Crystallization of lithium disilicate-based multicomponent glasses - effect of silica/lithia ratio

Two glass compositions were prepared from the system SiO2-Li2O-K2O-ZrO2-P2O5with different SiO2/Li2O ratio (2.39 and 3.39) and the crystallization behavior was investigated by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallization kinetic parameters (activation energy of crystallization and Avrami exponent) were evaluated by different methods from the data obtained by DTA performed at different heating rates. For both glasses, two exothermic peaks were observed in the DTA curves, and the crystallization peak temperatures increased with SiO2/Li2O ratio. XRD analysis revealed that the first peak corresponds to the crystallization of lithium metasilicate (Li2SiO3) and the second to the formation of lithium disilicate (Li2Si2O5). After heating the glasses at a temperature above the second crystallization peak (900°C), both Li2Si2O5and Li2SiO3were found in samples having the lowest SiO2/Li2O ratio, whereas no Li2SiO3was detected in samples with the highest SiO2/Li2O ratio. For both glasses, the value obtained by different methods for the activation energy of crystallization was in the range of 225–275kJmol−1for the first exothermic peak and in the range of 425–500kJmol−1for the second peak. The estimated Avrami exponent was close to 1 for the first exothermic peak, indicating surface crystallization, and close to 3 for the second exothermic peak, suggesting volume crystallization. This was confirmed by the morphological study made by SEM that showed needle-like crystals in the microstructure of samples with lithium metasilicate and granular crystals in the microstructure of samples having lithium disilicate.