Glass-forming ability of La₂O₃-Nb₂O₅ evaluated via thermophysical properties under microgravity

The La₂O₃-Nb₂O₅ binary system is a unique glass-forming system without conventional network former oxides, exhibiting two distinct glass-forming regions: La₂O₃-rich and Nb₂O₅-rich compositions. To evaluate its glass-forming ability, the temperature dependence of density, viscosity, and surface tension was measured using the electrostatic levitation furnace aboard the International Space Station (ISS-ELF). Melt density showed linear temperature dependence, and thermal expansion coefficients at 2000 K varied from 2.5 × 10^-5 to 4.0 × 10^-5 K^-1. Substantial undercooling was observed for glass-forming compositions. Viscosity measurements above the melting point revealed that both La₂O₃-rich and Nb₂O₅-rich melts behave as fragile liquids. Activation energy derived from viscosity data was higher for glass-forming compositions. These results suggest that glass-forming ability can be assessed based on undercooling and activation energy across a wide compositional range, including non-glass-forming melts. The ISS-ELF experiments provide a valuable platform for understanding glass formation in systems inaccessible by terrestrial techniques.