Structural and electrical properties of ZnO varistors containing differentspinel phases

Structural and electrical properties of ZnO varistors were investigated as a function of spinel composition. Six varistor mixtures differing only in c hemical composition of spinel, were prepared by mixing separately synthesiz ed constituent phases (DSCP method). Compositions of constituent phases in sintered samples were investigated by changes of lattice parameters of the phases, as well as by EDS analysis of the constituent phases. It was found that compositions of ZnO, intergranular and spinel phases were partially ch anged during sintering due to redistribution of additives, that was control led by starting spinel composition and its stability. Electrical characteri zation showed significant difference in electrical properties of investigat ed varistors: nonlinearity coefficients ranging from 22 to 55 and leakage c urrents differing by the order of magnitude. Activation energies of conduct ion were obtained from ac impedance spectroscopy measurements. Calculated v alues of activation energies were in the range 0.61-1.0 eV confirming diffe rence in defect structure of ZnO grain boundaries in varistors containing d ifferent spinel phases. (C) 2001 Elsevier Science Ltd and Techna S.r.l. All rights reserved.