JP7002610B2 - Gas sensor element - Google Patents

Description

The present invention relates to a gas sensor element and a gas sensor.

Conventionally, a gas sensor including a sensor element for detecting the concentration of a predetermined gas such as NOx in a gas to be measured such as an exhaust gas of an automobile is known. Further, in such a gas sensor, it is known to form a porous protective layer on the surface of the sensor element. For example, Patent Documents 1 and 2 describe that heat-resistant particles such as alumina are attached to the surface of a sensor element by plasma spraying to form a porous protective layer. By forming this porous protective layer, for example, it is possible to suppress cracking of the sensor element due to adhesion of moisture in the gas to be measured.

Japanese Unexamined Patent Publication No. 2013-54025 Patent No. 3766572

It has been desired that the sensor element of such a gas sensor has a high temperature (for example, 800 ° C.) during normal driving, and further suppresses cracking of the sensor element due to rapid cooling due to adhesion of moisture.

The present invention has been made to solve such a problem, and an object of the present invention is to improve the water resistance of a gas sensor element.

The present invention has taken the following measures to achieve the above-mentioned main object.

The first gas sensor element of the present invention is
A device body with an oxygen ion conductive solid electrolyte layer and
A protective layer that covers at least a part of the element body and has a unit thickness direction interface number of 15 or more and 250 or less, which is the number of particle interfaces of constituent particles per 100 μm in the thickness direction.
It is equipped with.

In this first gas sensor element, at least a part of the element body is covered with a protective layer. Here, the larger the number of interface in the unit thickness direction, which is the number of particle interfaces of the constituent particles per 100 μm in the thickness direction of the protective layer, the less likely it is that heat conduction occurs in the thickness direction of the protective layer. That is, there is a tendency that the cooling of the element body when moisture adheres to the surface of the protective layer is suppressed. The reason for this is considered to be that heat conduction between the constituent particles is less likely to occur than heat conduction within the constituent particles. When the number of interfaces in the unit thickness direction is 15 or more, the effect of suppressing the cooling of the element body, that is, the effect of improving the water resistance of the gas sensor element can be obtained. The larger the number of interfaces in the unit thickness direction, the larger the number of constituent particles per unit thickness, and the more difficult it is for the gas to be measured to pass through the protective layer and reach the element body. If the number of interfaces in the unit thickness direction is 250 or less, the gas can pass through the protective layer. In this first gas sensor element, the thickness of the protective layer may be 50 μm or more, or the thickness of the protective layer may be 100 μm or more. Further, the protective layer may have the number of interfaces in the unit thickness direction of 17 or more. The protective layer may have the number of interfaces in the unit thickness direction of 200 or less, 150 or less, 100 or less, and 50 or less.

The second gas sensor element of the present invention is
A device body with an oxygen ion conductive solid electrolyte layer and
At least a part of the element body is covered, and the number of interface in the unit thickness direction, which is the number of interface of constituent particles per 100 μm in the thickness direction, and the particle interface of the constituent particles per 100 μm in the surface direction perpendicular to the thickness direction. A protective layer in which the ratio of the number of interfaces in the unit surface direction (= the number of interfaces in the unit surface direction / the number of interfaces in the unit thickness direction) is more than 0 and 0.7 or less.
It is equipped with.

In this second gas sensor element, at least a part of the element body is covered with a protective layer. Here, the smaller the interface number ratio of the protective layer (= the number of interfaces in the unit surface direction / the number of interfaces in the unit thickness direction), the more the surface direction (thickness direction) of the protective layer rather than the heat conduction in the thickness direction of the protective layer. Heat conduction in the vertical direction) tends to occur. The reason for this is considered to be that heat conduction between the constituent particles is less likely to occur than heat conduction within the constituent particles. As a result, the smaller the interface number ratio, the more it tends to be suppressed that only a part of the element main body is rapidly cooled when moisture adheres to the surface of the protective layer. When the interface number ratio is 0.7 or less, the effect of suppressing the generation of cracks due to the rapid cooling of only a part of the element main body, that is, the effect of improving the water resistance of the gas sensor element can be obtained. The interface number ratio may be a value of 0.6 or less, preferably a value of 0.4 or less and 0.3 or less. The interface number ratio may be a value of 0.15 or more. Further, in the second gas sensor element, the thickness of the protective layer may be 50 μm or more, or the thickness of the protective layer may be 100 μm or more. Further, the number of interfaces in the unit thickness direction may be a value of 15 or more, or may be a value of 250 or less. The number of interfaces in the unit surface direction may be a value of 5 or more, a value of 10.5 or less, and a value of 10 or less.

In the first and second gas sensor elements of the present invention, the protective layer may have the number of interfaces in the unit thickness direction of 30 or more. By doing so, the water resistance of the gas sensor element is further improved.

In the first and second gas sensor elements of the present invention, the protective layer may have a thickness of 500 μm or less. Here, when the thickness of the protective layer is as thin as 500 μm or less, the water resistance of the gas sensor element tends to be insufficient. Since the first gas sensor element of the present invention can improve the water resistance when the number of interfaces in the unit thickness direction of the protective layer is 15 or more, the significance of applying the present invention to such a relatively thin protective layer is significant. Is high. As for the second gas sensor element of the present invention, the water resistance can be improved by setting the interface number ratio to 0.7 or less, so that it is highly significant to apply the present invention to such a relatively thin protective layer.

In the first and second gas sensor elements of the present invention, the protective layer may contain ceramic particles as the constituent particles. Ceramic particles are suitable as a protective layer for gas sensor elements from the viewpoint of strength, heat resistance, and corrosion resistance of at least one.

In the first and second gas sensor elements of the present invention, the protective layer may contain at least one of alumina, zirconia, spinel, cordierite, titania, and magnesia as the constituent particles. Since these materials have high heat resistance, they are suitable as a protective layer for gas sensor elements.

In the first and second gas sensor elements of the present invention, the element body has a long rectangular parallelepiped shape, and the protective layer has a longitudinal end surface of the element body and 4 perpendicular to the end surface. The region from the one end surface side of one surface to the distance L in the longitudinal direction of the element body may be covered (however, 0 <distance L <length in the longitudinal direction of the element body). By covering the five surfaces with the protective layer in this way, the effect of protecting the element main body by the protective layer is enhanced as compared with the case where the protective layer covers only four or less surfaces, for example.

The gas sensor of the present invention
The gas sensor element of any of the above embodiments,
It is equipped with.

This gas sensor includes the gas sensor element of any of the above-described embodiments of the first and second gas sensor elements of the present invention. Therefore, the same effect as that of the first and second gas sensor elements of the present invention described above, for example, the effect of improving the water resistance of the gas sensor element can be obtained. In this case, the gas sensor element may have a rectangular parallelepiped shape in which the element body is long. Further, the gas sensor of the present invention may include a fixing member for fixing the gas sensor element and a protective cover for covering one end of the gas sensor element in the longitudinal direction.

A vertical sectional view of the gas sensor 100. The perspective view which showed the example of the structure of the sensor element 101 schematically. FIG. 2 is a sectional view taken along the line AA of FIG. A conceptual diagram showing how to measure the number of interfaces in the unit thickness direction. Explanatory drawing of plasma spraying using plasma gun 170.

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a vertical sectional view of a gas sensor 100 according to an embodiment of the present invention. FIG. 2 is a perspective view schematically showing an example of the configuration of the sensor element 101. FIG. 3 is a sectional view taken along the line AA of FIG. The sensor element 101 has a long rectangular shape, the longitudinal direction of the sensor element 101 (horizontal direction in FIG. 2) is the front-rear direction, and the thickness direction of the sensor element 101 (vertical direction in FIG. 2) is up and down. The direction. Further, the width direction (direction perpendicular to the front-rear direction and the vertical direction) of the sensor element 101 is defined as the left-right direction. Further, the structure of the gas sensor 100 as shown in FIG. 1 is known and is described in, for example, Japanese Patent Application Laid-Open No. 2012-210637.

The gas sensor 100 includes a sensor element 101, a protective cover 110 that covers and protects the front end side (lower end side in FIG. 1) which is one end side in the longitudinal direction of the sensor element 101, and an element sealing body that encloses and fixes the sensor element 101. It includes a 120 and a nut 130 attached to the element encapsulant 120. As shown in the figure, the gas sensor 100 is attached to a pipe 140 such as an exhaust gas pipe of a vehicle, and is used to measure the concentration of a specific gas such as NOx or O ₂ contained in the exhaust gas as a gas to be measured. .. In the present embodiment, the gas sensor 100 measures the NOx concentration as the specific gas concentration. The sensor element 101 includes a sensor element main body 101a and a porous protective layer 91 that covers the sensor element main body 101a. The sensor element main body 101a refers to a portion of the sensor element 101 other than the porous protective layer 91.

The protective cover 110 includes a bottomed tubular inner protective cover 111 that covers one end of the sensor element 101, and a bottomed tubular outer protective cover 112 that covers the inner protective cover 111. The inner protective cover 111 and the outer protective cover 112 are formed with a plurality of holes for allowing the gas to be measured to flow in the protective cover 110. One end of the sensor element 101 is arranged in a space surrounded by the inner protective cover 111.

The element sealing body 120 includes a cylindrical main metal fitting 122, a ceramic supporter 124 enclosed in a through hole inside the main metal fitting 122, and a talc or the like enclosed in the through hole inside the main metal fitting 122. It is provided with a green compact 126 formed by molding a ceramic powder. The sensor element 101 is located on the central axis of the element encapsulant 120 and penetrates the element encapsulant 120 in the front-rear direction. The green compact 126 is compressed between the main metal fitting 122 and the sensor element 101. As a result, the green compact 126 seals the through hole in the main metal fitting 122 and fixes the sensor element 101.

The nut 130 is fixed coaxially with the main metal fitting 122, and a male screw portion is formed on the outer peripheral surface. The male threaded portion of the nut 130 is inserted into a mounting member 141 welded to the pipe 140 and provided with a female threaded portion on the inner peripheral surface. As a result, the gas sensor 100 can be fixed to the pipe 140 with one end of the sensor element 101 and the portion of the protective cover 110 protruding into the pipe 140.

As shown in FIG. 3, the sensor element 101 includes a first substrate layer 1, a second substrate layer 2, and a third substrate layer 3, each of which is composed of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO ₂ ). , The element has a structure in which six layers of the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6 are laminated in this order from the lower side in the drawing. Further, the solid electrolyte forming these six layers is a dense airtight one. The sensor element 101 is manufactured, for example, by performing predetermined processing, printing of a circuit pattern, or the like on a ceramic green sheet corresponding to each layer, laminating them, and further firing and integrating them.

A gas inlet 10 and a first diffusion between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4 which is one tip portion (end portion in the front direction) of the sensor element 101. In a manner in which the rate-determining section 11, the buffer space 12, the second diffusion rate-controlling section 13, the first internal vacant space 20, the third diffusion rate-controlling section 30, and the second internal vacant space 40 communicate in this order. It is formed adjacent to each other.

The gas introduction port 10, the buffer space 12, the first internal vacant space 20, and the second internal vacant space 40 are provided in such a manner that the spacer layer 5 is hollowed out, and the upper portion thereof is the lower surface of the second solid electrolyte layer 6. The lower part is the upper surface of the first solid electrolyte layer 4, and the side part is the space inside the sensor element 101 partitioned by the side surface of the spacer layer 5.

The first diffusion rate control section 11, the second diffusion rate control section 13, and the third diffusion rate control section 30 are all provided as two horizontally long slits (openings have a longitudinal direction in the direction perpendicular to the drawing). .. The portion from the gas introduction port 10 to the second internal vacant space 40 is also referred to as a gas distribution section.

Further, at a position far from the tip side of the gas flow portion, the side portion is partitioned between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5 by the side surface of the first solid electrolyte layer 4. A reference gas introduction space 43 is provided at such a position. For example, the atmosphere is introduced into the reference gas introduction space 43 as a reference gas for measuring the NOx concentration.

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

The reference electrode 42 is an electrode formed in such a manner that it is sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, the reference electrode 42 is connected to the reference gas introduction space 43 around the reference electrode 42. An atmosphere introduction layer 48 is provided. Further, as will be described later, 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 gas flow section, the gas introduction port 10 is a portion that is open to the external space, and the gas to be measured is taken into the sensor element 101 from the external space through the gas introduction port 10. The first diffusion rate-controlling unit 11 is a portion that imparts a predetermined diffusion resistance to the gas to be measured taken in from the gas introduction port 10. The buffer space 12 is a space provided for guiding the gas to be measured introduced from the first diffusion rate control unit 11 to the second diffusion rate control unit 13. The second diffusion rate controlling unit 13 is a portion that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 20. When the gas to be measured is introduced from the outside of the sensor element 101 to the inside of the first internal space 20, the pressure fluctuation of the gas to be measured in the external space (if the gas to be measured is the exhaust gas of an automobile, the pulsation of the exhaust pressure). ), The gas to be measured that is suddenly taken into the inside of the sensor element 101 from the gas introduction port 10 is not directly introduced into the first internal vacant space 20, but the first diffusion rate controlling unit 11, the buffer space 12, and the second. After the fluctuation in the concentration of the gas to be measured is canceled through the diffusion rate controlling unit 13, the gas is introduced into the first internal vacant space 20. As a result, the concentration fluctuation of the measured gas introduced into the first internal space 20 becomes almost negligible. The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced through the second diffusion rate controlling unit 13. The oxygen partial pressure is adjusted by operating the main pump cell 21.

The main pump cell 21 has an inner pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the lower surface of the second solid electrolyte layer 6 facing the first internal space 20, and the upper surface of the second solid electrolyte layer 6. An electrochemical pump cell composed of an outer pump electrode 23 provided in a region corresponding to the ceiling electrode portion 22a so as to be exposed to an external space, and a second solid electrolyte layer 6 sandwiched between these electrodes. be.

The inner pump electrode 22 is formed so as to straddle the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that partition the first internal space 20 and the spacer layer 5 that provides the side wall. There is. 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 portion is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. The electrode portion 22b is formed, and the side electrode portion (not shown) constitutes a spacer layer on both side walls of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b. It is formed on the side wall surface (inner surface) of 5, and is arranged in a structure in the form of a tunnel at the arrangement portion of the side electrode portion.

The inner pump electrode 22 and the outer pump electrode 23 are formed as a porous cermet electrode (for example, a cermet electrode of Pt containing 1% Au and ZrO ₂ ). The inner pump electrode 22 that comes into contact with the gas to be measured is formed by using a material having a weakened reducing ability for the NOx component in the gas to be measured.

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, and a pump current is applied in the positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23. By flowing Ip0, the oxygen in the first internal space 20 can be pumped into the external space, or the oxygen in the external space can be pumped into the first internal space 20.

Further, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4 are used. The third substrate layer 3 and the reference electrode 42 constitute an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 80 for controlling a main pump.

By measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump, the oxygen concentration (oxygen partial pressure) in the first internal space 20 can be known. Further, the pump current Ip0 is controlled by feedback-controlling the pump voltage Vp0 of the variable power supply 25 so that the electromotive force V0 becomes constant. As a result, the oxygen concentration in the first internal space 20 can be maintained at a predetermined constant value.

The third diffusion rate controlling unit 30 imparts a predetermined diffusion resistance to the measured gas whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal space 20, and transfers the measured gas. It is a part leading to the second internal vacant space 40.

The second internal space 40 is provided as a space for performing a process related to the measurement of the nitrogen oxide (NOx) concentration in the gas to be measured introduced through the third diffusion rate controlling unit 30. The NOx concentration is mainly measured in the second internal vacant space 40 in which 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, after the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, the auxiliary pump cell 50 is further applied to the gas to be measured introduced through the third diffusion rate controlling unit 30. The oxygen partial pressure is adjusted by. As a result, the oxygen concentration in the second internal space 40 can be kept constant with high accuracy, so that the NOx concentration can be measured with high accuracy in the gas sensor 100.

The auxiliary pump cell 50 has an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, and an outer pump electrode 23 (outer pump electrode 23). It is an auxiliary electrochemical pump cell composed of a sensor element 101 and an appropriate electrode on the outside) and a second solid electrolyte layer 6.

The auxiliary pump electrode 51 is arranged in the second internal space 40 in a structure having a tunnel shape similar to that of the inner pump electrode 22 provided in the first internal space 20. That is, the ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40 is formed. , The bottom electrode portion 51b is formed, and the side electrode portion (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b provides a side wall of the second internal space 40 of the spacer layer 5. It has a tunnel-like structure formed on both walls. The auxiliary pump electrode 51 is also formed by using a material having a weakened reducing ability for the NOx component in the gas to be measured, similarly to the inner pump electrode 22.

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

Further, in order to control the oxygen partial pressure in the atmosphere in the second internal space 40, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte are used. The layer 4 and the third substrate layer 3 constitute an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 81 for controlling an auxiliary pump.

The auxiliary pump cell 50 pumps at the 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. As a result, the oxygen partial pressure in the atmosphere in the second internal space 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

At the same time, 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 to the oxygen partial pressure detection sensor cell 80 for main pump control as a control signal, and the electromotive force V0 is controlled, so that the third diffusion rate control unit 30 to the second internal space are vacant. The gradient of the oxygen partial pressure in the gas to be measured introduced into the 40 is controlled to be 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 gas to be measured in the second internal space 40. The measurement pump cell 41 includes a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the second internal space 40 and at a position separated from the third diffusion rate controlling unit 30, and an outer pump electrode 23. , The electrochemical pump cell composed of the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

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

The fourth diffusion rate controlling unit 45 is a film made of a porous ceramic body. The fourth diffusion rate controlling unit 45 plays a role of limiting the amount of NOx flowing into the measuring electrode 44, and also functions as a protective film of the measuring electrode 44. In the measurement pump cell 41, oxygen generated by the decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44 can be pumped out, and the amount generated can be detected as the pump current Ip2.

Further, in order to detect the oxygen partial pressure around the measurement electrode 44, an electrochemical sensor cell, that is, a reference electrode 42 is provided by the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42. The oxygen partial pressure detection sensor cell 82 for controlling the measurement pump is configured. 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 gas to be measured guided into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion rate controlling unit 45 under the condition that the oxygen partial pressure is controlled. Nitrogen oxides in the gas to be measured around the measurement electrode 44 are reduced (2NO → N ₂ + O ₂ ) to generate oxygen. The generated oxygen is pumped by the measurement pump cell 41, and at that time, a variable power source is used so that the control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 becomes constant. The voltage Vp2 of 46 is controlled. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured, the nitrogen oxides in the gas to be measured are used by using the pump current Ip2 in the pump cell 41 for measurement. The concentration will be calculated.

Further, if the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 are combined to form an oxygen partial pressure detecting means as an electrochemical sensor cell, the measurement electrode 44 can be formed. It is possible to detect the electromotive force according to the difference between the amount of oxygen generated by the reduction of the NOx component in the surrounding atmosphere and the amount of oxygen contained in the reference atmosphere, thereby the concentration of the NOx component in the measured gas. It is also possible to ask for.

Further, the electrochemical sensor cell 83 is composed of the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42. The electromotive force Vref obtained by the 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 oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50. The gas to be measured is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the measured gas is determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out from the measurement pump cell 41 in substantially proportional to the concentration of NOx in the measured gas. You can know it.

Further, the sensor element 101 includes a heater unit 70 which plays a role of temperature adjustment for heating and retaining the temperature of the sensor element 101 in order to enhance the oxygen ion conductivity of the solid electrolyte. The heater unit 70 includes a heater connector electrode 71, a heater 72, a through hole 73, a heater insulating layer 74, and a pressure dissipation hole 75.

The heater connector 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 connector electrode 71 to an external power source, power can be supplied to the heater unit 70 from the outside.

The heater 72 is an electric resistor formed by being sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater connector electrode 71 via a through hole 73, and generates heat by being supplied with power from the outside through the heater connector electrode 71 to heat and retain heat of the solid electrolyte forming the sensor element 101. ..

Further, the heater 72 is embedded over the entire area from the first internal space 20 to the second internal space 40, and the entire sensor element 101 can be adjusted to a temperature at which the solid electrolyte is activated. ing.

The heater insulating layer 74 is an insulating layer formed on the upper and lower surfaces of the heater 72 by an insulator such as alumina. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and electrical insulation between the third substrate layer 3 and the heater 72.

The pressure dissipation hole 75 is a portion provided so as to penetrate the third substrate layer 3 and communicate with the reference gas introduction space 43, and for the purpose of alleviating the increase in internal pressure due to the temperature rise in the heater insulating layer 74. It is formed.

Further, as shown in FIGS. 2 and 3, the sensor element main body 101a includes a coating layer 24. The coating layer 24 is a coating that covers the upper surface side (upper surface of the second solid electrolyte layer 6) of the sensor element main body 101a and the lower surface side (lower surface of the first substrate layer 1) of the sensor element main body 101a. The layer 24b and the like are provided. The coating layer 24a also covers the surface of the outer pump electrode 23. The coating layer 24 is made of porous ceramics such as alumina, zirconia, spinel, cordierite, and magnesia. In the present embodiment, the coating layer 24 is made of porous ceramics made of alumina. Although not particularly limited, the film thickness of the coating layer 24 is, for example, 5 to 50 μm, and the porosity of the coating layer 24 is, for example, 10% by volume to 60% by volume. Further, the arithmetic mean roughness Ra of the surface of the coating layer 24 (the upper surface of the coating layer 24a and the lower surface of the coating layer 24b) is preferably 2.0 to 5.0 μm. Although not particularly limited, the arithmetic mean roughness Ra of the surface of the main body of the sensor element 101 on which the coating layer 24 is formed (the upper surface of the second solid electrolyte layer 6 and the lower surface of the first substrate layer 1) is determined. For example, it is 0.3 to 1.0 μm.

Further, as shown in FIGS. 2 and 3, the sensor element main body 101a is partially covered with the porous protective layer 91. The porous protective layer 91 includes porous protective layers 91a to 91e formed on five of the six surfaces of the sensor element main body 101a, respectively. The porous protective layer 91a covers a part of the upper surface (upper surface of the coating layer 24a) of the sensor element main body 101a. The porous protective layer 91b covers a part of the lower surface (lower surface of the coating layer 24b) of the sensor element main body 101a. The porous protective layer 91c covers a part of the left surface of the sensor element main body 101a. The porous protective layer 91d covers a part of the right surface of the sensor element main body 101a. The porous protective layer 91e covers the entire front end surface of the sensor element main body 101a. Each of the porous protective layers 91a to 91d is a region of the surface of the sensor element main body 101a on which the porous protective layers 91a to 91d are formed, from the front end surface of the sensor element main body 101a to the rearward distance L (see FIG. 3). Covers everything. Further, the porous protective layer 91a also covers the portion where the outer pump electrode 23 is formed. The porous protective layer 91e also covers the gas inlet 10, but since the porous protective layer 91e is a porous body, the gas to be measured flows through the inside of the porous protective layer 91e and reaches the gas inlet. It is possible. The porous protective layer 91 covers a part of the sensor element main body 101a (a part from the front end surface to the distance L including the front end surface of the sensor element main body 101a) to protect the part. The porous protective layer 91 plays a role of suppressing, for example, moisture in the gas to be measured from adhering to the sensor element main body 101a to cause cracks. Further, the porous protective layer 91a plays a role of suppressing deterioration of the outer pump electrode 23 by suppressing the oil component and the like contained in the gas to be measured from adhering to the outer pump electrode 23. The distance L is (0 <distance L <length in the longitudinal direction of the sensor element main body 101a ) based on the range in which the sensor element main body 101a is exposed to the gas to be measured in the gas sensor 100, the position of the outer pump electrode 23, and the like. ) Is defined.

The porous protective layer 91 is a porous body and preferably contains ceramic particles as constituent particles, and more preferably contains at least one of alumina, zirconia, spinel, cordierite, titania, and magnesia. In the present embodiment, the porous protective layer 91 is made of an alumina porous body. The porosity of the porous protective layer 91 is, for example, 5% by volume to 40% by volume. Since the adhesion is high, it is preferable that the coating layers 24a and 24b and the porous protective layers 91a and 91b formed on the surface thereof are made of the same material.

The porous protective layer 91 has a unit thickness direction interface number of 15 or more and 250 or less, which is the number of particle interfaces of the constituent particles per 100 μm in the thickness direction. Hereinafter, the number of interfaces in the unit thickness direction will be described. The number of interfaces in the unit thickness direction is measured using an image (SEM image) obtained by observing the porous protective layer 91 with a scanning electron microscope (SEM). FIG. 4 is a conceptual diagram showing how the number of interfaces in the unit thickness direction of the porous protective layer 91a is measured. A conceptual diagram of the SEM image is shown in the upper part of FIG. Such an image can be obtained as follows. First, the sensor element 101 is cut so that the cross section (see the lower part of FIG. 4) along the left-right direction (width direction) of the sensor element 101 is the observation surface. Next, the cut surface is filled with resin and polished to prepare a sample for observation. Then, by setting the magnification of SEM to 3000 times and photographing the observation surface of the observation sample, an SEM image as shown in the upper part of FIG. 4 is obtained.

Next, a reference line in the thickness direction is drawn in this SEM image. The thickness direction is defined as the direction perpendicular to the surface of the solid electrolyte layer on which the porous protective layer 91 is formed in the SEM image. A straight line along the thickness direction is drawn as a reference line in the thickness direction. In FIG. 4, the direction (vertical direction) perpendicular to the upper surface of the second solid electrolyte layer 6 is the thickness direction. Although FIG. 4 shows an example of an SEM image including both the upper surface of the second solid electrolyte layer 6 and the porous protective layer 91a, a reference line in the thickness direction is drawn on the SEM image of the porous protective layer 91. If possible (if the thickness direction in the SEM image is determined). For example, the second solid electrolyte layer 6 and the porous protective layer 91a may be photographed as separate SEM images. Subsequently, in the SEM image, a region of 100 μm in the thickness direction is defined along the reference line in the thickness direction, and the constituent particles of the porous protective layer 91 intersect with the reference line in the thickness direction in this region. Identify the particles that are present. Even when the porous protective layer 91 contains a plurality of types of constituent particles, all the particles intersecting the reference line in the thickness direction are specified without particularly distinguishing the types of the constituent particles. Then, the number of interfaces between the specified particles is defined as the number of interfaces in the unit thickness direction. In FIG. 4, nine particles A to I among the constituent particles of the porous protective layer 91a intersect the reference line in the thickness direction within the region of 100 μm in the thickness direction. Therefore, in the example of FIG. 4, the value 8 is the number of interfaces between particles A and I (interfaces between particles A and B, interfaces between particles B and C, ..., Interfaces between particles H and I). Is the number of interfaces in the unit thickness direction of the porous protective layer 91a. Even when the particles are separated from each other in the SEM image as between the particles A and B shown in FIG. 4, the calculation is performed as one interface between the particles A and B. Therefore, the number of interfaces in the unit thickness direction is the number obtained by subtracting the value 1 from the number of particles (9 in FIG. 4) that intersect the reference line in the thickness direction within the region of 100 μm in the thickness direction. When the thickness of the porous protective layer 91a is less than 100 μm, the number of interfaces between the constituent particles in the entire thickness direction of the porous protective layer 91a is measured along the reference line in the thickness direction. Then, the value obtained by converting the measured value per 100 μm thickness is defined as the number of interfaces in the unit thickness direction.

For the porous protective layers 91b to 91e, the number of interfaces in the unit thickness direction can be derived in the same manner as described above. The thickness direction is determined from the SEM images of the porous protective layers 91b to 91e. For example, the thickness direction of the porous protective layer 91c is the left-right direction. When measuring the number of interfaces in the unit thickness direction of the porous protective layer 91e, the cross section of the sensor element 101 along the front-rear direction (longitudinal direction) is used as the observation surface.

In the present embodiment, the condition that all of the porous protective layers 91a to 91e (that is, the entire porous protective layer 91) has the above-mentioned "number of interfaces in the unit thickness direction is a value of 15 or more and 250 or less" (hereinafter, both the first condition). It is assumed that the condition (referred to as) is satisfied. The number of interfaces in the unit thickness direction of any one or more of the porous protective layers 91a to 91e may be a value of 17 or more, and more preferably a value of 30 or more. The number of interfaces in the unit thickness direction of any one or more of the porous protective layers 91a to 91e may be 200 or less, 150 or less, 100 or less, and 50 or less. Although not particularly limited to this, the thickness of the porous protective layer 91 is, for example, 1000 μm or less, and may be 700 μm or less, 500 μm or less, 250 μm or less, 200 μm or less, 150 μm or less. Further, the thickness of the porous protective layer 91 may be 50 μm or more and 100 μm or more. Here, when the distribution of the constituent particles in the thickness direction of the porous protective layer 91a is not biased, the number of particle interfaces of the constituent particles over the entire thickness direction of the porous protective layer 91a is the total number of interfaces in the thickness direction. Then, it can be derived as the total number of interfaces in the thickness direction = the number of interfaces in the unit thickness direction × the thickness (μm) of the porous protective layer 91a / 100 μm. Further, the porous protective layer 91a having a thickness of 50 μm or more and satisfying the first condition has a total number of interfaces in the thickness direction of 7.5 or more. The same applies to the porous protective layers 91b to 91e.

Further, the porous protective layer 91 has an interface number ratio (= unit) of the number of interfaces in the unit thickness direction and the number of interfaces in the unit surface direction, which is the number of interface of constituent particles per 100 μm in the surface direction perpendicular to the thickness direction. The number of interfaces in the surface direction / the number of interfaces in the unit thickness direction) is greater than 0 and 0.7 or less. The number of interfaces in the unit surface direction is measured in the same manner as the number of interfaces in the unit thickness direction. First, as shown in FIG. 4, a straight line perpendicular to the above-mentioned reference line in the thickness direction in the SEM image is drawn as a reference line in the surface direction. Subsequently, in the SEM image, a region of 100 μm in the surface direction is defined along the reference line in the surface direction. The region of 100 μm in the surface direction is defined so that the reference line in the thickness direction is located at the center of the region of 100 μm in the surface direction in the surface direction (left-right direction in the figure). Then, among the constituent particles of the porous protective layer 91, the particles that intersect the reference line in the surface direction in this region are specified, and the number of interfaces between the specified particles is defined as the number of interfaces in the unit surface direction. The unit surface-direction interface number is the number obtained by subtracting the value 1 from the number of particles (6 in FIG. 4) that intersect the surface-direction reference line in the region of 100 μm in the surface direction. Even when there are a plurality of interfaces (two in FIG. 4) intersecting the reference line in the surface direction between the two particles, such as between the particles J and K in FIG. 4, the space between the particles J and K is reached. The interface is calculated as one. This idea is the same when measuring the number of interfaces in the unit thickness direction.

In the present embodiment, the condition that all of the porous protective layers 91a to 91e (that is, the entire porous protective layer 91) has the above-mentioned "interface number ratio of more than 0 and 0.7 or less" (hereinafter, also referred to as a second condition). ) Satisfied. The interface number ratio of any one or more of the porous protective layers 91a to 91e may be a value of 0.6 or less, preferably 0.4 or less and 0.3 or less. The interface number ratio of any one or more of the porous protective layers 91a to 91e may be a value of 0.15 or more. The number of interfaces in the unit surface direction of any one or more of the porous protective layers 91a to 91e may be a value of 5 or more, a value of 10.5 or less, and a value of 10 or less. When the interface number ratio of the porous protective layer 91a is less than 1, as shown in FIG. 4, the constituent particles of the porous protective layer 91a are crushed in the thickness direction (thickness direction of the constituent particles). The size of the particles is smaller than the size in the direction perpendicular to the thickness direction). The same applies to the porous protective layers 91b to 91e.

Next, a method of manufacturing such a gas sensor 100 will be described. In the method of manufacturing the gas sensor 100, first, the sensor element main body 101a is manufactured, and then the porous protective layer 91 is formed on the sensor element main body 101a to manufacture the sensor element 101.

First, a method of manufacturing the sensor element main body 101a will be described. First, six unfired ceramic green sheets are prepared. Then, corresponding to each 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, respectively. Print patterns such as electrodes, insulating layers, and resistance heating elements on a ceramic green sheet. Further, a paste to be the coating layer 24a after firing is screen-printed on the surface of the ceramic green sheet to be the second solid electrolyte layer 6 (the surface to be the upper surface of the sensor element main body 101a). Similarly, a paste to be the coating layer 24b after firing is screen-printed on the surface of the ceramic green sheet to be the first substrate layer 1 (the surface to be the lower surface of the sensor element main body 101a). As the paste to be the coating layers 24a and 24b, a mixture of a raw material powder (alumina powder in this embodiment) made of the material of the coating layer 24 described above, an organic binder, an organic solvent and the like is used. Further, it is preferable that this paste is adjusted in advance so that the arithmetic average roughness Ra of the surface of the coating layer 24 after firing is 2.0 to 5.0 μm. Although not particularly limited to this, for example, the particle size of the raw material powder is D50 = 2 to 20 μm, the volume ratio is 5 to 20 vol%, the binder solution is 20 to 40 vol%, and the auxiliary solvent is 30 to 50 vol%. The arithmetic mean roughness Ra of the surface of the coating layer 24 after firing was determined by using a paste prepared by blending these with a dispersant of 1 to 5 vol% and mixing at a rotation speed of 50 to 250 rpm for 2 to 6 hours. It can be 0 to 5.0 μm. After forming various patterns in this way, the green sheet is dried. Then, they are laminated to form a laminated body. The laminated body thus obtained includes a plurality of sensor element main bodies 101a. The laminate is cut into pieces of the size of the sensor element main body 101a and fired at a predetermined firing temperature to obtain the sensor element main body 101a. A method for manufacturing a sensor element main body 101a by laminating a plurality of green sheets is known, and is described in, for example, JP-A-2008-164411, JP-A-2009-175099 and the like.

Next, a method of forming the porous protective layer 91 on the sensor element main body 101a will be described. In the present embodiment, the porous protective layers 91a to 91e are formed one by one by plasma spraying. FIG. 5 is an explanatory diagram of plasma spraying using a plasma gun 170. Note that FIG. 5 shows the formation of the porous protective layer 91a as an example, and the plasma gun 170 is shown in cross section. The plasma gun 170 includes an anode 176 and a cathode 178 that serve as electrodes for generating plasma, and a substantially cylindrical outer peripheral portion 172 that covers them. The outer peripheral portion 172 includes an insulating portion (insulator) 173 for insulating the anode 176. At the lower end of the outer peripheral portion 172, a powder supply portion 182 for supplying the powder spraying material 184, which is a material for forming the porous protective layer 91, is formed. A water-cooled jacket 174 is provided between the outer peripheral portion 172 and the anode 176, whereby the anode 176 can be cooled. The anode 176 is formed in a cylindrical shape and has a nozzle 176a that opens downward. A plasma generating gas 180 is supplied from above between the anode 176 and the cathode 178. Such a plasma gun 170 is known and is described in, for example, the above-mentioned Patent Document 1.

When forming the porous protective layer 91a, a voltage is applied between the anode 176 and the cathode 178 of the plasma gun 170, and an arc discharge is performed in the presence of the supplied plasma generation gas 180 to generate plasma. Put the gas 180 in a high temperature plasma state. The gas in the plasma state is ejected from the nozzle 176a to the lower part of FIG. 5 as a high-temperature and high-speed plasma jet. On the other hand, the powder spraying material 184 is supplied together with the carrier gas from the powder supply unit 182. As a result, the powder sprayed material 184 is heated, melted and accelerated by plasma, collides with the surface (upper surface) of the sensor element main body 101a, and rapidly solidifies, thereby forming the porous protective layer 91a. The direction of thermal spraying of the plasma gun 170 (direction of the nozzle 176a) is not particularly limited, and it is sufficient that the porous protective layer 91a can be formed. The porous protective layers 91b to 91e are also formed one by one in the same manner except that the surfaces formed on the sensor element main body 101a are different. The porous protective layers 91a and 91b are formed on the surfaces of the coating layers 24a and 24b, respectively, as described above. The porous protective layers 91c to 91e are directly formed on the surface of the solid electrolyte layer (each layer 1 to 6) of the sensor element main body 101a. Plasma spraying is performed, for example, in an atmosphere or an atmosphere at room temperature. When the porous protective layers 91c to 91e are prepared one by one, the porous protective layers 91a and 91b to be formed on the coating layer 24 are formed first, and the porous protective layers 91c to 91e are the formed porous protective layers. It is preferable to form so that the end portion is connected to the. Further, two or more of the porous protective layers 91a to 91e may be formed at the same time. As a result, the porous protective layers 91a to 91e are formed on the upper, lower, left and right surfaces and the front end surface of the sensor element main body 101a, respectively, to form the porous protective layer 91, and the sensor element 101 is obtained.

Here, as the plasma generating gas 180, an inert gas such as argon gas can be used. Further, since plasma is likely to be generated, it is preferable to use a mixture of argon and hydrogen as the plasma generation gas 180. A mixture of argon and nitrogen may be used as the plasma generating gas 180. Although not particularly limited, the flow rate of argon gas is, for example, 40 to 50 L / min, the supply pressure is, for example, 0.5 to 0.6 MPa, the flow rate of hydrogen is, for example, 9 to 11 L / min, and the supply pressure is, for example, 0. It is .5 to 0.6 MPa. The voltage applied between the anode 176 and the cathode 178 is, for example, a DC voltage of 50 to 70 V, and the current is, for example, 500 to 550 A. By adjusting the conditions at the time of plasma generation (flow rate of plasma generation gas 180, supply pressure, applied voltage, current), the number of interfaces and the number of interfaces in the unit thickness direction of the porous protective layer 91 can be adjusted. be able to. The conditions at the time of plasma generation are preferably adjusted so that the constituent particles of the porous protective layer 91 are crushed in the thickness direction (see FIG. 4). If the constituent particles are crushed in the thickness direction, the number of interfaces in the unit thickness direction tends to increase, so that the number of interfaces in the unit thickness direction can be relatively easily set to a value of 15 or more. Further, since the interface number ratio tends to be small if the constituent particles are crushed in the thickness direction, the interface number ratio can be relatively easily set to a value of more than 0 and 0.7 or less. For example, by setting the applied voltage and current to relatively high values as described above, the powder spraying material 184 is likely to melt, and when the constituent particles collide with the surface of the sensor element main body 101a, they are crushed in the thickness direction. It tends to have a different shape. Similarly, it is preferable to set the flow rate and the supply pressure of the plasma generating gas 180 to relatively high values as described above.

The powder spraying material 184 is a powder that is a material for the above-mentioned porous protective layer 91, and is an alumina powder in the present embodiment. Although not particularly limited, the particle size of the powder sprayed material 184 is, for example, 1 μm to 50 μm, more preferably 1 μm to 20 μm. As the carrier gas used for supplying the powder spraying material 184, for example, the same argon gas as the plasma generating gas 180 can be used. Although not particularly limited, the flow rate of the carrier gas is, for example, 3 to 4 L / min, and the supply pressure is, for example, 0.5 to 0.6 MPa. By adjusting the particle size of the powder sprayed material 184, the number of interfaces in the unit thickness direction and the number of interfaces in the unit surface direction of the porous protective layer 91 can be adjusted. The smaller the particle size, the larger the number of interfaces in the unit thickness direction and the number of interfaces in the unit surface direction. However, if the particle size is too small, the powder supply unit 182 is likely to be clogged, so it is preferable to increase the sphericity of the particles of the powder spraying material 184 or increase the supply pressure of the carrier gas.

When performing plasma spraying, the distance W between the nozzle 176a, which is the outlet of the plasma gas in the plasma gun 170, and the surface (upper surface of the coating layer 24a in FIG. 5) forming the porous protective layer 91 in the sensor element main body 101a is set. It is preferably 50 mm to 300 mm. The distance W may be 120 mm to 250 mm. Further, plasma spraying may be performed while appropriately moving the plasma gun 170 (moving in the left-right direction in FIG. 5) according to the area forming the porous protective layer 91, but the distance W is also in the above-mentioned range. It is preferable to keep it at. The time for plasma spraying may be appropriately determined according to the film thickness and area of the porous protective layer 91 to be formed. When the porous protective layer 91 is formed on a part of the surface of the sensor element main body 101a (the region from the front end to the rear to a distance L) such as the porous protective layer 91a to the porous protective layer 91d. May cover the region that does not form the porous protective layer 91 with a mask.

When the sensor element 101 is obtained, the sensor element 101 is passed through the prepared supporter 124 and the thick powder 126, and these are inserted into the through holes inside the main metal fitting 122 from the upper side of FIG. 1, and the sensor element 101 is obtained. Is fixed with the element sealant 120. Then, by attaching the nut 130, the protective cover 110, or the like, the gas sensor 100 can be obtained. A method for manufacturing such a gas sensor is known and is described in, for example, International Publication No. 2013/005491.

When the gas sensor 100 configured in this way is used, the gas to be measured in the pipe 140 flows into the protective cover 110, reaches the sensor element 101, passes through the porous protective layer 91, and flows into the gas introduction port 10. .. Then, the sensor element 101 detects the NOx concentration in the gas to be measured that has flowed into the gas introduction port 10. At this time, the moisture contained in the gas to be measured may also enter the protective cover 110 and adhere to the surface of the porous protective layer 91. As described above, the sensor element main body 101a is adjusted to a temperature at which the solid electrolyte is activated by the heater 72 (for example, 800 ° C.), and when moisture adheres to the sensor element 101, the temperature drops sharply and the sensor element main body 101a. Cracks may occur in 101a. Here, the porous protective layer 91 of the present embodiment has a value of 15 or more interfaces in the unit thickness direction. As the number of interfaces in the unit thickness direction increases, heat conduction in the thickness direction of the porous protective layer 91 tends to be less likely to occur. That is, there is a tendency that the cooling of the sensor element main body 101a when moisture adheres to the surface of the porous protective layer 91 is suppressed. The reason for this is considered to be that heat conduction between the constituent particles is less likely to occur than heat conduction within the constituent particles. When the number of interfaces in the unit thickness direction is 15 or more, the effect of suppressing the cooling of the sensor element main body 101a, that is, the effect of improving the water resistance of the sensor element 101 can be obtained, and the occurrence of cracks can be suppressed. As the number of interfaces in the unit thickness direction increases, the number of constituent particles per unit thickness in the porous protective layer 91 increases, and the gas to be measured passes through the porous protective layer 91 and the sensor element main body 101a. It becomes difficult to reach. When the number of interfaces in the unit thickness direction is 250 or less, the gas to be measured can pass through the porous protective layer 91. Further, by setting the number of interfaces in the unit thickness direction of the porous protective layer 91 to a value of 100 or less and 50 or less, the gas to be measured can more easily pass through the porous protective layer 91, and the responsiveness of the sensor element 101 is lowered. Can be suppressed.

Further, the smaller the interface number ratio, the more likely it is that heat conduction in the surface direction (direction perpendicular to the thickness direction) of the porous protective layer 91 occurs rather than heat conduction in the thickness direction of the porous protective layer 91. be. The reason for this is considered to be that heat conduction between the constituent particles is less likely to occur than heat conduction within the constituent particles. As a result, the smaller the interface number ratio, the more it tends to be suppressed that only a part of the sensor element main body 101a is rapidly cooled when moisture adheres to the surface of the porous protective layer 91. When the interface number ratio is 0.7 or less, the effect of suppressing the generation of cracks due to the rapid cooling of only a part (the portion to which moisture adheres) of the sensor element body 101a, that is, the resistance of the sensor element 101. The effect of improving water coverage can be obtained.

Here, the correspondence between the constituent elements of the present embodiment and the constituent elements of the present invention will be clarified. The sensor element 101 of the present embodiment corresponds to the gas sensor element of the present invention, the sensor element main body 101a corresponds to the element main body, and the porous protective layer 91 corresponds to the protective layer. Further, the element sealing body 120 corresponds to the fixing member .

According to the gas sensor 100 of the present embodiment described above, the sensor element 101 includes a sensor element main body 101a provided with an oxygen ion conductive solid electrolyte layer (each layer 1 to 6) and at least a part of the sensor element main body 101a. A porous protective layer 91 for covering is provided. When the number of interfaces in the unit thickness direction of the porous protective layer 91 is 15 or more, the water resistance of the sensor element 101 can be improved. Further, when the number of interfaces in the unit thickness direction of the porous protective layer 91 is 250 or less, the gas can pass through the porous protective layer 91.

Further, the water resistance of the sensor element 101 is improved by setting the interface number ratio (= number of interface in the unit surface direction / number of interfaces in the unit thickness direction) of the porous protective layer 91 to be more than 0 and 0.7 or less. Can be done.

Further, by setting the number of interfaces in the unit thickness direction of the porous protective layer 91 to a value of 30 or more, the water resistance of the sensor element 101 can be further improved. Further, even if the thickness of the porous protective layer 91 is 500 μm or less, the effect of improving the water resistance of the sensor element 101 can be obtained. Here, when the thickness of the porous protective layer 91 is as thin as 500 μm or less, the water resistance of the sensor element 101 tends to be insufficient. Since the water resistance of the sensor element 101 is improved by satisfying at least one of the first condition and the second condition of the porous protective layer 91, the present invention is applied to such a relatively thin porous protective layer 91. Significant.

Further, the porous protective layer 91 contains ceramic particles as constituent particles. The ceramic particles are suitable for the protective layer of the sensor element 101 from the viewpoint of at least one of strength, heat resistance, and corrosion resistance. Further, the porous protective layer 91 contains at least one of alumina, zirconia, spinel, cordierite, titania, and magnesia as constituent particles. Since these materials have high heat resistance, they are suitable for the protective layer of the sensor element 101.

Furthermore, the sensor element body 101a has a long rectangular shape, and the porous protective layer 91 has one end surface (front end surface) in the longitudinal direction of the sensor element body 101a and four surfaces perpendicular to one end surface (the front end surface). It covers a region from one end surface side of the upper, lower, left, and right surfaces to a distance L in the longitudinal direction of the sensor element main body 101a (however, 0 <distance L <length of the sensor element main body 101a in the longitudinal direction). By covering the five surfaces of the porous protective layer 91 in this way, for example, the porous protective layer 91 protects the sensor element main body 101a as compared with the case where the porous protective layer 91 covers only four or less surfaces. The effect of

It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be carried out in various embodiments as long as it belongs to the technical scope of the present invention.

For example, in the above-described embodiment, the porous protective layer 91 satisfies both the first condition and the second condition, but at least one of the conditions may be satisfied.

In the above-described embodiment, all of the porous protective layers 91a to 91e satisfy the first condition, but at least one or more of the porous protective layers 91a to 91e may satisfy the first condition. If at least one of the porous protective layers 91a to 91e satisfies the first condition, the above-mentioned effect can be obtained for the porous protective layer satisfying at least the first condition. Therefore, for example, the number of interfaces in the unit thickness direction of any of the porous protective layers 91a to 91e may be less than 15 or more than 250. However, among the porous protective layers 91a to 91e, it is preferable that there are many porous protective layers satisfying the first condition, and all of the porous protective layers 91a to 91e (that is, the entire porous protective layer 91) satisfy the first condition. It is more preferable to satisfy. Similarly, if at least one or more of the porous protective layers 91a to 91e satisfy the second condition, the above-mentioned effect can be obtained for the porous protective layer satisfying the second condition.

In the above-described embodiment, for example, it has not been mentioned at which point in the porous protective layer 91a the number of interfaces in the unit thickness direction is measured, but this will be described. The following description is the same for the porous protective layers 91b to 91e. If any one of the porous protective layers 91a has a portion satisfying the first condition, the above-mentioned effect can be obtained as compared with the case where there is no portion satisfying the first condition. However, it is preferable that the first condition is satisfied at a plurality of locations having different surface directions. For example, it is preferable that the number of interfaces in the unit thickness direction is measured at any five points in the porous protective layer 91a separated from each other to some extent in the surface direction, and the average value thereof is 15 or more and 250 or less. Similarly, regarding the interface number ratio, if there is a portion satisfying the second condition in any one of the porous protective layers 91a, the above-mentioned effect can be obtained as compared with the case where there is no portion satisfying the second condition. Be done. However, it is preferable that the second condition is satisfied at a plurality of locations having different surface directions. For example, it is preferable that the interface number ratio is measured at any five points in the porous protective layer 91a separated from each other to some extent in the surface direction, and the average value thereof is more than 0 and 0.7 or less. When the porous protective layer 91a has a plurality of portions having different modes of constituent particles (for example, the material, ratio, average particle size, etc. of the constituent particles), the first condition is one or more of the portions. , At least one of the second conditions may be satisfied. For example, when the porous protective layer 91a includes a plurality of layers in the thickness direction, at least one of the first condition and the second condition may be satisfied by any one or more layers. The same applies to the case where the porous protective layer 91a includes a plurality of portions having different modes of constituent particles in the surface direction (front-back and left-right directions).

In the above-described embodiment, the porous protective layer 91 has the porous protective layers 91a to 91e, but the present invention is not limited to this. The porous protective layer 91 may cover at least a part of the sensor element main body 101a. For example, the porous protective layer 91 may not include one or more of the porous protective layers 91a to 91e. Further, the porous protective layer 91 is a porous body, but the present invention is not limited to this.

In the above-described embodiment, the porous protective layer 91 is formed by plasma spraying, but the present invention is not limited to this. For example, the porous protective layer 91 may be formed by other thermal spraying such as high-speed frame spraying, arc spraying, and laser spraying. Alternatively, a porous protective layer is formed by forming a coating film on the surface of the sensor element main body 101a using a slurry by another manufacturing method (for example, screen printing, dipping, gel casting method, etc.), not limited to thermal spraying, and firing the coating film. 91 may be formed. Such a slurry can be prepared, for example, by dispersing the raw material powder (ceramic particles or the like) of the porous protective layer 91 in a solvent. Further, it is preferable to add at least one of a sintering aid (binder) and a pore-forming material to the slurry. When the gel casting method is used, an organic solvent, a dispersant and a gelling agent (for example, isocyanates and polyols) are further added to the slurry. When the coating film is fired to form the porous protective layer 91, the coating film may be fired and the sensor element main body 101a may be fired at the same time.

In the above-described embodiment, the constituent particles of the porous protective layer 91 tend to have a crushed shape as shown in FIG. 4, but the present invention is not limited to this. Even if the constituent particles are not crushed, if the number of interfaces in the unit thickness direction is 15 or more, the above-mentioned effect can be obtained. Further, in the above-described embodiment, the conditions at the time of plasma generation are adjusted so that the constituent particles of the porous protective layer 91 are crushed, but the present invention is not limited to this. For example, the raw material particles that are the constituent particles of the porous protective layer 91 may be previously sheared, crushed, polished, or the like using a roll or the like so that the cross section of the particles becomes elliptical. For example, when the porous protective layer 91 is formed by screen printing, if the raw material particles are made elliptical, the direction of the raw material particles becomes the direction along the surface direction due to the shear stress due to the squeegee pressure. Since it is easy, the constituent particles of the porous protective layer 91 can tend to be crushed in the thickness direction. When forming the porous protective layer 91 by dipping, if the raw material particles are made elliptical, the direction of the raw material particles will be changed by dragging (pulling the sensor element main body 101a from the slurry) or shear stress due to the surface tension of the slurry. The long side of the ellipse tends to be oriented along the surface direction. When the porous protective layers 91a to 91d are formed by dipping, it is preferable to pull up the sensor element body 101a from the slurry along the front-rear direction (longitudinal direction of the sensor element body 101a). When the porous protective layer 91e is formed by dipping, it is preferable to pull up the sensor element body 101a from the slurry along the vertical direction and the horizontal direction (the direction perpendicular to the longitudinal direction of the sensor element body 101a). Further, when the porous protective layer 91 is formed by the gel casting method, for example, if the slurry covering the sensor element main body 101a is pressed in the thickness direction before the slurry is solidified, the constituent particles of the porous protective layer 91 are thickened. It tends to have a shape that is crushed in the vertical direction.

In the above-described embodiment, the sensor element main body 101a has coating layers 24a and 24b, but the present invention is not limited to this. The coating layer 24 may be formed on at least a part of the surface of the sensor element main body 101a covered with the porous protective layer 91. For example, one of the coating layers 24a and 24b may not be provided, or a coating layer that covers a surface other than the upper and lower surfaces of the sensor element main body 101a may be further provided. Further, the sensor element main body 101a does not have to be provided with a coating layer.

Hereinafter, an example in which the sensor element is specifically manufactured will be described as an example. Experimental Examples 3 to 11, 16 to 19, 22 to 25 correspond to the first embodiment of the gas sensor element of the present invention, and Experimental Examples 3 to 9, 12 to 13, 16 to 19, 22 to 25 of the present invention correspond to the first embodiment of the present invention. Examples of the second gas sensor element correspond to Examples, and Experimental Examples 1, 2, 14, 15, 20, and 21 correspond to Comparative Examples. The present invention is not limited to the following examples.

[Experimental Example 1]
According to the method for manufacturing the sensor element 101 of the above-described embodiment, the sensor element 101 shown in FIGS. 2 and 3 was created and used as Experimental Example 1. Specifically, first, a sensor element main body 101a having a length of 67.5 mm in the front-rear direction, a width of 4.25 mm in the left-right direction, and a thickness of 1.45 mm in the up-down direction was manufactured. In manufacturing the sensor element main body 101a, the ceramic green sheet was formed by mixing zirconia particles to which 4 mol% of yttria, a stabilizer was added, an organic binder, and an organic solvent, and tape forming. The paste for forming the coating layer 24 was adjusted as follows. The particle size of the raw material powder (alumina powder) is D50 = 5 μm, the volume ratio is 10 vol%, the binder solution (polyvinyl acetal and butyl carbitol) is 40 vol%, the auxiliary solvent (acetone) is 45 vol%, and the dispersant (poly). Oxyethylene styrene phenyl ether) was prepared at 5 vol%, and the mixture was mixed at a pot mill mixer rotation speed of 200 rpm for 3 hours to adjust the paste.

Subsequently, the porous protective layers 91a, 91b, 91c, 91d, 91e were formed in this order on the surface of the sensor element main body 101a to form the porous protective layer 91, which was used as the sensor element 101 of Experimental Example 1. The conditions for plasma spraying to form the porous protective layer 91 were as follows. As the plasma generation gas 180, a mixture of argon gas (flow rate 50 L / min, supply pressure 0.5 MPa) and hydrogen (flow rate 10 L / min, supply pressure 0.5 MPa) was used. The voltage applied between the anode 176 and the cathode 178 was a DC voltage of 70 V. The current was 500A. As the powder spraying material 184, alumina powder having an average particle size of 30 μm was used. The carrier gas used for supplying the powder spraying material 184 was argon gas (flow rate 4 L / min, supply pressure 0.5 MPa). The distance W was set to 150 mm. The distance L was set to 10 mm. Plasma spraying was performed in the atmosphere and the atmosphere at room temperature. The direction of thermal spraying of the plasma gun 170 (direction of the nozzle 176a) was perpendicular to the formation surface of the porous protective layer 91 in the sensor element 101. When the thicknesses of the formed porous protective layers 91a to 91e were measured with a micrometer, they were all 100 μm. Moreover, when the number of interfaces in the unit thickness direction of the porous protective layers 91a to 91e was measured by the above-mentioned method, all of them had a value of 10. When the number of interfaces in the unit surface direction of the porous protective layers 91a to 91e was measured by the above-mentioned method, all of them had a value of 13. The interface number ratio of the porous protective layers 91a to 91e was 1.30 in each case.

[Experimental Examples 2-9]
The sensor element 101 was created in the same manner as in Experimental Example 1 except that the conditions for plasma spraying (average particle size of powder spraying material 184, distance W, applied voltage, current) were variously changed as shown in Table 1. Experimental examples 2 to 9 were used. In each of Experimental Examples 2 to 9, the thickness of the porous protective layers 91a to 91e was 100 μm.

[Evaluation of water resistance]
The performance of the porous protective layer 91 (water resistance of the sensor element 101) was evaluated for the sensor elements of Experimental Examples 1 to 9. Specifically, first, the heater 72 was energized to set the temperature to 800 ° C., and the sensor element 101 was heated. In this state, the main pump cell 21, the auxiliary pump cell 50, the oxygen partial pressure detection sensor cell 80 for main pump control, the oxygen partial pressure detection sensor cell 81 for auxiliary pump control, etc. are operated in the atmospheric atmosphere, and the inside of the first internal vacant space 20 is operated. The oxygen concentration of the water was controlled to be kept at a predetermined constant value. Then, after waiting for the pump current Ip0 to stabilize, water droplets are dropped on the porous protective layer 91, and the crack of the sensor element 101 is based on whether the pump current Ip0 changes to a value exceeding a predetermined threshold value. The presence or absence of was determined. When a crack occurs in the sensor element 101 due to a thermal shock caused by a water droplet, oxygen easily flows into the first internal space 20 through the crack portion, so that the value of the pump current Ip0 becomes large. Therefore, when the pump current Ip0 exceeds a predetermined threshold value determined in the experiment, it is determined that the sensor element 101 is cracked by water droplets. Further, the amount of water droplets was gradually increased to 30 μL and a plurality of tests were performed, and the maximum amount of water droplets in which cracks did not occur was defined as the water resistance. Then, five sensor elements 101 of Experimental Examples 1 to 9 were prepared, and the average value of the water resistance of the five sensors was derived for each of Experimental Examples 1 to 9. When the average value of the water resistance is less than 10 μL, it is defective, when it is 10 μL or more, it is good, and when it is 30 μL (= no cracks occur), it is very good. Was evaluated.

Conditions for plasma spraying of Experimental Examples 1 to 9 (mean particle size, distance W, applied voltage, current of powder spraying material 184), measurement results (number of interfaces in the unit thickness direction, number of interfaces in the unit surface direction, ratio of the number of interfaces), And the evaluation results (average value of water resistance, evaluation of water resistance) are summarized in Table 1. Note that x means defective, ○ means good, and ◎ means very good. Further, as in Experimental Example 1, the number of interfaces in the unit thickness direction of the porous protective layers 91a to 91e of Experimental Example 2 is the same value, and the unit surface direction of the porous protective layers 91a to 91e of Experimental Example 2 The number of interfaces was the same in all cases. The same was true for Experimental Examples 3 to 9.

As can be seen from Table 1, from the results of Experimental Examples 1 to 9, when the number of interfaces in the unit thickness direction of the porous protective layer 91 is 15 or more and 17 or more, the generation of cracks can be suppressed and the water resistance is improved. I was able to confirm that it was done. Further, it was confirmed that the water resistance was further improved when the number of interfaces in the unit thickness direction of the porous protective layer 91 was 30 or more.

Further, when the interface number ratio of the porous protective layer 91 was 0.7 or less and 0.6 or less, it was confirmed that the generation of cracks could be suppressed and the water resistance was improved. Further, when the interface number ratio of the porous protective layer 91 was 0.4 or less and 0.3 or less, it was confirmed that the water resistance was further improved.

[Experimental Examples 10 to 13]
The sensor element 101 was created in the same manner as in Experimental Example 1 except that the conditions for plasma spraying were variously changed as shown in Table 2, and used as Experimental Examples 10 to 13. In all of Experimental Examples 10 to 13, the thickness of the porous protective layers 91a to 91e was 100 μm. The water resistance of Experimental Examples 10 to 13 was evaluated in the same manner as in Experimental Examples 1 to 9. Table 2 summarizes the plasma spraying conditions, measurement results, and evaluation results of Experimental Examples 10 to 13. From the results of Experimental Examples 10 and 11, if the porous protective layer 91 satisfies the first condition, even if the second condition is not satisfied, the first and second conditions are not satisfied together with Experimental Examples 1 and 2. It was confirmed that the water resistance was improved in comparison with the above. Further, from the results of Experimental Examples 12 and 13, if the porous protective layer 91 satisfies the second condition, even if the first condition is not satisfied, both the first and second conditions are not satisfied. It was confirmed that the water resistance was improved as compared with 2.

[Experimental Examples 14-19]
Sensor elements 101 were prepared in the same manner as in Experimental Examples 1 to 4, 7 and 9, except that the thicknesses of the porous protective layers 91a to 91e were all 50 μm, and used as Experimental Examples 14 to 19. The water resistance of Experimental Examples 14 to 19 was evaluated in the same manner as in Experimental Examples 1 to 9. Table 3 summarizes the plasma spraying conditions, measurement results, and evaluation results of Experimental Examples 14 to 19. The value of the number of interfaces in the unit thickness direction in Experimental Examples 14 to 19 was converted from the measured value by measuring the number of interfaces between the constituent particles in the region of 50 μm in the thickness direction along the reference line in the thickness direction. It is a value (a value twice the measured value). Even if the thickness of the porous protective layer 91 was 50 μm, the evaluation of water resistance was good in Experimental Examples 16 to 19, and it was confirmed that the effect of the present invention could be obtained. Further, it was confirmed that the water resistance of Experimental Examples 16 to 19 was improved as compared with Experimental Examples 1 and 2 having a thickness of 100 μm.

[Experimental Examples 20 to 25]
Sensor elements 101 were prepared in the same manner as in Experimental Examples 1 to 4, 7 and 9, except that the thicknesses of the porous protective layers 91a to 91e were all set to 500 μm, and used as Experimental Examples 20 to 25. The water resistance of Experimental Examples 20 to 25 was evaluated in the same manner as in Experimental Examples 1 to 9. Table 4 summarizes the plasma spraying conditions, measurement results, and evaluation results of Experimental Examples 20 to 25. In Experimental Examples 20 to 25, the thickness of the porous protective layer 91 is 500 μm, so that the average value of the water resistance is larger as a whole as compared with Experimental Examples 1 to 4, 7, and 9 having a thickness of 100 μm. However, the water resistance of Experimental Examples 20 and 21 which did not satisfy both the first and second conditions was poor. On the other hand, Experimental Examples 22 and 23 had good water resistance, and Experimental Examples 24 and 25 had very good water resistance.

1 1st substrate layer, 2 2nd substrate layer, 3 3rd substrate layer, 4 1st solid electrolyte layer, 5 spacer layer, 6 2nd solid electrolyte layer, 7a-7e adhesive layer, 10 gas inlet, 11 1st Diffusion rate control section, 12 buffer space, 13 second diffusion rate control section, 20 first internal space, 21 main pump cell, 22 inner pump electrode, 22a ceiling electrode section, 22b bottom electrode section, 23 outer pump electrode, 24, 24a, 24b coating layer, 25 variable power supply, 30 third diffusion rate control section, 40 second internal space, 41 measurement pump cell, 42 reference electrode, 43 reference gas introduction space, 44 measurement electrode, 45 fourth diffusion rate control section, 46 variable Power supply, 48 atmosphere introduction layer, 50 auxiliary pump cell, 51 auxiliary pump electrode, 51a ceiling electrode part, 51b bottom electrode part, 52 variable power supply, 70 heater part, 71 heater connector electrode, 72 heater, 73 through hole, 74 heater insulation layer , 75 pressure dissipation hole, 80 oxygen partial pressure detection sensor cell for main pump control, 81 oxygen partial pressure detection sensor cell for auxiliary pump control, 82 oxygen partial pressure detection sensor cell for measurement pump control, 83 sensor cell, 91, 91a to 91e porous Protective layer, 100 gas sensor, 101 sensor element, 101a sensor element body, 110 protective cover, 111 inner protective cover, 112 outer protective cover, 120 element sealant, 122 main metal fittings, 124 supporters, 126 powder powder, 130 nuts, 140 piping, 141 mounting members, 170 plasma gun, 172 outer circumference, 173 insulation, 174 water-cooled jacket, 176 anode, 176a nozzle, 178 cathode, 180 plasma generating gas, 182 powder supply, 184 powder spray material.

Claims (1)

A device body with an oxygen ion conductive solid electrolyte layer and
At least a part of the element body is covered, and the number of interface in the unit thickness direction, which is the number of interface of constituent particles per 100 μm in the thickness direction, and the particle interface of the constituent particles per 100 μm in the surface direction perpendicular to the thickness direction. A protective layer in which the ratio of the number of interfaces in the unit surface direction (= the number of interfaces in the unit surface direction / the number of interfaces in the unit thickness direction) is more than 0 and 0.7 or less.
Gas sensor element equipped with.

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