Effects of lithium oxide on the crystallization kinetics of Na2O·2CaO·3SiO2 glass

The crystal nucleation (I) and growth (U) rates of Na2O·2CaO·3SiO2 glasses with increasing additions of Li2O were measured for the primary crystalline phase over wide and overlapping temperature intervals. A partial section of the phase diagram Na2O·2CaO·3SiO2-Li2O was constructed via DSC analysis and shows a narrow range of solid solution formation that is close to that of Na2O·2CaO·3SiO2. In the four-component system (Na2O-Li2O-CaO-SiO2), a pseudo-binary eutectic system of Na2O·2CaO·3SiO2-Li2O·SiO2 exists. The role of Li2O in the formation of the crystalline phases was investigated using DSC and X-ray analyses. The addition of Li2O results in decreases in the glass transition (Tg) and liquidus (TL) temperatures. With increasing Li2O content, both the reduced glass transition temperature Tgr (Tgr = Tg/TL) and the fragility index, m, pass through a minimum. These findings extend and confirm the known inverse correlation between Tgr and Imax (lower values of Tgr correspond to higher values of Imax) and provide evidence for a similar correlation between m and Imax. When using the low temperature data for the crystal growth rate (U) the viscosity (η) and diffusivity (D) were decoupled at T < 1.1 Tg. Next, a parameter (U·η at Tg) that characterized the decoupling phenomenon was proposed.