Viscosity of Molten Alkaline-Earth Fluorides

Abstract

The viscosities of molten alkaline-earth fluorides \((\hbox {MgF}_{2}, \hbox {CaF}_{2}, \hbox {SrF}_{2},\)\(\hbox {BaF}_{2})\) were measured using the oscillating crucible method, which is especially suitable for measuring molten salts with low viscosity. The results showed a good Arrhenius linearity over a wide temperature range. The measured viscosities and activation energies increased in the following order: \(\hbox {MgF}_{2} < \hbox {CaF}_{2} < \hbox {BaF}_{2} < \hbox {SrF}_{2}\). Judging by the charge density, the viscosity of alkaline-earth fluorides should increase from molten \(\hbox {BaF}_{2}\) to \(\hbox {MgF}_{2}\). However, the results indicate a different tendency, which may be explained by a Coulomb force that is very strong. The low viscosity of \(\hbox {MgF}_{2}\) can be attributed to a decreased cohesive force, due to a partial loss of the Coulomb force caused by a higher charge density of the material. The viscosities were also compared to those of molten alkali fluorides and alkaline-earth chlorides. The viscosities of molten alkaline-earth fluorides were higher than those of molten alkali fluorides and alkaline-earth chlorides. The \(\hbox {CaF}_{2 }\) viscosity determined in this study was compared to literature values and showed a reasonable value in the relatively low-viscosity region.