Study of kinetics of the phase separation in sodium borate glasses

The kinetics of the phase separation in 15Na2O-85B2O3 binary glasses was investigated using 11B 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°C, the 15Na2O-85B2O3 glasses was finally separated into Na2O·9B2O3 and 3Na2O·B2O3 rather than those being proposed by the conventional phase diagram.