimental Examples 1 to 4, the porous protective layer 91 had a thickness of 300 μm and a porosity of 30% by volume. Except for not performing the pressing process, it was fabricated in the same manner as Experimental Examples 1 to 4. sensor element 101 was fabricated.

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

[0141] [Evaluation of water resistance] For the sensor elements 101 of Experimental Examples 1 to 5, the performance of the porous protective layer 91 (water resistance of the sensor element 101) was evaluated. Specifically, first, the heater 72 was energized to set the temperature to 800 °C, and the sensor element 101 was heated. In this state, in the air atmosphere, the main pump cell 21, the auxiliary pump cell 50, the oxygen partial pressure detection sensor cell 80 for main pump control, the oxygen partial pressure detection sensor cell 81 for auxiliary pump control, etc. were operated, and the first internal air location 2 0 was controlled to maintain the oxygen concentration at a predetermined constant value. Then, after waiting for the pump current Ip0 to stabilize, water droplets were dripped onto the upper surface of the porous protective layer 91 (porous protective layer 91a), and based on whether the pump current Ip0 changed to a value exceeding a predetermined threshold, the presence or absence of cracks in the sensor element 101 was determined. Note that if a crack occurs in the sensor element 101 due to the thermal shock of the water droplets, oxygen easily flows into the first internal air location 2 0 through the crack portion, so the value of the pump current Ip0 increases. Therefore, when the pump current Ip0 exceeds the predetermined threshold value determined in the experiment, it was determined that a crack occurred in the sensor element 101 due to the water droplets. Also, the amount of water droplets was gradually increased up to 30 μL and multiple tests were conducted, and the maximum amount of water droplets without cracks was taken as the water resistance amount. Then, five sensor elements 101 of Experimental Examples 1 to 5 were prepared each, and the average value of the water resistance amounts of the five was derived for each of Experimental Examples 1 to 5. If the average value of this water resistance amount was less than 10 μL, it was judged as defective, and if it was 10 μL or more, it was judged as good, and the water resistance of the sensor elements 101 of Experimental Examples 1 to 5 was evaluated.

[0142] [Evaluation of peel resistance] For the sensor elements 101 of Experimental Examples 1 to 5, the peel resistance of the porous protective layer 91 was evaluated. Specifically, first, for the sensor elements 101 of Experimental Examples 1 to 5, five gas sensors 100 of Experimental Examples 1 to 5 in which each was incorporated were manufactured. The heat vibration test was conducted under the following conditions with the gas sensors 100 of Experimental Examples 1 to 5 attached to the exhaust pipe of a propane burner installed in a vibration tester.

[0143] Gas temperature: 850 °C; Gas-air ratio λ: 1.05; Vibration conditions: Sweep from 50 Hz → 100 Hz → 150 Hz → 250 Hz for 30 minutes; Acceleration: 30 G, 40 G, 50 G; Test time: 150 hours.

[0144] After the heating vibration test, the sensor elements 101 were taken out from the gas sensors 100 of Experimental Examples 1 to 5 respectively. For each of the five sensor elements 101 of Experimental Examples 1 to 5 after the heating vibration test, the porous protective layer 91 was observed using a scanning electron microscope (SEM). Specifically, the sensor element 101 was cut in a region where the porous protective layer 91 exists so as to be orthogonal to the longitudinal direction of the sensor element 101. The cut surface was resin-embedded and polished, and the magnification of the SEM was set to 80 times and 500 times, 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, with no peeling being considered good and peeling being considered bad.

[0145] For the sensor elements 101 of Experimental Examples 1 to 5, the evaluation results of the ratio (Lt / Lf) value, water resistance, and peel resistance are shown in Table 1.

[0146] [Table 1]

[0147] As shown in Table 1, it was confirmed that in the porous protective layer 91, good water resistance can be obtained if the ratio (Lt / Lf) is 0.9 or less. Also, it was confirmed that good peel resistance can be obtained if the ratio (Lt / Lf) is 0.6 or more.

Explanation of symbols

[0148] 1 First substrate layer 2 Second substrate layer 3 Third substrate layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion rate-limiting section 12 Buffer space 13 Second Diffusion Rate Limiting Section 15 Measured Gas Flow Section 20 First Internal Void 21 Main Pump Cell 22 Inner Main Pump Electrode 22a Ceiling Electrode Section (of Inner Main Pump Electrode) 22b Bottom Electrode Section (of Inner Main Pump Electrode) 23 Outer Pump Electrode 24 Variable Power Supply (for Main Pump Cell) 30 Third Diffusion Rate Limiting Section 40 Second Internal Void 41 Measurement Pump Cell 42 Reference Electrode 43 Reference Gas Introduction Space 44 Measurement Electrode 46 Variable Power Supply (for Measurement Pump Cell) 48 Atmosphere Introduction Layer 50 Auxiliary Pump Cell 51 Auxiliary Pump Electrode 51a Ceiling Electrode Section (of Auxiliary Pump Electrode) 51b Bottom Electrode Section (of Auxiliary Pump Electrode) 52 Variable Power Supply (for Auxiliary Pump Cell) 60 Fourth Diffusion Rate Limiting Section 61 Third Internal Void 70 Heater Section 71 Heater Electrode 72 Heater 73 Through Hole 74 Heater Insulation Layer 75 Pressure Diffusion 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 Measurement Pump Control 83 Sensor Cell 91, 91a~91e Porous Protective Layer 100 Gas Sensor 101 Sensor Element 101a Element Body 102 Substrate Section

Claims

1. An element body including an oxygen ion conductive solid electrolyte layer, A protective layer covering at least a part of the surface of the element body, A sensor element comprising: The protective layer is made of a porous body having pores inside, For the pores of the protective layer, the 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 is 0.6 to 0.9, The protective layer includes a surface layer and an inner layer formed inside the surface layer, The inner layer has a higher porosity than the surface layer, The surface layer of the protective layer has a thickness of 100 μm to 300 μm and a porosity of 10% to 40% by volume, and is a sensor element for detecting a gas to be measured in a gas to be measured.

2. The sensor element according to claim 1, wherein the inner layer of the protective layer has a thickness of 300 μm to 700 μm.

3. The sensor element according to claim 1 or 2, wherein the inner layer of the protective layer has a porosity of 40% to 70% by volume.

4. The element body is A long plate-shaped base portion including a plurality of laminated oxygen ion conductive solid electrolyte layers, A gas to be measured flow portion formed from one end in the longitudinal direction of the base portion, At least one inner electrode disposed on the inner surface of the gas to be measured flow portion, An outer electrode disposed so as to be in contact with the inner electrode through at least one of the plurality of solid electrolyte layers, The sensor element according to any one of claims 1 to 3.

5. A method for manufacturing the sensor element according to any one of claims 1 to 4, A step of applying a composition for forming a protective layer containing a pore-forming material to at least a part of the surface of the element body to form a coating layer, A step of pressurizing the coating layer, A step of degreasing the pressurized coating layer to obtain a protective layer made of a porous body, A method for manufacturing a sensor element.

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