Carbon Nanotubes: Thermal Properties
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As nanoscale graphitic structures, carbon nanotubes are of great interest not only for their electronic and mechanical properties, but also for their thermal properties. Because of their small size, quantum effects are important, and the low-temperature specific heat and thermal conductivity show direct evidence of 1-D quantization of the phonon bandstructure. Modeling of the low-temperature specific heat allows for determination of the on-tube phonon velocity, the splitting of phonon subbands on a single tube, and the interaction between neighboring tubes in a bundle. The thermal conductivity of nanotubes has been examined both theoretically and experimentally. Theoretical work predicts a room-temperature thermal conductivity that is larger than graphite or diamond. Measurements show a room-temperature thermal conductivity over 200 W/m K for bulk samples of single-walled nanotubes (SWNTs), and over 3000 W/m K for individual multiwalled nanotubes (MWNTs). Addition of nanotubes to epoxy resin can double the thermal conductivity for a loading of only 1%, showing that nanotube composite materials may be useful for thermal management applications.
The first part of this manuscript discusses theoretical and experimental work on the specific heat of nanotubes. The section “Specific Heat” provides an introduction to specific heat. In the section “Phonon Density of States,” the theoretically derived phonon density of states of nanotubes and nanotube bundles is compared to that of 2-D graphene and 3-D graphite. In “Theoretically Derived Specific Heat,” the measured specific heat of nanotubes is compared to theoretical models.
The second part of this manuscript reviews the thermal conductivity of nanotubes. The first section provides an introduction to thermal conductivity. The section “Thermal Conductivity: Theory” discusses theoretical treatments of the thermal conductivity. “Measured K(T) of SWNTs” reviews measurements of the thermal conductivity of single-walled nanotubes, and “Measured K(T) of MWNTs” reviews measurements of the thermal conductivity of multiwalled nanotubes. Finally, “Applications” describes thermal conductivity measurements of nanotube-based composites.