JP7544262B2 - Gas Sensors - Google Patents

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on Japanese Patent Application No. 2021-70519, filed on April 19, 2021, the contents of which are incorporated by reference.

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

Gas sensors are disposed, for example, in the exhaust pipe of an internal combustion engine of a vehicle, and are used to determine the air-fuel ratio of the internal combustion engine based on the exhaust gas flowing through the exhaust pipe as the detection target gas. Gas sensors are also used, for example, to detect whether the air-fuel ratio of the internal combustion engine is on the rich side or lean side of the theoretical air-fuel ratio. The sensor element that constitutes the gas sensor may be a plate-shaped solid electrolyte body or a cylindrical solid electrolyte body with a bottom.

In a cylindrical solid electrolyte body with a bottom, a detection electrode (measurement electrode) exposed to a gas to be detected, such as exhaust gas, is provided on the outer surface, and a reference electrode exposed to a reference gas, such as air, is provided on the inner surface. The detection electrode and reference electrode have catalytic activity against oxygen. In addition, a porous layer is provided on the outer surface of the solid electrolyte body, covering the detection electrode, and captures water, poisonous substances, etc., while allowing the gas to be detected to pass through.

A heater element is disposed inside the sensor element having a cylindrical solid electrolyte body with a bottom for heating the detection electrode and reference electrode provided at the bottom of the sensor element. The heater element has a heating portion of the heating element at a position facing the detection electrode and reference electrode.

For example, the gas sensor described in Patent Document 1 includes a sensor element made of a solid electrolyte body having a cylindrical portion with a closed tip, and a heater element disposed inside the sensor element and generating heat to heat the sensor element. In order to reduce the time required to activate 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 in contact with the inner circumferential surface of the cylindrical portion of the sensor element.

WO2013/024775 publication

In the gas sensor of Patent Document 1, the central axis of the heater element must be intentionally offset from the central axis of the sensor element to bring the side of the heater element into contact with the inner circumferential surface of the cylindrical portion of the sensor element. Therefore, when assembling the gas sensor, it is necessary to devise a method to prevent the sensor element and heater element from being damaged, and assembling the gas sensor is not easy. Also, in the gas sensor of Patent Document 1, a part of the circumferential direction of the side of the heater element makes linear contact with 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 promote heat transfer from the heater element to the sensor element and reduce the temperature difference between the heater element and the sensor element, and further ingenuity is required for this purpose.

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

A first aspect of the present disclosure is a method for manufacturing a semiconductor device comprising:
A gas sensor comprising a sensor element including a solid electrolyte body having a bottomed cylindrical shape and made of a solid electrolyte material having ion conductivity, a detection electrode provided on the outside of the solid electrolyte body and exposed to a detection target gas, and a reference electrode provided on the inside of the solid electrolyte body and exposed to a reference gas,
a porous layer having a large number of pores is provided between the solid electrolyte body and the reference electrode or on a surface of the reference electrode;
the solid electrolyte body has a cylindrical portion and a curved bottom portion formed on a bottom side of the cylindrical portion,
In the gas sensor , the thickness of the portion of the porous layer provided on the inside of the bottom portion is greater than the thickness of the portion of the porous layer provided on the inside of the cylindrical portion .
A second aspect of the present disclosure is a method for manufacturing a semiconductor device comprising:
A gas sensor comprising a sensor element including a solid electrolyte body having a bottomed cylindrical shape and made of a solid electrolyte material having ion conductivity, a detection electrode provided on the outside of the solid electrolyte body and exposed to a detection target gas, and a reference electrode provided on the inside of the solid electrolyte body and exposed to a reference gas,
a porous layer having a large number of pores is provided between the solid electrolyte body and the reference electrode or on a surface of the reference electrode;
the porous layer is provided between the solid electrolyte body and the reference electrode, and is made of particles of a solid electrolyte material having a higher porosity than a solid electrolyte material constituting the solid electrolyte body,
In the gas sensor, the reference electrode has a surface having projections and recesses corresponding to the projections and recesses formed on the surface of the porous layer.

The gas sensor of the above embodiment includes a sensor element having a detection electrode and a reference electrode provided on 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. The formation of the porous layer can reduce the gap between the sensor element and the heater element arranged inside the sensor element, making it easier for the heater element to contact the sensor element over a wide area.

As a result, 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, no special measures are required when placing the heater element inside the sensor element, making it easier to assemble the gas sensor.

Therefore, according to the gas sensor of the above-mentioned embodiment, the temperature difference between the heater element and the sensor element can be reduced and the gas sensor can be easily assembled.

Note that the symbols in parentheses for each component shown in one aspect of the present disclosure indicate a correspondence with the symbols in the figures in the embodiment, but do not limit each component to only the contents of the embodiment.

The objects, features, advantages, etc. of the present disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is an explanatory diagram showing a gas sensor according to a first embodiment. FIG. 2 is an explanatory diagram showing a cross section of the sensor element according to the first embodiment. FIG. 3 is an explanatory diagram showing a state in which a detection electrode is formed on a surface of the sensor element according to the first embodiment. FIG. 4 is an explanatory diagram showing an enlarged cross section of a tip side portion of a sensor element and a heater element according to the first embodiment. FIG. 5 is an explanatory diagram showing an enlarged cross section of a part of a cylindrical portion of the sensor element according to the first embodiment. FIG. 6 is an explanatory diagram showing a cross section of a sensor element according to the second embodiment.

A preferred embodiment of the gas sensor 1 described above will now be described with reference to the drawings.
<Embodiment 1>
1 to 4, the gas sensor 1 of this embodiment includes a sensor element 2, which includes a solid electrolyte body 3, a detection electrode 4A, and a reference electrode 4B. The solid electrolyte body 3 has a cylindrical shape with a bottom, and is made of a solid electrolyte material having ion conductivity. The detection electrode 4A is provided on the outside of the solid electrolyte body 3 in a state where it is 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 where it is exposed to a reference gas A. 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 gas sensor 1 of this embodiment, the direction along the central axis O of the sensor element 2 is referred to as the axial direction L, the direction around the central axis O of the sensor element 2 is referred to as the circumferential direction C, and the direction spreading radially from the central axis O of the sensor element 2 is referred to as the radial direction R. In addition, in the sensor element 2, the side on which the bottom 32 of the sensor element 2 is provided is referred to as the bottom side L1, and the side opposite the bottom side L1 is referred to as the opening side L2. In the gas sensor 1, the side corresponding to the bottom side L1 is referred to as the tip side L1, and the side corresponding to the bottom side L1 is referred to as the base side L2.

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

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

In particular, the gas sensor 1 of this embodiment can maintain an appropriate temperature distribution in the axial direction L of the sensor element 2, and can therefore be effectively used even when the exhaust gas from an internal combustion engine has a lower temperature. In addition, the temperature of the exhaust gas is lower downstream of the position of the three-way catalyst in the exhaust pipe than upstream of the position of the three-way catalyst. It is preferable to place the gas sensor 1 of this embodiment downstream of the position of the three-way catalyst, where the temperature of the exhaust gas is lower. An air-fuel ratio sensor that detects the air-fuel ratio may be placed upstream of the position of the three-way catalyst, and the air-fuel ratio sensor and oxygen sensor may be used together in the combustion control of the internal combustion engine.

1 and 4, in addition to the sensor element 2, the gas sensor 1 includes a heater element 5 for heating a portion of the bottom side L1 of the sensor element 2 in the axial direction L. The heater element 5 is formed by providing a heating element 52 on substrates 51A and 51B. The heater element 5 has a heating portion 521 for heating the portion of the bottom side L1 of the sensor element 2 at a portion of the tip side L1 in the axial direction L, and the tip 501 of the heater element 5 is in contact with the inside of the bottom 32 of the sensor element 2.

In addition to the sensor element 2 and heater element 5, the gas sensor 1 further comprises a housing 61 having an insertion hole 611 through which the sensor element 2 is inserted, an outer terminal 71 attached to the outside of the opening side L2 of the solid electrolyte body 3 and connected to the detection 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)
2 and 3, the solid electrolyte body 3 of the sensor element 2 is mainly composed of zirconia, and is made of stabilized zirconia or partially stabilized zirconia in which a part of the zirconia is replaced by a rare earth metal element or an alkaline earth metal element. The solid electrolyte body 3 may be made of yttria stabilized zirconia or yttria partially stabilized zirconia. The solid electrolyte body 3 has ion conductivity that conducts oxide ions (O ²⁻ ) at a predetermined activation temperature. The detection electrode 4A and the reference electrode 4B contain platinum that exhibits catalytic activity against oxygen, and a solid electrolyte material equivalent to the solid electrolyte material constituting 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 of the cylindrical portion 31 is closed by the curved (hemispherical) bottom portion 32. An opening 33 is formed at a position opposite the bottom portion 32 in the axial direction L of the solid electrolyte body 3 to the axial direction L of the solid electrolyte body 3 for allowing the reference gas A to flow inside the solid electrolyte body 3. The outer diameter of each portion of the cylindrical portion 31 in the axial direction L is appropriately changed in consideration of the attachment to the housing 61.

The cylindrical portion 31 of the solid electrolyte body 3 has a straight cylindrical portion 311 on the bottom side L1 in the axial direction L where an outer detection portion 41 (described later) is provided, a tapered cylindrical portion 312 that expands in diameter as it approaches the opening side L2 of the straight cylindrical portion 311, and a flange portion 313 that is the portion of the solid electrolyte body 3 with the largest outer diameter on the opening side L2 of the tapered cylindrical portion 312. The flange portion 313 is a portion that engages with a step portion 614 formed in the insertion hole 611 of the housing 61.

1 and 3, the detection electrode 4A has an outer detection portion 41, an outer connection portion 43, and an outer lead portion 42. The outer detection portion 41 is provided at the bottom-most position L1 of the detection electrode 4A, over the entire circumference in the circumferential direction C centered on the central axis O of the cylindrical portion 31. The outer connection portion 43 is provided at the opening-most position L2 of the detection electrode 4A, over the entire circumference in the circumferential direction C, and is connected to the outer terminal 71. The outer lead portion 42 is provided in a part of the circumferential direction C at a position connecting the outer detection portion 41 and the outer connection portion 43.

2, the reference electrode 4B is provided on the surface of the porous layer 23 and on the inner surface 302 of the solid electrolyte body 3. In this embodiment, the reference electrode 4B is provided on almost 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 in the portion on the opening side L2 in the axial direction L, and is provided on the surface of the porous layer 23 in the portion other than the portion on the opening side L2 in the axial direction L.

The reference electrode 4B may be partially provided inside the solid electrolyte body 3, similar to the detection electrode 4A. The reference electrode 4B may be provided around the entire circumference at a portion of the bottom side L1 that faces the outer detection unit 41 via a portion of the solid electrolyte body 3, and at a portion of the opening side L2, and may be provided in a state in which the portion of the bottom side L1 and the portion of the opening side L2 are connected in a portion of the circumferential direction C.

The surface formation ratio of the reference electrode 4B in 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. It is preferable that the reference electrode 4B is provided on the entire circumference of the solid electrolyte body 3 in the circumferential direction C at the portion located on the inner side of the outer detection part 41 and at the portion where the inner terminal 72 is attached. If the surface formation ratio of the reference electrode 4B is less than 50%, the effect of heat conduction by the reference electrode 4B becomes small, the temperature rise rate of the sensor element 2 due to heat transfer of the heater element 5 becomes slow, and the time to activate the sensor element 2 may be long. The surface formation ratio of the reference electrode 4B in this embodiment is 100%.

(Porous layer 23)
2 , the porous layer 23 in 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. The porous layer 23 is provided on the inner surface 302 of the solid electrolyte body 3 except for the opening side L2 in the axial direction L. The porous layer 23 is made of a solid electrolyte material having a higher porosity than the solid electrolyte material constituting 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 due to the formation of numerous pores 231 between the solid electrolyte particles. The porosity of the porous layer 23 is preferably large to enhance the heat insulating effect, and small to enhance the strength. The porosity of the porous layer 23 in this embodiment is 5% or more and 40% or less. If the porosity of the porous layer 23 is less than 5%, the porous layer 23 may not be effective as a heat insulating layer. On the other hand, if the porosity of the porous layer 23 is more than 40%, the strength of the porous layer 23 is low, and the porous layer 23 may be damaged when the gas sensor 1 is assembled.

It is preferable that the porosity of the porous layer 23 is 10% or more and 30% or less. In this case, the insulating effect is more adequately obtained and a decrease in the rate of temperature rise of the sensor element 2 by the heater element 5 is appropriately prevented.

The porosity of the porous layer 23 may be measured as follows. A part of the porous body is taken out to make a thin sample, and the dry mass, hydrated mass, and submerged mass of the sample are measured. The porosity K of the porous layer 23 is then calculated as K = (saturated mass of sample - dry mass of sample) / (saturated mass of sample - submerged mass of sample) x 100 [%].

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

At the bottom side L1 of the sensor element 2 in the axial direction L, the reference electrode 4B is provided on the surface of the porous layer 23. With this configuration, the tip of the heater element 5 can be brought into contact with the reference electrode 4B provided on the inside of the bottom 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 transferred to the sensor element 2 via the reference electrode 4B by thermal conduction.

5, by providing the reference electrode 4B on the surface of the porous layer 23, a portion of the reference electrode 4B penetrates into the pores 231 in the porous layer 23, in other words, into the gaps between the solid electrolyte particles. This improves the adhesion between the reference electrode 4B and the porous layer 23, making it difficult for the reference electrode 4B to peel off from the solid electrolyte body 3. It also makes it possible to suppress agglomeration of the reference electrode 4B due to heat.

The numerous solid electrolyte particles that make up 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 formed by numerous solid electrolyte particles. The solid electrolyte particles that make up the porous layer 23 are formed by applying a slurry to the inner surface 302 of the solid electrolyte body 3. The numerous pores 231 in the porous layer 23 are formed by the gaps between the solid electrolyte particles.

2, the porous layer 23 in this embodiment is formed in a portion of the solid electrolyte body 3 closer to the bottom side L1 than the attachment 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 has been molded into a dense state. This ensures the strength of the attachment portion of the solid electrolyte body 3 where the inner terminal 72 is attached.

In addition, the heat of the heater element 5 heated by the heating portion 521 of the heating body 52 is not only conducted from the tip of the heater element 5 to the sensor element 2, but also conducted to the sensor element 2 via the inner terminal 72. 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 can be effectively transferred to the sensor element 2 at the bottom side L1 and the opening side L2 of 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.

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

In this embodiment, the thickness of the bottom 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 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 Figure 4, the thickness of the bottom portion 233 of the porous layer 23 inside the bottom 32 is relatively large, so that when the heater element 5 is placed inside the sensor element 2, a wide area around the heat generating portion 521 of the heater element 5 is likely to come into contact with the sensor element 2. It is also possible to bring a wide area around the heat generating portion 521 of the heater element 5 into contact with the sensor element 2. This can effectively promote heat transfer from the heater element 5 to the sensor element 2.

In this embodiment, the portion of the reference electrode 4B and the portion of the porous layer 23 provided on the inside of the cylindrical portion 31 are separated from the heater element 5, and the portion of the reference electrode 4B provided on the inside of the bottom portion 32 is in contact with the heater element 5. This configuration allows for effective heat conduction from the tip of the heater element 5 in the axial direction L to the sensor element 2, and effectively reduces the temperature difference between the heater element 5 and the sensor element 2.

In addition, a corner of the bottom side L1 portion of the heater element 5 in the axial direction L may be in contact with the reference electrode 4B on the inside of the bottom 32 of the sensor element 2. In addition, the tip 501 of the heater element 5 in the axial direction L may be in contact with the reference electrode 4B on the inside of the bottom 32 of the sensor element 2. By configuring the bottom side portion 233 of the porous layer 23 on the inside of the bottom 32 to be relatively thick, a wide range of the tip of the heater element 5 in the axial direction L can be in surface contact with the reference electrode 4B on the inside of the bottom 32 of the sensor element 2.

5, the porous layer 23 is formed of a large number of solid electrolyte particles, and thus the surface of the porous layer 23 is formed with unevenness 232 due to the solid electrolyte particles. The surface of the reference electrode 4B provided on the surface of the porous layer 23 is formed with unevenness 401 corresponding to the unevenness 232 formed on the surface of the porous layer 23. This configuration makes it easier for the convex portions of the unevenness 401 on the reference electrode 4B to come into contact with the heater element 5. Even if the convex portions of the unevenness 401 on 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.

In addition, the dimensions of the bottom 32 of the sensor element 2 and the thickness of the porous layer 23 may be devised so that the heater element 5 is intentionally brought into contact with the convex portion of the unevenness 401 formed on the surface of the reference electrode 4B. When the heater element 5 comes into contact with the convex portion of the unevenness 401 formed on the surface of the reference electrode 4B, the contact portion or a part of the convex portion may be deformed. The deformation of the contact portion or a part of the convex portion occurs due to the deformation of at least one of the porous layer 23 and the reference electrode 4B. In particular, the porous layer 23 is in a state where it is easily deformed because it has a large number of pores 231. The deformation of the porous layer 23 increases the contact area between the heater element 5 and the sensor element 2, and increases the degree of adhesion between them. The configuration in which the heater element 5 comes into 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, and more effectively alleviate the temperature difference between the heater element 5 and the sensor element 2.

In this embodiment, the opening side portion 234 of the porous layer 23 is formed to a thickness within the range of 1 to 20 μm. The bottom side portion 233 of the porous layer 23 is formed to a thickness within the range of 20 to 50 μm, which is greater than the thickness of the opening side portion 234. 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 in the bottom side L1 portion of the solid electrolyte body 3 can be appropriately obtained.

(Undercoat layer 21)
As shown in Fig. 5, an underlayer 21 made of solid electrolyte particles formed into a plurality of particle sizes and made of a solid electrolyte material is formed on an outer surface 301 of the solid electrolyte body 3. In the present embodiment, solid electrolyte particles having a relatively small particle size and solid electrolyte particles having a relatively large particle size are used for the underlayer 21. In the present embodiment, the detection electrode 4A is provided on the surface of the underlayer 21. The surface of the underlayer 21 is formed into an uneven shape made of solid electrolyte particles in order to increase the surface area of the detection electrode 4A and to make the detection electrode 4A less likely to peel off.

(Protective Layer 22)
2 to 4, a protective layer 22 made of a porous ceramic material is provided on the outer surface 301 of the solid electrolyte body 3, covering at least the entire outer detection portion 41 of the detection electrode 4A. The protective layer 22 is permeable to the detection target gas G, while preventing the detection electrode 4A from being poisoned or exposed to water. The protective layer 22 in this embodiment is formed of a plurality of layers having different porosities, which are stacked on top of each other.

5, the protective layer 22 in this embodiment is formed by 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 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 target gas G. The second protective layer 222 has a function of adsorbing the poisoning substances of the detection electrode 4A in the target gas G, and supports catalyst particles that adjust the components of the target gas G. The third protective layer 223 has a function of adsorbing the poisoning substances of the detection electrode 4A in the target gas G, and 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 heating portion 521 of the heating element 52 is formed to have the smallest cross-sectional area in the heating element 52, and is a portion that generates heat by Joule heat when electricity is applied to the heating element 52. The heating portion 521 is formed in a shape that meanders in the axial direction L at the tip of the heating element 52. The heater element 5 is formed by wrapping a sheet-like substrate 51B provided with the heating element 52 around the substrate 51A that serves as a core. The tip 501 of the substrate 51A that serves as a core is in contact with the reference electrode 4B at the bottom 32 of the solid electrolyte body 3.

(Other configurations of gas sensor 1)
As shown in FIG. 1, in addition to the sensor element 2 and heater element 5, the gas sensor 1 includes a housing 61 for holding the sensor element 2, a tip cover 62 attached to a bottom side L1 portion of the housing 61, a base end cover 63 attached to a base end side L2 portion of the housing 61, an inner terminal 72 attached to an inner surface 302 of the opening side L2 portion of the sensor element 2, and an outer terminal 71 attached to an outer surface 301 of the opening side L2 portion of the sensor element 2.

(Housing 61)
1, the housing 61 is formed with an insertion hole 611 penetrating in the axial direction L in order to hold the sensor element 2. The insertion hole 611 has a small diameter hole portion 612 located on a tip end side L1 in the axial direction L, and a large diameter hole portion 613 located on a base 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 is held via a sealant 64 such as talc powder or a sleeve that is arranged in a gap between the sensor element 2 and the large diameter hole portion 613.

In addition, the flange 313, which is the part of the sensor element 2 with the largest outer diameter, is engaged with the end of the small diameter hole 612, thereby preventing the sensor element 2 from slipping out of the insertion hole 611 of the housing 61 to the tip side L1. A crimped portion 615 bent inwardly is formed in a portion of 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 313, thereby holding the sensor element 2 in the housing 61. The portion of the bottom side L1 of the sensor element 2, particularly the portion where the outer detection portion 41 is formed, is disposed so as to protrude from the housing 61 to the tip side L1 in the axial direction L.

(Distal end cover 62 and base end cover 63)
As shown in Fig. 1, a tip cover 62 is attached to a tip side L1 of the housing 61 in the axial direction L to cover a portion of the sensor element 2 protruding from the housing 61 to the tip side L1 and protect the sensor element 2. The tip cover 62 is disposed in the exhaust pipe. The tip cover 62 is formed with a gas passage hole 621 for passing the detection target gas G. The tip cover 62 may have a double structure or a single structure. Exhaust gas as the detection target gas G flowing into the tip cover 62 from the gas passage hole 621 of the tip cover 62 passes through the protective layer 22 of the sensor element 2 and is guided to the detection electrode 4A.

A base end cover 63 is attached to the opening side L2 of the housing 61 in the axial direction L. The base end cover 63 is disposed outside the exhaust pipe. An inlet hole 631 is formed in a part of the base end cover 63 for introducing air as a reference gas A into the base end cover 63. A filter 632 that does not allow liquid to pass but allows gas to pass is disposed in the inlet hole 631. The reference gas A introduced into the base end cover 63 from the inlet hole 631 passes through the space in the base end cover 63 and is guided to the reference electrode 4B inside the sensor element 2.

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

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

When 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), a powder of the solid electrolyte material is mixed and stirred with ion-exchanged water, polyvinyl alcohol, an alkylsulfonic acid-based dispersant, and acrylic resin microbeads as a burn-off material for forming the pores 231 to generate a first slurry. Next, the first slurry is injected into the solid electrolyte body 3 using a syringe, and then a pin for adjusting the thickness is inserted inside the solid electrolyte body 3. At this time, the first slurry is filled in the gap between the pin and the inner surface 302 of the solid electrolyte body 3.

Next, before the first slurry dries completely, the pin is removed from inside the solid electrolyte body 3. At this time, because the bottom 32 of the sensor element 2 faces downward, the first slurry flows by gravity to the bottom side L1 in the axial direction L. As a result, the thickness of the first slurry at the bottom side L1 in the axial direction L is greater than the thickness of the first slurry at the opening side L2 in the axial direction L.

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

When forming the underlayer 21 on the outer surface 301 of the solid electrolyte body 3, the powder of the solid electrolyte material is mixed and stirred with ion-exchanged water, polyvinyl alcohol, an alkylsulfonic acid-based dispersant, and acrylic resin microbeads as a burn-off material for forming the pores 231, to generate a second slurry. In addition, a part of the powder of the solid electrolyte material is spray-dried with a spray dryer to produce a granulated powder, and the granulated powder is mixed with the second slurry to produce a granulated powder slurry. Next, the solid electrolyte body 3 is immersed in the granulated powder slurry, and the granulated powder slurry is dip-coated on the outer surface 301 of the solid electrolyte body 3.

In this embodiment, the base layer 21 is formed so that the thickness of the portion where the powder of the solid electrolyte material is arranged, excluding the portion where the granulated powder is arranged, is within a range of 10 to 50 μm after firing. The porosity of the base layer 21 is adjusted to about 10% by the same method as 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 underlayer 21 are the same as the solid electrolyte material used for the solid electrolyte body 3. The unfired solid electrolyte body 3 on which the porous layer 23 and the underlayer 21 were formed was fired for 2 hours in an air environment at 1450°C to obtain a fired body of the solid electrolyte body 3.

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

Next, the degreased sintered body is plated using an electroless platinum plating solution, and then heat-treated for one hour in an atmospheric environment at 1000 to 1200° C. In this way, 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, a powder of magnesium aluminate having a spinel structure is plasma sprayed on the outside of the solid electrolyte body 3 to form a first protective layer 221. 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., to form a second protective layer 222 on the surface of the first protective layer 221. Next, the solid electrolyte body 3 is immersed in a mixed slurry containing alumina particles, alumina sol, ion-exchanged water, etc., to form a third protective layer 223 on the surface of the second protective layer 222. Thereafter, the solid electrolyte body 3 is heat-treated at 500 to 900 ° C. to manufacture the sensor element 2.

(Action and Effect)
The gas sensor 1 of this embodiment includes a sensor element 2 in which a detection electrode 4A and a reference electrode 4B are provided on a solid electrolyte body 3 having a bottomed cylindrical shape. In the sensor element 2, a porous layer 23 having a large number of pores 231 is provided between the solid electrolyte body 3 and the reference electrode 4B. The 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, making it easier for the heater element 5 to come into contact with the sensor element 2 over a wide area.

As a result, when the sensor element 2 is heated by the heater element 5, 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. In addition, no special measures are required when arranging the heater element 5 inside the sensor element 2, making it easier to assemble the gas sensor 1.

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 gas sensor 1 can be easily assembled.

<Embodiment 2>
In this embodiment, as shown in FIG. 6, a porous layer 23 is provided on the surface of the reference electrode 4B in the solid electrolyte body 3. Since the porous layer 23 in this embodiment is provided on the surface of the reference electrode 4B, it does not need to have the properties of a solid electrolyte between the detection electrode 4A and the reference electrode 4B. The porous layer 23 in this embodiment is made of a metal oxide such as aluminum oxide, not a solid electrolyte material. The porous layer 23 is formed of a large number of metal oxide particles. The porous layer 23 in this embodiment is formed to reduce the gap between the heater element 5 and the sensor element 2.

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

As with the first embodiment, the gas sensor 1 of this embodiment can reduce the temperature difference between the heater element 5 and the sensor element 2 and facilitate the assembly of the gas sensor 1. Other configurations, functions, and effects of the gas sensor 1 of this embodiment are similar to those of the first embodiment. Also, in this embodiment, components denoted by the same reference numerals as those in the first embodiment are similar to those in the first embodiment.

<Embodiment 3>
This embodiment shows a case where an improvement is made to the composition of the solid electrolyte material constituting the porous layer 23. The solid electrolyte material constituting the solid electrolyte body 3 of this embodiment and the solid electrolyte material constituting the porous layer 23 are composed of stabilized zirconia or partially stabilized zirconia in which part of zirconia (zirconium oxide) is replaced by a rare earth oxide as a rare earth metal element or an alkaline earth oxide as an alkaline earth metal element.

Because the porous layer 23 is formed on the inner surface 302 of the solid electrolyte body 3, the oxide ion conductivity between the detection electrode 4A and the reference electrode 4B through the solid electrolyte body 3 and the porous layer 23 may be reduced compared to when the porous layer 23 is not formed. In this embodiment, in order to compensate for this reduction in oxide ion conductivity, the content ratio of the substitute of the solid electrolyte material in the porous layer 23 is adjusted.

Specifically, the content of rare earth oxides or alkaline earth oxides in the solid electrolyte material constituting the porous layer 23 is greater than the content of rare earth oxides or alkaline earth oxides in the solid electrolyte material constituting the solid electrolyte body 3. The porous layer 23 has a large number of pores 231, and therefore has lower oxide ion conductivity than 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 oxide ion conductivity in the porous layer 23 can be increased.

In this embodiment, the performance of the sensor element 2 can be improved due to the high oxide ion conductivity of the porous layer 23. Other configurations, functions, and effects of the gas sensor 1 of this embodiment are similar to those of the first and second embodiments. Also, in this embodiment, components denoted by the same reference numerals as those in the first and second embodiments are similar to those in the first and second embodiments.

<Confirmation test>
In this confirmation test, the temperature difference occurring between the heater element 5 and the sensor element when heating with the heater element 5 was measured for the sensor element 2 (embodiment) of embodiment 1 in which the porous layer 23 is provided on the solid electrolyte body 3, and a conventional sensor element (comparison product) in which the porous layer 23 is not provided on the solid electrolyte body 3. In the sensor element 2 of the embodiment, 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, with the heater element 5 disposed inside each sensor element of the embodiment and comparison product, the heater element 5 was controlled so that the temperature of the outer detection part 41 of the detection electrode 4A was 650°C. The temperature of the outer detection part 41 of the detection electrode 4A was measured as the temperature of each sensor element, and the temperature of the heating part 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 comparison product, the temperature of the sensor element was 651°C and the temperature of the heater element 5 was 947°C, and the temperature difference between the two was 296°C. On the other hand, in the sensor element 2 of the embodiment product, the temperature of the sensor element 2 was 652°C and the temperature of the heater element 5 was 891°C, and the temperature difference between the two was 239°C.

These results show that the temperature difference between the heater element 5 and the sensor element of the implemented sensor element 2 is 57°C smaller than that of the comparative sensor element. This result is due to the fact that the gap between the heater element 5 and the sensor element 2 of the implemented sensor element 2 is smaller due to the formation of the porous layer 23.

In addition, in this confirmation test, the degree of deterioration of the heater element 5 arranged on each sensor element was also measured when the sensor element 2 of the example product and the sensor element of the comparison product were continuously heated by intermittent current flow to the heating element 52 of the heater element 5. As a result, it was found that the heater element 5 used in the example product had a lifespan 3.6 times longer than that of the heater element 5 used in the comparison product. This result is due to the fact that in the sensor element 2 of the example product, more heat was transferred from the heater element 5 to the sensor element 2, reducing the temperature difference between the heater element 5 and the sensor element 2, thereby lowering the temperature of the heater element 5 that heats the sensor element 2 of the example product.

The above confirmation test results show that the gas sensor 1 using the sensor element 2 of this embodiment reduces the temperature difference between the heater element 5 and the sensor element 2, and enables early activation of the gas sensor 1. It was also found that the life of the heater element 5 is improved by reducing the temperature of the heater element 5 when the detection section formed by the detection electrode 4A and reference electrode 4B of the sensor element 2 is heated to a target temperature.

This disclosure is not limited to the embodiments, and further different embodiments may be constructed without departing from the spirit of the disclosure. This disclosure also includes various modified examples, modifications within the scope of equivalents, etc. Furthermore, the technical ideas of this disclosure also include combinations of various components, forms, etc. envisioned from this disclosure.

Claims (8)

A gas sensor (1) comprising a sensor element (2) including a solid electrolyte body (3) having a bottomed cylindrical shape and made of a solid electrolyte material having ion conductivity, a detection electrode (4A) provided on the outside of the solid electrolyte body and exposed to a detection target gas (G), and a reference electrode (4B) provided on the inside of the solid electrolyte body and exposed to a reference gas (A),
a porous layer (23) having a large number of pores (231) is provided between the solid electrolyte body and the reference electrode or on a surface of the reference electrode;
The solid electrolyte body has a cylindrical portion (31) and a curved bottom portion (32) formed on a bottom side of the cylindrical portion,
A gas sensor , wherein a thickness of the portion of the porous layer provided on the inside of the bottom portion is greater than a thickness of the portion of the porous layer provided on the inside of the cylindrical portion . A heater element (5) disposed within the sensor element for heating the sensor element,
a portion of the reference electrode and a portion of the porous layer provided inside the cylindrical portion are spaced apart from the heater element;
2. The gas sensor according to claim 1 , wherein a portion of the reference electrode or a portion of the porous layer provided on the inside of the bottom portion is in contact with the heater element. the porous layer is provided between the solid electrolyte body and the reference electrode, and is made of particles of a solid electrolyte material having a higher porosity than a solid electrolyte material constituting the solid electrolyte body,
3. The gas sensor according to claim 1 , wherein said reference electrode has a surface on which projections and recesses (401) are formed in accordance with projections and recesses (232) formed on the surface of said porous layer. a solid electrolyte material constituting the solid electrolyte body and a solid electrolyte material constituting the porous layer are composed of stabilized zirconia or partially stabilized zirconia in which a part of zirconia is replaced by a rare earth metal element or an alkaline earth metal element,
4. The gas sensor according to claim 3, wherein a content of a rare earth metal element or an alkaline earth metal element in the solid electrolyte material constituting the porous layer is greater than a content of a rare earth metal element or an alkaline earth metal element in the solid electrolyte material constituting the solid electrolyte body. A gas sensor (1) comprising a sensor element (2) including a solid electrolyte body (3) having a bottomed cylindrical shape and made of a solid electrolyte material having ion conductivity, a detection electrode (4A) provided on the outside of the solid electrolyte body and exposed to a detection target gas (G), and a reference electrode (4B) provided on the inside of the solid electrolyte body and exposed to a reference gas (A),
a porous layer (23) having a large number of pores (231) is provided between the solid electrolyte body and the reference electrode or on a surface of the reference electrode;
the porous layer is provided between the solid electrolyte body and the reference electrode, and is made of particles of a solid electrolyte material having a higher porosity than a solid electrolyte material constituting the solid electrolyte body,
A gas sensor , wherein the reference electrode has a surface on which irregularities (401) corresponding to the irregularities (232) formed on the surface of the porous layer are formed. a solid electrolyte material constituting the solid electrolyte body and a solid electrolyte material constituting the porous layer are composed of stabilized zirconia or partially stabilized zirconia in which a part of zirconia is replaced by a rare earth metal element or an alkaline earth metal element,
6. The gas sensor according to claim 5, wherein a content of a rare earth metal element or an alkaline earth metal element in the solid electrolyte material constituting the porous layer is greater than a content of a rare earth metal element or an alkaline earth metal element in the solid electrolyte material constituting the solid electrolyte body. An inner terminal (72) is attached to the inside of the open side portion of the solid electrolyte body and connected to the reference electrode,
The gas sensor according to claim 1 , wherein the porous layer is formed in a portion of the solid electrolyte body closer to a bottom side (L1) than a portion where the inner terminal is attached. 8. The gas sensor according to claim 1, wherein the porous layer has a porosity of 5% or more and 40% or less.

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