JP2019002866A - Sensor element and gas sensor - Google Patents

Abstract

To provide a sensor element that stabilizes output of an outside electrode so as to suppress both a decrease in durability of the outside electrode and a decrease in operability of a gas sensor at low temperature, and the gas sensor.SOLUTION: The present invention relates to a sensor element 3 which has an element body 3s formed in a hollow shaft shape having its tip closed and having a collar part, and an outside electrode 50 formed on an outer peripheral surface of the element body, and the collar part can engage the inside of a metal fittings body. The outside electrode has an electrode part 51 having an electrode function and arranged on a tip side with respect to the collar part, and a lead part 53 electrically connected to the electrode part to extend to a rear end side, and also straddling the collar part, and further comprises a porous protection layer 81 covering the electrode part and an insulation layer 82 made of a different material from the protection layer and covering the collar part in an insulated state. The insulation layer has smaller porosity than the protection layer, and the protection layer and insulation layer come into contact with each other along an axis O; and at least one of the protection layer and insulation layer covers a lead part on a tip side with respect to the collar part, and thermal conductivity of the protection layer is equal to or less than thermal conductivity of the element body.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a sensor element and a gas sensor for detecting the concentration of a gas to be detected.

As shown in FIG. 4, a sensor element is configured by providing an outer electrode and an inner electrode (not shown) on the outer inner surface of a cylindrical solid electrolyte body as a gas sensor for detecting the oxygen concentration in the exhaust gas of an automobile or the like. And what hold | maintains this sensor element inside a cylindrical metal fitting main body is known (patent document 1).
In this sensor element, a detection cell is formed between the outer electrode and the inner electrode sandwiching the solid electrolyte body, and the oxygen concentration is detected by bringing the solid electrolyte body to an activation temperature or higher.

Here, the outer electrode is integrated with the electrode part that covers the front end side of the solid electrolyte body and has an electrode function, the lead part that extends from the electrode part to the rear end side across the collar part, and the rear end side of the lead part. And a ring portion that is connected and electrically connected to a terminal fitting (not shown).
By the way, when the electrical resistance between the metal fitting main body and the lead portion close to the metal fitting main body is lowered, current may flow between them, and noise may be generated in the output of the outer electrode. Therefore, a region from the surface of the electrode portion to the rear end side of the lead portion across the collar portion is covered with a porous protective layer made of a heat resistant material mainly composed of spinel.
The porous protective layer described above also has an effect of preventing poisoning of the electrode part.

JP 2013-36940 A

The spinel used in the porous protective layer described above has higher thermal conductivity than YSZ that is generally used as a solid electrolyte body. For this reason, as shown in FIG. 4, when the electrode part is covered with a porous protective layer mainly composed of spinel, the heat of the electrode part escapes from the porous protective layer to the rear end side of the element main body. It becomes difficult to raise the temperature of the detection cell including the body, and the operability of the gas sensor at a low temperature is lowered.
On the other hand, since the porous protective layer described above is porous, noble metal components such as Pt contained in the lead portion when the high-temperature exhaust gas rises from the sensor element side to the lead portion side and the lead portion becomes hot. Is sublimated, and there is a problem that the durability of the lead portion, and thus the outer electrode, is lowered.
Accordingly, an object of the present invention is to provide a sensor element and a gas sensor that stabilizes the output of the outer electrode and suppresses both a decrease in durability of the outer electrode and a decrease in operability of the gas sensor at a low temperature.

  In order to solve the above-described problems, a sensor element of the present invention includes an element main body that has a flange portion that extends in the axial direction and that is formed in a hollow shaft shape with a closed tip and projects radially outward, and an outer peripheral surface of the element main body. An outer electrode, and a sensor element in which the flange can be engaged inside a cylindrical metal fitting body having a through hole in the axial direction, the outer electrode having an electrode function And an electrode portion disposed on the front end side of the flange portion, and a lead portion that is electrically connected to the electrode portion and extends to the rear end side of the electrode portion and straddles the flange portion. And a porous protective layer that covers the electrode part, and an insulating layer that is made of a material different from that of the protective layer and covers the flange part so that it can be insulated. The insulating layer is more than the protective layer. The porosity is small, the protective layer and the insulating layer are in contact with the axial direction, At least one of Mamoruso and the insulating layer, said covering the lead portion of the distal end side from the flange portion, the thermal conductivity of the protective layer becomes equal to or lower than the thermal conductivity of the element body.

According to this sensor element, since the insulating layer insulates between the flange portion and the metal fitting body, current flows between the lead portion formed on the flange portion and the metal fitting body, and noise is generated in the output of the outer electrode. Can be suppressed, and the output of the outer electrode can be stabilized.
Further, the protective layer covers the electrode portion, and further, an insulating layer having a lower porosity than the protective layer covers the flange portion while being in contact with the rear end side of the protective layer. That is, at least from the tip of the lead portion to the flange portion is covered with the protective layer or the insulating layer. For this reason, even if high-temperature gas to be measured (exhaust gas, etc.) rises from the front end side of the sensor element to the buttocks and the lead portion becomes hot, the lead portion is covered with an insulating layer denser than the protective layer. Sublimation of noble metals such as Pt can be suppressed, and a decrease in durability of the outer electrode can be suppressed.
Furthermore, since the protective layer having the same or lower thermal conductivity as the element body covers the electrode part (completely), the rate at which the heat of the electrode part escapes to the rear end side of the element body is reduced. It becomes easy to raise temperature, and can suppress the fall of the operativity of a gas sensor at low temperature.

In the sensor element of the present invention, the electric resistance of the material constituting the insulating layer may be higher than the electric resistance of the material constituting the protective layer.
According to this sensor element, the insulation between the flange portion and the metal fitting body by the insulating layer becomes more reliable.

In the sensor element of the present invention, a distance L in the axial direction from a rear end of the electrode portion to a front end of the insulating layer is preferably 7.7 mm or more.
When the distance L is 7.7 mm or more, the periphery of the electrode part is covered with a protective layer having a lower thermal conductivity, and the rate at which the heat of the electrode part escapes to the rear end side of the element body is further reduced.

  The gas sensor of the present invention has a cylindrical metal fitting body having a through-hole in the axial direction, a hollow shaft shape having a closed tip, an outer electrode on the tip side of the outer peripheral surface, and a rear end with respect to the outer electrode. A sensor element having a flange portion protruding radially outward on the outer peripheral surface of the side, and a sensor element that is inserted while engaging the flange portion on the inside of the metal fitting body, as the sensor element The sensor element as described in any one of Claims 1-3 is used.

  According to the present invention, it is possible to obtain a sensor element that stabilizes the output of the outer electrode and suppresses both a decrease in the durability of the outer electrode and a decrease in the operability of the gas sensor at a low temperature.

It is sectional drawing which cut | disconnected the gas sensor which concerns on embodiment of this invention by the surface which follows an axial direction. It is sectional drawing which shows the structure of an inner side electrode, an outer side electrode, and a porous protective layer. It is a figure which shows the actual internal resistance between an outer side electrode and an inner side electrode when the distance L is changed. It is a figure which shows the insulating layer in the conventional sensor element.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 shows a cross-sectional structure in which a gas sensor 100 according to an embodiment of the present invention is cut along a plane along an axis O direction (a direction from the front end toward the rear end). In this embodiment, the gas sensor 100 is an oxygen sensor that is inserted into an exhaust pipe of an automobile and has a tip exposed to the exhaust gas to detect the oxygen concentration in the exhaust gas. The sensor element 3 assembled in the gas sensor 100 constitutes an oxygen concentration cell in which a pair of electrodes are laminated on an element body 3s made of an oxygen ion conductive solid electrolyte body, and outputs a detection value corresponding to the amount of oxygen. It is an oxygen sensor element.
In addition, let the lower side of FIG. 1 be the front end side of the gas sensor 100, and let the upper side of FIG.

The gas sensor 100 is assembled so that a substantially cylindrical (hollow shaft-shaped) sensor element 3 (in this example, an oxygen sensor element) 3 with its tip closed is inserted and held inside a cylindrical metal fitting body (metal fitting body) 20. It has been. The sensor element 3 includes a cylindrical element body 3s that is tapered toward the tip, an outer electrode 50 and an inner electrode 60 that are respectively formed on the inner and outer peripheral surfaces of the solid electrolyte body, and the element body 3s. A porous protective layer 81 and an insulating layer 82 covering the outer surface of the substrate.
A cylindrical outer tube that holds a lead wire 41 and terminals 74 and 94 (described later) provided on the rear end side of the sensor element 3 and covers the rear end portion of the sensor element 3 at the rear end portion of the metal fitting body 20. 40 is joined. Further, an insulating cylindrical separator 121 is caulked and fixed inside the outer cylinder 40 on the rear end side of the sensor element 3. On the other hand, the detection part at the tip of the sensor element 3 is covered with a protector 7. Then, the male screw portion 20d of the metal fitting body 20 of the gas sensor 100 manufactured in this way is attached to a screw hole such as an exhaust pipe, so that the detection portion at the tip of the sensor element 3 is exposed in the exhaust pipe and the gas to be detected (exhaust gas) Gas). A polygonal flange 20c for engaging a hexagon wrench or the like is provided near the center of the metal fitting body 20, and the step between the flange 20c and the male thread 20d is attached to the exhaust pipe. A gasket 14 is inserted to prevent the gas from leaking when it is blown.

A flange portion 3 a is provided on the center side of the sensor element 3, and a step portion 20 f that is reduced in diameter on the inner peripheral surface near the tip of the metal fitting body 20 is provided. The sensor element 3 is inserted inside the metal fitting body 20, and the flange portion 3 a is positioned in the stepped portion 20 f via the washer 12 and a rear end portion of the protector 7 described later.
Further, a cylindrical talc powder 6 and a cylindrical ceramic sleeve 10 are arranged in the radial gap between the sensor element 3 and the metal fitting body 20 on the rear end side of the flange 3a. And the ceramic sleeve 10 is pressed to the front end side by arranging the metal ring 30 on the rear end side of the ceramic sleeve 10 and bending the rear end portion of the metal fitting body 20 inward to form the crimped portion 20a. As a result, the talc powder 6 is crushed and the ceramic sleeve 10 and the talc powder 6 are swaged and fixed, and the gap between the sensor element 3 and the metal fitting body 20 is sealed.

The separator 121 disposed on the rear end side of the sensor element 3 is provided with insertion holes (four in this example), and two of the insertion holes are plate-like base portions of the outer terminal fitting 71 and the inner terminal fitting 91, respectively. 74 and 94 are inserted and fixed. Lead wires 41 and 41 are caulked and connected to the rear ends of the plate-like base portions 74 and 94, respectively. Further, heater lead wires (not shown) drawn from the heater 15 are inserted into two insertion holes (heater lead holes) (not shown) of the separator 121.
Inside the outer cylinder 40 on the rear end side of the separator 121, a cylindrical grommet 131 that is spaced apart from the separator 121 is fixed by caulking, and two lead wires 41 and 2 are respectively inserted from the four insertion holes of the grommet 131. The heater lead wires are drawn out to the outside.

  A metal cover 42 is attached to the outer surface of the outer cylinder 40 between the separator 121 and the grommet 131 via a water repellent ventilation filter 140, and vent holes (on the side surfaces of the outer cylinder 40 and the metal cover 42 ( A reference gas (atmosphere) is introduced into the internal space of the sensor element 3 through a ventilation filter 140 from an unillustrated) without passing external water.

  On the other hand, a cylindrical protector 7 is fitted on the front end side of the metal fitting body 20, and the front end side of the sensor element 3 protruding from the metal fitting body 20 is covered with the protector 7. The protector 7 has a bottomed cylindrical shape having a plurality of holes and is made of metal (for example, stainless steel), and is attached to the metal fitting body 20 by welding or the like.

Next, the configuration of the outer electrode 50, the protective layer 81, and the insulating layer 82 in the sensor element 3 will be described with reference to FIG. First, the outer electrode 50 has an electrode function and is disposed closer to the front end side than the collar portion 3a of the element body 3s, and is electrically connected to the electrode portion 51 and rearward of the electrode portion 51. A lead portion 53 that extends to the side and straddles the flange portion 3a, and a ring portion 55 that is integrally connected to the rear end side of the lead portion 53 and electrically connected to the outer terminal fitting 71 (see FIG. 1).
The porous protective layer 81 covers the tip side of the electrode part 51 and the lead part 53. Furthermore, the insulating layer 82 is in contact with the rear end side of the protective layer 81 in the axis O direction and covers the region up to the flange 3a in the axis O direction so as to be insulated.

Further, the thermal conductivity of the protective layer 81 is the same as or lower than the thermal conductivity of the element body 3s, and the thermal conductivity of the element body 3s is lower than the thermal conductivity of the insulating layer 82.
As the protective layer 81, similarly to the element body 3s, partially stabilized zirconia (YSZ) in which yttria is dissolved as a stabilizer can be used. And after apply | coating the protective layer 81 paste which added the burnable organic particle etc. in the paste containing YSZ to the element main body 3s, the protective layer 81 can be made into a porous layer by baking the whole. Even if the protective layer 81 and the element main body 3s are formed of the same component, by making the protective layer 81 porous, the thermal conductivity of the protective layer 81 becomes lower than the thermal conductivity of the element main body 3s.
As the insulating layer 82, ceramics such as alumina and spinel can be used, and these may be dense layers having a porosity lower than that of the protective layer 81 in order to ensure insulation.
The outer electrode 50 can be formed by electroless plating with a noble metal such as Pt or by baking after applying a paste containing the noble metal.

Here, “covering the flange portion 3a” means that the maximum outside of the flange portion 3a from the point C at which the curvature changes from the outer surface 3f of the element body 3s on the front end side to the rear end side with respect to the flange portion 3a. It means covering at least the part R up to the point E at the most tip showing the diameter Dmax.
This is because there is a possibility that the point C to the part R of the sensor element 3 may be in contact with the washer 12 or the metal fitting body 20, and therefore it is necessary to ensure the insulation of that part.

As described above, in the present embodiment, since the insulating layer 82 insulates the flange portion 3a from the metal fitting body 20, a current flows between the lead portion 53 formed on the flange portion 3a and the metal fitting body 20. Generation of noise in the output of the outer electrode 50 can be suppressed, and the output of the outer electrode 50 can be stabilized.
Further, the protective layer 81 covers the electrode part 51, and the insulating layer 82 covers up to the flange part 3 a while being in contact with the rear end side of the protective layer 81. That is, at least the portion R of the flange portion 3 a from the tip of the lead portion 53 is covered with the protective layer 81 or the insulating layer 82. For this reason, even if high-temperature gas to be measured (exhaust gas or the like) rises from the front end side of the sensor element 3 to the region R and the lead portion 53 becomes hot, Pt in the lead portion 53 covered with the insulating layer 82 It is possible to suppress the sublimation of noble metals such as the like, and to suppress a decrease in durability of the outer electrode 50.
Furthermore, the protective layer 81 having the same or lower thermal conductivity as the element body 3s covers the electrode part 51 (completely), so that the heat of the electrode part 51 is transferred from the protective layer 81 to the rear end side of the element body 3s. The escape rate is reduced, the temperature of the electrode part 51 is easily raised, and a decrease in the operability of the gas sensor at a low temperature can be suppressed.

In this embodiment, the electric resistance of the material (alumina, spinel, etc.) constituting the insulating layer 82 is higher than the electric resistance of the material (YSZ) constituting the protective layer 81.
Thereby, the insulation between the collar part 3a and the metal fitting main body 20 by the insulating layer 82 becomes more reliable.

Further, in the present embodiment, as shown in FIG. 2, the distance L in the axis O direction from the rear end of the electrode part 51 to the front end of the insulating layer 82 is 7.7 mm or more.
When the distance L is 7.7 mm or more, the periphery of the electrode portion 51 is covered with the protective layer 81 having a lower thermal conductivity, and the rate at which the heat of the electrode portion 51 escapes to the rear end side of the element main body 3s. Even less.
The porosity is measured by obtaining a SEM image of 3000 times by SEM (Scanning Electron Microscope) observation of the cross section of the element, and binarizing the obtained image with image analysis software. This was carried out by calculating the porosity from the area occupied by this part.

It goes without saying that the present invention is not limited to the above-described embodiment, but extends to various modifications and equivalents included in the spirit and scope of the present invention.
For example, the material of the protective layer 81 and the insulating layer 82 is not limited to the above.

YSZ obtained by adding 5 mol% of yttria to zirconia is granulated, and then formed into a bottomed cylindrical shape shown in FIG. A slurry containing platinum (Pt) and zirconia is applied to the molded body and dried to form the outer electrode 50, the inner electrode, the lead portion 53, and the ring portion 55. Next, the insulating layer 82 is formed by applying a slurry made of alumina on the collar and drying it. Next, the protective layer 81 is formed by applying and drying the YSZ / 10 vol% pore former so as to cover the insulating layer 82 from the front end of the detection unit.
The molded body on which each of the above-described slurries was applied was fired at 1300 ° C. to 1500 ° C. to produce the sensor element 3. And as shown in FIG. 1, this sensor element 3 was assembled | attached and the gas sensor 100 was obtained.
The obtained gas sensor 100 was attached to a known burner measurement device, and the internal resistance between the outer electrode 50 and the inner electrode was measured by a burner measurement method. Specifically, the sensor output at an air-fuel ratio λ = 0.9 (rich) at a gas temperature of 300 ° C. was detected with two types of input impedances (1 MΩ, 100 kΩ), and the internal resistance was calculated based on the output difference.

The obtained results are shown in FIG.
As shown in FIG. 3, it can be seen that when the distance L is 7.7 mm or more, the internal resistance between the outer electrode 50 and the inner electrode decreases. The lower the internal resistance, the smaller the rate at which the heat of the electrode portion 51 escapes from the protective layer 81 to the rear end side of the element body 3s, and the operability of the gas sensor at low temperatures becomes better.

3 sensor element 3a collar 3s element body 20 metal fitting body 50 outer electrode 51 electrode part 53 lead part 81 protective layer 82 insulating layer 100 gas sensor O axis

Claims (4)

An element body having a flange extending in the axial direction and having a closed end and protruding radially outward; and an outer electrode formed on an outer peripheral surface of the element body, the axis A sensor element in which the flange can be engaged inside a cylindrical metal fitting body having a through hole in a direction,
The outer electrode has an electrode function and is disposed on the front side of the collar part, and is electrically connected to the electrode part and extends to the rear end side of the electrode part, and the collar part And a lead portion straddling
Furthermore, a porous protective layer that covers the electrode portion, and an insulating layer that is made of a material different from the protective layer and covers the flange portion so as to be insulated,
The insulating layer has a lower porosity than the protective layer,
The protective layer and the insulating layer are in contact with the axial direction, and at least one of the protective layer and the insulating layer covers the lead portion on the tip side from the flange portion,
A sensor element in which the thermal conductivity of the protective layer is equal to or lower than the thermal conductivity of the element body.   The sensor element according to claim 1, wherein an electrical resistance of a material constituting the insulating layer is higher than an electrical resistance of a material constituting the protective layer.   The sensor element according to claim 1 or 2, wherein a distance L in the axial direction from a rear end of the electrode portion to a front end of the insulating layer is 7.7 mm or more. A cylindrical metal fitting body having a through hole in the axial direction;
The metal fitting main body is formed in a hollow shaft shape having a closed tip, has an outer electrode on the distal end side of the outer peripheral surface, and has a flange projecting radially outward on the outer peripheral surface on the rear end side of the outer electrode. A gas sensor comprising a sensor element that is inserted while engaging the flange portion inside,
A gas sensor using the sensor element according to claim 1 as the sensor element.

Images