Thermophysical Properties of Fragile Liquid Oxides and Structure of Their Glasses Processed in Microgravity and on Earth

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–Al2O3–SiO2 and R2O3–Al2O3 (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.