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

Abstract

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 (B.E.T.), mercury porosimetry and inductively coupled plasma (I.C.P.) 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.

Effect On The Processing Characteristics Of ZnO Varistors Produced Using Vibratory Milling

“…The slight increase in powder density with increasing milling duration of the MMOA appears to correspond with the reduction in size and distribution of the MMOA 10 . The reduction in size and distribution of the MMOA may have resulted in reduced friction between the spray-dried agglomerates causing them to flow easier and move closer together.…”

“…Six batches of MMOA were prepared using a vibratory mill for durations of 0, 1, 3, 6, 12 and 18 hours in random order. This stage, called first milling, was carried out using a vertical mill (Sweeco Ind., model M-18) and has been described earlier by the authors 10 . Each batch of MMOA was shear mixed with the main constituent ZnO, binder and lubricant for one hour to form six slurries ready for drying.…”

“…The specific surface area, particle size, pore size and Zr concentration were determined by Brunauer, Emmett and Teller (BET), laser diffractometry, mercury porosimetry and inductively coupled plasma (ICP) analysis respectively. These techniques have been described previously by the authors 10 .…”

“…One such variable may be the concentration of elements introduced indirectly as a result of media wear during milling. A previous publication by Kelleher et al (2008) showed that zirconium was present in the metal oxide additive powders in the parts per million (ppm) concentration levels as a result of vibratory milling. Knowledge of the effect of zirconium concentration on the electrical properties of ZnO varistors is scarce.…”

“…Mixed metal oxide additives (MMOA) were milled using a vibratory mill with cylindrical ZrO 2 media in the first milling stage for different durations. The effect of different milling durations on the powder properties of the first milling stage was reported 10 . This study focuses on the effect of different milling durations on the powder characteristics of the subsequent processing stages, second spray drying and second milling.…”