Effect of Covalent Functionalization on Thermal Transport across Paraffin/Graphene Nanocomposite Interfaces

The low interfacial heat transfer efficiency limits the thermal conductivity (TC) of the paraffin/graphene composite phase change material. In this study, n-octadecane is used to represent paraffin. The interfacial thermal conductance (ITC) and overall thermal conductance (OTC) of pristine graphene/n-octadecane (PG/OD) and graphene/n-octadecane (G/OD) grafted with hydroxy (-OH), carboxyl (-COOH), methyl (-CH₃), butyl (-C₄H₉), and heptyl (-C₇H₁₅) are explored using nonequilibrium molecular dynamic simulation methods. The results show that alkyl functional groups can significantly improve ITC and OTC, and the improvement effect increases with the increase of chain length. However, -OH and -COOH only slightly increase ITC, and -OH actually decreases OTC. At the grafting density of 0.01497 Å^-2, -C₇H₁₅ increases the ITC and OTC by 128 and 34%, respectively. The phonon density of states analysis explains that -C₇H₁₅ demonstrates the greatest effect in enhancing interfacial thermal coupling, followed by -C₄H₉, -CH₃, -COOH, and -OH. Moreover, the long-chain alkane functional groups may improve the interlayer thermal conductance of the near-wall layer, and thus the OTC. Finally, according to the calculated ITC and effective medium theory, the TC of G/OD composites is predicted. This work provides valuable guidance for exploiting the potential of the TC of paraffin/graphene composites.