Densification Kinetics and In Situ Electrical Resistivity Measurements of Hematite Nanopowders during High-Frequency Microwave Sintering

Abstract

This paper discusses the systematic study about the densification sintering kinetics and to electrical resistivity of hematite nanopowders (30 nm) during sintering by high frequency microwave dilatometry (30 GHz). To analyze the sintering kinetics, non-isothermal methods were used, such as the classic models of Woolfrey-Bannister and Dorn for the initial stage of sintering, and the model of Wang-Raj for the intermediate stage, both at heating rates ranging from 5 to 20 °C/min. The results show that in all sintering conditions evaluated the onset temperatures of shrinkage and the temperature ranges at both the initial and intermediate stages are lower, if compared to microwave-assisted sintering at 2.45 GHz and conventional sintering reported in the literature. In addition, the resulting low activation energies, both for the initial stage (39 – 66 kJ/mol) and for the intermediate stage (68 kJ/mol), proved to be sufficient for material densification with less grain growth, compared to conventional heating. The in situ resistance measurements revealed a decrease in electrical resistivity as a function of material densification, as well as a semiconductor behavior of the sintered hematite.