Development of a microencapsulated Al-Si phase change material with high-temperature thermal stability and durability over 3000 cycles

Development of highly durable phase change materials (PCMs) above 500 degrees C is essential in future high-temperature thermal energy storage systems. In this study, we report the fabrication of microencapsulated PCM (MEPCM) microspheres with high-temperature stability and cycling durability over 3000 cycles. The MEPCM consists of an Al-Si alloy core (Al-25 wt% Si; melting point of 577 degrees C) and a self-repairing Al2O3 shell. The uniform and highly durable Al2O3 shell is processed in three indispensable steps. Firstly, a boehmite treatment in an Al(OH)(3) turbid solution under an optimal pH value of 8 is used for the formation of AlOOH and Al(OH)(3) shell precursors. Secondly, additional Al(OH)(3) is further precipitated on the surface to enhance the formation of a thicker shell. Finally, a stable and self-repairing two-phase (-Al2O3 and -Al2O3) Al2O3 shell is formed by heat-oxidation in an O-2 atmosphere. The surface morphology, crystal structure of the shell, thermal durability, cycling stability and the shell formation mechanism are carefully investigated. The newly introduced boehmite and precipitation pre-treatments under optimal conditions can reinforce the formation of a thick and highly compact shell with small -Al2O3 and -Al2O3 grains, which are beneficial to disperse the thermal stress during high-temperature cycling and restrain crack propagation. The excellent achievement of durability over 3000 cycles can promote the practical applications of the MEPCM for high-temperature thermal storage, for example, it can be applied to the thermal storage system of a concentrated solar power plant for more than 6 years based on the reported durability.