Facile Synthesis of High Thermal Conductive h-BN Microspheres/Paraffin Wax Shape-Stabilized Phase Change Materials for Intelligent Heat Storage System

The entire humanity is experiencing a severe energy consumption dilemma. As a result, increasing the efficiency of energy utilization and avoiding its depletion as a critical solution has gained increasing global attention. Thermal energy storage technology plays a significant role in reducing the disparity between energy demand and supply as a highly efficient technique for management of energy [1]. Among all the materials, phase change materials (PCMs) have garnered the most attention in numerous fields such as building and temperature control equipment due to their ability to capture and release a large amount of thermal energy during their melting and freezing processes, with superb advantages of storing or releasing the thermal energy at a constant temperature or within a narrow temperature range. Among a wide spectrum of prospective organic and inorganic PCMs, paraffin wax (PW) is particularly attractive because of their low price, large latent heat, chemical stability, no toxicity, and lack of phase separation. However, one of the main undesirable drawbacks of PW is low thermal conductivity, which decreases the efficiency of heat transfer. Moreover, the leakage of the PCMs when the application temperature exceeds the melting point hinders the direct application to numerous fields. In order to tackle the challenges, a method of encapsulation or shape stabilization of PCMs using supporting materials was considered, which allows the solid state to be maintained during the phase-change process of PCMs.<br/>In this work, we utilized a spray-dry strategy to fabricate thermally conductive hexagonal-boron nitride microspheres (sph-BN) as a shape-stable porous support using the as-synthesized nano-BN (nano-BN) powders. Sph-BN was assembled with polyvinylpyrrolidone (PVP) as an organic binder during the process followed by calcination to form porous structures in sph-BN after elimination of PVP between nano-BN. High thermally conductive sph-BN/PW shape-stabilized PCMs (SSPCMs) composites were successfully obtained by simple solution process that PW is incorporated into the pore sites in sph-BN at above of melting temperature of PW followed by mold-pressing into disk-shaped specimen. The leakage tests by macroscopic photographs of the SSPCMs and pure PW at 25^oC and elevated temperature up to 100^oC were performed, resulting in pure PW was in a molten state and the primary shape was completely changed while sph-BN/PW composites perfectly kept their primary form even at temperatures much beyond the melting point of PW. The phase change enthalpies and heat storage capacities of sph-BN/paraffin SSPCMs were determined by differential scanning calorimetry. In addition, the latent heats of the SSPCMs increased with increasing contents of paraffin in the composites. The thermal conductivity shows a slight decrease as the ratio of paraffin increases; yet it is still higher than that of pure paraffin, due to the existence of thermally conductive sph-BN in the organic matrix. As mentioned above, SSPCMs we developed represent an important step to develop leakage-protective PCMs with high performance and enhanced thermal conductivity. Therefore, we believe our work can be widely applied to latent heat thermal energy storage.<br/><br/>[1] Dimberu G. Atinafu et al., J. Mater. Chem. A, 6, 8969 (2018)