JP3670846B2 - Method for forming conductive film of ceramic body - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of forming a conductive film on the surface of a ceramic body, and more particularly, to a method of forming a conductive film of a ceramic body that can be used for forming electrodes in various sensors such as oxygen sensors for an oxygen concentration battery type automobile engine. .
[0002]
[Prior art]
Conventionally, as an oxygen sensor for automobiles, for example, a cylindrical oxygen detecting element with one end closed is used. The oxygen detection element includes an oxygen ion conductive solid electrolyte body (ceramic body) 6 such as zirconia, and a porous electrode 7 having heat resistance such as platinum formed on the inner and outer surfaces of the solid electrolyte body, 8 (see FIG. 2).
[0003]
In this type of oxygen sensor, the porous electrode 8 formed on the outer surface thereof is brought into contact with, for example, exhaust gas that is a gas to be detected, and the porous electrode 7 formed on the other inner surface is contacted with air or the like. The reference gas is brought into contact, the electromotive force generated between the porous electrodes 7 and 8 is detected from the difference in oxygen partial pressure between the exhaust gas and the reference gas, and the oxygen concentration in the exhaust gas is measured.
[0004]
As a method for forming the above-described porous electrodes 7 and 8 on the surface of the solid electrolyte body 6, for example, the following methods a) and b) are known.
a) A method of forming the porous electrodes 7 and 8 by performing electroless plating directly on the surface of the solid electrolyte body 6 and depositing platinum on the surface of the solid electrolyte body 6 from the electroless plating solution.
[0005]
b) After the surface of the solid electrolyte body 6 is roughened by etching, and then a thin solution containing platinum and a reducing agent is brought into contact with the surface of the solid electrolyte body 6 and reacted by heating to deposit platinum nuclei on the surface. A method of forming the porous electrodes 7 and 8 by performing electroless plating (using platinum nuclei as growth nuclei).
[0006]
[Problems to be solved by the invention]
However, the above-described method is not always sufficient, and further improvement has been demanded. That is, in the method a) , the adhesion between the solid electrolyte body 6 and the porous electrodes 7 and 8 is inferior, and when used as an oxygen sensor, the porous electrodes 7 and 8 are peeled off when exposed to severe temperature changes. As a result, the life of the oxygen sensor may be reduced.
[0007]
Further, the method b) has a problem that although the adhesion is improved to some extent, the adhesion state of platinum is uneven. In other words, platinum deposition occurs when the reaction between the platinum in the solution and the reducing agent is promoted by heating. In this case, platinum is deposited in any part of the solution, so that the platinum is uniformly deposited on the surface of the solid electrolyte body 6. There was a problem that the nucleus did not adhere easily.
[0008]
If the adhesion state of the platinum nuclei is uneven, the adhesion of the porous electrodes 7 and 8 to be formed later by electroless plating is deteriorated, and there is a problem in the denseness of the porous electrodes 7 and 8, resulting in a problem. In some cases, the measurement accuracy of the oxygen sensor is lowered or the life thereof is lowered.
[0009]
Furthermore, when platinum is generated in the solution, there is another problem that platinum floating in the solution adheres to the apparatus and contaminates the apparatus, or it takes time to collect platinum.
The present invention has been made in view of the above-mentioned problems, and can form platinum nuclei uniformly on the surface of the ceramic body, and can further prevent unnecessary platinum deposition. It aims to provide a method.
[0010]
[Means for Solving the Problems]
According to the first aspect of the invention for achieving the above object, in the method of forming a conductive film on the surface of the ceramic body, (1; step 1) roughening the surface of the ceramic body by etching; (2; step 2) ) A step of impregnating the roughened ceramic body with a platinum solution by heating; (3; step 3) a step of extracting the platinum solution from the surface of the ceramic body; and (4; step 4) a ceramic from which the platinum solution has been extracted. A step of bringing a reducing agent solution containing a reducing agent heated to 50 to 90 ° C. into contact with the body surface to deposit platinum nuclei on the ceramic surface; and (5; step 5) on the surface of the ceramic body on which the platinum nuclei are deposited. And a step of forming a conductive film by performing electroless plating, and a gist of the method for forming a conductive film of a ceramic body.
[0011]
In the present invention, in step 1, and roughening the ceramic surface by etching to form a fine small scratches (penetration path) to the surface. In step 2, when the surface of the roughened ceramic body is impregnated with the platinum solution, the platinum solution is sufficiently impregnated into the permeation path by heating. In step 3, the platinum solution is extracted from the surface of the ceramic body. By this extraction, the platinum solution is thinly attached to the surface of the ceramic body including the surface of the permeation path. In step 4, a reducing agent solution containing a reducing agent heated to 50 to 90 ° C. is brought into contact with the surface of the ceramic body. Thereby, since the platinum solution adhering on the ceramic surface reacts with the reducing agent, platinum nuclei are uniformly deposited on the surface of the ceramic body including the surface of the permeation path. In step 5, since the electroless plating is performed on the surface of the ceramic body on which the platinum nuclei are deposited, a uniform conductive film without unevenness is formed in which the platinum nuclei are firmly adhered as growth nuclei.
[0012]
That is, according to the present invention, platinum nuclei are deposited not only on the whole solution, but only on the surface of the ceramic body as in the prior art. Therefore, the platinum nuclei are uniformly and uniformly formed on the surface of the ceramic body including the surface of the permeation path. There is a feature that adheres. Therefore, the adhesiveness and the denseness of the conductive film formed later by electroless plating using platinum nuclei as growth nuclei are excellent. Therefore, for example, even when exposed to a drastic temperature change, the conductive film is difficult to peel off, and for example, when applied to an oxygen sensor, it is possible to prevent a decrease in measurement accuracy and lifetime.
[0013]
Further, in the present invention, since platinum nuclei are not generated in the entire solution, the amount of platinum floating in the solution is extremely small, so that the device does not adhere to the device and contaminate the device. Moreover, there is an advantage that the used platinum solution can be reused almost as it is.
Furthermore, in this invention, the reducing agent solution containing a reducing agent is heated and used at 50-90 degreeC. Therefore, within this temperature range, there is an effect that platinum nuclei can be deposited more uniformly on the surface of the ceramic body including the inside of the permeation path, and unevenness in the precipitation of platinum nuclei hardly occurs.
Thus, in the present invention, there is a remarkable effect that platinum nuclei can be deposited on the surface of the ceramic body without any unevenness, and unnecessary platinum deposition can be greatly suppressed.
[0014]
As the ceramic body, a ceramic body having oxygen ion conductivity is preferably used. For example, a material that can withstand use at high temperatures such as a zirconia solid electrolyte described in Japanese Patent Application Laid-Open No. 54-4913 is used. be able to.
A method for producing a zirconia solid electrolyte in the case of using this zirconia solid electrolyte is, for example, adding a predetermined amount of yttria (Y ₂ O ₃ ) to zirconia (ZrO ₂ ), pulverizing it, and pre-sintering it. It can be manufactured by pressing into a desired shape and firing.
[0015]
(1) When etching is performed in step 1, a hydrofluoric acid (HF) solution is suitable, but other etching solutions can be used.
The degree of etching can be adjusted by the concentration of the etching solution, the etching time, and the like.
[0016]
(2) In step 2, the portion impregnated with the platinum solution is not particularly limited as long as the surface of the ceramic body (etched) is a flat surface, a curved surface, an uneven surface, etc. What is necessary is just to impregnate the surface. The impregnation may be performed by any method as long as the platinum solution can be brought into contact with the surface of the ceramic body (impregnated) for a predetermined time, and may be applied or dipped, and the method is not particularly limited.
[0017]
(3) In step 3, extraction may be performed by suction using negative pressure, but may be suction using a pump or pipette-like rubber.
(4) In step 4, the reducing agent solution may be contacted by, for example, diluting the reducing agent in water, buffer solution, etc., and then applying or dipping, or spraying in the form of a mist. There is no particular limitation as long as it is a method capable of bringing the reducing agent solution containing the reducing agent into contact with the surface of the ceramic body to which the platinum solution is adhered (precipitating platinum nuclei) for a predetermined time.
[0018]
(5) In step 5, when the electroless plating is performed, the method of bringing the electroless plating solution into contact is a state where the electroless plating solution is in contact with the ceramic body for a predetermined time (capable of electroless plating) such as coating and immersion. There is no particular limitation as long as it can be used.
In invention of Claim 2, the heating temperature at the time of impregnating a platinum solution is 70-110 degreeC.
[0019]
The present invention exemplifies the heating temperature of the platinum solution, and within this temperature range, the platinum solution can be sufficiently and rapidly soaked into the permeation path formed by etching. Below this range, impregnation into the permeation path may not be sufficiently performed, and above this range, impregnation unevenness may occur due to a decrease in the level of the platinum solution and concentration of the platinum solution.
[0020]
In the invention of claim 3, the platinum solution is extracted by suction using a negative pressure. The present invention exemplifies a method of extracting a platinum solution, and an excess platinum solution can be removed sufficiently and quickly by suction using a negative pressure.
By this extraction, the platinum solution is removed from the inside of the permeation path and the like, but it is simply removed by suction and not cleaned, so the surface of the ceramic body including the surface of the permeation path is thinly coated with the platinum solution. Will be attached.
[0021]
In invention of Claim 4, the density | concentration of the reducing agent solution containing a reducing agent is 2 to 20 weight%.
The present invention exemplifies the concentration of the reducing agent. Within this concentration range, platinum nuclei can be uniformly deposited on the surface of the ceramic body including the inside of the permeation path. Outside this range, unevenness in the precipitation of platinum nuclei tends to occur.
[0023]
In the invention of claim 5 , the reducing agent is hydrazine.
The present invention exemplifies the types of reducing agents. When this hydrazine (N ₂ H ₄ ) is used, unnecessary reaction with the platinum solution or the ceramic body is not caused, and the ceramic body including the surface of the infiltration path is used. Platinum nuclei can be deposited more uniformly on the surface.
[0024]
As the reducing agent, various reducing agents other than hydrazine can be used as long as they do not cause unnecessary reactions with the platinum solution and the ceramic body.
In the invention of claim 6, the electroless plating solution used for electroless plating is a solution containing a platinum ammine compound and hydrazine.
[0025]
The present invention exemplifies the types of electroless plating solutions. By using a solution containing this component, the surface of the ceramic body including the inside of the permeation path is used as a growth nucleus, with a uniform and dense platinum nucleus. A conductive film can be formed.
The electroless plating solution is preferably a plating solution containing the above-described platinum complex salt as a main component, but various other plating solutions can be used.
[0026]
In the invention of claim 7, the electroless plating solution contains 3 to 30 g / L of platinum and 2 to 20% by weight of hydrazine. L represents liters (the same applies hereinafter).
The present invention exemplifies the component concentration of the electroless plating solution. With this component concentration, the conductive film can be formed more uniformly and densely.
[0027]
In addition, when platinum is less than the above range, the adhesion of plating may be insufficient, and when it exceeds the above range, peeling or the like is likely to occur. Moreover, when hydrazine is less than the above range, adhesion of plating may be insufficient, and when it exceeds the range, unevenness of the plating film is likely to occur.
[0028]
In the invention according to claim 8 , the ceramic body is an oxygen detection element made of a zirconia solid electrolyte.
The present invention exemplifies a ceramic body. According to the present invention, a conductive film that functions as an electrode (solidly adheres and is uniform without unevenness) is formed on the surface of an oxygen detecting element made of a zirconia solid electrolyte. can do.
[0029]
According to the ninth aspect of the present invention, the oxygen detection element has a cylindrical shape with one end closed, and the conductive film is an inner electrode among the porous electrodes inside and outside the oxygen detection element.
The present invention illustrates the shape of the ceramic body and the position of the conductive film. When forming the inner electrode of a cylindrical oxygen sensing element with one end blocked by a conventional method, it is difficult to stir the platinum solution, reducing agent solution, and electroless plating solution injected into the recess inside the element. Therefore, it is not easy to form a homogeneous inner electrode, but according to the present invention, a uniform inner electrode without unevenness can be easily formed.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an example (Example) of a method for forming a conductive film of a ceramic body according to the present invention will be described with reference to the drawings.
(Example)
In this embodiment, a method for forming a porous electrode of a solid electrolyte body constituting an oxygen detection element of an oxygen sensor will be described.
[0031]
a) First, the structure and function of the oxygen sensor will be briefly described.
As shown in FIG. 1, the oxygen sensor 1 detects the oxygen concentration in the exhaust gas of an automobile engine, and a cylindrical oxygen detection element 2 with one end closed is connected via a tubular member 3 and a filler 4. The heat-resistant steel housing 5 is fixed.
[0032]
As schematically shown in FIG. 2, the oxygen detection element 2 includes a solid electrolyte body (ceramic body) 6 made of an oxygen ion conductive solid electrolyte such as zirconia, and inner and outer surfaces of the solid electrolyte body. And formed porous electrodes (conductive films) 7 and 8 having heat resistance such as platinum.
[0033]
In this oxygen sensor 1, exhaust gas is brought into contact with the porous electrode (outer electrode) 8 formed on the outer surface of the solid electrolyte body 6, and the porous electrode (inner electrode) 7 formed on the inner surface is exposed to the atmosphere. In this way, the electromotive force generated between the porous electrodes 7 and 8 is detected from the difference in oxygen partial pressure between the exhaust gas and the reference gas, and the oxygen concentration in the exhaust gas is measured.
[0034]
b) Next, a method for forming the porous electrodes 7 and 8 will be described. In particular, the method for forming the inner electrode 7 will be described as an example with reference to FIG.
(Process 1)
First, the inside of the solid electrolyte body 6 was treated with hydrofluoric acid (HF) having a concentration of 5% by weight.
[0035]
Specifically, at room temperature, the hydrofluoric acid is injected into the concave portion 9 (see FIG. 2) inside the solid electrolyte body 6 and held in that state for 7 minutes to roughen the inner surface of the solid electrolyte body 6; A permeation path 10 was formed (so that a chloroplatinic acid solution described below can enter). Thereafter, the hydrofluoric acid was removed by suction, and then washed with water.
[0036]
In addition, the density | concentration of a hydrofluoric acid should just be the range of 1-20 weight% (preferably 5-10 weight%).
(Process 2)
Next, 0.05% by weight (corresponding to 0.5 g / L) of chloroplatinic acid solution (H ₂ PtCl) with 0.03 normality hydrochloric acid (HCl) added to the surface of the roughened solid electrolyte body 6. ₆ ; aqueous solution) was heated to 85 ° C. and impregnated.
[0037]
Specifically, the chloroplatinic acid solution is injected into the recess 9 inside the solid electrolyte body 6 and held in that state for 10 minutes, and the permeation path 10 on the inner surface of the solid electrolyte body 6 is impregnated with platinum chloride solution. I let you. The heating was performed by placing the solid electrolyte body 6 containing the chloroplatinic acid solution in a heater (not shown) and maintaining the temperature. By this heating, sufficient impregnation is performed.
[0038]
The platinum concentration in the chloroplatinic acid solution is in the range of 0.01 to 0.1% by weight (corresponding to 0.1 to 1 g / L) (preferably 0.03 to 0.07% by weight). The heating temperature may be in the range of 70 to 110 ° C. (preferably 80 to 90 ° C.).
[0039]
(Process 3)
Next, the chloroplatinic acid solution was extracted and removed from the surface of the solid electrolyte body 6.
Specifically, the chloroplatinic acid solution is removed from the concave portion 9 of the solid electrolyte body 6 by suction using negative pressure. As a result, the chloroplatinic acid solution is extracted, but is not cleaned, so the chloroplatinic acid solution is placed on the surface of the solid electrolyte body 6, specifically, the inside of the permeation path 10 formed by roughening. It is in a very thin film 11 attached so as to cover the surface.
[0040]
(Process 4)
Next, a reducing agent was brought into contact with the surface of the solid electrolyte body 6 to which the chloroplatinic acid solution was thinly attached, and platinum nuclei (Pt nuclei) were deposited on the surface.
Specifically, a solution containing a reducing agent, that is, a reducing agent solution (aqueous solution) containing hydrazine (N ₂ H ₄ ) having a concentration of 4% by weight as a reducing agent is injected into the recess 9 of the solid electrolyte body 6, and 70 ° C. And kept in that state for 30 minutes. Thereby, platinum nuclei were deposited on the inner surface of the solid electrolyte body 6 (including the surface of the permeation path 10). Thereafter, the reducing agent solution was removed from the recess 9 by suction using a negative pressure.
[0041]
The concentration of hydrazine in the reducing agent solution may be in the range of 2 to 20% by weight (preferably 3 to 10% by weight), and the heating temperature is in the range of 50 to 90 ° C. (preferably 60 to 80 ° C.). If it is.
(Process 5)
Next, the inner electrode 7 was formed by performing electroless plating on the inner surface of the solid electrolyte body 6 on which platinum nuclei were deposited.
[0042]
Specifically, an electroless plating solution containing a platinum ammine compound and hydrazine in the recess 9 of the solid electrolyte body 6, that is, electroless plating containing 1.5 wt% platinum and 4 wt% hydrazine. The liquid was poured, heated to 85 ° C., and kept in that state for 180 minutes. Thereby, the inner electrode 7 made of layered platinum was formed on the inner surface of the solid electrolyte body 6 using the platinum nucleus as a growth nucleus.
[0043]
Thereafter, the electroless plating solution was removed by suction. Further, the inside of the recess 9 was washed with water and then dried.
The amount of platinum in the electroless plating solution may be 3 to 30 g / L (preferably 7 to 20 g / L), and the concentration of hydrazine should be 2 to 20% by weight (preferably 3 to 10% by weight). The heating temperature may be in the range of 60 to 95 ° C (preferably 70 to 90 ° C).
[0044]
In addition, the outer electrode 8 is formed in substantially the same procedure, and then the solid electrolyte body 6 is heat-treated at 600 to 1000 ° C. to detect oxygen with porous electrodes 7 and 8 on both the inner and outer sides of the solid electrolyte body 6. Element 2 was completed.
In the present embodiment, since the inner electrode 7 is formed as in the above (Step 1) to (Step 5), the following effects are obtained.
[0045]
(1) In the present embodiment, unlike the conventional case, the reducing agent does not act on the entire platinum solution injected into the recess 9, but platinum chloride thinly adhered to a specific place, that is, the surface of the solid electrolyte body 6. The reducing agent acts only on the acid solution. Therefore, platinum nuclei are deposited only on the inner surface of the solid electrolyte body 6 (including the inner surface of the permeation path 10), so that the platinum nuclei are formed on the entire inner surface of the solid electrolyte body 6 along the inner surface of the permeation path 10. It adheres uniformly without unevenness. Thereby, the adhesiveness of the inner electrode 7 to be formed later by electroless plating is improved, and the denseness of the inner electrode 7 is improved. As a result, the measurement accuracy of the oxygen sensor 1 is improved and the life thereof is extended.
[0046]
(2) Further , in this embodiment, platinum nuclei are precipitated only on the inner surface of the solid electrolyte body 6, so that the platinum that is precipitated and suspended in the solution adheres to the apparatus and is thus installed as in the conventional case. There is no fouling.
(3) Further, the reducing agent solution used for depositing platinum is sucked and recovered, but as described above, the deposition of platinum is performed on the inner surface of the solid electrolyte body 6, and the platinum is Since it adheres to the inner surface, the recovered reducing agent solution does not contain floating platinum. Therefore, there is no need to recover platinum, and the reducing agent solution can be easily reused, contributing to resource saving.
[0047]
(4) In particular, since the concave portion 9 of the solid electrolyte body 6 has a considerably small inner diameter and is deep, stirring is difficult in the case of precipitation of platinum nuclei. However, if the method of the present embodiment described above is employed, unevenness is less likely to occur, which is optimal for forming the inner electrode 7.
(Experimental example)
Next, an experimental example performed to confirm the effect of the present embodiment will be described.
[0048]
Twenty samples were prepared by each of the conventional method (electroless plating immediately after etching) and the method of this example within the scope of the present invention, and each position A of the element shown in FIG. , B, C, and D were measured by a calibration curve method using a fluorescent X-ray apparatus.
[0049]
The result is shown in FIG. 4 (b). In the case of the conventional method, the film thickness varied greatly at all the positions A to D. However, in the case of the method of this example (this method), It was suitable with little variation in film thickness.
In addition, this invention is not limited to the said Example at all, and it cannot be overemphasized that it can implement with a various aspect in the range which does not deviate from the summary of this invention.
[0050]
(1) For example, in the above-described embodiment, the method for forming the inner electrode among the porous electrodes of the oxygen detection element has been described. However, it can be applied to the formation of the outer electrode, and other ceramic bodies other than the oxygen sensor (for example, It can be applied to the formation of a conductive film of an alumina plate.
[0051]
(2) The shape of the ceramic body forming the conductive film can take various forms such as a cylinder, a plate, a bar, a cylinder, etc., but when used as an oxygen sensor for automobiles, a cylinder with one end closed. It is preferable to use a shape (see FIG. 2).
[0052]
【The invention's effect】
As described above in detail, according to the present invention, platinum nuclei are precipitated not on the whole solution but only on the surface of the ceramic body as in the prior art. There is a feature that platinum nuclei adhere uniformly. Therefore, the adhesiveness and the denseness of the conductive film formed later by electroless plating using platinum nuclei as growth nuclei are excellent. Therefore, for example, even when exposed to a drastic temperature change, the conductive film is difficult to peel off, and for example, when applied to an oxygen sensor, it is possible to prevent a decrease in measurement accuracy and lifetime.
[0053]
Further, in the present invention, since platinum nuclei are not generated in the entire solution, the amount of platinum floating in the solution is extremely small, so that the device does not adhere to the device and contaminate the device. Moreover, there is an advantage that the used platinum solution can be reused almost as it is.
Furthermore, in this invention, the reducing agent solution containing a reducing agent is heated and used at 50-90 degreeC. Therefore, within this temperature range, there is an effect that platinum nuclei can be deposited more uniformly on the surface of the ceramic body including the inside of the permeation path, and unevenness in the precipitation of platinum nuclei hardly occurs.
Thus, in the present invention, there is a remarkable effect that platinum nuclei can be deposited on the surface of the ceramic body without unevenness, and unnecessary platinum deposition can be greatly suppressed.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing a part of an oxygen sensor in a broken state.
FIG. 2 is an explanatory view schematically showing an oxygen detection element.
FIG. 3 is an explanatory diagram showing a procedure of an inner electrode forming method according to an embodiment.
FIG. 4 shows an experimental example, (a) is an explanatory view showing a film thickness measurement position of the oxygen detection element, and (b) is a graph showing a film thickness measurement result.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Oxygen sensor 2 ... Oxygen detection element 6 ... Solid electrolyte 7 ... Inner electrode (porous electrode)
8 ... Outer electrode (porous electrode)
9 ... concave 10 ... penetration path

Claims (9)

In the method of forming a conductive film on the surface of the ceramic body,
(1) a step of roughening the surface of the ceramic body by etching;
(2) a step of impregnating the roughened ceramic body surface with a platinum solution by heating; (3) a step of extracting the platinum solution from the surface of the ceramic body;
(4) contacting a reducing agent solution containing a reducing agent heated to 50 to 90 ° C. with the surface of the ceramic body from which the platinum solution has been extracted to deposit platinum nuclei on the ceramic surface;
(5) forming a conductive film by performing electroless plating on the surface of the ceramic body on which the platinum nucleus is deposited;
A method for forming a conductive film of a ceramic body, comprising:  The method for forming a conductive film of a ceramic body according to claim 1, wherein a heating temperature when impregnating the platinum solution is 70 to 110 ° C.  The method for forming a conductive film of a ceramic body according to claim 1 or 2, wherein the platinum solution is extracted by suction using a negative pressure. The method for forming a conductive film of a ceramic body according to any one of claims 1 to 3, wherein the concentration of the reducing agent solution containing the reducing agent is 2 to 20% by weight . Wherein the reducing agent is a conductive film forming method of a ceramic body according to any one of claims 1 to 4, characterized in that a hydrazine. The electroless plating solution used in carrying out the electroless plating, the conductive film forming method of a ceramic body according to any one of claims 1 to 5, characterized in that a solution containing a platinum ammine compound with hydrazine. The method for forming a conductive film of a ceramic body according to claim 6 , wherein the electroless plating solution is 3 to 30 g / L of platinum and 2 to 20% by weight of hydrazine. Said ceramic body, the conductive film forming method of a ceramic body according to claim 1-7, characterized in that the oxygen sensing element made of zirconia solid electrolyte. Said oxygen sensing element, the one end a tubular that has been occluded, the conductive film, of the inner and outer porous electrodes of the oxygen sensing element, according to claim 8, characterized in that the inner electrode A method for forming a conductive film of a ceramic body.

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