Shape Stabilization and Photothermal Conversion of Paraffin Wax as a Phase Change Material Encapsulated by Large-Diameter Carbon Tubes

One of the most important requirements for the industrial application of phase change material (PCM)-based thermal energy storage is its thermal stability. Based on the confined pyrolysis strategy, electrospun polyacrylonitrile (PAN) precursor fibers are converted into carbon tubes with a large diameter (LDCT) of 200–300 nm, which have excellent heat transfer performance and can be a good option to avoid the low thermal conductivity of polymer-based microencapsulation. Due to the unique features of the large-diameter carbon tube, paraffin wax can easily enter the inner space. Unlike carbon fiber- or carbon nanotube-based composites, no obvious overflow or melting of liquid paraffin wax is observed. The LDCT (20 wt %)/paraffin wax composite exhibits a melting enthalpy of 95.7 J/g and a solidification enthalpy of 96.2 J/g. The composite shows significant thermal stability up to 200 °C, maintaining its overall shape unchanged during the heating process. Photothermal conversion results show an average conversion rate of approximately 0.23 °C/s, with the surface temperature reaching 79.9 °C within 220 s under solar radiation. The LDCT/paraffin composite achieves a conversion efficiency of 91.9%. The findings presented here combine the heat transfer performance and encapsulation design together, which not only have the potential for thermal storage applications but also provide important contributions to the fabrication and manufacture of carbon-based composites with inner and outer interfaces.