Fabrication and thermoelectric properties of Mg2Si-based composites using reduction reaction with additives

Mg2Si-based composites were successfully synthesized by a reduction reaction of Mg, Si, and a small amount of metal oxide, carbonate, or hydroxide additive (Al2O3, Bi2O3, Sb2O3, La(OH)3, Li2CO3, Ag2O, CuO, Ga2O3, In2O3, Na2CO3, or Y2O3). The reaction was carried out under a flowing Ar–5%H2 atmosphere at 1073 K and the resulting composites were densified under vacuum at 993−1053 K using spark plasma sintering (SPS). The thermoelectric properties of the dense composites were characterized using Hall effect measurements at room temperature, and by measurements of electrical resistivity (ρ), the Seebeck coefficient (S), and thermal conductivity (κ) at 300–900 K. XRD measurements of the composites revealed that the major phase was Mg2Si, and the minor phases were MgO, Si, intermetallics, and metal, obtained by the reduction of additives, and unreacted oxides. Doping of Al, Bi, Sb, La, Li, Ag, In, and Y into Mg2Si was successfully achieved. The microstructure and thermoelectric properties of the composites were drastically influenced by the additive used. The maximum values of dimensionless thermoelectric figure of merit (ZT) for the Mg2Si composites using Al2O3, Bi2O3, and Sb2O3 were found to be 0.58, 0.68, and 0.63 at ∼865 K, respectively, which are comparable to the previously reported values of Al-, Bi-, and Sb-doped Mg2Si.