Improving the thermal conductivity and shape-stabilization of phase change materials using nanographite additives

Improvements in the thermal conductivity and shape-stability of paraffin phase change materials (PCMs) by adding exfoliated graphite nanoplatelets (xGnP) or graphene were compared. The composite PCMs were fabricated by mixing paraffin with xGnP or graphene in hot toluene, followed by solvent evaporation and vacuum drying. A larger increase in thermal conductivity was observed for paraffin/xGnP, with a 10 wt.% xGnP loading producing a more than 10-fold increase. Graphene shows a lower electrical percolation threshold and offers a much larger increase in the electrical conductivity of paraffin than xGnP. However, its thermal conductivity increase is much lower. Despite the excellent thermal conductivity of single-flake graphene, the large density of nanointerfaces due to the small size of the graphene flakes significantly impedes heat transfer. We also found that graphene is much more effective than xGnP as a shape-stabilizing filler. At 2 wt.% graphene loading, paraffin maintains its shape up to 185.2 degrees C, well above the operating temperature range of paraffin PCMs, while the paraffin/xGnP counterpart is shape-stable up to 67.0 degrees C only. Small amounts of graphene and xGnP can be used in combination as a low-cost and effective improver for both the heat diffusion and shape-stabilization of paraffin PCMs. (C) 2012 Elsevier Ltd. All rights reserved.