Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors

In present study, impact of CaO doping on microstructure and phase electrical properties of a ZnO varistor is being reported. The doped ZnO nanopowders were prepared by solution combustion followed by conventional sintering. The ZnO particle size of powders decreased from 22 to 17 nm with CaO doping was noted. Decrease in grain size of the sintered samples from 4.54 to 1.19 mu m with CaO doping was observed. This is attributed to Ca4Bi6O13 and Ca0.89Bi3.11O5.58 secondary phase formations apart from pyrochlore and spinet phases. A breakdown field as high as 21 kVcm(-1) in X = 1.00 sample was obtained. The Coefficient of nonlinearity (alpha) decreased from 95 to 42 is due to decrease in carrier concentrations. Increase in resistivity with CaO doping and decrease in resistivity beyond X = 2.5 were noted. The electric modulus plots yielded two relaxations at a temperature in the range 100-300 degrees C and frequency in the range 0.1 Hz-1MHz. Activation energy (E-a) decreased from 0.65 to 0.50 eV for a region I and from 0.89 to 0.75 eV for region II with CaO doping was observed. (C) 2019 Elsevier B.V. All rights reserved.