Vitrification behavior of liquid salt droplet emulsions

Abstract

The subdivision of a liquid into fine droplets yields an effective nucleant isolation and a substantial increase in undercooling. At extreme levels of undercooling, vitrification can supercede crystallization in droplets even at a slow cooling rate. The droplet emulsion technique has been extended recently to the production of stable dispersions of molten salts including ZnCl 2. In fine droplet form (1–6 μ) ZnCl 2 vitrifies easily during slow cooling at 10°C/min. Thermal analysis measurements yield a glass transition for ZnCl 2 at T g = 114°C which is followed during heating by a broad crystallization starting at 190°C and involving metastable and stable products. In ZnCl 2KCl solutions, T g is lowered to 40°C near the eutectic at 28% KCl. From an analysis of the droplet size distribution characteristics, the undercooling behavior of dispersions has been modeled in terms of a collection of active heterogeneous surface sites which tend to promote crystallization of the large droplet size classes.