High Thermoelectric Performance in Mg₂(Si_0.3Sn_0.7) by Enhanced Phonon Scattering

Owing to the high thermoelectric (TE) conversion efficiency, low cost, and environmental friendliness Mg2Si0.3Sn0.7 solid solution has emerged as the material of choice for the n-leg of a TE generator for mid-temperature (room temperature to 800 K) applications. Dimensionless TE figure-of-merit (ZT) values of 1.3 (at 700 K) have been reported in this compound when optimally doped. High ZT values in this compound are due to a combination of improved electrical properties (band convergence effect) and reduced lattice thermal conductivity (alloy scattering of phonons). Here we demonstrate that the TE performance in this solid solution can be improved further (ZTmax = 1.7 at T = 673 K, which is a record in silicide-based TE materials) by enhancing phonon scattering with embedded nanoprecipitates. The high ZT values are obtained when Mg2Si0.3Sn0.7 is codoped with Bi and Cr. While Bi helps in optimizing the electrical transport properties, Cr results in formation of nanoprecipitates. Transmission electron microscopy (TEM) studies indicate embedded nanoprecipitates rich in elemental Cr and Sn. The nanoprecipitates also result in a strained interface as confirmed from high-resolution TEM (HRTEM) and grazing incidence XRD (GIXRD) studies. This results in an ∼15% reduction in the lattice thermal conductivity (κL) compared to the only Bi-doped sample while retaining similar power factor (S2σ) values. The overall effect is a ZTeng value of 0.73, which corresponds to a conversion efficiency (η) of 12% with the cold side temperature of 323 K and a ΔT = 350 K.