MULTIPLE-PEAKED STRUCTURE IN THE NONLINEARITY COEFFICIENT OF ZNO VARISTORS

Computer simulations of the current conduction through the granular mi crostructure of polycrystalline ZnO varistors are used to explain the multiple-peaked structure observed in the nonlinearity coefficient mea sured as a function of voltage for low-voltage varistors and for varis tors with microelectrode contacts on their surface. Two-dimensional Vo ronoi networks with random mixtures of electrically nonlinear and ohmi c elements are used to model the varistors. It is shown that the stati stical dispersion of the grain sizes has no significant effect on the nonlinearity coefficient. The multiple-peaked structure is caused by a small amount of randomly distributed grain boundaries with ohmic beha vior; i.e., linear current-voltage characteristics of low resistivity. The observed reduction of the maximum nonlinearity coefficient of pol ycrystalline varistors, relative to that of isolated grain boundaries, is attributed to the presence of varistor microjunctions with poor no nlinearity.