JP7631335B2 - Sensor element - Google Patents

Description

The present invention relates to a sensor element using an oxygen ion conductive solid electrolyte.

Gas sensors are used to detect and measure the concentration of target gas components (oxygen _O2 , nitrogen oxides NOx, ammonia _NH3 , hydrocarbons HC, carbon dioxide _CO2, etc.) in measurement gases such as automobile exhaust gas. For example, the concentration of target gas components in automobile exhaust gas is measured, and the exhaust gas purification system installed in the automobile is optimally controlled based on the measured value.

As such a gas sensor, a gas sensor having a sensor element using an oxygen ion conductive solid electrolyte such as zirconia (ZrO ₂ ) is known. In order to detect a target gas component, the gas sensor is used by heating the sensor element to a temperature at which the solid electrolyte exhibits oxygen ion conductivity. For this reason, a structure in which a heater is embedded inside the sensor element is widely used. However, peeling may occur in the internal structure of the sensor element during use.

For example, WO2018/230703 discloses a sensor element having multiple stacked oxygen ion conductive solid electrolyte layers. The sensor element is disclosed to include a measurement gas flow section for introducing and flowing a measurement gas, multiple electrodes for detecting the target gas components, a reference gas introduction space for introducing a reference gas, and a heater section for heating and keeping the sensor element warm. The publication also discloses a pressure relief hole formed so that a heater insulating layer present in the heater section communicates with the reference gas introduction space.

Japanese Patent Publication No. 4313027 discloses a gas sensor in which the heater section includes a heating element and a support that supports at least the heating element, and the gas sensor has an opening that is provided to reduce the pressure generated between the heating element and the support. In particular, the gas sensor shows a structure in which the opening opens into an air introduction space (Figure 1).

WO2018/230703 publication Patent No. 4313027

The reason why peeling occurs in the internal structure of the sensor element is thought to be as follows. The heater insulator surrounding the heater heating element is a porous body made of an insulator such as alumina. When the sensor element is not heated, liquid components such as moisture are present in addition to gas components such as air inside the heater insulator itself, which is a porous body, and at the interface with the heater heating element. When the heater is turned on to heat the sensor element, these moisture components evaporate due to the heat generated by the heater, and the generated water vapor causes a localized pressure increase. It is thought that peeling occurs due to this pressure increase. 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 thought that it is necessary to suppress the localized pressure increase.

In addition, gas sensors are used in exhaust gas purification systems mounted on automobiles, and are required to accurately measure the concentration of the target gas. Even when the gas sensor is started, it is required to be able to perform accurate measurement as quickly as possible. Through the study by the present inventors, it has been confirmed that the conventional gas sensor has a problem that the time from starting the gas sensor to starting accurate measurement (i.e., the start-up time) may be long. As a result of the study by the present inventors, the reason for the long start-up time is considered to be as follows. The reference gas introduction space is filled with a reference gas (e.g., air) having a certain oxygen concentration in order to detect the target gas component. According to WO2018/230703, in the conventional sensor element, in order to alleviate the internal pressure increase caused by the temperature increase in the heater insulator, a pressure diffusion hole is provided so that the heater insulator and the reference gas introduction space communicate with each other. Therefore, when moisture or the like present inside the heater insulator itself or at the interface with the heater heating element evaporates when the sensor is started, water vapor or the like flows into the reference gas introduction space from the pressure diffusion hole through the heater insulator, which is a porous body. As a result, the oxygen concentration in the reference gas fluctuates, making it impossible to accurately measure the target gas component in the measured gas. After that, when water vapor and the like are completely discharged from the reference gas introduction space, the oxygen concentration in the reference gas becomes constant, enabling accurate measurement. Because the heater insulator, which is a porous body, has a high diffusion resistance, it takes time for water vapor and the like to be completely discharged into the reference gas introduction space and then completely discharged from the reference gas introduction space. As a result, it is considered that the time from when the gas sensor is started until accurate measurement can begin (i.e., the start-up time) becomes long.

Therefore, the present invention aims to provide a sensor element that does not cause peeling in the internal structure of the sensor element and has a short start-up time from when the gas sensor is started until accurate measurement can begin.

The inventors have discovered that by forming a pressure relaxation space between a heater-containing layer consisting of a heater and a heater insulator embedded in a base portion containing a solid electrolyte of the sensor element and the base portion, peeling of the internal structure of the sensor element can be suppressed and the start-up time of the gas sensor can be shortened.

The present invention includes the following inventions.
(1) A long plate-like substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers;
a measurement gas flow section for introducing a measurement gas from one end of the base section in a longitudinal direction thereof and allowing the measurement gas to flow therethrough;
The measurement gas flow portion includes a heater-containing layer including a heater including a heater heat generating portion and a heater lead portion, and a heater insulator, the heater-containing layer being embedded in the base portion via at least one of the plurality of solid electrolyte layers;
a pressure relaxation space formed at least partially between the base portion and the heater-containing layer;
A sensor element for detecting a target gas in a measurement gas, comprising:

(2) The pressure relief space is
The base portion;
the heater-containing layer in an area where the heater heat generating portion is present;
The sensor element according to (1) above, wherein the sensor element is formed at least partially between the sensor elements.

(3) A sensor element described in (1) or (2), wherein the pressure relaxation space is formed adjacent to the surface of the heater-containing layer closer to the measured gas flow portion.

(4) A sensor element described in any of (1) to (3) above, wherein the pressure relaxation space is formed in contact with the surface of the heater-containing layer that is farther from the measured gas flow portion.

(5) A sensor element described in any of (1) to (4) above, wherein the pressure relaxation space is formed adjacent to the side of the heater-containing layer.

(6) A sensor element described in any of (1) to (5) above, wherein in a cross section perpendicular to the longitudinal direction of the base portion, the ratio of the cross-sectional area of the pressure relaxation space to the cross-sectional area of the heater-containing layer is 0.10 or more.

(7) A sensor element described in any of (1) to (6) above, wherein in a cross section perpendicular to the longitudinal direction of the base portion, the ratio of the cross-sectional area of the pressure relaxation space to the cross-sectional area of the heater-containing layer is 0.80 or less.

(8) A sensor element described in any of (1) to (7) above, wherein in a cross section perpendicular to the longitudinal direction of the base portion, the ratio of the cross-sectional area of the pressure relaxation space to the cross-sectional area of the heater-containing layer is 0.3 or more and 0.6 or less.

(9) The sensor element described in any of (1) to (8) above further includes a reference gas introduction space that is separated from each of the measured gas flow portion and the heater-containing layer, has an opening at the other longitudinal end of the base portion, and is formed to extend in the longitudinal direction of the base portion.

(10) A sensor element described in (9) above, wherein the pressure relaxation space is not open to the reference gas introduction space.

(11) A sensor element described in any of (1) to (8) above, wherein the pressure relaxation space is a space closed inside the base portion.

(12) A sensor element described in (9) above, wherein the pressure relaxation space is a space that is open to the outside of the base portion through a space other than the reference gas introduction space.

According to the present invention, it is possible to suppress peeling of the internal structure of the sensor element and shorten the start-up time of the gas sensor. When the gas sensor is started in a state where liquid components such as moisture are present inside the heater insulator itself, which is a porous body, or at the interface with the heater heating element, the moisture etc. rapidly evaporates due to the heat generated by the heater. However, gas components such as water vapor generated by the evaporation can move from the inside of the heater insulator itself or the interface with the heater heating element to the pressure relaxation space, so that the pressure increase near the heater insulator can be suppressed. As a result, peeling of the internal structure of the sensor element can be suppressed. Therefore, by using the sensor element according to the present invention, the sensor element is less likely to be destroyed by peeling of its internal structure and can withstand repeated use, improving the durability of the gas sensor.

Furthermore, according to the present invention, the pressure relaxation space may be a closed space inside the base portion of the sensor element. Therefore, gas components such as water vapor generated by the heat of the heater do not flow into the reference gas introduction space, and do not fluctuate the oxygen concentration in the reference gas. As a result, the start-up time of the gas sensor can be shortened. Therefore, by using the sensor element according to the present invention, accurate measurement can be started immediately after the gas sensor is started up.

In addition, according to the present invention, the presence of the pressure relaxation space can further improve the insulation between the heater and the solid electrolyte constituting the base. By further improving the insulation, the leakage current from the heater to the solid electrolyte can be further reduced, and the signal accuracy of the electrical signal when detecting the target gas can also be improved.

1 is a schematic vertical cross-sectional view showing an example of a schematic configuration of a gas sensor 100 in a longitudinal direction. 2 is a schematic vertical cross-sectional view in the longitudinal direction showing another example of the schematic configuration of the gas sensor 100. The sensor element 101 in Fig. 2 is a modified example having a measurement gas flow portion having a different configuration from the sensor element 101 in Fig. 1. 2 is a schematic diagram showing an example of a schematic planar arrangement of a heater 72 (heater heat generating portion 72a and heater lead portion 72b) and a pressure relaxation space 76. FIG. 13 is a schematic diagram showing another example of a schematic planar arrangement of the heater 72 (heater heat generating portion 72a and heater lead portion 72b) and the pressure relaxation space 76. FIG. 1. That is, it is a schematic vertical cross-sectional view perpendicular to the longitudinal direction of the sensor element 101, and is a schematic view showing an example of a pressure relaxation space 76. 13 is a schematic diagram showing another example of the pressure relaxation space 76 of the sensor element 101. FIG. 13 is a schematic diagram showing another example of the pressure relaxation space 76 of the sensor element 101. FIG. 13 is a schematic diagram showing another example of the pressure relaxation space 76 of the sensor element 101. FIG. FIG. 2 is a schematic plan view showing the position of a cut section in an example. FIG. 11 is a schematic vertical cross-sectional view perpendicular to the longitudinal direction of a conventional sensor element.

The sensor element of the present invention comprises:
A long plate-like base portion including a plurality of stacked oxygen ion conductive solid electrolyte layers;
a measurement gas flow section for introducing a measurement gas from one end of the base section in a longitudinal direction thereof and allowing the measurement gas to flow therethrough;
The measurement gas flow portion includes a heater-containing layer including a heater including a heater heat generating portion and a heater lead portion, and a heater insulator, the heater-containing layer being embedded in the base portion via at least one of the plurality of solid electrolyte layers;
a pressure relaxation space formed at least partially between the base portion and the heater-containing layer;
Includes.

[Outline of gas sensor configuration]
The sensor element of the present invention will be described below with reference to the drawings. Fig. 1 is a schematic vertical cross-sectional view in the longitudinal direction showing an example of a schematic configuration of a gas sensor 100 including a sensor element 101. In the following, with reference to Fig. 1, the upper side of Fig. 1 is referred to as the upper side, the lower side is referred to as the lower side, the left side of Fig. 1 is referred to as the front end side, and the right side is referred to as the rear end side.

In Figure 1, the gas sensor 100 shows an example of a limiting current type NOx sensor that detects NOx in the measured gas using a sensor element 101 and measures its concentration.

The sensor element 101 is a long plate-like element including a base portion 102 having a structure in which a plurality of oxygen ion conductive solid electrolyte layers are laminated. The long plate-like shape is also referred to as a long plate-like shape or a strip-like shape. The base portion 102 has a structure in which six layers, namely a _first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is made of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO 2 ), are laminated in this order from the bottom as viewed in the drawing. The solid electrolyte forming these six layers is dense and airtight. The six layers may all have the same thickness, or each layer may have a different thickness. The layers are bonded to each other via an adhesive layer made of a solid electrolyte, and the base portion 102 includes the adhesive layer. In FIG. 1, a layer structure made of the six layers is illustrated as an example, but the layer structure in the present invention is not limited to this and may be any layer structure.

The sensor element 101 is manufactured, for example, by stacking ceramic green sheets corresponding to each layer after carrying out predetermined processing and printing a circuit pattern, and then firing the sheets to integrate them.

A measured gas inlet 10 is formed at one end (hereinafter referred to as the tip) in the longitudinal direction 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 section is formed by adjacently forming, in the longitudinal direction from the measured gas inlet 10, a first diffusion rate-controlling section 11, a buffer space 12, a second diffusion rate-controlling section 13, a first internal space 20, a third diffusion rate-controlling section 30, and a second internal space 40 in this order in a manner that they are in communication with each other.

The measured gas inlet 10, the buffer space 12, the first internal space 20, and the second internal space 40 are spaces inside the sensor element 101, which are defined by a hollowed-out space in the spacer layer 5, with an upper portion defined by the underside of the second solid electrolyte layer 6, a lower portion defined by the upper surface of the first solid electrolyte layer 4, and a side portion defined by the side surface of the spacer layer 5.

The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13 and the third diffusion rate-controlling section 30 are all provided as two horizontally elongated slits (with the opening having its longitudinal direction perpendicular to the drawing in Figure 1).

Furthermore, a reference gas introduction space 43 is provided at a position farther from the tip side than the measured gas flow portion, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and at a position partitioned at the side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

The air introduction layer 48 is a layer made of porous alumina, and a reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is also formed to cover the reference electrode 42.

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, and as described above, an air introduction layer 48 connected to the reference gas introduction space 43 is provided around it. In addition, as described below, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40 using the reference electrode 42.

In the measured gas flow section, the gas inlet 10 is a portion that opens to the external space, and the measured gas is taken into the sensor element 101 from the external space through the gas inlet 10.

The first diffusion rate-controlling section 11 is a section that imparts a predetermined diffusion resistance to the measured gas taken in through the gas inlet 10.

The buffer space 12 is a space provided to guide the measured gas introduced from the first diffusion rate-controlling section 11 to the second diffusion rate-controlling section 13.

The second diffusion rate-controlling section 13 is a section that provides a predetermined diffusion resistance to the measured gas introduced from the buffer space 12 into the first internal space 20.

When the measured gas is introduced from the outside of the sensor element 101 into the first internal space 20, the measured gas is suddenly taken into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the measured gas in the external space (exhaust pressure pulsations if the measured gas is automobile exhaust gas), but is not introduced directly into the first internal space 20. Instead, the pressure fluctuations of the measured gas are canceled through the first diffusion rate-controlling section 11, the buffer space 12, and the second diffusion rate-controlling section 13, and then the measured gas is introduced into the first internal space 20. As a result, the pressure fluctuations of the measured gas introduced into the first internal space are almost negligible.

The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the second diffusion-controlling section 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

The main pump cell 21 is an electrochemical pump cell comprising an inner pump electrode 22 having a ceiling electrode portion 22a provided over almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 provided in a region corresponding to the ceiling electrode portion 22a on the upper surface of the second solid electrolyte layer 6 in a manner that exposes it to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20, and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Then, side electrode portions 22c (not shown in FIGS. 5 and 1) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that constitute both side walls of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a tunnel-shaped structure at the arrangement portion of the side electrode portion.

The inner 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 ). The inner pump electrode 22, which comes into contact with the measurement gas, is formed using a material with a weakened ability to reduce the NOx component in the measurement gas.

In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 by a variable power supply 24, and a pump current Ip0 is passed in a positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, thereby making it possible to pump oxygen from the first internal space 20 out to the external space, or to pump oxygen from the external space into the first internal space 20.

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

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

The third diffusion rate-controlling section 30 is a section that imparts a predetermined diffusion resistance to the measured gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the main pump cell 21 in the first internal space 20, and directs the measured gas to the second internal space 40.

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

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

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

The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to the inner pump electrode 22 disposed in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40, and side electrodes (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 that provides the side walls of the second internal space 40, forming a tunnel-shaped structure.

Like the inner pump electrode 22, the auxiliary pump electrode 51 is formed using a material that has a weakened reduction ability for the NOx components in the measured gas.

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

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

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

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

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

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

The fourth diffusion rate-controlling part 45 is a film made of a porous body mainly composed of alumina (Al ₂ O ₃ ). The fourth diffusion rate-controlling part 45 serves to limit the amount of NOx flowing into the measurement electrode 44 and also functions as a protective film for the measurement electrode 44.

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

In order to detect the oxygen partial pressure around the measurement electrode 44, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump, is configured 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 oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

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

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

In addition, an electrochemical sensor cell 83 is formed from 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 electromotive force Vref obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measured gas outside the sensor.

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

The main pump cell 21, the oxygen partial pressure detection sensor cell 80 for controlling the main pump, the auxiliary pump cell 50, the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, the measurement pump cell 41, the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump, the sensor cell 83, and the measurement gas flow section are collectively referred to as the gas detection section. The configuration of the gas detection section is not particularly limited, and it may be configured to detect the target gas component by utilizing the oxygen ion conductivity of the solid electrolyte.

In FIG. 1, the measurement gas flow section has two internal spaces (first internal space 20 and second internal space 40), but the structure of the measurement gas flow section is not limited to this. FIG. 2 is a vertical cross-sectional view showing another example of the general configuration of the gas sensor 100. The sensor element 101 in FIG. 2 is a modified example in which the measurement gas flow section has a different configuration from the sensor element 101 in FIG. 1. As illustrated in FIG. 2, the second internal space 40 may be further divided into two chambers by the fifth diffusion control section 60 to form a third internal space 61. In this case, the auxiliary pump electrode 51 may be disposed in the second internal space, and the measurement electrode 44 may be disposed in the third internal space. In addition, when the third internal space 61 is formed (three-chamber structure), the fourth diffusion control section 45 may not be provided. In FIG. 2, the same reference numerals are used for the same parts as in FIG. 1, and therefore the description will be omitted.

Furthermore, the sensor element 101 is provided with a heater section 70 that adjusts the temperature by heating and keeping the sensor element 101 warm in order to increase the oxygen ion conductivity of the solid electrolyte. In addition, the sensor element 101 is provided with a pressure relief space 76 to suppress the occurrence of peeling of the internal structure of the sensor element. This will be explained in detail below.

[Heater section]
The heater section 70 includes a heater electrode 71 , heaters 72 ( 72 a , 72 b ), a through hole 73 , and a heater insulator 74 .

The heater electrode 71 is an electrode formed in contact with the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to an external power source, it is possible to supply power to the heater section 70 from the outside.

3 is a schematic diagram showing an example of a general planar arrangement of the heater 72 and the pressure relaxation space 76 described later. The heater 72 includes a heater heat generating portion 72a and a heater lead portion 72b connected to the heater heat generating portion 72a and extending to the longitudinal rear end side of the sensor element 101. Referring to FIG. 1, the heater heat generating portion 72a is an electrical resistor formed in a manner sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater heat generating portion 72a is connected to the heater electrode 71 via the heater lead portion 72b and the through hole 73, and generates heat when power is supplied from the outside through the heater electrode 71, heating and keeping warm the solid electrolyte that forms the sensor element 101.

The heater heating portion 72a is embedded so as to face the entire area in the element thickness direction from the first internal space 20 to the second internal space 40 over almost the entire area, and it is possible to adjust the gas detection portion of the sensor element 101 to a temperature at which the solid electrolyte is activated. As shown in FIG. 3, the heater heating portion 72a is a single meandering linear electric resistor when viewed from above. In FIG. 3, the heater heating portion 72a is the portion of the heater 72 surrounded by the dashed line. The two heater lead portions 72b are connected to both ends of the heater heating portion 72a, respectively, and extend toward the longitudinal rear end of the sensor element 101 (FIG. 3). The shape of the heater heating portion 72a, such as the number of meanders and line width, can be set arbitrarily so that the gas detection portion of the sensor element 101 can be adjusted to a predetermined temperature. In addition, it is not necessary for the entire gas detection portion to be adjusted to the same temperature, and there may be a temperature distribution in the gas detection portion.

A modified example of the heater 72 is shown in FIG. 4. The heater heating portion 72a may have a so-called comb shape as shown in FIG. 4. Both ends of the heater heating portion 72a are thickened in a generally triangular shape in a plan view, and each is connected to a heater lead portion 72b. In FIG. 4, the heater heating portion 72a is the portion of the heater 72 surrounded by a dashed line. The shape of the heater heating portion 72a, such as the length and line width, can be set arbitrarily so that the gas detection portion of the sensor element 101 can be adjusted to a predetermined temperature. In addition, the entire gas detection portion does not need to be adjusted to the same temperature, and there may be a temperature distribution in the gas detection portion.

The heater insulator 74 is an insulating layer formed of an insulator such as alumina in layers on the upper and lower surfaces of the heater 72. The heater insulator 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72, and between the third substrate layer 3 and the heater 72. The heater insulator 74 is a porous body. Although not shown in FIG. 3, the heater insulator 74 is present in an area that covers the entire heater 72 (heater heat generating portion 72a and heater lead portion 72b), and its planar shape is approximately rectangular.

Of the heater section, the heater 72 and heater insulator 74 embedded between the second substrate layer 2 and the third substrate layer 3 in a manner sandwiched from above and below are referred to as the heater-containing layer 75.

The thickness of the heater heating portion 72a may be appropriately set to a desired electrical resistance value. The thickness of the heater insulator 74 may be appropriately set within a range in which electrical insulation between the second substrate layer 2 and the heater 72 and electrical insulation between the third substrate layer 3 and the heater 72 are maintained. For example, the thickness of the heater-containing layer 75 may be 15 to 50 μm. Here, the thickness of the heater-containing layer 75 is the combined thickness of the heater 72 and the heater insulator 74 in the portion where the heater 72 is present. The length of the heater-containing layer 75 in the longitudinal direction may be shorter than the length of the sensor element, and may be, for example, 40 to 80 mm. The length of the heater-containing layer 75 in the width direction perpendicular to the longitudinal direction may be shorter than the width of the sensor element, and may be, for example, 2 to 5 mm. In addition, the heater-containing layer 75 in the region including the heater heating portion 72a may have a longitudinal length of, for example, 2 to 15 mm.

[Pressure relief space]
The sensor element 101 includes a pressure relaxation space 76. As described above, Fig. 3 is a schematic diagram showing an example of a general planar arrangement of the heater 72 and the pressure relaxation space 76. In Fig. 3, the area of the pressure relaxation space 76 in a plane is illustrated by a dashed line. Fig. 5 is a schematic cross-sectional view taken along line VV in Fig. 1. That is, it is a schematic vertical cross-sectional view perpendicular to the longitudinal direction of the sensor element 101, and shows an example of the pressure relaxation space 76.

The pressure relaxation space 76 is formed in a manner sandwiched between the lower surface of the third substrate layer 3 constituting the base portion 102 of the sensor element 101 and the upper surface of the heater insulator 74. That is, it is formed in contact with the surface 75a of the heater-containing layer 75 that is closer to the measurement gas flow portion. As shown in FIG. 5, the side surfaces of the heater insulator 74 and the pressure relaxation space 76 are each 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, and is a layer composed of an oxygen ion conductive solid electrolyte, like the second substrate layer 2 and the third substrate layer 3. As shown by the dashed line in FIG. 3, the pressure relaxation space 76 is arranged on the upper surface of the heater insulator 74 (not shown in FIG. 3) so as to cover almost the entire area of the heater heat generating portion 72a in a plane. The pressure relaxation space 76 is a layered space extending in the longitudinal direction of the sensor element 101 , and is a closed space inside the base portion 102 of the sensor element 101 .

Since the heater insulator 74 is a porous body, liquid components such as moisture may accumulate inside the heater insulator 74 or at the interface with the heater heating portion 72a. When the gas sensor is in use, the sensor element becomes hot due to the heat generated by the heater heating portion 72a, but when the gas sensor is not in use (when the sensor element is not heated), the sensor element is at the same temperature as the ambient temperature. Therefore, in addition to gas components such as air, liquid components such as moisture are present inside the heater insulator 74, which is a porous body, and at the interface with the heater heating portion 72a. If liquid components such as moisture are present inside the heater insulator 74 or at the interface with the heater heating portion 72a when the gas sensor is started, the liquid components such as moisture will rapidly evaporate due to the heat generated by the heater heating portion 72a. Gas components such as water vapor generated by evaporation can move from the inside of the heater insulator 74 or the interface with the heater heating portion 72a to the pressure relaxation space 76, so that the pressure rise near the heater insulator 74 can be suppressed. As a result, it is believed that it is possible to suppress peeling of the sensor element 101 between two adjacent solid electrolyte layers, mainly between the second substrate layer 2 and the third substrate layer 3. Therefore, by using the sensor element 101 having the pressure relaxation space 76, the sensor element 101 is less likely to be destroyed due to peeling of its internal structure and can withstand repeated use, thereby improving the durability of the gas sensor 100.

The pressure relief space 76 only needs to be able to relieve pressure caused by evaporation of liquid components such as moisture present inside the heater insulator 74 and at the interface with the heater heat generating portion 72a. The thickness of the pressure relief space 76 can be, for example, 10 to 50 μm. The longitudinal length of the pressure relief space 76 can be, for example, 2 to 15 mm. The widthwise length of the pressure relief space 76 perpendicular to the longitudinal direction only needs to be shorter than the width of the sensor element, and can be, for example, 2 to 5 mm.

The pressure relaxation space 76 may be present at least partially between the solid electrolyte constituting the base portion 102 and the heater-containing layer 75. The pressure relaxation space 76 may be arranged so as to cover almost the entire area of the heater heat generating portion 72a in a plane, or may be arranged so as to cover a part of the heater heat generating portion 72a. It may be arranged so as to cover all or a part of the heater lead portion 72b. The pressure relaxation space 76 may be present only in the area of the heater heat generating portion 72a, or may extend from the area of the heater heat generating portion 72a to the rear end of the sensor element 101 including the heater lead portion 72b. Alternatively, the pressure relaxation space 76 may be present only in the area of the heater lead portion 72b. The pressure relaxation space 76 may be a single continuous space, or may be composed of multiple spaces independent of each other.

In Figure 5, a configuration is described in which the pressure relaxation space 76 is formed adjacent to the surface 75a of the heater-containing layer 75 that is closer to the measured gas flow portion.

As another form of the pressure relief space 76, as shown in Fig. 6, the pressure relief space 76 may 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 farther from the measurement gas flow portion). In this case, the pressure relief space 76 is a layered space extending in the longitudinal direction of the sensor element 101.

In this way, when the pressure relaxation space 76 is formed in contact with the surface 75b of the heater-containing layer 75 that is far from the measurement gas flow portion, the strength (water resistance) of the sensor element 101 when water is splashed on it during use of the gas sensor (when the sensor element is kept at a high temperature) can be improved. It is presumed that the pressure relaxation space acts to relieve stress that occurs when water is splashed on the surface of the high-temperature sensor element.

Alternatively, as another form of the pressure relief space 76, as shown in FIG. 7, the pressure relief space 76 may be formed on the side of the heater-containing layer 75, i.e., between the side surface 75c of the heater-containing layer 75 and the inner side surface 90a of the adhesive layer 90 (i.e., 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 the cross section perpendicular to the longitudinal direction may be, for example, rectangular (FIG. 7) or any other shape. The pressure relief space 76 may be present on both sides of the heater-containing layer 75, or on only one side.

8, the pressure relief space 76 may be formed so that the upper and side surfaces of the heater heat generating portion 72a are exposed to the pressure relief space 76. In this case, the heater-containing layer 75 is composed of a heater insulator 74 and a heater 72. Only the upper surface of the heater heat generating portion 72a may be exposed to the pressure relief space 76.

The size (volume) of the pressure relaxation 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 ratio R (A 76 /A ₇₅ ) of the cross-sectional area A 76 of the pressure relaxation space 76 to the cross-sectional area A ₇₅ of the heater-containing layer 75 (i.e., the sum of the cross-sectional area of the heater ₇₂ and the cross _- sectional area of the heater insulator 74) may be 0.10 or more as a lower limit. 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, the cross-sectional area ratio R may be 0.80 or less as an upper limit. Preferably, it may be 0.60 or less. The upper limit may be set from the viewpoint of the adhesion strength between the second substrate layer 2 and the third substrate layer 3 of the sensor element. A preferable range may be 0.10 or more and 0.80 or less, or 0.30 or more and 0.80 or less. More preferably, it may be set to 0.30 or more and 0.60 or less. The above-mentioned cross-sectional area ratio R may be set in various arrangements of the pressure relaxation space 76 as exemplified in Figures 5 to 8. 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.

Here, the ratio R (A ₇₆ /A ₇₅ ) of the cross-sectional area A ₇₆ of the pressure relaxation space 76 to the cross-sectional area A ₇₅ of the heater-containing layer 75 can be calculated as follows. A vertical cross section perpendicular to the longitudinal direction of the sensor element 101 is polished and photographed by an SEM. The obtained SEM image is then binarized to obtain the cross-sectional area A ₇₅ of the heater-containing layer 75. Furthermore, the binarization conditions are changed to obtain the cross-sectional area A ₇₆ of the pressure relaxation space 76. The cross-sectional area ratio R=A ₇₆ /A 75 is calculated from the cross-sectional area A ₇₅ of the heater-containing layer 75 and the cross-sectional area A ₇₆ of the pressure relaxation space ₇₆ thus obtained.

1, the pressure relaxation space 76 is a closed space inside the sensor element 101. Therefore, even if liquid components such as moisture present inside the heater insulator 74 or at the interface with the heater heat generating portion 72a evaporate and gas components such as water vapor are generated, they are only released into the pressure relaxation space 76 and do not affect the reference gas introduction space 43. In other words, the start-up time from when the gas sensor is started to when accurate measurement can be started is not affected, and the start-up time of the gas sensor can be shortened. Alternatively, the pressure relaxation space 76 may be open to the outside of the base portion 102 through a space other than the reference gas introduction space 43. If the pressure relaxation space 76 is open to the outside of the base portion 102, the pressure due to gas components such as water vapor generated can be further reduced. If it is open through a space other than the reference gas introduction space 43, the gas components such as water vapor generated do not directly affect the reference gas introduction space 43. Therefore, even in this case, it is considered that the start-up time of the gas sensor can be shortened.

The pressure relaxation space 76 can further provide the following effects. The presence of the pressure relaxation space 76 provides a space between the solid electrolyte constituting the base portion 102 and the heater in addition to the heater insulator 74, so that the insulation between the solid electrolyte and the heater can be further improved. This further improvement in insulation can further reduce the leakage current from the heater to the solid electrolyte, improving the signal accuracy of the electrical signal when detecting the target gas.

[Sensor element manufacturing method]
Next, an example of a method for manufacturing the above-mentioned sensor element will be described. After _performing predetermined processing and printing of a circuit pattern on a plurality of unfired sheet-shaped molded products (so-called green sheets) containing an oxygen ion conductive solid electrolyte such as zirconia (ZrO 2 ) as a ceramic component, the plurality of sheets are stacked, the stack is cut, and then fired to produce the sensor element 101.

In the following, we will use as an example the case of manufacturing a sensor element 101 consisting of six layers as shown in Figure 1.

First, six green sheets containing an oxygen ion conductive solid electrolyte such as zirconia (ZrO ₂ ) as a ceramic component are prepared. A known forming method can be used to prepare the green sheets. All six green sheets may have the same thickness, or the thickness may differ depending on the layer to be formed. Each of the six green sheets is previously formed with sheet holes or the like for positioning during printing or lamination by a known method such as punching processing with a punching device (blank sheet). In the blank sheet used for the spacer layer 5, a through portion such as an internal void is also formed by the same method. Necessary through portions are also previously formed in other layers.

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 are printed and dried with various patterns required for each layer. A known screen printing technique can be used to print the patterns. A known drying method can also be used for the drying process.

The formation of the pressure relaxation space 76 will be described in detail below in the case of producing a sensor element 101 in the form shown in Figure 5. First, prepare a blank sheet to be used for the second substrate layer 2. At this time, the blank sheet to be used for the second substrate layer 2 may be prepared in a state in which it is pre-laminated with the blank sheet to be used for the first substrate layer 1.

A heater-containing layer 75 is formed by printing on a blank sheet used for the second substrate layer 2. A paste for forming the heater insulator 74 (heater insulator paste) is printed and dried in a predetermined pattern. Depending on the desired thickness, printing and drying may be repeated multiple times. A paste for forming the heater 72 (heater paste) is printed and dried in a predetermined pattern on the printed heater insulator paste. When forming the heater 72, the heater heat generating portion 72a and the heater lead portion 72b may be printed with the same heater paste or may be printed with different heater pastes. Depending on the desired thickness, printing and drying may be repeated multiple times, or the heater heat generating portion 72a and the heater lead portion 72b may be printed and dried different times. Furthermore, a heater insulator paste is printed and dried in a predetermined pattern on the printed heater paste. Depending on the desired thickness, printing and drying may be repeated multiple times.

Next, the pressure relief space 76 is formed. A paste for forming the pressure relief space 76 (a paste made of a material that disappears when fired in a later process) is printed and dried in a predetermined pattern on the printed heater-containing layer 75. Depending on the desired thickness, printing and drying may be repeated multiple times.

In addition, a paste for forming adhesive layer 90 (adhesive paste) is printed and dried in a predetermined pattern on a blank sheet used for second substrate layer 2. Depending on the desired thickness, printing and drying may be repeated multiple times. Adhesive layer 90 is a layer for adhering second substrate layer 2 and third substrate layer 3, and preferably has a thickness approximately equal to the combined thickness of heater-containing layer 75 and pressure relief space 76.

The heater insulator paste is made by mixing Al ₂ O ₃ with a resin and an organic solvent to adjust the viscosity to a predetermined value. The heater paste is made by mixing Pt as the main component with a resin and an organic solvent to adjust the viscosity to a predetermined value. The vanishing material paste is made by mixing a material that vanishes by firing in a later process with an organic solvent to adjust the viscosity to a predetermined value. The vanishing material paste may be any paste that vanishes by firing in a later process. For example, theobromine, acrylic resin, carbon, etc. may be used. The adhesive paste includes a solid electrolyte. The same solid electrolyte as that of the second substrate layer 2 and the third substrate layer 3 may be used. For example, ZrO ₂ is used as the main component with a resin and an organic solvent to adjust the viscosity to a predetermined value.

After printing and drying various patterns on each of the six blank sheets, the six printed blank sheets are stacked in a specified order while being positioned using sheet holes, etc., and then pressed under specified temperature and pressure conditions to form a laminate. The pressing process is carried out by applying heat and pressure using a laminator such as a well-known hydraulic press. The temperature, pressure and time for heating and pressing depend on the laminator used, but can be determined as appropriate to achieve good lamination.

The obtained laminate contains a plurality of sensor elements 101. The laminate is cut into units of sensor elements 101. The cut laminate is fired at a prescribed firing temperature to obtain the sensor elements 101. The firing temperature may be any temperature at which the solid electrolyte constituting the base portion 102 of the sensor element 101 is sintered into a dense body and the electrodes etc. maintain the desired porosity. For example, firing is performed at a firing temperature of about 1300 to 1500°C. As described above, the vanishing material paste is vanished by firing, and the area where the vanishing material paste was present becomes a space, forming the pressure relaxation space 76.

As shown in Figure 6, when forming a pressure relief space 76 between the second substrate layer 2 and the heater-containing layer 75, a vanishing material paste for forming the pressure relief space 76 is first printed in a predetermined pattern on a blank sheet used for the second substrate layer 2 and dried. The heater-containing layer 75 is then formed on top of the blank sheet by printing as described above. The pressure relief space 76 is then formed by firing.

As shown in Figure 7, when the pressure relief space 76 is formed adjacent to the side surface 75c of the heater-containing layer 75, a vanishing material paste for forming the pressure relief space 76 and a heater insulator paste for forming the heater insulator 74 are printed and dried in a predetermined pattern on a blank sheet used for the second substrate layer 2. The vanishing material paste is printed so as to extend in the longitudinal direction of the sensor element 101 along the inside of the adhesive layer 90. Heater paste and heater insulator paste are further printed and dried on top of the heater insulator paste to form the heater-containing layer 75. The pressure relief space 76 is then formed by firing.

In the case of an embodiment in which the heater heat generating portion 72a is exposed in the pressure relief space 76 as shown in Figure 8, a heater insulator paste is printed and dried, and then a heater paste is printed and dried in a predetermined pattern. A vanishing material paste for forming the pressure relief space 76 is then printed and dried on top of that. The pressure relief space 76 is then formed by firing.

5, 6, 7, and 8 show examples of the arrangement of the pressure relief space 76 inside the sensor element 101, but the present invention is not limited to the forms shown in the above examples. The present invention may include sensor elements including pressure relief spaces 76 of various forms as long as the object of the present invention, which is to suppress peeling of the internal structure of the sensor element and to shorten the start-up time of the gas sensor, is achieved.

Below, we will explain examples of how sensor elements were specifically fabricated and tested. Note that the present invention is not limited to the following examples.

[Examples 1 to 6]
In Examples 1 to 6, a sensor element (FIG. 5) was fabricated in which a pressure relaxation space 76 was formed between the heater insulator 74 and the third substrate layer 3 (i.e., the surface 75a of the heater-containing layer 75 closer to the measured gas flow portion) according to the manufacturing method of the sensor element 101 described above. The vanishing material paste used was a mixture of theobromine and an organic solvent adjusted to a predetermined viscosity. The pressure relaxation space 76 was formed in an area covering almost the entire heater heat generating portion 72a. The width direction lengths of the pressure relaxation space 76 and the heater insulator 74 were almost the same. In Examples 1 to 6, the thickness of the heater-containing layer 75 was kept constant, and the thickness of the pressure relaxation space 76 was changed to six different values.

In a vertical cross section perpendicular to the longitudinal direction of the sensor element 101 and including the heater heat generating portion 72a, the ratios R of the cross-sectional area _A76 of the pressure relaxation space ₇₆ to the cross-sectional area A75 of the heater-containing layer 75 were 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.

The cross-sectional area ratio R was calculated as follows. First, for Example 1, polished cross sections were prepared by polishing cut cross sections at three locations perpendicular to the longitudinal direction of the sensor element 101 and including the heater heat generating portion 72a. That is, referring to FIG. 9, three polished cross sections were prepared at the tip (S1), center (S2), and rear end (S3) of the heater heat generating portion 72a in the longitudinal direction of the sensor element 101. SEM images (backscattered electron images, magnification 30 times, approximately 1.3 million pixels) of each of the three polished cross sections were taken. The magnification of the SEM images may be changed as appropriate depending on the size of the sensor element 101, etc. Next, one of the captured SEM images was binarized by adjusting the threshold to the average value of RGB using image processing software Pick Map (URL: https://fishers.mydns.jp/software/pickmap/index.html) so that only the heater-containing layer 75 (i.e., the heater heat generating portion 72a and the heater insulator 74) in the SEM image was extracted. The latest version of the image processing software Pick Map can be obtained and used from the above URL. Furthermore, the cross-sectional area A ₇₅ of the heater-containing layer 75 in the binarized SEM image was calculated using the image processing software Pick Map. Similarly, the SEM image of the pressure relaxation space 76 was binarized using the image processing software Pick Map to calculate the cross-sectional area A ₇₆ . Thereafter, the cross-sectional area ratio R: (cross-sectional area A ₇₆ of the pressure relaxation space 76)/(cross-sectional area A ₇₅ of the heater-containing layer 75) was calculated from the calculated cross-sectional area A 75 of the heater-containing layer 75 and the cross-sectional area A ₇₆ of the pressure relaxation space _76. 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 ratios R were also calculated in the same manner for Examples 2 to 6.

[Example 7]
As Example 7, a sensor element was fabricated in which a pressure relaxation space 76 was formed between the second substrate layer 2 and the heater insulator 74 (i.e., the surface 75b of the heater-containing layer 75 on the side farther from the measurement gas flow portion) (FIG. 6). The planar shape of the pressure relaxation space 76 was the same as in Examples 1 to 6. Except for the formation of the pressure relaxation space 76, the sensor element was fabricated in the same manner as in Examples 1 to 6. In a vertical cross section perpendicular to the longitudinal direction of the sensor element 101 and including the heater heat generating portion 72a, the ratio R (cross-sectional area ratio) of the cross-sectional area A ₇₆ of the pressure relaxation space 76 to the cross-sectional area A ₇₅ of the heater-containing layer 75 was 0.31 (Example 7). The cross-sectional area ratio R was calculated in the same manner as in Examples 1 to 6.

[Example 8]
As Example 8, a sensor element was fabricated in which a pressure relaxation 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-containing layer 75) (FIG. 7). The pressure relaxation space 76 was a space extending in the longitudinal direction of the sensor element on both sides of the heater-containing layer 75, and the longitudinal length was the same as in Examples 1 to 7. Except for the formation of the pressure relaxation space 76, the sensor element was fabricated in the same manner as in Examples 1 to 6. In a vertical cross section perpendicular to the longitudinal direction of the sensor element 101 and including the heater heat generating portion 72a, the ratio R (cross-sectional area ratio) of the cross-sectional area A ₇₆ of the pressure relaxation space 76 to the cross-sectional area A ₇₅ of the heater-containing layer 75 was 0.40 (Example 8). The cross-sectional area ratio R was calculated in the same manner as in Examples 1 to 6.

[Comparative Example 1]
As Comparative Example 1, a sensor element without a pressure relaxation space 76 was produced (FIG. 10). The sensor element was produced in the same manner as in Examples 1 to 6, except that the pressure relaxation space 76 was not formed.

In addition, as shown in Figures 5 and 6, when the pressure relaxation space 76 is formed to extend in the longitudinal direction of the sensor element 101 along the upper or lower surface of the heater-containing layer 75 (Examples 1 to 7), the above-mentioned cross-sectional area ratio R is substantially the same as the cross-sectional area ratio in a vertical cross section along the longitudinal direction of the sensor element 101 as shown in Figure 1.

[Measurement of peeling voltage]
Using the sensor elements obtained in Examples 1 to 8 and Comparative Example 1, the peeling voltage was measured. First, the rear end side of the sensor element, i.e., the side opposite to the front end side where the gas inlet is located, was immersed in water for 4 hours. After wiping off the moisture adhering to the surface, a voltage of 6 V was applied to the heater 72 of the sensor element 101 for 30 seconds to check whether peeling of the sensor element occurred. If peeling did not occur, the applied voltage was increased in 1 V increments up to 12 V to check 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.

Table 1 shows the position of the pressure relaxation space 76 and the cross-sectional area ratio R (A ₇₆ /A ₇₅ ) at each level in Examples 1 to 8 and Comparative Example 1. In addition, for each level, the number of peeled off pieces at each applied voltage (6 V, 7 V, 8 V, 9 V, 10 V, 11 V, and 12 V) is shown.

In Examples 1 to 8, no samples peeled at 6 V. In Examples 2 to 8, no samples peeled at 10 V or less. In Examples 3 and 6, not one of the five samples peeled when 6 to 11 V was applied, and only one sample peeled when 12 V was applied. The remaining four samples did not peel even when 12 V was applied. In the four levels of Examples 4, 5, 7, and 8, none of the five samples peeled even when 6 to 12 V was applied.

In Comparative Example 1, 6 V was applied to the heater for each of the five samples, and in one sample, peeling occurred in the sensor element 101 when 6 V was applied. In the four samples in which no peeling occurred at 6 V, 7 V was applied to the heater for each of the four samples, and all four samples peeled when 7 V was applied.

This peeling voltage measurement test 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, while the heater heating part 72a is usually heated with a controlled temperature curve, in this test, a constant voltage is applied to rapidly heat the heater heating part 72a and rapidly generate water vapor. Therefore, the peeling voltage in this test does not directly and quantitatively estimate the degree of peeling during actual use. However, if the peeling voltage is higher than that of Comparative Example 1, or if peeling does not occur even when the applied voltage is increased, it has been confirmed that peeling is suppressed compared to Comparative Example 1 even during actual use.

In Examples 1 to 8, due to the presence of the pressure relaxation space 76, the peeling voltage was higher than in Comparative Example 1, and peeling of the sensor element 101 was suppressed. Furthermore, as can be seen from the results of Examples 4, 7, and 8, peeling of the sensor element 101 was suppressed regardless of the position of the pressure relaxation space 76.

Furthermore, the peeling voltage increased as the cross-sectional area ratio increased (i.e., the volume of the pressure relaxation space 76 increased) (Examples 1 to 5). Heat generated by the heater heating portion 72a generates water vapor from inside or at the interface of the heater insulator 74, and it is believed that the larger the volume of the pressure relaxation space 76, the more the pressure rise is suppressed, and the more the peeling of the sensor element 101 is suppressed. On the other hand, when the cross-sectional area ratio becomes too large, the suppression effect tends to become slightly smaller (Example 6). When the cross-sectional area ratio is too large, the effect of reducing pressure is sufficient, but it is presumed that the space inside the sensor element is too large, which reduces the structural strength of the sensor element and causes peeling between the second substrate layer 2 and the third substrate layer 3 with less stress.

As described above, it has become clear that the presence of the pressure relief space 76 can suppress the occurrence of peeling of the sensor element during use. It has been shown that the occurrence of peeling can be suppressed regardless of whether the pressure relief space 76 is in contact with any surface (upper surface, lower surface, or side surface) of the heater insulator 74.

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 controlling portion 12 Buffer space 13 Second diffusion rate controlling portion 20 First internal space 21 Main pump cell 22 Inner pump electrode 22a Ceiling electrode portion 22b (of inner pump electrode) Bottom electrode portion 22c (of inner pump electrode) Side electrode portion 23 Outer pump electrode 24 Variable power source 30 (of main pump cell) Third diffusion rate controlling portion 40 Second internal space 41 Measurement pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measurement electrode 45 Fourth diffusion rate controlling portion 46 Variable power source 48 (of measurement pump cell) Air introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51a Ceiling electrode portion 51b (of auxiliary pump electrode) Bottom electrode portion 52 (of auxiliary pump electrode) Variable power source 60 (of the auxiliary pump cell) Fifth diffusion rate-controlling portion 61 Third internal space 70 Heater portion 71 Heater electrode 72 Heater 72a Heater heat generating portion 72b Heater lead portion 73 Through hole 74 Heater insulator 75 Heater-containing layer 75a Upper surface 75b (of the heater-containing layer) Lower surface 75c (of the heater-containing layer) Side surface 76 (of the heater-containing layer) Pressure relaxation space 80 Oxygen partial pressure detection sensor cell for controlling main pump 81 Oxygen partial pressure detection sensor cell for controlling auxiliary pump 82 Oxygen partial pressure detection sensor cell for controlling measurement pump 83 Sensor cell 90 Adhesive layer 90a Inner side surface 100 (of the adhesive layer) Gas sensor 101 Sensor element 102 Base portion

Claims (8)

A long plate-like base portion including a plurality of stacked oxygen ion conductive solid electrolyte layers;
a measurement gas flow section for introducing a measurement gas from one end of the base section in a longitudinal direction thereof and allowing the measurement gas to flow therethrough;
The measurement gas flow portion includes a heater-containing layer including a heater including a heater heat generating portion and a heater lead portion, and a heater insulator, the heater-containing layer being embedded in the base portion via at least one of the plurality of solid electrolyte layers;
a pressure relaxation space formed at least partially between the base portion and the heater-containing layer;
Including,
A sensor element for detecting a target gas in a measurement gas , wherein the pressure relaxation space is a space enclosed within the base portion . The pressure relief space is
The base portion;
the heater-containing layer in an area where the heater heat generating portion is present;
The sensor element according to claim 1 , wherein the sensor element is formed at least partially between the first and second electrodes. The sensor element according to claim 1 or 2, wherein the pressure relaxation space is formed in contact with the surface of the heater-containing layer that is closer to the measurement gas flow section. The sensor element according to any one of claims 1 to 3, wherein the pressure relaxation space is formed in contact with the surface of the heater-containing layer that is farther from the measurement gas flow section. The sensor element according to any one of claims 1 to 4, wherein the pressure relief space is formed adjacent to the side of the heater-containing layer. The sensor element according to any one of claims 1 to 5, wherein the ratio of the cross-sectional area of the pressure relaxation space to the cross-sectional area of the heater-containing layer in a cross section perpendicular to the longitudinal direction of the base portion is 0.10 or more. The sensor element according to any one of claims 1 to 6, wherein the ratio of the cross-sectional area of the pressure relaxation space to the cross-sectional area of the heater-containing layer in a cross section perpendicular to the longitudinal direction of the base portion is 0.80 or less. The sensor element according to any one of claims 1 to 7, wherein the ratio of the cross-sectional area of the pressure relaxation space to the cross-sectional area of the heater-containing layer in a cross section perpendicular to the longitudinal direction of the base portion is 0.3 to 0.6.