Abstract
Microstructural disorder in the form of variations in grain size, grain boundary area between adjacent grains and grain boundary barrier heights are characteristic features of semiconducting ceramics exhibiting varistor behavior. We investigate the effect of such disorder by computing the current flow through a network of grains whose boundaries are described by nonlinear electrical characteristics. Disorder is found to decrease the maximum attainable nonlinearity of the varistor current-voltage characteristics and also cause a rounding of the breakdown portion of the curve. Disorder also has the effect of introducing high conductance percolative paths into the microstructure which can cause localization in current flow. We show that current localization can be quantitatively described by two statistical parameters: an effective number of current paths through the network and a participation ratio. The former is related to the infinite moment of the current distribution whereas the latter is related to the second moment.
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