Density Measurement of Molten Alkaline-Earth Fluorides Using Archimedean Dual-Sinker Method

The densities of molten alkaline-earth fluorides (MgF₂, CaF₂, SrF₂, and BaF₂) were measured over the temperature range from 1526 K to 1873 K at ambient pressure using an Archimedean dual-sinker densitometer designed and set up by the authors. The volume difference between two sinkers was precisely determined by considering the wetting conditions between tungsten sinkers and water; appropriate experimental techniques were developed. The wetting condition became unstable when the sinkers were being moved for immersion in water, because the sinkers were moved in a direction that increased the contact angle. The wetting condition became stable when the sinkers were pulled up from the water, because the sinkers were moved in a direction that decreased the contact angle. The force exerted by the surface tension was efficiently canceled, and the volume difference became constant when the sinkers were pulled up. In this study, the total uncertainty was about 0.3 % at a maximum. The densities measured at high temperatures showed good linearity, with small scatter, over a wide temperature range. The densities and molar volumes increased in the following order: MgF₂, CaF₂, SrF₂, and BaF₂. The thermal-expansion coefficients showed anomalous behavior. The large thermal-expansion coefficient of MgF₂ is attributed to a decrease in the cohesive force as a result of a partial loss of the coulombic force, because of the high charge density.