Mechanical Activation of Spinel and Pyrochlore Phases in ZnO Based Varistors

Abstract

Mixtures of ZnO, Sb2O3 and Bi2O3 powders corresponding to the stoichiometric compositions of Zn7Sb2O12 (spinel) and Zn2Bi3Sb3O14 (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% Sb2O3 + 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-Sb2O3-Bi2O3 systems at temperatures below 1200∘C is reduced by milling the powder mixtures.