Physical aging of lithium disilicate glass

Refractive index measurements were used to probe the structural relaxation behavior during the physical aging of a lithium disilicate glass at temperatures below the glass transition. The structural relaxation process was recorded using a single sample, starting from 5 K below the initial fictive temperature (Tf) of 720 K and gradually lowering it by 5 K increments until reaching an impractical experimental time to obtain complete relaxation at Tf − 35 K. The aging curves exhibited a stretched exponential relaxation behavior characteristic of glassy systems and showed good reproducibility in relaxation kinetics at three temperatures. A long continuous aging experiment conducted at 705 K demonstrated that the interruptions between isothermal treatments and property determinations did not affect the results. Finally, it was impossible to complete all experiments with the same sample, as after several cycles of cumulative relaxation experiments, the glass began to crystallize with the appearance of a small fraction of lithium disilicate crystals. This result indicates a relationship between structural relaxation and crystallization, corroborating recent studies in the literature.