The effects of amorphous phase separation on crystal nucleation kinetics in BaO-SiO₂ glasses

Abstract

The nucleation kinetics of barium disilicate spherulites were determined by optical microscopy in BaO-SiO₂ glasses containing 27.0 to 33.3 mol % BaO for isothermal treatments at 718 to 760‡ C. Amorphous phase separation in two glasses was studied by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The average diameter, number and surface area of amorphous droplets were obtained by SAXS as a function of time at 743 and 760‡ C. In glasses not showing amorphous phase separation the crystal nucleation rate,I, was constant at a given temperature. In glasses undergoing phase separation,I increased with time approaching a constant value, which was identical for different phase-separated glasses. There was a striking correlation between the time to reach a constantI and the time required to attain the equilibrium composition of the amorphous (baria-rich) matrix phase, as revealed by a constant integrated SAXS intensity. The crystal nucleation rate,I, depended primarily on the composition of the baria-rich phase. There was no apparent relationship betweenI and the surface area or number of droplets from SAXS. However, possible evidence of additional crystal nucleation at droplet interfaces was found, although the effect was small. Viscosities and crystal growth rates for the phase-separated glasses are also discussed.