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CC6.02 - Phonon Transport in SiGe-Based Nanocomposites and Nanowires for Thermoelectric Applications 
December 3, 2014   8:15am - 8:30am
  Synced Audio / Video / Slides

Silicon-germanium (SiGe) superlattices (SLs) have been proposed for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk SiGe alloys. However, the cost of growing SLs is prohibitive, so nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Lattice thermal conductivity in SiGe SLs is reduced by scattering from the rough interfaces between layers. Therefore, it is expected that interface properties, such as roughness, orientation, and composition, will play a significant role in thermal transport in nanocomposites and offer many additional degrees of freedom to control the thermal conductivity in nanocompoosites by tailoring grain size, shape, and crystal angle distributions. We previously demonstrated the sensitivity of the lattice thermal conductivity in SLs to the interface properties, based on solving the phonon Boltzmann transport equation under the relaxation time approximation. Here we adapt the model to a broad range of SiGe nanocomposites. We model nanocomposite structures using a Voronoi tessellation to mimic the grains and their distribution in the nanocomposite and show excellent agreement with experimentally observed structures. In order to accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite, we employ a momentum-dependent specularity parameter. Our results show highly anisotropic thermal transport in SiGe nanocomposites below their bulk alloy counterparts. SiGe nanowires (NWs) have also attracted attention due to their unique properties such as unusually long phonon mean-free-paths and the possiblity of quasi-ballistic transport over micron lenghts. This counterintuitive feature is a consequence of alloy scattering having a strong frequency dependence so that, while most phonons are strongly scattered by mass disorder, long wavelength phonons having low vibrational frequencies are left contribute the majority of the thermal conductivity. In order to capture the interplay between alloy, anharmonic, and boundary scattering in SiGe nanowires, we develop and compare results based on the improved Callaway model and the Monte Carlo method, both utilizing the full phonon dispersion and a momentum-dependent roughness scattering model. We find good agreement between the two models and show that they capture the increased specularity of low-angle phonon scattering at the boundaries, which leads to long mean-free-paths for phonons directed along the wire. We conclude that our Monte Carlo simulations accurately capture the quasi-ballistic transport in SiGeNWs and that heat is carried predominantly by quasi-ballistic long-wavelenth phonons travelling along the direction of the wire, thus producing a pronounced length dependence, especially in NWs having smooth surfaces.

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