Dramatically reduced lattice thermal conductivity of Mg ₂ Si thermoelectric material from nanotwinning

Tuning phonon transport to reduce the lattice thermal conductivity (κ _L ) is crucial for advancing thermoelectrics (TEs). Traditional strategies on κ _L 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 κ _L of Mg ₂ Si 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 κ _L reduction to (i) the lower phonon velocities and larger Grüneisen parameter, (ii) “rattling” of the Mg–Mg 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 Mg ₂ Si matrix for the most effective κ _L 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 Mg ₂ Si TE materials with low κ _L for the energy conversion applications.