Encapsulation of Phase Change Materials By Microemulsion Processes

Phase change materials (PCMs) play a crucial role in thermal energy storage systems by utilizing the latent heat of fusion to store and release energy. KF·4H₂O is a particularly promising PCM due to its phase change temperature of 18°C, making it well-suited for enhancing the energy density of air conditioning systems. However, its low thermal conductivity, a common drawback of salt-based materials, limits its power density and hinders its potential for commercialization. Another challenge with PCMs is the risk of leakage, which can compromise the reliability of the storage system. Encapsulation has been proposed as a solution to prevent leakage, but scalable encapsulation methods face difficulties. For instance, the impregnation method, which involves embedding liquid PCM into micro-sized matrices, requires expensive solid adsorbents. While effective in incorporating PCMs, this approach necessitates careful consideration of cost and commercialization viability.

To overcome these challenges, we investigated KF·4H₂O-based microemulsion techniques to develop innovative nano- and micro-sized core-shell structured PCMs, designed to deliver high power density and leak-free thermal energy storage systems. This size scale was chosen to optimize the available heat transfer through the coating layer. Using a microemulsion process involving oil, water, and surfactants, we successfully synthesized TiO₂-encapsulated KF·4H₂O core-shell structures. TiO₂ was chosen as the coating material due to its larger thermal conductivity compared to KF·4H₂O and high strength. As the TiO₂ precursor diffused to the oil-water interfaces, it reacted with water, resulting in the formation of TiO₂, which fully encapsulated the KF·4H₂O. After purifying the TiO₂/KF·4H₂O core-shell structures, no residual liquids were detected. Transmission electron microscopy (TEM) confirmed the complete encapsulation of KF·4H₂O by TiO₂. Particle sizes ranged from 60 nm to 120 nm and had coating thicknesses of 6 nm to 12 nm. Furthermore, differential scanning calorimetry (DSC) studies demonstrated the phase change behavior of KF·4H₂O (solid to liquid) without any evidence of water leakage during thermal cycling, validating the effectiveness of this encapsulation method.