JPH1038840A - Manufacture of oxygen sensor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To heighten adhesion of an electrode at the inner surface of an element, particularly a closed end face part so as to prevent the generation of bulging and improve yield. SOLUTION: In a method of forming a ceramic molding forming an element body by a rubber pressing method, a tip face 25 out of the surface of a male mold 22 for forming the inner surface 2b of the molding 1b is made Ra: 0.35μm or more so as to have the roughened surface compared to other parts. At the time of forming the inner surface 2b of the molding 1b by the male mold 22, the inner surface is roughened at the same time. When the molding 1b is baked, surface roughness is slightly improved, but a closed end face part 5b is roughened on the surface at a nearly fixed rate compared with the other parts. Afterwards, when an electrode is formed by electroless plating, the electrode at the closed end face part is higher in adhesion than the other parts.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an oxygen sensor element used for an oxygen sensor for detecting an oxygen concentration in exhaust gas of an internal combustion engine.

[0002]

2. Description of the Related Art A cylindrical element having one end closed and used for an oxygen sensor made of a solid electrolyte material of an oxygen ion conductor such as zirconia has a shape as shown in FIGS. 5 and 6, and is not shown. It is incorporated in a holder or the like and attached to an exhaust gas pipe of an internal combustion engine. The electrode layer (reference electrode layer) 12 on the inner surface 2 of the element is replaced with a reference oxygen gas (atmosphere), and the electrode layer (measurement electrode layer) 13 on the outer surface 3. Is brought into contact with the exhaust gas to generate an electromotive force (potential difference) between the two electrodes in accordance with the oxygen concentration difference between the inner and outer surfaces of the element 1, and outputs a signal based on the electromotive force to a control circuit to control the air-fuel ratio. Have been to be. And
Such an oxygen sensor element (hereinafter, also referred to as a sensor element or simply an element) 1 is conventionally manufactured as follows. First, a ceramic molded body forming an oxygen sensor element body is molded into a cylindrical shape having one end closed by a rubber press method. Next, the outer peripheral surface is processed into a predetermined shape, and thereafter, a metallized paste such as platinum (Pt) is printed and fired so as to form the lead 6 of the electrode layer (hereinafter, also referred to as an outer electrode) 13 on the outer surface 3. . Then, electrode layers 12 and 13 made of platinum or the like are respectively formed on the inner and outer surfaces by electroless plating or the like. Further, after the formation of the electrode layers 12 and 13, a predetermined heat treatment is performed for activation thereof, and a ceramic porous body such as spinel is formed by thermal spraying to protect the electrode layer 13, thereby forming the oxygen sensor element 1. Will be completed as

In this type of oxygen sensor element, when the electrodes are separated, the electromotive force characteristics are deteriorated, and it becomes impossible to appropriately control the air-fuel ratio. Therefore, it is extremely important to increase the bonding strength of the electrodes formed on the surface of the element body (sintered body). Among these electrodes, the outer electrode 13 that is directly exposed to a high-temperature blast such as exhaust gas has a particularly high risk of peeling.

As a countermeasure against such a problem, for example, Japanese Patent Application Laid-Open
No. 137394 discloses an outer surface of a ceramic body before sintering of a solid electrolyte (hereinafter referred to as a body before firing) in order to improve the adhesive strength of an outer electrode exposed to high-temperature exhaust gas. The surface is roughened by blasting or machining to enhance the anchor effect of the electrode layer to be plated (hereinafter, also referred to as plating) and to improve the bonding strength of the outer electrode formed on the outer surface of the ceramic after sintering. I have to. As described above, conventionally, the adhesion (adhesion strength) of the outer electrode to the ceramic has been considered, but the electrode on the inner surface 2 (hereinafter, also referred to as the inner electrode).
As for the outer electrode 13, the bonding strength was not as important as that of the outer electrode 13 because, for example, the outer electrode 12 did not directly contact the exhaust gas.

[0005]

However, if the bonding strength of the inner electrode is low, it is easy for the inner electrode to peel off depending on the conditions of use, resulting in lowering the performance and reliability of the element. Further, when an electrode is formed on a fired ceramic molded body (hereinafter, the fired molded body is also simply referred to as a sintered body) by electroless plating, thereafter, as described above, A heat treatment is performed. By this treatment, a closed end face 5 (hereinafter also referred to as a closed end face) 5 of the inner surface 2 of the element is formed on a plating surface (electrode) 12 called blister F as shown in FIG. There is a problem that the production yield cannot be increased because swelling (separation of the electrodes) occurs.

[0006] The cause of blister F is that the adhesion of the plating is poor at the closed end face portion 5 of the inner surface 2, and the gas in the minute gap existing between the surface (base) of the element body and the plating by the subsequent heat treatment. It is considered that this is due to the expansion of.
The poor adhesion of the plating is because the closed end face portion 5 on the inner surface has a deep closed hole, so that the plating solution is poorly recirculated, so that the plating is liable to be deposited. It is considered that the internal stress of the plating layer increases due to the pressing.

It is considered that this blister can be eliminated by roughening the inner surface to a predetermined roughness to enhance the adhesion of plating, but the inner surface of the element has an inner diameter of several mm.
, The particles do not reach by the blast treatment, and it is difficult to roughen the surface to a desired surface roughness. In particular, the closed end face is the innermost part, and its effect can hardly be expected. On the other hand, it is conceivable to attach a predetermined ceramic powder (nota) to the inner surface of the molded body in order to obtain a constant surface roughness, or to roughen the inner surface by machining the inner surface separately. The number of steps is increased, the production efficiency is reduced, and the production cost is increased.

Under these circumstances, the inventors of the present invention have variously changed the surface roughness of a portion forming a molding surface of a male die (press pin) used in a rubber press method, molded a molded product, and fired a molded product. When the surface roughness of the inner surface was measured, it was thought that the surface roughness of the sintered body was due to sintering shrinkage, but the roughness (roughness) was smaller than the surface roughness of the male mold, but the ratio was almost constant. We came to find that it was stable.

[0009] In addition, the occurrence of blisters was confirmed on the sintered body obtained by plating and heat-treating the sintered body. The occurrence of blisters was remarkable due to slight differences in the surface roughness of the sintered body. They have come to the conclusion that they are different. That is, the inventor of the present application has found that by appropriately setting the surface roughness of the male die in the rubber press method that affects the surface roughness of the inner surface of the sintered body, the adhesion of the electrode (plating) is remarkably stable and improved. It has been found that the occurrence of blisters can be significantly and effectively reduced.

The present invention has been made based on such findings, and it is an object of the present invention to improve the adhesion of the inner electrode of the element, particularly the inner electrode at the closed end face, to prevent the occurrence of blisters and to improve the yield. Another object of the present invention is to provide a method capable of manufacturing such an element without requiring an independent step.

[0011]

According to a first aspect of the present invention, there is provided a method of manufacturing an oxygen sensor element, comprising forming a ceramic molded body constituting an oxygen sensor element body by a rubber press method. In the method, among the surfaces of the male mold for molding the inner surface of the ceramic molded body, at least the front end surface is roughened from other parts, and when the inner surface of the ceramic molded body is molded by the male mold, At least the closed end face is to be roughened at the same time.

As shown in FIG. 2, a molding die for molding a cylindrical molded body whose one end is closed by a rubber press method is as follows.
A female mold 23 having a rubber cylindrical shape for molding the outer surface of the element,
A metal mold (press pin) 22 formed into a cylindrical shape with a predetermined tapered taper (pulling taper) for forming the inner surface, and forming a closed end face of the molded body;
A predetermined amount of raw material powder is put into a mold through the opening 24 at the upper end of
After the opening 24 is closed, a water pressure is applied to the outside of the rubber female mold 23 to press-mold the molded body 1b. Then, after the pressure reduction, as shown in FIG. 3, the male mold 22 is removed together with the molded body 1b in the axial direction, and thereafter, the molded body 1b is pulled out from the male mold 22 in the axial direction and released. Molded product 1b released
On the inner surface 2b, the surface roughness state of the male mold 22 is transferred almost as it is.

Therefore, the front end surface (hatched area in the enlarged view of FIG. 3) 25 of the surface of the male mold 22 is appropriately roughened, so that the closed end surface of the inner surface 2b of the molded body 1b forming the molding surface is formed. 5b exhibits a moderately roughened surface roughness. And, since there is sintering shrinkage after firing, the surface roughness of the molded body (male surface roughness)
Although the surface roughness becomes smaller with less irregularities, a sintered body in which at least the closed end face portion of the inner surface is roughened can be obtained without requiring a separate roughening step. Therefore, when the electrode is formed by electroless plating or the like thereafter, since the electrode at the roughened closed end face portion has high adhesion, the occurrence of blisters is reduced, and the production yield is improved. .

In the above-mentioned manufacturing method, at least the surface roughness of the front end face 25 of the surface of the male mold 22 is preferably set to a center line average roughness Ra: 0.35 μm or more. This is because the surface has a center line average roughness Ra of 0.35 μm or more, and the inner surface of the sintered body has a surface roughness effective for enhancing the adhesion of plating. If the surface roughness of the male mold is too rough, the ceramic raw material powder is likely to adhere to the surface at the time of release from the mold, and it is impossible to continuously perform molding to remove the ceramic raw material powder, thereby lowering productivity. . Accordingly, the surface roughness of the male mold is adjusted so that the powder does not adhere to the center line plane roughness R.
a: It is good to be 1.2 μm or less, and in that sense, the surface roughness of the roughened portion is the center line average roughness Ra: 0.35 to
Suitably, it is in the range of 1.2 μm.

In this specification, the center line average roughness Ra refers to the surface roughness defined in JIS B0601-1994. That is, the center line average roughness Ra is obtained by taking a portion of the measurement length L in the direction of the center line from the roughness curve, setting the center line of this portion as the X axis, and the direction of the vertical magnification as the Y axis. When the curve is represented by Y = f (x), the value obtained by the following equation (1) is expressed in μm. Hereinafter, the center line average roughness Ra is also simply referred to as surface roughness.

[0016]

(Equation 1)

A second means of the production method according to the present invention is a method for molding a ceramic molded body forming an oxygen sensor element main body by a rubber press method, wherein a male mold for molding an inner surface of the ceramic molded body is provided. Of these, approximately the entire surface of the inner surface of the ceramic molded body is roughened, and when the inner surface of the ceramic molded body is molded by the male mold,
The purpose is to roughen the inner surface at the same time.

It is the closed end face of the element where blisters are likely to occur that the adhesion of the electrode tends to be a problem. Therefore, if at least this part is roughened, there is almost no problem in preventing the occurrence of blisters. However, according to the second means, the risk of dust increases, but a separate and independent roughening step is required. Almost the entire inner surface is roughened without necessity. Therefore, when the inner electrode is formed by electroless plating or the like after the baking, an element having high adhesion strength can be obtained as a whole. Therefore, stable sensor characteristics can be maintained even under use conditions such as a higher temperature environment than before. Also in this case, the surface roughness of the substantially entire surface of the male mold surface on which the inner surface of the ceramic molded body is formed is preferably center line average roughness Ra: 0.35 μm or more. For the same reason as described above, the surface roughness of the roughened portion is preferably set to a center line average roughness Ra: 0.3
Suitably, it is in the range of 5 to 1.2 μm.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for manufacturing an oxygen sensor element according to the present invention will be described in detail with reference to FIGS. First, in the drawing, reference numeral 1 denotes a sensor element manufactured by the manufacturing method of the present embodiment, which is a ceramic sintered body mainly composed of partially stabilized zirconia and has a bottomed cylindrical shape, and has an inner surface 2 Pt plating is applied to substantially the entire surface and the outer surface 3 below the flange 4 of the outer surface 3 by electroless plating, respectively, to form the inner electrode 12 and the outer electrode 13. In this example, the closed end face portion (hatched area in the enlarged view of FIG. 1) 5 of the inner surface 2 is formed in a substantially hemispherical concave shape, and the inner electrode ( The surface roughness of the sintered body 1 before the formation of the plating layer 12 is, in this example, a center line average roughness Ra: 0.23 μm or more.
The other portion of the inner surface 2 has a center line average roughness Ra of 0.20 μm or less.

The ceramic molded body 1b constituting such an oxygen sensor element 1 is manufactured as follows. First,
In molding the molded body 1b by the rubber press method, regarding the surface roughness of the male die (press pin) 22 of the molding die 21,
The surface roughness of the hemispherical tip surface (hatched area in the enlarged view of FIG. 3) 25 is, for example, the center line average roughness Ra:
The surface roughness of the other part of the male mold 22 is set to be not more than the center line average roughness Ra: 0.23 μm. Then, a predetermined amount of raw material powder is put into the mold through the opening 24 at the upper end of the molding die 21, and after the opening 24 is closed, the rubber female mold 23 is closed.
The raw material powder is press-molded by applying water pressure to the outside. After depressurization, as shown in FIG.
b, and the molded body 1b is pulled out from the male mold 22 in the axial direction and released. Then, as shown in FIG. 4, the surface roughness of the male mold 22 is transferred to the inner surface 2b of the molded body 1b, which has been released, almost as it is. Therefore, the closed end surface portion 5b of the inner surface 2b of the molded body 1b is formed with the center line average roughness R
a: about 0.35 μm, and the surface roughness (R
a: about 0.23 μm or less). The outer surface of the molded body 1b is processed into a predetermined shape and dimensions and, at the same time, has an appropriate surface roughness.
Printing and baking metallizing paste for use.

In FIG. 1, a sintered body 1 obtained after firing
The surface roughness of the inner surface 2 of a is due to sintering shrinkage due to firing.
The surface roughness of the molded body 1b is smaller than the surface roughness of the molded body 1b, that is, the surface roughness of the male mold 22 by a small percentage. In this example, the closed end face portion 5 forming the hemispherical surface has a center line average roughness Ra: 0.23 μm, the other portions have a center line average roughness Ra: 0.20 μm or less, and the closed end face of the inner surface 2. The part 5 becomes the sintered body 1a whose surface is roughened more than other parts. That is, the closed end surface portion 5 of the inner surface 2 is formed after molding.
Without performing any special treatment before sintering, the surface is roughened to a desired surface roughness simultaneously with the molding of the molded body 1b. Therefore, after that, the inner and outer electrodes 12, 13 are formed by electroless plating, respectively, and are subjected to a predetermined heat treatment to obtain the element 1 shown in FIG. And the element 1 thus obtained.
Since the surface roughness of the closed end surface portion 5 of the inner surface 2 before plating is appropriately roughened, the adhesion of plating is high, and the occurrence of blisters in heat treatment is prevented.

As described above, according to the present manufacturing method, the inner surface 2b of the molded body 1b is roughened simultaneously with the molding of the molded body 1b by using the male mold 22 having the roughened front end surface 25 in the rubber pressing method. Therefore, the inner surface of the element body (molded body and sintered body) can be roughened with high reproducibility without requiring blasting or a special process.

[0023]

Next, the tip surface (semi-spherical surface) 25 of the male mold 22 is made to have an appropriate surface roughness (0.23 to 1.2) by the above-mentioned manufacturing method.
6 μm), and a molded body (sample) 1b having a different surface roughness of the closed end face portion 5b of the inner surface 2b is produced using the roughened surface. Electrodes 12 and 13 are formed on each of the inner and outer surfaces of the sintered body 1a by electroless plating to form electrodes 12 and 13 and are subjected to a predetermined heat treatment. The occurrence of blisters (defectives) on the closed end face 5 of the inner electrode 12 Was confirmed by an endoscope, and the inner electrode (plating) 12 at the closed end face 5 was cut off by cutting the device along a plane including the axis of the element 1, and the surface roughness was confirmed. There are 100 samples each. The results are as shown in Table 1. The tip surface 25 of the male mold 22 is obtained by finishing the entire surface of the male mold 22 to a mirror surface with a surface roughness Ra: 0.20 μm, and then only the tip surface (hemispherical portion) 25 of a predetermined number of sandpapers. Alternatively, the surface was roughened by a predetermined abrasive file.

[0024]

[Table 1]

As shown in Table 1, the surface roughness Ra of the tip end surface 25 of the male mold 22 of the sample No. 1 (comparative example) was 0.23.
In the case of μm, the occurrence of blisters (defective rate) was 56%. Then, the inner surface 2 of the element (sintered body) 1a in that case
The surface roughness of the closed end face portion 5 is Ra: 0.17 to 0.20
It was in the range of μm. On the other hand, in the case of the sample No. 2 in which the front surface has a surface roughness Ra: 0.35 μm, the occurrence of blisters is drastically reduced to only 1%. In this case, the surface roughness of the closed end face portion 5 of the inner surface 2 of the element (sintered body) 1a was in the range of Ra: 0.23 to 0.29 μm. Further, the surface roughness R of the tip end surface 25 of the male mold 22 of the sample No. 3
In the case of a: 0.41 μm, the defect rate is 0%. In this case, the surface roughness of the closed end face 5 of the inner surface 2 of the element (sintered body) 1a was in the range of Ra: 0.29 to 0.35 μm.

From these results, the closed end face 5 of the inner surface 2 of the sintered body 1a was locally reduced to have a surface roughness Ra: 0.23 μm.
By doing so, it can be seen that the adhesion strength of the electrode (plating) is significantly improved. And for that, male type 2
2 is a center line average roughness R
a: It is understood that it is better to roughen the surface to 0.35 μm or more. Further, from this result, it is understood that the surface roughness of the closed end face portion 5 of the element after firing is about 70 to 90% with respect to the surface roughness of the male tip face 25.

As shown in this example, the ceramic molded body 1 forming the oxygen sensor element body by the rubber press method.
In the method of molding b, it is sufficient as a measure against blistering of plating if at least the surface roughness of the front end surface 25 of the surface of the male mold 22 for molding the inner surface of the element is appropriately roughened. However, substantially the entire surface of the male mold for molding the inner surface of the ceramic molded body, that is, substantially the entire shaft-shaped outer peripheral surface (range H in FIG. 3) is roughened. The inner surface of the ceramic molded body may be formed by the male mold. By doing so, substantially the entire inner surface is roughened, and the adhesion strength of the entire inner electrode is improved.

However, if the entire inner surface is roughened in this manner, powder tends to adhere when the male mold is released from the rubber press, and it takes time to remove the powder.
Depending on the surface roughness, productivity may decrease. Therefore, in this case, it is preferable to make the surface roughness as fine as possible, preferably Ra: 0.35 μm to 1.26 μm.
m.

The shape of the closed end face on the inner surface of the element is as follows:
In the above description, the shape of the closed semi-spherical surface of the inner surface of the element is other than the above. However, even in such a case, there are problems of occurrence of blistering of the electrode and adhesion strength. According to the present invention, even if the shape of the closed end face portion of the inner surface is such a shape, at least the front end surface of the male mold for forming the inner surface of the ceramic molded body is roughened more than other portions. Alternatively, it is only necessary to roughen almost the entire surface of the ceramic molded body on which the inner surface is molded.

[0030]

According to the first manufacturing method of the present invention, the closed end face of the inner surface is roughened simultaneously with the molding of the molded body, so that a separate and independent roughening step is not required. . In addition, a molded body and a sintered body that are roughened to a substantially constant surface roughness with good reproducibility can be obtained. Therefore, when the electrodes are formed thereafter by electroless plating or the like, high adhesion is ensured at the closed end face portion, so that the occurrence of blisters is reduced and the yield is improved.

According to the second production method of the present invention,
Although the risk of dusting increases, almost the entire inner surface is roughened without the need for a separate and independent roughening step. Therefore, when the inner electrode is formed by electroless plating or the like after the firing, an element having a high adhesion strength as a whole is obtained.

[Brief description of the drawings]

FIG. 1 is a semi-longitudinal front view and an enlarged view of a closed end face of a sensor element manufactured by a manufacturing method according to the present invention.

FIG. 2 is a schematic configuration diagram for explaining a state in which a molded body is molded by a rubber press method.

FIG. 3 is an explanatory view and a partially enlarged view of a state in which a male mold and a molded body are removed in the axial direction thereof.

FIG. 4 is a sectional view of a molded body before processing.

FIG. 5 is a semi-longitudinal front view of a conventional oxygen sensor element.

FIG. 6 is an enlarged view of a portion A (closed end surface portion) in FIG. 5;

[Explanation of symbols]

 Reference Signs List 1 oxygen sensor element 1a oxygen sensor element main body (sintered body) 1b ceramic molded body 2 inner surface of sintered body 2b inner surface of ceramic molded body 3 outer surface of sintered body 5 closed end face of sintered body 5b closed ceramic molded body End face part 12 Inner electrode 13 Outer electrode 22 Male type 23 Female type 25 Male type tip surface

Claims (4)

[Claims] 1. A method for molding a ceramic molded body constituting an oxygen sensor element body by a rubber press method, wherein at least a tip surface of a male mold surface for molding an inner surface of the ceramic molded body is rougher than other portions. A method of manufacturing an oxygen sensor element, wherein at least when the inner surface of a ceramic molded body is formed by the male mold, at least a closed end face of the inner surface is roughened. 2. The method for manufacturing an oxygen sensor element according to claim 1, wherein the surface roughness of at least the front end surface of the male mold surface is set to a center line average roughness Ra: 0.35 μm or more. A method for manufacturing an oxygen sensor element. 3. A method for molding a ceramic molded body constituting an oxygen sensor element body by a rubber press method, wherein substantially the entire surface of a male mold for molding an inner surface of the ceramic molded body is molded on an inner surface of the ceramic molded body. Characterized in that when the inner surface of the ceramic molded body is formed by the male mold, the inner surface is simultaneously roughened. 4. A method of manufacturing an oxygen sensor element according to claim 3, wherein the surface roughness of substantially the entire surface of the male mold on which the inner surface of the ceramic molded body is formed is determined by calculating a center line average roughness R.
a: A method for manufacturing an oxygen sensor element, wherein the thickness is 0.35 μm or more.