Dramatically reduced lattice thermal conductivity of Mg2Si thermoelectric material from nanotwinning

Abstract

Tuning phonon transport to reduce the lattice thermal conductivity (κL) is crucial for advancing thermoelectrics (TEs). Traditional strategies on κL reduction focus on introducing scattering sources such as point defects, dislocations, and grain boundaries, that may degrade the electrical conductivity and Seebeck coefficient. We suggest here, a novel twin boundary (TB) strategy that can decrease the κL of Mg2Si by ∼90%, but which may not degrade the electrical properties significantly. We validate this suggestion using density functional theory (DFT). We attribute the mechanism of TB induced κL reduction to (i) the lower phonon velocities and larger Grüneisen parameter, (ii) “rattling” of the MgMg pair induced soft acoustic and optical modes, (iii) shorter phonon lifetime and higher phonon scattering rate. We predict that the size of nanotwinned structure should be controlled between 3 nm and 100 nm in the Mg2Si matrix for the most effective κL reduction. These results should be applicable for other TE or non TE energy materials with desired low thermal conductivity, suggesting rational designs of high-performance Mg2Si TE materials with low κL for the energy conversion applications.