JPH06105239B2 - Oxygen concentration detector and method of manufacturing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an oxygen concentration detection device used for detecting the oxygen concentration in exhaust gas of an internal combustion engine, and a method for manufacturing the same.

[Technical background of the invention and its problems]

There are two types of oxygen concentration detectors.
One is a stoichiometric air-fuel ratio sensor, and the other is a lean sensor that operates in a lean region of the air-fuel ratio. The above theoretical air-fuel ratio sensor cannot sufficiently perform its function unless the solid electrolyte element, which is the main part thereof, is heated to a temperature of about 400 ° C. or more, and the above lean sensor is also called a limiting current type sensor. The limit current value of this sensor has a characteristic that it changes with temperature.

Therefore, the resistance heating element is indeterminate in any of the above sensors, and is therefore also called an oxygen sensor with a heater.

As a conventional oxygen sensor with a heater, Japanese Patent Laid-Open Publication No. 55-116248 is known. This is a structure in which a plate-shaped solid electrolyte element is provided with a detection electrode and a reference electrode exposed to a detection gas, and another plate-shaped solid electrolyte element is provided with a resistance heating element, and these plate-shaped solid electrolyte elements are laminated. It is a thing.

In this device, since the electrode and the resistance heating element are laminated, the electrode and the heating element are provided at the same place on the projection surface, and thus the element has a very high electrode heating efficiency. However, since this is basically a plate-shaped element, it has the following fatal drawbacks. That is, 1
However, if you try to detect the oxygen concentration in the exhaust gas of an internal combustion engine, for example, the exhaust gas will be received only on a certain level, so it will be affected by the variation between the cylinders. The problem is that it cannot detect various oxygen concentrations and is not practical. Since the other one is a thin plate-shaped element, it has the drawbacks that mechanical strength in the thickness direction is basically weak, and thermal shock resistance is inferior, so that it cannot be put to practical use. Therefore, if the wall thickness of the solid electrolyte element is increased in order to improve the mechanical strength, the resistance of the element becomes large and the minimum operating temperature becomes high. As a result, it is necessary to increase the power supplied to the resistance heating element, which conflicts with the structure for improving the thermal efficiency.

Considering the above-mentioned problems, the solid electrolyte element is basically cylindrical or columnar in order not to receive directionality per gas and to have strong mechanical strength and thermal shock resistance. desirable.

For this reason, the present applicant has proposed Japanese Patent Laid-Open Publication No. 58-76757. This one has a solid electrolyte element formed in a cup shape, so it has high mechanical strength and thermal shock resistance, and since electrodes are provided on the outer and inner peripheral surfaces of this element, there are restrictions on the directionality of gas contact. There are no advantages.

However, the above-mentioned conventional one of the present applicant has a structure in which a resistance heating element and an electrode are installed on the same surface of a solid electrolyte element, and the resistance heating element directly heats the solid electrolyte element. The resistive heating element is located elsewhere on the projection surface and is therefore thermally efficient and lossy.

[Object of the Invention]

The present invention has been made based on the above circumstances,
The first object thereof is to provide an oxygen concentration detecting device which is improved in mechanical strength and thermal shock resistance of the solid electrolyte element without being restricted by the directionality of gas contact and is excellent in heating efficiency by the resistance heating element. Is to provide.

A second object of the present invention is to provide a manufacturing method capable of easily manufacturing the above-mentioned oxygen concentration detecting device and having excellent mass productivity.

[Outline of Invention]

In order to achieve the first object, the device of the present invention comprises a cylindrical or column-shaped solid electrolyte element which is composed of an oxygen ion conductive metal oxide and forms a multilayer structure adhered in the radial direction, and a solid electrolyte element outside the solid electrolyte element. A thin film-like porous detection electrode provided on the surface over the entire circumference in the circumferential direction, and a layer of the solid electrolyte element with respect to the detection electrode inside the detection electrode, and almost the entire circumference in the circumferential direction. A thin-walled porous reference electrode facing the detection electrode, a ventilation part connecting the reference electrode and the outside of the solid electrolyte element, and the solid electrolyte element with respect to the reference electrode inside the reference electrode. While substantially uniform heat generation in the circumferential direction across the layer of, is sandwiched by the outer and inner layers of the solid electrolyte element, one of the outer or the inner solid electrolyte is an insulating layer. Sandwiched through A resistive heating element covers the outer surface of the detection electrode, to provide an oxygen concentration detecting device comprising a porous coating layer made of refractory material having a predetermined diffusion resistance to oxygen molecules.

Further, in order to achieve the second object, in the production method of the present invention, an unfired ceramic slurry that constitutes a solid electrolyte element of an oxygen ion conductive metal oxide is formed into a sheet to form a sheet member. A groove is formed in the sheet member, a resistance heating element having a predetermined area is formed by a conductive paste on one side surface of one end side of the sheet member, and a lead wire connected to this is formed, and the sheet member is formed. By forming a reference electrode and a detection electrode made of a thin film-shaped porous conductor with lead wires facing each other on both end surfaces on the other end side, and closely winding the sheet member in a roll shape from the heating resistance element side. , The groove constitutes a ventilation part that allows the reference electrode and the atmosphere to communicate with each other, and the detection electrode is wound so as to face the outer surface, the wound sheet is fired, and the firing is performed. The detection electrode provided on the outer surface of the winding circular sheet is intended to provide a method for producing the oxygen concentration detecting device is covered with a porous coating layer having a diffusion resistance.

The present invention will be described below based on an embodiment shown in the drawings.

FIG. 1 shows the structure of a sensor assembly, and FIGS. 2 to 4 are detailed configuration diagrams of the oxygen sensor 10 with a heater.

First, the oxygen sensor 10 with a heater will be described. Since the structure thereof can be easily understood by the manufacturing method, the manufacturing method will be described based on FIGS. 5 to 8.

In FIGS. 5 to 8, 50 is ZrO ₂ —Y ₂ O ₃ system, ZrO ₂ −
A sheet made of a non-fired ceramic slurry of an oxygen ion conductive metal oxide such as a Yb ₂ O ₃ system or a ZrO ₂ —Sc ₂ O ₃ system is shown. As a specific example, a sheet 50, which is the above-mentioned sheet member, is made by forming an unfired ceramic slurry of ZrO ₂ containing 5 mol of Y ₂ O ₃ to a thickness of 50 to 400 μm by a known doctor blade method. This sheet 50 has grooves 51 at predetermined positions.
To form.

On one side surface located on one end side and both side surfaces located on the other end side of the sheet 50, a paste containing an insulating powder such as alumina as a main component is applied to form insulating layers 52, 53, 54. After drying the insulating paste layers 52, 53, 54 at 120 ° C. for 1 hour, a conductive paste containing platinum powder and a heat-resistant metal oxide as a main component is applied to the surface of the insulating layer 52 on the one end side. This forms the resistance heating element 55 and the leads 56a, 56b continuous with this element 55. The resistance heating element 55 has, for example, a meandering shape with equal intervals.

Further, a thin film porous reference electrode 57 and a detection electrode 58 made of platinum-zirconia paste or the like are formed on both surfaces of the other end of the sheet 50, and lead wires 57a and 58a are formed on the insulating layers 53 and 54, respectively. To do. These formations can be made by known printing means. In this embodiment, the reference electrode 57 is formed on the same surface as the resistance heating element 55, and the detection electrode 58 is formed on the other surface. These two electrodes 57 and 58 are seat 5
0 was face opposite sides, each of the areas is at an equivalent area in 10 mm ² 100 mm ^2.

The groove 51 is formed so as to extend from the upper edge of the reference electrode 57 to the upper edge of the sheet 50.

After forming the electrodes 57 and 58, dry at 120 ° C for 1 hour,
An adhesive such as methylcellulose is applied to one side of the sheet 50, that is, the side provided with the resistance heating element 55, over the entire surface.

Then, at one end of the seat 50, as shown in FIG.
Apply a solid core material 59 made of the same material as the seat 50,
The sheet 50 is wound up in a roll from one end thereof in close contact with each other in the direction of the arrow. The sheet 50 rolled up in a roll shape has a contact multi-layer structure, and the detection electrode 58 is exposed on the outer peripheral surface over substantially the entire circumference, and the reference electrode 57 is also formed on the inner side of the sheet 50 through the layer of the sheet 50. Face the lap.

Further, by being wound in this roll shape, the ventilation hole 13 for conducting the reference electrode 57 and the atmosphere above the sheet 50 in the groove 51.
Is configured.

After that, by winding a ZrO ₂ sheet on the upper portion of the wound sheet 50, the housing shown in FIG.
A fixing flange portion 12 (shown in FIG. 2) for 20 is made.

Also, heater lead wires 56a, 56b and electrode lead wires 57a, 58a
Of the platinum lead wires 61,
Insert 2, 63, 64 and make electrical connection, solidify with platinum paste and dry at 120 ° C for 1 hour, then 400 ° C for 5 hours. By baking this by heating at 1400 ° C. for 10 hours, a sintered body of the sheet 50, that is, the solid electrolyte element 11 is obtained. By this sintering, platinum lead wires 61,62,63,6
4 is tightly bound.

Next, in the outer region of the detection electrode 58 exposed on the surface of the solid electrolyte element 11, a porous oxygen molecule diffusion layer composed of MgO.Al ₂ O ₃ (spinel) or the like which also acts as a protective layer of the electrode 58 is diffused. A ceramic coating layer 14 serving as a resistance layer is formed, and the detection electrode 58 is covered with this coating layer 14. This coating layer 14
Is formed to a thickness of 200 to 600 μ by plasma spraying or the like, and also covers the lower surface of the solid electrolyte element 11 in this embodiment.

The oxygen sensor 10 obtained by such a method has a structure as shown in FIGS. That is, the solid electrolyte element 11 is in the form of a contact multilayered roll, and the detection electrode 58 made of thin film porous platinum is provided on the outer surface over the entire circumference in the circumferential direction. And this detection electrode 58
Inside, a reference electrode 57 made of a thin film porous platinum sandwiching the solid electrolyte layer is formed over the entire circumference in the circumferential direction, and the reference electrode 57 is a surface with respect to the detection electrode 58. Facing each other. The upper edge of the reference electrode 57 is communicated with the atmosphere by the vent hole 13. A resistance heating element 55 is arranged inside the reference electrode 57 via a layer of a solid electrolyte, and the resistance heating element 55 is spirally arranged in a plurality of turns across the layer of the solid electrolyte. That is, the resistive heating element 55 is sandwiched by layers of solid electrolyte on its inner and outer sides.

The positional relationship between the resistance heating element 55 and the electrode parts 57 and 58 is set so that the electrode parts 57 and 58 are in a region uniformly heated by the resistance heating element 55 in both the axial direction and the circumferential direction. In the axial direction, for example, the positions of the electrodes 57 and 58 are specifically located slightly below the center of the heating pattern portion of the heater 55, and in the circumferential direction, the resistance heating element 55 is in the entire circumferential direction. The pattern was set so as to be wound exactly in a single layer or in a double layer so as to uniformly heat. The outside of the detection electrode 58 is covered with a porous oxygen molecule diffusion resistance coating layer 14.

The oxygen sensor having such a structure is assembled as shown in FIG.

That is, the oxygen sensor 10 is attached to the inner taper portion of the tubular housing 20 made of heat-resistant metal by the flange portion 12 through the packing 21.
It is attached to, for example, an exhaust pipe (not shown) by 20a.
The oxygen sensor 10 is pressed against the powder talc 22, and is firmly fixed to the housing 20 by caulking one end 20b of the housing 20 via the asbestos ring 23, the alumina porcelain insulator 24, the cover protection 25 and the crimping ring 26. ing.

Each platinum lead wire 61, 62, 63, 64 connected to the lead wires 56a, 56b of the resistance heating element 55 of the oxygen sensor 10 and the lead wires 57a, 58a of the electrodes 57, 58 is from the upper end surface of the solid electrolyte element 11. These four lead wires 61,62,63,64 are
It passes through the four holes provided in the insulator 24, is guided to the four holes of the resin insulator 27 without coming into contact with each other, and at the upper part of the insulator 27,
Each is welded to terminals 28a, 28b, 28c, 28d (28c and 28d are not shown). These terminals 28a, 28b, 28c,
28d is held by a rubber bush 29 having four holes, and is fixed by caulking the cover dust 30 with the cover protection 25. The four terminals 28
The a, 28b, 28c and 28d are connected to the connector 32 via the coated lead wires 31a, 31b, 31c and 31d (31d is not shown). In this way, the oxygen sensor 10 and the connector 32 are electrically connected.

On the other hand, in order to prevent the oxygen sensor 10 from being directly exposed to the exhaust gas, the tip of the mounting screw 20a side of the housing 20 is
A double protective cover 33 made of heat-resistant metal is attached. The protective cover 33 is provided with a plurality of detection gas intake holes 33a and 33b for introducing the detection gas to the oxygen sensor 10. Here, this hole is formed between the outer cover and the inner cover of the double cover.
The positions of 33a and 33b are staggered so that the gas to be detected is the oxygen sensor 10 directly.
Considered not to hit.

The operation of the oxygen concentration detecting device configured as described above will be described.

The housing 20 is attached to the detection gas pipe by its threaded portion 20a, the resistance heating element 55 is connected to the power supply via the connector 32, the reference electrode 57 is connected to the positive power supply, and the detection electrode 58 is connected to the negative power supply. This causes current to flow from the reference electrode 57 to the detection electrode 58. Where solid electrolyte element
Since 11 is oxygen ion conductive, holes 33a, 33 in cover 33 are
Oxygen of the detection gas that has entered into the vicinity of the sensor 10 from b is detected by the detection electrode 58 via the ceramic coating layer 14 serving as the diffusion resistance layer.
Then, the electrodes 58 are supplied with electrons to become oxygen ions. Then, the oxygen ions diffuse inside the solid electrolyte element 11, emit electrons at the reference electrode 57 located inside, and return to oxygen molecules. This oxygen molecule is released into the atmosphere through a vent hole 13 provided in the solid electrolyte element 11.

In such a reaction process, the thickness of the ceramic coating layer 14 that becomes diffusion resistance is a certain thickness or more, for example, 200μ, and the area of the detection electrode 58 is substantially reduced to 40 mm ² .
When the voltage between the electrodes 57 and 58 is gradually increased, a region where the current does not change even if the voltage is changed due to the influence of the ceramic coating layer 14, that is, a limiting current is generated. This limiting current
Il is expressed by the following formula.

However, F ... Faraday constant R ... Gas constant DO ₂ ... Oxygen diffusion constant T ... Absolute temperature S ... Electrode area l ... Diffusion resistance coating layer effective diffusion distance PO ₂ ... Oxygen molecule That is, the limiting current is the oxygen concentration in the detection gas ( Voltage) and therefore applying a fixed voltage,
By measuring this limiting current Il, the oxygen concentration in the detection gas can be measured.

Next, the experimental results of the present invention will be described.

The result of the experiment using the oxygen sensor 10 described in the example is B, and the result of the conventional type of the cup-shaped solid electrolyte element in which the detection electrode and the heater are arranged side by side (JP-A-58-76757) is A. And their respective characteristics are shown in FIGS. 9 and 10. The two types of sensors have an electrode area of 40 mm.
^2. The thickness of the diffusion layer is 500μ, and the thickness of the solid electrolyte sandwiched between the electrodes is 200μ in the present invention product and 600μ in the conventional product. The protective cover 33 has a double round hole.

Figure 9 shows the surface temperature of the sensor electrode with respect to the power consumption of the heater at the lean burn condition of an actual vehicle (2000cc, 4 cylinders), that is, when the exhaust gas temperature is 400 ° C. The characteristics that reach, that is, show the minimum operating temperature are shown in FIG. From FIG. 9, for example, the power consumption of the heating element required for raising the electrode surface temperature to 700 ° C. is 20 W or more as in the case of the conventional case as shown in the graph A, but the present invention uses the graph B. 13W
The power consumption can be reduced to about 2/3.

The operating temperature in FIG. 10 is about 610 ° C. for the conventional product A, whereas it is 520 ° C. for the product of the present invention.

Therefore, the power consumption of the heating element required to reach the minimum operating temperature of both is even smaller in the case of the product of the present invention, which is less than half that of the conventional product.

In the example, the sheet-shaped solid electrolyte element 50 was wound around the solid core material 59 so that there was no internal space, but the core material 59 has a hollow shaft-like heat capacity as long as the exhaust gas seal is taken into consideration. It may be kept small, or the core may not be used.

In addition, the resistance heating element 55 is provided inside the element 10 as in the embodiment.
Not only one pattern is provided, but two or more patterns may be provided for the purpose of equalizing the temperature, improving heating efficiency, and improving reliability. Not only heating from the inside of the electrode but also heating from the outside is combined. It doesn't matter. However, when the auxiliary heating element is provided outside the detection electrode, the influence on the diffusion resistance must be sufficiently considered.

Further, as the method for forming the diffusion resistance coating layer 14, the plasma spraying method has been described, but it may be formed by a method of baking after dipping into powder slurry, or the element 11 may be formed by forming the diffusion layer before firing. It may be sintered at the same time.

Further, for the resistance heating element, insulating layers may be formed directly on both sides of the heating element. The method of forming the flange portion 12, which is a fixing member to the housing of the oxygen sensor 10, is not only by sheet winding and simultaneous firing, but a ceramic body made of the same material as the solid electrolyte element 11 is sintered after the solid electrolyte element 11 is sintered. It may be formed by a general ceramic joining method such as brazing.

The lead wire may be taken out from the oxygen sensor 10 by a brazing method, or may be formed by making a hole in the sheet before firing, fitting the lead wire therein, and then performing simultaneous sintering.

Further, the solid electrolyte element 11 of the present invention is not limited to being wound in a roll shape, and pipe bodies having different diameters may be fitted to each other to form a contact multilayer structure.

And when the resistance heating element is wound into multiple layers, the innermost heating element is not limited to sandwiching it with a layer of solid electrolyte.

Further, by forming a notch-shaped groove in the sheet 50, or by disposing a porous ceramic material having air permeability on the sheet 50, a ventilation part that connects the reference electrode and the atmosphere is formed. May be.

Further, although an example in which the present invention is applied to the lean sensor has been described in the above-described embodiment, the present invention is not limited to the lean sensor, and can be applied to an air-fuel ratio sensor as is clear from the description of the related art. However, in the case of the lean sensor, the ventilation passage 13 and the diffusion resistance layer 14 are necessary constituent elements, but in the case of the theoretical air-fuel ratio sensor, the ventilation passage 13 and the diffusion resistance layer 14 are not necessarily required.

[Advantages of the Invention] As described above, according to the structure of the device of the present invention, since the detection electrode and the reference electrode are provided face-to-face and circumferentially across the solid electrolyte layer, the directionality with respect to the detection gas is improved. Not only is not regulated, but because the resistance heating element is arranged in the circumferential direction by sandwiching the solid electrolyte layer inside these electrodes, it was sandwiched by both electrodes that are the output control part of the solid electrolyte element. Higher heating efficiency as the layer is heated directly by the resistance heating element, less power consumption, and higher reliability as the temperature of the heating resistance element does not need to be too high To do. Moreover, since at least a part of the resistance heating element is sandwiched by the solid electrolyte layer, the mechanical support of the heating element is ensured. In addition, since the solid electrolyte element has a contact multilayer structure, it has advantages such as high mechanical strength and excellent thermal shock resistance.

Further, according to the manufacturing method of the present invention, the material forming the solid electrolyte element is formed into a sheet of unsintered ceramic slurry, and both electrodes, resistance heating elements and lead wires thereof are formed on this sheet. In addition, since the sheet is wound into a roll and the wound one is fired, the oxygen sensor having the above-described structure can be easily manufactured and the mass productivity is high. Moreover, by controlling the thickness of the sheet, the thickness of the solid electrolyte layer can be reduced,
Since the thermal conductivity can be improved, the minimum operating temperature can be lowered.

[Brief description of drawings]

The drawings show one embodiment of the present invention. FIG. 1 is a sectional view showing an assembly structure of an oxygen concentration detecting device, FIG. 2 is an enlarged sectional view of the oxygen sensor, and FIGS.
Sectional views taken along lines III-III and IV-IV in the drawings, FIGS. 5 to 8 are explanatory views of the manufacturing method, FIG. 5 is a front view, and FIGS. 6 and 7 are VI in FIG. -VI and VII-VII line cross-sectional views, FIG. 8 is a cross-sectional view illustrating a state of forming into a roll shape, and FIGS. 9 and 10 are characteristic diagrams showing experimental results, respectively. 10 ... Oxygen sensor, 11 ... Solid electrolyte element, 12 ... Flange part, 13 ... Vent hole, 14 ... Diffusion resistance coating layer, 50 ...
Seat, 55 ... Resistance heating element, 56a, 56b ... Lead wire, 5
7: Reference electrode, 58: Detection electrode, 57a, 58b: Lead wire.

Claims (3)

[Claims] 1. A cylindrical or column-shaped solid electrolyte element made of an oxygen ion conductive metal oxide, which has a multilayer structure adhered in a radial direction, and an outer surface of the solid electrolyte element over the entire circumference in the circumferential direction. A thin film-like porous detection electrode provided, and a thin-walled shape that sandwiches the layer of the solid electrolyte element with respect to the detection electrode inside the detection electrode and faces the detection electrode over substantially the entire circumference in the circumferential direction. A porous reference electrode, a ventilation part that connects the reference electrode to the outside of the solid electrolyte element, and substantially in the circumferential direction inward of the reference electrode with the layer of the solid electrolyte element sandwiched with respect to the reference electrode. A resistance heating element that produces uniform heat generation and is sandwiched between the outer and inner layers of the solid electrolyte element, and is sandwiched between either the outer or inner solid electrolyte through an insulating layer. , The inspection Covers the outer surface of the electrode, the oxygen concentration detecting device characterized by comprising a porous coating layer made of refractory material having a predetermined diffusion resistance to oxygen molecules. 2. The oxygen concentration detecting device according to claim 1, wherein the solid electrolyte element has a cylindrical shape or a pillar shape by a roll-shaped contact multilayer structure. 3. An unfired ceramic slurry forming a solid electrolyte element of an oxygen ion conductive metal oxide is formed into a sheet to form a sheet member, and a groove is formed in the sheet member, and one end of the sheet member is formed. On one side of the sheet member, a resistance heating element having a predetermined area is formed of a conductive paste and lead wires connected to the resistance heating element are formed. A reference electrode and a detection electrode made of a porous conductor are formed with lead wires respectively, and the sheet member is closely wound in a roll shape from the heating resistance element side, so that the groove makes the reference electrode communicate with the atmosphere. While configuring the ventilation part,
The detection electrode is wound so as to face the outer surface, the wound sheet is fired, and the detection electrode provided on the outer surface of the fired wound sheet is covered with a porous coating layer having diffusion resistance. A method for manufacturing an oxygen concentration detecting device, comprising: