JPH06283313A - Manufacture of reduced-reoxidized varistor - Google Patents

Abstract

PURPOSE:To reduce the voltage fluctuation of a varistor by heat-treating a sintered body in an oxidizing atmosphere after forming the sintered body by sticking a material composed mainly of aluminum to the surface of a molded body composed mainly of strontium titanate and baking the molded body in a reducing gas atmosphere. CONSTITUTION:A sintered body is formed by baking a molded body in a reducing gas atmosphere after molding the powder of a raw material composed mainly of strontium titanate in a desired shape and sticking a material composed mainly of aluminum to the surface of the molded body. Then the sintered body is heat-treated in an oxidizing atmosphere. Powder, paste, or a sheet composed mainly of aluminum is used as the material to be stuck to the surface of the molded body. Aluminum oxide can be used in place of the aluminum. Therefore, a varistor which is suppressed in voltage fluctuation and has a large nonlinear coefficient can be manufactured at a low cost, since the contrast between the high-resistance layer and semiconductor layer constituting the varistor becomes distinct.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a reduction / reoxidation type varistor.

[0002]

2. Description of the Related Art Conventionally, as a method for producing a reduction-reoxidation type varistor containing strontium titanate as a main component, a raw material powder containing strontium titanate as a main component is formed into a compact having a desired shape, and then, for example, H _{2 is used.} Heat treatment is performed at a temperature of 1300 to 1500 ° C in a reducing gas atmosphere of 5 vol% and N ₂ 95 vol%, and the strontium titanate semiconductor ceramic obtained by forced reduction is reoxidized in the atmosphere at a temperature of 800 to 1100 ° C. However, a method of reoxidizing the thin film on the surface of the semiconductor ceramic to increase the resistance is known.

Further, sodium (Na) or bismuth oxide (Bi) is formed on the surface of the semiconductor ceramic obtained by forced reduction.
A method is known in which ₂ O ₃ ) is applied and then heat treatment is performed at a temperature of 1000 to 1200 ° C. in the atmosphere to diffuse sodium and bismuth oxide from the surface of the ceramic along the grain boundaries.

[0004]

Among the above-mentioned methods, the varistor of the type manufactured by the former heat treatment in a reducing gas atmosphere and then the reoxidation heat treatment is generally used in the atmosphere after forced reduction. Since varistor characteristics can be obtained only by reoxidation heat treatment, it is suitable for mass production at low cost, but it has a drawback that the index showing the performance of varistor, that is, the nonlinear coefficient is small.

On the other hand, the surface of the semiconductor ceramic obtained by the latter forced reduction of the above method is coated with sodium or bismuth oxide, and then heat-treated in the atmosphere to remove sodium along the grain boundaries from the surface of the ceramic. Varistors of the type manufactured by the method of diffusing bismuth oxide or bismuth oxide have a large non-linear coefficient, but a step of applying sodium or bismuth oxide is required, and when further heat-treating in air
Since the applied sodium and bismuth oxide partly evaporate, the atmosphere changes depending on the heat treatment amount, and there are drawbacks such as large variations in the characteristic values of the resulting varistor, which greatly reduces the production cost. There is a problem that it becomes high.

An object of the present invention is to solve the above problems and to provide a method for producing a reductive reoxidation type varistor containing strontium titanate as a main component and having a large nonlinear coefficient at a low cost.

[0007]

The method for producing a reductive reoxidation type varistor according to the present invention comprises forming a raw material powder containing strontium titanate as a main component into a compact having a desired shape, and applying aluminum to the surface of the compact. The method is characterized in that after the material containing the main component is attached, the compact is fired in a reducing gas atmosphere to form a sintered body, and the sintered body is heat-treated in an oxidizing atmosphere. Further, the material to be attached to the surface of the molded body may be any of powder, paste or sheet containing aluminum as a main component. Further, aluminum may be aluminum oxide.

[0008]

[Function] A strontium titanate semiconductor in which aluminum is segregated on the surface of the compact when a material containing aluminum as a main component is adhered to the surface of the compact containing strontium titanate and heat-treated in a reducing gas atmosphere A ceramic is obtained.

When aluminum segregated on the surface of the strontium titanate sintered body made into a semiconductor, aluminum ions whose ionic radius is slightly different from the ionic radius of titanium ions are strontium titanate (SrTiO 3).
It forms a solid solution in the titanium (Ti) site of ₃ ), and this acts as an acceptor for the strontium titanate semiconductor ceramic. Therefore, it is considered that if aluminum is segregated near the surface of the semiconductor ceramic sintered body, this acts as an acceptor and forms an electrical depletion layer on the surface.

In the reduction / reoxidation type varistor, the more distinct the high resistance layer and the semiconductor layer inside the sintered body are by reoxidation,
Although the non-linear coefficient becomes large, if the metal element acting as an acceptor is segregated on the surface of the sintered body having a high resistance,
It is believed that this forms a depletion layer. Therefore, when heat treatment is performed on the reduced semiconductorized ceramic sintered body in the atmosphere, the high resistance layer on the surface of the sintered body and the internal semiconductor layer The contrast becomes clearer.

[0011]

EXAMPLES The material containing strontium titanate as a main component used in the method of the present invention is not particularly limited as long as it is a material usually used as a varistor material. Further, as a material containing aluminum as a main component to be attached to the surface of the strontium titanate molded body, aluminum oxide (Al
₂ O ₃ ), aluminum sulfate (Al ₂ (SO ₄ ) ₃ ), aluminum hydroxide (Al (OH) ₃ ) and the like. Aluminum oxide is most suitable because it is inexpensive and excellent in stability, considering that it does not evaporate during heat treatment and that it is inexpensive.

As a gas for firing the strontium titanate compact in a reducing gas atmosphere, for example, a mixed gas of hydrogen gas and nitrogen gas is used, and the ratio of hydrogen gas to nitrogen gas in the mixed gas is 1 to 10% by volume: 99 to 90% by volume, and the temperature may be about 1350 to 1500 ° C.
Further, for example, air is used as the atmosphere for the reoxidation heat treatment of the sintered body in the oxidizing atmosphere, and the heat treatment temperature is 80
It may be about 0 to 1100 ° C.

Next, specific examples of the present invention will be described together with comparative examples.

Example 1 This example shows an example in which the material containing aluminum as a main component is attached to the surface of a molded body containing strontium titanate as a main component in the form of powder.

First, strontium carbonate (SrCO ₃ ) 6
4.76 parts by weight and titanium oxide (TiO ₂ ) 35.05 parts by weight, niobium oxide (Nb ₂ O ₅ ) 0.06 parts by weight and silicon oxide (SiO ₂ )
₂ ) 0.13 parts by weight was added to prepare a raw powder for the varistor. Next, this raw material powder was wet-mixed for 10 hours by a ball mill using a ball of stabilized zirconia and water. This mixture was calcined in the air at a temperature of 1200 ° C. for 5 hours, and then wet-milled again with the same ball mill as above to obtain a powder mixture.

To this powder mixture, polyvinyl alcohol mixed with a small amount of glycerin as a binder was added and mixed, and this was press-molded using a dry molding press at a pressure of 1.5 t / cm ² to have a diameter of 12.5 mm and a thickness of A 1.1 mm disk-shaped molded body 1 (see FIG. 1) was prepared. Powder 2 containing aluminum oxide (Al ₂ O ₃ ) as a main component on the surface of the formed disk-shaped compact 1 (trade name AL-7, average particle size 0.5 μm, aluminum oxide content 99.7%, Sumitomo Chemical Manufactured by Co., Ltd. (see FIG. 1) was sprayed and adhered to both surfaces of the molded body 1.

A compact 1 (see FIG. 1) having aluminum material powders 2 adhered on both surfaces was placed in a reducing gas atmosphere containing 4% by volume of hydrogen gas (H ₂ ) and 96% by volume of nitrogen gas (N ₂ ). Temperature 1
A sintered body made of semiconducting ceramic porcelain was prepared by carrying out a firing treatment of forced reduction at 450 ° C. for 5 hours. The semiconductor-made ceramic porcelain sinter that was created was exposed to a temperature of 10
A reoxidation-type varistor was prepared by performing a reoxidation heat treatment at 00 ° C. for 2 hours.

Then, an In-Ga alloy paste is applied to both surfaces of the produced reduction-reoxidation varistor in accordance with a conventional method and baked to form upper and lower electrodes, and then, as varistor characteristics, a varistor voltage (E ₁₀ value). And its variation, and the nonlinear coefficient (α) were measured. The measurement results are shown in Table 1.

According to the method of this embodiment, it is possible to inexpensively manufacture a large number of varistors having a large nonlinear coefficient.

Comparative Example 1 A reduction-reoxidation type varistor was prepared according to the above-described Example 1 without adhering powder containing aluminum as a main component to the surface of a molded product containing strontium titanate as a main component.

Then, an In-Ga alloy paste was applied to both surfaces of the produced reduction-reoxidation type varistor according to a conventional method, and baked to form upper and lower electrodes, and then, as varistor characteristics, a varistor voltage (E ₁₀ value). The variation, and the non-linear coefficient (α) were measured under the same conditions as in Example 1. The measurement results are shown in Table 1.

Example 2 This example shows an example in which a material containing aluminum as a main component is attached to the surface of a molded product containing strontium titanate as a main component with a paste.

First, according to the procedure of Example 1, a disk-shaped molded body 3 (see FIG. 2) was prepared.

Next, on the surface of the formed molded body 3, a powder containing aluminum oxide (Al ₂ O ₃ ) as a main component (trade name: AL-7, average particle size: 0.5 μm, aluminum oxide content: 9).
9.7%, manufactured by Sumitomo Chemical Co., Ltd.), nitrocellulose as a binder, and butyl carbitol as a solvent were mixed at a mixing ratio of 2: 2: 1 by weight to prepare a paste 4 (see FIG. 2) on both surfaces of the molded body 1. 0.1 wt% in total (based on 100 molded bodies) was applied according to a conventional method.

A molded body 3 (see FIG. 2) having an aluminum material paste 4 adhered on both surfaces is hydrogen gas (H ₂ ) 4% by volume.
And nitrogen gas (N ₂ ) in a reducing gas atmosphere of 96% by volume,
At a temperature of 1450 ° C, firing treatment of forced reduction for 5 hours was performed,
A sintered body made of semiconductorized ceramic porcelain was prepared.

The produced semiconductor ceramic porcelain sintered body was subjected to reoxidation heat treatment at a temperature of 1000 ° C. for 2 hours in the atmosphere to prepare a reduction reoxidation type varistor.

Then, an In-Ga alloy paste was applied to both surfaces of the produced reduction-reoxidation type varistor in accordance with a conventional method and baked to form upper and lower electrodes, and then, as varistor characteristics, a varistor voltage (E ₁₀ value). And its variation, and the nonlinear coefficient (α) were measured. The measurement results are shown in Table 1.

According to the method of this embodiment, it is possible to manufacture a varistor having a small variation in varistor voltage (E ₁₀ value) and a large non-linear coefficient.

Example 3 In this example, a sheet is used to deposit a material containing aluminum as a main component on the surface of a molded product containing strontium titanate as a main component.

First, according to the procedure of Example 1, a disk-shaped molded body 5 (see FIG. 3) was prepared.

Separately from this, powder containing aluminum oxide (Al ₂ O ₃ ) as a main component (trade name: AL-7, average particle size 0.5 μm, aluminum oxide content 99.7%, Sumitomo Chemical Co., Ltd.) A), a polyacrylic acid-based binder as a binder, ammonium polyacrylate as a dispersant, and water with stirring at a mixing ratio of 1: 0.5: 2: 10 by mixing, and then degassed to obtain a slurry. Thickness by doctor blade method
Sheeted to 50 μm, and the sheeted sheet has a diameter of 12.5
A disc-shaped aluminum sheet material 6 (see FIG. 3) was prepared by punching into a disc having a size of mm.

Then, the disk-shaped molded body 5 and the disk-shaped aluminum sheet material 6 are alternately stacked and adhered to each other to form a laminated varistor material 7 (10 layers of the molded body 5 and the aluminum sheet material 6). 11 layers, see FIG. 3).

The laminated varistor material 7 (see FIG. 3) of the molded body 5 and the aluminum sheet material 6 was placed in a reducing gas atmosphere containing 4% by volume of hydrogen gas (H ₂ ) and 96% by volume of nitrogen gas (N ₂ ). Then, a sintering treatment of forced reduction was performed at a temperature of 1450 ° C. for 5 hours to prepare a sintered body made of laminated semiconductor ceramic porcelain.

The produced semiconductor ceramic porcelain sintered body was subjected to reoxidation heat treatment at a temperature of 1000 ° C. for 2 hours in the atmosphere to produce a reduction reoxidation type varistor.

Then, an In--Ga alloy paste was applied to both surfaces of the produced reduction-reoxidation type varistor in accordance with a conventional method and baked to form upper and lower electrodes, and then, as varistor characteristics, a varistor voltage (E ₁₀ value). And its variation, and the nonlinear coefficient (α) were measured. The measurement results are shown in Table 1.

According to the method of this embodiment, it is possible to mass-produce a varistor having a large variation in varistor voltage (E ₁₀ value) and a large non-linear coefficient as in the case of the second embodiment.

[0037]

[Table 1]

The E ₁₀ value in Table 1 indicates the varistor voltage (V) when the varistor current is 10 mA.

The non-linear coefficient (α) was calculated by the following equation (Equation 1).

[0040]

[Equation 1]

In the equation (1), E ₁₀ is the varistor voltage (V) when the varistor current is 10 mA, and E ₁ is the varistor voltage (V) when the varistor current is 1 mA.

As is apparent from Table 1, the varistor voltage (E ₁₀ value) of each of the inventive examples is almost the same as that of the comparative example 1, but the variation is small and the varistor voltage is large. It was confirmed to have a linear coefficient (α).

Although aluminum oxide (Al ₂ O ₃ ) was used as the aluminum material to be adhered to the surface of the molded body containing strontium titanate as a main component in the above-mentioned embodiment, a material which becomes an acceptor such as sodium instead of aluminum oxide. Any material may be used, and examples of the material include sodium chloride and sodium fluoride.

Further, in the above-mentioned embodiment, the material made of the aluminum material is adhered to both surfaces of the molded body containing strontium titanate as a main component. A material made of an aluminum material may be attached to the side surface in the form of powder, paste, or sheet.

Further, in order to segregate the aluminum material (aluminum oxide Al ₂ O ₃ ) on both surfaces of the molded body containing strontium titanate as a main component, the aluminum material (aluminum oxide Al ₂ O ₃ ) may be contacted at high temperature, for example.

[0046]

As described above, according to the method of the present invention, the contrast between the high resistance layer and the semiconductor layer forming the varistor is clarified. Has the effect that the reduction / reoxidation type varistor having a small varistor characteristic and a very large non-linear coefficient can be manufactured very easily.

[Brief description of drawings]

FIG. 1 is a manufacturing process 1 of an embodiment of the manufacturing method of the present invention.
Explanatory diagram showing a part,

FIG. 2 is an explanatory view showing a part of a manufacturing process of another embodiment of the manufacturing method of the present invention,

FIG. 3 is an explanatory view showing a part of a manufacturing process of another embodiment of the manufacturing method of the present invention.

[Explanation of symbols]

1,3,5 compact, 2 powder, 4
Paste, 6 sheets.

Claims (3)

[Claims] 1. A raw material powder containing strontium titanate as a main component is formed into a compact having a desired shape, and a material containing aluminum as a main component is attached to the surface of the compact, and the compact is then reducible. A method for producing a reductive reoxidation type varistor, which comprises firing in a gas atmosphere to form a sintered body, and heat-treating the sintered body in an oxidizing atmosphere. 2. The material adhered to the surface of the molded body is
The reduction-reoxidation type varistor according to claim 1, which is one of a powder containing aluminum as a main component, a paste containing aluminum as a main component, and a sheet containing aluminum as a main component. Method. 3. The method for manufacturing a reductive reoxidation type varistor according to claim 1, wherein the aluminum is aluminum oxide.