Mechanical activation of spinel and pyrochlore phases in ZnO based varistors

Mixtures of ZnO, Sb₂O₃ and Bi₂O ₃ powders corresponding to the stoichiometric compositions of Zn ₇Sb₂O₁₂ (spinel) and Zn₂Bi ₃Sb₃O₁₄ (pyrochlore) were milled in a shaker mill (SPEX) up to 180 minutes. The influence of the mechanical activation of the powders, as a result of the milling, was determined by Differential Thermal Analysis, Thermogravimetric Analysis and X-ray Diffraction. Additionally, in order to compare to commercial recipes the mixture ZnO + 2.7 wt% Sb ₂O₃ + 4.5 wt% Bi₂O₃ was milled in a shaker mill up to 180 minutes. Varistor devices were fabricated with both milled and unmilled mixtures. The devices were characterized by I-V studies, Scanning Electron Microscopy and X-ray Diffraction. Mechanical activation on the system ZnO-Bi₂O₃-Sb₂O₃ produced a chemical reaction and amorphization in the powder mixture. The weight loss due to volatilization of Bi₂O₃ and Sb₂O₃ that occurrs in both ZnO-Sb₂O₃ and ZnO-Sb ₂O₃-Bi₂O₃ systems at temperatures below 1200°C is reduced by milling the powder mixtures.