JPH0249521B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention has a zinc oxide grain boundary segregated phase in a sintered body containing zinc oxide as a main component and having voltage nonlinearity, and is composed mainly of a bismuth titanate layered compound, thereby achieving a low voltage rise voltage. Concerning voltage nonlinear resistors with thermal stability and thermal stability. [Technical Background of the Invention] In recent years, with the rapid development of semiconductor elements and circuits such as ICs, transistors, and thyristors, and their applications, the use of semiconductors and semiconductor circuits in control, communication equipment, and power equipment has become widespread. Devices are rapidly becoming smaller and more sophisticated. However, on the other hand, with such progress, the withstand voltage, surge resistance, and noise resistance of these devices and their components cannot be said to be sufficient. Therefore, it is necessary to protect these devices and components from abnormal surges and noise, or to stabilize the circuit voltage. Until now, SiC and Si varistors have been widely used for these purposes. Recently, varistors have been developed that use zinc oxide as a main component and add additives to it. However, recently, with the increase in density and integration of semiconductors and semiconductor circuits, lower voltage and lower power consumption of semiconductors and semiconductor circuits have been promoted. In order to protect these semiconductors and semiconductor circuits or to stabilize circuit voltage, varistors that operate at lower voltages have become necessary. The voltage-current characteristics of a varistor are generally expressed by the following relational expression: I=(V/C). Here, V is the voltage applied to the varistor body, and I is the current flowing through the varistor body. Further, C is a constant corresponding to the voltage when a given current is passed. α=1 is an ordinary resistor that follows Ohm's law, and the larger α is, the better the nonlinearity is. Here, instead of being expressed by C and α, the varistor characteristics are expressed by a rising voltage V1mA and α when a current is passed at a current density of 1 mA/cm ² . SiC varistors, which are a typical sintered body with voltage nonlinearity, are made by baking SiC particles with a ceramic binder.
The nonlinearity is due to the voltage dependence of the contact resistance of SiC particles. If the thickness of the varistor body is 1 mm, the rising voltage when a current of 1 mA/ ^cm2 flows is V1.
Assuming mA/mm, SiC varistors can reduce V1mA to several V by controlling the particle size and element thickness of SiC particles.
It is possible to manufacture products with voltages ranging from 1000V to several thousand volts.
Since the non-linear coefficient α is as small as 3 to 7, this is not sufficient.
Also, like SiC varistors, there are zinc oxide varistors whose sintered bodies themselves have nonlinearity. This is mainly composed of zinc oxide and contains small amounts of bismuth oxide, cobalt oxide, manganese oxide, antimony oxide, etc. as additives. The nonlinearity coefficient of zinc oxide-based varistors is about 20 to 50, which shows extremely superior nonlinearity compared to SiC varistors. For this reason, it is very suitable for protecting semiconductors and semiconductor circuits, but the voltage used in semiconductor circuits is becoming increasingly lower. [Problems in the background technology] Conventional zinc oxide varistors have been developed as low voltage varistors (V1m
A is about 20V). The reason for this is that, as shown in FIG. 1, the fine structure of the zinc oxide-based varistor is such that low-resistance zinc oxide crystal grains 1 are surrounded by high-resistance grain boundary layers 2 that exhibit nonlinearity. In the figure, 3 is an electrode and 4 is a terminal. Therefore, V1mA per 1mm of sintered body thickness/
mm is determined by the number of grain boundary layers 2 present in the thickness direction of the element body. In a zinc oxide varistor, the rising voltage per grain boundary layer is about 2.5V. When zinc oxide mixed with bismuth oxide, cobalt oxide, manganese oxide, etc. is sintered at a temperature of 1100 to 1300°C, the size of the zinc oxide crystal grains 1 in the sintered body is about 30 to 40 μm. Therefore, V1mA/
mm is 40V or more. Therefore, in order to manufacture a varistor with V1mA = 20V, it is necessary to make an element body with a thickness of 0.5mm. In this case, the microstructure is such that ten zinc oxide crystal grains 1 are connected in series and eight grain boundary layers 2 are present between the grains. Thickness 0.5mm
As a method of manufacturing the element body, there is a method of polishing the sintered body to obtain a predetermined thickness. However, the mechanical strength of the sintered body is weak because the zinc oxide crystal grains 1 are surrounded by grain boundary layers 2 mainly made of bismuth oxide.
As a result, zinc oxide crystal grains 1 fall off during polishing, microcracks occur, and defective areas with locally low voltage occur. Another method is to mold the sintered body into a thin plate shape, taking into consideration the shrinkage rate after sintering so that the sintered body has a predetermined thickness. In this case, it must be molded to a thickness of 0.6 to 0.7 mm, taking into account the shrinkage rate. However, the mechanical strength of the molded product is very low, and cracks and chips occur, making it extremely difficult to handle the molded product. In order to eliminate these drawbacks, the grain size of the zinc oxide crystal grains 1 of the sintered body should be grown to about 100 μm, so that a varistor with a thickness of about 1.0 mm and V1mA = 20V can be produced, and the strength of the molded body should be increased. I need to make it stronger. Recently, as a method for manufacturing zinc oxide-based low voltage varistors, there is a method of using zinc oxide crystal grains 1 having a grain size of about 100 μm, as proposed in, for example, Japanese Patent Application Laid-open No. 140995/1983. This is because it is difficult to grow zinc oxide crystal grains 1 to a size of about 100 μm during the sintering process, so zinc oxide crystal grains 1 are grown large from the beginning in a part of the main component, zinc oxide.
is used. However, in order to obtain large zinc oxide crystal grains 1, a mixture of zinc oxide and barium carbonate or strontium carbonate is molded.
It is necessary to sinter and boil this sintered body in pure water to dissolve Ba and Sr surrounding the zinc oxide crystal grains 1 in water and decompose the sintered body. Further, the grain size of the raw materials other than the zinc oxide crystal grains 1 is several μm or less. Therefore, during the sintering process, there is a difference in shrinkage rate between one part of the zinc oxide crystal grains and the other part of the raw material powder, making the sintered body porous. Therefore, in order to obtain a dense element body with a low V1mA/mm, the sintering temperature of the zinc oxide crystal grains 1,
Selection of conditions such as particle size, addition ratio, composition of raw materials other than crystal nuclei becomes complicated. Furthermore, a drawback of zinc oxide-based varistors containing bismuth oxide as an additive is that the life characteristics vary greatly depending on the heat treatment conditions of the sintered body (including the electrode baking process). This is because the grain boundary layer 2 that exhibits nonlinearity is mainly composed of bismuth oxide. The bismuth oxide crystal phase in the sintered body before heat treatment includes at least one of an α phase, a β phase, and a δ phase. By heat-treating this sintered body, all bismuth oxide is converted into the γ phase. It is thought that the life characteristics vary greatly due to this process of change to the γ phase. As described above, manufacturing a low voltage varistor using a composition system containing zinc oxide as a main component and bismuth oxide as an additive has the disadvantage that the number of steps is extremely large, and the selection and management of conditions are also complicated. [Purpose of the invention] The present invention uses zinc oxide as the main component, and the zinc oxide grain boundary segregated phase is mainly given to bismuth oxide, and is composed of a bismuth titanate layered compound, thereby growing zinc oxide crystal particles in a sintered body to a size of 100 μm or more. It is an object of the present invention to provide a voltage nonlinear resistor with low voltage. [Summary of the Invention] The voltage nonlinear resistor of the present invention is a voltage nonlinear resistor mainly composed of zinc oxide and in which the sintered body itself has voltage nonlinearity, in which the zinc oxide crystal grain boundary segregated phase of the sintered body is It mainly consists of a bismuth layered compound represented by (Bi2O2) ²⁺ (Bi2Ti3O10) ^2- . [Embodiments of the Invention] The details of the present invention will be described below. In other words, the fine structure of the varistor in which the zinc oxide grain boundary segregated phase is mainly composed of bismuth titanate layered compound is similar to that of conventional bismuth oxide, and the grain boundary layer that exhibits nonlinearity is composed of bismuth titanate layered compound. It is. This has made it possible to eliminate crystal phase changes in the grain boundary layer during the heat treatment process. As described above, the present invention provides a highly reliable, low-voltage nonlinear resistor obtained by having the zinc oxide grain boundary segregated phase mainly composed of a bismuth titanate layered compound instead of bismuth oxide. Next, embodiments of the present invention will be described. Starting materials zinc oxide, bismuth oxide, bismuth titanate layered compound, cobalt oxide, manganese oxide, and nickel oxide were weighed to the composition ratios shown in Table 1.
After mixing, drying, granulation, and molding, the mixture was sintered at a temperature of 1100 to 1300°C to obtain a sintered body with a diameter of 15 mm and a thickness of 1.0 mm.
A 1.0 cm ² electrode showing ohmic contact was formed on this, and V1 mA/mm and nonlinear coefficient α were measured. Table 1 shows the results. Examples of the present invention are composition Nos. 5 to 8 marked with a circle, and these were sintered at a temperature of 1220° C. for 2 hours.

[Table] In Table 1, compositions No. 1 to 4 are reference examples in which the amount of bismuth oxide was changed, and compositions No. 5 to 8 are examples in which the amount of bismuth titanate compound was changed. From Table 1, compositions No. 5 to 8 of Examples have the highest V1m
It can be seen that the A/mm is lower and the nonlinear coefficient α is also higher, so the effect is remarkable. Also bismuth oxide,
FIG. 2 shows the change in V1mA/mm of the sintered body with respect to the amount of bismuth titanate compound added, and FIG. 3 shows the change in the nonlinear coefficient α. In FIGS. 2 and 3, curve A shows the case of bismuth oxide, and curve B shows the case of bismuth titanate compound. From FIG. 2 and FIG. 3, the example of curve B in which bismuth titanate layered compound is added is compared with the reference example of curve A in which bismuth oxide is added. I know it will grow significantly. Figure 4 shows the life characteristics with respect to heat treatment temperature, that is, the rate of change ΔV1mA in the rise voltage V1mA when a current of 1mA/cm ² is applied for 1000 hours in an atmosphere of 25°C in air.
Curve A2 is a reference example of composition No. 2, curve B6 is composition No. 6
The case of the example is shown below. It can be seen from FIG. 4 that the example of curve _B6 containing a bismuth titanate layered compound as the zinc oxide grain boundary segregated phase is not affected by the heat treatment temperature. Furthermore, if the amount of the bismuth titanate layered compound added is less than 0.05 mol%, it is not sufficient to grow zinc oxide crystal grains and V1mA/mm becomes high, which is not appropriate. In addition, if it exceeds 3.0 mol%, it will exceed the required amount and cause damage such as precipitation on the surface of the sintered body and fusion of the sintered body, V1mA/mm will also increase, and the nonlinear coefficient α will also start to decrease, so it is not appropriate. Not. In addition to the additives used in the above examples,
Sb, Cr, Sn, Al, Mg, Ba, B, Si, Pb, Fe,
Nonlinearity can be greatly improved by adding a small amount of Sr oxide. [Effects of the Invention] As detailed above, the present invention uses zinc oxide as the main component, and uses bismuth titanate layered compound as the zinc oxide crystal grain boundary segregated phase in the form of Bi ₄ Ti ₃ O ₁₂ in terms of 0.05~ By containing 3.0 mol%, zinc oxide crystal grains can be grown to 100 μm or more, and there is no crystal phase change in the grain boundary layer during the heat treatment process, making it possible to create a voltage nonlinear resistor with high reliability and stable characteristics. can be provided.

[Brief explanation of the drawing]

Figure 1 is an enlarged cross-sectional view showing the fine structure of a zinc oxide varistor, Figures 2 to 4 are comparisons of characteristics between examples of the present invention and reference examples, and Figure 2 shows the amount of additives added. FIG. 3 is a curve diagram showing the change in V1mA/mm with respect to heat treatment temperature, FIG. 3 is a curve diagram similarly showing the change in the nonlinear coefficient α, and FIG. 4 is a curve diagram showing the rate of change in V1mA with respect to the heat treatment temperature. 1...Zinc oxide crystal grain, 2...Grain boundary layer, 3...
Electrode, 4...terminal.

Claims (1)

[Scope of Claims] 1. In a voltage nonlinear resistor whose main component is zinc oxide and in which the sintered body itself has voltage nonlinearity, the zinc oxide grain boundary segregated phase of the sintered body is mainly (Bi ₂ O ₂ ) ²⁺ (Bi ₂ Ti ₃ O ₁₀ ) ^2- A voltage nonlinear resistor comprising a bismuth layered compound represented by (Bi 2 Ti 3 O 10 ) 2-. 2 Bismuth layered compound is converted into Bi ₄ Ti ₃ O ₁₂ type.
The voltage nonlinear resistor according to claim 1, characterized in that the voltage nonlinear resistor contains 0.05 to 3.0 mol%.