In situ observation of metastable phase separation for the development of high-temperature latent heat energy storage materials

Abstract Miscibility gap alloys (MGAs) are promising candidates for high‑temperature thermal energy storage owing to their high latent heat and intrinsic phase separation. In this study, the liquid–liquid phase separation and subsequent solidification of Fe–Cu alloys were experimentally investigated using an aerodynamic levitator in a reducing atmosphere to suppress oxidation. In situ observations using a high-speed camera revealed that Fe‑rich liquid domains separated first from the undercooled homogeneous liquid, followed by the formation of Cu‑rich liquid domains. These observations are consistent with the asymmetry of the Gibbs free energy of mixing in liquid Fe–Cu alloys. The energy densities of these alloys exceeded the upper range of IRENA’s 2050 target (50–85 kWh m⁻ 3 ) for high-temperature latent-heat storage at Cu concentrations above 40 at. % (Fe60Cu40 and higher), indicating the potential of Fe–Cu alloys as high‑temperature latent heat storage materials. Our results provide insights into the role of microstructural control and, together with favorable thermal properties, offer a promising strategy for the design of MGA‑based thermal energy storage materials produced by casting.