Electrical nonlinearity of (Ni, Ta) doped SnO 2 varistors

Abstract

The electrical nonlinearity of (Ni, Ta) doped SnO 2 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% Ni 2O 3 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% Ni 2O 3 and 0.05 mol% Ta 2O 5 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 φ B of 0.693 eV. An atom mode for the acceptors and donors to penetrate into SnO 2 lattice was put forward. To illustrate the grain-boundary barrier formation of (Ni, Ta) doped SnO 2 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 SnO 2 lattice sites of depletion layers.