Electrical-Impulse-Induced Fracture of Zinc Oxide Varistor Ceramics

When varistors are subject to very short (˜1 μ s), high-current pulses, they occasionally fracture. We attribute the occurrence of fracture to stress waves, created by inertial forces in response to the rapid Joule heating, causing the propagation of preexisting microstructural flaws. The amplitude of the stress oscillation is shown to be proportional to the second derivative of temperature with respect to time which, in turn, is directly related to the rate of increase of power. An upper limit for the magnitude of the peak stress is derived in terms of material parameters, varistor geometry, and the maximum electrical power. From the magnitude of the peak stress, a critical defect size is estimated which will cause fracture under given pulse conditions. Conclusions for the design and testing of varistors are drawn.