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 (T _g). The qualitative nature of aging is found to be almost independent of the structural differences on the micrometer scale. A well-expressed step-like behavior in the enthalpy recovery kinetics is observed for aging temperatures T _a ∼ 0.90T _g and T _a ∼ 0.85T _g, which, however, disappears when the aging occurs at T _a ∼ 0.95T _g. The overall kinetics are described by a stretched-exponential function with stretching exponent close to 3/7 at T _a ∼ 0.95T _g, and 1/3 when the aging temperature drops to ∼0.90T _g and below. The values correlate well with the predictions of Phillips' diffusion-to-traps and percolating fractals models. Appearance of step-like behavior at larger departure from T _g is attributed to the hierarchical scheme of approaching equilibrium based on an alignment-shrinkage mechanism of physical aging proposed earlier for chalcogenide glasses.