The phase separation phenomenon in the Na2O–B2O3–SiO2–Fe2O3 glass-forming system and its application for producing porous glasses

Glasses in the Na₂O–B₂O₃–SiO₂–Fe₂O₃ system with a constant SiO₂ content 70 mol% were synthesized using conventional melting in platinum crucibles in SiC-furnace in air. After synthesis and annealing, glasses were heat treated at 550°C for 96–144 hrs to promote phase separation. A tentative region of phase separation for this temperature was outlined. X-ray powder diffractometry results showed three iron-containing phases (Fe₃O₄, FeSiO₃ and β-Fe₂O₃) forming in the investigated glasses with magnetite being the main phase as it is observed in most of the glasses. Chemical durability studies showed that compositions of phase-separated glasses suitable for synthesis of porous glasses, both iron-free and iron-containing lie in between 4 and 8 mol% of Na₂O. Bulk samples of porous glasses were obtained within the chosen region having the following parameters: specific surface area 40–185 m²/g, porosity 30%–45%, pore diameter 3–14 nm. The parameters of porous structure of iron-containing porous glasses are of the same order of magnitude as the porous glass used for the multiferroic nanocomposite synthesis.