Nucleation and growth of precipitates in demixing glass melts in the system NA2O-CAO-SIO2.

Abstract

As an example of a first order phase transition with conserved order parameter the demixing of glass-forming quasibinary systems has been investigated by in situ measurement of light scattering during heat treatment. The relative supercooling is consistent with theoretical expectations. Using a two stage heat treatment, nucleation and growth are decoupled. Nuclei formed at a nucleation temperature Tn can grow to sizes detectable by light scattering at a higher temperature Tgr, where no more nuclei are formed. Reducing the nucleation time, one at last ends up with an incubation time, the temperature dependence of which is described by an Arrhenius ansatz. For low particle densities, the growth mechanism can be deduced from the initial increase of the scattering intensity. The proportionality of the latter to t5 is characteristic for diffusion controlled growth of spheres in a depleted zone. From extrema in the scattering a definite size is assigned to the aggregate. Then, the number of particles can be deduced from the absolute intensity. For the systems investigated, however, growth time is limited by the occurrence of crystallisation, thus allowing only the determination of relative nucleation rates