Thermal transport from nanoscale heat-sources and in nano-structured materials

Nano-structured materials find practical applications in microelectronics, optoelectronics, micro-mechanical-electrical systems, thermoelectric energy-conversion and lab-on-a-chip devices, to name a few. Fundamental research on thermal transport properties at length scale where the classical transport laws are not applicable anymore and the effective transport properties of nano-structured materials are affected primarily by the characteristic length and interface properties in these low-dimensional materials is crucial for engineering high performance devices. Even though reduction in thermal conductivity within a nano-structure has been extensively studied, experimental investigation of heat-dissipation from a nanoscale heat-source or "hot-spots" to its bulk surrounding substrate has been virtually unexplored. A technique based on joule heating thermometry was developed to capture the non-equilibrium thermal transport across nanowires deposited on silicon substrate. Employing this technique the effect of heater size on the effective thermal conductivity of bulk substrate was shown for the first time in silicon as a function of temperature. The effective thermal conductivity of bulk silicon substrate, measured by a heater-wire with characteristic width ∼ 96 nm, varied from a strongly reduced value of 57 W/mK at 300 K to only 77 W/mK at 80 K in sharp contrast to that of bulk silicon which ranges from 148 W/mK--1292 W/mK for the same temperature range. A proof-of-concept demonstration of the DC measurement of thermal resistance from nanowires using AFM based 'moving' probe technique was performed. Extensive study on improving the spatial resolution to map any local variation of thermal resistance along the length nanoscale heat-source is the objective of future studies.;The second part of the thesis investigates thermal transport in two nanostructured materials, polymer infiltrated aligned carbon nanotube arrays and Si/SiC superlattices. Thermal transport in polymer infiltrated multiwalled carbon nanotubes arrays showed at least 600% enhancement in thermal conductivity of the nano-composite compared to the matrix. Aligning the tubes is critical to limit the scattering at the nanotube-polymer interface and obtain the measured enhancement along the alignment direction. Nevertheless, in the next system investigated (superlattice system), resistance across multiple interfaces stemming from numerous interface-phonon scattering usually proves beneficial in enhancing thermoelectric figure of merit. Temperature dependent thermal conductivity of Si/SiC superlattices was studied for the first time employing a differential 3? method. Results showed a strong reduction in the cross-plane thermal conductivity varying from 1--2 W/mK which much smaller than that of the constituent materials. Estimations of the relative contributions of interface and intrinsic layer thermal resistance based on microscopic phonon transport models indicated that mean free path reductions within the superlattice layers were responsible for the observed experimental trends. The phonon mean free path was strongly reduced by structural disorder in each of the superlattice layers as confirmed by high resolution transmission electron microscopy.