Thermochemistry of Glasses in the Y₂O₃-Al ₂O₃-SiO₂ System

Enthalpies of drop solution in molten 2PbO-B₂O₃ at 1078 K were measured for glasses along the 2YAlO₃-3SiO₂ and 1/2Y₃Al₅O₁₂-3SiO₂ joins. The onset glass transition temperature increases slightly with increasing silica content and Y/Al. Enthalpies of mixing were calculated on the basis of amorphous end members. Strongly negative heats of mixing support the absence of miscibility gaps except possibly for very high silica content, consistent with experimental phase analyses, which indicate much narrower miscibility gaps compared with the phase diagrams calculated on the basis of previous data and the CALPHAD formalism.