Preparation and thermal energy storage properties of highly thermally conductive and evaporation-resistant n-heptane phase change capsules via buoyancy microfluidics

Low-temperature phase change materials (PCMs) are promising for cold-energy storage but are limited by volatilization-induced leakage and the low thermal conductivity of polymer shells. To overcome these limitations, a buoyancy-driven coaxial microfluidic device integrated with ultraviolet curing was developed to encapsulate n-heptane (C 7 ) into phase change capsules. After the optimal flow rate ratio was determined via the comprehensive evaluation index ( CPI ), SiO 2 -KH570 nanoparticles were incorporated into the UV-curable resin shell to improve its compactness and barrier performance. Surface ag was subsequently deposited through a silver mirror reaction to enhance heat transfer, forming a C 7 @polymer-SiO 2 @ag hierarchical structure. At the optimal core-to-wall flow-rate ratio of 40:20, the capsules exhibited a solidification temperature of −103.68 °C and a solidification enthalpy of 74.13 J/g. the addition of 1 wt% SiO 2 improved volatilization resistance and reduced supercooling, increasing solidification and melting enthalpies to 84.74 J/g and 90.15 J/g respectively. After 400 h of storage, the mass retention increased by 16.94% compared with the unmodified capsules. Furthermore, surface silver plating preserved the phase-change enthalpy while increasing overall thermal conductivity by 62.7%. After 25 consecutive thermal cycles, the solidification enthalpy decreased by only 0.47% compared with that of the first cycle, further demonstrating the good thermal cycling stability of the EPCMs. In a demonstrative cold-retention test, the ag-coated capsules extended the retention time by 47.4% at −80 °C and 19.4% at −60 °C compared with the system without EPCMs. These results demonstrate that the combined SiO 2 and ag modifications effectively improve volatilization resistance, heat-transfer performance, thermal cycling stability, and low-temperature thermal buffering, supporting the potential application of the developed capsules in low-temperature cold-energy storage and thermal management