Dynamics of structural relaxation in bioactive 45S5 glass.

Kinetics of physical aging in archetypic 45S5 bioactive silicate glass composition with different types of phase separation are studied in situ below the glass transition temperature (Tg). The qualitative nature of aging is found to be almost independent of the structural differences on the micrometer scale. A well-expressed step-like behaviour in the enthalpy recovery kinetics is observed for aging temperatures Ta~0.90Tg and Ta~0.85Tg, which, however, disappears when the aging occurs at Ta ~0.95Tg. The overall kinetics is described by a stretched-exponential function with stretching exponent close to 3/7 at Ta ~0.95Tg, and 1/3 when the aging temperature drops to ~0.90Tg and below. The values correlate well with the predictions of Phillips' diffusion-to-traps and percolating fractals models. Appearance of step-like behaviour at larger departure from Tg is attributed to the hierarchical scheme of approaching equilibrium based on an alignment-shrinkage mechanism of physical aging proposed earlier for chalcogenide glasses.