Viscosity of the Molten Mixtures FLiNaK-LnF3 (Ln = La, Ce, Nd)

Abstract

Molten salt eutectic FLiNaK containing lanthanide fluorides (LnF₃) is widely used in various industrial applications and research projects, so knowledge of the physicochemical properties of such complex ionic melts is crucial. The dynamic viscosity of the FLiNaK-LnF₃ (Ln = La, Ce, Nd) molten salt mixtures with content of LnF₃ 3, 7 and 15 mol% was studied by rotational viscometry using rheometer FRS 1600 (Anton Paar). Viscosity measurements were carried out in the temperature range from 1023 K to a temperature close to the liquidus of each composition. The viscosity of the FLiNaK-LnF₃ melts increases significantly both with decreasing temperature and with increasing LnF₃ content. The viscosity values of molten mixtures with the same content of LaF₃, NdF₃ or CeF₃ do not differ insignificantly. However, a tendency for a more substantial decrease in viscosity is observed when introducing the LnF₃ additives in the sequence CeF₃ > NdF₃ > LaF₃. This tendency is violated for the dependence of viscosity on the superheating temperature (20, 100, 200 degrees above the liquidus): the viscosity of molten mixtures with any lanthanide fluoride in an amount greater than 7 mol% does not change. For the FLiNaK-NdF₃ melts, the dependence of the fluidity on the molar volume was determined using the available data on the density of these melts. The fluidity of the FLiNaK-NdF3 decreases with increasing NdF₃ concentration. However, when comparing the fluidity values versus the relative molar volume, it turns out that it stops changing in melts with an NdF₃ content of more than 7 mol%. Such a change in the viscosity and fluidity of FLiNaK-LnF₃ melts is explained by their complex ionic structure.