The effect of vibratory milling on the powder properties of zinc oxide varistors

The formation of powders from multi-elemental oxides for the subsequent formation of a green and sintered compact body is a major processing operation in the manufacture of ZnO varistors. The physical and chemical properties of the metal oxides are crucial not only for the formation of a varistor component with optimum microstructure and thus electrical characteristics but also for avoiding the introduction of flaws and reduced yield during subsequent manufacturing stages. The effect of vibratory milling using cylindrical zirconia media on the physical and chemical properties of the mixed multi-elemental oxide additives of a commercial ZnO varistor formulation was studied. These properties include particle size distribution, specific surface area, pore size distribution and zirconium concentration. They were evaluated by laser diffraction, Braunauer, Emmett and Teller (BET), mercury porosimetry and inductively coupled plasma (ICP) analysis, respectively. It was found that all of the physical and chemical properties of the metal oxide additives changed with increased duration of vibratory milling. The change in these properties was compared, a model of the powder morphology constructed and their significance analysed.