Metastable segregation in the Al2O3-B2O3-SiO2 system

Glasses of twenty-one compositions grouped in the highly siliceous part of the diagram representing the Al2O3-B2O3-SiO2 system between approximately 66 and 85 mole% SiO2 were synthetized and investigated. The boundary of the metastable liquation region was determined by heat treatment of the glasses at various temperatures with subsequent examination of their structures with the aid of an electron microscope. The system was found to contain a wide metastable liquation range, which extends from Al2O3-SiO2 in the diagram almost to B2O3-SiO2, attaining compositions of about 3 mole% Al2O3. In this case, the liquation temperature smoothly declines, and near B2O3-SiO2 tends to fall sharply. A study of the infrared absorption spectra of glasses not subjected to heat treatment permitted to conclude that liquation in the Al2O3-B2O3-SiO2 system is apparently associated with formation of AlO6 and BO4 groups, which are incompatible with SiO4 tetrahedra