JPH0773082B2 - Method for producing zinc oxide varistor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a zinc oxide varistor containing zinc oxide as a main component.

2. Description of the Related Art Zinc oxide varistor elements have a large surge current withstanding capability and excellent voltage nonlinearity, and low-voltage type elements are widely used as surge absorbers, and high-voltage type elements are widely used as gapless arrester elements.

Conventionally, zinc oxide type varistor elements are mainly composed of zinc oxide (Zn
O) to bismuth oxide (Bi ₂ O ₃ ) and antimony oxide (Sb
₂ O ₃ ), cobalt oxide (Co ₂ O ₃ ), manganese oxide (Mn
O ₂ ), silicon oxide (SiO ₂ ), etc. are added, an appropriate binder is added, and wet pulverization is performed with a ball mill or the like, followed by granulation, molding and sintering processes. When manufacturing a zinc oxide varistor for an arrester, generally, a material with a varistor voltage per unit thickness (V _1mA / mm) of 200V or more is required for downsizing and cost reduction of the element. Among the above additives, silicon oxide (SiO ₂ ) is known as a substance that suppresses the grain growth of zinc oxide, and also improves the temperature characteristics and the voltage life characteristics, and is a material for high pressure type arrester elements. Is an essential ingredient. As a result of analysis by X-ray microanalyzer and X-ray diffraction, silicon oxide was segregated in the vicinity of triple points between zinc oxide grains in the sintered body in the same manner as antimony oxide, and was present as zinc silicate (Zn ₂ SiO ₄ ). Was confirmed.

Problems to be Solved by the Invention However, when silicon oxide (SiO ₂ ) is used as a starting material for a zinc oxide type varistor, its bulk specific gravity is significantly lighter than other additives, and even if a sedimentable material is used, it is sufficiently crushed. As a result, voids are generated in the sintered body, and there is a problem that the rectangular wave current application test of surge withstand (2 ms), which is one of the most important characteristics of zinc oxide varistor, is low. It was In addition, when silicon oxide (SiO ₂ ) is added in an amount of 1 mol% or more in order to increase the pressure of the material, the viscosity of the pulverized raw material slurry remarkably increases, making it difficult to granulate.

The present invention solves such conventional problems,
The main purpose of the present invention is to improve the performance of a zinc oxide varistor, in particular, to greatly improve the surge withstand capability, and also to provide a method for producing a zinc oxide varistor without gelling of a slurry.

Means for Solving the Problems In order to achieve this object, the present invention adds water and a binder to a raw material powder containing zinc oxide as a main component and at least 1.0 to 10 mol% of powdered silicon as an accessory component. A step of pulverizing to obtain a slurry, a step of drying and granulating the slurry with a spray dryer to obtain a granulated powder, and a step of molding the granulated powder and firing in air Is.

Action According to the above method, since powdered silicon is used, gelation of the slurry can be prevented and granulation can be performed by a spray dryer, so that a large amount of raw material can be processed and stable mass production can be performed.

Further, in the crushing process, since the crushing of silicon proceeds rapidly and uniformly, defects (voids etc.) inside the sintered body are less likely to occur, the surge resistance is extremely high, and the zinc oxide type varistor with few characteristic variations. Can be obtained.

Examples Hereinafter, the manufacturing method of the present invention and the zinc oxide type varistor obtained thereby will be described in detail based on Examples.

First, zinc oxide powder, bismuth oxide 0.
5 mol%, antimony oxide 1.0 mol%, cobalt oxide 0.5
Mol%, manganese oxide 0.5 mol% with 0.5 to 20.0 mol% of powdered silicon added to the raw material powder, solid content ratio of about 60 wt%
Pure water was added so that PVA (polyvinyl alcohol) was added as a binder in an amount of 0.5% by weight based on the solid content, and the whole was put in a ball mill together with zirconia boulders and pulverized for 30 hours to obtain a slurry. This slurry was dried with a spray dryer and granulated to prepare raw material powder. The raw material powder was compression-molded to a size of 40 mm in diameter and 30 mm in thickness, and sintered in air at 1200 ° C. Both end surfaces of the thus obtained sintered body were polished to form a sprayed aluminum electrode. The viscosity of the slurry was measured with a rotary viscometer. Also, for comparison, a system using silicon oxide as a starting material was similarly prepared by the manufacturing process. The figure shows the change in slurry viscosity over time. When 1 mol% of silicon oxide is used as a starting material,
The viscosity exceeds 500 cps in about 5 hours, making it impossible to granulate with a spray dryer. Further, when 5 mol% of silicon oxide was used, it was confirmed that the initial viscosity was already over 500 cps. On the other hand, when 1 mol% of powdered silicon was used as the starting material, it was found that the thickening phenomenon of the slurry was hardly seen. Similarly, no thickening phenomenon was observed when 10 mol% of silicon was used. Therefore, it can be seen that the addition method of powdered silicon enables granulation by a spray dryer for a long time, which is convenient for processing a large amount of raw material.

Next, Table 1 shows the initial characteristics of the sample in which the electrode was formed on the sintered body. V _1mA and V _10μA were measured using a DC constant current power supply. Also, the limiting voltage characteristics are: current peak value 5000A, waveform 8 / 20μs
The impulse was used for measurement. Regarding the initial characteristics, it can be seen that there is almost no difference between the method of adding silicon oxide and the method of adding powdered silicon, except for the thickening phenomenon of the slurry. Further, by adopting the powdered silicon addition method, it is possible to add a large amount of 1 to 10 mol% or more, and it is possible to easily realize the downsizing and cost reduction of the device. Further, as a result of material analysis of the sintered body to which powdered silicon was added, it was confirmed that zinc silicate was present in the vicinity of the triple point between zinc oxides, just as in the case of adding silicon oxide.

Next, a surge withstand test was performed on the above sample. The test condition was that a rectangular wave current of 300 A for 2 ms was repeatedly applied in the same direction at intervals of 5 minutes until the device was broken. The results are shown in Table 2. The values in the table represent the number of times a rectangular wave current was applied without breaking the element, the average value of 5 samples, and the distribution range (maximum value-minimum value).

From Table 2, it can be seen that by changing the starting material from silicon oxide to silicon, the surge resistance doubles and the performance is remarkably improved. Further, it can be seen that the distribution range is narrower than the average value and the reliability of the element is improved. This is because, when silicon is used as the starting material, crushing progresses more quickly and uniformly than when silicon oxide is used, so that the occurrence of voids inside the sintered body is reduced and the zinc silicate is evenly distributed. It is considered that the current distribution was made uniform by the distribution. Moreover, the surge withstand amount gradually decreases in contrast to the increase in the amount of silicon added, because the addition of silicon increases the varistor voltage per unit thickness (V _1mA / mm), which increases the energy applied per unit volume. Although it is considered that the sample is easily destroyed by thermal stress, it has a great effect in terms of performance and size and cost reduction of the device as compared with the conventional sample.

Effects of the Invention According to the present invention, since at least 1.0 to 10 mol% of powdered silicon is used, gelation of the slurry is prevented, and granulation by a spray dryer is possible, so that a large amount of raw material can be processed and stable. Mass production is possible.

Further, in the crushing process, since the crushing of silicon proceeds rapidly and uniformly, defects (voids etc.) inside the sintered body are less likely to occur, the surge resistance is extremely high, and the zinc oxide type varistor with few characteristic variations. Can be obtained.

[Brief description of drawings]

The figure is a diagram showing changes with time in viscosity of raw material slurries obtained by the present invention and the conventional manufacturing method.

Claims (1)

[Claims] 1. A zinc oxide varistor containing zinc silicate in a sintered body, wherein water and a binder are added to a raw material powder containing zinc oxide as a main component and at least 1.0 to 10 mol% of powdery silicon as an accessory component. Oxidation comprising a step of pulverizing to obtain a slurry, a step of drying and granulating the slurry with a spray dryer to obtain a granulated powder, and a step of molding the granulated powder and firing it in air. Manufacturing method of zinc varistor.