Thermodynamic and kinetic parameters for glass transition of alkali-borate and alkali-silicate melts

Differential scanning calorimetry was used to study glasses and crystals whose compositions correspond to congruently melting compounds: B2O3, Rb2O . nB(2)O(3) (n = 2, 3, 5); Cs2O . nB(2)O(3) (n = 3, 5, 9), M2O . 2B(2)O(3) (M = Li, Na); and K2O . 4SiO(2). The fictive glass-transition temperature T-f, the excess of the entropy of glass over that of the crystal at Tf for various prehistories of the glasses (the rate of melt cooling was varied from 0.5 to 10(6) K/min), and the activation energies of glass transition Deltah* were evaluated. Based on the data obtained, the thermodynamic (the ratio of the excess of the entropy of glass over the entropy of the corresponding crystal at Tf to the entropy of the melting of the crystal) and kinetic (fragility m =f(Deltah*, T-f)) parameters were compared for alkali-borate and alkali-silicate melts. The thermodynamic parameter alone is insufficient to describe the process of glass transition; both parameters should be used simultaneously.