Effects of Er₂O₃ on Electrical Properties of the SnO₂.CoO.Ta₂O₅ Varistor System

We investigate the effects of Er₂O₃ on electrical properties of the SnO₂.CoO.Ta₂O₅ varistor system sintered at 1400°C. It is found that all the samples have excellent nonlinear electrical characteristics and the sample with 0.50 mol% Er₂O₃ has the best nonlinear electrical property and the highest nonlinear coefficient (α = 43.7). The high nonlinear coefficient value obtained in the system indicates that the SnO₂-based varistor is a candidate for ZnO-based varistors in commercial applications. Er₂O₃ additive can significantly affect the average grain size. With increasing Er₂O₃ concentration from 0.10 mol% to 1.00 mol%, the average grain size decreases from 21.2μm to 10.6μm, the breakdown electrical field increases from 208 V/mm to 459 V/mm, and the relative electrical permittivity decreases from 2440 to 1210. The reason that the grain size decreases with increasing Er₂O₃ concentration is explained. Also, we present a modified defect barrier model to illustrate the grain-boundary barrier formation of Er₂O₃-doped SnO₂ based varistors.