Graphite-based shape-stabilized composites for phase change material applications

Phase Change Materials (PCM) possess high heat storage density, but commonly have low thermal conductivity that results in poor heat transfer. Another common problem is the shape stabilization of the storage medium. These problems could be solved by loading the medium with thermally conductive fillers, such as graphite, and encapsulating it in a thermally conductive polymer matrix shell, hence enhancing the thermal properties of both. We suggest a ternary system in which a graphite-epoxy composite provides an encapsulating scaffold to the paraffin, which is also loaded with graphite-based filler. Various graphite-based fillers differing in sizes and geometry were explored, aiming at optimizing their intrinsic properties, such as defect density, and consequently enhancing the thermal properties of the PCM as a whole. It was demonstrated that by judicious choice of the filler, enhancement is achieved for the thermal conductivity of: (1) the shell (epoxy-graphite flakes composite) by 4000% compared to the neat epoxy; (2) the medium (paraffin-graphite flakes composite) by > 2000% compared to the neat paraffin; and (3) the integrated PCM system (paraffin-epoxy-graphite flakes composite) by 1000% compared to that of neat paraffin. The PCM composite is completely preserved during phase change cycling.