Enhanced Thermal Durability of Microencapsulated Phase Change Material via Shell-Localized Oxide Nanoparticles

Thermal energy storage (TES) is an indispensable technology for enhancing the efficiency and reliability of renewable energy systems, particularly in solar thermal power generation. By harnessing phase change materials (PCMs), TES can effectively store and release thermal energy, thereby balancing energy supply and demand. Among various PCMs, inorganic materials such as metal alloys are favored for their high thermal conductivity and latent heat capacity. However, the practical application of PCMs is often hampered by issues such as supercooling and material degradation over repeated thermal cycles.<br/><br/>Microencapsulated PCM (MEPCM) has emerged as a promising solution to these challenges, offering enhanced thermal stability, ease of handling, and improved durability. Encapsulation involves coating PCM particles with a protective shell, which not only prevents leakage during the liquid phase but also mitigates the adverse effects of thermal expansion and contraction. The choice of shell material is critical, as it must exhibit high thermal conductivity, chemical stability, and mechanical strength. Among various encapsulation materials, α-Al₂O₃ is particularly attractive due to its exceptional thermal and structural properties. However, the immiscibility between α-Al₂O₃ nanoparticles and certain PCMs, such as Sn, poses significant challenges in achieving uniform encapsulation and optimal thermal performance.<br/><br/>This study addresses these challenges by employing a high-speed impact blending (HIB) technique to synthesize Sn@α-Al₂O₃ MEPCMs. The HIB method facilitates the dry synthesis of MEPCMs, ensuring high yield and environmental friendliness. Through rapid rotation and collision, Sn and α-Al₂O₃ particles are blended to form a uniform shell structure. The resulting MEPCMs demonstrate remarkable thermal durability, withstanding up to 100 cycles of melting-solidification without significant degradation. The encapsulation process introduces lattice defects within the α-Al₂O₃ nanoparticles, creating voids that promote the localization of Sn nanoparticles. This localized Sn, observed as nanosized particles within the α-Al₂O₃ lattice, forms a metastable Sn−Al−O surface that is crucial for the subsequent formation of SnO and SnO₂ nanoparticles during thermal cycling.<br/><br/>Thermal cyclic tests reveal that the Sn@α-Al₂O₃ MEPCMs maintain their structural integrity and thermal properties over 100 cycles, with the α-Al₂O₃ shell remaining resilient even under ultrafast heating/cooling rates. Differential scanning calorimetry (DSC) measurements indicate that the encapsulated Sn core exhibits a latent heat capacity of approximately 50 J g⁻¹, which is marginally lower than that of pure Sn due to the encapsulation. The degree of supercooling decreases from 72°C to 61°C after 100 cycles, demonstrating the effectiveness of the SnO and SnO₂ nanoparticles in accelerating the nucleation rate during solidification and thus suppressing supercooling.<br/><br/>Density functional theory calculations provide insights into the electronic states at the SnO₂/α-Al₂O₃ interface, highlighting the role of oxygen vacancies in lattice strain and electron delocalization. The formation of the SnO₂/α-Al₂O₃ interface reduces the energy barrier for Sn nucleation, thereby enhancing the nucleation velocity and contributing to supercooling suppression. The SnO and SnO₂ nanoparticles within the α-Al₂O₃ lattice also act as nucleation sites, promoting faster Sn nuclei formation and reinforcing the shell structure, which stabilizes the latent heat and improves thermal cyclic stability.<br/><br/>Our findings highlight the significance of lattice defects and localized oxide nanoparticles in enhancing the thermal performance of MEPCMs. This study provides valuable insights into the interplay between nanoscale crystal lattice imperfections and their implications for energy storage applications, paving the way for the development of advanced MEPCMs with improved properties and durability.