JPH04322402A - Manufacturing method of voltage nonlinear resistor and lightning arrester - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention uses ZnO as a main component,
The present invention relates to a method for manufacturing a voltage nonlinear resistor containing a metal oxide such as Bi2O3 with excellent life characteristics, and to a lightning arrester using the voltage nonlinear resistor.

0002

[Prior art] Non-linear resistor (ZnO) mainly composed of ZnO
nO elements) have excellent nonlinearity and are widely used as elements for lightning arresters. In particular, ZnO elements used in gapless lightning arresters are constantly energized, so a small amount of leakage current occurs in the element, and when energized for a long period of time, the leakage current gradually increases, causing the element to heat up and cause thermal runaway. This may cause a phenomenon. In order to prevent thermal runaway of the element (improve its life), it is important to reduce the rate of increase in leakage current. For this reason, as a method for obtaining long-life ZnO elements whose characteristics do not deteriorate even when constantly charged, we proposed
According to Publication No. 8-159303, 1050 to 1300
A ZnO element sintered at a high temperature of ℃ is reheated to 500 to 700℃, held for one or two hours, and then the temperature decrease rate is 100 to 300℃.
A method is disclosed in which deterioration of the characteristics of the element is prevented by performing a so-called one-time heat treatment after sintering, in which the material is re-cooled to room temperature at a rate of .degree. C./h. Furthermore, according to Japanese Patent Application Laid-Open No. 58-200508, ZnO is the main component and has a composition containing at least Bi2O3, and is sintered at a high temperature of 1050 to 1300°C.
The nO element was reheated to 850 to 950°C, held for one or two hours, cooled to below 300°C at a cooling rate of 300°C/h, heated again to 500 to 700°C, held for one or two hours, Re-cooling to room temperature at a cooling rate of 50 to 150°C/h,
A method for preventing deterioration of device characteristics by performing so-called two-time heat treatment after sintering is disclosed.

0003

[Problems to be solved by the invention] ZnO elements are made of ZnO
It has a structure in which the particles are surrounded by a high-resistance boundary layer mainly composed of Bi2O3, and the resistance of this boundary layer exhibits nonlinearity with respect to voltage.

Generally, the voltage-current characteristics of a ZnO element are approximately expressed by the following equation.

[0005]

[Math 1]

where I is current, V is voltage, K is constant, α
represents the nonlinear coefficient. α of the ZnO element is in the range of 10 to 70.

[0007] The larger α is, the smaller the leakage current that flows in the ZnO element when it is constantly energized. Therefore, it is desirable that α be large. In addition, to suppress the increase in leakage current due to long-term energization, ZnO can be reduced by heat treatment of the sintered ZnO element.
It is known that it is effective to generate a γ-type Bi2O3 phase in an nO element.

However, with this prior art, sintered ZnO
In one heat treatment in which the element is heated at 500 to 700°C,
Even if γ-type Bi2O3 is generated in the ZnO element and characteristic deterioration can be prevented, there is a problem in that the voltage-current characteristics of the element are poor.

On the other hand, when a sintered ZnO element is heat treated twice to improve the life of the ZnO element, the first heat treatment
When γ-type Bi2O3 is not generated in the nO element, there is a problem that the charged life characteristics of the ZnO element do not improve even if the heat treatment is performed twice. For example, if ZnO is the main component and Bi
Even in compositions containing 2O3, Sb2O3, MnCO3, Cr
2O3, Co2O3, SiO2, NiO, B2O3,
For compositions containing many types of metal oxides such as Al(NO3)3, in the first heat treatment step of the sintered ZnO element, the cooling rate is reduced to 300°C as disclosed in the prior art.
/h, γ-type Bi2O3 was sometimes difficult to generate in the ZnO element.

[0010] For these reasons, the conventional techniques have been insufficient in terms of reliability, particularly for multi-component ZnO elements used at high charging rates in DC systems.

[0011] An object of the present invention is to provide a method for manufacturing a ZnO element and a lightning arrester that are highly reliable and stable against long-term energization without deterioration of characteristics.

0012

[Means for Solving the Problems] The above object is to use ZnO as a main component and various kinds of metal oxides (for example, Sb2O3, MnCO3, Co2O3, Cr2O) in addition to Bi2O3.
3, SiO2, NiO, B2O3, Al(NO3)3
etc.) and sintering the molded body at 1050 to 1300°C, then cooling the sintered body to 300°C or less, and then sintering at 800 to 950°C.
A step of performing a first heat treatment in which the temperature is raised to 1 to 3 hours and then lowered to 300 °C or less;
It consists of a second heat treatment process in which the temperature is raised to 900°C, held for 1 to 3 hours, and then recooled to room temperature. This is achieved by a manufacturing method that reduces the temperature to 100°C/h or less and 150°C/h or less.

[0013] Another object is achieved by a lightning arrester in which electrodes are formed on the end faces of the disc or cylindrical ZnO element except for the outer circumferential surface produced by the above manufacturing method, and the electrodes are placed in an insulator tube.

[0014]

[Operation] In the present invention, the sintering and heat treatment pattern shown in FIG. 1 is used.

[0015] A molded product made by mixing and molding raw materials containing ZnO as a main component and metal oxides such as Bi2O3, first,
Sinter at 1050-1300°C for 1 to 12 hours. The rate of temperature rise and fall in this sintering step is set to 300° C./h or less so that the ZnO element does not undergo thermal breakdown. At the end of sintering, the temperature is lowered to 300° C. or less for cooling to stabilize the crystal and grain boundary structure of the device. After the temperature is lowered to 300° C. or lower, a heat treatment step is started either after a holding time T or immediately.

In the first heat treatment step, the sintered ZnO element is heated to 800 to 950°C and heat treated for 1 to 3 hours to dissolve Bi2O3 in the ZnO element, and when the temperature is lowered, γ-type Bi2O3 is formed in the ZnO element. to be generated. When γ-type Bi2O3 is generated in the device, the life characteristics of the device are improved. Although the reason is not necessarily clear, it is inferred as follows. (1) Characteristic deterioration of ZnO elements due to long-term energization is caused by Zn
The boundary layer and ZnO
It is thought that oxygen ions existing on the surface of the crystal grains were dissipated to the outside due to heat generation of the element during energization, and as a result, the electrostatic potential of the boundary layer decreased (varistor voltage decreased). (2) γ-type Bi2O3 is generally α-type Bi2O3, β
It has higher crystallinity than type Bi2O3 and δ type Bi2O3,
ZnO has few internal defects and has a large volume.
It has the effect of preventing oxygen from diffusing through the boundary layer of the crystal. Therefore, the movement of oxygen ions present on the surface of the ZnO particles is inhibited, and the dissipation of oxygen to the outside is reduced, making the ZnO element stable against charging.

[0017] The cooling rate of the ZnO element in the first heat treatment step is set to 100°C/h or less in order to generate γ-Bi2O3 in the ZnO element. If it exceeds 100℃/h, γ
Type Bi2O3 is no longer generated. Further, by dissolving Bi2O3 in the first heat treatment, voids in the sintered body are reduced, and a decrease in varistor voltage is prevented, thereby preventing characteristic deterioration of the ZnO element. Below 800°C, the Bi2O3 layer at the grain boundaries of the ZnO element does not dissolve sufficiently, and above 950°C, the thermal activation of the ZnO crystal becomes too high and the Bi2O3
The dissolution of the layer is not limited to the grain boundary region, and the ZnO
This is undesirable because oxygen ions adsorbed at grain boundaries tend to dissipate. In addition, if the heat treatment time is less than one hour, the effect of maintaining the temperature is small, and if it is more than three hours, the Zn
The problem of activation of O crystals arises.

Next, as a second heat treatment, after the temperature decrease in the first heat treatment reaches 300 °C or less, the temperature is raised to 650 to 900 °C for an appropriate holding time T or immediately, and the temperature is raised to 650 to 900 °C for 1 to 3 seconds. Hold for a while and cool down.

By this second heat treatment, the remaining Bi2O3 that could not be transformed into γ-type Bi2O3 in the first heat treatment is transformed into γ-type Bi2O3, and the grain growth of the grain boundary layer is adjusted. The temperature at this time of 650 to 900 DEG C. is the temperature necessary for Bi2O3 to change to .gamma.-type Bi2O3, and the holding time at this time of 1 to 3 hours is determined for the same reason as described above. The temperature decreasing rate of the second heat treatment was 150°C/h.
Do the following. This is effective in removing thermal distortion of the ZnO element and improving the characteristics of the element.

It is also effective to repeat the same heat treatment as the second heat treatment several more times.

[0021]

【Example】

<Example 1> ZnO9 with a purity of 99% or more as a starting material
4-95 mol%, Bi2O3 0.1-1 mol%, Sb2
O30.8-1.2 mol%, MnCO30.8-1.2
Mol%, Co2O30.8-1.2mol%, Cr2O3
0.1-1.0 mol%, SiO2 1.0-1.5 mol%
, NiO0.5-1.0 mol%, B2O30.1-0.
15 mol%, Al(NO3)3 0.005-0.00
A predetermined amount of each powder was weighed so as to have a concentration of 9 mol %, mixed, granulated, and pressure molded, and then sintered in air at 1190° C. for about 4 hours. The dimensions of the ZnO element after sintering are φ90×20t. The rate of temperature rise and fall at this time is approximately 70°C/h,
The temperature was lowered to room temperature and cooled. Next, the sintered body was heated to 950°C and held for two hours, and then heated to about 70°C/h and about 300°C.
Each sintered body was cooled to room temperature at a cooling rate of /h, and then heated again for 7 hours.
After heating to 00°C and holding for 2 hours, heat treatment was performed twice in which the temperature was lowered to room temperature at a rate of about 70°C/h. Electrodes were attached to the sintered body thus obtained to produce a ZnO element.

A direct current voltage was applied to the manufactured ZnO element and the element which was not subjected to heat treatment after sintering. FIG. 2 shows the change over time in the leakage current flowing through the ZnO element at this time. The charging conditions were an ambient temperature of 115° C. and a charging rate (=applied voltage/voltage when 1 mA flows through the ZnO element)=0.85.

In FIG. 2, in the element not subjected to heat treatment (characteristic A), the leakage current changes greatly over time and thermal runaway occurs in a short period of time. In addition, in the first heat treatment step, the element (characteristic B) obtained by the conventional heat treatment method, which was cooled at a temperature decreasing rate of 300 °C/h exceeding 100 °C/h, was cooled for about 30 h.
Thermal runaway occurred in a short period of time. The device produced by the heat treatment method of the present invention (characteristic C) exhibits almost no increase in leakage current due to long-term energization, and has a long service life. It should be noted that γ was measured by X-ray diffraction method for the device after the first heat treatment.
As a result of examining the presence or absence of the formation of Bi2O3, it was confirmed that γ-type Bi2O3 was not generated in the device using the conventional heat treatment method, and that γ-type Bi2O3 was definitely generated in the device using the heat treatment method of the present invention.

<Example 2> Among the first and second heat treatment steps shown in Example 1, the heating temperature of the first heat treatment step was changed to 750, 800, 900, 950, and 1000°C,
ZnO obtained by heat treatment twice at a cooling rate of 70°C/h
Electrodes were formed on the sintered body, and DC voltage was applied under the same conditions as in Example 1. FIG. 3 shows the change over time in the leakage current flowing through the ZnO element at this time.

[0025] The heating temperature in the first heat treatment step of the element was 7.
For temperatures of 50 and 1000°C, characteristics A and B in Figure 3
As shown in , thermal runaway occurred in a short period of time. The reason for this is that at 750°C, Bi2O contained in the ZnO element
It was determined that this was due to the fact that No. 3 was not dissolved and that γ-type Bi2O3 was not generated in the ZnO element at 1000°C.

When the heating temperature is 800, 900 and 950°C, as shown in characteristics C, D and E in FIG.
Although the leakage current is larger at 00°C and 900°C than at 950°C, in both cases there is almost no increase in leakage current due to long-term energization, and a long life of the element is achieved. Therefore, the heating temperature is preferably between 800 and 950°C. <Example 3> In the first and second heat treatment steps shown in Example 1, the temperature decreasing rate was set to 70°C/70°C in the first heat treatment step.
In the second heat treatment step, the heating temperature was set to 600 h.
, 650, 750, 900 and 950°C twice to form an electrode on the ZnO sintered body obtained. Example 1
DC voltage was applied under the same conditions as . FIG. 4 shows the change over time in the leakage current flowing through the ZnO element at this time.

When the heating temperature in the second heat treatment step was 600° C. and 950° C., as shown in characteristics A and E in FIG. 4, the leakage current was large and thermal runaway occurred in a short time in both cases. On the other hand, as shown in characteristics B, C, and D, the elements heated at heating temperatures of 650, 750, and 900°C have no significant difference in leakage current, and the leakage current increases due to long-term energization. Since there is almost no oxidation, the device has a long life. From this, the heating temperature of the element in the second heat treatment is
A temperature of 650 to 900°C is desirable.

Example 4 22 elements produced in Example 3 (elements with characteristic C in FIG. 4) were stacked and housed in an insulator tube to produce a DC 125 kV lightning arrester as shown in FIG. 5. Due to the life characteristics of the elements, this lightning arrester is guaranteed to have a lifespan of 100 years under actual usage conditions.

[0029]

Effects of the Invention According to the present invention, by realizing a two-time heat treatment method in which the combination of the reheating temperature and cooling rate after sintering of the ZnO element is optimized, the energized life characteristics are superior to the conventional method. It is possible to produce ZnO elements and lightning arresters.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of sintering and heat treatment patterns according to the present invention.

FIG. 2 is an explanatory diagram showing the life characteristics of the element according to the present invention together with those of the conventional method.

FIG. 3 is a diagram showing the life characteristics of the element when the heating temperature of the first heat treatment in the present invention is changed.

FIG. 4 is a diagram showing the life characteristics of the element when the heating temperature of the second heat treatment in the present invention is changed.

FIG. 5 is a perspective view showing the structure of a lightning arrester using a voltage nonlinear resistor according to the method of the present invention.

[Explanation of symbols]

1... Voltage nonlinear resistor, 2... Insulator tube, 3... Shield, 4
…Insulated base.

Claims (2)

[Claims] Claim 1: The main component is ZnO, Bi2O3, Sb2
O3, MnCO3, Cr2O3, Co2O3, SiO2
, NiO, B2O3, and Al(NO3)3 are mixed and molded, and this molded body is sintered at 1050 to 1300°C, and then the sintered body is heated to 300°C or lower. temperature drops to 800-95
After raising the temperature to 0℃ and holding it for 1 to 3 hours, it was heated to 100℃/
A first heat treatment step in which the temperature is lowered to 300°C or less at a cooling rate of 150°C/h or less, and the temperature is raised to 650 to 900°C, held for 1 to 3 hours, and then cooled to room temperature at a cooling rate of 150°C/h or less. A method for manufacturing a voltage non-linear resistor, comprising at least two heat treatment steps: a second heat treatment step. 2. The lightning arrester according to claim 1, wherein the voltage non-linear resistor having electrodes formed on the end face except for the outer circumferential face of the prepared disc or cylindrical sintered body is placed in an insulator tube.