Devitrification kinetics of bulk and pulverized sodium metaphosphate glassy phase

Crystallization kinetics of bulk and pulverized glass samples of the system mol% 50Na2O:50P2O5 have been investigated through isothermal as well as non-isothermal DSC studies. Analysis of the crystallization exotherms reveals that the bulk sodium metaphosphate glass undergoes single-stage crystallization with an order parameter (Avrami constant) of 2.7 ± 0.1 and an activation energy for crystallization equal to 142.5 kJ/mol, suggesting internal (bulk) crystallization of the glass. The activation energy for structural relaxation, evaluated by the analysis of the dependence of Tg on different heating rates, turns out to be 534.8 kJ/mol. The exotherm peaks of pulverized glasses exhibit a shift in the crystallization temperature, Tc along with a change in the height and width of the peak. The pulverized glass sample undergoes surface crystallization, with almost the same value of activation energy for crystallization as that of bulk glass. The single-stage crystallization mechanism of sodium metaphosphate (bulk and pulverized) glass is closely associated with Johnson–Mehl–Avrami (JMA) model.