JP3223462B2 - Method for producing reduction-reoxidation type varistor - Google Patents

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 re-oxidation 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 molded body having a desired shape, and then, for example, H _{2 is} added. 5 vol%, heat-treated at a temperature of 1300 to 1500 ° C. in N ₂ 95 vol% of a reducing gas atmosphere, reoxidation heat treatment at 800 to 1100 ° C. at a strontium titanate semiconductor ceramic obtained by forced reduction again in the air Then, a method of reoxidizing a thin film on the surface of the semiconductor ceramic to increase the resistance is known.

Further, sodium (Na) or bismuth oxide (Bi) is applied to the surface of a semiconductor ceramic obtained by forced reduction.
₂ O ₃ ) is applied, followed by heat treatment at a temperature of 1000 to 1200 ° C. in the air to diffuse sodium and bismuth oxide from the surface of the ceramic along the grain boundaries.

[0004]

Among the above-mentioned methods, varistors of the type manufactured by the former method of performing a heat treatment in a reducing gas atmosphere and then performing a re-oxidation heat treatment are generally subjected to forced reduction and then performed in air. Varistor characteristics can be obtained only by re-oxidation heat treatment, so that it is suitable for mass production at low cost. However, a disadvantage is that the index indicating the performance of the varistor, that is, the nonlinear coefficient is small.

On the other hand, sodium or bismuth oxide is applied to the surface of the semiconductor ceramic obtained by the latter forced reduction in the above-mentioned method, and then heat-treated in the air to remove sodium from the ceramic surface along the grain boundaries. And varistors manufactured by the method of diffusing bismuth oxide have a large nonlinear coefficient, but require a process of applying sodium or bismuth oxide.
Since the applied sodium and bismuth oxide partially evaporate, the atmosphere changes depending on the amount of heat treatment, and the resulting varistors have disadvantages such as large variations in the characteristic values, resulting in extremely low production costs. There is a problem that becomes higher.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for manufacturing a reduced reoxidation type varistor containing strontium titanate as a main component, which has a large nonlinear coefficient at a low cost and which solves the above problem.

[0007]

According to the present invention, there is provided a method for producing a reduction-reoxidation type varistor, wherein a raw material powder containing strontium titanate as a main component is formed into a compact having a desired shape, and aluminum is formed on the surface of the compact. The method is characterized in that after the material as the main component is adhered, the molded body is fired in a reducing gas atmosphere to form a sintered body, and the sintered body is subjected to a heat treatment in an oxidizing atmosphere. The material to be adhered to the surface of the molded body may be any one of powder, paste, and sheet containing aluminum as a main component. Further, aluminum may be aluminum oxide.

[0008]

The 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 segregated aluminum is contained in the surface of a strontium titanate sintered body which has been made into a semiconductor, aluminum ions having an ionic radius slightly different from that of titanium ions are converted into strontium titanate (SrTiO).
₃ ) It forms a solid solution in the titanium (Ti) site, and this works as an acceptor of the strontium titanate semiconductor ceramic. Therefore, it is considered that if aluminum is segregated near the surface of the ceramic sintered body that has been made into a semiconductor, this acts as an acceptor and forms an electric depletion layer on the surface.

[0010] The re-oxidation type varistor is such that as the distinction between the high resistance layer and the semiconductor layer inside the sintered body becomes clear by re-oxidation,
The nonlinear coefficient increases, but if the metal element acting as an acceptor is segregated on the surface of the sintered body that increases resistance,
It is thought that this forms a depletion layer. Therefore, when heat treatment is performed on the ceramic sintered body that has been reduced and converted to a semiconductor, the high resistance layer on the surface of the sintered body after the heat treatment and the internal semiconductor layer are formed. Becomes clearer.

[0011]

The strontium titanate-based material used in the method of the present invention is not particularly limited as long as it is a material normally used as a varistor material. Further, as a material containing aluminum as a main component to be adhered to the surface of the strontium titanate compact, aluminum oxide (Al
₂ O ₃ ), aluminum sulfate (Al ₂ (SO ₄ ) ₃ ), aluminum hydroxide (Al (OH) ₃ ) and the like. Aluminum oxide, which is inexpensive and has excellent stability, taking into consideration that it does not evaporate during heat treatment, low cost, and the like, is most suitable.

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 as follows. 1 to 10% by volume: 99 to 90% by volume, and the temperature may be about 1350 to 1500 ° C.
In addition, for example, air is used as an atmosphere when the sintered body is subjected to the re-oxidation heat treatment in the oxidizing atmosphere.
The temperature may be about 0 to 1100 ° C.

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

Embodiment 1 This embodiment shows an example in which a material containing aluminum as a main component is applied to the surface of a molded product containing strontium titanate as a main component by powder.

First, strontium carbonate (SrCO ₃ ) 6
4.76 parts by weight, 35.05 parts by weight of titanium oxide (TiO ₂ ), 0.06 parts by weight of niobium oxide (Nb ₂ O ₅ ) and silicon oxide (SiO ₂ )
₂ ) 0.13 parts by weight was added to prepare a varistor raw material powder. Next, the raw material powder was wet-mixed by a ball mill using stabilized zirconia balls and water for 10 hours. The mixture was calcined at 1200 ° C. for 5 hours in the air, and then wet-pulverized again by 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 is added and mixed. The mixture is press-molded with a dry molding press at a pressure of 1.5 t / cm ² , and has a diameter of 12.5 mm and a thickness of 12.5 mm. A 1.1 mm disk-shaped compact 1 (see FIG. 1) was prepared. Powder 2 (trade name AL-7 of aluminum oxide created disc-shaped surface of the molded body 1 (Al ₂ O ₃₎ is the main component, the average particle diameter of 0.5 [mu] m, the aluminum oxide content of 99.7%, Sumitomo Chemical Made by Co., Ltd., see FIG. 1) and adhered to both surfaces of the molded body 1.

The compact 1 having aluminum powder 2 adhered to both surfaces (see FIG. 1) was placed in a reducing gas atmosphere of 4% by volume of hydrogen gas (H ₂ ) and 96% by volume of nitrogen gas (N ₂ ). Temperature 1
A firing process of forced reduction was performed at 450 ° C. for 5 hours to produce a sintered body made of ceramic ceramics. The sintered body made of semiconducting ceramic porcelain was
A reoxidation heat treatment was performed at 00 ° C. for 2 hours to produce a reduced reoxidation type varistor.

[0018] Then a paste of In-Ga alloy was applied in accordance with respective ordinary method on both surfaces of the created reduced reoxidation varistor, after forming the upper and lower electrodes and baking, the varistor voltage ₍₁₀ value E) as varistor characteristics And its variation, and the nonlinear coefficient (α) were measured. Table 1 shows the measurement results.

According to the method of this embodiment, varistors having a large nonlinear coefficient can be manufactured inexpensively and in large quantities.

Comparative Example 1 A reduction / reoxidation type varistor was prepared in the same manner as in Example 1 except that no powder mainly containing aluminum was attached to the surface of the molded article mainly containing strontium titanate.

[0021] Then a paste of In-Ga alloy was applied in accordance with respective ordinary method on both surfaces of the created reduced reoxidation varistor, after forming the upper and lower electrodes and baking, the varistor voltage ₍₁₀ value E) as varistor characteristics And its variation and the nonlinear coefficient (α) were measured under the same conditions as in Example 1. Table 1 shows the measurement results.

Embodiment 2 This embodiment shows an example in which a material containing aluminum as a main component is applied to the surface of a molded product containing strontium titanate as a main component by using a paste.

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

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

A molded body 3 (see FIG. 2) having an aluminum material paste 4 adhered to both surfaces was converted to a 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, a firing treatment of forced reduction for 5 hours is performed,
A sintered body made of semiconducting ceramic porcelain was prepared.

The resulting sintered ceramic ceramic body was subjected to a re-oxidation heat treatment at a temperature of 1000 ° C. for 2 hours in the atmosphere to produce a reduced re-oxidation type varistor.

[0027] Then a paste of In-Ga alloy was applied in accordance with respective ordinary method on both surfaces of the created reduced reoxidation varistor, after forming the upper and lower electrodes and baking, the varistor voltage ₍₁₀ value E) as varistor characteristics And its variation, and the nonlinear coefficient (α) were measured. Table 1 shows the measurement results.

[0028] According to the method of this embodiment, a small variation in varistor voltage ₍₁₀ values E), it is possible to produce a large varistor nonlinear coefficient.

Embodiment 3 This embodiment 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 by using a sheet.

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

Next, separately from this, a powder mainly composed of aluminum oxide (Al ₂ O ₃ ) (trade name: AL-7, average particle size: 0.5 μm, aluminum oxide content: 99.7%, Sumitomo Chemical Co., Ltd.) ), A polyacrylic acid-based binder as a binder, a polyammonium acrylate as a dispersant, and water in a mixing ratio of 1: 0.5: 2: 10 by weight and stirring. Thickness by doctor blade method
Sheet into 50μm, and sheeted sheet is 12.5mm in diameter
An aluminum sheet material 6 having a disk shape (see FIG. 3) was formed by punching into a disk shape 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 (the molded body 5 has 10 layers, and the aluminum sheet material 6 has 11 layers, see FIG. 3).

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

The resulting sintered ceramic ceramic body was subjected to a re-oxidation heat treatment in air at 1000 ° C. for 2 hours to produce a reduced re-oxidation type varistor.

[0035] Then a paste of In-Ga alloy was applied in accordance with respective ordinary method on both surfaces of the created reduced reoxidation varistor, after forming the upper and lower electrodes and baking, the varistor voltage ₍₁₀ value E) as varistor characteristics And its variation, and the nonlinear coefficient (α) were measured. Table 1 shows the measurement results.

[0036] According to the method of this embodiment, the second embodiment similarly to small variations in the varistor voltage ₍₁₀ values E), it is possible to produce a large varistor nonlinear coefficients for mass production.

[0037]

[Table 1]

[0038] A varistor voltage (V) when the varistor current of 10mA and E ₁₀ values in the Table 1.

The nonlinear coefficient (α) was obtained by the following equation (Equation 1).

[0040]

(Equation 1)

[0041] In equation 1, the varistor voltage when the varistor current of 10mA and E ₁₀ (V), also varistor voltage when the varistor current of 1mA (V) of each indicating the E _1.

[0042] As apparent from Table 1, the present invention embodiment in comparison with Comparative Example 1, but none varistor voltage ₍₁₀ value E) are substantially the same value, the variation is small and a large non It was confirmed that it had a linear coefficient (α).

In the above embodiment, 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, but instead of aluminum oxide, a material serving as an acceptor such as sodium was used. Any material may be used, for example, sodium chloride, sodium fluoride, and the like.

In the above embodiment, a material made of aluminum is attached to both surfaces of a molded body containing strontium titanate as a main component. For example, in order to manufacture a varistor using a side surface, the molded body is made of the strontium titanate. A material made of an aluminum material may be attached to the side surface in powder, paste, or sheet form.

Further, in order to segregate an aluminum material (aluminum oxide Al ₂ O ₃ ) on both surfaces of a molded body containing strontium titanate as a main component, an aluminum material (aluminum oxide) is applied to a sintered body made of semiconductor ceramic ceramic. Al ₂ O ₃ ) may be brought into contact, for example, at a high temperature.

[0046]

As described above, according to the method of the present invention, the contrast between the high-resistance layer and the semiconductor layer constituting the varistor is clarified, so that the varistor voltage varies despite the reduction / reoxidation type varistor. In addition, there is an effect that it is possible to extremely easily manufacture a reduction-reoxidation type varistor having varistor characteristics having a small varistor characteristic and a very large nonlinear coefficient.

[Brief description of the drawings]

FIG. 1 shows 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 according to another embodiment of the manufacturing method of the present invention.

[Explanation of symbols]

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

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01C ^7/ 02-7/22

Claims (3)

(57) [Claims] 1. Forming a raw material powder containing strontium titanate as a main component into a compact having a desired shape, attaching a material containing aluminum as a main component to the surface of the compact, and then reducing the compact with a reducing property. A method for producing a reduced reoxidation type varistor, characterized in that a sintered body is formed by firing in a gas atmosphere, and the sintered body is subjected to a heat treatment in an oxidizing atmosphere. 2. The material adhered to the surface of the molded body,
2. The reduction / reoxidation type varistor according to claim 1, wherein the varistor 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 according to claim 1, wherein the aluminum is aluminum oxide.