Abstract
Energy conversion from waste heat to electric power is a promising approach for energy harvest, and the clathrates crystals have received lots of attentions in this field from the concept of ‘phonon-glass and electron-crystal’. However, the thermal transport mechanisms and roles of rattlers have yet been clearly revealed in clathrates. By using iterative solution of Peierls-Boltzmann transport equation and first principle calculations, we have systematically revisited the thermal transport properties of a simple binary representative of clathrates, Ba8Si46. Our results confirm that the suppressed phonon lifetime is responsible for the huge reduction of lattice thermal conductivity (κ_l) in clathrates, in addition to the decrease of phonon group velocity. Furthermore, we clarify that phonon scatterings in a wide frequency range and the resonant characteristic scatterings coexist in clathrates, due to the emergence of hybridized modes introduced by the rattlers. We also elucidate that the hybridized modes dramatically suppress the acoustic phonon contribution to κ_l, leading to the non-negligible relative contribution from optical phonon to thermal transport in clathrates. Moreover, the impacts of the hybridized modes on different scattering channels in the phase space are also discussed. Our study provides fundamental physical insights into the impacts of rattlers on thermal conductivity of clathrates, which is valuable towards the design of efficient thermoelectric materials based on the concept of ‘phonon-glass and electron-crystal’.
Highlights
Thermoelectric devices can convert waste heat into electricity, as a promising approach to address two urgent problems worldwide, energy crisis and greenhouse effect, and have received lots of research interests (Zhao et al, 2014; Zhang and Zhao, 2015)
This structure character results in a remarkable feature in phonon dispersion, as show in Figure 2A, that is the existence of the hybridized modes with frequency ωh as a result of the so-called avoided crossing between the acoustic phonon mode in the host system and the flat guest modes (Nakayama and Kaneshita, 2011)
The complex orientations of tetrakaidecahedrons cages in typeI clathrates result in the broadened frequency distribution of hybridized modes in phonon dispersion (Figure 2A), which is different from the single-frequency hybridized mode in hostguest system of YbFe4Sb12 (Li and Mingo, 2014)
Summary
The previous studies have demonstrated that host-guest systems are ideal platform for this concept, such as clathrates (Takabatake et al, 2014), perovskites (Lee et al, 2017), and skutterudites (Ren et al, 2017), in which the lattice thermal conductivity (κl) can be dramatically reduced to a glass-like value while maintaining excellent electric properties. Because of their desirable thermoelectric behaviors, host-guest systems are identified as one of the most promising materials in practical thermoelectric applications
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