Effects of In2O3on the properties of (Co, Nb)-doped SnO2varistors

The effects of In2O3 on the properties of (Co, Nb)-doped SnO2 varistors were investigated. It was found by characterizing the samples sintered at 1350°C that the nonlinear coefficient presents a peak of α = 20.4 for the concentration of 0.05 mol% In2O3, the average grain size decreases from 8.4 to 3.9 μm, the breakdown electrical field increases from 206 to 821 V mm−1 and relative electrical permittivity decreases from 2.3 to 0.16 k with increasing In2O3 from 0.00 to 0.10 mol%. The increase of the breakdown electrical field with increasing In2O3 concentration is mainly attributed to the decrease of the grain size. The reason why the permittivity decreases with increasing In2O3 concentration was originated from the ratio of the grain size to the barrier width. To illustrate the grain-boundary barrier formation of (In, 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 instead at SnO2 lattice sites of depletion layers.