| dc.description.abstract | Thermal conductivities of individual nanostructures have been measured recently with several newly-developed experimental schemes. However, quantifying contact thermal resistance between a nanostructure and related heat source/sink and extracting the intrinsic thermal conductivity of the nanostructure remain a significant challenge. In this dissertation, we develop a measurement method based on a suspended microdevice to derive intrinsic thermal conductivities of individual nanostructures such as multi-walled carbon nanotubes (MWCNTs) and silicon nanoribbons. Through multiple measurements of the same sample with different suspended lengths between the heat source and heat sink, the intrinsic thermal conductivity can be extracted. For MWCNTs, the intrinsic thermal conductivity is significantly higher than the effective ones without eliminating the contact thermal resistance between the CNT and the heat source/sink. For silicon nanoribbons, however, due to the flat contact configuration with relatively large contact area between silicon nanoribbons and suspended membranes, the contact thermal resistance is negligible. Results indicate that for nanoribbons with rectangular cross-sections, one single parameter (e.g. the Casimir length) is not enough to characterize the phonon-boundary scattering effects.
This dissertation also studies the thermal conductivity of boron carbide nanowires, a promising high temperature thermoelectric nanomaterial. One-on-one thermal property-structure characterization results suggest that the thermal conductivity of individual boron carbide nanowires can be affected by several different factors such as crystalline structure, carbon content, fault orientation and density, as well as wire diameter.
To address the less than expected thermal conductivity enhancement for CNT-based nanocomposites, this dissertation presents an innovative approach to measure the contact thermal conductance between individual MWCNTs. For bare MWCNTs, contrary to the common expectation, the normalized contact thermal conductance per unit area still depends linearly on the tube diameter. The intriguing observation is explained by showing that the phonon mean free path in the c-axis direction of graphite is two orders of magnitude higher than the commonly believed value of just a few nanometers. For MWCNTs with humic acid coating, no diameter dependence was observed even for the total contact thermal conductance, likely due to the variation in the coated humic acid layer, which can significantly affect thermal transport at the contact. | |
