Hydrophobic nanosilica-stabilized graphite particles for improving thermal conductivity of paraffin wax-based phase-change materials

Phase change materials based on paraffin wax are characterized with low thermal conductivity, which could be improved by the addition of particles of a heat-conducting material, for example, graphite. However, graphite particles agglomerate in molten paraffin wax matrix and settle. In this study, the use of hydrophobic nano-sized silicon dioxide as a stabilizer was considered to obtain stable dispersions of graphite in paraffin wax. Rheological and thermophysical properties of paraffin-based mixtures containing from 1 to 5 vol. % of hydrophobic silica and from 1 to 15 vol. % of graphite were investigated. At the concentration of 3 vol. %, silica nanoparticles form a percolation structure, which is expressed in the emergence of the yield stress that is capable to prevent the sedimentation of graphite particles introduced into the mixture. It allows obtaining dispersions that are stable both in time and at passage of cooling–heating cycles. The joint influence of silica and graphite fillers on the ability of paraffin wax to accumulate and conduct heat was investigated. Thermal conductivity of dispersions at the introduction of graphite increases according to the Maxwell's model, which allows improving the thermal conductivity on 33 % from initial value at the introduction of 15 vol. % of graphite.