Propriedades elétricas da alfa hematita ( α-Fe2O3) dopada com óxido de bismuto (Bi2O3)

Abstract

In this work, the influence of the Bi2O3 dopant on the varistor electrical properties of the α-Fe2O3-Bi2O3 binary system was investigated. The used concentrations of doping had varied in 0.5%, 1.0%, 1.5%, 1.8%, 2.0%, 2.5% and 3.0% by mol. The processing used was the conventional method of misture of oxides. Five samples were formed for each composition at a pressure of 4.5 Mpa and sintered at 1280°C for 1 hour in ambient atmosphere with an intermediate level at 350 °C for 1 hour. After sintering, the densification of the ceramic samples was verified, with a value of 5.07 g/cm for the pure α-Fe2O3 system and 4.78 g/cm for the system with the addition of 3.0% in mol of Bi2O3. The study of varistor behavior properties for systems α-Fe2O3-Bi2O3 was carried out in direct current, in the ambient temperature and, depending on the temperature (up to 300 ° C). It was observed that the dopant content alters the electrical behavior of ceramic samples based on α-Fe2O3. The electrical parameters found are α = 1.45, Er = 1102 V/cm and 4,7 Vb V/cm for the binary system with 2.0% in mol of Bi2O3, α = 1.48, Er = 1156 V/cm and 5,0 Vb V/cm for the binary system with 2.5% in mol of Bi2O3 and, α = 1.62, Er = 1143 V/cm and 4,4 Vb V/cm with the addition of 3.0% in mol of the dopant, since all binary systems had a relatively high leakage current (If). Only for concentrations above 2.0% in mol of Bi2O3, the increase in the dopant concentration becomes favorable for the non-linear varistor behavior. This effect was due to the increase in the concentration of the bismuth ferrite crystalline phase (FeBiO3), of low electrical resistivity, characterized by DRX, together with the analysis EDS coupled to the FEG. With the results obtained from the characterizations, it was possible to identify that the FeBiO3 phase is formed in the grain boundary region of the microstructure. This crystalline phase was considered as the electrical conduction mechanism responsible for the nonlinear behavior of the α-Fe2O3-Bi2O3 binary varistor systems, with resistive grain and conductive grain boundary.