Characteristics of SnO2-doped ZnO-based varistor ceramics

The Sb2O3 in a ZnO-based varistor composition was systematically substituted with SnO2 in the range from 0 to 100%. The microstructural characteristics, average grain size and the general phase composition of the samples were not influenced by the substitutions. In all the samples we observed ZnO, Bi2O3-rich and spinel phase, where the composition of the spinel depends on the Sb2O3 to SnO2 ratio in the starting composition. The non-linear coefficient a of the samples was not influenced by the SnO2 substitution, while the threshold voltage decreases and the leakage current increased with larger additions of SnO2. The capacitance-voltage measurements indicated strong increase in the donor density and the density of interface states at the grain boundaries while the depletion-layer width decreased and the barrier height was comparable in all the investigated samples. According to the electrical properties, SnO2 had a strong donor effect in the ZnO varistor ceramics. (C) 2001 Elsevier Science Ltd. All rights reserved.