WO2021100572A1

WO2021100572A1 - Sensor element, gas sensor, and method for producing sensor element

Info

Publication number: WO2021100572A1
Application number: PCT/JP2020/042047
Authority: WO
Inventors: 嘉彦山村; 中垣　邦彦
Original assignee: 日本碍子株式会社
Priority date: 2019-11-22
Filing date: 2020-11-11
Publication date: 2021-05-27
Also published as: JPWO2021100572A1

Abstract

A sensor element 101 used for detecting specific gas concentration in a to-be-measured gas is provided with: a sensor element body 101a; and a protective layer 90 which at least partially covers the surface of the sensor element body 101a. Residual compressive stress is present in the surface of the protective layer 90 at least at a temperature T1 [°C] within a temperature range of 500-900°C.

Description

Manufacturing method of sensor element, gas sensor and sensor element

The present invention relates to a sensor element, a gas sensor, and a method for manufacturing the sensor element.

Conventionally, a gas sensor having a sensor element for detecting the concentration of a specific gas such as NOx in the gas to be measured such as the exhaust gas of an automobile has been known. Patent Document 1 describes a sensor element of such a gas sensor in which a protective layer is provided so as to cover the element main body and a space is provided between the protective layer and the element main body. It is explained that the heat conduction in the thickness direction of the protective layer can be insulated by the space, so that the cooling of the element body when water adheres to the surface of the protective layer is suppressed and the water resistance is improved.

Japanese Unexamined Patent Publication No. 2016-188853

However, in the sensor element of Patent Document 1, when water adheres to the surface of the protective layer, the cooling of the element body can be suppressed, but the cooling is concentrated on the portion of the surface of the protective layer to which water adheres. As a result, on the surface of the protective layer, when water adheres, the local temperature drop becomes large, and tensile stress acts between the locally lowered part and the surrounding part, and the protective layer is damaged. There was something.

The present invention has been made to solve such a problem, and a main object thereof is to improve the water resistance of the protective layer of the sensor element.

The sensor element of the present invention
A sensor element used to detect a specific gas concentration in the gas to be measured.
An element body having an oxygen ion conductive solid electrolyte layer and having a longitudinal direction,
A protective layer that covers at least a part of the surface of the element body,
With
The protective layer has residual compressive stress on its surface at at least any temperature T1 [° C.] included in the temperature range of 500 ° C. or higher and 900 ° C. or lower.

In this sensor element, residual compressive stress exists on the surface of the protective layer at at least one temperature T1 [° C.] included in the temperature range of 500 ° C. or higher and 900 ° C. or lower. Therefore, when water adheres to the surface of the protective layer at the temperature T1 [° C.], the above-mentioned tensile stress is canceled by the residual compressive stress. As a result, damage to the protective layer is suppressed, and the water resistance of the protective layer is improved. The temperature T1 [° C.] may be at least one temperature included in the temperature range of 500 ° C. or higher and 900 ° C. or lower, but for example, at least one temperature included in the temperature range of 700 ° C. or higher and 900 ° C. or lower. May be. The temperature T1 [° C.] is the temperature of the protective layer when the sensor element is used. The temperature T1 [° C.] may be, for example, an actually measured value of the temperature on the surface of the protective layer when the sensor element is used or a predicted value by simulation. The temperature of the surface of the protective layer may be the temperature of the hottest portion of the surface of the protective layer.

In the sensor element of the present invention, the residual compressive stress is preferably 15 MPa or more. In this way, the water resistance is further improved. The upper limit of the residual compressive stress is not particularly limited, but is, for example, 300 MPa or less, preferably 200 MPa or less.

In the sensor element of the present invention, the element body has a coefficient of linear thermal expansion of 10 ppm / K or more and 15 ppm / K or less at 40 ° C. to 700 ° C., and the protective layer has a linear thermal expansion of 40 ° C. to 700 ° C. The coefficient may be 1 ppm / K or more and 9 ppm / K or less. While the sensor element is used at a high temperature such as 700 ° C., it reaches room temperature when not in use, so that thermal expansion and contraction are repeated in the element body and the protective layer. Even when such thermal expansion and contraction are repeated, if the coefficient of thermal expansion of the element body and the protective layer is within the above range, the difference between the coefficient of linear thermal expansion between the two is not too large, so that the difference between the coefficient of linear thermal expansion is not too large. Peeling due to the difference in thermal expansion with the above is unlikely to occur.

In the sensor element of the present invention, the element body may contain zirconia as a main component, and the protective layer may contain one or more selected from the group consisting of alumina, spinel, cordierite and mullite as the main component. The principal component means the component contained most, for example, the component having the highest mass ratio. The coefficient of linear thermal expansion from 40 ° C. to 700 ° C. is 11 ppm / K for zirconia, 8 ppm / K for alumina and spinel, 2 ppm / K for cordierite, and 7 ppm / K for mullite.

In the sensor element of the present invention, the protective layer is between a bottomed tubular main body portion that covers the tip end portion, which is one end portion in the longitudinal direction of the element main body, and the main body portion and the element main body. It may have a space support portion provided so as to form a space in the space. In such a device, since the protective layer having the bottomed tubular main body covers the tip of the element main body, the entire tip of the element main body can be protected. In addition, a space is formed between the main body of the protective layer and the main body of the element by the space support portion, and this space can suppress heat conduction from the protective layer to the main body of the element, so that water adheres to the surface of the protective layer. It is possible to suppress the cooling of the element body in the case of this.

In the sensor element of the present invention, the protective layer has a side columnar portion provided inside the side portion of the main body portion as the space support portion, and (a) the side columnar portion is the main body portion. Is provided in a range of 2% or more and 35% or less inside the side portion of the element, or (b) the side columnar portion is a portion of the element body covered with the side portion of the main body portion of the protective layer. It may satisfy at least one of 2% or more and 35% or less of the above. In this way, the residual compressive stress on the surface of the protective layer tends to be a desired value in the range of, for example, 15 MPa or more and 300 MPa or less.

In the sensor element of the present invention, the protective layer has a side columnar portion provided inside the side portion of the main body portion as the space support portion, and the side columnar portion is a side portion of the main body portion. The length protruding from the height H is defined as the height H, the length in the direction perpendicular to the height H and the shortest length is defined as the width W, and the length in the direction perpendicular to the height H and the width W. The width W and the length L may be 200 μm or more when the length L is defined as. When the width W and the length L of the side columnar portion are 200 μm or more, the residual compressive stress on the surface of the protective layer tends to be a desired value in the range of, for example, 15 MPa or more and 300 MPa or less.

In the sensor element of the present invention, the protective layer has a side columnar portion provided inside the side portion of the main body portion as the space support portion, and the side columnar portion is a side portion of the main body portion. Assuming that the height H is the protrusion length from the height H, the height H may be 400 μm or less. When the height H of the side columnar portion is 400 μm or less, the residual compressive stress on the surface of the protective layer tends to be a desired value in the range of, for example, 15 MPa or more and 300 MPa or less. The height H may be 10 μm or more. In this way, heat conduction from the protective layer to the element body can be further suppressed.

In the sensor element of the present invention, the protective layer may have a thickness t of a side portion of the main body portion of 600 μm or less. The thickness t of the side portion is the average thickness of the side portion, and if there is a portion (for example, a corner portion) whose thickness is significantly different from that of the other portion, the average thickness of the portion excluding such a portion is used. When the thickness t of the side portion is 600 μm or less, the residual compressive stress on the surface of the protective layer tends to be a desired value in the range of, for example, 15 MPa or more and 300 MPa or less.

The gas sensor of the present invention includes the sensor element of any of the above-described aspects. Therefore, this gas sensor has the same effect as the sensor element of the present invention described above, for example, the effect of improving the water resistance of the protective layer of the sensor element.

The method for manufacturing the sensor element of the present invention is
A method for manufacturing a sensor element according to any one of the above embodiments.
An arrangement step of arranging the protective layer material to be the protective layer by firing on at least a part of the surface of the element main body material which is the element main body before or after firing.
A firing step of heating the protective layer material and the element main body material in a state where the arrangement step has been performed at a temperature of T2 [° C.] and firing at least the protective layer material.
A temperature lowering step of lowering the temperature of the protective layer and the element body in the state where the firing step has been performed from the temperature T2 [° C.] to a temperature of the temperature T1 [° C.] or lower.
Including
In the arrangement step, the temperature T1 [° C.] to the temperature T2 [° C.] of the solid electrolyte layer contained in the element body material after performing the same steps as the firing step as the element body material and the protective layer material. ], The coefficient of linear thermal expansion αa'[ppm / K] and the linear thermal expansion of the protective layer material at the temperature T1 [° C.] to the temperature T2 [° C.] after the same step as the firing step is performed. A coefficient αb [ppm / K] that satisfies the relationship of αa'> αb is used.

In this manufacturing method, in the arrangement process, the solid electrolyte layer of the element body material after the same process as the firing step is used as the element body material and the protective layer material at a temperature T1 [° C.] to a temperature T2 [° C.]. The coefficient of linear thermal expansion αa'[ppm / K] and the coefficient of linear thermal expansion αb [ppm / K] at the temperature T1 [° C.] to the temperature T2 [° C.] of the protective layer material after performing the same process as the firing step. Since the one that satisfies the relationship of αa'> αb is used, compressive stress can be applied to the protective layer in the temperature lowering step. This point will be described below. When the element body material and the protective layer material are individually cooled from the temperature T2 [° C.] to the temperature T1 [° C.] after the same process as the firing step, the linear shrinkage rate Sa'[of the solid electrolyte layer of the element body material. %] Is Sa'= αa' × (T2-T1) × 10 ^-4 , and the linear shrinkage rate Sb [%] of the protective layer material is Sb = αb × (T2-T1) × 10 ^-4 . Further, since most of the element body is formed of the solid electrolyte layer, the linear shrinkage rate Sa [%] of the element body material after the same step as the firing step is the linear shrinkage rate Sa [%] of the solid electrolyte layer. 'It is almost the same as [%]. Therefore, when the relationship of αa'> αb is satisfied, it can be said that the relationship of Sa (≈Sa')> Sb is satisfied. When the relationship of Sa> Sb is satisfied, when the element main body material and the protective layer material are individually cooled from the temperature T2 [° C.] to the temperature T1 [° C.] after the same process as the firing step, the element is more than the protective layer material. The main body material shrinks more. By the way, in the firing step, the protective layer material and the element main body material in the state where the arrangement step has been performed are heated at a temperature T2 [° C.] to fire at least the protective layer material, so that the protective layer material in the state where the firing step has been performed is protected. The layer and the element body are in close contact with or bonded to each other at least in part. Therefore, when the temperature of the protective layer and the element body in which the firing step has been performed is lowered from the temperature T2 [° C.] to the temperature T1 [° C.] in the subsequent temperature lowering step, the protective layer is dragged by the large shrinkage of the element body. Residual compressive stress is applied to the surface. In this way, a protective layer having residual compressive stress on the surface is obtained. Therefore, a sensor element having a protective layer having high water resistance can be obtained.

In the method for manufacturing a sensor element of the present invention, the element body after firing may be used as the element body material in the arrangement step. In this case, since it is not necessary to fire the element main body material in the firing step, the temperature T2 [° C.] can be set relatively low.

A vertical sectional view of the gas sensor 100. FIG. 5 is a perspective view schematically showing an example of the configuration of the sensor element 101. FIG. 2 is a vertical cross-sectional view of FIG. FIG. 2 is a cross-sectional view taken along the line AA of FIG. BB sectional view of FIG. FIG. 2 is a partial cross-sectional view of the vertical cross section of FIG. FIG. 2 is a partial cross-sectional view of the cross section of FIG. The development view which shows the inside state of the side part 90c and the side part connection part 90e. FIG. 3 is a cross-sectional view of the mesh sealing portion 94. It is explanatory drawing which shows the state of making the unfired body 190 by the molding die 150. It is explanatory drawing which shows the state of inserting the tip end part 101b of the sensor element main body 101a into the unfired body 190, and firing. It is explanatory drawing which shows the state of inserting the tip end part 101b of the sensor element main body 101a into the unfired body 190, and firing. Explanatory drawing of the protection layer 90 of the modification. Explanatory drawing of the protection layer 90 of the modification. Explanatory drawing of the protection layer 90 of the modification. Explanatory drawing of the protection layer 90 of the modification. The explanatory view of the sensor element 101 of the modification. The explanatory view of the eye sealing part 94 of the modification. The explanatory view of the arrangement process of the modification. A graph showing the relationship between residual compressive stress and water resistance.

Next, an embodiment of the present invention will be described with reference to the drawings. 1 is a vertical sectional view of the 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 vertical sectional view of FIG. 2, and FIG. 2A is a sectional view taken along the line AA, 5 is a sectional view taken along the line BB of FIG. 2, FIG. 6 is a partial sectional view of a vertical sectional view of FIG. 2, FIG. 7 is a partial sectional view of a cross section of FIG. A developed view showing the inside of the side portion 90c and the side portion connecting portion 90e, and FIG. 9 is a cross-sectional view of the eye sealing portion 94.

The gas sensor 100 includes a sensor element 101, a protective cover 110 that covers and protects one end (lower end in FIG. 1) of the sensor element 101 in the longitudinal direction, an element encapsulant 120 that encloses and fixes the sensor element 101, and element encapsulation. It includes a nut 130 attached to the body 120. As shown in the figure, this 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 (NOx in this embodiment) contained in the exhaust gas as a gas to be measured. Be done. The sensor element 101 includes a sensor element main body 101a, a porous protective layer 90 that covers the sensor element main body 101a, and a mesh sealing portion 94.

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 into 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 sealed in a through hole inside the main metal fitting 122, and a talc or the like sealed in the through hole inside the main metal fitting 122. It includes 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 a portion of the protective cover 110 protruding into the pipe 140.

The sensor element main body 101a of the sensor element 101 has a long rectangular parallelepiped shape as shown in FIGS. 2 and 3. The sensor element 101 will be described in detail below, but for convenience of explanation, the longitudinal direction of the sensor element body 101a is referred to as a front-rear direction, the thickness direction of the sensor element body 101a is referred to as a vertical direction, and the width direction of the sensor element body 101a is referred to as a left-right direction. I will do it. 3 and 6 show a cross section parallel to the front-back and up-down directions, FIG. 7 shows a cross section parallel to the front-back and left-right directions, and FIGS. ing.

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 having a laminated body (sensor element main body 101a) 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. is there. Further, the solid electrolyte forming these six layers is a dense and airtight one. The sensor element main body 101a 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 gas inlet 10 are located between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4 at one tip (end in the front direction) of the sensor element main body 101a. The diffusion rate-determining section 11, the buffer space 12, the second diffusion rate-determining section 13, the first internal space 20, the third diffusion rate-determining section 30, and the second internal space 40 communicate in this order. It is formed adjacent to each other.

The gas inlet 10, the buffer space 12, the first internal space 20, and the second internal space 40 are provided with the spacer layer 5 hollowed out so that the upper portion 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 main body 101a partitioned by the side surface of the spacer layer 5.

The first diffusion rate-determining section 11, the second diffusion rate-determining section 13, and the third diffusion rate-determining section 30 are all provided as two horizontally long slits (the openings have a longitudinal direction in the direction perpendicular to the drawing). .. The space from the gas introduction port 10 to the second internal vacant space 40 is referred to as a gas distribution unit 9 to be measured. The gas flow unit 9 to be measured is formed in a substantially rectangular parallelepiped shape. The longitudinal direction of the gas flow section 9 to be measured is parallel to the front-rear direction.

Further, at a position far from the tip side of the gas flow portion 9 to be measured, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, the side portion is the side surface of the first solid electrolyte layer 4. A reference gas introduction space 43 is provided at a position partitioned by. 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 so as to be 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 air 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 9 to be measured, the gas introduction port 10 is a portion that is open to the external space so that the gas to be measured is taken into the sensor element main body 101a from the external space through the gas introduction port 10. It has become. The first diffusion rate-determining 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-determining unit 11 to the second diffusion rate-determining unit 13. The second diffusion rate-determining 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 main body 101a 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 exhaust pressure The gas to be measured, which is rapidly taken into the inside of the sensor element main body 101a from the gas introduction port 10 by pulsation), is not directly introduced into the first internal space 20, but the first diffusion rate-determining unit 11, the buffer space 12, and the buffer space 12. After the pressure fluctuation of the gas to be measured is canceled through the second diffusion rate-determining unit 13, the gas is introduced into the first internal space 20. As a result, the pressure fluctuation of the gas to be measured 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-determining 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 an upper surface of the second solid electrolyte layer 6. Electricity composed of an outer pump electrode 23 provided in a region corresponding to the ceiling electrode portion 22a so as to be exposed to the outside of the sensor element main body 101a, and a second solid electrolyte layer 6 sandwiched between these electrodes. It is a chemical pump cell. The outer pump electrode 23 is provided on the upper surface of the sensor element main body 101a.

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. A spacer layer in which electrode portions 22b are formed, and side electrode portions (not shown) form both side wall portions of the first internal space 20 so as to connect the ceiling electrode portions 22a and the bottom electrode portions 22b. It is formed on the side wall surface (inner surface) of No. 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 the target value. As a result, the oxygen concentration in the first internal space 20 can be maintained at a predetermined constant value.

The third diffusion rate-determining unit 30 imparts a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal space 20, and applies the gas to be measured. It is a part leading to the second internal 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-determining unit 30. The NOx concentration is mainly measured in the second internal space 40 whose oxygen concentration is adjusted by the auxiliary pump cell 50, and further 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-determining 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 gas sensor 100 can measure the NOx concentration with high accuracy.

The auxiliary pump cell 50 includes 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 suitable electrode on the outside of the sensor element main body 101a) 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 with 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 partial pressure of oxygen 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.

Along with this, 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 controlling the main pump as a control signal, and the above-mentioned target value of the electromotive force V0 is controlled from the third diffusion rate-determining unit 30. The gradient of the oxygen partial pressure in the gas to be measured introduced into the second internal space 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-determining portion 30, and an outer pump electrode 23. , A second solid electrolyte layer 6, a spacer layer 5, and a 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 existing in the atmosphere in the second internal space 40. Further, the measurement electrode 44 is covered with the fourth diffusion rate-determining portion 45.

The fourth diffusion rate-determining unit 45 is a film made of a ceramic porous body. The fourth diffusion rate-determining unit 45 plays a role of limiting the amount of NOx flowing into the measurement electrode 44, and also functions as a protective film of the measurement electrode 44. In the measurement pump cell 41, oxygen generated by decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44 can be pumped out, and the amount of oxygen 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 used 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-determining 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. Then, the generated oxygen is pumped by the measurement pump cell 41, and at that time, a variable power source is used so that the electromotive force 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 measurement pump cell 41. The concentration will be calculated.

Further, if the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 are combined to form an oxygen partial pressure detecting means as an electrochemical sensor cell, the measuring 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, and thereby the concentration of the NOx component in the gas to be measured. 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 partial pressure of oxygen in the gas to be measured 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 gas to be measured 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 gas to be measured. You can know it.

Further, the sensor element main body 101a is provided with a heater unit 70 which plays a role of temperature adjustment for heating and keeping the sensor element main body 101a warm 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 in a manner of 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 when power is supplied from the outside through the heater connector electrode 71 to heat and retain heat of the solid electrolyte forming the sensor element main body 101a. Do.

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 main body 101a can be adjusted to a temperature at which the solid electrolyte is activated. It has become.

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 the atmosphere introduction layer 48 and communicate with the reference gas introduction space 43, and the internal pressure rises with the temperature rise in the heater insulating layer 74. It is formed for the purpose of alleviating.

Here, NOx is detected by utilizing the oxygen ion conductivity of the solid electrolyte layer (third substrate layer 3, first solid electrolyte layer 4, spacer layer 5 and second solid electrolyte layer 6) in the sensor element main body 101a. The portion provided with the electrode group (inner pump electrode 22, outer pump electrode 23, auxiliary pump electrode 51, and measurement electrode 44) used for the operation is referred to as a tip portion 101b. The tip portion 101b is a portion (front end portion) from the front end surface (the surface including the gas introduction port 10) of the sensor element main body 101a to a predetermined position exceeding the measurement electrode 44. The rear end of the tip portion 101b is located behind the rear end of the gas flow portion 9 to be measured. That is, the gas flow unit 9 to be measured is included in the tip 101b. A buffer layer 84 is provided on the upper surface and the lower surface of the tip portion 101b. Further, the periphery of the tip portion 101b is covered with a protective layer 90.

As shown in FIGS. 2 and 3, the sensor element main body 101a includes a buffer layer 84. The buffer layer 84 includes an upper buffer layer 84a that covers at least a part of the upper surface of the second solid electrolyte layer 6, and a lower buffer layer 84b that covers at least a part of the lower surface of the first substrate layer 1. There is. The upper buffer layer 84a also covers the outer pump electrode 23. The upper buffer layer 84a is arranged on the upper surface portion of the tip portion 101b, and exists to the rear of the tip portion 101b. The lower buffer layer 84b is arranged on the lower surface portion of the tip portion 101b, and exists to the rear of the tip portion 101b. Therefore, as shown in FIGS. 2 and 3, each of the upper buffer layer 84a and the lower buffer layer 84b protrudes behind the protective layer 90. The buffer layer 84 is made of porous ceramics such as alumina, zirconia, spinel, cordierite, and magnesia. The main component of the buffer layer 84 is preferably the same as the main component of the protective layer 90. In the present embodiment, the buffer layer 84 is made of porous ceramics made of alumina. Although not particularly limited, the film thickness of the buffer layer 84 is, for example, 5 to 50 μm. The porosity of the buffer layer 84 is preferably 10% to 71%. The porosity of the buffer layer 84 may be 70% or less, or 60% or less. Further, the arithmetic mean roughness Ra of the surface of the buffer layer 84 (the upper surface of the upper buffer layer 84a and the lower surface of the lower buffer layer 84b) is preferably 2.0 to 5.0 μm. The buffer layer 84 plays a role of enhancing the adhesion between the sensor element main body 101a and the protective layer 90.

The protective layer 90 includes a main body 90a and a columnar portion 91 arranged between the main body 90a and the sensor element main body 101a. The main body 90a covers the entire front end surface of the sensor element main body 101a provided with the gas introduction port 10, and covers a part of the upper surface, the lower surface, the left surface, and the right surface of the sensor element main body 101a connected to the front end surface. It is provided in a bottomed tubular shape (also called a cap shape). The portion of the main body 90a that covers the front end surface of the sensor element main body 101a is referred to as a bottom 90b. Of the main body 90a, the portion that covers the upper, lower, left, and right surfaces of the sensor element main body 101a is referred to as a side 90c. The main body 90a also covers the portion of the buffer layer 84 included in the tip 101b. The portion of the main body 90a that connects the bottom 90b and the side 90c is referred to as a bottom connecting portion 90d. Of the main body 90a, the portion that connects the side 90c to each other is referred to as a side connection 90e. Further, among the side portions 90c of the main body portion 90a, the portion covering the upper surface of the sensor element main body 101a is the upper side portion 92a, the portion covering the lower surface of the sensor element main body 101a is the lower side portion 92b, and the left surface of the sensor element main body 101a is. The covering portion is referred to as a left side portion 92c, and the portion covering the right surface of the sensor element main body 101a is referred to as a right side portion 92d. The main body 90a also covers the outer pump electrode 23 provided on the upper surface of the sensor element main body 101a. Therefore, the protective layer 90 plays a role of suppressing the adhesion of toxic substances such as oil components contained in the gas to be measured to the outer pump electrode 23 and suppressing the deterioration of the outer pump electrode 23. The main body 90a also covers the gas inlet 10, but since the protective layer 90 is made of a porous body, the gas to be measured can flow through the inside of the protective layer 90 and reach the gas inlet 10. ..

The thickness t (see FIG. 4) of the side portion 90c of the main body portion 90a may be, for example, 1.5 mm or less, 1 mm or less, 600 μm or less, or 400 μm or less. The thickness t of the side portion 90c of the main body portion 90a may be, for example, 100 μm or more, or 200 μm or more. Further, among the side portions 90c, the thickness t of any one or more of the upper side portion 92a, the lower side portion 92b, the left side portion 92c and the right side portion 92d satisfies at least one of the above ranges. May be good. Further, the thickness t of the bottom 90b of the main body 90a may also satisfy at least one of the above ranges, or the thickness t may satisfy at least one of the above ranges over the entire area of the main body 90a.

A space 95 exists between the surface of the main body portion 90a and the surface of the tip portion 101b. The space 95 includes an upper space 95a, a lower space 95b, a left space 95c, a right space 95d, and a front space 95e. The upper space 95a is a space between the main body 90a and the upper surface of the sensor element main body 101a. The lower space 95b is a space between the main body 90a and the lower surface of the sensor element main body 101a. The left side space 95c is a space between the main body 90a and the left surface of the sensor element main body 101a. The right side space 95d is a space between the main body 90a and the right surface of the sensor element main body 101a. The front space 95e is a space between the main body 90a and the front surface of the sensor element main body 101a. The height of the upper space 95a may be 10 μm or more, 20 μm or more, or 50 μm or more. The height of the upper space 95a is preferably 1 mm or less, more preferably 400 μm or less, and even more preferably 300 μm or less. The numerical range of these heights is the same for the lower space 95b, the left space 95c, the right space 95d, and the front space 95e. The height direction of the upper space 95a and the lower space 95b is the vertical direction, the height direction of the left side space 95c and the right side space 95d is the left-right direction, and the height direction of the front side space 95e is the front-rear direction. This also applies to the height directions of the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, the right columnar portion 91d, and the front columnar portion 91e, which will be described later.

The columnar portion 91 is provided inside the main body portion 90a. The columnar portion 91 is integrally molded with the main body portion 90a, and protrudes from the connection portion with the main body portion 90a toward the sensor element main body 101a. The columnar portion 91 includes one or more upper columnar portions 91a, one or more lower columnar portions 91b, one or more left side columnar portions 91c, one or more right side columnar portions 91d, and one or more front side columnar portions 91e. Of the columnar portions 91, those provided inside the side portion 90c of the main body portion 90a, that is, the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d are formed into side columnar portions. It is called. The columnar portion 91 serves as a space support portion that supports the space 95 between the protective layer 90 and the sensor element main body 101a. The upper columnar portion 91a supports the upper space 95a in a direction perpendicular to the upper surface of the sensor element main body 101a. That is, the upper columnar portion 91a supports the upper space 95a up and down. In the present embodiment, there are two upper columnar portions 91a, which are arranged side by side. The lower columnar portion 91b supports the lower space 95b in a direction perpendicular to the lower surface of the sensor element main body 101a. That is, the lower columnar portion 91b supports the lower space 95b up and down. In the present embodiment, there are two lower columnar portions 91b, which are arranged side by side. The left columnar portion 91c supports the left side space 95c in a direction perpendicular to the left surface of the sensor element main body 101a. That is, the left columnar portion 91c supports the left side space 95c to the left and right. In this embodiment, there is one left columnar portion 91c. The right columnar portion 91d supports the right side space 95d in a direction perpendicular to the right surface of the sensor element main body 101a. That is, the right columnar portion 91d supports the right side space 95d to the left and right. In this embodiment, there is one right columnar portion 91d. The front columnar portion 91e supports the front space 95e in a direction perpendicular to the front surface of the sensor element main body 101a. That is, the front columnar portion 91e supports the front space 95e back and forth. In the present embodiment, there are two front columnar portions 91e, which are arranged side by side.

The longitudinal direction of the upper columnar portion 91a is along the longitudinal direction of the sensor element main body 101a, that is, the front-rear direction. As shown in FIGS. 4 to 6, the upper columnar portion 91a has a length and a width from the connection portion with the side portion 90c to the facing surface facing the upper surface of the sensor element main body 101a (here, the lower surface of the upper columnar portion 91a). Is constant, and the lower surface of the upper columnar portion 91a is flat. The front end of the upper columnar portion 91a is connected to the bottom portion 90b of the main body portion 90a. In other words, there is no space 95 between the front of the upper columnar portion 91a and the bottom portion 90b of the main body portion 90a. The upper columnar portion 91a is not arranged up to the rear end of the main body portion 90a. Therefore, as shown in FIGS. 3 and 6, the rear end portion 93a of the upper columnar portion 91a and the eye sealing portion 94 are separated from each other in the front-rear direction, and the upper columnar portion 91a and the eye sealing portion 94 are in contact with each other. Absent. The lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d also have the same configuration as the upper columnar portion 91a.

As shown in FIG. 4, the two upper columnar portions 91a are 1 on the left and right sides of the region 102a so as to avoid the region 102a in which the gas flow portion 9 to be measured is projected onto the upper surface of the sensor element main body 101a. They are arranged one by one. Similarly, one of the two lower columnar portions 91b is arranged on each of the left and right sides of the region 102b so as to avoid the region 102b of the lower surface of the sensor element main body 101a where the gas flow portion 9 to be measured is projected onto the lower surface. Has been done. Similarly, each of the left columnar portion 91c and the right columnar portion 91d is arranged so as to avoid the regions 102c and 102d on which the gas flow portion 9 to be measured is projected on the left surface and the right surface of the sensor element main body 101a. As shown in FIG. 4, in the present embodiment, of the upper surface, the lower surface, the left surface, and the right surface of the sensor element main body 101a, the upper surface is the surface closest to the gas flow unit 9 to be measured. Further, the outer pump electrode 23 is arranged in the region 102a on the upper surface of the sensor element main body 101a.

As shown in FIGS. 3 and 5, the longitudinal direction of the front columnar portion 91e is along the vertical direction. As shown in FIGS. 6 and 7, the front columnar portion 91e extends from the connection portion of the main body portion 90a with the bottom portion 90b to the facing surface facing the front surface of the sensor element main body 101a (here, the rear surface of the front side columnar portion 91e). The width and width are constant, and the rear surface of the front columnar portion 91e is flat. As shown in FIGS. 5 and 7, the two front columnar portions 91e are formed on both the left and right sides of the region 102e so as to avoid the region 102e in which the gas flow portion 9 to be measured is projected onto the front surface of the sensor element main body 101a. One is placed in each. Since the front columnar portion 91e avoids the region 102e, the gas introduction port 10 is also avoided.

The height of the upper space 95a can be adjusted by adjusting the height H of the upper columnar portion 91a (see FIG. 4). Therefore, the height H of the upper columnar portion 91a may be 10 μm or more, 20 μm or more, or 50 μm or more. The height H of the upper columnar portion 91a is preferably 1 mm or less, more preferably 400 μm or less, still more preferably 300 μm or less. The numerical range of these heights H is the same for the lower columnar portion 91b, the left columnar portion 91c, the right columnar portion 91d, and the front columnar portion 91e. The height H of the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d is a length protruding from the side portion 90c of the main body portion 90a. Further, the height H of the front columnar portion 91e is a length protruding from the bottom portion 90b of the main body portion 90a.

The width W (see FIG. 4) of the upper columnar portion 91a is preferably at least 1 times its own height H, for example. The width W of the upper columnar portion 91a is preferably 100 μm or more, more preferably 200 μm or more, still more preferably 300 μm or more. Further, the width W of the upper columnar portion 91a may be 500 μm or less, or 400 μm or less. The numerical range of these widths W is the same for the lower columnar portion 91b, the left columnar portion 91c, the right columnar portion 91d, and the front columnar portion 91e. The width W of each of the columnar portions 91a to 91e is the length in the shortest length direction (in the present embodiment, each short direction) in the direction perpendicular to each height H. Here, the width W of the upper columnar portion 91a, the lower columnar portion 91b, and the front columnar portion 91e is the length in the left-right direction, and the width W of the left columnar portion 91c and the right columnar portion 91d is the length in the vertical direction. ..

The length L (see FIG. 6) of the upper columnar portion 91a is preferably, for example, one or more times its own height H. The length L of the upper columnar portion 91a is preferably 100 μm or more, more preferably 200 μm or more, still more preferably 300 μm or more. The length L of the upper columnar portion 91a may be 20000 μm or less, or 15000 μm or less. The numerical range of these lengths L is the same for the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d. The length L of the front columnar portion 91e is preferably 100 μm or more, more preferably 200 μm or more, still more preferably 300 μm or more. The length L of the front columnar portion 91e may be 1300 μm or less, or 1000 μm or less. The length L of each of the columnar portions 91a to 91e is the length in the direction perpendicular to the height H and the width W (in the present embodiment, each longitudinal direction). Here, the length L of the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d is the length in the front-rear direction, and the length L of the front columnar portion 91e is the length in the vertical direction. Is.

The side columnar portions, that is, the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d are provided in a range of 2% or more and 35% or less inside the side portion 90c of the main body portion 90a. Is preferable. This range will be described with reference to FIG. FIG. 8 is a view in which the side portion 90c and the side portion connecting portion 90e of the main body portion 90a are developed so that the inside is in front of the paper surface. Here, the area of the upper side portion 92a is Aa, the area of the lower side portion 92b is Ab, the area of the left side portion 92c is Ac, the area of the right side portion 92d is Ad, and the upper side portion of each upper columnar portion 91a is defined as Ad. The area of the connection portion with the 92a is Ba, the area of the connection portion between each lower columnar portion 91b and the lower side portion 92b is Bb, the area of the connection portion of the left columnar portion 91c with the left side side portion 92c is Bc, and the right side. Let Bd be the area of the connecting portion of the columnar portion 91d with the right side portion 92d. Here, the ratio of the total area (2Ba + 2Bb + Bc + Bd in this embodiment) of the portion where each columnar portion 91a to 91d is provided to the total area inside each side portion 92a to 92d (Aa + Ab + Ac + Ad in this embodiment), that is, , (2Ba + 2Bb + Bc + Bd) / (Aa + Ab + Ac + Ad) × 100 [%] is preferably within the above range. The area Aa of the upper side portion 92a includes the area of the portion where the upper columnar portion 91a is formed, and is equal to the area of the upper surface of the tip portion 101b of the sensor element main body 101a in the present embodiment. The same applies to the area Ab of the lower side portion 92b, the area Ac of the left side portion 92c, and the area Ad of the right side portion 92d. Further, the upper columnar portion 91a is preferably provided in a range of 2% or more and 35% or less inside the upper side portion 92a. Here, the ratio of the total area (2Ba in the present embodiment) of the portion provided with the two upper columnar portions 91a to the area of the upper side portion 92a (Aa in the present embodiment), that is, 2Ba / Aa. It is preferable that the value of × 100 [%] is within the above range. The same applies to the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d. Further, the front columnar portion 91e is preferably provided in a range of 2% or more and 35% or less inside the bottom portion 90b of the main body portion 90a. In the present embodiment, when the bottom 90b of the main body 90a is viewed from the rear, the area of the bottom 90b is Ae, and the area of the connection portion of each front columnar portion 91e with the bottom 90b is Be. In the embodiment, the ratio of the total area (2Be in the present embodiment) of the portion provided with the two front columnar portions 91e to Ae), that is, the value of 2Be / Ae × 100 [%] is in the above range. Is preferable. The area Ae of the bottom portion 90b includes the area of the portion where the front columnar portion 91e is formed, and is equal to the area of the front end surface of the sensor element main body 101a in the present embodiment.

The side columnar portion, that is, the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d is 2% of the portion of the sensor element main body 101a covered by the side portion 90c of the protective layer 90. It is preferable that it is in close contact with or bonded to the range of 35% or more. Further, the upper columnar portion 91a is in close contact with or coupled with a range of 2% or more and 35% or less of the portion of the sensor element main body 101a covered with the upper side portion 92a of the protective layer 90 (here, the upper surface of the tip portion 101b). Is preferable. The same applies to the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d. Further, the front columnar portion 91e is in close contact with or coupled with a range of 2% or more and 35% or less of the portion of the sensor element main body 101a covered with the bottom portion 90b of the protective layer 90 (here, the front end surface of the tip portion 101b). It is preferable to have. In the present embodiment, the area of the portion of the sensor element main body 101a covered by the upper side portion 92a is the same as the area Aa described above, and the area of the sensor element main body 101a covered by the lower side portion 92b is the same. The area of the above-mentioned area Ab is the same as the above-mentioned area Ab, and the area of the portion of the sensor element main body 101a covered by the left side portion 92c is the same as the above-mentioned area Ac. The area of the covered portion is the same as the above-mentioned area Ad, and the area of the portion of the sensor element main body 101a covered by the bottom 90b is the same as the above-mentioned area Ae. Further, the area of each upper columnar portion 91a that is in close contact with or coupled to the sensor element main body 101a (here, the lower surface of each upper columnar portion 91a) is the same as the above-mentioned area Ba, and in each lower columnar portion 91b. The area of the portion (here, the upper surface of each lower columnar portion 91b) that is in close contact with or coupled to the sensor element main body 101a is the same as the area Bb described above, and the left columnar portion 91c is in close contact or coupled with the sensor element main body 101a. The area of the portion (here, the right surface of the left columnar portion 91c) is the same as the area Bc described above, and the portion of the right columnar portion 91d that is in close contact with or coupled to the sensor element main body 101a (here, the right columnar portion 91d). The area of the left surface) is the same as the area Bd described above, and the area of the portion of each front columnar portion 91e that is in close contact with or coupled to the sensor element main body 101a (here, the rear surface of each front columnar portion 91e) is the area Be described above. Is the same as.

As shown in FIGS. 4 and 5, one or more upper columnar portions 91a and one or more lower columnar portions 91b are arranged so that their left and right positions are at least partially overlapped with each other at positions corresponding to each other. It is installed. That is, the left and right positions of the upper left columnar portion 91a and the left lower columnar portion 91b overlap at least partially, and the left and right positions of the right upper columnar portion 91a and the right lower columnar portion 91b are At least some overlap. Further, in the present embodiment, the width W of the upper columnar portion 91a and the width W of the lower columnar portion 91b are the same values, and the left and right positions of the upper columnar portion 91a on the left side and the lower columnar portion 91b on the left side are one. However, the left and right positions of the upper columnar portion 91a on the right side and the lower columnar portion 91b on the right side are the same.

Similarly, the one or more upper columnar portions 91a and the one or more front columnar portions 91e are also arranged so that the left and right positions of the ones or more corresponding to each other are at least partially overlapped with each other (FIG. 5). Further, in the present embodiment, the width W of the upper columnar portion 91a and the width W of the front columnar portion 91e are the same values, and the left and right positions of the upper columnar portion 91a on the left side and the front columnar portion 91e on the left side coincide with each other. The left and right positions of the upper columnar portion 91a on the right side and the front columnar portion 91e on the right side coincide with each other.

As shown in FIGS. 4 and 5, the upper and lower positions of the left columnar portion 91c and the right columnar portion 91d overlap at least partially. Further, in the present embodiment, the width W of the left columnar portion 91c and the width W of the right columnar portion 91d are the same values, and their upper and lower positions are the same.

The protective layer 90 has a residual compressive stress on its surface at at least one temperature T1 [° C.] included in the temperature range of 500 ° C. or higher and 900 ° C. or lower. The temperature T1 [° C.] is the temperature of the protective layer 90 when the sensor element 101 is used. The temperature T1 [° C.] is, for example, the same as the temperature of the hottest portion of the surface of the protective layer 90 (here, the portion directly above the outer pump electrode 23) when the sensor element 101 is used, for example, 700. ℃. For example, when the hottest portion of the sensor element main body 101a is 800 ° C., the surface of the protective layer 90 is about 700 ° C. The residual compressive stress is preferably present on the entire surface of the protective layer 90, but may be present on at least a part of the surface, for example, the upper side portion 92a, the lower side portion 92b, the left side portion 92c, and the right side portion. It may be present on at least one surface of the side portion 92d. The location where the residual compressive stress is measured is not particularly limited, but may be, for example, at least one of the upper side portion 92a, the lower side portion 92b, the left side portion 92c, and the right side portion 92d. The location where the residual compressive stress is measured may be, for example, a portion of the surface of the protective layer 90 where the temperature becomes particularly high when the sensor element 101 is used (here, a portion directly above the outer pump electrode 23). The residual compressive stress on the surface of the protective layer 90 is the sin ² ψ method using X-ray diffraction (J. Soc. Mat. Sci., Japan, Vol. 48, No. 10, pp. 1147-1154, Oct. 1999. See). In this sensor element 101, the residual compressive stress is, for example, 15 MPa or more and 300 MPa or less. The residual compressive stress of the sensor element 101 of the present embodiment can be measured as follows, for example. First, the sensor element 101 is held at a uniform temperature of temperature T1 (for example, 700 ° C.), and the residual stress of the portion of the surface of the protective layer 90 that becomes the highest temperature when the sensor element 101 is used is determined by using an X-ray diffractometer. To measure. Specifically, each sensor element 101 is placed on a Pt heater in a high temperature chamber and measured by a wide-angle X-ray diffraction method. As the X-ray diffractometer, D8ADVANCE (encapsulated tube type) manufactured by Bruker AXS is used, and the X-ray source is CuKα ray (Gobel mirror (parallel beam)). The scan method is 2θ / θ scan, and the measurement range is 133 to 138 ° (Al ₂ O ₃ (146)). In the stress evaluation by the X-ray diffraction method, the sin ² ψ method is used, and the stress value σ of the sample is calculated using Eq. (1) from the change in the lattice plane spacing (d value) when tilted by a certain ψ from the symmetric reflection. calculate.
σ =-[E / {2 (1 + ν)}] cotθ ₀ [∂ (2θ) / ∂ (sin ² ψ)]
... Equation (1)
Here, σ is the stress (MPa), E is the Young's modulus (MPa), ν is the Poisson's ratio, and θ ₀ is the standard Bragg angle. 340 GPa is used as the Young's modulus, and 0.2 is used as the Poisson's ratio. The standard Bragg angle θ ₀ is the value of symmetric reflection (ψ = 0 °). Further, ∂ (2θ) / ∂ (sin ² ψ) is calculated from the slope of ^{the 2θ-sin 2} ψ diagram in which 2θ is taken on the horizontal axis and sin ^{2 ψ is taken on the vertical axis.} In addition, σ obtained by using the equation (1) has a negative value in the case of compressive stress, but in the present specification, when the value of residual compressive stress is indicated, it indicates the absolute value of residual compressive stress. ..

The protective layer 90 is a porous body and preferably contains ceramic particles as constituent particles, and more preferably contains at least one of alumina, mullite, cordelite, spinel, zirconia, titania, and magnesia. In the present embodiment, the protective layer 90 is made of a porous alumina body. The porosity of the protective layer 90 is, for example, 5% to 45%. The porosity of the protective layer 90 may be 20% or more.

The mesh sealing portion 94 is a porous body that covers one or more of the surface along the longitudinal direction of the sensor element main body 101a, that is, the upper surface, the lower surface, the left surface, and the right surface of the sensor element main body 101a. In the present embodiment, as shown in FIG. 9, the eye-sealing portion 94 is the upper surface (here, the upper surface of the upper buffer layer 84a), the lower surface (here, the lower surface of the lower buffer layer 84b), and the left surface of the sensor element main body 101a. Both the right side and the right side are covered and are in close contact with each of these sides. Adjacent portions of the sealing portion 94 that cover the upper surface, the lower surface, the left surface, and the right surface of the sensor element main body 101a are connected to each other. Further, the seal sealing portion 94 is arranged so as to be in contact with the rear end of the protective layer 90. More specifically, the eye-sealing portion 94 is in contact with the rear end surface of the main body portion 90a. As a result, the sealing portion 94 closes the opening at the rear end of the main body portion 90a, that is, the rear ends of the upper space 95a, the lower space 95b, the left side space 95c, and the right side space 95d. However, since the sealing portion 94 is also a porous body, the gas to be measured can pass through the sealing portion 94. The porosity of the sealing portion 94 is, for example, 10% to 50%. The porosity of the sealing portion 94 may exceed 20%.

The porosity of the protective layer 90 and the sealing portion 94 is a value derived as follows using an image (SEM image) obtained by observing with a scanning electron microscope (SEM). First, the protective layer 90 is cut along the thickness direction of the measurement target so that the cross section of the measurement target (for example, the protective layer 90) is the observation surface, and the cut surface is resin-filled and polished to prepare an observation sample. .. Subsequently, an SEM image to be measured is obtained by photographing the observation surface of the observation sample with an SEM photograph (secondary electron image, acceleration voltage 15 kV, magnification 2000 times). Next, by performing image analysis on the obtained image, a threshold value is determined by a discriminant analysis method (binarization of Otsu) from the brightness distribution of the brightness data of the pixels in the image. After that, each pixel in the image is binarized into an object portion and a pore portion based on the determined threshold value, and the area of the object portion and the area of the pore portion are calculated. Then, the ratio of the area of the pore portion to the total area (total area of the object portion and the pore portion) is derived as the porosity (unit:%).

The sealing portion 94 preferably contains ceramic particles as constituent particles, and more preferably contains at least one of alumina, zirconia, spinel, cordierite, titania, and magnesia. Further, the main component of the sealing portion 94 is preferably the same as the main component of the protective layer 90. In the present embodiment, the sealing portion 94 is made of an alumina porous body containing alumina ceramic particles as a main component.

Next, a method of manufacturing such a gas sensor 100 will be described. First, a method of manufacturing the sensor element 101 of the gas sensor 100 will be described. FIG. 10 is an explanatory view showing how the unfired body 190 is produced by the molding die 150. 11 and 12 are explanatory views showing a state in which the tip portion 101b of the sensor element main body 101a is inserted into the unfired body 190 and fired. In the present embodiment, the manufacturing method of the sensor element 101 includes a preparation step, an arrangement step, a firing step, and a temperature lowering step. In the firing step, heating is performed at a temperature of T2 [° C.], and in the temperature lowering step, the temperature is lowered from the temperature T2 [° C.] to a temperature of T1 [° C.] or lower.

[Preparation process]
In this step, an element body material which is a sensor element body 101a before or after firing and a protective layer material which becomes a protective layer 90 by firing are prepared. As the element body material and the protective layer material, the coefficient of linear thermal expansion αa'[ppm] at the temperature T1 [° C.] to the temperature T2 [° C.] of the solid electrolyte layer of the element body material after the same step as the firing step is performed. / K] and the coefficient of linear thermal expansion αb [ppm / K] at the temperature T1 [° C.] to the temperature T2 [° C.] of the protective layer material after performing the same step as the firing step are αa'> αb. Prepare something that satisfies the relationship. Here, the temperature T1 [° C.] is as described above, for example, 700 ° C. The temperature T2 [° C.] is a temperature at which the protective layer material is fired in a firing step described later, and is, for example, any temperature included in a temperature range of 1100 ° C. or higher and 1200 ° C. or lower.

In the preparation step of the present embodiment, the sensor element main body 101a as the element main body material is prepared by manufacturing the sensor element main body 101a after firing. In the preparatory step, first, six unfired ceramic green sheets are prepared. The ceramic green sheet is produced, for example, by mixing ceramic particles, an organic solvent, a plasticizer, a binder, a sintering aid, etc., which are the main components of the sensor element main body 101a, to form a paste, and molding the sheet into a sheet. 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 heaters on a ceramic green sheet. Further, 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), a paste to be the upper buffer layer 84a after firing is screen-printed. Similarly, 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), a paste to be the lower buffer layer 84b after firing is screen-printed. The paste to be the upper buffer layer 84a and the lower buffer layer 84b is, for example, a raw material powder (alumina powder in this embodiment) made of the material of the buffer layer 84 described above, a pore-forming material, an organic binder, and an organic material. Use a mixture of solvents. Next, six ceramic green sheets having various patterns formed in this way are laminated to form a laminated body. The laminate is cut and cut into small laminates having the size of the sensor element main body 101a. This small laminate is the sensor element main body 101a before firing. Then, the small laminate is fired at a predetermined firing temperature (for example, 1300 to 1500 ° C.) to obtain the sensor element main body 101a. The paste that becomes the upper buffer layer 84a and the lower buffer layer 84b after firing may be printed after the above-mentioned laminate is prepared. Further, the raw material powder used for the paste to be the upper buffer layer 84a and the lower buffer layer 84b may be a raw material powder of cordierite instead of the alumina powder. The coefficient of linear thermal expansion αa ′ of the solid electrolyte layer of the element body material after the same step as the firing step at the temperature T1 [° C.] to the temperature T2 [° C.] is the solid electrolyte layer of the sensor element body 101a. Here, the material 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), that is, the main component of the sensor element main body 101a. It is determined by the material of the ceramic particles. Therefore, the coefficient of linear thermal expansion αa'can be set to a desired value by appropriately selecting the material of the ceramic particles.

In the preparatory step of the present embodiment, the unfired body 190 which becomes the protective layer 90 by firing is produced by a mold casting method (also referred to as a gel casting method) using a molding die 150, so that the unfired body 190 as a protective layer material is not fired. Prepare the body 190. Further, in the present embodiment, the unfired body 190 is integrally molded. The mold casting method is a method of solidifying a slurry by a chemical reaction of the slurry itself to form a molded product, and is described in, for example, Japanese Patent Application Laid-Open No. 2016-95287. When producing the unfired body 190, first, a predetermined molding die 150 is prepared (FIG. 10A). The molding die 150 has a first outer die 151 and a second outer die 152 having a shape obtained by dividing the outer die into two, and an insertion portion 153a inserted inside the first outer die 151 and the second outer die 152. It is equipped with an inner mold 153. The first outer mold 151 and the second outer mold 152 have recesses corresponding to the outer shape of the unfired body 190. Further, the first outer mold 151 and the second outer mold 152 have a notch 151a and a notch 152a for allowing the slurry to flow in. The outer shape of the insertion portion 153a of the inner mold 153 corresponds to the inner shape of the unfired body 190, and the insertion portion 153a has grooves and recesses for forming the columnar portion 91. Subsequently, the first outer mold 151, the second outer mold 152 and the inner mold 153 are brought into close contact with each other, and the molding mold 150 is inserted in a state where the insertion portion 153a is inserted inside the first outer mold 151 and the second outer mold 152. Fix (Fig. 10B). In this state, the slurry used for mold casting is allowed to flow into the molding die 150 from the injection port composed of the notch 151a and the notch 152a. The components of the slurry will be described later, but those containing a gelling agent. Then, after the slurry is gelled by the polymerization reaction of the gelling agent to form the unfired body 190, the inner mold 153 is pulled out and the first outer mold 151 and the second outer mold 152 are removed to form the molding mold 150. The mold is released to obtain an unfired body 190 (FIG. 10C). Like the protective layer 90, the unfired body 190 has a main body 90a including a bottom 90b and a side 90c, and a columnar portion 91 (see FIGS. 11 and 12), and has a cap-like shape. There is. The unfired body 190 is preferably dried before or after the mold release of the mold 150.

The slurry will be explained in detail. The slurry used in the mold casting method contains, for example, ceramic particles, sintering aids, organic solvents, dispersants, and gelling agents, which are constituent particles of the protective layer 90 described above. The gelling agent is not particularly limited as long as it contains at least two types of polymerizable organic compounds, and examples thereof include those containing two types of organic compounds capable of urethane reaction. Examples of such two types of organic compounds include isocyanates and polyols. In preparing the slurry, first, a ceramic particle, a sintering aid, an organic solvent and a dispersant are added at a predetermined ratio and mixed over a predetermined time to prepare a slurry precursor. Then, immediately before using the slurry, a gelling agent is added to the slurry precursor and mixed to obtain a slurry. After the gelling agent is added to the slurry precursor, the chemical reaction (urethane reaction) of the gelling agent starts to proceed with the passage of time, so that it is preferable to quickly pour the slurry into the molding die 150. The coefficient of linear thermal expansion αb of the protective layer material at a temperature T1 [° C.] to a temperature T2 [° C.] after the same step as the firing step depends on the material of the protective layer 90, that is, the material of the ceramic particles contained in the slurry. It is decided. Therefore, the coefficient of linear thermal expansion αb can be set to a desired value by appropriately selecting the material of the ceramic particles.

[Placement process]
Subsequently, an arrangement in which the protective layer material (unfired body 190 in the present embodiment) is arranged on at least a part of the surface of the element main body material (sensor element main body 101a after firing in the present embodiment) prepared in the preparation step. Perform the process. In the arranging step of the present embodiment, the unfired body 190 and the sensor element main body 101a are arranged so that the tip portion 101b of the sensor element main body 101a is inserted inside the unfired body 190. Specifically, the tip portion 101b of the sensor element main body 101a is inserted inside the unfired body 190 (FIGS. 11A and 12A) until the front end of the sensor element main body 101a abuts on the front columnar portion 91e. 101b is inserted (FIGS. 11B and 12B). In this insertion, as shown in FIGS. 11 and 12, the longitudinal direction (here, the front-rear direction) of the sensor element main body 101a is along the vertical direction, and the unfired body 190 is located vertically above the sensor element main body 101a. It is preferable to carry out in a state.

By performing the arrangement step in this way, the tip portion 101b of the sensor element main body 101a is covered with the unfired body 190. Further, a space is formed between the unfired body 190 and the sensor element main body 101a by the space support portion (here, the columnar portion 91) that the unfired body 190 has inside. Specifically, the presence of the front columnar portion 91e inside the bottom portion 90b of the unfired body 190 causes the bottom portion 90b and the sensor element main body 101a to be separated from each other, and a space is formed between them. Similarly, the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d are present inside the side portion 90c of the unfired body 190, so that the side portion 90c and the sensor element main body 101a are combined. Are separated and a space is formed between them. The space between the side portion 90c and the sensor element main body 101a opens toward the rear end of the front end portion 101b.

[Baking process]
When the arranging step is performed, a firing step of firing the unfired body 190, which is a protective layer material, is performed. In the firing step, the unfired body 190 is fired by heating the protective layer material and the element main body material in the state where the arrangement step has been performed at a temperature T2 [° C.]. As a result, the unfired body 190 is fired to become the protective layer 90, the space between the unfired body 190 and the sensor element main body 101a becomes the space 95, and the space 95 between the protective layer 90 and the sensor element main body 101a. Is formed (FIGS. 11C, 12C). Since the unfired body 190 shrinks during firing, for example, even if there is a gap between the unfired body 190 and the columnar portion 91 (FIG. 12B), the gap disappears and the columnar portion 91 of the protective layer 90 and the sensor element main body 101a Can be brought into close contact with or combined with each other (Fig. 12C). Further, in consideration of the shrinkage of the cap of the unfired body 190 in the depth direction (vertical direction of FIG. 12), the unfired body 190 is provided so that the protective layer 90 after shrinkage can cover the tip portion 101b (FIG. 12C). It is preferable that the size of the protective layer 90 is longer than that of the protective layer 90 (FIG. 12B). At the time of firing, as shown in FIGS. 11C and 12C, the longitudinal direction (here, the front-rear direction) of the sensor element body 101a is along the vertical direction, and the unfired body 190 is located vertically above the sensor element body 101a. It is preferable to carry out in a state. Further, since the sensor element main body 101a has already been fired in the preparation step, it is preferable to fire the unfired body 190 at a temperature lower than the firing temperature of the sensor element main body 101a in the firing step. The firing temperature of the unfired body 190 is preferably 100 ° C. to 200 ° C. lower than the firing temperature of the sensor element main body 101a.

[Temperature lowering process]
When the firing step is performed, the temperature of the protective layer material (here, the protective layer 90) and the element main body material (here, the sensor element main body 101a) in the state where the firing step has been performed is lowered from the temperature T2 [° C.] to the temperature T1 [° C.]. Perform the temperature lowering process. Here, as described above, the element body material (here, the sensor element body 101a) and the protective layer material (here, the unfired body 190) used in the arrangement process are the element body materials after the same process as the firing step. The coefficient of linear thermal expansion αa'[ppm / K] from the temperature T1 [° C.] to the temperature T2 [° C.] of the solid electrolyte layer, and the temperature T1 [° C.] of the protective layer material after the same step as the firing step. The coefficient of linear thermal expansion αb [ppm / K] at the temperature T2 [° C.] satisfies the relationship of αa'> αb. When the element body material and the protective layer material are individually cooled from the temperature T2 [° C.] to the temperature T1 [° C.] after the same process as the firing step, the linear shrinkage rate Sa'[of the solid electrolyte layer of the element body material. %] Is Sa'= αa' × (T2-T1) × 10 ^-4 , and the linear shrinkage rate Sb [%] of the protective layer material is Sb = αb × (T2-T1) × 10 ^-4 . Further, since most of the element main body material after the firing step is formed of the solid electrolyte layer, the linear shrinkage rate Sa [%] of the element main body material is the linear shrinkage rate Sa'[%] of the solid electrolyte layer. It almost matches. Therefore, when the relationship of αa'> αb is satisfied, it can be said that the relationship of Sa (≈Sa')> Sb is satisfied. When the relationship of Sa> Sb is satisfied, when the element main body material and the protective layer material are individually cooled from the temperature T2 [° C.] to the temperature T1 [° C.] after the same process as the firing step, the element is more than the protective layer material. The main body material shrinks more. By the way, the protective layer 90 and the sensor element main body 101a in the state where the firing step has been performed are in close contact with or bonded to each other at least in a part. Therefore, when the protective layer 90 and the sensor element main body 101a in the state where the firing step has been performed are cooled from the temperature T2 [° C.] to the temperature T1 [° C.] in the temperature lowering step, they are dragged by the large shrinkage of the sensor element main body 101a. Residual compressive stress is applied to the surface of the protective layer 90. In this way, in the temperature lowering step, residual compressive stress is applied to the surface of the protective layer 90.

The residual compressive stress on the surface of the protective layer 90 includes the linear shrinkage rates Sa', Sa, Sb due to the linear thermal expansion coefficients αa'and αb, as well as the linear shrinkage rate of the protective layer material due to firing shrinkage during the firing step. It is possible that the linear shrinkage rate (referred to as the firing shrinkage rate) of the element body material has an effect, but it is presumed that the effect is negligible. However, when the firing shrinkage rate Cb [%] of the protective layer material is larger than the firing shrinkage rate Ca [%] of the element main body material (Ca <Cb), the difference is taken into consideration when αa'or Sa'is used. , An element main body material having a larger Sa, or a protective layer material having a smaller αb or Sb may be used. Further, the firing shrinkage rate Cb of the protective layer material is adjusted by adjusting the particle size of the ceramic particles contained in the slurry used for the protective layer material, the type and blending ratio of other materials contained in the slurry, the firing conditions in the firing step, and the like. [%] may be reduced.

The linear shrinkage rate Sa'[%] and the linear shrinkage rate Sb [%] can be theoretically obtained from the above-mentioned equations, but may be measured as follows, for example. For example, when determining the linear shrinkage rate Sa'[%], first, the same material as the solid electrolyte layer of the element main body material after the same step as the firing step, that is, the same as the solid electrolyte layer of the sensor element main body 101a. Make an anti-folding rod from the material. Next, using a coefficient of thermal expansion measuring device (push rod type expansion meter), the length of the anti-folding rod L2 [mm] at the temperature T2 [° C.] and the length L1 [of the anti-folding rod at the temperature T1 [° C.] mm] is measured. Then, the linear shrinkage rate Sa'[%] is derived using the formula Sa'= (L2-L1) / L2 × 100 [%]. When determining the linear shrinkage rate Sb, when producing the anti-folding rod, the anti-folding rod may be produced from the same material as the protective layer material after the same process as the firing step, that is, the same material as the protective layer 90. .. The linear shrinkage rate Sa is considered to be substantially the same as Sa'as described above, but if the buffer layer 84 may affect the linear shrinkage rate Sa, a layer made of the same material as the buffer layer 84 and having the same thickness. It is also possible to prepare an anti-folding rod similar to the anti-folding rod used to obtain the linear shrinkage rate Sa'except that the anti-folding rod is provided on the surface, and use this anti-folding rod to derive the linear shrinkage rate Sa [%]. .. When there is a difference between the calculated value of the linear shrinkage rate Sa [%], the linear shrinkage rate Sa'[%], and the linear shrinkage rate Sb [%] and the actually measured value, the actually measured value may be prioritized.

The value of the residual compressive stress applied to the surface of the protective layer 90 can also be adjusted by the shape and dimensions of the protective layer 90. For example, the ratio of the side portion 90c of the protective layer 90 to which the side columnar portion is provided, or the ratio of the portion of the side surface of the tip portion 101b of the sensor element main body 101a that is in close contact with or bonded to the side portion columnar portion. When the above range is set, the residual compressive stress can be easily set to a suitable value. The larger the ratio, the larger the value of the residual compressive stress tends to be. Further, for example, when the thickness t of the main body portion 90a, the height H, the length L, and the width W of the columnar portion 91 are within the above-mentioned ranges, the residual compressive stress can be easily set to a suitable value. The smaller the thickness t, the smaller the height H, and the larger the length L and the width W, the larger the value of the residual compressive stress tends to be.

[Eye sealing process]
When the temperature lowering step is performed, the sealing portion 94 is formed so as to close the opening on the rear end side of the sensor element main body 101a in the space 95 between the side portion 90c of the protective layer 90 and the sensor element main body 101a. Perform a stop process. In the present embodiment, the eye-sealing portion 94 is formed by plasma spraying. Such plasma spraying can be performed in the same manner as the plasma spraying described in, for example, Japanese Patent Application Laid-Open No. 2016-109685. As shown in FIG. 9, the mesh sealing portion 94 is formed on any of the upper, lower, left, and right surfaces of the sensor element main body 101a, and the portions formed on the respective surfaces are formed so as to be connected to each other. When the eye-sealing portion 94 is formed in this way, the sensor element 101 is obtained.

When the sensor element 101 is obtained, the sensor element 101 is passed through the prepared supporter 124 and the green compact 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, the gas sensor 100 can be obtained by attaching the nut 130, the protective cover 110, or the like.

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 protective layer 90, 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 protective layer 90. 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 the protective layer 90 is also at a high temperature (for example, 700 ° C.). Therefore, when moisture adheres to the surface of the protective layer 90, tensile stress acts on the boundary between the portion that has been rapidly cooled and shrunk due to the adhesion of the moisture and the other portion, and the protective layer 90 may crack. However, in the present embodiment, since the residual compressive stress exists on the surface of the protective layer 90 when the gas sensor 100 is used, the above-mentioned tensile stress is canceled by the residual compressive stress. Therefore, the occurrence of cracks in the protective layer 90 is suppressed, and the water resistance of the protective layer 90 is improved.

Further, when moisture adheres to the sensor element 101, the temperature may drop sharply and cracks may occur in the sensor element main body 101a. However, in the present embodiment, since the space 95 exists between the protective layer 90 and the sensor element main body 101a, the space 95 can block the heat conduction from the protective layer 90 to the sensor element main body 101a. Therefore, the cooling of the sensor element main body 101a when water adheres to the surface of the protective layer 90 is suppressed.

Further, in the sensor element 101 of the present embodiment, the columnar portion 91 supports the space 95. Therefore, it is possible to suppress a decrease in the strength of the protective layer 90 due to the presence of the space 95. Further, since the longitudinal direction of each of the side columnar portions is along the longitudinal direction of the sensor element main body 101a, it is easy to increase the residual compressive stress on the surface of the protective layer 90. In the temperature lowering step, the sensor element main body 101a and the protective layer 90 shrink as the temperature drops, and the change in the dimensions of the sensor element main body 101a due to this shrinkage is in the direction along the longitudinal direction of the sensor element main body 101a (here, the front-rear direction). Occurs larger in. Therefore, when the longitudinal direction of the side columnar portion is along the longitudinal direction of the sensor element main body 101a, it is dragged by such a large dimensional change (shrinkage) of the sensor element main body 101a, and the surface of the protective layer 90 has a larger residual compression. It is considered that stress is applied.

Further, in the sensor element 101 of the present embodiment, the upper columnar portion 91a has a length L and a length L from the connection portion with the side portion 90c to the facing surface facing the upper surface of the sensor element main body 101a (lower surface of the upper columnar portion 91a). The width W is constant. In this case, since the contact (or coupling) area between the upper columnar portion 91a and the sensor element main body 101a is larger than when the length L and width W on the lower surface side of the upper columnar portion 91a are smaller, the protective layer 90 The residual compressive stress on the surface can be increased. Similarly, the lower columnar portion 91b has a constant length L and width W from the connection portion with the side portion 90c to the facing surface facing the lower surface of the sensor element main body 101a (upper surface of the lower columnar portion 91b). .. Therefore, the residual compressive stress on the surface of the protective layer 90 can be increased. The same effect can be obtained by having the same shape for each of the left columnar portion 91c, the right columnar portion 91d, and the front side columnar portion 91e.

Here, the correspondence between the components of the present embodiment and the components of the present invention will be clarified. The sensor element main body 101a of the present embodiment corresponds to the element main body of the present invention, the tip portion 101b corresponds to the tip portion, and the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, and the first solid electrolyte The layer 4, the spacer layer 5 and the second solid electrolyte layer 6 correspond to the solid electrolyte layer, and the protective layer 90 corresponds to the protective layer. Further, the columnar portion 91 corresponds to the space support portion, and the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d correspond to the side columnar portion. Further, the sensor element main body 101a after firing corresponds to the element main body material, and the unfired body 190 corresponds to the protective layer material.

In the sensor element 101 of the present embodiment described above, residual compressive stress exists on the surface of the protective layer 90 at at least one temperature T1 [° C.] included in the temperature range of 500 ° C. or higher and 900 ° C. or lower. Therefore, the tensile stress generated on the surface of the protective layer 90 when water adheres to the surface of the protective layer 90 at the temperature T1 [° C.] is canceled by the residual compressive stress existing on the surface of the protective layer 90. As a result, damage to the protective layer 90 is suppressed, and the water resistance of the protective layer 90 is improved.

Further, if the residual compressive stress existing on the surface of the protective layer 90 is 15 MPa or more, the water resistance is further improved. Further, when the residual compressive stress is 300 MPa or less, the tensile stress generated on the surface of the sensor element main body 101a can be relatively small. Therefore, the sensor element main body 101a is also less likely to be damaged.

Further, zirconia, which is the main component of the sensor element body 101a, has a coefficient of linear thermal expansion of 11 ppm / K at 40 ° C. to 700 ° C., and alumina, which is the main component of the protective layer 90, is at 40 ° C. to 700 ° C. The coefficient of linear thermal expansion is 8 ppm / K. That is, the sensor element main body 101a has a linear thermal expansion coefficient of 10 ppm / K or more and 15 ppm / K or less at 40 ° C. to 700 ° C., and the protective layer 90 has a linear thermal expansion coefficient of 1 ppm / K at 40 ° C. to 700 ° C. It is K or more and 9 ppm / K or less. Therefore, peeling or the like due to the difference in thermal expansion between the sensor element main body 101a and the protective layer 90 is unlikely to occur. The coefficient of linear thermal expansion from 40 ° C. to 700 ° C. is about 8 ppm / K for spinel, 7 ppm / K for mullite, and about 2 ppm / K for cordierite, which are also suitable as the main components of the protective layer 90.

Furthermore, since the tip portion 101b of the sensor element main body 101a is covered with the protective layer 90 having the bottomed tubular main body portion 90a, the entire tip portion 101b can be protected by the protective layer 90. Further, since the space 95 formed between the main body 90a of the protective layer 90 and the sensor element main body 101a by the space support portion can suppress heat conduction from the protective layer 90 to the sensor element main body 101a, the protective layer 90 It is possible to suppress the cooling of the sensor element main body 101a when water adheres to the surface.

Then, the side columnar portion is provided in a range of 2% or more and 35% or less inside the side portion 90c of the main body portion 90a, or the side of the main body portion 90a of the protective layer 90 in the sensor element main body 101a. The residual compressive stress on the surface of the protective layer 90 can be easily set to a suitable value by adhering or bonding with the range of 2% or more and 35% or less of the portion covered with the portion 90c.

Further, if the height H of the side columnar portion is set to 400 μm or less, the length L or width W of the side columnar portion is set to 200 μm or more, or the thickness t of the side portion 90c is set to 600 μm or less, protection is provided. The residual compressive stress on the surface of the layer 90 can be easily set to a suitable value.

Further, the height of one or more of the upper space 95a, the lower space 95b, the left space 95c, the right space 95d, and the front space 95e is 10 μm or more, that is, the upper columnar portion 91a, the lower columnar portion 91b, By setting the height H of one or more of the left columnar portion 91c, the right columnar portion 91d, and the front columnar portion 91e to 10 μm or more, the effect of blocking the heat conduction from the protective layer 90 to the sensor element main body 101a by the space 95 can be obtained. Easy to be enough. The higher the height of the space 95, the higher the effect of blocking heat conduction.

Furthermore, the upper columnar portion 91a is arranged so as to avoid the region 102a on which the gas flow portion 9 to be measured is projected on the upper surface of the sensor element main body 101a. Here, the portion of the sensor element main body 101a between the gas flow portion 9 to be measured and the upper surface of the sensor element main body 101a is a portion having weak strength and relatively prone to cracking. Since the upper columnar portion 91a does not exist in the region 102a, direct heat conduction to the region 102a via the upper columnar portion 91a does not occur. Cracks in the portion between the gas flow section 9 to be measured are less likely to occur. The same effect can be obtained by arranging each of the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d so as to avoid the

regions

102b, 102c, 102d, and 102e, respectively. Further, as described above, since the upper surface of the upper surface, the lower surface, the left surface, and the right surface of the sensor element main body 101a is the surface closest to the gas flow portion 9 to be measured, the region 102a is particularly weak in the regions 102a to 102d. is there. In the present embodiment, since the upper columnar portion 91a is arranged avoiding this region 102a, cracks are less likely to occur in the region 102a having particularly weak strength.

Further, in the method for manufacturing the sensor element 101 of the present embodiment, the temperature T1 [° C.] of the solid electrolyte layer of the element body material after the same process as the firing step is performed as the element body material and the protective layer material in the arrangement process. ] -The coefficient of linear thermal expansion αa'[ppm / K] at the temperature T2 [° C] and the line at the temperature T1 [° C] to the temperature T2 [° C] of the protective layer material after performing the same process as the firing step. Since the coefficient of thermal expansion αb [ppm / K] satisfies the relationship of αa'> αb, compressive stress can be applied to the protective layer 90 in the temperature lowering step.

Furthermore, since the sensor element body 101a after firing is used as the element body material, that is, it is not necessary to fire the element body material in the firing step, the temperature T2 [° C.] can be set relatively low.

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

For example, in the above-described embodiment, the temperature T1 [° C.] is 700 ° C., but the temperature is not limited to 700 ° C. The temperature T1 [° C.] may be at least one temperature included in the temperature range of 500 ° C. or higher and 900 ° C. or lower, and the temperature T1 [° C.] may be at least one temperature included in the temperature range of 700 ° C. or higher and 900 ° C. or lower. May be. The higher the temperature, the smaller the residual compressive stress on the surface of the protective layer 90. For example, when the residual compressive stress exists at 700 ° C., it is considered that the residual compressive stress exists even at a temperature of 700 ° C. or lower.

In the above-described embodiment, the temperature T2 [° C.] is any temperature included in the temperature range of 1100 ° C. or higher and 1200 ° C. or lower, but the temperature is not limited to these. For example, the temperature T2 [° C.] may be any temperature included in the temperature range of 1100 ° C. or higher and 1600 ° C. or lower, or may be any temperature included in the temperature range of 1100 ° C. or higher and 1300 ° C. or lower.

In the above-described embodiment, the protective layer 90 has a cap shape, but the protective layer 90 is not limited to the cap shape as long as it covers at least a part of the surface of the sensor element main body 101a. The protective layer 90 may have, for example, a tubular shape in which the bottom portion 90b is omitted, a shape in which a part of the side portion 90c (for example, a portion covering the left and right sides of the sensor element main body 101a) is omitted, or both of them are omitted. It may have a shaped shape.

In the above-described embodiment, the longitudinal direction of the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d is assumed to be along the longitudinal direction of the sensor element main body 101a. May be perpendicular to the longitudinal direction of the sensor element main body 101a, or the longitudinal direction may be oriented in another direction. Further, the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d do not have a longitudinal direction, that is, the length L and the width W may satisfy L = W. ..

For example, in the above-described embodiment, the rear end portions 93a to 93d are separated from the eye sealing portion 94, but the present invention is not limited to this. For example, as in the protective layer 90 of the modified example shown in FIG. 13, the upper columnar portion 91a and the lower columnar portion 91b exist up to the rear end of the main body portion 90a, and the rear end and the lower columnar portion of the upper columnar portion 91a The rear end of 91b may be in contact with the sealing portion 94. The same applies to the left columnar portion 91c and the right columnar portion 91d.

In the above-described embodiment, the columnar portion 91 of the protective layer 90 includes the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, the right columnar portion 91d, and the front columnar portion 91e. It does not have to be included. The columnar portion 91 is preferably arranged on the surfaces of the sensor element main body 101a on opposite sides so as to support at least two spaces located on opposite sides of the sensor element main body 101a. For example, as in the protective layer 90 of the modified example shown in FIG. 14, the protective layer 90 includes columnar portions 91 arranged on the surfaces opposite to each other, that is, the left columnar portion 91c and the right columnar portion 91d, and has an upper columnar portion. It is not necessary to have the part 91a. In FIG. 14, the lower columnar portion 91b may be omitted, or the upper columnar portion 91a may be provided in place of the lower columnar portion 91b. Further, as the columnar portions 91 arranged on the surfaces opposite to each other, the upper columnar portion 91a and the lower columnar portion 91b may be provided.

In the above-described embodiment, the upper columnar portion 91a has a constant length L and width W from the connection portion of the main body portion 90a with the side portion 90c to the lower surface of the upper columnar portion 91a, and the lower surface of the upper columnar portion 91a is flat. Yes, but not limited to these. For example, the lower surface of the upper columnar portion 91a has a surface facing the upper surface of the sensor element main body 101a (here, the lower surface of the upper columnar portion 91a) in a cross-sectional view perpendicular to the longitudinal direction of the upper columnar portion 91a. It may have a shape that bulges in an arc toward the upper surface of the. The "arc shape" includes various arc shapes such as an arc shape and an elliptical arc shape. Further, the upper columnar portion 91a may have a chamfered shape portion such as a C chamfer or an R chamfer around a flat lower surface. If the length L and width W from the connection portion of the main body 90a with the side 90c to the lower surface of the upper columnar portion 91a are not constant, the lower surface of the upper columnar portion 91a may be in close contact with the upper surface of the sensor element main body 101a. It is preferable that the length L and the width W of the portions to be joined satisfy the above-mentioned ranges of the length L and the width W.

In the above-described embodiment, the protective layer 90 includes a front columnar portion 91e that supports the front space 95e, but is not limited to this. For example, as in the modified protective layer 90 shown in FIGS. 15 and 16, the protective layer 90 may include a front stepped portion 92e instead of the front columnar portion 91e. The front step portion 92e is in contact with the four corners of the front surface of the sensor element main body 101a in the front-rear direction, and the height of the step (length in the front-rear direction) of the front step portion 92e determines the front surface and the main body portion of the sensor element main body 101a. 90a and 90a are separated from each other via the front space 95e.

In the above-described embodiment, the columnar portions 91a to 91e are arranged so as to avoid the regions 102a to 102e on which the gas flow portion 9 to be measured is projected on the surface of the sensor element main body 101a, but the present invention is not limited to this. .. One or more of the columnar portions 91a to 91e may be arranged at positions overlapping with the respective regions 102a to 102e.

In the above-described embodiment, of the upper surface, the lower surface, the left surface, and the right surface of the sensor element main body 101a, the upper surface is the surface closest to the gas flow unit 9 to be measured, but the other surface is not limited to this and is the gas flow unit to be measured. It may be closest to 9. Further, not only when the upper surface is the surface closest to the gas flow portion 9 to be measured, but also the surface closest to the gas flow unit 9 to be measured among the upper surface, the lower surface, the left surface and the right surface of the sensor element main body 101a is formed on the surface. It is preferable that the columnar portion 91 is arranged so as to avoid the region on which the gas flow portion 9 to be measured is projected. Further, regardless of whether or not the upper surface is the surface closest to the gas flow portion 9 to be measured, the upper columnar portion 91a is arranged on the upper surface, which is the surface on which the outer pump electrode 23 is arranged, avoiding the region 102a. It may be installed. Further, in the above-described embodiment, the gas introduction port 10 which is an opening serving as an inlet of the gas flow unit 9 to be measured is arranged on the front surface of the sensor element main body 101a, but the present invention is not limited to this. For example, the gas introduction port 10 may be arranged on any of the upper surface, the lower surface, the left surface, and the right surface of the sensor element main body 101a. In this case, of the upper surface, lower surface, left surface, and right surface of the sensor element main body 101a, the surface on which the gas introduction port 10 is arranged is defined as the "surface closest to the gas flow portion to be measured".

In the above-described embodiment, the columnar portions 91a to 91e are arranged so as to avoid the regions 102a to 102e on which the gas flow portion 9 to be measured is projected on the surface of the sensor element main body 101a, but the present invention is not limited to this. Of the columnar portions 91a to 91e, the portions that come into contact with the sensor element main body 101a may be arranged so as to avoid the respective regions 102a to 102e. For example, if at least the portion of the upper columnar portion 91a in contact with the sensor element main body 101a avoids the region 102a, the entire upper columnar portion 91a may not avoid the region 102a. In this case as well, as in the above-described embodiment, direct heat conduction to the region 102a via the upper columnar portion 91a does not occur, so that the upper surface of the sensor element main body 101a and the gas flow to be measured are flown among the sensor element main bodies 101a. Cracks in the portion between the portion 9 and the portion 9 are less likely to occur. In each of the lower columnar portion 91b, the left columnar portion 91c, the right columnar portion 91d, and the front columnar portion 91e, the portions in contact with the sensor element main body 101a are arranged so as to avoid the

regions

102b, 102c, 102d, 102e. By doing so, the same effect can be obtained. In the above-described embodiment, the portions of the columnar portions 91a to 91e that come into contact with the sensor element main body 101a avoid the regions 102a to 102e, but one or more of the columnar portions 91a to 91e The portion that comes into contact with the sensor element main body 101a may be arranged at a position that overlaps with each of the regions 102a to 102e. Further, regarding the upper surface, lower surface, left surface, and right surface of the sensor element main body 101a that are closest to the gas flow portion 9 to be measured, the portion of the columnar portion 91 that contacts the surface of the sensor element main body 101a covers the surface. It is preferable that the measurement gas flow unit 9 is arranged so as to avoid the projected region. Further, at least the front columnar portion 91e of the columnar portions 91a to 91e may be arranged so that the portion in contact with the sensor element main body 101a avoids the region 102e, or the entire front columnar portion 91e avoids the region 102e. It may be arranged. Further, regardless of whether or not the upper surface of the sensor element main body 101a is the surface closest to the gas flow portion 9 to be measured, the upper surface, which is the surface on which the outer pump electrode 23 is arranged, is among the upper columnar portions 91a. The portion in contact with the sensor element main body 101a may be arranged so as to avoid the region 102a.

In the above-described embodiment, the space 95 does not have to exist between the surface of the main body 90a and the surface of the tip 101b. In this case, since the area where the sensor element main body 101a and the protective layer 90 are in close contact with each other or are bonded to each other is large, the residual compressive stress on the surface of the protective layer 90 tends to be large.

In the above-described embodiment, the size of the space 95 has not been particularly described, but the space 95 is a space larger than the pores different from the pores in the protective layer 90, and has a size distinguishable from the pores in the protective layer 90. is there. For example, in the above-described embodiment, each of the spaces 95a to 95e has a size that can be distinguished from the pores in the protective layer 90. For example, the volume of the part present in the region directly above the top surface of the sensor element body 101a of the upper space 95a, may be 0.03 mm ³ or more, may be 0.04 mm ³ or more, 0.07 mm ³ It may be 0.5 mm ³ or more, ^{or 1.5 mm 3} or more. The volume of the part present in the region beneath the lower surface of the sensor element body 101a of the lower space 95b, may be 0.03 mm ³ or more, may be 0.04 mm ³ or higher, as 0.07 mm ³ or higher It may be 0.5 mm ³ or more, ^{or 1.5 mm 3} or more. Volume of the part present in the left area of the left surface of the sensor element body 101a of the left space 95c may be a 0.015 mm ³ or more, may be 0.2 mm ³ or more, as 0.4 mm ³ or more May be good. Volume of the portion present in the right area of the right surface of the sensor element body 101a of the right space 95d may be a 0.015 mm ³ or more, may be 0.2 mm ³ or more, as 0.4 mm ³ or more May be good. The volume of the portion in front of the area of the front surface of the sensor element body 101a of the front space 95e, may be 0.010 mm ³ or more, may be 0.1 mm ³ or more, even 0.2 mm ³ or more It may be 0.3 mm ³ or more. Here, the "region directly above the upper surface" of the sensor element main body 101a means a region existing in a direction perpendicular to the upper surface with respect to the upper surface, and does not include the upper left, upper right, and the like of the upper surface. The same applies to "the area directly below the lower surface", "the area on the left side of the left surface", "the area on the right side of the right surface", and "the area in front of the front surface". When the upper space 95a has a plurality of spaces, the volume of the portion existing in the region directly above the upper surface of the sensor element main body 101a is 0.03 mm ³ or more, and 0. 04Mm ³ above, 0.07 mm ³ or more, 0.5 mm ³ or more, or may be at 1.5 mm ³ or more, as the sum of a plurality of spaces, present in the region directly above the top surface of the sensor element body 101a volume portions 0.03 mm ³ or more, 0.04 mm ³ or more, or 0.07 mm ³ or more, 0.5 mm ³ or more, or may be 1.5 mm ³ or more. Similarly, when each of the spaces 95b to 95e has a plurality of spaces, at least one of the plurality of spaces may satisfy the above numerical range of the volume, and the total of the plurality of spaces may be described above. It may satisfy the numerical range of the volume of. Further, the height of the upper space 95a may be 40% or more and 70% or less of the height from the upper surface of the sensor element main body 101a to the upper surface of the protective layer 90. Similarly, the height of the lower space 95b may be 40% or more and 70% or less of the height from the lower surface of the sensor element main body 101a to the lower surface of the protective layer 90. The height of the left side space 95c may be 40% or more and 70% or less of the height from the left surface of the sensor element main body 101a to the left surface of the protective layer 90. The height of the right side space 95d may be 40% or more and 70% or less of the height from the right surface of the sensor element main body 101a to the right surface of the protective layer 90. The height of the front space 95e may be 40% or more and 70% or less of the height from the front surface of the sensor element main body 101a to the front surface of the protective layer 90. The height of the upper space 95a may be 5 times or more or 10 times or more the average pore diameter (by the mercury injection method) of the protective layer 90. Similarly, the heights of the lower space 95b, the left space 95c, the right space 95d, and the front space 95e may be 5 times or more or 10 times or more the average pore diameter of the protective layer 90. ..

In the above-described embodiment, the eye-sealing portion 94 is arranged so as to cover a part of the surface of the sensor element main body 101a along the longitudinal direction and contact the end surface of the protective layer 90 on the rear end side. , Not limited to this. For example, the eye-sealing portion 94 may not be in contact with the rear end surface of the protective layer 90, as in the modified example of the eye-sealing portion 94 shown in FIG. In FIG. 18, the eye sealing portion 94 is arranged between the protective layer 90 and the sensor element main body 101a. Also in FIG. 18, the eye sealing portion 94 closes the opening toward the rear end side of the space 95 existing between the protective layer 90 and the surface of the sensor element main body 101a, as in the above-described embodiment. In FIG. 18, the rear end of the sealing portion 94 and the rear end of the protective layer 90 are at the same position in the front-rear direction, but the present invention is not limited to this. For example, a part of the eye-sealing portion 94 may protrude rearward from the rear end of the protective layer 90, or the rear end of the eye-sealing portion 94 may be located in front of the rear end of the protective layer 90. Good. Further, in FIG. 18, the sealing portion 94 is separated from the upper columnar portion 91a and the lower columnar portion 91b, but may be in contact with each other. Further, the eye-sealing portion 94 is provided so that a part of the sealing portion 94 is in contact with the end surface on the rear end side of the protective layer 90, and a part of the sealing portion 94 enters between the protective layer 90 and the sensor element main body 101a to open an opening of the space 95. It may be blocked. For example, the eye-sealing portion 94 shown in FIG. 18 masks the rear end surface of the protective layer 90 in the above-mentioned eye-sealing step, performs plasma spraying, and between the protective layer 90 and the sensor element main body 101a. It can be formed by allowing the powder sprayed material 184 to penetrate. Alternatively, the eye-sealing portion 94 shown in FIG. 18 can be formed by using a paste that becomes the eye-sealing portion 94 after firing. For example, before the tip portion 101b of the sensor element main body 101a is inserted inside the unfired body 190 in the above-mentioned arrangement step, a paste that becomes the eye-sealing portion 94 after firing is applied to the surface of the sensor element main body 101a. It may be applied by printing or the like. Alternatively, after the tip portion 101b of the sensor element main body 101a is inserted inside the unfired body 190 in the above-mentioned arrangement step, a paste that becomes the eye-sealing portion 94 after firing is applied after the unfired body 190. It may be injected from the end side into the gap between the unfired body 190 and the sensor element main body 101a. Further, in the above-described embodiment, the sealing portion 94 may be omitted.

In the case of forming the eye-sealing portion 94 shown in FIG. 18, a method of forming the eye-sealing portion 94 by using a paste instead of plasma spraying has been described, but the method is not limited to the eye-sealing portion 94 of FIG. For example, when forming the eye-sealing portion 94 of another aspect such as the eye-sealing portion 94 of FIG. 3, the eye-sealing portion 94 may be formed by using a paste instead of plasma spraying. Further, when the eye-sealing portion 94 is formed by using the paste, the unfired body 190 and the paste that becomes the eye-sealing portion 94 after firing may be fired at the same time. Alternatively, after the unfired body 190 is fired, a paste to be the sealing portion 94 may be applied after the firing, and the paste may be fired to form the sealing portion 94.

In the above-described embodiment, the sensor element 101 of the gas sensor 100 is provided with the measurement electrode 44 coated with the fourth diffusion rate-determining portion 45 in the second internal space 40, but the present invention is not particularly limited to this configuration. For example, as in the sensor element 101 of the modified example of FIG. 17, the measurement electrode 44 is exposed without being covered, and a slit-shaped fourth diffusion rate-determining portion 60 is provided between the measurement electrode 44 and the auxiliary pump electrode 51. You may. The fourth diffusion rate-determining unit 60 imparts a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump cell 50 in the second internal space 40, and transfers the gas to be measured. This is the part that leads to the third internal space 61 in the back. The fourth diffusion rate-determining unit 60 plays a role of limiting the amount of NOx flowing into the third internal space 61. Even with the sensor element 101 having such a configuration, the NOx concentration can be detected by the measurement pump cell 41 as in the above-described embodiment.

In the above-described embodiment, the sensor element main body 101a is provided with the buffer layer 84, but the present invention is not limited to this. For example, as shown in FIGS. 15 and 16, the sensor element main body 101a does not include the buffer layer 84, and includes the upper surface of the second solid electrolyte layer 6, the lower surface of the first substrate layer 1, the protective layer 90, and the sealing portion 94. May be in direct contact.

In the above-described embodiment, the sensor element main body 101a includes the reference gas introduction space 43 and the atmosphere introduction layer 48, but the present invention is not limited to this. For example, the reference gas introduction space 43 may be filled with the same porous body as the atmosphere introduction layer 48. Alternatively, the sensor element main body 101a may not include the reference gas introduction space 43, and the atmosphere introduction layer 48 may exist up to the rear end of the sensor element main body 101a. Also in these embodiments, the reference gas can be introduced into the sensor element body 101a from the rear end of the sensor element body 101a and reach the reference electrode 42.

In the above-described embodiment, the sensor element main body 101a is a laminated body having a plurality of solid electrolyte layers (layers 1 to 6), but the present invention is not limited to this. The sensor element main body 101a may include at least one oxygen ion conductive solid electrolyte layer. For example, in FIG. 3, the layers 1 to 5 other than the second solid electrolyte layer 6 may be a structural layer made of a material other than the solid electrolyte (for example, a layer made of alumina). In this case, each electrode of the sensor element main body 101a may be arranged on the second solid electrolyte layer 6. For example, the measurement electrode 44 of FIG. 3 may be arranged on the lower surface of the second solid electrolyte layer 6. Further, the reference gas introduction space 43 is provided in the spacer layer 5 instead of the first solid electrolyte layer 4, and the atmosphere introduction layer 48 is provided between the first solid electrolyte layer 4 and the third substrate layer 3 instead of being provided in the second solid. It may be provided between the electrolyte layer 6 and the spacer layer 5, and the reference electrode 42 may be provided behind the third internal space 61 and on the lower surface of the second solid electrolyte layer 6.

In the above-described embodiment, the target value of the electromotive force V0 is set based on the pump current Ip1, and the pump voltage Vp0 is feedback-controlled so that the electromotive force V0 becomes the target value, but other control may be performed. For example, the pump voltage Vp0 may be feedback-controlled based on the pump current Ip1 so that the pump current Ip1 becomes the target value Ip1 *. That is, the pump voltage Vp0 is directly controlled based on the pump current Ip1 by omitting the acquisition of the electromotive force V0 from the main pump control oxygen partial pressure detection sensor cell 80 and the setting of the target value of the electromotive force V0 (and thus the pump). The current Ip0 may be controlled).

In the above-described embodiment, the gas sensor 100 that detects the NOx concentration is illustrated, but the present invention may be applied to a gas sensor that detects the oxygen concentration or a gas sensor that detects the ammonia concentration.

In the above-described embodiment, the unfired body 190 is integrally molded, but the present invention is not limited to this. For example, a plurality of members having a shape obtained by dividing the unfired body 190 shown in FIG. 10C into a plurality of members may be formed. In this case, the plurality of members and the sensor are joined so that the obtained plurality of members are joined to form a cap-shaped unfired body 190 and the sensor element main body 101a is inserted inside the unfired body 190. It may be arranged with the element body 101a. For example, a member having a shape obtained by dividing the unfired body 190 into an upper half and a lower half is formed, and the two members are arranged so as to sandwich the sensor element main body 101a from above and below, and between the two members. The two members and the sensor element main body 101a may be arranged so as to be adhered to each other. When the unfired body 190 is divided into a plurality of members and molded in this way, the sealing portion 94 may also be included in the unfired body 190 as a part of the protective layer 90 for molding. FIG. 19 shows how the

unfired bodies

190a and 190b, which are formed by dividing the unfired body 190 into two, are arranged so that the

unfired bodies

190a and 190b sandwich the sensor element main body 101a from above and below. .. In FIG. 19, as shown in the drawing, the

unfired bodies

190a and 190b are molded into a shape including the eye-sealing portion 94 of the above-described embodiment, respectively. In this case, the sealing step performed after the firing step can be omitted.

Here, in the above-described embodiment, since the unfired body 190 is integrally formed, the unfired body 190 and the protective layer 90 obtained by firing the unfired body 190 face the bottom 90b side inward (inner peripheral surface). The shape is such that there is no surface (the surface facing the front of the sensor element 101). When integrally molding the cap-shaped unfired body 190, it is necessary to pull out the inner mold 153 in the direction opposite to the bottom 90b after molding as described with reference to FIG. 10, and the bottom portion is inside the unfired body 190. This is because the inner mold 153 cannot be pulled out if there is a surface facing the 90b side. It is also for this reason that the eye-sealing portion 94 is formed later in the above-described embodiment. That is, for example, as shown in FIG. 3, the front surface of the eye-sealing portion 94 exists inside the protective layer 90 and faces the bottom 90b side. Therefore, the cap-shaped protective layer 90 and the eye-sealing portion 94 are integrated. It cannot be molded. On the other hand, when the unfired body 190 is divided into a plurality of members and molded as described above, the shape is such that the unfired body 190 and the protective layer 90 have a surface facing the bottom 90b side inward. It can also be. For example, as shown in FIG. 19, an unfired body 190 (unfired body 190a and unfired body 190b) having a shape such that the protective layer 90 includes a portion having the same shape as the sealing portion 94 can be formed. Similarly, in the above-described embodiment, the front ends of the upper columnar portion 91a, the lower columnar portion 91b, the left columnar portion 91c, and the right columnar portion 91d are connected to the bottom portion 90b of the main body portion 90a, but are not fired. When the body 190 is not integrally molded, an unfired body 190 having a shape in which the front end and the bottom 90b are not connected can be formed for one or more of these columnar portions 91a to 91d. For example, the unfired body 190 and the protective layer 90 have a shape such that the front end surface of the upper columnar portion 91a exists inside the protective layer 90 and a space exists between the front end surface of the upper columnar portion 91a and the bottom portion 90b. Can also be formed. Alternatively, one or more of the columnar portions 91a to 91d may have a shape in which the longitudinal direction does not follow the longitudinal direction of the sensor element main body 101a, that is, the front-rear direction (for example, along the left-right direction).

In the above-described embodiment, the unfired body 190 was produced by the mold casting method, but the present invention is not limited to this. For example, the unfired body 190 may be produced by using the powder compaction method. In the powder compaction method, the unfired body 190 is molded as a powder molded body by sandwiching the raw material powder with a mold and pressing it. The

unfired bodies

190a and 190b in which the unfired body 190 is divided as shown in FIG. 19 may also be molded by using the powder compaction method. A method of pressing and molding the raw material powder in this way is described in, for example, Japanese Patent Application Laid-Open No. 2018-146470.

In the above-described embodiment, the fired sensor element main body 101a and the unfired body 190 are prepared in the preparatory step, but the preparatory step is omitted, and the separately prepared sensor element main body 101a and the unfired body 190 are prepared in the arrangement step. 190 may be used. Further, in the preparation step, only the sensor element main body 101a after firing is prepared, and in the arrangement step, the raw material paste of the unfired body 190 is applied to a predetermined position of the sensor element main body 101a to prepare the unfired body 190. And the arrangement may be performed at the same time.

In the above-described embodiment, the sensor element main body 101a after firing is prepared as the element main body material in the preparation step, but the present invention is not limited to this, and the sensor element main body 101a before firing may be prepared. In this case, both the sensor element main body 101a and the unfired body 190 may be fired in the firing step. When the firing shrinkage rate Cb [%] of the protective layer material is large, the firing shrinkage rate Ca [%] of the sensor element body 101a before firing as the element body material is increased to increase the firing shrinkage rate Ca [%]. The difference between%] and Cb [%] may be small. For example, the firing shrinkage rate Ca [%] can be adjusted by adjusting the particle size of the ceramic particles contained in the ceramic green sheet, the type and blending ratio of other materials contained in the ceramic green sheet, and the firing conditions in the firing process. It may be increased.

Hereinafter, an example in which the sensor element 101 of the present invention is specifically examined will be described as an example. Here, all of Experimental Examples 1 to 18 correspond to the examples of the present invention. It goes without saying that the present invention is not limited to the following examples, and can be carried out in various embodiments as long as it belongs to the technical scope of the present invention.

[Structure of sensor element 101]
In Experimental Examples 1 to 18, the sensor element 101 of the above-described embodiment was examined. In each of the sensor elements 101 of Experimental Examples 1 to 18, the solid electrolyte layer contained in the sensor element main body 101a is mainly composed of zirconia, and the protective layer 90 is mainly composed of alumina. Regarding the sensor element main body 101a, the thickness (length in the vertical direction) was 1.4 mm, the width (length in the horizontal direction) was 4.1 mm, and the length of the tip portion 101b was 11 mm. The buffer layer 84 is mainly composed of alumina, and the thickness of the upper buffer layer 84a and the lower buffer layer 84b is 20 μm, respectively. The temperature T1 was 700 ° C., and the temperature T2 was 1100-1200 ° C. In this case, the coefficient of linear thermal expansion αa'is 11 ppm / K, and the coefficient of linear thermal expansion αb is 8 ppm / K. The linear shrinkage rate Sa'is 0.44 to 0.55%, and the linear shrinkage rate Sb is 0.32 to 0.40%. Also, Sa ≈ Sa'is satisfied. That is, the relationship of αa'> αb and the relationship of Sa (≈Sa')> Sb are satisfied. When the protective layer 90 is changed to one containing cordierite as a main component, the linear thermal expansion coefficient αb is 2 ppm / K, and the linear contraction coefficient Sb is 0.08 to 0.10%. When the protective layer 90 contains mullite as a main component, the linear thermal expansion coefficient αb is 7 ppm / K, and the linear contraction coefficient Sb is 0.28 to 0.35%. In these cases as well, the relationship of αa'> αb and the relationship of Sa (≈Sa')> Sb are satisfied.

In the sensor elements 101 of Experimental Examples 1 to 18, as shown in Table 1, the height H, length L, width W of each side columnar portion, and the thickness t of the side portion 90c of the protective layer 90 were adjusted. .. The columnar portion ratio in Table 1 is the ratio of the area of the area inside the side portion 90c of the main body portion 90a to which the side columnar portion is provided, that is, (2Ba + 2Bb + Bc + Bd) × 100. / (Aa + Ab + Ac + Ad). This value is the same as the ratio of the portion of the side surface of the tip portion 101b of the sensor element main body 101a that is in close contact with or coupled to the side columnar portion.

[Residual compressive stress on the surface]
The residual compressive stress at T1 = 700 ° C. was examined for each of the sensor elements 101 of Experimental Examples 1 to 18. The surveyed part was the part of the surface of the protective layer 90 that became the hottest when the sensor element 101 was used (the part directly above the outer pump electrode 23, and here, the position 4 mm from the tip of the sensor element 101). The results are shown in Table 1.

[Water resistance]
For each of the sensor elements 101 of Experimental Examples 1 to 18, the portion of the surface of the protective layer 90 that becomes the hottest when the sensor element 101 is used (the portion directly above the outer pump electrode 23, and here, the tip of the sensor element 101). The amount of water received was examined in a state where the output of the heater 72 was adjusted so that the temperature at (4 mm position) was 700 ° C. The water resistance of the protective layer 90 is such that water droplets are dropped to a position 4 mm from the tip of the sensor element 101, and the amount of each drop is increased each time, and the amount of drops when a crack occurs in the protective layer 90. And said. The results are shown in Table 1.

FIG. 20 is a graph showing the relationship between the residual compressive stress existing on the surface of the protective layer 90 and the water resistance of the protective layer 90. As shown in FIG. 20, the larger the residual compressive stress, the higher the water resistance. From this, it was found that the water resistance of the protective layer 90 can be enhanced by the presence of residual compressive stress on the surface of the protective layer 90 as in the present invention. It was also found that the water resistance was further improved when the residual compressive stress was 15 MPa or more. It was found that the larger the area where the sensor element main body 101a is in close contact with or bonded to the side columnar portion, or the larger the proportion of the columnar portion, the larger the residual compressive stress and the better the water resistance. Further, the larger the width W of the side columnar portion, the lower the height H of the side columnar portion, and the thinner the thickness t of the side portion 90c of the main body portion 90a, the greater the residual compressive stress and the water resistance. Was found to improve.

This application is based on Japanese Patent Application No. 2019-211701 filed on November 22, 2019, and all of its contents are included in the present specification by citation.

The present invention can be used in the manufacturing industry of a gas sensor provided with a sensor element for detecting the concentration of a specific gas such as NOx in a gas to be measured such as the exhaust gas of an automobile.

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, 9 measurement gas flow section, 10 gas inlet, 11th 1 diffusion rate control part, 12 buffer space, 13 second diffusion rate control part, 20 first internal space, 21 main pump cell, 22 inner pump electrode, 22a ceiling electrode part, 22b bottom electrode part, 23 outer pump electrode, 25 variable power supply , 30 3rd diffusion rate control section, 40 2nd internal space, 41 measurement pump cell, 42 reference electrode, 43 reference gas introduction space, 44 measurement electrode, 45 4th 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, 60 4th diffusion rate control part, 61 3rd internal space, 70 heater part, 71 heater connector electrode, 72 heater, 73 Through hole, 74 heater insulation layer, 75 pressure dissipation hole, 80 main pump control oxygen partial pressure detection sensor cell, 81 auxiliary pump control oxygen partial pressure detection sensor cell, 82 measurement pump control oxygen partial pressure detection sensor cell, 83 sensor cell, 84 buffer layer, 84a upper buffer layer, 84b lower buffer layer, 90 protective layer, 90a main body, 90b bottom, 90c side, 90d bottom connection, 90e side connection, 91 columnar, 91a upper column, 91b lower columnar part, 91c left side columnar part, 91d right side columnar part, 91e front side columnar part, 92a upper side part, 92b lower side part, 92c left side part, 92d right side part, 92e front side stepped part, 93a to 93d Rear end, 94th seal, 95 space, 95a upper space, 95b lower space, 95c left space, 95d right space, 95e front space, 100 gas sensor, 101 sensor element, 101a sensor element body, 101b tip, 102a-102d area, 110 protective cover, 111 inner protective cover, 112 outer protective cover, 120 element sealant, 122 main metal fittings, 124 supporter, 126 green compact, 130 nut, 140 piping, 141 mounting member, 150 molding Mold, 151 first outer mold, 151a notch, 152 second outer mold, 152a notch, 153 inner mold, 153a insertion part, 190 unfired body.

Claims

A sensor element used to detect a specific gas concentration in the gas to be measured.
An element body having an oxygen ion conductive solid electrolyte layer and having a longitudinal direction,
A protective layer that covers at least a part of the surface of the element body,
With
The protective layer has a residual compressive stress on its surface at at least one temperature T1 [° C.] included in the temperature range of 500 ° C. or higher and 900 ° C. or lower.
Sensor element.
The residual compressive stress is 15 MPa or more.
The sensor element according to claim 1.
The element body has a linear thermal expansion coefficient of 10 ppm / K or more and 15 ppm / K or less at 40 ° C. to 700 ° C.
The protective layer has a linear thermal expansion coefficient of 1 ppm / K or more and 9 ppm / K or less at 40 ° C. to 700 ° C.
The sensor element according to claim 1 or 2.
The element body is mainly composed of zirconia.
The protective layer contains one or more selected from the group consisting of alumina, spinel, cordierite and mullite as a main component.
The sensor element according to any one of claims 1 to 3.
The protective layer is provided so that a space is formed between the bottomed tubular main body portion that covers the tip end portion, which is one end portion in the longitudinal direction of the element main body, and the main body portion and the element main body. With a space support provided in
The sensor element according to any one of claims 1 to 4.
The protective layer has a side columnar portion provided inside the side portion of the main body portion as the space support portion.
(A) Is the side columnar portion provided in a range of 2% or more and 35% or less inside the side portion of the main body portion?
(B) Whether the side columnar portion is in close contact with or bonded to a range of 2% or more and 35% or less of the portion of the element main body covered by the side portion of the main body portion of the protective layer.
Meet at least one of
The sensor element according to claim 5.
The protective layer has a side columnar portion provided inside the side portion of the main body portion as the space support portion, and the side columnar portion has a high protrusion length from the side portion of the main body portion. When the length in the direction perpendicular to the height H and the shortest length is the width W, and the length in the direction perpendicular to the height H and the width W is the length L. In addition, the width W and the length L are 200 μm or more.
The sensor element according to claim 5 or 6.
The protective layer has a side columnar portion provided inside the side portion of the main body portion as the space support portion, and the side columnar portion has a high protrusion length from the side portion of the main body portion. If H, the height H is 400 μm or less.
The sensor element according to any one of claims 5 to 7.
The protective layer has a thickness t of a side portion of the main body portion of 600 μm or less.
The sensor element according to any one of claims 5 to 8.
The sensor element according to any one of claims 1 to 9.
With, gas sensor.
The method for manufacturing a sensor element according to any one of claims 1 to 9.
An arrangement step of arranging the protective layer material to be the protective layer by firing on at least a part of the surface of the element main body material which is the element main body before or after firing.
A firing step of heating the protective layer material and the element main body material in a state where the arrangement step has been performed at a temperature of T2 [° C.] and firing at least the protective layer material.
A temperature lowering step of lowering the temperature of the protective layer and the element body in the state where the firing step has been performed from the temperature T2 [° C.] to a temperature of the temperature T1 [° C.] or lower.
Including
In the arrangement step, the temperature T1 [° C.] to the temperature T2 [° C.] of the solid electrolyte layer contained in the element body material after performing the same steps as the firing step as the element body material and the protective layer material. ], The coefficient of linear thermal expansion αa'[ppm / K] and the linear thermal expansion of the protective layer material at the temperature T1 [° C.] to the temperature T2 [° C.] after the same step as the firing step is performed. A coefficient αb [ppm / K] that satisfies the relationship of αa'> αb is used.
Manufacturing method of sensor element.
In the arrangement step, the element body after firing is used as the element body material.
The method for manufacturing a sensor element according to claim 11.