Microwave sintering of doped nanocrystalline ZnO and characterization for varistor applications

Doped nanocrystalline zinc oxide powder with particle sizes ranging from 30 nm to 200 nm was synthesized by spray pyrolysis technique. The powder after calcination at 750 °C was consolidated and sintered using microwaves by employing different sintering schedules. The effect of heating rate on the density and electrical properties was studied. An optimized sintering temperature of 1100 °C was employed to obtain well-densified samples by both microwave and conventional processing. The sintered disks were characterized for their densities, microstructure, phase composition by X-ray diffraction and varistor properties like breakdown voltage, leakage current and non-linearity coefficient. The electrical properties obtained using microwave sintering technique was compared with those obtained on a conventionally sintered material as well as those of a commercially available product. The microwave sintered doped nanocrystalline ZnO yielded 98% dense bodies and a breakdown voltage of 9.6 kV cm−1, a leakage current of 0.25 μA cm−2 and a coefficient of non-linearity of 70 when compared to 4.3 kV cm−1, 1.2 μA cm−2 and 118 as measured for a conventionally sintered compact.