STATISTICS AND GRAIN-SIZE IN ZINC-OXIDE VARISTORS

A ZnO varistor is a ceramic with grains of various sizes. Therefore, some of the electrical breakdown characteristics of the ceramic reflect variations in the voltage impressed upon a grain, rather than the breakdown characteristics of an average grain boundary. It is found that the mean breakdown voltage per grain in the ceramic is less than that of an isolated grain boundary because there are chains of long grains through the ceramic; the density of such chains is finite even in a thick sample. The maximum value of the coefficient of nonlinearity αmax, which measures the rapidity of breakdown, is found to be characteristic of the grain boundaries when αmax is moderate, that is, if αmax <50. The high values of αmax observed at low temperatures appear to be limited by statistical effects rather than being representative of the boundaries.