This invention relates to a method for manufacturing polycrystalline metal oxide varistors. More particularly, this invention relates to a novel process for producing the varistor powder which is used in the pressing stage of varistor manufacture. The process disclosed produces varistor powder having a very high degree of homogeneity. This property of homogeneity is important both in varistors for low voltage applications and in varistors for high voltage application.
The primary ingredient in varistor manufacture is zinc oxide. To this zinc oxide is added the oxides of several other metals, notably bismuth, antimony, cobalt, manganese, tin, barium, boron, nickel, and magnesium. Other metal oxide additives include oxides of the rare earth elements. The zinc oxide is however, the largest single ingredient, the grain size of which largely determines the breakdown voltage in varistors.
Once the varistor powder with all of the desired ingredients is produced, the powder is pressed into a disk, slug, or wafer of the desired shape. Typically, the powder is pressed into a cylindrical shape. Then the pressed varistor wafer is sintered for several hours at temperatures between 1,000.degree. C. and 1,400.degree. C. The sintered ceramic is now coated with a suitable electrode material and wires are attached to the surfaces of the wafer for connection to external circuitry.
In particular, this invention relates to an improved process for producing the varistor powder to be used at the pressing stage of varistor manufacture. Currently, the additive oxides are milled and pre-reacted at a temperature of approximately 900.degree. C., then the pre-reacted product is crushed and mixed with ZnO. This mixture is then introduced into a ball mill whose objective is the further crushing of the powder mix and most importantly, the homogeneous dispersion of each component amongst the others. However, the ball milling process is expensive, time consuming, and inefficient. Ball milling is an attempt to achieve, by means of a mechanical process, the dispersion of the particles of each component of the powder mix with the particles of the other components.
In addition to being an inefficient mechanical procedure, the process of ball milling can introduce contamination from the ball material into the varistor powder mix. For instance, ZrO.sub.2 balls that are used presently in ball milling of metal oxide varistor powders release zirconium oxide into the varistor powder mix as a result of the impacts sustained by the balls. The presence of zirconium oxide in the varistor powder mix is highly deleterious in that this substance inhibits zinc oxide grain growth. It is this grain growth, which occurs during the sintering of the pressed varistor powder mix, which is responsible in large part for the characteristics of the varistor device, most notably, its breakdown voltage.
Between each zinc oxide grain there is an intergranular layer and each such layer contributes approximately two to three volts to the breakdown voltage of the varistor. For example, a one millimeter thick slab of varistor material having zinc oxide grains approximately 25 microns in diameter will yield a device with approximately 40 intergranular layers between the electrodes of the varistor, resulting in a breakdown voltage of approximately 120 volts, depending upon the exact composition of the varistor powder. The zirconia that is introduced from the ball milling process is not well dispersed in the powder mix and therefore produces localized regions of inhibited grain growth. This inhomogeneous distribution of zinc oxide grain size results in a varistor device with poor leakage and breakdown characteristics. In particular, it results in so-called hot spots in the varistor wafer which will conduct most of the current through the device rather than having the current density distribution spread evenly across the surface of the device. These hot spots lead to thermal heating and eventual fracturing of the varistor material.
As mentioned above, the ball milling process, even after many hours of milling, does not produce a molecularly homogeneous mixture. This lack of dispersion of the components of the mixture also produces, in a highly complex fashion, a discontinuity of grain growth. As indicated with the zirconia above, this too results in poor electrical properties.
This discontinuity of grain sizes produces highly undesirable electrical properties in both low breakdown voltage and in high breakdown voltage varistor devices. In low breakdown voltage varistors, the thickness of the wafer is small and therefore the number of grains and intergranular boundaries is small. Hence, the breakdown voltage of the device is more sensitive to the number of grains actually present. Similarly, in the high breakdown voltage varistor devices, the energy that must be dissipated is greater and an inhomogeneous varistor will result in the current being channeled through very narrow regions of the device which often fail, either electrically or mechanically, when these narrow regions are forced to channel these higher energies.
Metal oxide varistors exhibit a very non-linear current-voltage characteristic which is expressed by the equation ##EQU1## where I is current flowing through the material, V is the voltage across the material, C is a constant which is a function of the physical dimensions of the body of the device, its composition, and the parameters of the process employed to form the body, and is a measure of the voltage at which breakdown occurs, and .alpha. is a constant for a given range of current and is a measure of the non-linearity of the resistance characteristic of the varistor. Hence for voltage values below the breakdown voltage, the device behaves like an ohmic resistor of very large value (approximately 10,000 M .OMEGA.) but when the breakdown voltage is exceeded, the device behavior is very much like that of a low resistance conductor. For a wide range of current values, .alpha. is approximately constant.
When any of the above-mentioned metal oxides need to be added to the varistor powder mix in trace amounts, the ball milling process becomes particularly inefficient and incapable of insuring a homogeneous distribution of the trace material.