Varistor behaviour of Mn-doped ZnO ceramics

Polycrystalline ZnO doped with MnO, from 0.1 to 0.6 mol%, was prepared by conventional ceramic processing. The effect of Mn doping on the electrical properties of ZnO was investigated. Samples quenched from the sintering temperature show ohmic behaviour, while pronounced varistor behaviour is found in Mn-doped ZnO obtained by slow cooling from the sintering temperature or by annealing at a lower temperature. The origin of the varistor effect in the samples of polycrystalline ZnO with MnO as the only additive is discussed. The defect equilibrium analysis suggests that the varistor behaviour in these samples is due to the oxidation of the double ionised zinc interstitial defects present at grain boundaries by ambient oxygen during cooling or annealing, and the presence of Mn in the ZnO grains induces this process.