SYNTHESIS OF ZINC-OXIDE VARISTORS THROUGH MICROWAVE-DERIVED PRECURSOR

A rapid microwave decomposition method has been reported for the prepa ration of ZnO varistor precursors which on subsequent heat treatment f or short period results in non-ohmic properties. Thus a mixture of nit rates of zinc and praseodymium along with minor additives such as coba lt, chromium and potassium when subjected to microwaves of 2.45 GHz in a domestic oven for 8 min, resulted in the decomposition of nitrates and formation of the precursor oxide mixtures. The precursor powder co nsists of easily friable agglomerates of size about 5 mu m, having the major phase as ZnO. Such powder on further compaction at about 150 MP a and sintering in the range of 1200-1350 degrees C, resulted in high density (> 95%) varistors. The electrical measurements of the varistor sample showed nonlinearity coefficient (proportional to) above 30. El ectrical field (E) vs current density (J) measurements showed breakdow n fields of 135 V/mm for Pr2O3-doped varistors. The fractograph of the sintered varistor sample indicates an average grain size of 6-8 mu m.