Thermal conductivity and phonon transport in empty and water-filled carbon nanotubes

The thermal conductivities of empty and water-filled single-walled carbon nanotubes (CNTs) with diameters between 0.83 and 1.36 nm and lengths ranging from 200 to 1400 nm are predicted using molecular dynamics simulation. Using a direct application of the Fourier law, we explore the transition to fully diffusive phonon transport with increasing CNT length. For empty CNTs, we find that the CNT length required to obtain fully diffusive phonon transport decreases from 1090 nm for the 0.83-nm-diameter CNT to 510 nm for the 1.36-nm-diameter CNT. The magnitude of the fully diffusive thermal conductivity also decreases monotonically with increasing CNT diameter. We find that the fully diffusive thermal conductivity of water-filled CNTs is 20%–35% lower than that of empty CNTs. By examining the empty and water-filled CNT density of states, we attribute the thermal conductivity reductions to an increase in low-frequency acoustic phonon scattering due to interactions with the water molecules.