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 w
as 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-ric
h phase. Although the XRD results showed that the development of the boron-
rich phase was through nucleation and growth, the NMR spectra indicated tha
t the sodium-rich phase always occurred as amorphous glasses with fluctuati
on in composition during the phase separation. This may result from that th
e over-coordination of oxygen atoms in the sodium-rich phase, which renders
difficulties in the crystallization of the sodium-rich phase with less tha
n 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.