Structural Response to Sub-Tg Annealing in a Hyperquenched SiO2-Al2O3 Glass

We explore the connection between the structural change and the sub-T _g enthalpy relaxation in 61SiO ₂–39Al ₂O ₃ (SA) glass by using the hyperquenching-annealing-calorimetry approach, nuclear magnetic resonance (NMR) and high-resolution transmission electron microscopy (HRTEM). The differential scanning calorimetry (DSC) measurements show a sub-T _g relaxation peak with a long tail starting from the onset temperature. With increasing the degree of sub-T _g annealing, the tail evolves into a broad peak. The 5- and 6- fold coordinated Al species in the SA glass decrease with sub-T _g annealing, while the 4-fold coordinated Al species increases. The coordination change leads to the formation of the oxygen triclusters, which act as the nucleation sites for mullite formation. This is confirmed by the existence of ordered structural nanodomains (∼10 nm). The extremely unstable behavior of the SA glass against crystallization could be attributed to both the higher content of the 5-fold coordinated Al (∼64%) and the easily formed oxygen triclusters. This work provides insights into both the structural evolution during the sub-T _g relaxation, and the structural heterogeneity of an unstable glass system.