JPS5941287B2 - Manufacturing method of voltage nonlinear resistance element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a voltage nonlinear resistance element containing zinc oxide (ZnO) as a main component.

BACKGROUND ART Voltage nonlinear resistance elements (hereinafter referred to as varistors) are used as lightning arresters and overvoltage protection elements.

When used for such purposes, the varistor should be (1
) The voltage nonlinearity index α is large (however, α is I=(
V/C)α, where ■ is current, ■ is voltage, and C is a constant.

) (2) Good limiting voltage characteristics (Limiting voltage is expressed as the ratio of voltage at large current to voltage at minute current,
In the case of normal lightning arresters, the voltage at 100A to 10KA ■1oooA to Vt0KA and the voltage V at 1mA
Expressed as a ratio of 1 mA.

) (3) Large surge energy withstand capacity (surge energy withstand capacity refers to the maximum surge energy that can be withstood at one time).

) is desirable. Among conventionally known varistors, SiC varistors made of silicon carbide baked and hardened at high temperatures have an α of 3 to 7.
The limiting voltage ratio is also extremely poor.

Therefore, in recent years, ZnO varistors containing ZnO as a main component have been increasingly used.

ZnO varistor has ZnO as the main component and 0.5
~1.0 mol% of 5i203. Co2032MnO
It is obtained by adding and mixing 2,5b203°Cr2O3, etc., molding it in a mold, and firing it in air at a temperature of 1000°C or higher.

This ZnO varistor has a large α, excellent limiting voltage characteristics and surge energy resistance, and is widely used in overvoltage protection elements and lightning arresters.

Therefore, recently, attempts have been made to apply it to lightning arresters for high-voltage power.

In order to apply it to high-voltage power, it is necessary to increase the size of the device.

However, as the size of the device increases, the excellent properties of the device deteriorate, and there is a phenomenon in which the characteristics do not improve much despite the increase in size.

In view of this situation, the present invention aims to provide a method for manufacturing a voltage nonlinear resistance element that does not deteriorate in characteristics even when increased in size and has excellent voltage nonlinearity index, limiting voltage characteristics, and surge energy withstand capacity. Details are shown together with examples.

The most standard material compositions often used for overvoltage protection and lightning arresters are Zn0 (97.0 mol%), B I
203 (0,5 mol%), Co203 (0,5
mol%), M n 02 (0.5 mol%), 5b20
3 (1.0 mol%) and Cr2O3 (0.5 mol%) were selected and the conventional method and the method of the present invention were applied, and the characteristics of the obtained devices were compared and studied.

A normal ZnO barisk is manufactured by the following procedure.

First, the above materials are thoroughly ground and mixed using a ball mill, etc., and an organic binder such as polyvinyl alcohol is added to maintain the strength of the molded product, so that the average particle size is 50 to 150.
The granules are granulated to a size of approximately μm, and then placed in a cylindrical mold and compressed into a disk shape.

The molding pressure is usually 250 to 400 kg/=.

Next, it is placed in an electric furnace, and the temperature is raised to 1150°C to 13000C at a heating rate of 100 to 200°C/hour, and sintering is performed for 2 to 5 hours.

After polishing both sides of the disk-shaped sintered body thus obtained,
A sprayed aluminum electrode is provided over the entire sintered body, leaving about two beads at the edges.

By such a method, disk-shaped pieces up to about 20 mm in diameter and 5 mm in thickness can be obtained with relatively good characteristics.

However, if the diameter is made larger or the thickness is made thicker, the characteristics per unit area or per unit volume deteriorate, and the overall characteristics do not improve much even though the element is made larger.

FIG. 1 shows changes in α, limiting voltage ratio, and surge energy withstand capacity when the thickness of the element is 10 mm and the diameter of the element is changed from 5 mm to 112 mm in the conventional manufacturing method.

In both cases, the area is corrected and the current flowing per unit area is kept constant, and α, the limiting voltage ratio, and the surge energy withstand capacity per unit volume are shown.

Specifically, α is 1c11t, 0.1mA and 1mA
The limiting voltage ratio is expressed as the voltage ratio when currents of 100 OA and 1 mA flow in 1d.

In addition, for voltage measurement at 1000A, 8×2
Measurements were performed using an impact current of 0, s, and direct current for other measurements.

In addition, the surge energy withstand capacity is determined by using an impact current with a long wave tail of 2 mS or more and determining the value at which the element is destroyed.

In Fig. 1, when the diameter of the element becomes 20 m or more, α,
It is recognized that both the limiting voltage ratio and surge energy withstand characteristics have decreased.

FIG. 2 shows α, limiting voltage ratio, and surge energy withstand capacity when the element diameter is 56 mm and the element thickness is varied from 5 mm to 50 mm.

When the thickness of the element becomes 5 mm or more, α, limiting voltage ratio, and surge energy withstand capacity also decrease.

We also analyzed the uniformity of devices manufactured using conventional methods.

FIG. 3 shows the characteristic distribution of a large-sized element manufactured by the conventional manufacturing method.

This was determined by cutting out five squares in the thickness direction and six squares in the diameter direction from the center of an element having a diameter of 56 mm and a thickness of 40 mm, and measuring the α and limiting voltage ratio of each part.

It can be seen that internally, the limiting voltage ratio is decreasing.

Moreover, FIG. 4 shows the results of measuring the distribution of voltage V1 mA at 1 mA in each part for an element having a diameter of 112 mm and a thickness of 1 mm.

It is thought that when the diameter is increased, the uniformity of the resistance distribution within the cross section becomes poor, resulting in concentration of current and a decrease in the surge energy withstand capacity per unit volume.

Next, the manufacturing method according to the present invention will be described.

The material with the same composition as above is crushed in the same way as the conventional method.
After mixing and granulation, 9/er to 100 using a cylindrical mold
Compression molding is performed into a disk shape at a pressure of a to preform.

Next, this preform is placed in a rubber bag, and after removing the air inside with a vacuum pump, the rubber bag is sealed.

Next, after storing this in a pressure vessel filled with water, 500 kg/cr? t.

Main molding is performed by applying pressure.

Next, the molded body was taken out from the rubber bag, heated in air at 500°C for about 2 hours to remove the binder, and then placed in an electric furnace and heated to 1200°C at a heating rate of 50°C/hour.
The temperature is maintained for 5 hours to allow sintering.

The subsequent steps are the same as the conventional method.

The characteristics of the device thus obtained are shown in Figures 5 and 6.
As shown in the figure.

Figure 5 shows the characteristics of each element when the thickness of the element is 10rrtm and the diameter is changed, and Figure 6 shows the characteristics of each element when the diameter of the element is 55mm.
The characteristics of each element are shown when the thickness is changed.

In either case, by manufacturing with this method, the deterioration in characteristics when the device is increased in size is significantly improved.

Next, we investigated the relationship between the preforming pressure by the mold and the main molding pressure by the pressurized liquid.

Table 1 shows the measurement results of each characteristic when the preforming pressure and the main molding pressure were varied for an element having a diameter of 56 mm and a thickness of 107 nm.

From Table 1, it can be seen that there is no effect of improving the properties unless the main molding pressure applied later is higher than the preforming pressure, and the preforming pressure is 500kL! It can be seen that when the value becomes larger than /d, the improvement effect becomes weaker.

In addition, when the preforming pressure is less than 50 kg/i, the strength of the molded product is weak and handling is extremely difficult1.Therefore, the preforming pressure is 50 to 500 kg/C7? The main molding pressure must be greater than the preforming pressure.

Moreover, the effect is weak unless the main molding pressure is 200 kg/cn or more, and the properties are saturated when it is 2000 kg/i or more, and there is not much improvement effect.

Therefore, the main molding pressure is preferably 200 to 2000 kg/i.

The above-mentioned improvements in properties take advantage of the fact that the pressure distribution during molding is strongly related to the properties of large elements.

In other words, in the case of mold forming, it is -axial pressure,
Additionally, due to the frictional force acting between the mold part and the molded object, the pressure applied on the surface and inside of the molded object will be very different, and if the diameter is increased, uneven filling of powder may occur. It is thought that the uniformity of the element is impaired and the characteristics are deteriorated due to the pressure distribution caused by this.

In contrast, in the method according to the present invention, the shape is determined by preforming with a mold, and then pressure is applied isotropically in a pressurized liquid to remove the pressure distribution caused by molding, so that uniformity is achieved. It is considered that the characteristics are improved and the characteristics deteriorate less even if the device is made large.

By the way, even if a molded body with a uniform pressure distribution obtained in this manner is used, the effect may be weakened depending on the firing conditions.

In other words, if the heating rate is too fast, a temperature difference will occur between the inside and the outside, and if that temperature difference becomes too large, the sintering reaction will be different between the inside and outside, and even if a uniform compact is used, This is thought to be because the sintered body becomes non-uniform.

Therefore, changes in characteristics were investigated by changing the heating rate during firing for an element with a diameter of 56 mm and a thickness of 50+u+.

The results are shown in Table 2. It can be seen from Table 2 that if the temperature increase rate per hour is 80° C. or less, the phenomenon of deterioration of properties during sintering can be avoided.

Furthermore, if the device becomes large in size, the binder may not be completely removed when the temperature is increased during firing, so if the binder is not removed before firing, the characteristics will deteriorate.

In particular, in the method of the present invention, the main molding pressure is higher than the preforming pressure, so high pressure is applied, for example,
If a pressure of 0.00 kg/cyyt or more is applied, it becomes difficult for the binder to escape when the temperature is increased during firing, so it is necessary to remove the binder before firing.

The temperature for removing the binder is preferably 200 to 700°C for about 1 to 5 hours.

This is because the binder does not easily dissipate at temperatures below 200°C, and sintering begins at temperatures above 700°C.

In addition, in the above embodiment, water was used as the pressurizing medium during main molding using pressurized liquid, but considering the principle, any medium that functions similarly to water, that is, any liquid may be used.
It is clear that the same effect can be expected by using oil, for example.

In addition, it is necessary to completely isolate the molded body from water during pressurization, and the isolation means must be such that the water pressure is sufficiently transmitted to the molded body and that it can deform to follow changes in the shape of the molded body. Must have contractility.

A rubber bag is most suitable for such a bag, and after housing the molded product, the inside of the bag is evacuated so that water pressure can be sufficiently transmitted to the molded product.

However, the bag is not limited to a rubber bag, and other means may be used as long as they meet the above conditions.

According to the method for manufacturing a voltage nonlinear resistance element of the present invention, as is clear from the above explanation, an element with no deterioration in characteristics can be obtained even in a large-sized element, and therefore a large-sized element such as a surge arrester for high-voltage power is not required. The effect in the field is dogs.

[Brief explanation of the drawing]

Figures 1 to 4 show the time characteristics of elements manufactured by conventional methods, Figure 1 is a graph showing the relationship between element diameter and characteristics, and Figure 2 is a graph showing the relationship between element thickness and characteristics. 3 is a diagram showing the characteristic distribution in the vertical cross section of the device, FIG. 4 is a diagram showing the characteristic distribution in the cross section, and FIGS. 5 and 6 are graphs showing the characteristic distribution in the longitudinal section of the device. The characteristics of the device are shown.
Figure 5 is a graph showing the relationship between element diameter and characteristics;
The figure is a graph showing the relationship between element thickness and characteristics.

Claims (1)

[Claims] 1. A mixture of zinc oxide and additives granulated with a binder is preformed in a mold, and this preform is subjected to main molding using a pressurized liquid at a pressure higher than the preforming pressure. A method for manufacturing a voltage nonlinear resistance element, which comprises removing the binder after completion of molding, and then sintering by gradually raising the temperature. 2. Is the preforming pressure 50 to 500 kg/Cr? The manufacturing method according to claim 1, characterized in that the main molding pressure is 200 to 2000 kg/Cr4. 3. The method according to claim 1, characterized in that the temperature increase rate during sintering is 80° C. or less per hour. 4. The preform is placed in a flexible and contractible airtight container, the container is evacuated and sealed, and the container is placed in a pressurized container filled with liquid, and then pressure is applied to the liquid. 2. The manufacturing method according to claim 1, wherein the final molding is carried out using a wafer. 5. The manufacturing method according to claim 1, wherein the binder removal temperature is 200 to 700°C.