The viscosity of SiO2-Na2O-NaF melts was measured by using a rotating crucible viscometer developed by the authors. The basicity (= in mol%) of the melts was 0.67, 0.82, and 1.00. The concentrations of NaF were 5, 10, and 15 mol%. As a result, the viscosity of the melts showed a good Arrhenius type linearity in any melts. It was found that the sample loss occurred in highly basic composition region due to a prolonged measurement at high temperature. The viscosity of the melts decreased with increasing the concentration of NaF, and the composition dependence is similar to that for SiO2-CaO-CaF2 melts. Viscosity decreasing ability of NaF is higher than that of Na2O within the composition region investigated in this study. Transitions of activation energy of viscous flow are different between Na2O addition and NaF addition. This implies that the mechanism of enhancement of fluidity is different between Na2O and NaF.

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