In this study, a copper-based capsule, encapsulated by a black alumina shell using a simple method, was developed for high-temperature heat storage over 1000 °C. The shell was filled with copper beads (diameter = ∼3 mm), the copper–aluminum (Cu–Al) atomized powder (particle size = 150 μm) was filled in the gap, and then it was heat-treated. This eventually formed a high-density, aluminum-rich network around the Cu beads in the shell. Morphological observations indicated that the corrosion of copper oxide (CuO) on the alumina shell was significantly reduced by mixing Al with Cu. When heat-treated in air at 1100 °C, Cu became CuO, which reacted with Al₂O₃ to form a new compound, CuAl₂O₄. Owing to this two-step reaction, the area around the hole provided at the top of the spherical shell gets sealed, thereby suppressing the flow of oxygen into the shell (self-sealing function). The mechanism of in situ sealing was detected using X-ray diffraction and scanning electron microscopy analyses. The Cu–(5–10%) Al capsule could endure a 400 h air exposure test at 1100 °C without leakage or cracking. A low weight increase ratio of 3.5% after the exposure revealed good oxidation resistance of the capsule, whereas the slight damage load change of the capsule after the exposure showed good mechanical stability. The results concluded that a Cu–(5–10%) Al capsule for high-temperature heat storage applications above 1000 °C can be fabricated by a simple process, and it demonstrates excellent durability during the long-term air exposure test. The data obtained in this study can be used as a reference for the design of packed beds in the future.

Developing thermal storage materials is crucial for the efficient recovery of thermal energy. Salt-based phase-change materials have been widely studied. Despite their high thermal storage density and low cost, they still face issues such as low thermal conductivity and easy leaks. Therefore, a new type of NaCl-Al₂O₃@SiC@Al₂O₃ macrocapsule was developed to address these drawbacks, and it exhibited excellent rapid heat storage and release capabilities and was extremely stable, significantly reducing the risk of leakage at high temperatures for industrial waste heat recovery and in concentrated solar power systems above 800 °C. Thermal storage macrocapsules consisted of a double-layer encapsulation of silicon carbide and alumina and a self-standing core of NaCl-Al₂O_3. After enduring over 1000 h at a high temperature of 850 °C, the encapsulated phase-change material exhibited an extremely low weight loss rate of less than 5% compared with NaCl@Al₂O₃ and NaCl-Al₂O₃@Al₂O₃ macrocapsules, for which the weight loss rate was reduced by 25% and 10%, respectively, proving their excellent leakage prevention. The SiC powder layer, serving as an intermediate coating, further prevented leakage, while the use of Al₂O₃ ceramics for encapsulation enhanced the overall mechanical strength. It was innovatively discovered that the Al₂O₃ particles formed a network structure around the molten NaCl, playing an important role in maintaining the shape and preventing leakage of the composite thermal storage phase-change material. Furthermore, the addition of Al₂O₃ significantly enhanced the rapid heat storage and release rate of NaCl-Al₂O₃ compared to pure NaCl. This encapsulated phase-change material demonstrated outstanding durability and rapid heat storage and release performance, offering an innovative approach to the application of salt phase-change materials in the field of high temperature rapid heat storage and release and encapsulating NaCl as a high-temperature thermal storage material in a packed bed system. Compared with conventional salt-based phase-change materials, the developed product is expected to significantly improve the reliability and thermal efficiency of thermal storage systems.