Effects of Sc₂O₃ on the microstructure and varistor properties of (Co, Nb)-doped SnO₂

Abstract

The effects of Sc₂O₃ on the properties of (Co, Nb)-doped SnO₂ varistors were investigated. It was found that by characterizing the samples, sintered at 1300 °C, the nonlinear coefficient presents a peak of α=28.6 for a concentration of 0.05 mol % Sc₂O₃. The average grain size decreases from 2.9 μm to 1.5 μm, and the breakdown electrical field increases from 655 V/mm to 2197 V/mm. The relative electrical permittivity decreases from 7574 to 792 on increasing Sc₂O₃ from 0.05 mol % to 0.08 mol %. The increase of the breakdown electrical field with increasing Sc₂O₃ concentration is mainly attributed to the decrease of the grain size. The reason why the permittivity decreases with increasing Sc₂O₃ concentration originates from the ratio of the grain size to the barrier width. To illustrate the grain-boundary barrier formation of (Sc, Co, Nb) 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, but at SnO₂ lattice sites of depletion layers instead.