JPH09278537A - Production of ceramic sintered compact - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a dense ceramic sintered compact with few pores by covering the surface of a ceramic molding containing substances scatterable during baking the molding with inert powder followed by conducting a heat treatment. SOLUTION: When a ceramic molding containing substances scatterable during its baking (e.g. bismuth) is to be heat-treated, its surface is covered with powder inert to the molding; for example, a ceramic molding 1 is embedded in aluminum oxide powder 2 and baked. Thereby, the low-melting substances in the molding 1 can be prevented from being scattered; therefore, the objective ceramic sintered compact with few pores can be obtained. Alternatively, a sagger may be packed with the ceramic molding and inert powder also inert to the sagger followed by heat treatment. This ceramic sintered compact thus obtained has few pores; therefore, a ceramic element produced using this sintered compact seldom causes deterioration of its properties under high humidity and voltage.

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 a ceramic sintered body used in, for example, a varistor element for the purpose of protecting an electric circuit from overvoltage.

[0002]

2. Description of the Related Art Conventionally, a ceramic sintered body used for a varistor element is formed by forming a mixture of a ceramic raw material containing ZnO as a main component and a sub-component such as Bi ₂ O ₃ and an organic binder, and forming the mixture. The body was manufactured by placing it in an electric furnace in a state where it was used alone or in a state where several bodies were stacked and fired.

[0003]

According to this method, it was found that a large number of holes are formed inside the sintered body due to the fact that the low melting point component such as bismuth scatters during firing. . Such a ceramic sintered body with a large number of pores has a reduced mechanical strength and is easily affected by the surrounding atmosphere, etc., or the plating solution penetrates inside the sintered body when a process such as plating is performed. , Causing a condition such as deterioration of electrical characteristics. In particular, in a varistor element, having many holes inside the element has a problem that the electrical characteristics under high humidity and high voltage are significantly deteriorated.

Therefore, an object of the present invention is to provide a ceramic sintered body having a small number of holes.

[0005]

In order to achieve this object, the present invention provides a surface of a ceramic compact containing a substance having a possibility of being scattered during firing, and a powder inert to the ceramic compact. According to this method, although the reason is not clear at this time, it may be difficult to evaporate the low melting point substance having a possibility of scattering in the ceramic molded body, In conclusion, it is possible to provide a ceramic sintered body having a small number of holes.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION According to the first aspect of the present invention, the surface of a ceramic molded body containing a substance having a possibility of being scattered during firing is covered with this ceramic molded body and an inert powder, and then heat treated. Since it is considered that the low melting point substance in the ceramic molded body is hard to evaporate, it is possible to provide a ceramic sintered body having a small number of pores.

According to the second aspect of the present invention, a powder containing at least one of aluminum oxide and magnesium oxide is used as the inert powder, and the low melting point substance in the ceramic molded body is hard to evaporate. Therefore, it is possible to provide a ceramic sintered body having a small number of holes.

According to the third aspect of the present invention, the ceramic molded body and the inert powder are heat-treated by filling the ceramic molded body and the inert powder in a sheath that does not react with the inert powder. The surface of the body can be uniformly coated with an inert powder.

According to a fourth aspect of the present invention, a ceramic molded body containing zinc oxide as a main component and Bi ₂ O ₃ as a secondary component is used, and Bi ₂ has a low melting point and easily scatters.
Since it is considered that O ₃ is less likely to evaporate, it is possible to provide a ceramic sintered body having a small number of holes.

According to a fifth aspect of the present invention, after the ceramic compact and the inert powder are charged into the sheath, pressure is applied to the ceramic compact and the inert powder to perform heat treatment. Even if the ceramic molded body shrinks, the surface can always be covered with an inert material.

The invention according to claim 6 uses a pod formed of a material containing at least one of aluminum oxide and zirconium oxide. The pod is formed of an inactive substance to form a pod. Does not react with the filled inert powder.

An embodiment of the present invention will be described below with reference to the drawings. (Embodiment 1) FIG. 1 is a sectional view showing a state in which a molded body is embedded in an inert powder.
Above, the surface of the molded body 1 is covered with the aluminum oxide powder 2 which is an inert powder. 2 uses a sheath 4 instead of the aluminum oxide plate 3 in FIG. 1, and by using the sheath 4, the surface of the molded body 1 can be uniformly covered with the aluminum oxide powder 2. FIG. 4 shows a cross-sectional view of a general disc type zinc oxide varistor, which is composed of a sintered body 5 and silver electrodes 6 provided on both upper and lower surfaces of the sintered body 5.

First, an organic binder is added to and mixed with a ceramic material containing at least zinc oxide and bismuth oxide, and then a ceramic material having a diameter of 10 mm and a thickness of 1.2 mm is prepared.
A disc-shaped compact 1 was obtained by applying a pressure of t / cm ² . After debinding the molded body 1 thus obtained at 500 ° C., 50 cc of the aluminum oxide powder 2 was placed on the aluminum oxide plate 3 having a length of 150 mm × width of 150 mm × thickness of 2 mm as shown in FIG. Then, the molded body 1 was placed, and the aluminum oxide powder 2 was further placed on the molded body 1 by 50 cc to completely embed the molded body 1 in the aluminum oxide powder 2 and fired at 930 ° C. to obtain a sintered body 5. . During firing, the molded bodies 1 were prevented from directly contacting each other.

The purpose of covering the surface of the molded body 1 with the aluminum oxide powder 2 which is an inactive substance is that Bi ₂ O ₃ contained in the molded body 1 has a temperature above its melting point (825 ° C.).
It is to prevent the vapor pressure from rapidly rising and evaporating. In order to improve the shielding effect, as shown in FIG. 2, a molded body 1 is formed by using a sheath 4 formed of aluminum oxide or magnesium oxide, which is stable at a predetermined firing temperature.
The surface of the molded body 1 is uniformly covered with the aluminum oxide powder 2 for embedding.

For comparison, the molded body 1 produced by the same method as above was debindered, placed on a Ni net as in the conventional case, and fired at 930 ° C. to prepare a sintered body.

Next, as shown in FIG. 4, a silver paste having a diameter of 7 mm was applied to the obtained sintered body 5 and then baked at 800 ° C. to form a silver electrode 6 to obtain a varistor element.

Table 1 shows the test results of the varistor element of this embodiment and the conventional varistor element under the conditions of porosity and humidity of 90%, 60 ° C., and applied voltage DCV 1 mA × 90%.

[0018]

[Table 1]

Here, the varistor voltage is a voltage when a current flows by 1 mA, and will be hereinafter referred to as V1 mA.
As shown in (Table 1), it can be seen that the varistor element of the present embodiment has good characteristics with respect to the conventional varistor element, in which the porosity is reduced. This indicates that there is a correlation between the porosity and the humidity resistance test.When the porosity is large, the vaporized water penetrates from the surface of the varistor element, which causes the conductive state of the current to flow under high voltage. It is thought that this has resulted in deterioration of the characteristics.

Although aluminum oxide powder is used as the inactive powder in this embodiment, the main component of the sintered body 5 such as zinc oxide and bismuth oxide as a secondary component, and zirconium oxide and oxide that are stable at the firing temperature are used. Any inert powder such as a powder obtained by mixing aluminum and zirconium oxide may be used.

The same applies to the sheath 4, and any material may be used as long as it is a substance that is stable at the firing temperature for the inert powder that covers the compact 1. Also sintered body 5
The shape is not limited to the disk type, and the same effect can be obtained with a molded body of any shape such as a laminated body in which internal electrodes and ceramic layers are alternately laminated. Further, in the zinc oxide varistor, as a method for preventing the scattering of bismuth oxide, the shielding was performed as shown in the present embodiment, but the aluminum oxide powder 2 was used to make the surroundings of the compact 1 a bismuth oxide atmosphere. The effect is further enhanced by adding an evaporation component such as bismuth oxide into the inert substance.

(Embodiment 2) FIG. 3 shows an aluminum oxide powder 5 installed on the aluminum oxide powder 2 shown in FIG. 2, and the aluminum oxide powder 5 has a constant pressure during firing. Acts so as to be loaded on the aluminum oxide powder 2.

First, the molded body 1 obtained in the same manner as in the first embodiment is debindered at 500 ° C., and then 40 c in 100 cc of aluminum oxide pod 4 as shown in FIG.
The aluminum oxide powder 2 of c is put, and the molded body 1 is placed thereon.
Was installed, and 40 cc of aluminum oxide powder 2 was further filled therein to completely embed the molded body 1 and fired at 930 ° C.

Since the sintering reaction of the molded body 1 proceeds during firing, the volume after firing is greatly reduced. Therefore, the surface of the molded body 1 should always be covered with the aluminum oxide powder 2 even during firing to control the scattering of components that easily evaporate. Therefore, as shown in FIG. The aluminum oxide burned material 5 was placed on the aluminum oxide powder 2 so as to be applied with the pressure (0.3 g / mm ² ) of FIG.

Next, a silver paste having a diameter of 7 mm was applied to the obtained sintered body 5 as shown in FIG. 4 and then baked at 800 ° C. to form an electrode 6 to obtain a varistor element.

The porosity and humidity of 90% of the varistor element of the present embodiment, which was subjected to the high humidity and high voltage application test in Table 2,
The test results are shown under the conditions of 60 ° C. and applied voltage DCV 1 mA × 90%.

[0027]

[Table 2]

As can be seen from (Table 2), the characteristics are in a favorable direction in comparison with the conventional examples.

In the case of the zinc oxide varistor molded body 1, when the temperature is lowered during firing (particularly, the melting point of bismuth oxide is 8).
A rapid temperature change (around 25 ° C.) may cause variations in characteristics, but in the present example, since the heat treatment is performed in the aluminum oxide powder 2, the heat capacity of the aluminum oxide powder 2 causes a rapid change. It becomes difficult to receive a temperature change, and a uniform sintered body 5 can be obtained. Therefore, the total heat capacity of the aluminum oxide powder 2, the sheath 4, and the aluminum oxide fuel binder 5 for pressure application should be large as long as the temperature rising rate of the compact 1 follows the temperature rising rate during firing.

In this embodiment, the aluminum oxide powder 5 is used on the aluminum oxide powder 2 when firing in the sheath 4 under constant pressure. However, as long as the molded body 1 is not affected, Even if such a substance is installed, it is more effective if the pressure per unit area is larger.

When aluminum oxide is used in Embodiments 1 and 2, excess bismuth oxide is removed by the reaction between aluminum oxide and bismuth oxide at 850 ° C. or higher, and bismuth oxide is uniformly distributed in the device. You can also see the effect.

Although aluminum oxide is used as the inert powder in the first and second embodiments, magnesium oxide or a mixture of aluminum oxide and magnesium oxide may be used. The same thing can be said about the sheath 4.

Further, in the present embodiment, the disk type zinc oxide varistor element is shown as an example, but any shape such as a laminated body in which the internal electrode layers and the ceramic layers are alternately laminated may be used. The same effect can be obtained with substances that may be scattered by heat treatment (substances having a melting point lower than the heat treatment temperature, that is, substances with high vapor pressure) made of materials containing bismuth, lead, boron, antimony, etc. .

As described above, in the zinc oxide varistor, bismuth oxide mainly forming the grain boundary phase has a low melting point (825 ° C.), and bismuth oxide is vaporized due to the high vapor pressure during the firing process. This is considered to be the cause of increasing the number of holes in the varistor element, and when firing the molded body of the varistor, by covering the surface of the molded body with an inert material, the evaporation of bismuth oxide is significantly increased. It is suppressed, the number of holes is reduced, and as a result, the deterioration of the characteristics under high humidity and high voltage is significantly improved. Further, in the case of a zinc oxide varistor, subjecting it to a rapid temperature change during temperature reduction during firing (particularly near 825 ° C. which is the melting point of bismuth oxide) may cause variations in characteristics,
In the present invention, since the heat treatment is performed in the powder, it is difficult to receive a rapid temperature change due to the heat capacity of the powder, and a uniform element can be obtained.

[0035]

As described above, according to the present invention, it is possible to provide a denser sintered body having a smaller number of holes.
As a result, it is possible to obtain a ceramic element having improved weather resistance such as reduction in deterioration of characteristics under high humidity and high voltage.

[Brief description of drawings]

FIG. 1 is a sectional view showing a firing method according to an embodiment of the present invention.

FIG. 2 is a sectional view showing a firing method according to another embodiment of the present invention.

FIG. 3 is a sectional view showing a firing method according to another embodiment of the present invention.

FIG. 4 is a sectional view of a general zinc oxide varistor.

[Explanation of symbols]

 1 molded body 2 aluminum oxide powder 4 pod

Claims (6)

[Claims] 1. A method for producing a ceramic sintered body, in which the surface of a ceramic molded body containing a substance that may scatter during firing is covered with the ceramic molded body and an inert powder for heat treatment. 2. The method for producing a ceramic sintered body according to claim 1, wherein a powder containing at least one of aluminum oxide and magnesium oxide is used as the inert powder. 3. The method for producing a ceramic sintered body according to claim 1, wherein the ceramic compact and the inert powder are filled in a sheath that does not react with the inert powder and heat-treated. 4. Zinc oxide as a main component and B as a secondary component
The method for producing a ceramic sintered body according to claim 1, wherein a ceramic molded body containing i ₂ O ₃ is used. 5. The ceramic sintered body according to claim 4, wherein after the ceramic molded body and the inert powder are filled in the sheath, pressure is applied to the ceramic molded body and the inert powder to perform heat treatment. Production method. 6. The method for producing a ceramic sintered body according to claim 4, wherein the pod is formed of at least one of aluminum oxide and zirconium oxide.