Investigation of Phonon Scattering and Thermal Conductivity Reduction in Thermoelectric Materials Using Ultrafast Time-Resolved Optical Measurements

Upon filling, caged compounds like skutterudites can have their lattice thermal conductivity reduced by 4∼5 times compared with unfilled structures [1]. Recently, it was found that the thermal conductivity in Bi₂Te₃/Sb₂Te₃ superlattice structure is also greatly reduced, even comparing with its corresponding alloy, in the cross-plane direction [2]. A fundamental understanding of thermal conductivity reduction in these structures is important due to their enhanced thermoelectric figure of merit. For filled skutterudites, “phonon-glass-electron-crystal (PGEC)” scheme was adopted to describe the role of guest atoms in the cages constructed by host atoms [3]. The localized and incoherent “rattling” behavior of guest atoms cuts down the mean free path of phonons, which results in reduced lattice thermal conductivity. In this study we apply ultrafast time resolved measurement technique to study coherent phonons in Bi₂Te₃/Sb₂Te₃ superlattice and coherent vibrations in misch metal filled skutterudites, aim to reveal the mechanisms behind thermal conductivity reduction.