Varistor properties of Sc2O3-doped Sn·Co·Nb ceramics

The varistor properties of Sc2O3-doped SnO2·Co2O3·Nb2O5 ceramics were investigated. It was found that the non-linear coefficient presents a peak of 17.1 at the concentration of 0.06 mol% Sc2O3, the average grain size decreases from 10 to 5 μm, the breakdown electrical field increases from 199 to 1790 V/mm and relative electrical permittivity decreases from 2600 to 60 as Sc2O3 concentration was increased up to 0.09 mol%. The increase of the breakdown electrical field with increasing Sc2O3 concentration is mainly attributed to the decrease of the average grain size. The reason why the permittivity decreases with increasing Sc2O3 concentration was originated 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 be located at SnO2 lattice sites in the depletion layers, instead at the grain interfaces.