WO2022224764A1

WO2022224764A1 - Gas sensor

Info

Publication number: WO2022224764A1
Application number: PCT/JP2022/015790
Authority: WO
Inventors: 朗齋藤; 康敬神谷; 直輝神村
Original assignee: 株式会社デンソー
Priority date: 2021-04-19
Filing date: 2022-03-30
Publication date: 2022-10-27
Also published as: JPWO2022224764A1

Abstract

This gas sensor is provided with a sensor element (2), and the sensor element (2) includes a solid electrolyte body (3), a detecting electrode (4A), and a reference electrode (4B). The solid electrolyte body (3) is in the shape of a bottomed cylinder, and is configured from a solid electrolyte material having ion conductivity. The detecting electrode (4A) is provided on the outside of the solid electrolyte body (3), in a state of being exposed to a detection target gas (G). The reference electrode (4B) is provided on the inside of the solid electrolyte body (3), in a state of being exposed to a reference gas (A). A porous layer (23) having multiple pores is provided between the solid electrolyte body (3) and the reference electrode (4B).

Description

gas sensor

Cross-reference to related applications

This application is based on Japanese Patent Application No. 2021-70519 filed on April 19, 2021, and the contents thereof are incorporated herein.

The present disclosure relates to a gas sensor including a sensor element.

A gas sensor is placed, for example, in an exhaust pipe of an internal combustion engine of a vehicle, and is used to obtain the air-fuel ratio of the internal combustion engine based on the detection target gas, with the exhaust gas flowing through the exhaust pipe as the detection target gas. Also, the gas sensor is used, for example, to detect whether the air-fuel ratio of the internal combustion engine is on the rich side or on the lean side with respect to the stoichiometric air-fuel ratio. A plate-shaped solid electrolyte body and a cylindrical solid electrolyte body with a bottom are used as the sensor element constituting the gas sensor.

In the bottomed cylindrical solid electrolyte body, a detection electrode (measurement electrode) that is exposed to a detection target gas such as exhaust gas is provided on the outer surface, and a reference electrode that is exposed to a reference gas such as the atmosphere is provided on the inner surface. It is The sensing and reference electrodes are catalytically active towards oxygen. Further, on the outer surface of the solid electrolyte body, a porous layer that captures water, poisonous substances, etc. and allows the gas to be detected to permeate is provided in a state of covering the detection electrode.

Further, inside the sensor element having a bottomed cylindrical solid electrolyte body, a heater element for heating the detection electrode and the reference electrode provided on the bottom side portion of the sensor element is arranged. The heater element has a heating portion of the heating element at a position facing the detection electrode and the reference electrode.

For example, the gas sensor described in Patent Document 1 includes a sensor element composed of a solid electrolyte body having a cylindrical portion with a closed tip, and a heater element arranged inside the sensor element to heat the sensor element by generating heat. Prepare. In order to shorten the time required for activation of the sensor element, the tip of the heater element is in contact with the inner surface of the tip of the sensor element, and the side of the heater element is located on the cylindrical portion of the sensor element. It is in contact with the inner peripheral surface.

WO2013/024775

In the gas sensor of Patent Document 1, it is necessary to intentionally eccentrically align the central axis of the heater element with respect to the central axis of the sensor element so that the sides of the heater element come into contact with the inner peripheral surface of the cylindrical portion of the sensor element. . Therefore, it is necessary to devise ways to prevent damage to the sensor element and the heater element when assembling the gas sensor, and assembling the gas sensor is not easy. In addition, in the gas sensor of Patent Document 1, a part of the side portion of the heater element in the circumferential direction linearly contacts the inner circumferential surface of the cylindrical portion of the heater element along the direction of the central axis. Therefore, it is not sufficient to accelerate the heat transfer from the heater element to the sensor element and to reduce the temperature difference between the heater element and the sensor element.

An object of the present disclosure is to provide a gas sensor that can reduce the temperature difference between the heater element and the sensor element and facilitate assembly of the gas sensor.

One aspect of the present disclosure is
A solid electrolyte body having a bottomed cylindrical shape and made of a solid electrolyte material having ionic conductivity, a detection electrode provided outside the solid electrolyte body and exposed to a gas to be detected, and an inside of the solid electrolyte body A gas sensor comprising a sensor element having a reference electrode provided in and exposed to a reference gas,
In the gas sensor, a porous layer having a large number of pores is provided between the solid electrolyte body and the reference electrode or on the surface of the reference electrode.

The gas sensor of the above aspect includes a sensor element in which a detection electrode and a reference electrode are provided in a solid electrolyte body having a bottomed cylindrical shape. In the sensor element, a porous layer having a large number of pores is provided between the solid electrolyte body and the reference electrode or on the surface of the reference electrode. By forming the porous layer, the gap between the sensor element and the heater element arranged inside the sensor element can be reduced, and the heater element can easily come into contact with the sensor element over a wide range.

Thereby, when the sensor element is heated by the heater element, heat transfer from the heater element to the sensor element can be effectively promoted, and the temperature difference between the heater element and the sensor element can be reduced. In addition, there is no need to take special measures when arranging the heater element inside the sensor element, and the assembly of the gas sensor can be facilitated.

Therefore, according to the gas sensor of one aspect, the temperature difference between the heater element and the sensor element can be reduced, and the assembly of the gas sensor can be facilitated.

It should be noted that the symbols in parentheses for each component shown in one aspect of the present disclosure indicate the correspondence relationship with the symbols in the drawings in the embodiment, but each component is not limited only to the contents of the embodiment.

Objects, features, advantages, etc. of the present disclosure will become clearer from the following detailed description with reference to the accompanying drawings. Drawings of the present disclosure are provided below.
FIG. 1 is an explanatory diagram showing a gas sensor according to Embodiment 1. FIG. FIG. 2 is an explanatory diagram showing a cross section of the sensor element according to the first embodiment. 3A and 3B are explanatory diagrams showing a formation state of detection electrodes on the surface of the sensor element according to the first embodiment. FIG. FIG. 4 is an explanatory view showing an enlarged cross-section of tip side portions of the sensor element and the heater element according to the first embodiment. FIG. 5 is an explanatory diagram showing an enlarged cross section of a portion of the cylindrical portion of the sensor element according to the first embodiment. FIG. 6 is an explanatory diagram showing a cross section of the sensor element according to the second embodiment.

A preferred embodiment of the gas sensor 1 described above will be described with reference to the drawings.
<Embodiment 1>
As shown in FIGS. 1 to 4, the gas sensor 1 of this embodiment includes a sensor element 2, and the sensor element 2 has a solid electrolyte body 3, a detection electrode 4A and a reference electrode 4B. The solid electrolyte body 3 has a bottomed cylindrical shape and is made of a solid electrolyte material having ionic conductivity. The detection electrode 4A is provided outside the solid electrolyte body 3 in a state of being exposed to the gas G to be detected. The reference electrode 4B is provided inside the solid electrolyte body 3 in a state of being exposed to the reference gas A. As shown in FIG. A porous layer 23 having a large number of pores 231 is provided between the solid electrolyte body 3 and the reference electrode 4B.

In the sensor element 2 and the gas sensor 1 of this embodiment, the direction along the central axis O of the sensor element 2 is called the axial direction L, and the direction around the central axis O of the sensor element 2 is called the circumferential direction C. 2 is called a radial direction R. Further, in the sensor element 2, the side on which the bottom portion 32 of the sensor element 2 is provided is called the bottom side L1, and the side opposite to the bottom side L1 is called the opening side L2. In the gas sensor 1, the side corresponding to the bottom side L1 is called the distal end side L1, and the side corresponding to the bottom side L1 is called the base end side L2.

The gas sensor 1 of this embodiment will be described in detail below.
(Gas sensor 1)
As shown in FIG. 1, a gas sensor 1 is arranged in an exhaust pipe through which exhaust gas discharged from an internal combustion engine of a vehicle flows. The gas sensor 1 uses the exhaust gas flowing in the exhaust pipe as the detection target gas G and the atmosphere as the reference gas A to perform gas detection. The gas sensor 1 of this embodiment detects the electromotive force generated between the detection electrode 4A and the reference electrode 4B via the solid electrolyte body 3, and the air-fuel ratio of the internal combustion engine obtained from the composition of the exhaust gas is different from the stoichiometric air-fuel ratio. It is used as an oxygen sensor to determine whether the air-fuel ratio is on the fuel-rich side, where the ratio of fuel to air is higher than the stoichiometric air-fuel ratio, or on the fuel-lean side, where the ratio of fuel to air is lower than the stoichiometric air-fuel ratio.

The gas sensor 1 is used to bring the air-fuel ratio in the internal combustion engine close to the stoichiometric air-fuel ratio at which the catalytic activity of the three-way catalyst arranged in the exhaust pipe is effectively maintained. The gas sensor 1 may be arranged either upstream or downstream of the exhaust gas flow relative to the position of the three-way catalyst in the exhaust pipe.

In particular, since the gas sensor 1 of this embodiment can maintain an appropriate temperature distribution in the axial direction L of the sensor element 2, it can be effectively used even when the temperature of the exhaust gas from the internal combustion engine becomes lower. Further, the temperature of the exhaust gas is lower at a position downstream than the three-way catalyst arrangement position in the exhaust pipe compared to a position upstream from the three-way catalyst arrangement position. The gas sensor 1 of the present embodiment is preferably arranged at a position downstream of the position where the three-way catalyst is arranged, where the temperature of the exhaust gas is low. An air-fuel ratio sensor for detecting the air-fuel ratio may be arranged upstream of the arrangement position of the three-way catalyst, and in the combustion control of the internal combustion engine, the air-fuel ratio sensor and the oxygen sensor may be used together. .

As shown in FIGS. 1 and 4, the gas sensor 1 includes, in addition to the sensor element 2, a heater element 5 for heating a portion of the sensor element 2 on the bottom side L1 in the axial direction L. The heater element 5 is formed by providing a heating element 52 on

base materials

51A and 51B. The heater element 5 has a heat generating portion 521 for heating a portion on the bottom side L1 of the sensor element 2 at a portion on the tip side L1 in the axial direction L. It contacts the inside of the bottom 32 .

In addition to the sensor element 2 and the heater element 5, the gas sensor 1 is attached to a housing 61 formed with an insertion hole 611 through which the sensor element 2 is inserted, and to the outside of the opening side L2 of the solid electrolyte body 3 for detection. It further includes an outer terminal 71 connected to the electrode 4A, and an inner terminal 72 attached to the inside of the opening side L2 of the solid electrolyte body 3 and connected to the reference electrode 4B.

(Sensor element 2)
As shown in FIGS. 2 and 3, the solid electrolyte body 3 of the sensor element 2 is mainly composed of zirconia, and stabilized by substituting a part of zirconia with a rare earth metal element or an alkaline earth metal element. It consists of zirconia or partially stabilized zirconia. The solid electrolyte body 3 may be composed of yttria-stabilized zirconia or yttria-partially-stabilized zirconia. The solid electrolyte body 3 has ion conductivity to conduct oxide ions (O ²⁻ ) at a predetermined activation temperature. The detection electrode 4A and the reference electrode 4B contain platinum, which exhibits catalytic activity with respect to oxygen, and a solid electrolyte material equivalent to the solid electrolyte material forming the solid electrolyte body 3 .

The solid electrolyte body 3 has a cylindrical portion 31 and a curved bottom portion 32 formed on the bottom side L1 of the cylindrical portion 31 . In other words, the solid electrolyte body 3 has a bottomed cylindrical shape in which the tip portion of the cylindrical portion 31 is closed by the curved (hemispherical) bottom portion 32 . An opening 33 for allowing the reference gas A to flow inside the solid electrolyte body 3 is formed at a position opposite to the bottom 32 in the axial direction L of the solid electrolyte body 3 . The outer diameter of each portion in the axial direction L of the cylindrical portion 31 is appropriately changed in consideration of attachment to the housing 61 .

The cylindrical portion 31 of the solid electrolyte body 3 has a straight cylindrical portion 311 provided with an outer detection portion 41 (to be described later) on the bottom side L1 in the axial direction L, and an opening side L2 of the straight cylindrical portion 311 toward the opening side L2. and a collar portion 313 which is a portion of the solid electrolyte body 3 having the largest outer diameter on the opening side L2 of the tapered cylindrical portion 312 . The collar portion 313 is a portion that is engaged with a stepped portion 614 formed in the insertion hole 611 of the housing 61 .

As shown in FIGS. 1 and 3, the detection electrode 4A has an outer detection portion 41, an outer connection portion 43 and an outer lead portion . The outer detection portion 41 is provided over the entire circumference in the circumferential direction C about the center axis O of the cylindrical portion 31 at the position closest to the bottom side L1 of the detection electrode 4A. The outer connection portion 43 is provided over the entire circumference in the circumferential direction C at the position closest to the opening side L2 of the detection electrode 4A, and is connected to the outer terminal 71 . The outer lead portion 42 is provided partly in the circumferential direction C at a position connecting the outer detection portion 41 and the outer connection portion 43 .

As shown in FIG. 2, the reference electrode 4B is provided on the surface of the porous layer 23 and the inner surface 302 of the solid electrolyte body 3. The reference electrode 4</b>B of this embodiment is provided on substantially the entire inner surface 302 of the solid electrolyte body 3 . The reference electrode 4B is provided on the inner surface 302 of the solid electrolyte body 3 at the portion on the opening side L2 in the axial direction L, and the porous layer 23 on the portion other than the portion on the opening side L2 in the axial direction L. is provided on the surface of

The reference electrode 4B may be partially provided inside the solid electrolyte body 3, similar to the detection electrode 4A. The reference electrode 4B is provided along the entire circumference of the bottom side L1 portion facing the outer detection portion 41 via the solid electrolyte body 3 and the opening side L2 portion. You may provide in the state which connects the site|part of L2 in a part of the circumferential direction C. FIG.

The ratio of surface formation of the reference electrode 4B to the entire inner surface 302 of the solid electrolyte body 3 is 50% or more. The inner surface 302 of the solid electrolyte body 3 when specifying the surface formation ratio of the reference electrode 4B also includes the surface of the porous layer 23 provided on the inner surface 302 of the solid electrolyte body 3 . The reference electrode 4B is preferably provided along the entire circumference of the solid electrolyte body 3 in the circumferential direction C at the portion located on the inner peripheral side of the outer detecting portion 41 and the portion at which the inner terminal 72 is attached. When the surface formation ratio of the reference electrode 4B is less than 50%, the effect of heat conduction by the reference electrode 4B is reduced, the temperature rise rate of the sensor element 2 due to the heat transfer of the heater element 5 is slowed, and the sensor element 2 is activated. It may take longer to let The surface formation ratio of the reference electrode 4B of this embodiment is 100%.

(Porous layer 23)
As shown in FIG. 2 , the porous layer 23 of this embodiment is provided on the inner surface 302 of the solid electrolyte body 3 . More specifically, the porous layer 23 is sandwiched between the inner surface 302 of the solid electrolyte body 3 and the reference electrode 4B. In addition, the porous layer 23 is provided at a portion of the inner surface 302 of the solid electrolyte body 3 excluding the portion on the opening side L2 in the axial direction L. As shown in FIG. The porous layer 23 is made of a solid electrolyte material having a higher porosity than the solid electrolyte material forming the solid electrolyte body 3 . The composition of the porous layer 23 is the same as the composition of the solid electrolyte body 3 .

The porous layer 23 functions as a heat insulating layer by forming numerous pores 231 between the solid electrolyte particles. The porosity of the porous layer 23 should be large in order to enhance the heat insulating effect, and should be small in order to enhance the strength. The porosity of the porous layer 23 of this embodiment is 5% or more and 40% or less. If the porosity of the porous layer 23 is less than 5%, the effect of the porous layer 23 as a heat insulating layer may not be obtained. On the other hand, when the porosity of the porous layer 23 exceeds 40%, the strength of the porous layer 23 is low, and the porous layer 23 may be damaged during assembly of the gas sensor 1 .

The porosity of the porous layer 23 is preferably 10% or more and 30% or less. In this case, a heat insulating effect can be obtained more appropriately, and a decrease in the rate of temperature increase of the sensor element 2 by the heater element 5 can be properly prevented.

The porosity of the porous layer 23 can be measured as follows. A part of the porous body is taken out to prepare a thin sample, and the dry mass, saturated water mass and underwater mass of the sample are measured. The porosity K of the porous layer 23 is calculated as K=(saturated mass of sample−dry mass of sample)/(saturated mass of sample−mass of sample in water)×100 [%].

This porosity measurement method complies with JIS R1634: 1998 "Method for measuring sintered body density and open porosity of fine ceramics" (Archimedes method). Further, this JIS standard corresponds to the ISO standard of ISO18754:2003 "Fine ceramics-Determination of density and apparent porosity".

The reference electrode 4B is provided on the surface of the porous layer 23 at the bottom side L1 in the axial direction L of the sensor element 2 . With this configuration, the tip portion of the heater element 5 can be brought into contact with the reference electrode 4</b>B provided inside the bottom portion 32 of the solid electrolyte body 3 . Therefore, the heat generated in the heat generating portion 521 of the heater element 5 can be effectively transmitted to the sensor element 2 through the reference electrode 4B by thermal conduction.

Further, as shown in FIG. 5, by providing the reference electrode 4B on the surface of the porous layer 23, a part of the reference electrode 4B becomes the pores 231 in the porous layer 23, in other words, between the solid electrolyte particles. penetrate into the gaps of As a result, the adhesion between the reference electrode 4B and the porous layer 23 is improved, and the reference electrode 4B is less likely to peel off from the solid electrolyte body 3 . Also, it is possible to suppress aggregation of the reference electrode 4B due to heat.

A large number of solid electrolyte particles that constitute the solid electrolyte body 3 are formed into a dense cylindrical shape with a bottom, and are sintered in close contact with each other. The porous layer 23 is made up of many solid electrolyte particles. The solid electrolyte particles forming the porous layer 23 are formed by applying slurry to the inner surface 302 of the solid electrolyte body 3 . A large number of pores 231 of the porous layer 23 are formed by gaps between solid electrolyte particles.

As shown in FIG. 2, the porous layer 23 of the present embodiment is formed at a portion of the solid electrolyte body 3 closer to the bottom side L1 than the mounting portion of the inner terminal 72 . With this configuration, the inner terminal 72 is attached in contact with the reference electrode 4B provided on the inner surface 302 of the solid electrolyte body 3 which is compactly formed. Therefore, it is possible to secure the strength of the mounting portion of the inner terminal 72 in the solid electrolyte body 3 .

Moreover, the heat of the heater element 5 heated by the heat generating portion 521 of the heating element 52 is conducted to the sensor element 2 via the inner terminal 72 in addition to being conducted from the tip of the heater element 5 to the sensor element 2 . . Since the inner terminal 72 is in contact with the reference electrode 4B provided on the inner surface 302 of the solid electrolyte body 3, the heat of the heater element 5 is transferred to the bottom side L1 and the opening side L2 of the sensor element 2. It can be effectively transmitted to the sensor element 2 . The heat of the heater element 5 may be transferred to the sensor element 2 by thermal radiation or thermal convection.

As shown in FIG. 2 , the thickness of the bottom portion 233 of the porous layer 23 provided on the inner surface 302 of the bottom portion 32 of this embodiment is equal to the thickness of the opening of the porous layer 23 provided on the inner surface 302 of the cylindrical portion 31 . It is greater than the thickness of side portion 234 . In other words, the inner surface 302 of the bottom portion 32 has the thickest portion of the entire porous layer 23 . In particular, in this embodiment, the thickness of the porous layer 23 is the thickest at the center position of the bottom portion 32 .

In addition, in this embodiment, the thickness of the bottom side portion 233 of the porous layer 23 provided on the inner surface 302 of the bottom portion 32 and the thickness of the portion 233A of the porous layer 23 provided on the inner surface 302 of the straight cylindrical portion 311 are Thickness is relatively large. On the other hand, the thickness of the opening side portion 234 of the porous layer 23 provided on the inner surface 302 of the tapered cylindrical portion 312 and the thickness of the opening side portion 234 of the porous layer 23 provided on the inner surface 302 of the flange portion 313 are , relatively small.

As shown in FIG. 4, the thickness of the bottom-side portion 233 of the porous layer 23 inside the bottom portion 32 is relatively large. A wide range around the heat generating portion 521 is likely to come into contact with the sensor element 2 . Also, a wide area around the heat generating portion 521 of the heater element 5 can be brought into contact with the sensor element 2 . Thereby, heat transfer from the heater element 5 to the sensor element 2 can be effectively promoted.

In this embodiment, the portion of the reference electrode 4B and the portion of the porous layer 23 provided inside the cylindrical portion 31 are separated from the heater element 5, and the reference electrode 4B provided inside the bottom portion 32 is separated from the heater element 5. is in contact with the heater element 5 . With this configuration, it is possible to effectively conduct heat from the tip portion of the heater element 5 in the axial direction L to the sensor element 2 , thereby effectively reducing the temperature difference between the heater element 5 and the sensor element 2 . can.

Further, the corner portion of the bottom side L1 in the axial direction L of the heater element 5 may be in contact with the reference electrode 4B inside the bottom portion 32 of the sensor element 2 . Also, the tip 501 of the heater element 5 in the axial direction L may be in contact with the reference electrode 4B inside the bottom portion 32 of the sensor element 2 . Due to the configuration in which the thickness of the bottom side portion 233 of the porous layer 23 inside the bottom portion 32 is relatively large, a wide range of the tip portion of the heater element 5 in the axial direction L serves as a reference inside the bottom portion 32 of the sensor element 2 . Surface contact with the electrode 4B becomes possible.

As shown in FIG. 5, since the porous layer 23 is formed of a large number of solid electrolyte particles, unevenness 232 is formed on the surface of the porous layer 23 by the solid electrolyte particles. On the surface of the reference electrode 4B provided on the surface of the porous layer 23, unevenness 401 corresponding to the unevenness 232 formed on the surface of the porous layer 23 is formed. With this configuration, the protrusions of the unevenness 401 of the reference electrode 4B are likely to come into contact with the heater element 5 . Further, even if the protrusions of the unevenness 401 of the reference electrode 4B come into contact with the heater element 5, the friction generated between the reference electrode 4B and the heater element 5 can be kept small.

Alternatively, the dimension of the bottom portion 32 of the sensor element 2, the thickness of the porous layer 23, etc. may be devised so that the heater element 5 is intentionally brought into contact with the projections of the irregularities 401 formed on the surface of the reference electrode 4B. good. Further, when the heater element 5 comes into contact with the projections of the irregularities 401 formed on the surface of the reference electrode 4B, the contact portion or the projections may be partially deformed. Partial deformation of the contact portion or protrusion is caused by deformation of at least one of the porous layer 23 and the reference electrode 4B. In particular, since the porous layer 23 has a large number of pores 231, it is easily deformed. By deforming the porous layer 23, the contact area between the heater element 5 and the sensor element 2 can be increased, and the degree of adhesion therebetween can be increased. The configuration in which the heater element 5 is in contact with the convex portion of the reference electrode 4B makes it possible to more effectively conduct heat from the heater element 5 to the sensor element 2, thereby reducing the temperature difference between the heater element 5 and the sensor element 2. , can be mitigated more effectively.

The opening side portion 234 of the porous layer 23 of this embodiment is formed with a thickness within the range of 1 to 20 μm. In addition, the bottom side portion 233 of the porous layer 23 is formed to have a thickness greater than that of the opening side portion 234 and within a range of 20 to 50 μm. Since the thickness of the bottom side portion 233 of the porous layer 23 is 2.5 times or more the thickness of the opening side portion 234 of the porous layer 23, the heat transfer effect at the bottom side L1 portion of the solid electrolyte body 3 can be properly obtained.

(Base layer 21)
As shown in FIG. 5, the outer surface 301 of the solid electrolyte body 3 is formed with a base layer 21 made of solid electrolyte particles having a plurality of types of particle diameters and made of a solid electrolyte material. Solid electrolyte particles with a relatively small particle size and solid electrolyte particles with a relatively large particle size are used for the underlayer 21 of this embodiment. The detection electrode 4A of this embodiment is provided on the surface of the underlying layer 21 . The surface of the base layer 21 is made uneven by solid electrolyte particles in order to increase the surface area of the detection electrode 4A and to prevent the detection electrode 4A from peeling off.

(Protective layer 22)
As shown in FIGS. 2 to 4, the outer surface 301 of the solid electrolyte body 3 is provided with a protective layer 22 made of a ceramic porous body that covers at least the entire outer detection portion 41 of the detection electrode 4A. . The protective layer 22 is permeable to the gas G to be detected and prevents the detection electrode 4A from being poisoned and wet. The protective layer 22 of this embodiment is formed of a plurality of laminated layers having different porosities.

As shown in FIG. 5, the protective layer 22 of this embodiment includes a first protective layer 221 provided on the surface of the detection electrode 4A, a second protective layer 222 provided on the surface of the first protective layer 221, and a second protective layer 222 provided on the surface of the first protective layer 221. 2 and a third protective layer 223 provided on the surface of the second protective layer 222 . The first protective layer 221 protects the detection electrode 4A from the heat of the gas G to be detected. The second protective layer 222 has a function of adsorbing the poisoning substance of the detection electrode 4A in the gas G to be detected, and supports catalyst particles for adjusting the components of the gas G to be detected. The third protective layer 223 has a function of adsorbing the poisoning substance of the detection electrode 4A in the gas G to be detected, and also protects the sensor element 2 from condensed water.

(Heater element 5)
As shown in FIG. 4, the heater element 5 has

ceramic substrates

51A and 51B, and a heating element 52 made of a conductor provided on the substrate 51B. The heat-generating portion 521 of the heat-generating member 52 is formed with the smallest cross-sectional area in the heat-generating member 52 and is a portion that generates heat by Joule heat when the heat-generating member 52 is energized. The heat generating portion 521 is formed in a meandering shape in the axial direction L at the tip portion of the heat generating body 52 . The heater element 5 is formed by winding a sheet-like base material 51B provided with a heating element 52 around a base material 51A serving as a mandrel. The reference electrode 4B on the bottom 32 of the solid electrolyte body 3 is in contact with the tip 501 of the base material 51A serving as the mandrel.

(Another Configuration of Gas Sensor 1)
As shown in FIG. 1, the gas sensor 1 includes, in addition to the sensor element 2 and the heater element 5, a housing 61 that holds the sensor element 2; The proximal side cover 63 attached to the proximal side L2 portion of the sensor element 2, the inner terminal 72 attached to the inner surface 302 of the opening side L2 portion of the sensor element 2, the outer surface of the opening side L2 portion of the sensor element 2 An outer terminal 71 and the like attached to 301 are provided.

(Housing 61)
As shown in FIG. 1, the housing 61 is formed with an insertion hole 611 penetrating in the axial direction L to hold the sensor element 2 . The insertion hole 611 has a small-diameter hole portion 612 positioned on the distal end side L1 in the axial direction L and a large-diameter hole portion 613 positioned on the proximal end side L2 in the axial direction L and having a larger diameter than the small-diameter hole portion 612. . The sensor element 2 is inserted into the small-diameter hole portion 612 and the large-diameter hole portion 613 of the insertion hole 611, and a sealing material such as talc powder or a sleeve is placed in the gap between the sensor element 2 and the large-diameter hole portion 613. 64.

Further, the sensor element 2 is pulled out from the insertion hole 611 of the housing 61 to the tip end side L1 by locking the collar portion 313, which is the portion of the sensor element 2 having the largest outer diameter, to the end portion of the small diameter hole portion 612. is prevented. A crimped portion 615 that is bent inward is formed at a portion on the opening side L2 in the axial direction L of the housing 61 . The sealing material 64 is compressed in the axial direction L between the crimped portion 615 and the flange portion 313 to hold the sensor element 2 in the housing 61 . A portion of the bottom side L1 of the sensor element 2, particularly a portion where the outer detection portion 41 is formed, is arranged to protrude from the housing 61 toward the tip side L1 in the axial direction L. As shown in FIG.

(Distal side cover 62 and proximal side cover 63)
As shown in FIG. 1, a distal end portion for protecting the sensor element 2 by covering the portion of the sensor element 2 projecting from the housing 61 toward the distal end side L1 is provided on the distal end side L1 in the axial direction L of the housing 61 . A cover 62 is attached. The tip side cover 62 is arranged inside the exhaust pipe. A gas passage hole 621 for allowing the gas G to be detected to pass is formed in the tip side cover 62 . The tip side cover 62 may be of a double structure or of a single structure. Exhaust gas as the detection target gas G flowing into the distal end cover 62 from the gas passage hole 621 of the distal end cover 62 passes through the protective layer 22 of the sensor element 2 and is guided to the detection electrode 4A.

A proximal end cover 63 is attached to the opening side L2 of the housing 61 in the axial direction L. The proximal end cover 63 is arranged outside the exhaust pipe. An introduction hole 631 for introducing air as the reference gas A into the proximal side cover 63 is formed in a part of the proximal side cover 63 . The introduction hole 631 is provided with a filter 632 that blocks the passage of liquid but allows the passage of gas. The reference gas A introduced into the base end cover 63 through the introduction hole 631 passes through the space inside the base end cover 63 and is guided to the reference electrode 4B inside the sensor element 2 .

As shown in FIG. 1, an inner terminal 72 that contacts the reference electrode 4B is attached to the inner surface 302 of the solid electrolyte body 3 on the opening side L2. An outer terminal 71 is attached to the outer surface 301 of the solid electrolyte body 3 on the opening side L2 to contact the outer connection portion 43 of the detection electrode 4A. Lead wires 65 are attached to the inner terminal 72 and the outer terminal 71 for electrically connecting the reference electrode 4B and the detection electrode 4A of the sensor element 2 to an external control device. The lead wire 65 is held by a bush 66 arranged inside the proximal cover 63 .

(Production method)
Next, a method for manufacturing the sensor element 2 will be described.
In manufacturing the solid electrolyte body 3, powder of a solid electrolyte material is prepared by solid-solution substitution of zirconium oxide with at least one of rare earth oxides such as yttrium oxide and alkaline earth metal oxides as additives. . Next, the powder of the solid electrolyte material is formed into a preliminary shape close to a cylindrical shape with a bottom by a rubber press, and then ground into a precursor of a cylindrical shape with a bottom as the final shape. Next, the precursor was heat-treated in an atmospheric environment at 1100 to 1200° C. for 2 hours to obtain a calcined solid electrolyte body 3 .

In forming the porous layer 23 on the inner surface 302 of the calcined body of the solid electrolyte body 3 (simply referred to as the solid electrolyte body 3), the powder of the solid electrolyte material is mixed with ion-exchanged water, polyvinyl alcohol, and an alkylsulfonic acid system. A first slurry is generated by mixing and stirring a dispersant, acrylic resin microbeads, etc. as a burn-off material for forming the pores 231 . Next, after injecting the first slurry into the solid electrolyte body 3 using a syringe, a pin for thickness adjustment is inserted inside the solid electrolyte body 3 . At this time, the gap between the pin and the inner surface 302 of the solid electrolyte body 3 is filled with the first slurry.

Next, the pins are pulled out from the inside of the solid electrolyte body 3 before the first slurry is completely dried. At this time, since the bottom portion 32 of the sensor element 2 is directed downward, the first slurry flows toward the bottom side L1 in the axial direction L due to gravity. As a result, the thickness of the first slurry on the bottom side L1 in the axial direction L is greater than the thickness of the first slurry on the opening side L2 in the axial direction L.

In this way, on the inner surface 302 of the fixed electrolyte body, the first slurry forms the porous layer 23 in which the bottom side portion 233 is thicker than the opening side portion 234 . The porous layer 23 of this embodiment was formed so that the thickness of the bottom side portion 233 was within the range of 20 to 50 μm and the thickness of the opening side portion 234 was within the range of 1 to 20 μm after firing. The porosity of the porous layer 23 was adjusted to be within the range of 5 to 40% by adjusting the amount of polyvinyl alcohol added and the amount of acrylic resin microbeads that burn during firing.

In forming the base layer 21 on the outer surface 301 of the solid electrolyte body 3, the powder of the solid electrolyte material is mixed with ion-exchanged water, polyvinyl alcohol, an alkylsulfonic acid-based dispersant, and a burnout material for forming the pores 231. A second slurry is produced by mixing and stirring acrylic resin microbeads and the like. Also, part of the powder of the solid electrolyte material is spray-dried with a spray dryer to obtain granulated powder, and the granulated powder is mixed with the second slurry to obtain granulated powder slurry. Next, the solid electrolyte body 3 is immersed in the granulated powder slurry, and the outer surface 301 of the solid electrolyte body 3 is dip-coated with the granulated powder slurry.

The base layer 21 of this embodiment was formed so that the thickness of the portion where the powder of the solid electrolyte material was placed, excluding the portion where the granulated powder was placed, was in the range of 10 to 50 μm after firing. Also, the porosity of the underlying layer 21 was adjusted to about 10% by the same method as for the porous layer 23 .

In this embodiment, the solid electrolyte material used for the porous layer 23 and the solid electrolyte material used for the underlying layer 21 are the same as the solid electrolyte material used for the solid electrolyte body 3 . Then, the unfired solid electrolyte body 3 having the porous layer 23 and the underlying layer 21 formed thereon was fired in an atmospheric environment at 1450° C. for 2 hours to obtain a fired body of the solid electrolyte body 3 .

In forming the detection electrode 4A and the reference electrode 4B on the solid electrolyte body 3, the fired body is immersed in an aqueous hydrofluoric acid solution in order to roughen the surface of the fired body of the solid electrolyte body 3. was washed with water and dried. Next, a paste-like electrode material containing an organoplatinum compound is applied to the sintered body so as to form a desired electrode pattern, and the sintered body is heat-treated to degrease organic substances in the electrode material.

Next, the fired body after degreasing is subjected to plating treatment using an electroless platinum plating solution, and then subjected to heat treatment in an atmospheric environment at 1000 to 1200° C. for 1 hour. Thus, the detection electrode 4A and the reference electrode 4B are provided on the sintered body of the solid electrolyte body 3 .

When forming the protective layer 22 on the outer surface 301 of the solid electrolyte body 3, the first protective layer 221 is formed on the outside of the solid electrolyte body 3 by plasma spraying magnesium aluminate powder having a spinel structure. Next, the solid electrolyte body 3 is immersed in a mixed slurry containing alumina particles, alumina sol, ion-exchanged water, catalyst particles made of platinum or platinum and rhodium, etc., and the second protective layer 222 is formed on the surface of the first protective layer 221. Form. Next, the solid electrolyte body 3 is immersed in a mixed slurry containing alumina particles, alumina sol, ion-exchanged water, etc. to form the third protective layer 223 on the surface of the second protective layer 222 . After that, the solid electrolyte body 3 is heat-treated at 500 to 900° C. to manufacture the sensor element 2 .

(Effect)
A gas sensor 1 of this embodiment includes a sensor element 2 in which a solid electrolyte body 3 having a bottomed cylindrical shape is provided with a detection electrode 4A and a reference electrode 4B. In the sensor element 2, a porous layer 23 having many pores 231 is provided between the solid electrolyte body 3 and the reference electrode 4B. Formation of the porous layer 23 can reduce the gap between the sensor element 2 and the heater element 5 arranged inside the sensor element 2, and facilitates the heater element 5 to come into contact with the sensor element 2 over a wide range. can be done.

As a result, when the sensor element 2 is heated by the heater element 5, the heat transfer from the heater element 5 to the sensor element 2 can be effectively promoted, and the temperature difference between the heater element 5 and the sensor element 2 can be reduced. can be made smaller. Moreover, it is not necessary to take special measures when arranging the heater element 5 inside the sensor element 2, and the assembly of the gas sensor 1 can be facilitated.

Therefore, according to the gas sensor 1 of this embodiment, the temperature difference between the heater element 5 and the sensor element 2 can be reduced, and the assembly of the gas sensor 1 can be facilitated.

<Embodiment 2>
This embodiment shows the case where the porous layer 23 is provided on the surface of the reference electrode 4B in the solid electrolyte body 3, as shown in FIG. Since the porous layer 23 of this embodiment is provided on the surface of the reference electrode 4B, it does not have to have properties as a solid electrolyte between the detection electrode 4A and the reference electrode 4B. The porous layer 23 of this embodiment is made of a metal oxide such as aluminum oxide instead of a solid electrolyte material. The porous layer 23 is formed of a large number of metal oxide particles. The porous layer 23 of this embodiment is formed to reduce the gap between the heater element 5 and the sensor element 2 .

Also in this embodiment, the thickness of the portion of the porous layer 23 provided inside the bottom portion 32 is greater than the thickness of the portion of the porous layer 23 provided inside the cylindrical portion 31 . Also, the portion of the porous layer 23 provided on the surface of the reference electrode 4B inside the bottom portion 32 is in contact with the heater element 5 . As shown in FIG. 5, on the surface of the porous layer 23, irregularities 232 are formed by a large number of metal oxide particles. Moreover, the contact portion of the heater element 5 or the convex portion of the unevenness 232 in the porous layer 23 may be deformed. Moreover, even if the porous layer 23 is provided on the surface of the reference electrode 4B, the reference gas A can come into contact with the reference electrode 4B through the pores 231 in the porous layer 23 .

According to the gas sensor 1 of this embodiment, as in the first embodiment, the temperature difference between the heater element 5 and the sensor element 2 can be reduced, and the assembly of the gas sensor 1 can be facilitated. Other configurations, effects, and the like of the gas sensor 1 of this embodiment are the same as those of the first embodiment. Also in the present embodiment, constituent elements indicated by the same reference numerals as those in the first embodiment are the same as those in the first embodiment.

<Embodiment 3>
This embodiment shows a case where the composition of the solid electrolyte material forming the porous layer 23 is devised. In the solid electrolyte material constituting the solid electrolyte body 3 and the solid electrolyte material constituting the porous layer 23 of the present embodiment, a part of zirconia (zirconium oxide) is a rare earth oxide as a rare earth metal element or as an alkaline earth metal element. stabilized zirconia or partially stabilized zirconia replaced by alkaline earth oxides.

Since the porous layer 23 is formed on the inner surface 302 of the solid electrolyte body 3, the conductivity of oxide ions by the solid electrolyte body 3 and the porous layer 23 between the detection electrode 4A and the reference electrode 4B is , there is a possibility of lowering compared to the case where the porous layer 23 is not formed. In the present embodiment, the content ratio of the solid electrolyte material substitute in the porous layer 23 is adjusted in order to compensate for the decrease in conductivity of oxide ions.

Specifically, the content of the rare earth oxide or alkaline earth oxide in the solid electrolyte material forming the porous layer 23 is equal to the content of the rare earth oxide or alkaline earth oxide in the solid electrolyte material forming the solid electrolyte body 3. It is large compared to the oxide content. Since the porous layer 23 has a large number of pores 231 , the conductivity of oxide ions is lower than that of the solid electrolyte body 3 . By relatively increasing the content of rare earth oxides or alkaline earth oxides in the solid electrolyte material constituting the porous layer 23, the conductivity of oxide ions in the porous layer 23 can be increased. .

In this embodiment, the performance of the sensor element 2 can be enhanced due to the high oxide ion conductivity of the porous layer 23 . Other configurations, effects, and the like of the gas sensor 1 of this embodiment are the same as those of the first and second embodiments. Further, in this embodiment as well, constituent elements indicated by the same reference numerals as those in the first and second embodiments are the same as those in the first and second embodiments.

<Confirmation test>
In this confirmation test, the sensor element 2 (implementation product) of Embodiment 1 in which the solid electrolyte body 3 is provided with the porous layer 23 and the conventional sensor element in which the solid electrolyte body 3 is not provided with the porous layer 23 (comparison The temperature difference generated between the heater element 5 and the sensor element when heated by the heater element 5 was measured. In the sensor element 2 of the practical product, the average thickness of the bottom side portion 233 of the porous layer 23 was set to 40 μm, and the average thickness of the opening side portion 234 was set to 10 μm.

In this confirmation test, the heater element 5 is controlled so that the temperature of the outer detection part 41 of the detection electrode 4A reaches 650° C. in a state where the heater element 5 is arranged inside each sensor element of the test product and the comparative product. did. Then, the temperature of the outer detection portion 41 of the detection electrode 4A was measured as the temperature of each sensor element, and the temperature of the heating portion 521 of the heating element 52 of the heater element 5 was measured as the temperature of the heater element 5. As a result, in the sensor element of the comparative product, the temperature of the sensor element was 651.degree. C., the temperature of the heater element 5 was 947.degree. On the other hand, in the sensor element 2 of the practical product, the temperature of the sensor element 2 was 652°C, the temperature of the heater element 5 was 891°C, and the temperature difference between them was 239°C.

From this result, it was found that the temperature difference between the heater element 5 and the sensor element was 57°C smaller than that of the sensor element of the comparative product. This result is due to the fact that the gap between the heater element 5 and the sensor element 2 was reduced by forming the porous layer 23 in the sensor element 2 of the embodied product.

Further, in this confirmation test, when the sensor element 2 of the practical product and the sensor element of the comparative product were continuously heated by intermittent energization to the heating element 52 of the heater element 5, the heater arranged in each sensor element The degree of deterioration of element 5 was also measured. As a result, it was found that the life of the heater element 5 used as the practical product was 3.6 times longer than that of the heater element 5 used as the comparative product. As a result, more heat was transferred from the heater element 5 to the sensor element 2 in the sensor element 2 of the practical product, and the temperature difference between the heater element 5 and the sensor element 2 was reduced. This is because the temperature of the heater element 5 that heats the sensor element 2 has decreased.

From the results of the confirmation test described above, it was found that the gas sensor 1 using the sensor element 2 of the present embodiment reduces the temperature difference between the heater element 5 and the sensor element 2, and early activation of the gas sensor 1 is achieved. rice field. Further, it was found that the life of the heater element 5 is improved by lowering the temperature of the heater element 5 when heating the detection portion by the detection electrode 4A and the reference electrode 4B of the sensor element 2 to the target temperature.

The present disclosure is not limited to only each embodiment, and further different embodiments can be configured without departing from the gist thereof. In addition, the present disclosure includes various modifications, modifications within the equivalent range, and the like. Furthermore, the technical idea of the present disclosure includes combinations of various constituent elements, forms, and the like assumed from the present disclosure.

Claims

A solid electrolyte body (3) having a bottomed cylindrical shape and made of a solid electrolyte material having ionic conductivity, a detection electrode (4A) provided outside the solid electrolyte body and exposed to the gas (G) to be detected ), and a sensor element (2) having a reference electrode (4B) provided inside the solid electrolyte body and exposed to a reference gas (A), wherein
A gas sensor, wherein a porous layer (23) having a large number of pores (231) is provided between the solid electrolyte body and the reference electrode or on the surface of the reference electrode.
further comprising an inner terminal (72) mounted inside the opening side portion of the solid electrolyte body and connected to the reference electrode;
2. The gas sensor according to claim 1, wherein said porous layer is formed on a portion of said solid electrolyte body closer to the bottom side (L1) than said inner terminal mounting portion.
The solid electrolyte body has a cylindrical portion (31) and a curved bottom portion (32) formed on the bottom side of the cylindrical portion,
3. The gas sensor according to claim 1, wherein the thickness of the portion of the porous layer provided inside the bottom portion is greater than the thickness of the portion of the porous layer provided inside the cylindrical portion.
further comprising a heater element (5) disposed within the sensor element to heat the sensor element;
the portion of the reference electrode and the portion of the porous layer provided inside the cylindrical portion are spaced apart from the heater element;
4. The gas sensor of claim 3, wherein the portion of the reference electrode or the portion of the porous layer provided inside the bottom is in contact with the heater element.
The porous layer is provided between the solid electrolyte body and the reference electrode, and is composed of particles of a solid electrolyte material having a higher porosity than the solid electrolyte material constituting the solid electrolyte body. and
The gas sensor according to any one of claims 1 to 4, wherein the surface of the reference electrode is formed with irregularities (401) corresponding to the irregularities (232) formed on the surface of the porous layer.
The solid electrolyte material constituting the solid electrolyte body and the solid electrolyte material constituting the porous layer are composed of stabilized zirconia or partially stabilized zirconia in which a part of zirconia is replaced with a rare earth metal element or an alkaline earth metal element. is composed of
The content of the rare earth metal element or alkaline earth metal element in the solid electrolyte material constituting the porous layer is the content of the rare earth metal element or alkaline earth metal element in the solid electrolyte material constituting the solid electrolyte body. 6. The gas sensor according to claim 5, wherein the number is greater than the .
The gas sensor according to any one of claims 1 to 6, wherein the porous layer has a porosity of 5% or more and 40% or less.