Thermodynamics and structure of single- and two-phase yttria-alumina glasses

Yttria-alumina (YA) glasses containing 59.8-75.6 mol % Al2O3 were synthesized in the form of 1-3.5 turn diameter spheroids using containerless techniques. The glasses formed at cooling rates ranging from <70 K/s at compositions near 72 mol % Al2O3 and up to 300 K/s at the ends of the composition range. Samples with compositions from 59.8 to similar to69.0 mol % Al2O3 contained two glass phases, with an immiscible droplet (similar to1- 20 mum diameter) phase in the matrix glass. A single glass phase was formed at greater alumina concentrations. Glasses near the alumina- and yttria-rich ends of the compositional suite also contained similar to5-20% and 1-2% crystals, respectively. Heat capacities (C-p) of the glasses and supercooled liquids, glass transition temperatures (T-g), configurational heat capacities at T-g (DeltaC(p)(T-g)), and glass structures were investigated using differential scanning calorimetry (DSC) and Al-27 MAS NMR spectroscopy. The T-g increases slightly with alumina content, from 1146 to 1156 K. The Cp increases similar to60% at T, to form a highly fragile supercooled liquid. The concentrations of 4-, 5-, and 6-coordinate Al3+ ions in a single-phase glass with 71.5 mol % Al2O3 (Y71.5A) are similar to68% +/- 4, 27% +/- 3, and 6% +/- 2, respectively. A minimum 6- and maximum 5-coordinate Al3+ concentration occurs near the Y71.5A composition, which is one of the best glass formers. In the context of the structural and thermodynamic properties reported herein, the formation mechanism and coordination of polyamorphic YA glasses are discussed.