Processing of ZnO-based varistors using oxide, alkoxide, nitrato-alkoxide and carboxylato precursors
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We have investigated the preparation of ZnO varistors by different chemical solution routes with the aim of improving the homogeneity of the phases formed and to obtain a better control of microstructure. One conventional oxide mixing route and four solution chemical routes have been used to prepare the precursor materials. In all cases, the same composition (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 smaller ZnO grains. Also the microstructure in the final compacts was improved, compared with that of compacts prepared from oxide mixing routes. The smaller ZnO grain size in the final compacts was attributed to the presence of spinel grains. The spinel grains are formed at lower temperatures and, when they reach a size of 1 μm, they hinder the growth of ZnO grains.
KeywordsZnO-varistor sintering grain growth solution routes
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