Sub-Tg enthalpy relaxation in an extremely unstable oxide glass and its implication for structural heterogeneity

We study the sub-Tg relaxation in an extremely unstable glass former, i.e., 65SiO2–35Al2O3, and its relation to structural heterogeneity (e.g., structurally ordered domains in glass matrix). This is done by hyperquenching (~ 106 K/s) the liquid, then annealing the hyperquenched (HQ) glass below Tg and subsequently scanning the annealed HQ glass in a differential scanning calorimeter (DSC). The results show that structural ordering can take place even below Tg. An endothermic pre-peak is observed when the HQ sample is annealed at 0.75Tg for sufficiently long time, which is, however, much weaker compared to that of stable glass formers subjected to same annealing conditions. We also investigate the effect of the sub-Tg annealing on crystallization above Tg. The results imply that some structurally ordered domains exist already in the liquid state. The ordered domains lower the activation energy for further structural ordering during the sub-Tg annealing, and thereby lowers the activation energy for crystal growth above Tg.