Mechanical–Thermal Synergy in Phase Change Microcapsules: Experimental and Numerical Study on Double-Shell Structure and Nanoreinforced Polyurea Composites

Phase change microcapsules (MCs) demonstrate significant application potential in thermal energy storage. However, conventional single-component shell materials exhibit limitations, including low mechanical strength, poor thermal conductivity, and inadequate sealing performance. In this study, octadecane was used as the core material, and four kinds of MC systems were constructed by interfacial polymerization and in situ polymerization: melamine–urea–formaldehyde (MUF) single shell, polyurea (PUA) single shell with nano-SiO 2 as Pickering emulsifier, MUF inner shell/PUA outer shell double shell, and PUA inner shell/MUF outer shell double shell. The packaging efficiency, thermal conductivity, leakage rate and heat-transfer characteristics of different structures are systematically compared. The results show that the comprehensive performance of the PUA inner shell/MUF outer shell double-shell structure is the best, the packaging efficiency is 76.2%, the thermal conductivity is 9.35% higher than that of pure PUA single shell, and the leakage rate is as low as 3.35% at 50°C for 20 h. The transient heat conduction simulation of ANSYS Fluent reveals the performance improvement mechanism: the heat conduction network formed by self-assembly of nano-SiO 2 at the oil–water interface accelerates the heat transfer, and the double-shell synergistic effect of “flexible inner shell attached to the core material and rigid outer shell provided protection” significantly inhibits the core material leakage. The collaborative optimization strategy of shell structure design and nano-Pickering emulsification proposed in this study provides a new technical path for realizing high sealing, high strength, and high thermal conductivity of phase change MCs at the same time and has application value in the fields of building energy saving and thermal management of electronic equipment.