Kinetics of phase separation in a 6.5 Na2O • 33.5 B2O3 • 60 SiO2 glass

Industrially melted glasses with the composition (in mol%) 6.5 Na2O . 33.5 B2O3 . 60 SiO2 Were thermally treated at temperatures in the range of 660 to 750 degrees C. This resulted in phase separation, i.e, in the formation of a silica- and a sodium berate-rich phase with an interconnected microstructure. Both, the volume content of the berate-rich phase and the mean structure thickness (the correlation length) increased with time as well as with temperature. The volume content approached to a limiting value at constant temperature. The correlation length increased with time according to a power law (similar to t(1/n)). By contrast to previous studies, n was in the range of 1 to 1.2 within the temperature range and time scale studied. The correlation lengths were much larger (up to 12 mu m) and the viscosities much lower than in most previous studies. The kinetic law was explained as controlled by visous flow.