Effect of Al2O3 and K2O content on structure, properties and devitrification of glasses in the Li2O–SiO2 system

The effect of Al2O3 and K2O content on structure, sintering and devitrification behaviour of glasses in the Li2O–SiO2 system along with the properties of the resultant glass–ceramics (GCs) was investigated. Glasses containing Al2O3 and K2O and featuring SiO2/Li2O molar ratios (3.13–4.88) far beyond that of lithium disilicate (Li2Si2O5) stoichiometry were produced by conventional melt-quenching technique along with a bicomponent glass with a composition 23Li2O–77SiO2 (mol.%) (L23S77). 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 Al2O3 and K2O content in the glasses. The crystallization of glasses richer in Al2O3 and K2O was dominated by surface nucleation leading to crystallization of lithium metasilicate (Li2SiO3) within the temperature range of 550–900 °C. On the other hand, the glass with lowest amount of Al2O3 and K2O and glass L23S77 were prone to volume nucleation and crystallization, resulting in formation of Li2Si2O5 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 L23S77 demonstrated high fragility along with low flexural strength and density. The addition of Al2O3 and K2O to Li2O–SiO2 system resulted in improved densification and mechanical strength.