Effect of Al2O3 on phase separation and microstructure of R2O-B2O3-Al2O3-SiO2 glass system (R = Li, Na)

•Addition of Al2O3 in lithium borosilicate glass changes the microstructure from isolated into bimodal and monomodal interconnected phases.•Lithium borosilicate glass accommodate higher Al2O3 content with phase separation ability compared to sodium borosilicate glass system.•Al2O3 decrease crystallization and phase separation in sodium borosilicate and lithium borosilicate glass systems.•The glass transition and liquidus temperature decrease with increase in Al2O3 content in sodium borosilicate and lithium borosilicate glass systems. The investigation of selected properties in non-porous and porous R2O-B2O3-Al2O3-SiO2 (R = Li, Na) glasses indicates different effects of Al2O3 on phase separation and microstructure. Decrease of [BO4]− units in Na2O-B2O3-SiO2 glass system reveals a constant inhibition of phase separation with addition of Al2O3. In Li2O-B2O3-SiO2 glass system, the addition of Al2O3 leads to a non-linear behaviour. XRD pattern and MAS-NMR studies display a microstructure-dependence on partial crystallization and phase separation. An addition of up to 4 mol.-% Al2O3 changes the microstructure from isolated into bimodal and monomodal interconnected phases. Based on the analysed properties a simplified model is set up which explains the influence of Al2O3 on phase separation, crystallization and microstructure in Li2O-B2O3-Al2O3-SiO2 glasses.