Effect of Al_2O_3 and K_2O content on structure, properties and devitrification of glasses in the Li_2O-SiO_2 system

摘要

The effect of Al_2O_3 and K_2O content on structure, sintering and devitrification behaviour of glasses in the Li_2O-SiO_2 system along with the properties of the resultant glass-ceramics (GCs) was investigated. Glasses containing Al_2O_3 and K_2O and featuring SiO_2/Li_2O molar ratios (3.13-4.88) far beyond that of lithium disilicate (Li_2Si_2O_5) stoichiometry were produced by conventional melt-quenching technique along with a bicomponent glass with a composition 23Li_2O-77SiO_2 (mol.%) (L_(23)S_(77)). The GCs were produced through two different methods: (a) nucleation and crystallization of monolithic bulk glass, (b) sintering and crystallization of glass powder compacts. Scanning electron microscopy (SEM) examination of as cast non-annealed monolithic glasses revealed precipitation of nanosize droplet phase in glassy matrices suggesting the occurrence of phase separation in all investigated compositions. The extent of segregation, as judged from the mean droplet diameter and the packing density of droplet phase, decreased with increasing Al_2O_3 and K_2O content in the glasses. The crystallization of glasses richer in Al_2O_3 and K_2O was dominated by surface nucleation leading to crystallization of lithium metasilicate (Li_2SiO_3) within the temperature range of 550-900 °C. On the other hand, the glass with lowest amount of Al_2O_3 and K_2O and glass L_(23)S_(77) were prone to volume nucleation and crystallization, resulting in formation of Li_2Si_2O_5 within the temperature interval of 650-800 °C. Sintering and crystallization behaviour of glass powders was followed by hot stage microscopy (HSM) and differential thermal analysis (DTA), respectively. GCs from composition L_(23)S_(77) demonstrated high fragility along with low flexural strength and density. The addition of Al_2O_3 and K_2O to Li_2O-SiO_2 system resulted in improved densification and mechanical strength.