Relationship between nonlinear resistivity and the varistor forming mechanism in ZnO ceramics

The use of a number of perovskite phases M″ M′O3−x, as the only forming additive in ZnO ceramics, produces a high nonlinearity index, α(up to 45), where M′ is a multivalent transition-metal ion and M″ is an alkaline earth or a rare-earth ion. From this study, the formation parameters crucial to high nonlinearity, such as nonstoichiometry in the as-received ZnO powder, low x values of the additives and fast cooling rate after the sintering, are explainable on the basis of a depletion layer formation at the presintering stage. This is because of the surface states arising out of the chemisorbed oxygen. The depletion layer is retained during sintering as a result of the higher valence state of M′ ions, preferentially present at the grain-boundary regions. The fast cooling freezes in the high-temperature concentration of donor-type defects, thereby decreasing the depletion layer width.