Takehiko Ishikawa

ABSTRACT The thermophysical properties and atomic structure of molten oxides are crucial data for advancing our understanding of the glass transition and for optimizing melt processes of advanced functional glasses. We report a variety of measurements on ten binary and ternary fragile liquid oxides selected from two compositional families, the CaO–Al ₂ O ₃ –SiO ₂ and R ₂ O ₃ –Al ₂ O ₃ (R = Y, La, and/or Yb) systems, using imaging techniques on droplets levitated and laser beam heated in microgravity. The liquids’ densities, thermal expansion coefficients, viscosities, and surface tensions are measured up to 2800 K, spanning several hundred kelvins above and below the equilibrium melting points. For binary and ternary rare‐earth aluminate melts, the molar volumes follow approximately a linear trend with the mean cube of the cation radii, consistent with their unary oxide endmembers. Melt‐quenched glasses are further characterized with x‐ray tomography and diffraction to assess internal porosity and structure. Glasses prepared in microgravity have atomic structures that are indistinguishable from terrestrially prepared analogues. Internal bubbles are occasionally present, and in microgravity, the bubbles do not migrate to external surfaces as is common for terrestrial processing of such high‐temperature, inviscid liquids. These findings provide useful insights into the nature of fragile oxide liquids and glass formation, with implications for space‐based manufacturing.

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.