Influence of Phonon Modes on the Thermal Conductivity of Single-wall, Double-wall, and Functionalized Carbon Nanotubes

Abstract

Carbon nanotubes (CNTs) are modeled using the Tersoff-Brenner potential and thermal conductivities were estimated using non-equilibrium molecular dynamics. Thermal conductivity for single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were studied for lengths from 25 nm to 1 μm. Thermal conductivity increases with length from the inclusion of additional phonon modes. To investigate influences of individual modes on thermal conductivity, DWNTs are used to model vibrational mode confinement in SWNTs. Also, various concentrations of phenyl united atom models and values for the Lennard-Jones parameter σ are used to model functionalization and the influence of bond strength. Thermal conductivity is largely influenced by longitudinal and flexural modes. Due to scattering from phonon-phonon interactions, the combination of the longitudinal and flexural modes results in a lower thermal conductivity than other phonon mode combinations. The influence of suppressing the flexural mode is also observed in the thermal conductivity behavior of functionalized CNTs. When using the united atom model, larger percentages of functionalization result in decreasing flexural modes and, consequently, higher thermal conductivity. Similarly, smaller values of σ, which indicate a stronger bond, showed better thermal conductivity. Overall the best performance resulted from functionalized DWNTs, which have the additional wall to transport energy.