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

Yttria-alumina (YA) glasses containing 59.8-75.6 mol % Al ₂O ₃ were synthesized in the form of 1-3.5 mm diameter spheroids using containerless techniques. The glasses formed at cooling rates ranging from <70 K/s at compositions near 72 mol % Al ₂O ₃ and up to 300 K/s at the ends of the composition range. Samples with compositions from 59.8 to ∼69.0 mol % Al ₂O ₃ contained two glass phases, with an immiscible droplet (∼1-20 μm 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 ∼5-20% 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 (ΔC _P(T _g)), and glass structures were investigated using differential scanning calorimetry (DSC) and ²⁷Al MAS NMR spectroscopy. The T _g increases slightly with alumina content, from 1146 to 1156 K. The C _p increases ∼60% at T _g to form a highly fragile supercooled liquid. The concentrations of 4-, 5-, and 6-coordinate Al ³⁺ ions in a single-phase glass with 71.5 mol % Al ₂O ₃ (Y71.5A) are ∼68% ± 4, 27% ± 3, and 6% ± 2, respectively. A minimum 6- and maximum 5-coordinate Al ³⁺ 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.