Phonon Transport in SiGe-Based Nanocomposites and Nanowires for Thermoelectric Applications
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Phonon Transport in SiGe-Based Nanocomposites and Nanowires for Thermoelectric Applications M. Upadhyaya and Z. Aksamija Electrical and Computer Engineering, University of Massachusetts-Amherst Amherst, MA 01003, U.S.A.
ABSTRACT 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 nanocomposites 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, while for nanowires we use the Monte Carlo method to solve the phonon Boltzmann equation. In order to accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite and at the boundaries of nanowires, we employ a momentum-dependent specularity parameter. Our results show thermal transport in SiGe nanocomposites and nanowires is reduced significantly below their bulk alloy counterparts.
INTRODUCTION The approach to maximizing the thermoelectric figure of merit ZT by utilizing lowdimensional nanostructures was first proposed theoretically by Hicks and Dresselhaus in 1993 [1]. Recent work in materials for energy has gone a step beyond individual nanostructures to show that most thermoelectric (TE) materials, including the traditional bulk thermoelectrics, such as Bi2Te3 (used mainly at room temperature), SiGe alloys (used at temperatures up to 900K [2]), and oxides (used typically above 900K) can be improved in a cost-effective way by making nanocomposites [3], where a macroscopic (or bulk) material is made up of nanoscale grains either interconnected or embedded in a host matrix. Si-based nanocomposites are especially attractive because the material is cheaper and more abundant than other TE materials and it can be easily integrated with other electronic components for on-chip cooling of computer central processing units (CPUs) of powering embedded sensors through waste heat. Nanocomposites are typically designed to improve ZT by reducing thermal conductivity through the scatte
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