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Microstructure and Electrical Properties of Co-Doped ZnO Varistors

S. Hamdelou, K. Guergouri, K. Guergouri

Journal of Ceramic Science and Technology, Vol. 7, No. 4 (2016), pp. 357-364

DOI: 10.4416/JCST2016-00045

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Abstract

Varistor ceramics based on Co-doped ZnO nanopowders were prepared with the sol-gel method. The analysis of the structure shows that the ZnO:Co (1 mol%, 3 mol%, 5 mol% and 7 mol%) has a wurtzite structure without any second phase, the Co ions successfully substitute Zn sites in the ZnO lattice. The grain size of the obtained nanopowders varies between 36 and 210 nm. The microstructure and electrical properties of varistors made of these powders have been studied as a function of the sintering temperature, which ranged between 900 °C and 1075 °C. The varistors were sintered by means of the microwave method. The results obtained show that the grain size of the ZnO nanopowders decreases with increasing Co content, and increases with rising of the sintering temperature, the threshold voltage increases with decreasing grain size with values varying between 99 V/mm and 681.3 V/mm.Typically, the varistor ceramics with 5 mol% Co sintered at 1000 °C exhibit the best electrical properties with a threshold voltage of 333.5 V/mm and a nonlinear coefficient of 15.32. The results also show that doping with Co atoms is a promising route to obtaining a higher threshold voltage of varistor ceramics based on ZnO nanopowders.

Keywords

Co-doped ZnO varistors, microwave sintering, microstructure, electrical properties

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