Surface Tension Measurement of SiO2-Na2O-NaF System by Maximum Bubble Pressure Method

The surface tension of the mold flux is important because it governs the interfacial phenomena between the mold and molten/solid steel. The maximum bubble pressure (MBP) method is often used to determine the surface tension of molten slags. However, for liquid samples with high viscosity, such as molten silicate, the MBP is overestimated. In this study, a simple relaxation function was applied to determine the MBP using the gas flow-rate as a parameter. Consequently, a static MBP was obtained, and the surface tension was reliably measured. The surface tension of the SiO2-Na2O-NaF melts were measured over a wide composition range using the developed method. When SiO2-40 mol% Na2O was added to NaF, the surface tension of the melts gradually increased with increase in SiO2-Na2O concentration. When the NaF in SiO2-40 mol% NaF was replaced by Na2O, the surface tension of the melts did not change significantly at the beginning of the addition. It was considered that F- ion was exposed in the surface of melts instead of O-2(-) ion. This result is consistent with the discussion of the relative strength of the bonding force between the constituent particles. The increase in the surface tension upon further replacement of NaF with Na2O was gradual, indicating that F- was relatively exposed more than O-2(-) on the surface of the investigated melts.