Study of phase separation in Na2O-B2O3 glass system by nuclear magnetic resonance

The B-11 nuclear magnetic resonance (NMR) spectra, together with X-ray diff raction (XRD) and scanning electron microscopy (SEM), have been used to inv estigate the phase separation in 15Na(2)O-85B(2)O(3) binary glass. Based on the present investigation we suggest that the phase equilibrium in sodium berate glasses is controlled by the rate competition of the nucleation and crystal growth between two immiscible phases rather than those being propos ed by the conventional phase diagram. The experimental results show that th e sodium-rich phase, after being heat-treated at 500 degrees C, was not Na2 O . 4B(2)O(3) as indicated by phase diagram, The over-coordination of oxyge n atoms in the sodium-rich phase rendered difficulties in the crystallizati on of the sodium-rich phase with less than 75 mol % sodium oxide. Therefore , the high growth rate of the boron-rich phase can force the residual glass toward much higher concentrations of sodium oxide. According to the NMR me asurements, it was found that, after reaching equilibrium,: the 15Na(2)O-85 B(2)O(3) glass separated into Na2O . 9B(2)O(3) and 3Na(2)O . B2O3. (C)2000 Academic Press.