Influence of melting and annealing conditions on the structure of borosilicate glasses

Structural changes of borate glasses have been intensively studied with regard to the boric oxide anomaly. Two glasses, NBS1 (74SiO₂.10B₂O₃.16Na₂O) and NBS2 (74SiO₂.20.7B₂O₃.4.3Na₂O.1Al₂O ) were investigated by several techniques in order to learn more about the less thoroughly investigated borosilicate glasses. 11B NMR spectroscopy established that the BO₄ tetrahedron is the predominant structural boron unit (>80%) in the glass NBS1. A mixture of several boron coordinations is found in the glass NBS2 (18% BO₄ 42% BO₃,-I, 40% BO₃ - II). The glasses were further doped with the structure indicator ions Co²⁺ and Ni²⁺. Both ions are octahedrally coordinated in annealed NBS2 and tetrahedrally in NBS1. Co²⁺ and Ni²⁺ are expected to be tetrahedrally coordinated in the melt. Quenching the highly viscous glass NBS2 freezes in a transitional coordination of the dopants. The optical spectra of the doped glasses are discussed in relation to the applied heat treatment. The structural changes observed upon reheating the quenched samples were especially pronounced when the applied temperature is 150 to 200°C above the T_g of NBS2. The viscosity versus temperature curve deviates from the Vogel-Fulcher-Tammann (VFT) equation in this temperature range indicating structural changes within the glass matrix. NMR studies show a slight increase of BO₃ units connected with SiO₄ tetrahedra for the quenched glasses compared to the annealed glasses where more of the BO₃ units are linked in boroxol rings.