Phonon Transport in Nanophononic Metamaterials Using Large-scale Atomistic Models

Gheitanbaf, Hossein Honarvar

Graduate Thesis Or Dissertation

Phonon Transport in Nanophononic Metamaterials Using Large-scale Atomistic Models Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/dr26xx626

Abstract

Understanding nanoscale thermal transport in materials is essential for developing efficient energy materials/devices for thermoelectric energy conversion. The performance of current thermoelectric materials (TEM) is relatively low and thus they are not cost- efficient compared to conventional technologies. One path towards improving the performance of TEM is the reduction of thermal conductivity in semiconducting materials, for which phonons are the dominant heat carriers. In the past two decades, the phonon thermal conductivity reduction by engineering the material at the atomistic level has shown remarkable promise due to the rapid progress in nano science and technology. In this thesis, using large-scale atomistic models, phonon transport in a recently discovered nanostructured material called nanophononic metamaterial (NPM) is extensively investigated at low-dimensional and bulk levels. A low-dimensional NPM can be created by attaching nanopillars to, for example, a thin silicon membrane. In this system, the leading mechanism is local resonance, whereby standing waves created by the nanopillars couple with the propagating phonons in the base membrane. These couplings affect the traveling phonons across the full frequency spectrum of the membrane, and are able to slow down the heat. These effects result in significantly low in-plane thermal conductivity. The low-dimensional NPM concept is unique because the nanoresonators are located outside the main medium of transport and are expected to have minimal impact on electron transport. Here, the resonance phenomenon, physical size effects, and design rules to achieve high TEM performance for realistic configurations and sizes of NPM are investigated, while ensuring that the emerging systems are amenable to fabrication and characterization by modern technologies. At the bulk level, phonon transport in crystalline silicon with resonant inclusions is also investigated. In this context, the resonance effects and the thermal conductivity reduction caused by two types of inclusions are studied. The first is an amorphous inclusion and the second is based on Van der Waals resonators. The resonant behavior of each of these systems is characterized directly from simulations. These unique configurations may provide a practical platform for thermal conductivity reduction using nanoresonators embedded in a bulk medium.

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