Electrical nonlinearity of (Ni, Ta) doped SnO2 varistors

The electrical nonlinearity of (Ni, Ta) doped SnO2 varistor system was investigated. The nonlinear coefficient α and the barrier height of this varistor system were calculated. It is found that the variations of electrical nonlinear coefficients with acceptor concentrations are in accordance with that of the barrier heights. 0.75 mol% Ni2O3 doped sample sintered at 1370 °C exhibits a high nonlinear coefficient α of 21 and a high breakdown electric field (697 V mm−1 at 1 mA cm−2), but presents a relatively low densification. Among the samples sintered at 1420 °C, the sample doped with 0.75 mol% Ni2O3 and 0.05 mol% Ta2O5 possesses the highest breakdown electric field (469 V mm−1 at 1 mA cm−2) and the highest electrical nonlinear coefficient α of 16.3, which is consistent to its highest defect barrier φB of 0.693 eV. An atom mode for the acceptors and donors to penetrate into SnO2 lattice was put forward. To illustrate the grain-boundary barrier formation of (Ni, Ta) doped SnO2 varistors, a modified defect barrier model was introduced, in which the negatively charged acceptors substituting for Sn ions should not be located at the grain interfaces instead at SnO2 lattice sites of depletion layers.