Progress in Preparation of ZnO Based Varistor Ceramics
Chemical preparation methods were used to fabricate ZnO-based varistor ceramics having nonlinearity coefficients between 35–45, breakdown fields ranging from 400–1000 kV/m and leakage currents far less than 10−2 A/m2.The processing steps employed include (1) chemical synthesis of powders having a complex initial composition of the form (100−y) ZnO+y (additives) with y selected from the group of soluble salts of Bi, Sb, Co, Mn, Ni and Cr, then (2) calcination of as-prepared powders within the temperature interval from 373 to 1473K and (3) sintering at temperatures from 1373 to 1573K. Two different methods for powder preparation were used: 1) evaporation of ZnO suspensions with additive solutions and 2) coprecipitation of additive solution and afterwards by adding ZnO.
Various analytical methods such as optical and scanning electron microscopy, DTA, TGA, X-ray and EDAX were used in characterizing the powders and the resulting ceramics. Processes of formation of the main phases as well as their influence on resulting non-Ohmic behaviour, was investigated.
KeywordsCalcination Temperature Spinel Phase Nonlinearity Coefficient Additive Solution Breakdown Field
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