Graphite nanofibers (GNFs) embedded in paraffin phase change material (PCM) are found to settle to the bottom of the sample when subjected to repeated thermal cycling. The majority of this settling is found to occur over the first heating/cooling cycle. A method is developed to prevent GNF settling by creating a shape stabilized phase change material using high density polyethylene (HDPE) as a stabilizing polymer. The shape stabilized samples reduced or eliminated GNF settling over repeated thermal cycles with as little as 10% HDPE by weight.
Shape-stabilization of PCM is done using a polymeric stabilizing agent to eliminate the contraction of the sample during solidification. The resulting material, referred to as shape-stable PCM (SS-PCM) is a polymer/paraffin blend. When a SS-PCM sample is heated above the PCM melt point, the paraffin liquefies but remains encapsulated within a solid polymeric matrix much like water in a sponge. It is proposed that the addition of GNFs to a shape-stabilized PCM will fix the location of the graphite nanofibers and prevent their migration during the phase change. This study examines the development of SS-PCM using high density polyethylene (HOPE) with embedded GNF. Any addition of HDPE to the paraffin reduces the overall energy storage of the system, so samples with varying levels of HDPE are studied to determine the minimum necessary level of polymeric addition.