Enhancement of non-linear properties in SnO2 varistors by ZnO doping

The effect of Zn doping content in SnO2-based varistors was investigated. The general formula was ((Sn0.985Co0.01Nb0.005Oβ)1−x(ZnO)x) ; x = 0, 0.005, 0.01, 0.02 and 0.04. The powders synthesized by chemical precipitation were 4.4 - 6.1 nm in crystallite size along both [110] and [101]. Only cassiterite (SnO2) phase was observed within the limit of detection of X-ray diffraction (XRD). Zn doping yielded great enhancement in sinterability. High densities were achieved in the x = 0.04 sample at 1150 °C whereas it took 100 °C higher to achieve the similar density for the undoped sample. Substitution of Zn cations into the Sn lattice was believed to generate oxygen vacancies, facilitating diffusion and mass transport. As a result, the average grain sizes initially increased at x = 0.005 and gradually decreased at higher Zn content possibly due to existence of secondary phases. The optimal composition with the greatest breakdown voltage (6384 V cm−1) and non-linear coefficient (17.2) occurred at x = 0.02. This improvement in varistor properties has been attributed by formation of more effective Schottky-type potential barrier. The correlations between varistor properties and other microstructural aspects were also discussed.