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B8: Modelling

Phonon-Carrier Coupling in Au-Substituted Type I Clathrates

Daniel Thompson, Zuxin Ye, and Chang Liu1James R. Salvador2Jihui Yang3Wei Cai, Melanie Kirkham, and Hsin Wang4
1 Optimal Inc.2 Chemical and Materials Systems Lab, GM R&D Center3 University of Washington4 Materials Science and Technology Division, ORNL

Type I clathrates A8M16X30 (A=Sr, Ba, Eu; M=Al, Ga, In; X=Si, Ge, Sn) form a promising thermoelectric (TE) materials system for high-temperature power generation applications. While the TE properties of Sr8Ga16Ge30 were reported as early as 1998, polycrystalline Ba8Ga16Ge30 is the current benchmark material with a ZT~0.8 at 1050 K. We present a study on the effects of Au substitution on the thermal conductivity of type I clathrates. Polycrystalline samples with nominal composition Ba8AuxGa16-3xGe30+2x (x = 0, 1, 2, 3, 4, 5, and 5.33) were synthesized by induction melting, annealing, and hot-pressing. At low temperatures the thermal conductivity behavior progresses from crystalline to amorphous as Au doping increases. The most significant behavior change occurs with the transition from n-type (x=3) to p-type (x=4) materials. The temperature dependence of lattice thermal conductivity implies a combination of point-defect and carrier-phonon scattering being the dominant phonon scattering mechanisms from 2 K to 15 K. Modelling the lattice thermal conductivity with the Debye approximation quantifies the increasing carrier-phonon scattering with increasing Au content. The nature of the phonon interactions with heavier holes or lighter electrons will be discussed in light of these findings.