Microstructural origin of current localization and “puncture’’ failure in varistor ceramics

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Abstract
The effect of microstructural disorder, such as variations in grain size and in the height of the electrostatic potential at grain boundaries, on current flow in varistors has been simulated by computing the currents in a network of grain boundaries each of which can be described by nonlinear electrical characteristics. Microstructural disorder causes current localization, which is analyzed in terms of two parameters, the infinite moment and the second moment of the current distribution. These parameters enable quantitative comparison of different microstructures. By incorporating the effect of Joule heating the temperature increases due to current localization can be simulated and the effect of short electrical pulses of different energies investigated. On the basis of the simulations a mechanism for current localization, due to microstructural variations, leading to thermal runaway and “puncture’’ failure by melting is proposed. The results suggest that electrode protrusions and large grains at the electrodes are likely sources for failure, and that some degree of microstructural disorder is required to resist thermal runaway for all but the most energetic pulses.