Enhanced Thermal Conductivity of Bi-filler Paraffin Composites Using Expanded Graphite and Graphene Nanoplatelets

Paraffin is one of the most widely used phase change materials (PCMs) for thermal energy conversion and storage. Due to its varied composition, its melting point can be tailored to the application, and its high chemical stability allows for the integration of virtually any filler material in the creation of phase change composites. However, its performance as a PCM is significantly hindered by its low thermal conductivity. In an effort to optimize the increase in thermal conductivity of paraffin with the least amount of filler, various nanocomposites were synthesized by adding a combination of graphene nanoplatelets (GNP) and expanded graphite (EG) at various ratios and weight fractions. The thermal conductivity of these nanocomposites were measured in the solid phase using the steady state method from 280-300 K. The latent heat of fusion and melting point were measured using a differential scanning calorimeter, and the distribution of fillers was investigated using SEM. We found that hybrid systems using both expanded graphite and graphene outperformed the fillers individually at the same filler fraction. At a low filler fraction of 4 wt.%, the 50/50 hybrid EG/GNP system exhibited above a 700% increase in thermal conductivity, roughly the same increase achieved by adding 8 wt.% GNP, indicating unique thermal transport mechanisms specific to hybrid systems. The thermal conductivity data was analyzed using an effective medium theory (EMT) approach to determine the interfacial thermal resistance. These results indicate that this hybrid system shows far superior performance to other single filler materials such as metal/oxide nanoparticles and carbon nanotubes. The thermal energy storage capacity of the composites were not significantly affected through the addition of the filler materials due to the low weight percentage used to enhance the thermal conductivity of the composite.