Effects of multiple current pulses on the microstructure and electrical properties of zinc oxide varistors

Multiple pulses were seen to be much more detrimental to the stability of a ZnO varistor than single pulses of the same magnitude. They were observed to lead to an increase in temperature and some vaporization. The degraded varistors were found to contain cracks in which an additional phase was situated. All experimental results indicate that this phase is amorphous in nature. A possible mechanism for its formation is proposed which assumes a nonuniform temperature distribution due to the development of extremely localized hot spots. The pulse energy is concentrated into very small regions, leading to high local temperatures which decrease rapidly due to conduction to the surrounding material. This leads to the formation of an amorphous phase. This phase could affect the electrical properties either by lowering the resistance of the material at low voltages or by affecting the metastable component of the potential barrier that exists at the grain boundaries of the varistor