Crystallization kinetics of sodium-niobium silicate glasses

The crystallization of glasses in the Na₂O-Nb₂O₅-SiO₂ system in the temperature range 690–750°C is investigated by measuring the material density and X-ray powder diffraction analysis. It is found that the kinetics of precipitation of the NaNbO₃ crystalline phase can be described by the Avrami equation with the Avrami exponentn ≈ 1. The model describing the crystallization kinetics is proposed. The crystallization of amorphous inhomogeneity regions, which are formed as a result of phase separation, is considered in the model. The consideration refers to the crystallization process governed by the nucleation of crystals in these regions. The calculated Avrami exponents are close to unity. The proposed model does not contradict the available data on the X-ray small-angle scattering that have been obtained upon crystallization of the samples and qualitatively explains a nonmonotonic variation in the material transparency during crystallization.