Dramatically reduced lattice thermal conductivity of Mg_2Si thermoelectric material from nanotwinning

Abstract

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_2Si 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 Mg2Si 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_2Si TE materials with low κ_L for the energy conversion applications.

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