The Auto‐Oxidation Synthesis of Bi@Bi ₂ O ₃ Phase Change Microcapsules for Medium Temperature Thermal Energy Storage

This study proposes a straightforward auto‐oxidation method for synthesizing metallic microencapsulated phase change materials (MEPCMs). Bi@Bi 2 O 3 MEPCMs are formed with Bi microspheres as the core material and Bi 2 O 3 as the outer shell by “boiling/steam surface pretreatment + thermal oxidation treatment.” The composition, micromorphology, and thermophysical properties of the samples are investigated. The results show that the process of boiling/steam surface pretreatment has no effect on the shape and surface deformation of the Bi micropowders. The pretreated powders indicate spherical shape and smooth surface. MEPCMs obtained after the final thermal oxidation treatment are still scattered in powder form. The thickness of the shell layer of Bi 2 O 3 is about 1 μm, which can effectively prevent leakage and agglomeration of the microcapsules. The Bi@Bi 2 O 3 MEPCMs exhibit a melting point of around 271 °C and a maximum latent melting heat of 47.9 J g −1 , representing only a 1.6% reduction relative to Bi microspheres. In addition, the MEPCMs can preserve an intact core‐shell structure over 300 tests without shell rupture or leaking, indicating the potential for application in medium‐temperature thermal energy storage and thermal management.