The effect of TiO ₂ concentration on the electrical and microstructural properties of ZnO-base varistor ceramic prepared from nanosize ZnO particles

This study investigated electrical properties and microstructure of zinc oxide varistors containing MnO2, Cr2O3, V2O5 and TiO2. To this end, nanosize ZnO particles were synthesised by the polyacrylamide-gel method and doped with minor amounts of MnO2 (0.5 mol-%), Cr2O3 (0.5 mol-%), V2O5 (2 mol-%) and TiO2 in the range of 2–15 mol-%. The prepared samples sintered at 1050°C in air for 2 h. It was observed that TiO2 promoted grain growth of ZnO particles. X-ray examination showed hexagonal ZnO structure as a main phase and Zn2TiO4 and ZnV2O4 as minor secondary phases. The nonlinear coefficient and threshold voltage of the prepared samples were in the range of 3.3–7.9 and 1120–4700 V cm−1 respectively, in which both of them increased with increase in TiO2 content. Among the prepared samples, the varistor with 15 mol-% TiO2 exhibited the best performance in terms of nonlinearity. The nonlinear electrical behaviour was explained by theory of formation of the potential barriers at grain boundaries.