In this project, we have investigated the thermal conductivity of polymer-carbon nanotube composite materials with an aim toward developing better materials for enhanced heat removal from high temperature electronics. Our results indicate that thermal conduction in the composites occurs through percolation, and suggest the intriguing possibility that thermal resistances in the system are dramatically reduced by polyethylene crystallites that nucleate at carbon nanotube surfaces and bridge adjacent nanotubes.

We have investigated the length, spacing, and overlap dependence of thermal interfacial resistance between carbon nanotubes. Interestingly, our studies have found that thermal tunneling between the nanotubes across a vacuum gap can occur even at large spacings due to intermittent thermal fluctuations that drive atoms in neighboring nanotubes toward each other.

We have investigated the thermal transport characteristics of superlattices and superlattice nanowires. The major accomplishment of this research was the identification of interfacial strain as a key factor influencing thermal transport. For example, we found that interfacial strain causes superlattice minimum conductivity to vanish. Also, we found that thermal resistance at individual interfaces in superlattice nanowires increased with increasing superlattice period due to pronounced strain variation across the interfacial region.

We have established a modulated thermoreflectance laser setup for thermal conductivity measurements of nanostructured (and other) materials. In the experiment, a modulated laser beam is used to heat the sample, and a second, continuous laser beam is used to probe the resultant reflectivity variation. The amplitude and phase of the measured reflectivity signal are used, in conjunction with an analytical model, to determine thermal conductivity. Additionally, the setup has a scanning capability for nondestructive structural evaluation.