Expanded graphite/paraffin/silica phase change composites with high thermal conductivity and low permeability prepared by the solid-state wet grinding method

Abstract

In this study, expanded graphite (EG)/paraffin (PA)/silica (SiO2) composite phase change materials (PCM), with good thermal conductivity and impermeability, were fabricated for potential applications in solar energy. The EG-PA was first prepared by vacuum adsorption and then encapsulated by SiO2 through solid-phase wet grinding in the presence of sodium silicate. Our results show that the EG-PA-SiO2 composite have optimal properties when the mass ratio of raw material is PA: sodium silicate: H2O = 1:9:2:8. The latent heat of melting of the EG-PA-SiO2 composite prepared at this mass ratio is 106.2 J/g. The core material loss ratio, after washing with petroleum ether, is 4.79%. The thermal conductivity is 2.053 W/m·K, which is 8.5 times higher than that of pure paraffin wax, and the melt permeability is 9.35% after 120 min at 60 °C. Under simulated sunlight, the time required for the composite material to change from 30 °C to 65 °C is reduced by 70.78% compared to PA and the heat absorption and release times are nearly unchanged under multiple cycles. These results indicate that the material has a stable photothermal conversion efficiency.