Electro-thermal simulation studies for pulsed voltage failures in microstructured ZNO varistors

Time-dependent, two-dimensional simulations based on random Voronoi networks have been developed to study the internal heating and related breakdown effects in ZnO varistors in response to high-voltage pulsing. The focus is on internal grain-size variations and relative disorder. Our results predict that parameters such as the device hold-off voltage, the average internal temperature, and average dissipated energy density would be higher with more uniform grains. This uniformity is also predicted to produce lower thermal stresses and to allow for the application of longer duration pulses. Finally, it is shown that the principle failure mechanism arises from internal localized melting, while thermal stresses are well below the thresholds for cracking.