Effect of bundling on phonon transport in single-walled carbon nanotubes

Thermally conductive bulk materials composed of single-walled carbon nanotubes (SWCNTs), such as fibers and mats, are characterized by a complex network formed by entangled SWCNTs. The fundamental unit of this network is a bundle, in which SWCNTs are assembled through weak van der Waals interactions. Although the thermal conductivity of a bundle is conventionally described as the sum of the heat conduction in the constituent SWCNTs, recent experiments have demonstrated that bundling may reduce the high thermal conductivity of SWCNTs, which is counterintuitive considering the weak interactions between SWCNTs in a bundle. Herein, by performing spectral phonon transport analysis for bundles of SWCNTs with radii of approximately 3.4 & Aring;, we explore the effect of the bundle size and chiral configuration on phonon transport in bundles. Results show that although the heat conduction properties of individual SWCNTs are maintained for bundles with a length of less than 1 mu m around room temperature in both the ballistic and quasi-ballistic regimes, intertube interactions between constituent SWCNTs coupled with their structural low dimensionality may yield additional thermal resistance. In particular, the thermal conductivity of a bundle decreases when low-frequency phonons influenced by intertube interactions substantially contribute to the overall heat conduction, namely, either at low temperatures or for long bundles. Although the observed additional thermal resistance cannot fully explain the experimental results of reduced thermal conductivity, our findings on the effect of bundling can advance the understanding of thermal transport in bundles and the thermal management of SWCNT bulk materials.