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

The effects of In 2 O 3 on the properties of (Co, Nb)-doped SnO 2 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% In 2 O 3 , 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 In 2 O 3 from 0.00 to 0.10 mol%. The increase of the breakdown electrical field with increasing In 2 O 3 concentration is mainly attributed to the decrease of the grain size. The reason why the permittivity decreases with increasing In 2 O 3 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 SnO 2 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 SnO 2 lattice sites of depletion layers.