Effect of variations in grain size and grain boundary barrier heights on the current-voltage characteristics of ZnO varistors

Throughout the microstructure at every varistor, there exist variations of the grain size and the electrical properties of the individual grain boundaries, To calculate the effect of such microstructural variations on the overall electrical transport properties, we describe a multibond percolation approach utilizing effective medium theory. The model presented takes into account these variations as well as the electrical conductivity of the ZnO grains themselves, To illustrate the predictions of the model, both the effect of continuous distributions in the grain boundary potential and the grain size are considered, as well as a bi-lognormal distribution in grain size to represent the effect of a population of anomalously large grains in a smaller grain size varistor.