Thermal history driven molecular structure transitions in alumino-borosilicate glass

The glass network structure governs various thermos-physical properties such as viscosity, thermal, and electrical conductivities, and crystallization kinetics. We investigated the effect of temperature on structural changes in a Na2O-CaO-Al2O3-SiO2-B2O3 glass system using Al-27 MAS NMR spectroscopy. Around the glass transition temperature, most of aluminate structures exist as AlO4, acting as a glass former. When the temperature is above the melt crystallization temperature, the AlO4 structure is drastically decreased and glass structures are mainly composed of AlO5 and AlO6, acting as glass modifiers. Thermodynamic assessment based on Gibbs energy minimization was used to confirm the dependency of aluminate structure's amphoteric characteristic on temperature by calculating the site fraction of aluminate molecular structures at different temperatures. Temperature-induced aluminate structural variation can also influence silicate and borate structural changes, which have been confirmed by the Si-29 and B-11 NMR spectra.