ZnO-based varistors prepared by spark plasma sintering

Varistor ceramics were prepared by spark plasma sintering (SPS) using two doped zinc oxide based powders. Optimized sintering cycle for the commercial powder (mixture of oxides) yielded dense parts (>99%) containing mainly ZnO grains (1 μm) and additional Bi2O3 and Zn7Sb2O12 oxides. Sintering at the low oxygen partial pressure inherent to SPS leads to the reduction of Bi2O3 into metallic Bi. To reduce the grain size, improve the distribution of dopants and the grain size distribution, we used co-precipitation to synthesize a powder with the same formulation as the commercial powder. Due to the high reactivity of this powder, fully dense parts were obtained at temperature as low as 400 °C under air atmosphere using carbide tools. Such conditions led to limited ZnO grain growth and bismuth reduction during the thermal cycle. The fine microstructure obtained led to varistances with to high electrical properties (threshold field higher than 2300 V/mm).