Paper SE-TuP3
Thermal Transport at Metal-Carbon Interfaces

Tuesday, October 30, 2012, 6:00 pm, Room Central Hall

Carbon nanotubes are appealing for diverse thermal management applications due to their high thermal conductivity (as high as 3,000 W m^-1 K^-1) coupled with interesting mechanical properties (stiff and strong, but also exhibiting foam-like deformation in arrays). Unfortunately, CNT surfaces are generally non-reactive and demonstrate weak bonding to other materials, limiting thermal interfacial conductance. Functionalization with “linker molecules” has been shown to enhance interfacial conductance, but reduces thermal conductivity of the nanotubes themselves by altering carbon bond hybridization within the nanotube. Metal coatings provide a way to increase thermal interface conductance associated with carbon nanostructures, while maintaining their high thermal conductivity. We used Molecular Dynamics and vibrational modes analysis to study heat transfer through carbon nanotube - metal interfaces in highly nonequilibrium conditions (NEMD). The simulation results were compared to experimental measurements of conductance for metallized highly oriented pyrolitic graphite (HOPG) substrates. HOPG was selected as a practical 2-dimensioinal analog for nanotube sidewalls to facilitate experimentation by analysis of the two-color time domain thermoreflectance (TDTR) data from the samples. The TDTR analysis of the different metals on HOPG was made possible by employing an optical parametric oscillator on the probe beam which allows for tuning the probe beam wavelength to match absorption bands for each metal studied. Metal films were selected to identify effects of atomic mass (inversely proportional to Debye temperature), chemical interactions (i.e., interfacial carbide formation) and electron configuration. Measurements of chemically inert metals at the carbon interface, including Al, Cu and Au demonstrated a strong dependence on Debye temperature, with conductance values differing by a factor of 3. The effects of interfacial carbide layers with varied areal densities on HOPG surfaces on thermal conductance were also examined, in addition to the presence of molecular interlayers. These results were applied to 3D assemblages of carbon nanotubes, such as dry spun CNT yarn, where metallization yielded significant enhancements of thermal conductivity in addition to increased tensile strength.