sensor element

Abstract

The present invention provides a sensor element whose internal structure does not peel off and whose startup time from activation to accurate measurement is short. A sensor element 101 for detecting a target gas in a gas to be measured comprises: a long, plate-shaped base portion 102 comprising a plurality of stacked oxygen ion conductive solid electrolyte layers 1, 2, 3, 4, 5, and 6; a gas flow portion through which the gas to be measured is introduced and circulated from one longitudinal end of the base portion 102; a heater-containing layer 75 embedded in the base portion 102 with at least one of the plurality of solid electrolyte layers 3 or 4 interposed therebetween, and comprising a heater 72 having a heater heating portion 72a and a heater lead portion 72b, and a heater insulator 74; and a pressure relief space 76 formed at least partially between the base portion 102 and the heater-containing layer 75.

Description

Technical Field

[0001] The present invention relates to a sensor element using an oxygen ion conductive solid electrolyte. 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), and carbon dioxide (CO2)) in gases such as automobile exhaust. For example, the concentration of target gas components in automobile exhaust is measured, and based on this value, the exhaust gas purification system installed in the vehicle is optimized and controlled.

[0003] One such gas sensor is one that uses a sensor element with an oxygen-ion-conducting solid electrolyte, such as zirconium oxide (ZrO2). These gas sensors monitor the composition of a target gas by heating the sensor element to a temperature at which the solid electrolyte exhibits its oxygen-ion conductivity. Therefore, a heater embedded within the sensor element is widely used. However, this can sometimes cause delamination of the sensor element's internal structure during use.

[0004] For example, WO2018 / 230703 discloses a sensor element having a plurality of stacked oxygen ion conductive solid electrolyte layers. It also discloses that the sensor element is provided with: a measured gas circulation portion for introducing and circulating the measured gas; a plurality of electrodes for detecting the target gas components; a reference gas introduction space for introducing a reference gas; and a heater portion for heating and maintaining the temperature of the sensor element. Furthermore, a pressure relief hole is disclosed, formed so that the heater insulation layer of the heater portion and the reference gas introduction space are connected.

[0005] Japanese Patent No. 4313027 discloses a gas sensor in which a heater section includes a heating element and a support for at least supporting the heating element, and has an opening provided to reduce the pressure generated between the heating element and the support. In particular, a structure in which the opening is open to the atmosphere introduction cavity is shown ( Figure 1 ).

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: WO2018 / 230703

[0009] Patent Document 2: Japanese Patent No. 4313027 Summary of the Invention

[0010] The reason why it is considered that peeling occurs in the internal structure of the sensor element is as follows. The heater insulator surrounding the heater heating element is a porous body formed of an insulator such as alumina. When the sensor element is not heated, in addition to gas components such as air, liquid components such as water are present inside the heater insulator itself, which is a porous body, or at the interface between the heater and the heater heating element. When the heater is heated to heat the sensor element, this water and other components evaporate due to the heat of the heater, and the pressure rises locally due to the generated water vapor and other components. It is considered that peeling occurs due to this pressure rise. Therefore, peeling of the internal structure of the sensor element often occurs when the gas sensor is started. In order to suppress peeling, it is considered necessary to suppress the local pressure rise.

[0011] In addition, gas sensors are used in exhaust gas purification systems installed in automobiles, etc., and are required to accurately measure the concentration of the target gas. When the gas sensor is started, it is also required to be able to perform accurate measurements as early as possible. The inventors of the present invention conducted research and confirmed the following problem, namely, the time from the start of the gas sensor to the time when accurate measurement can be started (i.e., the start-up time) in conventional gas sensors is sometimes longer. The research results of the inventors of the present invention are: The reasons for the longer start-up time are as follows. The reference gas introduction space is filled with a reference gas (such as the atmosphere) with a certain oxygen concentration in order to monitor the target gas composition. According to the publication No. WO2018 / 230703, the conventional sensor element is provided with a pressure relief hole in such a way that the heater insulator and the reference gas introduction space are connected, so as to alleviate the internal pressure increase associated with the temperature increase in the heater insulator. Therefore, when the sensor is started, if the moisture or the like present in the interior of the heater insulator itself or at the interface between the heater and the heating element evaporates, the water vapor or the like will pass through the heater insulator, which is a porous body, and flow into the reference gas introduction space from the pressure relief hole. As a result, since the oxygen concentration in the reference gas changes, it is impossible to accurately measure the target gas component in the measured gas. Then, when water vapor and the like are completely discharged from the reference gas introduction space, the oxygen concentration in the reference gas will be constant, allowing accurate measurement. Since the diffusion resistance of the heater insulator, which is a porous body, is high, it takes time for water vapor and the like to be completely discharged into the reference gas introduction space and further completely discharged from the reference gas introduction space. As a result, it is believed that the time from starting the gas sensor to the time when accurate measurement can begin (i.e., the startup time) is prolonged.

[0012] Therefore, an object of the present invention is to provide a sensor element in which the internal structure of the sensor element does not peel off and the activation time from activation of the gas sensor to the start of accurate measurement is short.

[0013] The inventors of the present invention have discovered that by forming a pressure relief space between a heater-containing layer including a heater and a heater insulator embedded in a base portion containing a solid electrolyte of a sensor element and the base portion, peeling of the internal structure of the sensor element can be suppressed and the startup time of the gas sensor can be shortened.

[0014] The present invention includes the following inventions.

[0015] (1) A sensor element for detecting a target gas in a gas to be measured, characterized by comprising:

[0016] a long plate-shaped base portion including a plurality of stacked oxygen ion conductive solid electrolyte layers;

[0017] a gas-to-be-measured flow portion into which a gas to be measured is introduced from one end portion in the longitudinal direction of the base portion and through which the gas to be measured flows;

[0018] a heater-containing layer embedded in the base portion with at least one of the plurality of solid electrolyte layers interposed therebetween from the measured gas flow portion, the heater-containing layer including a heater having a heater heat-generating portion and a heater lead portion, and a heater insulator; and

[0019] A pressure absorbing space is formed in at least a portion between the base portion and the heater-containing layer.

[0020] (2) The sensor element according to (1) above, characterized in that:

[0021] The pressure absorbing space is formed in at least a portion between the base portion and the heater-containing layer in a region where the heat generating portion of the heater is located.

[0022] (3) The sensor element according to (1) or (2) above, characterized in that:

[0023] The pressure buffering space is formed so as to be in contact with a surface of the heater-containing layer on a side close to the measured gas flow portion.

[0024] (4) The sensor element according to any one of (1) to (3) above, characterized in that:

[0025] The pressure buffering space is formed so as to be in contact with a surface of the heater-containing layer on a side away from the measured gas flow portion.

[0026] (5) The sensor element according to any one of (1) to (4) above, characterized in that:

[0027] The pressure buffering space is formed so as to be in contact with a side portion of the heater-containing layer.

[0028] (6) The sensor element according to any one of (1) to (5) above, characterized in that:

[0029] In a cross section of the base portion perpendicular to the longitudinal direction, a ratio of a cross-sectional area of ​​the pressure buffering space to a cross-sectional area of ​​the heater-containing layer is 0.10 or greater.

[0030] (7) The sensor element according to any one of (1) to (6) above, characterized in that:

[0031] In a cross section of the base portion perpendicular to the longitudinal direction, a ratio of a cross-sectional area of ​​the pressure buffering space to a cross-sectional area of ​​the heater-containing layer is 0.80 or less.

[0032] (8) The sensor element according to any one of (1) to (7) above, characterized in that:

[0033] In a cross section of the base portion perpendicular to the longitudinal direction, a ratio of a cross-sectional area of ​​the pressure buffering space to a cross-sectional area of ​​the heater-containing layer is 0.3 or more and 0.6 or less.

[0034] (9) The sensor element according to any one of (1) to (8) above, characterized in that:

[0035] The sensor element further includes a reference gas introduction space formed to be isolated from both the measured gas flow portion and the heater-containing layer, having an opening at the other end in the longitudinal direction of the base portion, and extending along the longitudinal direction of the base portion.

[0036] (10) The sensor element according to (9) above, characterized in that:

[0037] The pressure buffering space is not open to the reference gas introduction space.

[0038] (11) The sensor element according to any one of (1) to (8) above, characterized in that:

[0039] The pressure relief space is a space enclosed inside the base portion.

[0040] (12) The sensor element according to (9) above, characterized in that:

[0041] The pressure absorbing space is a space that passes through a portion other than the reference gas introduction space and is open to the outside of the base portion.

[0042] Effects of the Invention

[0043] According to the present invention, it is possible to suppress the peeling of the internal structure of the sensor element and shorten the startup time of the gas sensor. When the gas sensor is started in a state where liquid components such as water are present inside the heater insulator itself, which is a porous body, or at the interface between the heater and the heating element, the water and the like evaporate rapidly due to the heat generated by the heater. However, since gas components such as water vapor generated by evaporation can move from the inside of the heater insulator itself, or at the interface between the heater and the heating element to the pressure relief space, the pressure rise near the heater insulator can be suppressed. As a result, the peeling of the internal structure of the sensor element can be suppressed. Therefore, if the sensor element involved in the present invention is used, the sensor element is unlikely to be damaged due to the peeling of its internal structure and can withstand repeated use, thereby improving the durability of the gas sensor.

[0044] Furthermore, according to the present invention, the pressure buffering space can be a space enclosed within the base of the sensor element. Therefore, gas components such as water vapor generated by the heater's heat do not flow into the reference gas introduction space, thereby preventing changes in the oxygen concentration in the reference gas. Consequently, the startup time of the gas sensor can be shortened. Therefore, using the sensor element of the present invention enables accurate measurements from the moment the gas sensor is activated.

[0045] Furthermore, according to the present invention, the presence of the pressure relief space further improves the insulation between the heater and the solid electrolyte forming the base portion. This further improved insulation further reduces leakage current from the heater to the solid electrolyte, and also improves the accuracy of the electrical signal when detecting the target gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 1 is a schematic vertical cross-sectional view in the longitudinal direction showing an example of a schematic configuration of the gas sensor 100 .

[0047] Figure 2 FIG. 1 is a schematic vertical cross-sectional view in the longitudinal direction showing another example of the schematic configuration of the gas sensor 100 . Figure 2 The sensor element 101 is the structure of the gas flow part to be measured. Figure 1 Different modifications of the sensor element 101 in FIG.

[0048] Figure 3 1 is a schematic diagram showing an example of a schematic planar arrangement of the heater 72 (heating portion 72 a and heater lead portion 72 b ) and the pressure buffering space 76 .

[0049] Figure 4It is a schematic diagram showing another example of the schematic planar arrangement of the heater 72 (heating portion 72 a and heater lead portion 72 b ) and the pressure buffering space 76 .

[0050] Figure 5 It is along Figure 1 That is, it is a schematic cross-sectional view taken along the line VV of the sensor element 101 , taken along a vertical cross-sectional view perpendicular to the longitudinal direction, and is a schematic view showing an example of the pressure buffering space 76 .

[0051] Figure 6 1 is a schematic diagram showing another example of the pressure buffering space 76 of the sensor element 101 .

[0052] Figure 7 1 is a schematic diagram showing another example of the pressure buffering space 76 of the sensor element 101 .

[0053] Figure 8 1 is a schematic diagram showing another example of the pressure buffering space 76 of the sensor element 101 .

[0054] Figure 9 It is a schematic plan view showing the position of a cut cross section in the example.

[0055] Figure 10 This is a schematic diagram of a vertical cross section perpendicular to the longitudinal direction of a conventional sensor element. DETAILED DESCRIPTION

[0056] The sensor element of the present invention comprises:

[0057] a long plate-shaped base portion including a plurality of stacked oxygen ion conductive solid electrolyte layers;

[0058] a gas-to-be-measured flow portion, the gas-to-be-measured flow portion introducing the gas-to-be-measured from one end portion in the longitudinal direction of the base portion and allowing the gas-to-be-measured to flow;

[0059] a heater-containing layer embedded in the base portion with at least one of the plurality of solid electrolyte layers interposed therebetween from the measured gas flow portion, the heater-containing layer including a heater and a heater insulator, the heater having a heater heat generating portion and a heater lead portion; and

[0060] A pressure absorbing space is formed in at least a portion between the base portion and the heater-containing layer.

[0061] [General Structure of Gas Sensor]

[0062] Hereinafter, the sensor element of the present invention will be described with reference to the drawings. Figure 1FIG. 1 is a schematic diagram of a vertical cross section in the longitudinal direction showing an example of the schematic configuration of a gas sensor 100 including a sensor element 101. Figure 1 As a benchmark, the so-called up and down Figure 1 Set the upper side of the , set the lower side of the Figure 1 The left side of is set as the front end side, and the right side thereof is set as the rear end side.

[0063] Figure 1 1 , the gas sensor 100 is an example of a limiting current type NOx sensor that monitors NOx in a gas to be measured by a sensor element 101 and measures the concentration thereof.

[0064] The sensor element 101 is a long plate-shaped element, which includes a base portion 102 having a structure obtained by stacking multiple oxygen ion conductive solid electrolyte layers. The long plate shape is also called a long plate shape or a strip shape. The base portion 102 has a structure obtained by stacking six layers in the order of a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, an isolation layer 5, and a second solid electrolyte layer 6, which are respectively formed from oxygen ion conductive solid electrolyte layers such as zirconium oxide (ZrO2) from the bottom side in the accompanying drawings. The solid electrolyte forming these six layers is a dense and airtight solid electrolyte. The six layers can be all of the same thickness, or they can be of different thicknesses for each layer. The layers are bonded to each other by means of an adhesive layer containing a solid electrolyte, and the base portion 102 includes the adhesive layer. Figure 1 In the embodiment, the layer structure including the six layers is exemplified; however, the layer structure in the present invention is not limited thereto, and any layer structure may be employed.

[0065] The sensor element 101 is manufactured by, for example, subjecting ceramic green sheets corresponding to the respective layers to predetermined processing and printing circuit patterns, laminating the sheets, and then firing the sheets to integrate the sheets.

[0066] A measured gas inlet 10 is formed at one longitudinal end (hereinafter referred to as the front end) 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 measured gas flow portion is formed such that, starting from the measured gas inlet 10, the first diffusion rate control section 11, the buffer space 12, the second diffusion rate control section 13, the first internal cavity 20, the third diffusion rate control section 30, and the second internal cavity 40 are adjacently connected to each other in the order described above.

[0067] The measured gas inlet port 10, the buffer space 12, the first internal cavity 20, and the second internal cavity 40 are the internal spaces of the sensor element 101 arranged in a manner of hollowing out the isolation layer 5, wherein the upper portion of the internal space is divided by the lower surface of the second solid electrolyte layer 6, the lower portion is divided by the upper surface of the first solid electrolyte layer 4, and the side portion is divided by the side surface of the isolation layer 5.

[0068] The first diffusion rate control unit 11, the second diffusion rate control unit 13 and the third diffusion rate control unit 30 are each provided with two horizontally long slits ( Figure 1 , the direction perpendicular to the drawing constitutes the length direction of the opening).

[0069] Furthermore, a reference gas introduction space 43 is provided at a position further from the front end than the measured gas flow portion, between the upper surface of the third substrate layer 3 and the lower surface of the separator 5, and at a location defined laterally by the side surface of the first solid electrolyte layer 4. 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, atmospheric air is introduced into reference gas introduction space 43 as a reference gas for measuring NOx concentration.

[0070] The atmosphere introduction layer 48 is a layer made of porous alumina, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. The atmosphere introduction layer 48 is formed so as to cover the reference electrode 42.

[0071] The reference electrode 42 is formed by being sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, an atmosphere introduction layer 48 is provided around the reference electrode 42 and communicates with the reference gas introduction space 43. Furthermore, as will be described later, the reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) within the first internal cavity 20 and the second internal cavity 40.

[0072] In the measured gas flow portion, the gas inlet port 10 is a portion open to the external space, and the measured gas is introduced from the external space into the sensor element 101 through the gas inlet port 10 .

[0073] The first diffusion rate control unit 11 is a portion that applies a predetermined diffusion resistance to the gas to be measured introduced from the gas inlet 10 .

[0074] The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate control unit 11 to the second diffusion rate control unit 13 .

[0075] The second diffusion rate control unit 13 is a portion that applies a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal cavity 20 .

[0076] When the measured gas is introduced from outside the sensor element 101 into the first internal cavity 20, it is rapidly drawn into the sensor element 101 through the gas inlet 10 due to pressure changes in the measured gas in the external space (in the case of automobile exhaust, this is the pulsation of exhaust pressure). However, the measured gas is not introduced directly into the first internal cavity 20. Instead, it is introduced into the first internal cavity 20 after the pressure changes of the measured gas are eliminated by the first diffusion rate control section 11, the buffer space 12, and the second diffusion rate control section 13. As a result, the pressure changes of the measured gas introduced into the first internal space are reduced to a nearly negligible level.

[0077] The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced by the second diffusion rate control unit 13. The main pump unit 21 operates to adjust the oxygen partial pressure.

[0078] The main pump unit 21 is an electrochemical pump unit composed of an inner pump electrode 22, an outer pump electrode 23, and a second solid electrolyte layer 6 clamped by the inner pump electrode 22 and the outer pump electrode 23, wherein the inner pump electrode 22 has a top electrode portion 22a arranged on the lower surface of the second solid electrolyte layer 6 and facing the first internal cavity 20, and the outer pump electrode 23 is arranged on the upper surface of the second solid electrolyte layer 6 and in an area corresponding to the top electrode portion 22a in a manner exposed to the external space.

[0079] The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first inner cavity 20, and the isolation layer 5 that forms the side wall. Specifically, a top electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that forms the top surface of the first inner cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that forms the bottom surface. In addition, a side electrode portion 22c ( Figure 5 、 Figure 1 The top electrode portion 22a and the bottom electrode portion 22b are connected to each other in the side wall surface (inner surface) of the isolation layer 5 constituting the two side wall portions of the first internal cavity 20, thereby forming a tunnel-shaped structure at the location where the side electrode portion is arranged.

[0080] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (e.g., a cermet electrode containing 1% Au, Pt, and ZrO2). Furthermore, the inner pump electrode 22, which comes into contact with the gas being measured, is made of a material that has a reduced ability to reduce NOx components in the gas being measured.

[0081] In the main pump unit 21, a variable power supply 24 is used to apply the desired pump voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23, so that the pump current Ip0 flows between the inner pump electrode 22 and the outer pump electrode 23 in a positive direction or a negative direction. In this way, the oxygen in the first internal cavity 20 can be sucked out to the external space, or the oxygen in the external space can be sucked into the first internal cavity 20.

[0082] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, an electrochemical sensor unit, i.e., an oxygen partial pressure detection sensor unit 80 for main pump control, is formed by the inner pump electrode 22, the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42.

[0083] By measuring the electromotive force V0 of the main pump control oxygen partial pressure detection sensor unit 80, the oxygen concentration (oxygen partial pressure) within the first internal cavity 20 can be determined. Furthermore, feedback control of Vp0 is performed to maintain the electromotive force V0 constant, thereby controlling the pump current Ip0. This maintains the oxygen concentration within the first internal cavity 20 at a predetermined constant value.

[0084] The third diffusion rate control unit 30 applies a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump unit 21 in the first internal cavity 20 , and guides the gas to be measured to the second internal cavity 40 .

[0085] The second internal cavity 40 is provided as a space for measuring the concentration of nitrogen oxides (NOx) in the gas to be measured introduced through the third diffusion rate control unit 30. The NOx concentration is primarily measured by operating the measurement pump unit 41 in the second internal cavity 40 after the oxygen concentration has been adjusted by the auxiliary pump unit 50.

[0086] In the second internal cavity 40, the oxygen concentration (oxygen partial pressure) of the measured gas, which has been previously adjusted in the first internal cavity 20 and then introduced through the third diffusion rate control unit, is further adjusted by the auxiliary pump unit 50. This allows the oxygen concentration in the second internal cavity 40 to be kept constant with high precision, and thus, in such a gas sensor 100, it is possible to measure the NOx concentration with high precision.

[0087] The auxiliary pump unit 50 is an auxiliary electrochemical pump unit composed of an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, as long as it is a suitable electrode on the outside of the sensor element 101), and a second solid electrolyte layer 6, wherein the auxiliary pump electrode 51 has a top electrode portion 51a that is arranged on the lower surface of the second solid electrolyte layer 6 and is roughly entirely facing the second internal cavity 40.

[0088] This auxiliary pump electrode 51 is arranged within the second internal cavity 40 in a tunnel-shaped structure similar to the inner pump electrode 22 previously provided within the first internal cavity 20. Specifically, a top electrode portion 51a is formed relative to the second solid electrolyte layer 6 forming the top surface of the second internal cavity 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 forming the bottom surface of the second internal cavity 40. Furthermore, side electrodes (not shown) connecting the top electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the isolation layer 5 forming the side walls of the second internal cavity 40, resulting in a tunnel-shaped structure.

[0089] Furthermore, similarly to the inner pump electrode 22 , the auxiliary pump electrode 51 is formed of a material having a weakened reducing ability for NOx components in the measured gas.

[0090] In the auxiliary pump cell 50 , a desired voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23 , thereby absorbing oxygen in the atmosphere within the second internal cavity 40 to the external space or absorbing oxygen from the external space into the second internal cavity 40 .

[0091] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal cavity 40, an electrochemical sensor unit, that is, an auxiliary pump control oxygen partial pressure detection sensor unit 81, is composed of an auxiliary pump electrode 51, a reference electrode 42, a second solid electrolyte layer 6, an isolation layer 5, a first solid electrolyte layer 4 and a third substrate layer 3.

[0092] The auxiliary pump unit 50 performs pumping using a variable power supply 52, the voltage of which is controlled based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor unit 81. As a result, the oxygen partial pressure in the atmosphere within the second internal cavity 40 is controlled to a low enough level to have no substantial effect on NOx measurement.

[0093] Simultaneously, the pump current Ip1 is used to control the electromotive force of the main pump control oxygen partial pressure detection sensor unit 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor unit 80, controlling its electromotive force V0. This control ensures that the gradient of the oxygen partial pressure in the gas to be measured, introduced from the third diffusion rate control unit 30 into the second internal cavity 40, remains constant. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of approximately 0.001 ppm by the main pump unit 21 and the auxiliary pump unit 50.

[0094] The measurement pump cell 41 measures the NOx concentration in the gas being measured within the second internal cavity 40. The measurement pump cell 41 is an electrochemical pump cell composed of a measurement electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, a separator 5, and the first solid electrolyte layer 4. The measurement electrode 44 is located on the upper surface of the first solid electrolyte layer 4, facing the second internal cavity 40 and separated from the third diffusion rate control unit 30.

[0095] The measuring electrode 44 is a porous cermet electrode and also functions as a NOx reduction catalyst for reducing NOx present in the atmosphere within the second internal cavity 40. Furthermore, the measuring electrode 44 is covered by a fourth diffusion rate control unit 45.

[0096] The fourth diffusion rate control unit 45 is a porous membrane composed mainly of aluminum oxide (Al2O3). The fourth diffusion rate control unit 45 has the function of limiting the amount of NOx flowing into the measuring electrode 44 and also functions as a protective membrane for the measuring electrode 44.

[0097] The measurement pump cell 41 can absorb oxygen generated by decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44 , and its generated amount can be detected as the pump current Ip2 .

[0098] Furthermore, to detect the oxygen partial pressure around the measuring electrode 44, the second solid electrolyte layer 6, the separator layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42 constitute an electrochemical sensor unit, namely, a measuring pump control oxygen partial pressure detection sensor unit 82. The variable power supply 46 is controlled based on the electromotive force V2 detected by the measuring pump control oxygen partial pressure detection sensor unit 82.

[0099] The gas to be measured introduced into the second internal cavity 40 passes through the fourth diffusion rate control unit 45 while the oxygen partial pressure is controlled, reaching the measuring electrode 44. Nitrogen oxides in the gas to be measured are reduced (2NO → N2 + O2), generating oxygen. This generated oxygen is then pumped by the measurement pump unit 41. During this process, the voltage Vp2 of the variable power supply is controlled to maintain a constant control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor unit 82. The amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured. Therefore, the nitrogen oxide concentration in the gas to be measured is calculated using the pump current Ip2 of the measurement pump unit 41.

[0100] In addition, if the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 are combined to form an oxygen partial pressure detection mechanism in the form of an electrochemical sensor unit, it is possible to detect the electromotive force corresponding to the following difference, thereby also being able to calculate the concentration of the NOx component in the measured gas. The difference refers to: the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere.

[0101] In addition, the electrochemical sensor unit 83 is composed of the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23 and the reference electrode 42. The sensor unit 83 can be used to obtain the electromotive force Vref, and the electromotive force Vref can be used to detect the oxygen partial pressure in the measured gas outside the sensor.

[0102] In the gas sensor 100 having the above-described configuration, the main pump cell 21 and the auxiliary pump cell 50 are operated to supply the measurement gas, whose oxygen partial pressure is always maintained at a constant low value (a value that has substantially no effect on NOx measurement), to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas can be determined based on the pump current Ip2 flowing through the measurement pump cell 41 as oxygen generated by NOx reduction is drawn in, and is substantially proportional to the NOx concentration in the measurement gas.

[0103] The main pump unit 21, the main pump control oxygen partial pressure detection sensor unit 80, the auxiliary pump unit 50, the auxiliary pump control oxygen partial pressure detection sensor unit 81, the measurement pump unit 41, the measurement pump control oxygen partial pressure detection sensor unit 82, the sensor unit 83, and the measured gas flow section are collectively referred to as a gas monitoring unit. The configuration of the gas monitoring unit is not particularly limited, as long as it is configured to monitor the target gas components using the oxygen ion conductivity of the solid electrolyte.

[0104] Figure 1In the embodiment, the measured gas flow portion has a structure having two internal cavities (a first internal cavity 20 and a second internal cavity 40 ), but the structure of the measured gas flow portion is not limited thereto. Figure 2 FIG. 1 is a schematic vertical cross-sectional view in the longitudinal direction showing another example of the schematic configuration of the gas sensor 100 . Figure 2 The sensor element 101 is the structure of the gas flow part to be measured. Figure 1 Different variations of the sensor element 101. Figure 2 As shown in the example, the second internal cavity 40 can be further divided into two chambers by the fifth diffusion rate control unit 60 to form a third internal cavity 61. In this case, the auxiliary pump electrode 51 can be arranged in the second internal cavity, and the measurement electrode 44 can be arranged in the third internal cavity. In addition, when the third internal cavity 61 is formed (three-chamber structure), the fourth diffusion rate control unit 45 can be omitted. It should be noted that Figure 2 In, with Figure 1 The same components are marked with the same symbols, and the description thereof is omitted.

[0105] Sensor element 101 also includes a heater 70, which heats and maintains the sensor element 101 to improve its oxygen ion conductivity. Furthermore, sensor element 101 includes a pressure relief space 76, which prevents delamination of the sensor element's internal structure. This will be described in detail below.

[0106] [Heater section]

[0107] The heater portion 70 includes a heater electrode 71 , heaters 72 ( 72 a , 72 b ), a through hole 73 , and a heater insulating layer 74 .

[0108] The heater electrode 71 is an electrode formed so as to be in contact with the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to an external power source, power can be supplied to the heater portion 70 from the outside.

[0109] Figure 3 Schematic diagram showing an example of the schematic planar arrangement of the heater 72 and the pressure relief space 76 described later. The heater 72 includes a heater heating portion 72a and a heater lead portion 72b connected to the heater heating portion 72a and extending toward the rear end side in the longitudinal direction of the sensor element 101. Figure 1The heater portion 72a is a resistor formed by being sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater portion 72a is connected to the heater electrode 71 via a heater lead portion 72b and a through hole 73. The heater portion 72a generates heat when power is supplied from the outside through the heater electrode 71, thereby heating and maintaining the temperature of the solid electrolyte forming the sensor element 101.

[0110] The heater heating portion 72a is embedded in the substantially entire region from the first internal cavity 20 to the second internal cavity 40 and faces the entire region in the thickness direction of the element, and can adjust the gas monitoring portion of the sensor element 101 to the temperature at which the solid electrolyte is activated. Figure 3 As shown, the heater heating portion 72a is a linear resistor that meanders in a plan view. Figure 3 In FIG. 7 , the heater heating portion 72a is the portion of the heater 72 surrounded by the dotted line. The two heater lead portions 72b are connected to both ends of the heater heating portion 72a and extend toward the rear end portion in the longitudinal direction of the sensor element 101 ( Figure 3 The number of serpentine lines, line width, and other shapes of the heater heating portion 72a can be arbitrarily set so as to adjust the gas monitoring portion of the sensor element 101 to a predetermined temperature. Furthermore, the entire gas monitoring portion does not need to be adjusted to the same temperature; a temperature distribution can be provided within the gas monitoring portion.

[0111] Figure 4 A modified example of the heater 72 is shown in FIG. As the heater heating portion 72a, a Figure 4 The heater heating portion 72a has a so-called comb-shaped shape as shown. Both ends of the heater heating portion 72a are widened into a substantially triangular shape in plan view and are connected to the heater lead portions 72b, respectively. Figure 4 In the figure, heater heating portion 72a is the portion of heater 72 enclosed by the dotted line. The length, line width, and other shapes of heater heating portion 72a can be arbitrarily set so that the gas monitoring portion of sensor element 101 can be adjusted to a predetermined temperature. Furthermore, the entire gas monitoring portion does not need to be adjusted to the same temperature; a temperature distribution can be provided within the gas monitoring portion.

[0112] Heater insulating layer 74 is an insulating layer formed in layers on the upper and lower surfaces of heater 72 using an insulator such as alumina. Heater insulating layer 74 is formed to provide electrical insulation between second substrate layer 2 and heater 72, and between third substrate layer 3 and heater 72. Heater insulator 74 is a porous material. Figure 3 Although not shown in the figure, the heater insulator 74 exists in a region covering the entire heater 72 (heating portion 72a and heater lead portion 72b), and its planar shape is substantially rectangular.

[0113] The heater 72 and the heater insulator 74 embedded in the heater portion so as to be sandwiched between the second substrate layer 2 and the third substrate layer 3 are referred to as a heater-containing layer 75 .

[0114] The thickness of the heater heat-generating portion 72a can be appropriately set according to the desired resistance value. The thickness of the heater insulator 74 can be appropriately set within the range of maintaining the electrical insulation between the second substrate layer 2 and the heater 72, and the electrical insulation between the third substrate layer 3 and the heater 72. For example, the thickness of the heater-containing layer 75 can be 15 to 50 μm. Here, the thickness of the heater-containing layer 75 is the thickness of the heater 72 and the heater insulator 74 added together at the portion where the heater 72 is present. The length of the heater-containing layer 75 in the longitudinal direction can be shorter than the length of the sensor element, for example, it can be 40 to 80 mm. The length of the heater-containing layer 75 in the width direction perpendicular to the longitudinal direction can be shorter than the width of the sensor element, for example, it can be 2 to 5 mm. In addition, the length of the heater-containing layer 75 in the longitudinal direction of the region including the heater heat-generating portion 72a can be, for example, 2 to 15 mm.

[0115] [Stress Relief Space]

[0116] The sensor element 101 includes the pressure relieving space 76. As described above, Figure 3 It is a schematic diagram showing an example of a schematic planar arrangement of the heater 72 and the pressure buffering space 76 . Figure 3 In FIG. 1 , the area of ​​the pressure relief space 76 in the plane is illustrated by a dotted line. Figure 5 It is along Figure 1 A schematic cross-sectional view of the V-V line. That is, Figure 5 1 is a schematic diagram of a vertical cross section perpendicular to the longitudinal direction of the sensor element 101 , and is a schematic diagram showing an example of the pressure buffering space 76 .

[0117] The pressure absorbing space 76 is formed in a manner sandwiched between the lower surface of the third substrate layer 3 of the base portion 102 constituting the sensor element 101 and the upper surface of the heater insulator 74. In other words, it is formed so as to contact the surface 75a of the heater layer 75 on the side close to the measured gas flow portion. Figure 5 As shown, the side surfaces of the heater insulator 74 and the pressure relief space 76 are in contact with the adhesive layer 90. The adhesive layer 90 is an airtight layer that bonds the second substrate layer 2 and the third substrate layer 3 together, and is a layer made of an oxygen ion conductive solid electrolyte, similar to the second substrate layer 2 and the third substrate layer 3. Figure 3 As shown by the dotted line in FIG, the pressure relieving space 76 is arranged on the heater insulator 74 ( Figure 3The pressure absorbing space 76 is a layered space extending in the longitudinal direction of the sensor element 101 and is a space enclosed within the base portion 102 of the sensor element 101.

[0118] Because the heater insulator 74 is porous, liquid components such as water may accumulate within it or at the interface with the heater's heating element 72a. When the gas sensor is in use, the sensor element reaches a high temperature due to the heat generated by the heater's heating element 72a. However, when the gas sensor is not in use (when the sensor element is not heated), the sensor element maintains a temperature equal to the ambient temperature. Therefore, within the porous heater insulator 74 or at the interface with the heater's heating element 72a, liquid components such as water may also be present in addition to gas components such as air. When the gas sensor is activated while liquid components such as water are present within the heater insulator 74 or at the interface with the heater's heating element 72a, the water and other liquid components rapidly evaporate due to the heat generated by the heater's heating element 72a. The vaporized gas components such as water vapor can migrate from the heater insulator 74 or at the interface with the heater's heating element 72a to the pressure relief space 76, thereby suppressing pressure increases near the heater insulator 74. As a result, it is believed that the sensor element 101 can suppress delamination between two adjacent solid electrolyte layers, primarily between the second substrate layer 2 and the third substrate layer 3. Therefore, if the sensor element 101 having the pressure relief space 76 is used, the sensor element 101 is less likely to be damaged by delamination of its internal structure and can withstand repeated use, thereby improving the durability of the gas sensor 100.

[0119] The pressure buffering space 76 can be configured to relieve pressure caused by the evaporation of liquid components such as water within the heater insulator 74 or at the interface with the heater heating portion 72a. The thickness of the pressure buffering space 76 can be, for example, 10 to 50 μm. The length of the pressure buffering space 76 in the longitudinal direction can be, for example, 2 to 15 mm. The width of the pressure buffering space 76, perpendicular to the longitudinal direction, can be shorter than the width of the sensor element and can be, for example, 2 to 5 mm.

[0120] It is sufficient as long as the pressure relief space 76 exists in at least a portion between the solid electrolyte constituting the base portion 102 and the heater-containing layer 75. The pressure relief space 76 can be configured to roughly cover the entire area of ​​the heater heating portion 72a in a plane, or can be configured to cover a portion of the heater heating portion 72a. It can be configured to cover all or a portion of the heater lead portion 72b. The pressure relief space 76 can exist only in the area of ​​the heater heating portion 72a, or can extend from the area of ​​the heater heating portion 72a to the rear end portion of the sensor element 101 including the heater lead portion 72b. Alternatively, the pressure relief space 76 can exist only in the area of ​​the heater lead portion 72b. In addition, the pressure relief space 76 can be either a continuous space or composed of a plurality of spaces independent of each other.

[0121] Figure 5 In the description, a mode in which the pressure buffering space 76 is formed so as to be in contact with the surface 75 a of the heater-containing layer 75 on the side close to the measured gas flow portion will be described.

[0122] As another form of the pressure relief space 76, Figure 6 As shown, a pressure buffering space 76 can be formed between the upper surface of the second substrate layer 2 and the lower surface of the heater insulator 74 (i.e., in contact with the surface 75b of the heater-containing layer 75 that is away from the measured gas flow portion). In this case, the pressure buffering space 76 is a layered space extending in the longitudinal direction of the sensor element 101.

[0123] In this way, when the pressure relief space 76 is formed so as to contact the surface 75b of the heater-containing layer 75 on the side away from the measured gas flow portion, the strength (water resistance) of the sensor element 101 when water adheres to the sensor element 101 can be improved during use of the gas sensor (when the sensor element is kept at a high temperature). It is speculated that the pressure relief space can play a role in relieving the stress generated when water adheres to the high-temperature sensor element surface.

[0124] Alternatively, as another form of the pressure relief space 76, Figure 7 As shown, a pressure relief space 76 can be formed between the side of the heater-containing layer 75, that is, the side surface 75c of the heater-containing layer 75 and the inner side surface 90a of the adhesive layer 90 (that is, in contact with the side surface 75c of the heater-containing layer 75). In this case, the pressure relief space 76 is a space extending in the longitudinal direction, and for example, the cross section perpendicular to the longitudinal direction can be a rectangle ( Figure 7 The pressure relief space 76 may be present on both sides of the heater layer 75 or only on one side.

[0125] In addition, as another form of the pressure relief space 76, Figure 8 As shown, the pressure buffering space 76 can be formed so that the upper surface and side surfaces of the heater heating portion 72a are exposed in the pressure buffering space 76. In this case, the heater-containing layer 75 is composed of the heater insulator 74 and the heater 72. Only the upper surface of the heater heating portion 72a may be exposed in the pressure buffering space 76.

[0126] The size (volume) of the pressure buffering space 76 can be appropriately set by a person skilled in the art. For example, in a vertical cross section perpendicular to the longitudinal direction of the sensor element 101, the cross-sectional area A of the pressure buffering space 76 is 76 With respect to the cross-sectional area A of the heater-containing layer 75 75 (ie, the sum of the cross-sectional area of ​​the heater 72 and the cross-sectional area of ​​the heater insulator 74) 76 / A 75 ), as a lower limit, it may be 0.10 or more. If it is 0.10 or more, it is easy to suppress the pressure rise near the heater insulator 74. Preferably, the cross-sectional area ratio R may be 0.30 or more. On the other hand, as an upper limit, the cross-sectional area ratio R may be 0.80 or less. Preferably, it may be 0.60 or less. It is sufficient to set the upper limit from the perspective of the adhesion strength between the second substrate layer 2 and the third substrate layer 3 of the sensor element. As a preferred range, it may be 0.10 or more and 0.80 or less, or 0.30 or more and 0.80 or less. Preferably, it may be 0.30 or more and 0.60 or less. In Figures 5 to 8 The above-mentioned cross-sectional area ratio R can be set in the various arrangements of the pressure absorbing spaces 76 as exemplified in FIG. Note that the above-mentioned vertical cross section may be a vertical cross section at any position perpendicular to the longitudinal direction of the sensor element 101 .

[0127] Here, the cross-sectional area A of the pressure buffer space 76 is 76 With respect to the cross-sectional area A of the heater-containing layer 75 75 The ratio R(A 76 / A 75 ) can be calculated as follows. A vertical cross section perpendicular to the longitudinal direction of the sensor element 101 is ground and photographed using an SEM. Next, the obtained SEM image is binarized to determine the cross-sectional area A including the heater layer 75. 75 Furthermore, the conditions of the binarization process are changed to obtain the cross-sectional area A of the pressure relieving space 76. 76 According to the cross-sectional area A including the heater layer 75 75 and the cross-sectional area A of the pressure relief space 76 76 , calculate the cross-sectional area ratio R = A 76 / A 75 .

[0128] Figure 1 In the example, the pressure buffering space 76 is a space enclosed within the sensor element 101. Therefore, even if liquid components such as water vapor present within the heater insulator 74 or at the interface with the heater heating portion 72a evaporate and generate gas components such as water vapor, these components are released only into the pressure buffering space 76 and do not affect the reference gas introduction space 43. This means that the startup time from activation of the gas sensor to the start of accurate measurement is not affected, thereby shortening the gas sensor's startup time. Alternatively, the pressure buffering space 76 can be open to the exterior of the base 102 at a location other than the reference gas introduction space 43. Opening the pressure buffering space 76 to the exterior of the base 102 further reduces the pressure exerted by generated gas components such as water vapor. Opening the pressure buffering space 76 at a location other than the reference gas introduction space 43 prevents the generated gas components such as water vapor from directly affecting the reference gas introduction space 43. Therefore, even in this case, the startup time of the gas sensor can be shortened.

[0129] The pressure relief space 76 also provides the following benefits. The presence of the pressure relief space 76 creates a space between the solid electrolyte constituting the base 102 and the heater, in addition to the heater insulator 74. This further improves the insulation between the solid electrolyte and the heater. This improved insulation further reduces leakage current from the heater to the solid electrolyte, thereby improving the accuracy of the electrical signal when detecting the target gas.

[0130] [Sensor Element Manufacturing Method]

[0131] Next, an example of a method for manufacturing the sensor element described above will be described. Sensor element 101 can be produced by subjecting multiple unfired sheet-shaped molded products (so-called green sheets) containing an oxygen ion conductive solid electrolyte such as zirconium oxide (ZrO2) as a ceramic component to prescribed processing and printing of a circuit pattern. The multiple sheets are then stacked, cut, and fired.

[0132] The following is to make Figure 1 The case of a sensor element 101 comprising six layers is shown for explanation by way of example.

[0133] First, prepare six green sheets containing an oxygen ion conductive solid electrolyte such as zirconium oxide (ZrO2) as a ceramic component. The green sheets can be made by a known molding method. The six green sheets can be of the same thickness or of different thicknesses depending on the layers to be formed. Sheet holes (semi-finished sheets) for positioning during printing and stacking are formed in advance on each of the six green sheets using known methods such as punching with a punching device. In the semi-finished sheet used for the isolation layer 5, through-holes such as internal cavities are also formed using the same method. Necessary through-holes are also formed in advance in other layers.

[0134] The semi-finished sheets for the six layers (first substrate layer 1, second substrate layer 2, third substrate layer 3, first solid electrolyte layer 4, separator layer 5, and second solid electrolyte layer 6) are printed with the various patterns required for each layer and then dried. The patterns can be printed using known screen printing techniques. Drying can also be performed using known drying methods.

[0135] Regarding the formation of the pressure relief space 76, Figure 5 The case of the sensor element 101 in the form of a shaped sensor element 101 will be described in detail. First, a semi-finished sheet for the second substrate layer 2 is prepared. In this case, the semi-finished sheet for the second substrate layer 2 can be prepared in a form pre-stacked with the semi-finished sheet for the first substrate layer 1.

[0136] On the semi-finished sheet for the second substrate layer 2, a heater layer 75 is formed by printing. The paste for forming the heater insulator 74 (heater insulator paste) is printed in a prescribed pattern and dried. Depending on the desired thickness, printing and drying can be repeated multiple times. On top of the printed heater insulator paste, the paste for forming the heater 72 (heater paste) is printed in a prescribed pattern and dried. When forming the heater 72, the heater heating portion 72a and the heater lead portion 72b can be printed using the same heater paste or different heater pastes. In addition, depending on the desired thickness, printing and drying can be repeated multiple times, and the heater heating portion 72a and the heater lead portion 72b can be printed and dried different times. Furthermore, on top of the printed heater paste, the heater insulator paste is printed in a prescribed pattern and dried. Depending on the desired thickness, printing and drying can be repeated multiple times.

[0137] Next, pressure relief spaces 76 are formed by printing a paste (a volatile material paste that disappears during firing in a subsequent step) in a predetermined pattern on the printed heater-containing layer 75 to form the pressure relief spaces 76 and drying the paste. Printing and drying can be repeated multiple times to achieve the desired thickness.

[0138] Furthermore, a paste for forming an adhesive layer 90 (adhesive paste) is printed in a predetermined pattern and dried on the semi-finished sheet for the second substrate layer 2. Printing and drying can be repeated multiple times to achieve the desired thickness. The adhesive layer 90 is used to bond the second substrate layer 2 and the third substrate layer 3 together, and preferably has a thickness approximately equal to the combined thickness of the heater-containing layer 75 and the pressure relief space 76.

[0139] As a heater insulator paste, a paste obtained by mixing a resin and an organic solvent in Al2O3 and adjusting the viscosity to a specified value is used. As a heater paste, a paste obtained by mixing a resin and an organic solvent with Pt as the main component and adjusting the viscosity to a specified value is used. As a disappearing material paste, a paste obtained by mixing a material that disappears due to firing in a subsequent process with an organic solvent and adjusting the viscosity to a specified value is used. The disappearing material paste is a material that disappears due to firing in a subsequent process. For example, theobromine, acrylic resin, carbon, etc. can be used. The bonding paste contains a solid electrolyte. The same solid electrolyte as that used for the second substrate layer 2 and the third substrate layer 3 can be used. For example, a paste obtained by mixing a resin and an organic solvent with ZrO2 as the main component and adjusting the viscosity to a specified value is used.

[0140] After the six semi-finished sheets have been printed and dried, they are crimped. This involves positioning the six printed semi-finished sheets using the holes in the sheets, stacking them in a specified order, and then crimping them together under specified temperature and pressure conditions to form a laminate. This crimping is performed by applying heat and pressure using a known laminating machine, such as a hydraulic press. The temperature, pressure, and duration of the heating and pressing process depend on the laminating machine used and can be appropriately determined to achieve optimal lamination.

[0141] The obtained stacked body includes a plurality of sensor elements 101. The stacked body is cut and divided into units of the sensor element 101. Then, the divided stacked body is fired at a predetermined firing temperature, thereby obtaining the sensor element 101. The firing temperature only needs to 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, etc., maintain the desired porosity. For example, the firing temperature is about 1300 to 1500°C. As described above, the expendable material paste disappears due to firing, and therefore, the area where the expendable material paste exists becomes a space, which can form a pressure relief space 76.

[0142] like Figure 6As shown, when forming the pressure buffering space 76 between the second substrate layer 2 and the heater-containing layer 75, first, a eliminable material paste for forming the pressure buffering space 76 is printed in a predetermined pattern on the semi-finished sheet for the second substrate layer 2 and dried. The heater-containing layer 75 is then printed on the eliminable material paste as described above. Then, the pressure buffering space 76 is formed by firing.

[0143] like Figure 7 As shown, while forming pressure buffering space 76 in contact with side surface 75c of heater-containing layer 75, eliminative material paste for forming pressure buffering space 76 and heater insulator paste for forming heater insulator 74 are printed in predetermined patterns on the semi-finished sheet for second substrate layer 2 and dried. The eliminative material paste is printed along the inside of adhesive layer 90 and extending in the longitudinal direction of sensor element 101. Heater paste and heater insulator paste are further printed on top of the heater insulator paste and dried to form heater-containing layer 75. Then, the pressure buffering space 76 is formed by firing.

[0144] like Figure 8 As shown, with the heater heat generating portion 72a exposed in the pressure relief space 76, the heater insulator paste is printed and dried, and then the heater paste is printed in a predetermined pattern and dried. Furthermore, a condensing material paste for forming the pressure relief space 76 is printed and dried on top of the heater insulator paste and the heater paste. Then, the pressure relief space 76 is formed by firing.

[0145] Figure 5 、 6 , 7, and 8 illustrate examples of the arrangement of the pressure buffering space 76 within the sensor element 101. However, the present invention is not limited to the configurations shown in these examples. The present invention encompasses sensor elements including various configurations of the pressure buffering space 76 as long as they can achieve the objectives of the present invention of suppressing delamination of the internal structure of the sensor element and shortening the startup time of the gas sensor.

[0146] Example

[0147] Hereinafter, an example in which a sensor element was specifically manufactured and tested will be described as an embodiment. However, the present invention is not limited to the following embodiment.

[0148] [Examples 1 to 6]

[0149] As Examples 1 to 6, according to the manufacturing method of the sensor element 101 described above, a sensor element having a pressure buffering space 76 formed between the heater insulator 74 and the third substrate layer 3 (i.e., the surface 75a of the heater layer 75 close to the measured gas flow portion) was manufactured. Figure 5 A paste obtained by mixing theobromine and an organic solvent and adjusting the mixture to a predetermined viscosity was used as the eliminative material paste. The pressure buffering space 76 was formed in an area that covered substantially the entire area of ​​the heater heating portion 72a. The pressure buffering space 76 and the heater insulator 74 had approximately the same length in the width direction. In Examples 1 to 6, the thickness of the heater-containing layer 75 was constant, and the thickness of the pressure buffering space 76 was varied in six different ways.

[0150] In a vertical cross section of the sensor element 101 perpendicular to the longitudinal direction and including the heater heating portion 72a, the cross-sectional area A of the pressure buffering space 76 is 76 With respect to the cross-sectional area A of the heater-containing layer 75 75 The ratios R are 0.04 (Example 1), 0.14 (Example 2), 0.24 (Example 3), 0.33 (Example 4), 0.66 (Example 5), and 0.79 (Example 6), respectively.

[0151] It should be noted that the cross-sectional area ratio R is calculated as follows. First, regarding Example 1, a polished cross section is prepared by polishing three cross sections of the sensor element 101 that are perpendicular to the longitudinal direction and include the heater heating portion 72a. Figure 9 , three polished cross sections of the front end (S1), center (S2) and rear end (S3) of the heater heating portion 72a in the longitudinal direction of the sensor element 101 were prepared. SEM images (reflected electron image, magnification 30 times, about 1.3 million pixels) of each of the three polished cross sections were taken. The magnification of the SEM image can be appropriately changed according to the size of the sensor element 101, etc. Next, for one of the captured SEM images, the image processing software Pick Map (URL: https: / / fishers.mydns.jp / software / pickmap / index.html) was used to adjust the threshold to the average value of RGB, and the SEM image was binarized so that only the heater layer 75 (i.e., the heater heating portion 72a and the heater insulator 74) in the SEM image was selected. It should be noted that the latest version of the image processing software Pick Map can be obtained from the above URL for use. Furthermore, the image processing software Pick Map is used to calculate the cross-sectional area A of the heater layer 75 in the binarized SEM image. 75Regarding the pressure relief space 76, the image processing software Pick Map was used to binarize the SEM image and calculate the cross-sectional area A. 76 Then, according to the calculated cross-sectional area A of the heater layer 75 75 and the cross-sectional area A of the pressure relief space 76 76 , calculate the cross-sectional area ratio R: (the cross-sectional area A of the pressure relieving space 76 76 ) / (cross-sectional area A including the heater layer 75 75 ). The same operation was performed on the remaining two SEM images to calculate the cross-sectional area ratio R. The average value of the three calculated cross-sectional area ratios R was set as the cross-sectional area ratio R in Example 1. The cross-sectional area ratio R was calculated similarly for Examples 2 to 6.

[0152] [Example 7]

[0153] As Example 7, a sensor element was prepared in which a pressure relief space 76 was formed between the second substrate layer 2 and the heater insulator 74 (i.e., the surface 75b of the heater layer 75 away from the measured gas flow portion). Figure 6 The planar shape of the pressure absorbing space 76 is the same as that of Examples 1 to 6. Except for the formation of the pressure absorbing space 76, the same method as that of Examples 1 to 6 is used. In the vertical cross section of the sensor element 101 perpendicular to the longitudinal direction and including the heater heating portion 72a, the cross-sectional area A of the pressure absorbing space 76 is 76 With respect to the cross-sectional area A of the heater-containing layer 75 75 R

[0154] (Cross-sectional area ratio) is 0.31 (Example 7) The calculation of the cross-sectional area ratio R is performed in the same manner as in Examples 1 to 6.

[0155] [Example 8]

[0156] As Example 8, a sensor element was produced in which a pressure relief space 76 was formed between the side surface 90a of the adhesive layer 90 and the side surface of the heater insulator 74 (i.e., in contact with the side surface 75c of the heater layer 75). Figure 7 The pressure absorbing space 76 is a space extending toward both sides of the heater layer 75 in the longitudinal direction of the sensor element. The length in the longitudinal direction is the same as that of Examples 1 to 7. Except for the formation of the pressure absorbing space 76, the same method as that of Examples 1 to 6 is used. In the vertical cross section of the sensor element 101 perpendicular to the longitudinal direction and including the heater heating portion 72a, the cross-sectional area A of the pressure absorbing space 76 is 76 With respect to the cross-sectional area A of the heater-containing layer 75 75The ratio R (cross-sectional area ratio) is 0.40 (Example 8). The calculation of the cross-sectional area ratio R is performed in the same manner as in Examples 1 to 6.

[0157] [Comparative Example 1]

[0158] As Comparative Example 1, a sensor element without the pressure buffering space 76 was produced ( Figure 10 ) Except that the pressure buffering space 76 is not formed, the same method as in Examples 1 to 6 was carried out.

[0159] Should be explained, such as Figure 5 and Figure 6 As shown, when the pressure absorbing space 76 is formed to extend along the upper surface or the lower surface of the heater-containing layer 75 in the longitudinal direction of the sensor element 101 (Examples 1 to 7), the cross-sectional area ratio R and Figure 1 The cross-sectional area ratios of the sensor element 101 shown in the vertical cross section along the longitudinal direction are substantially the same.

[0160] [Measurement of peeling generation voltage]

[0161] The peeling voltage was measured using the sensor elements obtained in Examples 1 to 8 and Comparative Example 1. First, the rear end side of the sensor element, that is, the side opposite to the front end side with the gas inlet, was immersed in water for 4 hours. After wiping off the moisture attached to the surface, a voltage of 6V was applied to the heater 72 of the sensor element 101 for 30 seconds to investigate whether the sensor element peeled off. If peeling did not occur, the applied voltage was increased by 1V each time to 12V to investigate the voltage at which peeling occurred. The number of test samples for Examples 1 to 8 and Comparative Example 1 was 5 each. The results are shown in Table 1.

[0162] [Table 1]

[0163]

[0164] Table 1 shows the position of the pressure relief space 76 and the cross-sectional area ratio R (A76 / A75) for each of Examples 1 to 8 and Comparative Example 1. Furthermore, for each case, the number of stripes (strips) of stripes that occurred at each applied voltage (6 V, 7 V, 8 V, 9 V, 10 V, 11 V, and 12 V) is shown.

[0165] In Examples 1-8, no samples peeled at 6V. In Examples 2-8, no samples peeled at 10V or below. In Examples 3 and 6, none of the five strips peeled when 6-11V was applied. At 12V, one strip began to peel. The remaining four strips did not peel even at 12V. In Examples 4, 5, 7, and 8, none of the five strips peeled even at 6-12V.

[0166] In Comparative Example 1, 6 V was applied to the heaters of five samples, and one sample peeled off the sensor element 101 due to the application of 6 V. For the four samples that did not peel off at 6 V, 7 V was applied to the heaters of the four samples, and peeling occurred in all four samples.

[0167] The test for measuring the peeling voltage is an accelerated test. By immersing the rear end of the sensor element in water, water is forcibly accumulated inside or on the interface of the heater insulator 74. Furthermore, the heater heating portion 72a is usually heated at a controlled temperature curve. In this test, a certain voltage is applied to rapidly heat the heater heating portion 72a, causing it to quickly generate water vapor. Therefore, the peeling voltage in this test cannot be used to directly and quantitatively estimate the degree of peeling during actual use. However, if the peeling voltage is higher than that in Comparative Example 1, or if peeling does not occur even when the applied voltage is increased, it is confirmed that peeling is suppressed in actual use compared to Comparative Example 1.

[0168] In Examples 1 to 8, the presence of pressure buffering space 76 resulted in a higher peeling-initiating voltage than in Comparative Example 1, thus suppressing peeling of the sensor element 101. Furthermore, the results of Examples 4, 7, and 8 demonstrate that peeling of the sensor element 101 can be suppressed regardless of the location of the pressure buffering space 76.

[0169] In addition, as the value of the cross-sectional area ratio increases (i.e., the volume of the pressure relief space 76 increases), the peeling occurrence voltage further increases (Examples 1 to 5). It can be considered that: although water vapor is generated from the inside or the interface of the heater insulator 74 due to the heat generated by the heater heating portion 72a, the larger the volume of the pressure relief space 76, the more the pressure rise is suppressed, and the peeling of the sensor element 101 can be further suppressed. On the other hand, the following trend is found, that is, if the value of the cross-sectional area ratio is too large, the suppression effect is slightly reduced (Example 6). It can be inferred that: if the value of the cross-sectional area ratio is too large, although the pressure relief effect is fully obtained, since the space inside the sensor element is too large, the structural strength of the sensor element is reduced, and peeling occurs between the second substrate layer 2 and the third substrate layer 3 due to smaller stress.

[0170] This shows that the presence of the pressure absorbing space 76 can suppress the sensor element from peeling off during use. It also shows that peeling can be suppressed regardless of which surface (top, bottom, or side) of the heater insulator 74 the pressure absorbing space 76 contacts.

[0171] Explanation of symbols

[0172] 1. First substrate layer

[0173] 2 Second substrate layer

[0174] 3. Third substrate layer

[0175] 4 First solid electrolyte layer

[0176] 5 Isolation layer

[0177] 6 Second solid electrolyte layer

[0178] 10 Gas inlet

[0179] 11. First diffusion rate control unit

[0180] 12 Buffer Space

[0181] 13. Second diffusion rate control unit

[0182] 20 first inner cavity

[0183] 21 Main pump unit

[0184] 22 Inner pump electrode

[0185] 22a Top electrode portion (of inner pump electrode)

[0186] 22b Bottom electrode portion (of inner pump electrode)

[0187] 22c (side electrode of inner pump electrode)

[0188] 23 Outer pump electrode

[0189] 24 Variable power supply (for main pump unit)

[0190] 30 Third diffusion rate control unit

[0191] 40 Second inner cavity

[0192] 41 Measurement pump unit

[0193] 42 reference electrode

[0194] 43 Reference gas introduction space

[0195] 44 Measuring electrode

[0196] 45 Fourth diffusion rate control unit

[0197] 46 Variable power supply (for measurement pump unit)

[0198] 48 Atmospheric introduction layer

[0199] 50 Auxiliary pump unit

[0200] 51 Auxiliary pump electrode

[0201] 51a Top electrode portion (of auxiliary pump electrode)

[0202] 51b bottom electrode portion (auxiliary pump electrode)

[0203] 52 Variable power supply (for auxiliary pump unit)

[0204] 60 Fifth diffusion rate control unit

[0205] 61 Third internal cavity

[0206] 70 heater unit

[0207] 71 heater electrode

[0208] 72 Heater

[0209] 72a Heater heating part

[0210] 72b Heater lead

[0211] 73 through holes

[0212] 74 Heater Insulator

[0213] 75 with heater layer

[0214] 75a Upper surface (including heater layer)

[0215] 75b Lower surface (including heater layer)

[0216] 75c (including heater layer) side

[0217] 76 Stress Relief Space

[0218] 80 Main pump control oxygen partial pressure detection sensor unit

[0219] 81 Auxiliary pump control oxygen partial pressure detection sensor unit

[0220] 82 Oxygen partial pressure detection sensor unit for measurement pump control

[0221] 83 sensor unit

[0222] 90 adhesive layer

[0223] 90a Inner side (of adhesive layer)

[0224] 100 Gas Sensors

[0225] 101 sensor element

[0226] 102 Base part

Claims

1. A sensor element for detecting a target gas in a gas to be measured, characterized in that: include: a long plate-shaped base portion including a plurality of stacked oxygen ion conductive solid electrolyte layers; a gas-to-be-measured flow portion into which a gas to be measured is introduced from one end portion in the longitudinal direction of the base portion and through which the gas to be measured flows; a heater-containing layer embedded in the base portion with at least one of the plurality of oxygen ion conductive solid electrolyte layers interposed therebetween from the measured gas flow portion, the heater-containing layer including a heater having a heater heat generating portion and a heater lead portion, and a heater insulator; a pressure relieving space formed at least partially between the base portion and the heater-containing layer; The pressure relief space is a space enclosed inside the base portion.

2. The sensor element according to claim 1, characterized in that The pressure absorbing space is formed in at least a portion between the base portion and the heater-containing layer in a region where the heat generating portion of the heater is located.

3. The sensor element according to claim 1 or 2, characterized in that The pressure buffering space is formed so as to be in contact with a surface of the heater-containing layer on a side close to the measured gas flow portion.

4. The sensor element according to claim 1 or 2, characterized in that The pressure buffering space is formed so as to be in contact with a surface of the heater-containing layer on a side away from the measured gas flow portion.

5. The sensor element according to claim 1 or 2, characterized in that The pressure buffering space is formed so as to be in contact with a side portion of the heater-containing layer.

6. The sensor element according to claim 1 or 2, characterized in that In a cross section of the base portion perpendicular to the longitudinal direction, a ratio of a cross-sectional area of ​​the pressure buffering space to a cross-sectional area of ​​the heater-containing layer is 0.10 or greater.

7. The sensor element according to claim 1 or 2, characterized in that In a cross section of the base portion perpendicular to the longitudinal direction, a ratio of a cross-sectional area of ​​the pressure buffering space to a cross-sectional area of ​​the heater-containing layer is 0.80 or less.

8. The sensor element according to claim 1 or 2, characterized in that In a cross section of the base portion perpendicular to the longitudinal direction, a ratio of a cross-sectional area of ​​the pressure buffering space to a cross-sectional area of ​​the heater-containing layer is 0.3 or more and 0.6 or less.

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