Crystallization and microstructure in K2O substituted SiO2-MgO-Al2O3-Li2O-AlPO4 glass-ceramics

The substitution of K+ ion by Ca2+ or Sr2+ in K2O-SiO2-MgO-Al2O3-B2O3-MgF2-Li2O-AlPO4 glass is executed and the relevant effects on crystallization and microstructure are explored. The glass monoliths obtained by single-step melt-quenching at 1500 +/- 10 degrees C were transparent in nature. Density of base glass was evaluated as 2.66 +/- 0.03 g cm(-3) and found to be decreased by in presence of CaO or SrO. Opaque glass-ceramics were derived from the transparent glasses by controlled heat-treatment at 1000 +/- 10 degrees C and the predominant crystalline phase was identified as fluorophlogopite mica, KMg3AlSi3O10F2 by X-ray diffraction. Field emission scanning electron microscopy revel that the plate like fluorophlogopite mica crystals predominated in base glass-ceramic (without CaO or SrO doped) but changed to nanocrystalline morphology containing droplet like spherical shaped mica crystallites (average size 200-400 nm) on addition of CaO and SrO. Highest density (=2.74 +/- 0.03 g.cm-3) observed for SrO containing glass-ceramic is attributed to the nanocrystalline morphology.