Glass structure and crystallization of Al and B containing glasses belonging to the Li2O–SiO2 system

Abstract

The aim of the present work is to investigate the effect of substituting B₂O₃ for Al₂O₃ in a non-stoichiometric lithium disilicate (Li₂Si₂O₅, LS₂) glass composition belonging to the system Li₂O–K₂O–Al₂O₃–SiO₂. Addition of equimolar amounts of K₂O and Al₂O₃ to binary lithium silicate glass compositions improves chemical resistance, sintering behaviour and mechanical properties of the glass-ceramics produced from sintered glass powder compacts. However, in bulk (monolithic) glasses Al₂O₃ addition hinders bulk nucleation. It also suppresses crystallization of LS₂ and promotes formation of a meta-stable crystalline phase called lithium metasilicate (Li₂SiO₃, LS). The results showed that B substitution resulted in the depolymerisation of the glass network increasing the percentage of NBOs leading to decreasing viscosity, molar volumes, oxygen densities and glass transition temperatures. The simultaneous addition of Al and B into the glass composition resulted in decreased liquid–liquid phase segregation (LLPS) and lower crystal nucleation tendency when compared to Al pure or B pure compositions. Further, Al rich glasses featured lithium metasilicate crystallization at initial stages and then transformed into LS₂ at higher temperatures, while with B addition glasses crystallize directly into LS₂.