Thermal Transport in Low Dimensional Polar Nanostructures

Abstract

The thermal conductivities of nanostructures and materials are of both fundamental interest and practical implications for a broad variety of technologies. While the past two decades have witnessed remarkable progress in understanding thermal transport in nanostructures, emerging phenomena associated with scattering of energy carriers within extreme geometries and complex structures require continued investigation. Importantly, while limited attention has been paid to energy carriers beyond electrons and phonons for heat conduction as well as one-dimensional phonon transport, several efforts on these topics have demonstrated exotic observations. This dissertation starts with pursuing better understanding of the contact thermal resistance in polymer nanocomposites. Utilizing a well-established microthermal bridge method, we studied thermal transport at contacts between individual boron nitride nanotubes (BNNTs) with and without a thin interlayer. A polyvinylpyrrolidone interlayer leads to an interesting bidirectional modulation of the contact thermal resistance by eliminating phonon back reflection at the tube contact. More interestingly, we measured an unexpected negative contact thermal resistance with a gold interlayer between two BNNTs, which was attributed to the change of the tube resistance facilitated by a new energy transport channel mediated by surface phonon polaritons (SPhPs). Systematic studies of SPhP mediated heat conduction along SiC nanowires have been performed, revealing the SPhP launching mechanism, non-equilibrium thermal transport, and configurable thermal conductivity in 3C-SiC nanowires, which opens the door for engineering thermal conductivity in various polar nanostructures. In addition, thermal conductivity measurements have been done on low dimensional nanowires, i.e. NbSe3 and Ta2Se3 nanowires. The combination of weak intra-chain van der Waals interactions and strong along-chain covalent bonding yields exotic thermal transport features such as charge density waves (CDWs), tunable electron-phonon interactions, and one-dimensional superdiffusive phonon transport. Overall, this dissertation reports the first experimental demonstration of SPhPs as a type of new energy carriers for heat conduction and exotic thermal transport features in low dimensional polar nanostructures.