JPH07226307A - Manufacture of strontium titanate varistor - Google Patents

Abstract

PURPOSE:To manufacture a varisotr having less irregularity in a stable manner by adjusting the atmosphere of a diffusion heat treatment. CONSTITUTION:In order to enhance the crystal grain boundary of strontium titanate varistor in resistance a diffusion heat treatment is conducted on a crystal grain boundary using a diffusion agent. The surface of the sintered body obtained by firing is coated with sodium carbonate of 5wt.% against the sintered body of 100wt.% as a diffusion agent. A diffusion heat treatment is conducted for one hour using nitrogen-oxygen mixed gas of total pressure 2atm in the atmosphere of partial pressure of oxygen or total pressure of oxygen of 1.1 to 1.8atm. The temperature of the diffusion heat treatment is used to obtain the current-voltage characteristics of V1mA=240V.

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 strontium titanate varistor.

[0002]

2. Description of the Related Art The manufacturing process of a strontium titanate-based varistor is roughly described as follows. In particular, the characteristics are determined in the steps of firing and diffusion heat treatment. In order to reduce the internal resistance during yielding, firing is performed in a reducing atmosphere, the diffusing agent is diffused into the grain boundary layer of the sintered body, and a high resistance layer is formed on the extremely thin grain boundary surface to improve semiconductor characteristics. In order to obtain it, diffusion heat treatment is performed in the atmosphere.

[0003]

The process that most greatly affects the semiconductor characteristics is the diffusion heat treatment process. When the conventional diffusion heat treatment is performed in the atmosphere, strontium titanate-based varistors such as zinc oxide and The grain boundary layer tends to be unstable compared to a grain boundary layer type capacitor that obtains characteristics by using the grain boundary layer, and the type and amount of the diffusing agent, the distribution of the diffusing agent to the sintered body, and the sintered body The characteristics are affected by the amount of air and the airtightness of the box, and the characteristic values vary widely. Therefore, there is a problem that the characteristics of the varistor are greatly affected and mass production cannot be performed.

An object of the present invention is to provide a method capable of stably producing a varistor with little variation in characteristic values by adjusting the atmosphere in the diffusion heat treatment step.

[0005]

In the diffusion heat treatment step of a diffusing agent for increasing the resistance of a grain boundary in the production of a strontium titanate-based varistor, the atmosphere of the diffusion heat treatment step has an oxygen partial pressure or a total oxygen pressure of 1. The range was 1 to 1.8 atm.

[0006]

[Function] By adjusting the oxygen partial pressure or the total oxygen pressure in the diffusion heat treatment step to 1.1 to 1.8 atm, the current-voltage characteristics,
A varistor with a large capacitance, a small dielectric loss, a small variation in each, and stable electrical characteristics has come to be obtained.

[0007]

EXAMPLE Strontium carbonate, erbium oxide, and titanium oxide were prepared as raw materials, and the composition after firing was (Sr
_A predetermined amount of _0.995 Er _0.005 ) TiO ₃ was weighed and blended, wet-mixed and pulverized with a ball mill, dehydrated and dried. The calcined powder calcined at 1200 ° C for 1 hour is crushed by a dry mill, mixed with vinyl acetate as a binder, and molded and degreased into a disc with a diameter of 10 mm and a thickness of 1 mm after firing with a uniaxial press. Then, it was baked at 1400 ° C. for 3 hours in a gas atmosphere of nitrogen 98% and hydrogen 2%.

Sodium carbonate as a diffusing agent was applied to the surface of the obtained sintered body in an amount of 5% by weight based on 100% by weight of the sintered body, and an oxygen partial pressure was adjusted with a nitrogen-oxygen mixed gas having a total pressure of 2 atm. Diffusion heat treatment was performed for 1 hour in an atmosphere in which the values shown in Table 1 were changed. The diffusion heat treatment temperature has a current-voltage characteristic of V1mA = 240V
So that In Table 1, PO ₂ is oxygen partial pressure, PN ₂
Is the nitrogen partial pressure.

[0009]

[Table 1]

Silver electrodes having a diameter of 8 mm were formed on both surfaces of the sintered body after the diffusion heat treatment, and each sample had a current-voltage (V1mA), non-linear coefficient (α), capacitance (C), dielectric loss (DF). Was measured. The non-linear coefficient is the logarithm of the current ratio between 1mA and 10mA,
The capacitance was measured at 1 kHz-1V. Each variation (3 Cv) is represented by a value obtained from the following equation. 3Cv = (3σ / average value) × 100 (%).

When the oxygen partial pressure is set to 1.1 to 1.8 atm, the nonlinear coefficient, the electrostatic capacitance are large, the dielectric loss is small, and the respective variations are suppressed. This indicates that the high resistance layer is thinly and uniformly formed in the grain boundary layer within this range.

Table 2 shows the measurement results. In Tables 1 and 2, those marked with * are outside the scope of the invention, and others are within the scope of the invention.

[0013]

[Table 2]

The reason why the scope of the invention is limited to the scope of the claims is as follows. It is oxygen that selectively increases the resistance of the grain boundary layer of the sintered body, which is entirely made into a semiconductor, by firing in a reducing atmosphere. The diffusing agent has a function of promoting oxygen diffusion into the grain boundary layer, and the diffusion heat treatment of the diffusing agent is performed to form the grain boundary layer. If the oxygen partial pressure is less than 1.1 atm in the diffusion heat treatment step, the non-linear coefficient and the capacitance are small, the dielectric loss is large, and the respective variations are significantly large. Further, when the oxygen partial pressure is higher than 1.8 atm, the electrostatic capacitance becomes large, but the non-linear coefficient is small, the dielectric loss is large, and the respective variations are significantly large.

In the above embodiment, the diffusion heat treatment atmosphere has been described using the nitrogen-oxygen mixed gas.
The pressure may be 1.1 to 1.8 atm in an atmosphere of 00%.

[0016]

EFFECTS OF THE INVENTION The strontium titanate-based varistor obtained by oxygen pressure according to the present invention has a large capacitance value and a large non-linear coefficient value, and variations of the varistor can be suppressed. Can be produced.

Claims (1)

[Claims] 1. In a diffusion heat treatment step of a diffusing agent for increasing the resistance of a grain boundary in the production of a strontium titanate varistor, the atmosphere of the diffusion heat treatment step has an oxygen partial pressure or a total oxygen pressure of 1.1 to 1. A method for producing a strontium titanate-based varistor, which is in the range of 8 atm.