JPH0136961B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improving the electrical characteristics of a voltage nonlinear resistor containing zinc oxide as a main component. Voltage nonlinear resistors are widely used in surge absorbers, voltage stabilizing elements, lightning arresters, etc.
Recently, sintered bodies containing zinc oxide as a main component have become the mainstream. This voltage nonlinear resistor is made by thoroughly mixing raw materials such as zinc oxide and trace additives such as bismuth oxide, manganese oxide, and cobalt oxide, and then press-molding and firing at a high temperature of 1000℃ or more, followed by electrodes. It will be done. In this voltage nonlinear resistor whose main component is zinc oxide, an electrical barrier is formed at the contact area (grain boundary area) between zinc oxide particles, and this is thought to be the cause of the excellent voltage nonlinearity. . In general, a nonlinearity index α is used as an indicator of voltage nonlinearity, but in the case of nonlinear resistors, this index is significantly larger than the conventionally known 3 to 4 for silicon carbide, and has a value of about 5 to 100. Furthermore, since it does not use a single crystal junction like a Zener diode, the device characteristics, such as V _1nA
(The voltage when a current of 1 mA flows) has the advantage that it can be changed relatively freely by selecting the element shape and firing conditions, and the use of voltage nonlinear resistors is increasing. However, when applying a DC or AC voltage to a voltage nonlinear resistor that has such advantages,
Generally, the leakage current flowing through an element increases with time, and the element generates heat and exhibits a thermal runaway phenomenon, eventually leading to element destruction, which has reached a limit in terms of life. Particularly in the case of gear press zinc oxide type lightning arresters, element destruction due to increased leakage current is an extremely important problem, and it is essential to obtain long-life elements with extremely low deterioration due to electrical current. For this reason, in the past, the increase in leakage current was anticipated in advance, α was made as large as possible (that is, the initial leakage current was made small), and the charging rate (V ₀ /V _1nA : V ₀ applied voltage) was
Methods have been taken to slow down the increase in leakage current and ensure a long life by reducing V ₀ as much as possible (lowering V 0 , that is, applying a low voltage). However, from the viewpoint of improving the protection performance of the lightning arrester, the above-mentioned method of ensuring the lifespan is not desirable. This will be explained with reference to FIG. What is important in the current-voltage characteristics of lightning arrester elements is the ratio of the limiting voltage on the large current side determined by the standard, e.g. V _10KA , to the voltage V ₀ actually applied to the element (V _10KA /
V ₀ ), and it is better that this ratio is as small as possible. In other words, the limiting voltage should be as low as possible;
It is desirable that V ₀ be as high as possible.
However, as already mentioned, increasing V ₀ causes problems such as shortening the lifespan, and thus extending the lifespan of the element is extremely important in improving the protection characteristics of lightning arresters. In attempts to extend the service life, extensive studies are being conducted on firing conditions, selection of additives, combinations, etc. As a result, the nonlinear index α became slightly smaller, and although the initial leakage current became somewhat larger, the rate of increase in the leakage current could be reduced, and a certain degree of longevity became possible. Nevertheless, other characteristics, such as the limiting voltage ratio (V _10KA /
There were drawbacks such as the extremely large V _1nA ) value. Additionally, there is the problem of dynamic thermal stability when high energy surges are injected into the device while it is being energized at V ₀ . This is a problem related to the rise in element temperature due to surge absorption and heat dissipation, and when a certain critical temperature is exceeded, the element goes into thermal runaway. This will be explained with reference to FIG. Generally, the relationship between element temperature and leakage current I is expressed by equation (1): I=I ₀ exp (-Ea (T, V)/kT) (1) Here, Ea (T, V) is Activation energy (generally varies with temperature (T) and voltage (V), but is constant within a narrow temperature range), k is Boltzmann's constant, T is absolute temperature, and I ₀ is a constant. Assuming that the applied voltage is constant, the electrical input P to the element is expressed by equation (2): P = P ₀ exp (-Ea (T) / kT) (2) The amount of natural heat radiation Q is (3) It is expressed by the formula: Q=K(T-T ₀ ) (3) where K is a constant. This state is shown in FIG. 2 as a curve A (a curve corresponding to equation (2) and representing the electrical input P) and a straight line B (a straight line corresponding to equation (3) and representing the amount of heat dissipation Q). The straight line B is determined by the ambient conditions, but the heat generation curve A is determined by the temperature characteristics of the element. In the figure, between intersection points C and D, heat radiation exceeds heat generation and the device is thermally stable; however, when the temperature rise of the element exceeds point D during surge absorption, heat generation exceeds heat radiation, leading to thermal runaway. For this reason, it is desirable that point D be as high as possible. This means that it is desirable for E ₀ , P ₀ , and (I ₀ ) to have small values in equation (2). These values also vary depending on the composition and firing conditions, so this point also needs to be taken into consideration.
Although Ea generally decreases when the device has a longer life, conversely I ₀ increases (this is related to a decrease in the nonlinear index α), and the temperature between the thermal stability regions C and D in Figure 2 increases. The difference ΔT becomes small and becomes a problem. The object of the present invention is to obtain an element that (1) has a long life, (2) has excellent leakage current temperature characteristics, and (3) has a limiting voltage ratio as small as possible, especially when a voltage is applied. The present invention involves adding an appropriate amount of boric acid (H ₃ BO ₃ ) to a composition containing zinc oxide as a main component, sintering the mixture, and then oxidizing it by appropriate methods such as selecting firing conditions and heat treatment after firing. The above object is achieved by controlling the content ratio of body-centered cubic crystals in the grain boundary partial crystal phase containing bismuth as a main component. The present invention contains at least boric acid and bismuth oxide as additives, and the boric acid content is from 0.02 to
0.1 mol %, and 80% or more of the bismuth oxide is a body-centered cubic crystal. The amount of boric acid added is 002 as shown in Figure 3 below.
If it is less than mol%, the limiting voltage ratio will reach a maximum, which is not practical, and if it exceeds 0.1 mol%, the limiting voltage ratio will increase, which is not preferable, but 0.02 to 0.1 mol%.
In the range of , the limiting voltage ratio is constant. At the same time, by increasing the content of body-centered cubic crystals in bismuth oxide to 80% or more through heat treatment, excellent life characteristics (Figure 4) and temperature characteristics of leakage current (Figure 6) can be achieved. is achieved. Next, the present invention will be explained based on examples. Example Raw material composition is bismuth oxide (Bi ₂ O ₃ ) 0.5 mol%,
Cobalt oxide (Co ₂ O ₃ ) 1.0 mol %, manganese oxide (MnO ₂ ) 0.5 mol %, antimony oxide (Sb ₂ O ₃ ) 1.0 mol %, chromium oxide (Cr ₂ O ₃ ) 0.5 mol %, silicon oxide (SiO ₂ ) 0.5 mol% and the amount of boric acid (H ₃ BO ₃ ) added was varied in the range of 0 to 0.1 mol%, and the remainder was zinc oxide (ZnO). The above raw material powder and boric acid aqueous solution were thoroughly ground and mixed in a ball mill, a binder such as PVA (polyvinyl alcohol) was added, granulated, pressure molded, and fired at 1200° C. for 2 hours to obtain a sintered body. Here, the bismuth oxide crystal phase was controlled by annealing the sintered body at a temperature range of 450°C to 800°C, and then aluminum electrodes were formed and the electrical properties were measured. FIG. 3 shows the relationship between the amount of boric acid added and the limiting voltage ratio (V _2.5KA /V ₁₀ 〓 _A ) normalized to the case where no boric acid is added. Curve 1 in the diagram
is not annealed, and curves 2 to 4 have annealing temperatures of 600°C, 650°C, and 700°C, respectively. The limiting voltage ratio increases monotonically with respect to the amount of boric acid added without heat treatment, but when heat treatment is applied, it reaches a maximum around 0.01 mol%, and remains almost constant for boric acid in the range of 0.02 to 0.1 mol%. However, when the amount of boric acid is further increased, the limiting voltage ratio increases. Judging only from the limiting voltage ratio, it seems best not to add boric acid, but when boric acid is added, 0.02 to 0.1 mol % is suitable. Next, FIG. 4 shows a study of the lifespan of the element when charged with electricity. Relatively good flatness ratio without annealing and 600
Taking the case of °C annealing as an example, we investigated the change in leakage current of the element over time at a charge rate of 0.8 and an ambient temperature of 100 °C, using the amount of boric acid added as a parameter. The amount of boric acid added and the presence or absence of heat treatment for curves 1 to 8 in the figure are shown below:

[Table] When no heat treatment is performed, the initial leakage current value is small, but the current increases significantly in a short period of time regardless of the amount of boric acid added (0.01 mol% to 0.1 mol% is not shown in the figure, but the same applies) ).
In the device annealed at 600° C., boric acid of 0.02 mol % (curve 6 in the figure) to 0.04 mol % (curve 7 in the figure) showed the most stable change over time. Next, FIG. 5 shows the results of examining the annealing temperature and the amount of body-centered cubic bismuth oxide produced using X-ray diffraction. Each element to which 0.04 mol% of boric acid was added was annealed for 2 hours. Formation of body-centered cubic bismuth oxide begins at around 450°C, and at 650°C it becomes almost completely body-centered cubic (until then, it is a mixed crystal with tetragonal crystals). From these relationships, it can be seen that when the amount of body-centered cubic bismuth oxide produced is about 80% or more, the change in leakage current over time when the device is energized is stabilized, and its life is extended. Finally, FIG. 6 shows the temperature dependence of the DC leakage current in relation to the amount of boric acid added and the heat treatment conditions. The amount of boric acid added and heat treatment conditions for each curve in the figure are shown below:

[Table] The reciprocal of leakage current and absolute temperature is actually shown as a curve, and although the activation energy tends to be slightly smaller at higher leakage current levels, there does not seem to be a large difference. Rather, the magnitude of the I ₀ value becomes a problem, such as the small I ₀ value in equation (1) in the case without heat treatment. FIG. 6 shows the optimum boron addition amount (lower leakage current) in the case of heat treatment (curve 2). In this way, the present invention is designed to simultaneously satisfy the three conditions described above: (1) long life, (2) excellent temperature characteristics of leakage current, and (3) as small a limiting voltage ratio as possible. The optimum amount of boric acid added and the content ratio of body-centered cubic crystals of bismuth oxide were determined by integrating the results shown in FIGS. In this experiment, all boron was added as an aqueous boric acid solution, but the above effect could not be obtained unless it was an aqueous boric acid solution. This is presumably because boron oxide (B ₂ O ₃ ), for example, is difficult to mix uniformly due to its reaction with water. In addition, although the amount of body-centered cubic bismuth oxide was controlled by heat treatment in this example, the amount of body-centered cubic bismuth oxide was
Any method such as atmosphere firing or consideration of additives may be used as long as it is 80% or more. As described above, according to the present invention, by controlling the amount of boric acid added and the amount of body-centered cubic bismuth oxide produced, an element with a long life and excellent temperature characteristics of limiting voltage ratio and leakage current can be produced. Obtainable.

[Brief explanation of drawings]

Figure 1 shows the current-voltage characteristics of a zinc oxide voltage nonlinear resistor, Figure 2 shows the heat balance in the element, and Figure 3 shows the relationship between the amount of boric acid added and the normalized limiting voltage ratio. Figure 4 shows the change in element leakage current over time, Figure 5 shows the annealing temperature and body-centered cubic bismuth production rate, and Figure 6 shows the temperature dependence of element leakage current. FIG.

Claims (1)

[Claims] 1 Contains at least boric acid and bismuth oxide as additives, the boric acid content is 0.02 to 0.1 mol%,
A zinc oxide voltage nonlinear resistor characterized in that 80% or more of the bismuth oxide is a body-centered cubic crystal.