Electrical nonlinearity of (Cu, Ni, Nb)-doped SnO2 varistors system

A best nonlinearity properties is obtained in the varistors of Sn–Ni–Nb, via the investigation of the varistors with compositions of (99.2−x)% SnO2+0.75% Ni2O3+0.05% Nb2O5+x% CuO. The sample doped with 1.0 mol% CuO exhibits the highest electrical nonlinear coefficient (α=25) and the highest relative density (96.1%). SEM micrographs show that grain size increases with increasing CuO content. The phenomenon can be explained by interface defect model, the substitution of Cu2+ for Sn4+ facilitates the formation of the defect barriers and facilitates the grain growing. Breakdown voltage increases with increasing CuO when doping less than 1.0 mol% CuO, and reaches a maximum (946 V mm−1) at 1.0 mol% CuO dopant then decreases with increasing CuO. It can also be explained by the effect of Cu2+ substituting for Sn4+.