Development of wide-range viscometer and the viscosity measurement for SiO₂–Na₂O–NaF system

A rotating crucible viscometer previously developed by the authors was improved in order to measure wide-range of viscosity values with high reliability. Two sets of inner cylinder and crucible with different geometries were employed as an attempt to accomplish this objective. The centers of these components were matched with accuracy of ± 0.1 mm by adopting X-Y stage for positioning. Relative error in torque measurement was less than 1.5%, and calibration curves had a good consistency for all experiments. By using the improved viscometer, a typical molten salt, LiF–10 mol% NaF–45 mol% KF (FLiNaK), a high-temperature low viscosity liquid, was measured and the results agreed with the theoretical values estimated from pure substances relations. Viscosity of the SiO₂–Na₂O–NaF system (C_Na2O /C_SiO2 = 0.67 in mol%) was obtained by varying concentration range from 0 to 80 mol% NaF. All melts’ composition tested followed Arrhenius-type temperature dependence, and a viscosity relation at 1 473 K based on composition was also obtained. Drastic decreases in activation energy for viscous flow were observed at the concentrations of 5 and 80 mol% NaF.