Magnesioreduction Synthesis of Co-Doped β‑FeSi2: Mechanism, Microstructure, and Improved Thermoelectric Properties

β-FeSi2 and β-Co0.07Fe0.93Si2 thermoelectric silicides were synthesized from Fe2O3 and Si powders using a magnesiothermic process. Detailed study of the reaction mechanism by X-ray diffraction reveals that liquid Mg is mandatory to initiate the reduction. After completion of the reaction in relatively short time (10 h at 1173 K), the magnesiosynthesized iron disilicides are characterized as powders with grain sizes ranging from 30 to 400 nm and containing a high concentration of stacking faults quantified for the first time using a dedicated refinement software. The thermoelectric properties of spark plasma sintered pellets with submicrometric grain sizes, high stacking fault density, and residual micro- to nanoporosities are presented and compared to corresponding materials synthesized by conventional arc-melting process. Strong thermal conductivity reduction of 20% at 773 K has been achieved thanks to the mesostructure induced by the magnesioreduction. This results in an improved maximum figure-of-merit ZT reaching 0.18 at 773 K for β-Co0.07Fe0.93Si2.