VISCOSITY AND DENSITY IN MOLTEN BEF2-LIF SOLUTIONS

The viscosity of the molten salt system, BeF2-LiF, was determined with a coaxial-cylinder viscometer. The concentration range studied was 36-99 mole % BeF2, each composition being measured over a 200° temperature interval within over-all limits of 376°-967°C. The viscosity isotherms exhibit an exponential decrease in viscosity with increasing LiF concentration; the decreases are attributed to the rupturing of fluoride bridges in the BeF2 network structure. Each composition displayed Arrhenius behavior (Eη, energy of activation for viscous flow, was independent of temperature). Eη decreased with LiF concentration in a way that paralleled the decreases in the analogous systems, SiO2-alkaline-earth oxide. Densities were measured between 50 and 100 mole % BeF2. From these measurements and other density determinations in BeF2-LiF, it was found that the molar volumes were additive. Volume expansivity, however, increases markedly with increasing LiF concentration. The decrease of Eη with LiF content in this system is correlated with an increase in volume expansivity; for the composition range 100-70 mole % BeF2, Eη is linear with the reciprocal of expansivity.