Development of a new viscometer based on rotating crucible method and viscosity measurement of SiO ₂-CaO-CaF ₂ system

The development of a new viscometer based on rotating crucible method and the viscosity measurement of SiO ₂-CaO-CaF ₂ system were carried out. The viscometer was hermetically closed, and the atmosphere around the melts was highly controllable. In the viscosity measurement, an inner cylinder made of graphite was immersed into the melts in a graphite crucible under Ar at elevated temperatures (1 467-1 782 K). The torque given to the inner cylinder by rotating the crucible was measured by means of a supersensitive torque sensor, and the viscosity was determined. The minimum limit of the viscosity determined was 20 mPa·s. A special attention was paid to keep the temperature uniformity inside the furnace for precise measurement by placing tungsten plates as the thermal shield, and the excellent temperature uniformity (±0.2 K in a whole crucible) was obtained. The basicity (= C _CaO/C _SiO2 in mass) of the melts was 0.79 or 1.25, and the concentrations of CaF ₂ were 5, 10, 15, 20, 25 mass%. As the results, the viscosity of the melts showed a good Arrhenian type linearity in any samples. The loss of fluoride from the melts after the measurement was very small which suggests the consistency of melt composition. The viscosity of the melt decreased with increasing the concentration of CaF ₂, but the degree of decrease was smaller than literature values. The decrease in the viscosity of the melts with the addition of CaF ₂ in acid region is slightly greater than that in basic region.