Dielectric spectra of ZnO varistor ceramics

In this paper, the dielectric spectra of ZnO varistor ceramics are measured by Novocontrol wide band dielectric spectrometer in a temperature range of -160°C-200°C and frequency range of 0.1 Hz-0.1 MHz. It is found that electron transportation can be characterized by the flat region on a low frequency side of σ′-f curve. The Schottky barrier height is 0.77 eV obtained from σ′-f curve, which is consistent very well with the data from I-V curves given in other literature. On the basis of back-to-back double Schottky barrier model, Schottky barrier height corresponding to electron transportation across grainboundary is explained to be the energy difference between interface state and barrier top. According to this explanation, Schottky barrier height will increase linearly with the increase of DC voltage applied. The linear variation of barrier height with the increase of DC voltage applied is confirmed experimentally. Finally, the theoretical value of averaged grain size is obtained to be 6.8 μm, which is almost identical to 6.5 μm measured from SEM images. Therefore, the macroscopic electrical properties and the microstructure can be expressed at the same time by dielectric spectra.