Homogeneous and Heterogeneous Crystallization in Nanometer-Sized Regions of the Copper Halide Phase in Photochromic Glasses

Abstract

The halide phase in copper halide photochromic glasses is studied by the small-angle X-ray scattering (SAXS) technique. The dependences of the volume and composition of the halide phase on the temperature of primary heat treatment are determined. The temperature of the formation of a vacuum pore upon cooling from the temperature of primary heat treatment and the temperature of pore disappearance upon heating are investigated as functions of the size of the halide phase regions. The influence of the size of the halide phase regions and the temperature of secondary heat treatment on the crystallization temperature of the halide phase is elucidated. It is shown that the previously observed increase in the crystallization temperature of the halide phase with an increase in the size of the phase regions is associated with the change-over from homogeneous to heterogeneous crystallization. In turn, the mechanism of crystallization depends on the region where the vacuum pore is formed. In the case when the effective radii of drops are less than 130–140 A, the vacuum pore is formed in the bulk of a liquid drop and the homogeneous crystallization occurs. The nuclei of heterogeneous crystallization arise when the formation of the vacuum pore is accompanied by the destruction of the vitreous matrix. This takes place when the radii of liquid drops are larger than 165–170 A. The nuclei of heterogeneous crystallization disappear upon heat treatment of glasses at temperatures above 600°C. As a result, upon cooling from these temperatures, the homogeneous crystallization is observed in all cases and the crystallization temperature does not depend on the size of the halide phase regions.