A. Ekstrand et al., PROCESSING OF ZNO-BASED VARISTORS USING OXIDE, ALKOXIDE, NITRATO-ALKOXIDE AND CARBOXYLATO PRECURSORS, JOURNAL OF SOL-GEL SCIENCE AND TECHNOLOGY, 8(1-3), 1997, pp. 697-701
We have investigated the preparation of ZnO varistors by different che
mical solution routes with the aim of improving the homogeneity of the
phases formed and to obtain a better control of microstructure. One c
onventional oxide mixing route and four solution chemical routes have
been used to prepare the precursor materials. In all cases, the same c
omposition (mol%) was used; ZnO (95.9), Bi2O3 (1), Sb2O3 (1), NiO (1),
Co2O3 (0.5), MnO (0.5), Cr2O3 (0.1). The same sintering procedure was
also applied. It was found that the precursor materials consisting of
ZnO grains covered by a thin film of the additive oxides yielded smal
ler ZnO grains. Also the microstructure in the final compacts was impr
oved, compared with that of compacts prepared from oxide mixing routes
. The smaller ZnO grain size in the final compacts was attributed to t
he presence of spinel grains. The spinel grains are formed at lower te
mperatures and, when they reach a size of 1 mu m, they hinder the grow
th of ZnO grains.