The influence of $Bi_2O_3$ non-stoichiometry on the non-linear property of ZnO varistors

Proper choice of the starting compounds for the additive oxides enhances the non-linearity coefficient $(\alpha)$ to 40-60. High or-values can be achieved by using $Bi_2O_3$ containing $Bi^{5+}$ and consequently higher oxygen content. $Bi_20_3$ prepared by various low temperature chemical routes are more effective in increasing the a values. Ceramics formulated from ZnO + oxygen - excess $Bi_20_3$ have $\alpha$ = 6 to 14, whereas in presence of the transition metal oxides, the same fomulations exhibit $\alpha$-values up to 60, with the sharp turn-on point on the current-voltage curves. The oxygen-excess behaviour of $Bi_20_3$ is related to its defect-fluorite structure, with the excess oxygen located at the vacant tetrahedral voids and are charge com- pensated by $Bi^{5+}$ ions. This is supported by X-ray diffraction, opticaf 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 talent transition metal ions that are preserved through charge compensation by cation vacancies, after the sintering