Copper–Alumina Capsules for High-Temperature Thermal Energy Storage

High corrosivity, leakage, and oxidation of metallic phase-change materials (PCMs) have limited their applications in high-temperature thermal energy storage (TES) systems, regardless of their favorable benefits for high-temperature TES applications of over 1000 °C. To overcome these major challenges, this work presents a facile paraffin sacrificial layer approach for directly encapsulating copper (Cu) sphere PCMs with the alumina (Al2O3) shell, considering a buffer inner cavity. The cavity is formed by the decomposition of the paraffin layer through a pre-sintered process. It plays a key role in accommodating the volume expansion of the Cu core, thereby preventing the breakage of the shell and the leakage of the liquid PCM. A series of macrocapsules with different sizes (9.5–21 mm) containing the Cu core, cavity, and Al2O3 shell are successfully produced using the paraffin sacrificial layer method and deploying a two-step heat treatment. The experimental results show that the Al2O3 shell possesses a good structure, which can prevent the leakage of the Cu core. The Al2O3 shell also has strong compatibility with the Cu core without any chemical reaction between two materials. In a temperature range of 1000–1100 °C, the calculated mass and volume energy storage densities of the PCM macrocapsule with 21 mm outer diameter are found to be 222 kJ/kg and 745 J/cm3, which are 1.83 and 1.76 times, respectively, higher than those for the Al2O3 ceramic. After thermal cycle tests, the encapsulated Cu PCM shows superior shape stability, chemical stability, and thermal durability, which can be applied in long-term thermal storage systems for high-temperature TES systems.