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.

The viscosity of molten beryllium fluoride has been measured with coaxial-cylinder viscometers over five orders of magnitude (limits: 979°C, 28.6 P; 574°C, 2.24 × 106 P). Over the experimental range of shear stresses, BeF2 behaves as a Newtonian fluid. Like SiO2, its high-temperature analog, BeF2 shows an Arrhenius temperature dependence over several orders of magnitude in viscosity. Reported heat-capacity data for BeF2 and SiO2 and the Adam-Gibbs theory indicate that the Arrhenius viscosity behavior of these liquids may be explained in terms of the configurational entropy which is virtually constant at temperatures above the glass-transition temperature.

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