Phase and conductivity dynamics of strontium hexaferrite nanocrystals in a hydrogen gas flow

The phase and conductivity dynamics of strontium hexaferrite nanocrystals were isothermally studied at different temperatures during a constant flow of hydrogen gas at normal atmospheric pressure. The nanocrystals were prepared by self-flash combustion of acetate precursors. While the formed phase was characterized using XRD, TEM, and optical microscopy after hydrogen exposure, the electrical conductivity was in-situ measured during reduction. The temporal changes in conductivity as well as the formed phases at partial and complete reduction were found to be significantly affected by the operating temperature. Nanocrystals reduced at lower temperatures showed formation of lower oxygen content phases of strontium-iron oxides (SrFe12O19, Sr2Fe2O5, Sr7Fe10O22), and iron oxides (Fe3O4, FeO), while those reduced at higher reducing temperature showed the formation of metallic iron responsible for higher electric conductivity during reduction. Metallic iron nanocrystals of increased sizes were formed at higher reducing temperatures and longer reducing times. Temporal conductivity changes during hydrogen gas flow at different temperatures showed three regions corresponding to removal of surface oxygen, surface reduction, and bulk reduction of the nanocrystals. Nanocrystals reduced at temperatures higher than 400C showed three reduction regions corresponding to these mechanisms, whereas those reduced at 400C only two regions could be detected. The activation energies of the oxygen desorption, surface reduction and bulk reduction were found to be 55.5, 40.2, and 44.1 kJ mol respectively. This indicates that oxygen desorption follows a chemical reaction controlled mechanism, while surface and bulk reductions are of combined gas diffusion and interfacial chemical reaction controlled mechanisms. The results obtained from the conductivity measurements were further supported by thermo-gravimetric measurements.