Thermochemistry of Glasses in the Y 2 O 3 ‐Al 2 O 3 ‐SiO 2 System

Enthalpies of drop solution in molten 2PbO‐B 2 O 3 at 1078 K were measured for glasses along the 2YAlO 3 ‐3SiO 2 and return ½Y 3 Al 5 O 12 ‐3SiO 2 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.