HIGH-TEMPERATURE BEHAVIOR OF ZNO-BASED CERAMIC VARISTORS

Current-voltage data for metal oxide varistors are presented in the temperature range 100-1300°C and for current densities as high as 10 3 A/cm2. The current-voltage characteristic can be crudely represented at all temperatures by a breakdown line I=CVα where α≳30 (and is somewhat voltage dependent) and a temperature-dependent Ohmic leakage conduction, α=1, which is smoothly joined to the breakdown characteristic by a transition region (1<α<30) called the prebreakdown region. The Ohmic conductance is associated with a thermal excitation process over grain-boundary barriers and can be resolved into three distinct thermally activated regions which are correlated with similar features found in conductivity data for pure sintered ZnO. Low-field capacitance data on varistors show that all or part of these barriers can be attributed to depletion layers in the ZnO grains. Additional studies at very high average power levels show that the varistor characteristic can be degraded when the temperature is allowed to increase unduly due to self-heating. This degradation effects only the Ohmic and prebreakdown regions; the breakdown characteristic is unaltered. The varistor can be restored to its initial state by annealing.