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気液平衡を考慮した金属融体の正確な表面張力測定
Surface Tension Measurement of Molten Metals
Considering Gas-Liquid Equilibrium of Oxygen
Adsorption Reaction
清宮優作 1, 堀内豪暉 2, 西村美咲 2, 川島匠生 2, 栗林一彦 1, 小澤俊平 2
Yusaku SEIMIYA1, Gouki HORIUCHI2, Misaki NISHIMURA2, Takumi KAWASHIMA2, Kazuhiko KURIBAYASHI1,and Shumpei OZAWA2
1 千葉工業大学附属研究所 Research Liaison Centre, Chiba Institute of Technology
2 先端材料工学科，Department of advanced materials science and Engineering, Chiba Institute of Technology

1. Introduction
When measuring the surface tension of liquid metallic melts, accurate evaluation of oxygen activity (𝑎O )is crucial due to the strong surfactant effect of oxygen 1-3). This is especially important for samples with highoxygen solubility, where equilibrium between the atmospheric oxygen partial pressure (PO2 ) and the oxygen
concentration in the melt ([O]) may not be reached in a short time 4-5). 𝑎O can only be defined by the PO2
when this equilibrium is reliably established. Moreover , when PO2 is controlled through gas phase reactions,such as those between CO-CO2 and H2-CO2, it is essential to maintain the equilibrium of these gases.
Alternatively, defining 𝑎O by the [O] in the sample requires maintaining a constant amount of dissolved
oxygen throughout the measurement process, including during solidification and subsequent sampling. Thisis because the oxygen solubility in the molten sample decreases at lower temperatures.
Recent studies on surface tension measurements using containerless methods have assumed that thetemperature of the atmospheric gas is the same as that of the sample. However, since the atmospheric gas isheated only by heat transfer and radiation from the small levitated sample, controlling PO2 to target values
using gas phase equilibrium may be inadequate due to insufficient heating of the gas. In addition, if the sample
experiences a deep undercooling prior to solidification, not only is the oxygen solubility further reduced, but
the duration of the liquid phase state until solidification begins is also prolonged. This may result in a lower[O] after solidification compared to that during surface tension measurement.
In this study, we solidified iron sample using an electromagnetic levitation furnace, carefully considering
the equilibrium between PO2 and [O] in the sample, as well as the equilibrium gas reaction between H2 andCO2. The purpose of this investigation was to demonstrate a reliable method for measuring surface tension
using electromagnetic levitation while ensuring accurate accounting for oxygen activity.

2. Experimental product
Electrolytic iron with a mass purity of 99.99% was electromagnetically levitated and melted under theflow conditions (2 L·min-¹) of Ar-He-H2-CO2 mixed gas. The PO2 of the atmospheric gas was controlled
through the gas phase equilibrium between H2 and CO2 under the assumption that the gas temperature is thesame as the sample temperature. The PO2 of the atmospheric gas was monitored using a zirconia oxygensensor, which operated at 1008 K, installed at the inlet and outlet of the chamber.
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After the indicated values of the PO2 of the sensors are stabilized, the oscillation behavior and thetemperature of the levitated droplet were monitored from above using a high-speed video camera (500 FPS)
and a monochromatic pyrometer. Subsequently, He gas was directed onto the sample, immediately solidifying
the droplet in a containerless state. Furthermore, to minimize the liquid phase duration, the droplet was also
dropped and solidified on a sample holder with directed He gas.
The frequencies of the surface oscillations and the motion of the center of gravity of the droplet wereanalyzed from the time-sequential data of the observed images. The surface tension of liquid iron wascalculated from these frequencies by using the Rayleigh equation 7) and the Cummings and Blackburncalibration8). The oxygen concentration in the solidified sample was analyzed using the inert gas fusioninfrared absorption method.
3. Results and Discussion

Oxygen concentration log([O]), at%Figure 1 shows the analysis results of [O] in the solidified iron sample, maintained at 1900 K under various
PO2 conditions prior to solidification. The relationship between [O] and PO2 for sample solidified on thesample holder (◆) shows a good agreement with one of the most reliable reported data by Sakao et al. Thissuggests that PO2 of the atmospheric gas, flowing at 2 Lmin-1 in the vicinity of the electromagneticallylevitated droplet, was well controlled at the target values through the gas phase equilibrium between H2 andCO2: the droplet is heated to the same temperature as the droplet by heat transfer and radiation from thesample. Therefore, 𝑎O for the surface tension measurement can be defined using both PO2 and [O] at least at
temperatures above 1900 K.
On the other hand, [O] in the samples solidified under a containerless state (◇) are lower than these data,
indicating that [O] decreases during the cooling stage before solidification due to the prolonged liquid state.
In such cases, 𝑎O for the surface tension measurement should not be defined using [O]. These results confirm
the essential need to ensure the equilibrium state of [O] and PO2 to accurately define 𝑎O for surface tension
measurement of molten metallic melts.

0@ ~1900 K-1

Sakao et al.

He gas blown with
Sample dropped-2-3-4
He gas blown-5-13 -12 -11 -10-9-8-7
Oxygen partial pressure log(Po2), PaFigure 1. Relationship between Po2 and [O] in molten iron solidified by He gas blown with sample
dropped (◆) and by He gas blown (◇)

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Acknowledgements
This work was financially supported by JSPS KAKENHI under Grant No. P20H02453 and by JSPS Grant-inAid for Research Activity Start-up under Grant No. 24K22936.
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