Nanoscale heat transfer in carbon nanotube - sugar alcohol composites as heat storage materials

Nanoscale carbon structures such as graphene and carbon nanotubes
(CNTs) can greatly improve the effective thermal conductivity of thermally sluggish heat storage materials, such as sugar alcohols (SAs). The specific improvement depends on the heat transfer rate across the carbon structure. Besides, the heat transfer rate is further dependent on the material and the CNT diameter. In this paper, molecular dynamics simulations are applied to graphene/CNT-SA interfacial systems. Using erythritol and xylitol as model materials, we find the cross-plane thermal contact conductance to decrease as the CNT diameter decreases, with an exception for CNT(7,7). A phonon mode analysis is carried out to explain the general decreasing trend. The larger phonon mode mismatch observed between the molecules on both sides of smaller diameter CNTs is found to be a finite size effect of the confinement, instead of an interfacial effect. From the molecular collision point of view, a higher molecular density promotes heat transfer. In the case of CNT(7,7), the effective density of molecules enclosed in the CNT is found to be much higher than that of CNT(8,8). This may be the cause of the higher heat transfer rate across CNT(7,7). Molecular orientations and hydrogen bond structures of the molecules inside the CNTs are investigated to demonstrate the finite size effect
of the confinement. For graphene-SA composites, five model materials are considered and their cross-plane thermal contact conductance values fall into a narrow range.