Reaction mechanism and thermoelectric properties of In0·22Co4Sb12 prepared by magnesiothermy

Abstract The magnesioreduction synthesis of In0·22Co4Sb12 with high In rattler concentration from Sb2O4 and In-doped Co3O4 precursors is reported. This process directly yields a submicronic powder in a single step of 96 h at 810 K. The reaction mechanism has been investigated by stopping the reaction every 12 h and quantifying the existing phases by X-ray diffraction and Rietveld refinements. The precursors are first reduced in CoO and Sb2O3 lower oxides, then form CoSb2O6 and CoSb2O4 intermediates which are finally reduced in InxCo4Sb12. A powder with 350 nm average size and mostly composed of In-filled skutterudite phase with composition close to In0·17Co4Sb12 is obtained. Upon spark plasma sintering, small residual amount of InSb reacts with the skutterudite matrix to form a single-phase densified pellet with composition close to In0·22Co4Sb12. The resulting densified material with 1.8 μm average grain size shows a figure of merit ZTmax of 0.95 at 750 K.