Nanoencapsulated n-tetradecane phase change materials with melamine–urea–formaldehyde–TiO₂ hybrid shell for cold energy storage

A series of nanoencapsulated phase change materials with n-tetradecane as the core material and melamine–urea–formaldehyde (MUF)–TiO₂ composite as the shell material was developed using a two-step method. Sol-gel and blending modification methods were respectively utilized after the in situ polymerization. The physical, chemical, and thermal properties of the nanoencapsulated phase change material (NEPCM) samples were characterized. Furthermore, a comprehensive performance evaluation system was established based on the efficacy coefficient method. The results indicated that the average diameter of the nanocapsules was significantly increased by the sol-gel modification and was slightly influenced by the blending method. All NEPCMs exhibited high chemical and thermal stability. The NEPCMs modified by the sol-gel method (C₁₄@MUF–TiO₂-S1) exhibited the best performance in terms of the encapsulation ratio, melting enthalpy, and onset decomposition temperature, which were 68.6%, 156.2 J·g^-1, and 155.36 °C, respectively. C₁₄@MUF–TiO₂-S2 exhibited the lowest mass loss of 44.2% at 100 h. For the blending method, C₁₄@MUF–TiO₂-B1 had the highest yield of 53.87%. The thermal conductivity of C₁₄@MUF–TiO₂-B3 increased up to 88.15% compared with the unmodified NEPCMs. Therefore, modification by the sol-gel method with 2.2 g TiO₂ sol had the highest total efficacy coefficient of 0.843 among the samples.