Tuning Structure and Thermal Evolution of High-ZT Thermoelectrics Using First Principles.

Abstract

In this study, using the first-principles based atomistic simulations, we address tuning of the atomic structure of thermoelectric (TE) materials manifested through the roles of phonons and charge carriers in the TE figure-of-merit (ZT). This computational work suggests new, systematic and extensive methods to analyze such temperature-dependent phonon scatterings and charge carrier mechanisms mediated with temperature-evolved lattice vibration. Through a unified and integrated approach we show that, bulk, homogeneous high-ZT materials take advantage of atomic displacements through their bond softening, phase change, anharmonicity, thermal disorder, and phonon red shifting to reduce their lattice thermal conductivity, while enhancing (or non-deteriorating) their charge transport properties. The TE conversion of intermediate waste heat (600 < T < 900 K) is most practical and we consider a range of structures (rocksalt-/antifluorite-based chalcogenides, filled/substituted skutterudites, and icosahedral-based borides) and show how each structure allows for atomic tuning for improved ZT. Ab-initio molecular dynamics (AIMD) show the rocksalt structure (PbTe) experiences thermal disorder (high-temperatures, off-lattice dislocation due to anharmonicity) and this leads to temperature-dependent effective mass and band convergence and suppression or the long-range acoustic phonon transport, resulting in high ZT. Through ab-initio phase diagram prediction we reveal that point and phase-boundary scatterings significantly reduce lattice thermal conductivity in filled skutterudites (BaxCo4Sb12). With AIMD we show in atomic substitution Co4(Sb,Ge,Te)12 configuring the pnicogen rings lowers lattice thermal conductivity and suggest the combination of filler and substitution will further reduce lattice thermal conductivity. With direct non-equilibrium AIMD we predict the anomalous temperature-independent behaviors of the Seebeck coefficient and lattice thermal conductivity of the icosahedral-based borides (B13C2). Our statistical entropy analysis supports this significant vibrational contribution (phonon softening) to the Seebeck coefficient. In antifluorite-based chalcogenides (beta-Cu2Se), the large displacement of the Cu+ ions in the interstitial sites results in low lattice thermal conductivity, and we suggest alloying Se with Te for higher ZT. We compare these first-principles based predictions with the available experiments and find good agreement. In this study, we have demonstrated the relations, metrics and tuning of the thermal evolution of the atomic displacements for optimal phonon and charge carrier TE properties.