Study of kinetics of the phase separation in sodium borate glasses

The kinetics of the phase separation in 15Na(2)O-85B(2)O(3) binary glasses was investigated using B-11 nuclear-magnetic-resonance (NMR) spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM) observation. It was found that the equilibrium of the phase separation took long time due to the growth of a boron-rich phase and the composition fluctuation in Na-rich phase. Although the XRD results showed that the development of the boron-rich phase was through nucleation and growth, the NMR spectra indicated that the sodium-rich phase always occurred as amorphous glasses with fluctuation in composition during the phase separation. This may result from that the over-coordination of oxygen atoms in the sodium-rich phase, which renders difficulties in the crystallization of the sodium-rich phase with less than 75 mol% sodium oxides. It was found that, at 500 degrees C, the 15Na(2)O-85B(2)O(3) glasses was finally separated into Na2O . 9B(2)O(3) and 3Na(2)O . B2O3 rather than those being proposed by the conventional phase diagram. (C) 2000 Kluwer Academic Publishers.