Resumo:
The doping of the basic varistor system ZnO.Bi2O3.MnO2.Co3O4 by praseodymium was studied in this work. The individual and combined action of the dopant and atmosphere sintering on the electrical properties and degradation of zinc oxide varistors were studied. The varistor ceramics syntheses followed the ceramic process of powder metallurgy and were sinterized at the same temperature in nitrogen or air atmospheres to investigate the influence of the sintering atmosphere on varistor properties. The microestrutural and the ceramic phases were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrical characterization and degradation properties were made by the current density as a function of the electric field besides on low current measurements. The results show that the Pr contributes to high varistor properties when the ceramic is synterized in air atmosphere. The breakdown electric field has been shown to be 900V.cm-1 (nonlinear coefficient α=19) and such result indicated that the ceramics obtained in this work might be used as high voltage varistors. When subjecting the ZnO ceramics to an electric field at high temperatures it was observed an insulator to conductor transition. Even when the temperature becomes lower the conductivity remains high and this phenomenon is not related with the dopants (Pr) or another metallic oxide (Bi2O3, MnO2, Co3O4), because this high persistent conductivity was presented in pure ZnO samples. It was proposed that high persistent conductivity may be due to the redistribution of intrinsic point defects promoted by the electric field and favored by the high temperatures.
