The size effect on filling and phase change behavior of paraffin within carbon nanotube

Abstract

Filling organic phase change materials (PCM) into carbon nanotubes (CNT) to obtain nanocapsules (NCs) has been proven to be a useful strategy to meet the longstanding challenge of low thermal conductivity and liquid leakage. However, the filling and phase change mechanism of NCs has not been well understood yet. In this work, molecular dynamics simulations were adopted to reveal the effect of CNT diameter on the filling and phase change behavior of PCM. The NCs exhibit high filling ratio up to 73.5% when the diameter of CNT is 7.5 nm. Paraffin molecules could not enter in the central space of CNTs when the diameter of CNT is higher than 7.5 nm, which results in the low filling ratio. PCM in CNT could be divided into the surface absorbed state and the confined state. With the effect of the filling ratio, the local density and the order parameter, the thermal storage capability is high to 97.4% with diameter 6 nm. The diffusion of paraffin molecules in CNT belongs to Fickian diffusion and decreases with the increasing diameters. Even at limited length of 6 nm, filling PA within CNT increase the thermal conductivity 19–180 times concerning different fillling ratios. The present work helps for understanding the thermal property behavior of PCM in nanopores.