Effect of annealing on the ac leakage components of the ZnO varistor. I, Resistive current

The ZnO varistor is known to require a postsintering heat treatment (annealing) to impart stability to an otherwise unstable device, when subjected to an applied voltage stress. A large body of experimental data in the literature indicate that the most desirable temperature of annealing is between 600 and 700 °C at which the varistor becomes highly stable. This paper is aimed at understanding the significance of this annealing temperature from the analysis of the activation energies, estimated from the Arrhenius plots of the resistive current versus temperature in the prebreakdown region. It is found that the activation energy in the annealing temperature range of 600–700 °C is significantly higher than at any other annealing temperature between 500 and 800 °C. This conclusion is borne out by the statistical evaluation of the experimental data. When this result is combined with the equally notable observations in the literature that the interface state density [J. Mater. Sci. 20, 3487 (1985)], the grain-boundary trap density [J. Appl. Phys. 63, 5375 (1988)], and the density of the negatively charged zinc vacancy at the interface [J. Appl. Phys. 66, 6132 (1989)] are all reduced, also at the same annealing temperature range, it becomes clear that the annealing temperature of 600–700 °C is very unique to varistor stability. The significance of these observations is that the annealing and accompanying stability should be recognized as a very complex process that brings fundamental changes in the grain-boundary junction characteristic of the polycrystalline ceramic.