Current state of research on the viscosity of molten fluorides

The development of new high-precision equipment for the experimental study of the physical-chemical properties of molten salts has led to the emergence of new data. This review examines recent experimental results on the viscosity of molten salt mixtures based on alkali fluorides, which are currently used in modern technologies for the production of metals and alloys and are also promising media for application in molten salt nuclear reactors. Various approaches to the analysis of experimental data are discussed. A different mechanism of viscous flow in the series LiF – NaF – KF and KF – RbF – CsF is well explained from the position of the theory of the autocomplex structure of molten salts. An anomalous decrease in the viscosity of fluorides from lithium to cesium, in comparison with other alkali metal halides, the viscosity of which increases with increasing the cation radius, is confirmed by calculations of the formation energy of the autocomplex and the binding energy between the complex-forming ion and the free ion of the second coordination sphere. To study the mechanism of liquid flow, the temperature dependence of dynamic viscosity is analyzed using the fluidity parameter. Additional information regarding the mechanism of viscous flow can be obtained from the relationship between the viscosity and electrical conductivity of melts. It was assumed that the most promising approach to studying the kinematic properties and the structure of molten salts is the use of both high-precision experiment and mathematical modeling based on first principles.