EVOLUTION OF MICROSTRUCTURE AND ELECTRICAL NONLINEARITY OF ZnO-Pr6O11-Cr2O3 VARISTOR CERAMICS DERIVED THROUGH CITRATE GEL TECHNIQUE

Abstract

The evolution of microstructure and electrical nonlinear characteristics of ZnO doped with Pr6O11 and Cr2O3 varistor ceramics as a function of sintering temperature and time is discussed. The ceramics were prepared through combination of citrate gel process and solid phase sintering. Transformation in the grain microstructure and electrical behavior of the prepared ceramics was monitored by using field emission scanning electron microscope, X-ray diffractometer and J-E characteristic measurement. The findings showed that the elevation of solid phase sintering temperature between 1200 to 1275 ^oC has promoted the formation of secondary phase in the form of PrCrO₃ while minimally hindered the ZnO grain growth and densification rate of sintered ceramics. The nonlinear coefficient, α, that signifies the electrical nonlinearity of the ceramics decreased with increasing sintering temperature due to trade-off between effective number of Double Schottky Barrier at grain boundaries and grain size. Meanwhile, extending the sintering time from 1 to 7 hours caused the formation of the varistor ceramics with lower density and finer grains. Accumulation of intergrain pores and segregated phase at triple points led to deactivation of some grain boundaries. As a result, a decrement in both α and ΦB values was observed implying to the loss of electrical nonlinear behavior.