Molecular dynamics study of phase change properties of paraffin-based phase change materials with carbon nano-additives

Phase change materials (PCMs) with carbon nano-additives (CNs) are the key materials for energy storage and thermal control. However, the interaction mechanism between phase change characteristics and the microstructure remains unclear. The solidification/melting process of n-octadecane, n-octadecane/CNT (carbon nanotubes), n-octadecane/GNP (graphene nanoplatelets) and n-octadecane/GNP-CNT are studied by molecular dynamics method. The effect of microstructure evolution on phase change temperature and enthalpy of four PCMs is investigated. The results show that CNs induce alkane crystallization and enhance the order of alkane molecular orientation, leading to the increase of solidification and melting temperature of composite PCMs. Thermal hysteresis of CNT additive is intensified, while that of GNP or GNP-CNT additives is weakened due to the difference in crystallization capability of CNs-induced n-octadecane and the order of n-octadecane grain orientation during melting process. The solidification of composite PCMs depends on CNs-induced heterogeneous nucleation and n-octadecane self-crystallization, and the melting process starts from the grains far away from CNs and extends to the grains close to CNs. The addition of CNs reduces the phase change enthalpy and the decrement is related to the ratio of solid-like alkanes adsorbed around CNs. The phase change enthalpy ranked in ascending order is n-octadecane/GNP < n-octadecane/GNP-CNT < n-octadecane/CNT.