PULSE-DEGRADATION CHARACTERISTIC OF ZNO VARISTORS

We find that two factors bear on the high-energy pulse degradation of a varistor's electronic characteristic. First, the electronic structure of active grain-boundary segments depends solely on the peak temperature occurring there during a pulse, and second, this peak temperature is determined by the thermal transport properties of the microstructure. We find that the intrinsic electronic structure can be modified without changing the microstructure to greatly improve the pulse-degradation performance of a varistor by an anneal at low temperature (600 °C) in oxygen. The influence of microstructure is embodied in the grain-boundary temperature-magnification factor Zgb which is defined, measured, and studied theoretically. Theory shows that there are three regimes: (i) a long-pulse-width regime where Zgb=1, (ii) a short-pulse-width regime where Zgb∼ (pulse width)-1/2, independent of pulse energy, and (iii) a very-short-pulse-width (or high-energy-density) regime where Zgb∼ (pulse energy) -1/2, independent of the pulse width. The theory contains one microstructural parameter. Our data encompasses the first two regimes and agrees well with theory for a value of the microstructural parameter which is in good agreemet with earlier work.