Enhanced thermoelectric properties of Sn-rich quaternary Mg2Si0.25Ge0.05Sn0.7 based solid solutions by band convergence in addition to scattering of point defects

Abstract

Quaternary Mg2Si0.25Ge0.05Sn0.7 based solid solutions were synthesized by B2O3 flux method combined with spark plasma sintering (SPS) technique. The carrier concentration was tuned by adjustment of the SPS temperature. The electrical conductivity, Seebeck coefficient and thermal conductivity were measured in the temperature range from 320 to 820 K. The results show that the density, carrier concentration and lattice thermal conductivity monotonically increase with sintering temperature. With increasing carrier concentration the electron effective mass increases due to band convergence, while the electron mobility drops dramatically from 70.5 cm2 V−1 s−1 to 7.5 cm2 V−1 s−1. The phonon scattering mechanism in the quaternary solid solutions was analyzed using the Abeles model. The calculation shows that mass fluctuation scattering plays a crucial role in reducing the lattice thermal conductivity. Owing to band convergence combined with scattering of the phonons by point defects, a ZTmax of 1.1 at 700 K was obtained for Mg2(Si0.2Ge0.1Sn0.7)0.99Sb0.01.