THE INFLUENCE OF BI2O3 NONSTOICHIOMETRY ON THE NONLINEAR PROPERTY OF ZNO VARISTORS

Proper choice of the starting compounds for the additive oxides enhances the non-linearity coefficient (alpha) to 40-60. High alpha-values can be achieved by using Bi2O3 containing Bi5+ and consequently higher oxygen content. Bi2O3 prepared by various low temperature chemical routes are more effective in increasing the alpha values. Ceramics formulated from ZnO + oxygen - excess Bi2O3 have alpha = 6 to 14, whereas in presence of the transition metal oxides, the same formulations exhibit alpha-values up to 60, with the sharp turn-on point on the current-voltage curves. The oxygen-excess behaviour of Bi2O3 is related to its defect-fluorite structure, with the excess oxygen located at the vacant tetrahedral voids and are charge compensated by Bi5+ ions. This is supported by X-ray diffraction, optical reflectance spectra and electron paramagnetic resonance data. The depletion layer is formed at the pre-sintering stage as a result of electron trapping by oxygen evolved from the additives being chemisorbed on ZnO particles. The depletion layer is stabilized on either sides of the grain boundary by the higher valent transition metal ions that are preserved through charge compensation by cation vacancies, after the sintering.