Microstructures and thermoelectric properties of Co-doped iron disilicides prepared by rapid solidification and hot pressing

Iron disilicide based alloys Fe0.94Co0.06Si2.00 were prepared by rapid solidification and hot uniaxial pressing. The effects of pre-annealing of the powders and hot pressing temperature on the microstructures and thermoelectric properties were investigated. X-ray diffraction and scanning electron microscopy show that after pre-annealing at 800 °C for only 2 h, the high temperature phases (α-Fe2Si5 and ɛ-FeSi) in the rapidly solidified powders have been completely transformed to β-FeSi2 phase, while the fine grain structures were maintained. It is found that the grain growth during hot pressing was successfully suppressed for the pre-annealed samples, especially at a lower pressing temperature of 880 °C. The Seebeck coefficient was markedly enhanced by the pre-annealing treatment, also by a lower pressing temperature, while both the electrical conductivity and the thermal conductivity were decreased by them. The maximum dimensionless figure of merit ZT = 0.25 at 908 K was obtained for Fe0.94Co0.06Si2.00, which is pre-annealed, followed by hot pressing at 880 °C with 70 MPa for 30 min.