Influence of Nb2O5 on the varistor behavior of TiO2–Cr2O3 system

Abstract

This work investigated the influence of Nb2O5 dopant on the varistor behavior of the ternary system (99.95 − x)%TiO2, 0.05 %Cr2O3, x%Nb2O5, where x = 0.10; 0.15; 0.20; 0.25 % in mol. The processing was carried out using the conventional oxide mixture method. The initial oxides were homogenized in alcoholic media in a ball mill, for 3 h, dried in oven and isostatically pressed at 210 MPa and sintered at 1,400 °C for 2 h in air atmosphere and cooled at 5 °C/min cooling rate, resulting in pellets with high densification. Electrical measurements in continuous current at different temperatures revealed that for the lowest Nb2O5 concentration the breakdown electric field, EB = 4.41 V/cm and non-linear coefficient, α = 4.6 were obtained, and for the highest Nb2O5 concentration the breakdown electric field, EB = 9.71 V/cm and non-linear coefficient, α = 15.3 were obtained. These low values in the breakdown electric field enable these varistor systems to be used in protection systems for low-voltage energy grids. Changes in the potential barrier present in the grain–grain boundaries could also be observed, in which the height increased and the width decreased with the increase in the dopant concentration. However, for x = 0.25 % in mol Nb2O5, a significant reduction in the potential barrier height and the voltage per barrier was obtained. Furthermore, the tendency to increase the volume of the unitary cell with the increase of dopant concentration, which was interrupted in the system with 0.25 % Nb2O5, was another evidence that suggested the occurrence of Nb2O5 segregation in the grain boundaries, or even that the reduction in the average grain size could possibly dilute Nb2O5 concentration in the grain boundaries. The average grain size was calculated through SEM micrographs and ranged from 7 to 16 μm, with larger sizes occurring for lower dopant concentrations and presenting higher porosity and lower uniformity in the grains shape.