Thermochemistry of Y, Yb, and Gd CMAS glasses and melts

Calcium-magnesium-aluminosilicate (CMAS) glasses with and without yttrium (Y), ytterbium (Yb), and gadolinium (Gd) oxides were synthesized by a laser-beam heating and melt quenching technique and characterized by X-ray diffraction, simultaneous thermal analysis , and electron probe microanalysis . The formation enthalpies of the rare-earth CMAS have been investigated using high-temperature oxide melt calorimetry in molten lead borate solvent at 1073 K. The glass transition and crystallization temperatures increase with the decreasing ionic radius of the rare-earth cations. The enthalpy of formation of the CMAS glasses with higher rare-earth content becomes more exothermic with increasing ionic radius of the cations, which indicates increasing stability in this order. The free Gibbs energy of formation of the molten glasses was used to calculate the free Gibbs energy of corrosion of rare-earth silicate coatings. Rare-earth disilicates exhibited more negative free Gibbs energy of corrosion when compared to the monosilicates, indicating higher reactivity.