JPH0136681B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a voltage non-linear resistor containing zinc oxide as a main component and a metal oxide such as bismuth oxide.

Voltage nonlinear resistors are widely used in surge absorbing elements, voltage stabilizing elements, lightning arresters, etc., and in recent years, voltage nonlinear resistors made of oxide sintered bodies containing zinc oxide as a main component have been developed. This voltage nonlinear resistor is generally made by mixing zinc oxide with trace additives such as bismuth oxide, cobalt oxide, chromium oxide, antimony oxide, etc., press-forming it, and then sintering it at 1,100 to 1,300°C. It is made by attaching. For details on compounding compositions, manufacturing methods, etc., see Japanese Journal of Applied Physics [M. Matsuoka, “Nonohmic Properties of
Zinc Oxide Ceramics”, Jap.J.Appl.Phys., 10
[1971) 736].

This voltage nonlinear resistor whose main component is zinc oxide has a structure in which zinc oxide particles are surrounded by a boundary layer (grain boundary layer) whose main component is bismuth oxide.When a voltage is applied, this grain boundary layer It is believed that excellent voltage nonlinearity appears due to the electrical barrier formed between the zinc particles.

In general, the voltage-current characteristics of a voltage nonlinear resistor are approximately expressed by the following formula: I=KV〓 …(1) (1) In the formula, I is current, K is a constant, V is voltage, and α is a nonlinear coefficient. It is. The voltage nonlinear resistor whose main component is zinc oxide has an α value of 5 to 60, which is considerably larger than the α value of 3 to 4 for the conventional nonlinear resistor made of silicon carbide. Since the voltage applied between the zinc oxide particles is almost constant in the region, simply changing the thickness of the voltage nonlinear resistor
It has the characteristic of being able to freely change V1mA (the voltage required to flow 1mA of current), and its applications are increasingly being expanded.

However, when a constant DC or AC voltage is applied to this voltage nonlinear resistor element, which has such advantages, the leakage current flowing through the element generally increases with time, and as the leakage current increases, Due to the heat generation of the element, thermal runaway occurred, which eventually led to element destruction, which resulted in a limit to its lifespan. Particularly in the case of gear press zinc oxide type lightning arresters, element destruction due to increased leakage current is an extremely important issue, so the deterioration of the elements due to the above-mentioned voltage application should be suppressed as much as possible to minimize the increase in leakage current. It is desirable to obtain long-life devices.

As a method to improve the life of the voltage nonlinear resistor when DC or AC voltage is applied as described above, after sintering at 1100℃ or higher, this sintered body is heated to 400℃.
A method is known in which a so-called post-firing heat treatment is performed in which the material is cooled to a temperature lower than 100°C at a rate of 100°C/hour, then reheated to a temperature range of 400 to 700°C, and then recooled to room temperature. (Japanese Unexamined Patent Publication No. 52-87695) However, in this heat treatment method, it is necessary to cool the sintering temperature from a high sintering temperature of about 1100°C to a temperature lower than 400°C, and the cooling rate is about 100°C per hour. Therefore, it takes a long time to cool down, and it also has the drawback that there is a lot of energy loss.

In the present invention, as a result of repeated studies regarding the cooling temperature, reheating temperature, and temperature decreasing rate using the temperature pattern shown in Figure 1, the temperature decreasing rates R ₁ and R ₂ have changed from the sintering temperature T ₁ to 600℃. In the temperature range approximately 100-300 ° C per hour, cooling temperature T ₂ is 400-530 ° C, reheating temperature T ₃ regardless of its speed.
It has been found that an effective heat treatment that improves the above-mentioned energized life characteristics can be carried out by setting the temperature in the range of 600 to 900°C, leading to the completion of this invention. Moreover, since the cooling time can be significantly shortened compared to the heat treatment method described above, it has become possible to perform heat treatment with less energy loss.

Furthermore, it is known that the heat treatment conditions greatly depend on the composition of the sintered body (Japanese Unexamined Patent Application Publication No. 1989-1999).
61214), as a result of the firing treatment according to the present invention, in addition to additives containing zinc oxide as a main component and various metal oxides such as bismuth oxide and antimony oxide, zinc borosilicate glass or borosilicate It was found that a composition containing bismuth glass was more effective in improving the charging life characteristics.

The present invention was made in order to eliminate the drawbacks of the conventional heat treatment method as described above, and by considering the firing process, in particular, the cooling temperature after sintering, the reheating temperature, and the composition are selected within a preferable range. The purpose of this invention is to provide a manufacturing method that improves the energized life characteristics of a voltage nonlinear resistor element containing zinc oxide as a main component, can be produced with high yield, and has little energy loss.

This invention will be explained below with reference to Examples.

Example 1 Starting materials include zinc oxide (ZnO), bismuth oxide (Bi ₂ O ₃ ), cobalt oxide (Co ₂ O 3 ), manganese carbonate (MnCO ₃ ), and chromium oxide (Cr ₂ O ₃ ) with a purity of 99% _or more. , antimony oxide (Sb ₂ O ₃ ), silicon dioxide (SiO ₂ ), and powders of bismuth borosilicate glass or zinc borosilicate glass. Note that the bismuth borosilicate glass and the zinc borosilicate glass were each created by heating and melting at 1200 to 1300°C and then rapidly cooling, and after crushing, they were passed through a 400-mesh sieve, and only fine powders with uniform particle sizes were used.

A predetermined amount of these powders were weighed, mixed, granulated, pressure molded, and then sintered in air at 1200°C for about 2 hours, and then the sintered body was cooled at a rate of temperature drop of about 200°C per hour. Immediately after cooling to 500°C, reheating was started and maintained at 600 to 900°C for about 1 hour, followed by cooling at a temperature decreasing rate of about 200°C per hour. Electrodes were attached to the sintered body thus obtained to form a voltage nonlinear resistor element.

Direct current was applied to the voltage nonlinear resistor element whose main component was zinc oxide, and FIG. 2 shows the change over time in the leakage current flowing through the element at that time. The charging condition is the charging rate (=applied voltage/device
The voltage when 1mA flows) = 0.8, and the ambient temperature was 100°C. In the figure, 1 is an element that is cooled to room temperature after firing and is not subjected to heat treatment, and 2, 3, and 4 are elements that have been subjected to heat treatment according to the above-mentioned invention, and each has a reheating temperature T ₃ of 600.
℃, 700℃, and 800℃ are shown. Also 2′,
3' and 4' are elements that have been subjected to conventional heat treatment in which the elements are cooled to a temperature lower than 400°C and then reheated to 600°C, 700°C, and 800°C, respectively.

As can be seen from the figure, in the case of an element that is not subjected to heat treatment, the change in leakage current over time is large and a thermal runaway phenomenon occurs in a short time, whereas the element subjected to the above-mentioned heat treatment according to the present invention has a small change in leakage current over time. It can be seen that there is almost no difference when compared with elements made using conventional heat treatment methods, and that the life characteristics are excellent.

Example 2 The cooling temperature _T2 after sintering was kept at 510°C, the reheating temperature _T3 was changed from 550°C to 1000°C, and the cooling rate was set at 200°C.
FIG. 3 shows the change in leakage current over time when direct current was applied to the heat-treated element at a rate of .degree. C./hour. Note that the charging conditions are the same as in the first embodiment. In the figure, _T3 is 550℃, 600℃, 700℃, respectively.
Shows elements at 800℃, 900℃, and 1000℃.

As shown in the figure, those with _T3 of 600℃ to 900℃ have a small change in leakage current over time and have good charging life characteristics.
Those with T ₃ of 550°C and 1000°C are both undesirable because the leakage current changes significantly over time and thermal runaway occurs in a short period of time.

Therefore, the optimal reheating temperature _T3 is 600℃~900℃
℃ range.

Example 3 The cooling temperature _T2 after sintering was changed from 400°C to 550°C, the reheating temperature _T3 was 600°C, and the cooling rate was 200°C.
FIG. 4 shows the change in leakage current over time when direct current was applied to the device at ℃/hour. The charging conditions are the same as in the first embodiment. 1 to 5 in the figure are respectively
Devices with T ₂ of 400°C, 450°C, 500°C, 530°C, and 550°C are shown. As can be seen from the figure, all those with T ₂ of 530°C or less have small changes in leakage current over time and have good charging life characteristics, but those with T 2 of 550°C have large changes in leakage current over time, which is not preferable. Therefore the cooling temperature
It can be seen that T ₂ must be at least 530°C or lower.

Example 4 Temperature cooling rate R ₁ and R ₂ after sintering and reheating were set to 1
FIG. 5 shows the change in leakage current over time when direct current is applied to the device when heat treatment is performed by cooling the device at a temperature varying from 100° C. to 300° C. per hour. The charging conditions are the same as in the first embodiment. In the figure, 1, 2, and 3 have R ₁ (=R ₂ ) of 100℃/hour, 200℃/hour, and 300℃/hour, respectively.
Show the time. From the figure, there is almost no difference in the temperature decrease rate with respect to the change in leakage current over time.
It can be determined that the charging life characteristics are almost the same.

In this example, the cooling temperature was 500°C and the reheating temperature was 600°C, but if the cooling temperature was limited to 400 to 530°C and the reheating temperature to 600 to 900°C, almost the same results as above could be obtained. obtained, the cooling rate is 100~300
It has been found that when the heating rate is set to ℃/hour, heat treatment can be performed to obtain an element with good charging life characteristics regardless of the speed. It is not preferable to increase the cooling rate to more than 300° C./hour because it may cause variations in characteristics (voltage nonlinearity, energized life characteristics, etc.) between elements.

Example 5 When direct current was applied to a voltage nonlinear resistor element whose main component was zinc oxide, which was created by varying the amount of zinc borosilicate glass added as a starting material from 0 to 1.0 weight, Fig. 6 shows the change over time in the leakage current flowing in the capacitor. The heat treatment conditions are _T2
480°C, T ₃ is 800°C, and R ₁ and R ₂ are both 150°C/hour. Further, the charging conditions are the same as in the first embodiment. 1 in the diagram
~6, the amount of the above glass added is 0,
Elements with 0.01, 0.05, 0.1, 0.5, and 1.0% by weight are shown. As shown in the figure, when the amount of glass added is 0.01 to 0.5% by weight, the change in leakage current over time is small and the charging life characteristics are good, but when it is less than 0.01% by weight, leakage occurs despite heat treatment. The change in current over time becomes large and thermal runaway occurs in a relatively short period of time. If the amount exceeds 0.5% by weight, the change in leakage current over time is small, but the voltage nonlinearity deteriorates, making it unsuitable as a power device.

Therefore, the amount of the glass added that makes the heat treatment according to the present invention most effective in improving the charging life characteristics is in the range of 0.01 to 0.5% by weight.

Although zinc borosilicate glass was used as the glass additive in this example, similar results were obtained using bismuth borosilicate, and by implementing the heat treatment method of the present invention, an element with excellent charging life characteristics was obtained. I found out what I could get.

By the way, when AC voltage is applied to the voltage nonlinear resistor element that has been subjected to the heat treatment described above, the leakage current flowing through the element changes over time, showing almost the same characteristics as when DC voltage is applied. The above heat treatment effect was observed.

When performing heat treatment as described in Examples 1 to 5 above, even if a continuous furnace is used, if a cooling zone is provided by using forced cooling in a certain area of the furnace, energy can be saved in a short time. Sintering, cooling, reheating, and recooling can be performed continuously with little loss, and the same effect as using a batch furnace is obtained.

These heat treatment effects are strongly related to the crystalline phase of bismuth oxide, which is the main component of the boundary layer.
After considering X-ray diffraction method, high-temperature X-ray diffraction method, etc., we found that if no heat treatment is performed after firing, bismuth oxide will be face-centered cubic and tetragonal, but if heat treated according to the present invention, it will be body-centered cubic. Changes to bismuth oxide. It has also been found that the upper and lower limits of the heat treatment temperature T ₃ correspond to the range in which body-centered cubic bismuth oxide is produced from tetragonal and face-centered cubic bismuth oxides.

It was also found that the addition of glass is also effective in reducing variations in device electrical characteristics due to heat treatment by stably causing crystal transition.

As described above, according to the present invention, an effective heat treatment method has been realized through the correlation between the cooling temperature after sintering, the reheating temperature, the cooling rate during cooling, and the composition of the sintered body. In addition to being able to significantly shorten the time, voltage nonlinear resistor elements with excellent energized life characteristics can be produced with high yield.

[Brief explanation of drawings]

Figure 1 is a diagram showing the firing pattern of a voltage nonlinear resistor whose main component is zinc oxide, Figure 2 is a characteristic diagram showing the change in leakage current over time when DC voltage is applied, and Figure 3 is a diagram showing the change in leakage current over time when DC voltage is applied. Fig. 4 is a characteristic diagram showing the relationship between the change in leakage current over time and _the reheating temperature T ₃ when applying direct current; Figure 5 shows the change in leakage current over time and the cooling rate (R ₁
= R ₂ ), and FIG. 6 is a characteristic diagram showing the relationship between the change in leakage current over time and the composition when DC current is applied.

Claims (1)

[Claims] 1. After sintering a raw material molded product of a voltage nonlinear resistor containing zinc oxide as a main component at a temperature of 1050°C or higher, the sintered body is cooled to a temperature range of 400 to 530°C. ,again
A method for manufacturing a voltage nonlinear resistor, which comprises heating to a temperature range of 600 to 900°C and then recooling to room temperature. 2 The temperature cooling rate after sintering and reheating is at least 100 to 300℃/300℃ in the temperature range of 600℃ from the sintering temperature.
2. A method of manufacturing a voltage nonlinear resistor according to claim 1, wherein the voltage nonlinear resistor is manufactured at a time. 3 The raw material molded product contains bismuth borosilicate glass or zinc borosilicate glass in a proportion of 0.01 to 0.01 to the total amount.
3. A method for manufacturing a voltage nonlinear resistor according to claim 1 or 2, characterized in that the material is made of a powder molded product whose main component is zinc oxide containing 0.5% by weight.