Enhancement of thermoelectric performance driven by Ge substitution in SrSi2 alloy

Abstract

The effect of partial substitution of Ge in the Si sites of SrSi2 alloy has been investigated by means of electrical and thermal transport studies. Electrical resistivity (ρ), Seebeck coefficient (S) and thermal conductivity (κ) measurements were performed on the SrSi2−xGex alloys with x = 0–0.12. The room-temperature resistivity of these substituted alloys decreases with increasing Ge content up to x = 0.06 and then starts to increase upon further substitution. Seebeck coefficient study shows a substantial increase in S with Ge content, and a maximum of about 280 µV K−1 was obtained for x = 0.03 near 85 K. These observations can be realized as the broadening of band gap and the changes in the Fermi-level density of states (DOS) upon Ge doping. Analysis of the thermal conductivity of the SrSi2−xGex alloys suggests that the heat transport is essentially associated with the lattice phonons. In addition, low-temperature peak in the lattice thermal conductivity of these alloys drops markedly with increasing Ge content, attributed to the point-defect scattering of the phonons. Thermoelectric performance characterized by the figure-of-merit, ZT was evaluated for each alloy, and the largest ZT value of about 0.13 at room temperature was achieved for SrSi1.94Ge0.06 alloy, nearly three times higher than that of pure SrSi2.