Densification kinetics and in situ electrical resistivity measurements of hematite nanopowders during high frequency microwave sintering

The aim of this work was to study the densification kinetics and to evaluate the electrical resistivity of hematite nanopowders (30 nm) during sintering by high-frequency microwave dilatometry (30 GHz) in multimodal cavity, as the purpose of providing new reference data on the kinetic behavior of the densification of high dielectric loss ceramic materials when subjected to microwave sintering, which are few in the literature to date. To analyze the densification 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 an extreme very 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 lower frequencies (2.45 GHz). In addition, 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.