Novel (Zn, Nb)-doped SnO2 varistors

The effect of ZnO on the Nb-doped SnO2 varistors was investigated. The grain boundary barrier height (ΦB) of the varistors were determined by use of Schottky type of conduction mechanism. The grain boundary impedance measurements of the varistors were performed using the impedance spectroscopy technique. It was found that ZnO had a significant effect on the varistor properties of Nb-doped SnO2 ceramics. An optimal doping composition of 98.95SnO2–1.00ZnO–0.05 mol% Nb2O5 with the highest nonlinearity with α=12.3, the highest ΦB=0.73 eV and the largest grain boundary resistance RGB=3.43×106 Ω cm was obtained. A defect barrier model was introduced to illustrate the formation of the grain boundary barrier for SnO2–ZnO–Nb2O5 ceramic varistors. The key element of this model is that the depletion layers formed at the near-grain-boundary region due to the introduction of defects in the crystal lattice, are responsible for the formation of Schottky type potential barriers at the grain boundaries.