JPS6236611B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a voltage nonlinear resistor made of a sintered body containing zinc oxide as a main component. In recent years, molding is mainly made of zinc oxide, with the addition of bismuth oxide, manganese oxide, cobalt oxide, antimony oxide, chromium oxide, boron oxide, etc.
The sintered voltage nonlinear resistor is used as a voltage stabilizing element,
It has come to be used in surge absorbers, arresters, etc. The principle of this zinc oxide-based voltage nonlinear resistor is that a high-resistance layer containing bismuth oxide is formed at the grain boundaries of zinc oxide, which is an n-type semiconductor.
This causes electrical breakdown and a rapid increase in current. This zinc oxide-based voltage nonlinear resistor has a lower voltage difference than a voltage nonlinear resistor made of silicon carbide.
Although the nonlinearity of the current characteristics is excellent, there is a problem that the characteristics deteriorate due to surge absorption or constant voltage application for a long time, and the leakage current gradually increases, eventually leading to thermal runaway. Ta. This characteristic deterioration is due to the following experimental facts: (1) When a voltage nonlinear resistor element is heat-treated in a nitrogen atmosphere, the characteristics deteriorate in the same way as when a voltage is applied. (2) When an element with deteriorated characteristics is heat-treated in the air or an oxygen atmosphere, the characteristics return to their original state. As a result, the adsorbed oxygen in the grain boundary layer or on the surface of the crystal grain is desorbed when electricity is applied and dissipates to the outside world, reducing the electrostatic potential of the grain boundary layer.
This is thought to be due to an increase in leakage current. The following methods have been developed to reduce the deterioration of characteristics of such a zinc oxide-based voltage nonlinear resistor due to the application of electricity and to increase its stability. (1) Bismuth oxide is diffused from the entire surface of the sintered body. (2) Part or all of the bismuth oxide contained in the sintered body is contained as a γ-type bismuth oxide phase. However, products manufactured by such a method do not necessarily have sufficiently stable characteristics, and may also have drawbacks such as a decrease in other characteristics, especially the ability to withstand long-wave tail impulse currents. There is also. In view of these circumstances, the inventors of the present invention have conducted intensive research to find a voltage nonlinear resistor with stable characteristics. As a result, the inventors have found that by creating a specific gradient in the concentration of the γ-type bismuth oxide phase, an excellent The inventors have discovered that a voltage nonlinear resistor can be obtained, and have arrived at the present invention. That is, an object of the present invention is to provide a voltage nonlinear resistor that has stable characteristics even when applied with electricity for a long time and has a high long-wave tail impact current capacity.
This purpose is to provide a voltage nonlinear resistor in which electrodes are provided on the upper and lower main surfaces of a sintered body containing zinc oxide as a main component and at least bismuth oxide as an additive, in which the γ-type bismuth oxide phase concentration is the main component forming the electrodes. The γ-type bismuth oxide phase has a concentration gradient in the thickness direction that is highest on the surface and decreases toward the inside of the sintered body, and the concentration of the γ-type bismuth oxide phase at the peripheral portion of the electrode-forming main surface is lower than that inside the main surface. This can easily be achieved by using a non-linear resistor. Such a voltage nonlinear resistor is made by diffusing bismuth oxide from the upper and lower main surfaces of a sintered body whose main component is zinc oxide, excluding the peripheral edge, to form a γ-type bismuth oxide phase with the above concentration distribution. and then forming electrodes on the upper and lower main surfaces. The present invention will be described in detail below with reference to FIGS. 1 and 2, which show one embodiment of the present invention. 1 and 2 are schematic diagrams showing a cross section of a voltage nonlinear resistor of the present invention. The voltage nonlinear resistor of the present invention has a concentration gradient in the thickness direction in which the concentration of the γ-type bismuth oxide phase is highest on the main surface where the electrode is formed and decreases as it goes inside the sintered body, and The γ-type bismuth oxide phase concentration at the peripheral edge of the surface is lower than that at the inner side. 1 and 2, on the surface of the zinc oxide-based sintered body 1 containing bismuth oxide, on which the electrode 2 is formed, the concentration of the γ-type bismuth oxide phase contained in the surface layer 11 in the portion where bismuth oxide is diffused. is the highest. The high concentration of the γ-type bismuth oxide phase in the surface layer 11 makes it possible to improve the stability of the voltage nonlinear resistor against long-term energization. Possible reasons for this are as follows. (1) The resistance (operating region) of a voltage nonlinear resistor tends to decrease as the content of the γ-type bismuth oxide phase precipitated at the grain boundaries of zinc oxide increases. In the structure of the present invention, since the resistance of the surface layer 11 is low, the amount of heat generated in the surface layer 11 when electricity is applied is smaller than that inside, and of course less oxygen is dissipated to the outside, so that the characteristics of the surface layer 11 are less likely to deteriorate. On the other hand, in the interior where the content of the γ-type bismuth oxide phase is low, resistance is high and heat generation is large when electricity is applied, but since oxygen dissipates to the outside through a thick layer, the amount of oxygen dissipated is small and characteristics are less likely to deteriorate. (2) Bismuth oxide diffuses through the pores in the sintered body and at the grain boundaries of zinc oxide, filling these pores and preventing the movement of oxygen from the sintered body to the outside world. arise. The γ-type bismuth oxide phase has a body-centered cubic structure and has a larger volume than the α-type bismuth oxide phase (monoclinic) and the β-type bismuth oxide phase (tetragonal), so it has a pore-filling effect. big. (3) It is said that the γ-type bismuth oxide phase contains not only trivalent bismuth but also some pentavalent bismuth, and this pentavalent bismuth absorbs oxygen ions present in the grain boundary layer. It has the effect of stabilizing and preventing dissipation to the outside world. Further, in the voltage nonlinear resistor of the present invention, by lowering the concentration of the γ-type bismuth oxide phase in the side layer 12 including the peripheral edge 121 of the main surface of the sintered body, the resistance to long-wave tail impact current can be further improved. This is more preferable because it allows The reason is thought to be as follows. In FIG. 1, when a rectangular waveform impact current having a long wave tail of 2 ms, for example, flows through the sintered body 1, penetration failure is most likely to occur at the end 3 of the electrode 2. This is because electric field concentration occurs at the end of the electrode.
This is because an electric field approximately 4 to 5 times greater than that at the center of the electrode is applied, so a larger current flows through the electrode end of the sintered body than in other parts, making that part more likely to be thermally destroyed. According to the present invention, since the concentration of the γ-type bismuth oxide phase in the periphery of the main surface of the sintered body or the side layer including the periphery is lower than that in the center of the sintered body, the resistance value of the periphery or the side layer is lower than that in the center of the sintered body. is larger than the center of the sintered body, so even if electric field concentration occurs, the amount of heat generated will be smaller because the current flowing will be smaller.
Thermal damage can be easily avoided. Furthermore, as shown in FIG. 2, if the end 3 of the electrode 2 is placed above the peripheral edge 121 left during the bismuth oxide phase diffusion, the sintered body part in contact with the end of the electrode will be in contact with most of the other part of the electrode. It is particularly preferable because it has a higher resistance than the sintered body portion and is highly effective in alleviating current concentration and increasing the long-wave tail impact current withstand capacity. The above-mentioned surface layer with a high content of γ-type bismuth oxide phase is created by attaching or coating a diffusing agent containing bismuth oxide to the main surface of the sintered body, and then diffusing it through heat treatment and at the same time changing the phase to the γ phase. It can be formed by As the diffusion method, there are known methods such as a method of applying bismuth oxide using water or an organic solvent, a method of forming a diffusion layer by vapor deposition, etc.
Various methods can be adopted. In manufacturing the voltage nonlinear resistor of the present invention, bismuth oxide is diffused from the upper and lower main surfaces of a sintered body containing zinc oxide as a main component, excluding the peripheral edge, to obtain the concentration distribution. It is preferable to form a γ-type bismuth oxide phase, and then form electrodes on the upper and lower main surfaces. When the bismuth oxide phase is diffused from the entire main surface of the sintered body as in the conventional manufacturing method, there is a possibility that the bismuth oxide phase melted during the diffusion process will flow from the main surface to the side surfaces and adhere to the side surfaces. big. Also,
As the diffusion progresses, the concentration of the γ-type bismuth oxide phase in the side layer becomes higher than that in the inner side, and the resistance of the side layer becomes smaller. For this reason, the degree of current concentration at the electrode end near the side layer becomes even higher, and the withstand capability against long-wave tail impact current is significantly reduced. Furthermore, in this case, since the side surfaces have low resistance, creeping flashing is likely to occur when a short-wave tail impact current is applied, and the withstand capability at this time also decreases. It is difficult to completely control so that the bismuth oxide phase does not flow to the sides during the diffusion process, and even if careful fabrication is done, problems such as large variations in resistance to long-wave tail impact current depending on the device may occur. be. On the other hand, according to the method for manufacturing the voltage nonlinear resistor of the present invention, the bismuth oxide phase does not flow to the side surfaces during the diffusion process, and the above-mentioned disadvantages can be easily avoided. In addition, in the present invention, since the peripheral area that is removed when the bismuth oxide phase is diffused is only a small part of the surface area of the sintered body, the stability against long-term electrification is determined when the bismuth oxide phase is diffused from the entire main surface. It can be almost the same as. In addition to zinc oxide and bismuth oxide, the voltage nonlinear resistor of the present invention also contains 0.01 to 10 mol% of each of manganese oxide, cobalt oxide, chromium oxide, antimony oxide, nickel oxide, silicon oxide, and boron oxide.
It can contain 0.001 to 0.01 mol% of aluminum oxide, gallium oxide, etc. These additives are effective in improving the non-linearity coefficient of the device, as well as the lifespan of charging and impulse withstand capability. Although bismuth oxide is diffused into the sintered body, it is preferable to form and fire the body containing 0.05 mol % or more of bismuth oxide before sintering. By doing so, a sintered body can be obtained without impairing sinterability. The amount of bismuth oxide to be diffused is not particularly limited as long as it is sufficient to fill most of the pores in the sintered body, but is preferably 0.01 mol% or more. Moreover, it is particularly preferable that boron oxide be contained in the sintered body. The γ-type bismuth oxide phase is normally a metastable phase, but boron oxide has the effect of stabilizing the γ-type bismuth oxide phase, and therefore, it is particularly effective against the γ-phase due to thermal cycles associated with long-term voltage application or surge application. It is possible to prevent changes to other phases and achieve long-term stability. The preferred content of boron oxide is 0.01
~0.2 mol%. Note that the diffusion temperature is the melting point of bismuth oxide (approximately 820
℃), the diffusion rate is extremely slow, and when the temperature exceeds the sintering temperature of the sintered body, the γ-type bismuth oxide phase decreases and the diffusion effect becomes small. Therefore,
The diffusion temperature is preferably the melting point of bismuth oxide or the sintering temperature of the sintered body. In order to form a γ-type bismuth oxide phase with good reproducibility, it is particularly preferable that the diffusion temperature is 1100° C. or lower. A glass film or the like may be further provided on the side surface of the sintered body for the purpose of increasing short wave tail impact current resistance. As described above, according to the present invention, the life of the voltage nonlinear resistor is significantly improved compared to conventional elements, and the resistance to long-wave tail impact currents is also increased. EXAMPLES The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited to the following Examples unless it exceeds the gist thereof. Example 1 ZnO with 0.7 mol% Bi ₂ O ₃ and 0.5 mol% MnCO _{3
Co2O3 1.0mol} %, _{Cr2O3 0.5mol} %, _Sb2O3 1.0mol%, NiO1.0mol%, _SiO2 1.5mol%, _{B2O3 0.1mol} %, Al _{( NO3} ) ₃ 0.005 mol% was added and mixed for 10 hours using a ball mill. To this raw material powder, 10% of a 2% polyvinyl alcohol aqueous solution was added and granulated. This is formed into a disk shape and placed in the air at 1160°C.
It was baked at ℃ for 5 h. Both main surfaces of the obtained sintered body are 0.5
By polishing in mm increments, an element of 60 mm × 20 mm ^t was obtained.
Next, on both main surfaces of this element, 2 g of bismuth oxide, 0.05 g of ethyl cellulose, and 0.4 g of butyl carbitol were added.
The paste consisting of g was applied almost uniformly on the main surface, leaving a 3 mm wide area on the outer periphery, and heat treated at 950° C. for 2 hours. Finally, Al was sprayed on both main surfaces and the diameter was 56 mm.
The electrode 〓 was formed so that the electrode end was located on the part where the above paste was not applied. The nonlinear coefficient of the obtained element (current 3 × 10 ^-6 ~ 3
×10 ^-4 A/cm ² ) is 52, and the flatness rate (current 3 × 10 ^-3 A/cm 2 ) is 52.
(ratio of voltage at cm ² and voltage at 3×10 ^-4 A/cm ² )
was 1.54. Figure 3 shows the voltage nonlinear resistor of the present invention at a temperature of 90°C.
It shows how the resistance leakage current changes over time when AC is continuously applied at 100% charging rate (the same peak voltage as the voltage required to flow 1 mA of DC at 20°C). In the figure, A is the element obtained in this example, B is the element before bismuth oxide diffusion obtained by the same method as in this example, and C is the element with the same amount of paste as in this example applied to the entire surface of both main surfaces. D is an element in which the bismuth oxide phase is diffused by coating, D is an element that does not contain boron oxide as an additive obtained by the same method as in this example,
E is the characteristic of the device before bismuth oxide diffusion obtained by the same method as the device D. From FIG. 3, it can be seen that element C and element A of the present invention have small changes in resistance leakage current, and have significantly high stability against long-term energization. Considering the acceleration of the rate of characteristic deterioration due to temperature, it can be said that 10,000 hours of energization at 90°C is equivalent to more than 100 years at 40°C, which is the actual operating condition. Furthermore, in order to measure the withstand capacity of each element A to E against long wave tail impact current, the 2 ms rectangular wave withstand capacity was measured.
The results are shown in Table 1.

[Table] According to Table 1, it can be seen that the element A of the present invention has sufficient rectangular wave resistance. Element C is deficient in this respect. Generally, in order to use it as a UHV (1000 KV or higher) arrester, it is said that a rectangular wave withstand capacity of 3000 A is desirable for an element of about 60 mm × 20 mm ^t , and 4000 A or more when considering the safety factor. From these results, the device of the present invention has excellent instantaneous charging stability and long-wave tail impact current resistance.
It can be seen that it is insufficiently practical as a UHV arrester. Further, FIG. 4 shows the distribution of the γ-type bismuth oxide phase in the obtained voltage nonlinear resistor elements A and C. FIG. 4 shows the distribution when the concentration of the γ-type bismuth oxide phase in the central part of the element is normalized to 1. The distribution of the γ-type bismuth oxide phase is determined by cutting the device parallel to the electrode surface, cutting out 1 mm thick slices from the surface portion and the center portion of the device, and then cutting each slice 1 mm wide from the outside along the sides. Each fine piece was divided into powder, and the distribution in the radial direction was determined for the surface portion and the center portion of the element from the diffraction intensity of the γ-Bi ₂ O ₃ phase by X-ray powder diffraction method. Reflection lines with an interplanar spacing of 2.71 to 2.72 Å were used for measurement, and normalized by the diffraction line intensity of ZnO. FIG. 5 shows the resistance distribution of the voltage nonlinear resistor elements A and C obtained. To measure the resistance distribution, touch a 1 mm needle to corresponding points on both main surfaces of the sample (before electrode formation) and apply a current of 2 μA (current density 3×
10 ^-4 A/cm ² ) was applied, the voltage distribution was measured while shifting the needle in the radial direction, and the voltage distribution was determined from this voltage distribution. As seen in FIG. 4, in the voltage nonlinear resistor of the present invention, the element surface portion A1 has a higher concentration of γ-type bismuth oxide phase on average than the element central portion A2, but in any portion, the side surface portion When it gets closer, γ
The amount of bismuth oxide phase is reduced. In particular, it can be seen that the concentration of the γ-type bismuth oxide phase in the peripheral portion of the surface portion A1 is lower than that on the inside, and the content of the γ-type bismuth oxide phase in the entire side layer is also lower than that on the inside. As seen in FIG. 5, in accordance with this, the side layer of element A has a high resistance. On the other hand, in the element C, the concentration of the γ-type bismuth oxide phase is higher in the element surface part C1 than in the element central part C2, and the content is particularly high in the parts near the side surfaces. This is thought to be because the bismuth oxide phase covered some of the sides during diffusion, and there was also some diffusion from the sides. In element C, the resistance of the side layer is smaller, which can be explained by the same reason. Elements B, D, and E do not contain a γ-type bismuth oxide phase. As seen in Figure 5, the resistance of the side layer is much higher in these samples, but this is thought to be due to the density distribution of the sintered body and the volatilization of Bi ₂ O ₃ during sintering. . Example 2 After polishing both main surfaces of a sintered body obtained in the same manner as in Example 1, a paste similar to that in Example 1 was applied to both main surfaces of this element, leaving a 1 mm wide portion on the outer periphery of the main surface. The coating was applied almost uniformly using a wafer and heat treated at 950°C for 2 hours. Finally, Al was sprayed on both main surfaces to form a 56 mm electrode so that the electrode end was on the area where the paste was applied. The obtained element has a nonlinear coefficient of 50 and a flatness rate of 1.55.
It was hot. In addition, the energized lifespan was as long as in A and C in Fig. 3, and the resistive current did not reach twice the initial value even after 10,000 hours of energization. The square wave withstand capacity is 4100A,
It showed sufficient values as a UHV arrester. As a result of examining the distribution of the γ-type bismuth oxide phase in this device using the same method as in Example 1, it was found that the concentration of the γ-type bismuth oxide phase on the main surface where the electrode is formed is higher than in the center, and that It was found that the concentration of the γ-type bismuth oxide phase was lower in the 1 mm wide part than on the inside, and that the content of the γ-type bismuth oxide phase in the side layer was lower than that on the inside. It was also confirmed by the same method as in Example 1 that the side layer had higher resistance than the inside layer. In addition, when the distribution of the γ-type bismuth oxide phase was investigated by X-ray powder diffraction method on the sintered element that was heat-treated at 950°C for 2 hours without diffusing bismuth oxide into the sintered body, it was found that the distribution of the γ-type bismuth oxide phase in the element was did not contain any γ-type bismuth oxide phase. From this,
γ detected in the device in Example 1 and Example 2
The type bismuth oxide phase is presumed to be derived from the diffused bismuth oxide phase. Next, in order to examine the effect of diffusion temperature, the bismuth oxide phase was diffused using the same method as in this example, with only the diffusion temperature set to 750°C and 1150°C. In this case, at 750°C, diffusion was insufficient and the nonlinear coefficient was as small as 5 to 8. Moreover, at 1150℃, γ is present in the sintered body after diffusion.
The type bismuth oxide phase was small, and the life of charging was short.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views showing the structure of the voltage nonlinear resistor of the present invention, and FIGS. 3 to 5 show the voltage nonlinear resistor obtained in the embodiment of the present invention and the conventional voltage nonlinear resistor. It is a characteristic curve diagram comparing characteristics with a linear resistor. 1...Sintered body, 11...High concentration phase of γ-type bismuth oxide phase, 12... Side layer with low concentration of γ-type bismuth oxide phase, 121... Main surface peripheral part with low concentration of γ-type bismuth oxide phase, 2... Electrode, 3... Electrode end.

Claims (1)

[Scope of Claims] 1. A voltage nonlinear resistor comprising zinc oxide as a main component and electrodes provided on the upper and lower main surfaces of a sintered body containing bismuth oxide, wherein the γ-type bismuth oxide concentration is on the main surface on which the electrodes are formed. A voltage having a concentration gradient in the thickness direction that is highest and decreases as it goes inside the sintered body, and in the main surface where the electrode is formed, the concentration is lower at the peripheral edge of the surface than inside the surface. Nonlinear resistor. 2. An electrode provided on the main surface of the resistor is provided across a peripheral portion of the main surface having a low concentration of γ-type bismuth oxide and a high concentration portion inside the peripheral portion, and an end portion of the electrode is provided on the peripheral portion. 2. The voltage nonlinear resistor according to claim 1, wherein the voltage nonlinear resistor is formed so that the voltage nonlinear resistor has the following characteristics.