Effect of Al 2O 3 and K 2O content on structure, properties and devitrification of glasses in the Li 2O–SiO 2 system

Abstract

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 23S 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 23S 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 23S 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.