Investigation on defect structure in ZnO varistor ceramics by dielectric spectra

Defect structure of ZnO varistor ceramics plays a key role in determining the electrical properties. In this paper defect structure was investigated by dielectric response of pure ZnO ceramics, binary and multi-element ZnO-Bi ₂O₃ based varistor ceramics with the help of broad band dielectric spectroscopy. It was found that no loss peak appears around 10 ⁵Hz at room temperature in pure ZnO ceramics at all, while there is one and two loss peaks in the binary and multi-element ZnO-Bi₂O ₃ based varistors ceramics respectively. According to the activation energies of two peaks, it is deduced that the loss peaks originate from relaxation process of electrons trapped by oxygen vacancy and zinc interstitial. In order to distinguish the influence of zinc interstitial and oxygen vacancy, the effect of heat treatment in air, rich nitrogen and oxygen atmospheres was investigated, for the oxygen diffuses mainly along grainboundaries. Based on dependences of the two loss peaks on the temperature and the atmosphere of heat treatment, the degradation mechanism is explored.