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

Abstract

Development of highly durable phase change materials (PCMs) above 500 °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 °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 (α-Al₂O₃ and θ-Al₂O₃) Al₂O₃ shell is formed by heat-oxidation in an O₂ 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 α-Al₂O₃ and θ-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.