OS2-2微小重力環境での溶融鉄と酸化物融体によるコア・シェル
液滴体積変化Volume change of core-shell droplets of liquid iron andmolten oxide under microgravity conditions
渡邉匡人1,† ，佐藤令奈1 ，松本彩里1 ，松下泰士 2 ，石川毅彦 3 ，小山千尋 3 ，伊藤剛3
Masahito WATANABE1 , Reina SATO1 , Irori MATSUMOTO1 , Taishi MATSUSHITA2 , Takehiko ISHIKAWA3 ,Chiro KOYAMA3 , Tsuyoshi ITO3
1

学習院大学理学部, Department of Physics, Gakushuin University, Tokyo, Japan
Department of Materials and Manufacturing, Jönköping University, Jönköping, Sweden
3
宇宙航空研究開発機構, Japan Aerospace Exploration Agency (JAXA), Tsukuba, Japan∗ Correspondence: masahito.watanabe@gakushuin.ac.jp
2

1. Introduction
Under microgravity conditions, two immiscible liquids form a core-shell droplet dominated by surface
and interfacial free energies. Liquid iron and molten oxides are immiscible, and their immiscibility is utilized in
the steel industries, including smelting, continuous casting, and welding. Therefore, controlling the interfacial
tension between liquid iron and molten oxide is crucial in the steel process 1). We succeeded in forming thecore-shell droplet with liquid iron and molten oxide using the electrostatic levitation furnace (ELF) 2) installedin the KIBO module of the International Space Station (ISS). We performed the core-shell droplet oscillating
drop experiments under constant temperature conditions and found a volume increase during the experiments.
The volume increase is attributed to the dissolution of Fe from core-liquid iron to shell-molten oxide. In thisreport, we focus on the volume change of core-shell droplets by liquid iron and molten oxide under constanttemperature and discuss the volume increase from the viewpoint of oxidation of core-liquid iron.
2. Core-shell droplet forming using ELF in ISS
The samples for processing by ELF in ISS were prepared on the ground using the aerodynamic levitation
technique with two steps. We made each spherical sample of iron and oxides in the first step. In the second step,a CO2 laser melted an oxide sample, and then a spherical iron sample was dropped into the melted oxide. Theinterfacial tension incorporated a spherical iron sample into melted oxide samples, and after rapid cooling, we
obtained a solid compound drop. Using the ELF 2,3) installed in the ISS, the solid compound drop was heating
up and the melted forming a core-shell droplet using a semiconductor laser with a wavelength of 980 nm.
The oxide sample composition was the flux material model composition (SiO2 :CaO:Mn3 O4 :TiO2 :Fe2 O3 =25:7:20:18:30 mass% , named Flux-9) oxide in the welding process.
After the core-shell droplet formation, it was kept at a constant temperature at the center of the ELF
electrode to perform the oscillating drop experiments. During the experiments, the total volume of the core-shell
droplet was simultaneously obtained from its shape using backlight optics. The sample temperature wasdetermined as the apparent temperature using a single-color pyrometer with a wavelength of 1.45 − 1.8 µm byemissivity set to 1.0. The apparent temperature was corrected using sample emissivity and system constant asreported in 4). The emissivity of Flux-9 oxide is 0.76, which was obtained by our original method using liquid
iron melting temperature and its emissivity 5).
3. Volume change core-shell droplet by liquid Fe and molten oxides
The volume of core-shell droplets by liquid iron and molten oxide under constant temperature increased
linearly with time shown in Fig. 1. Before and after onboard experiments, we observed solidified samples ofProceedings of the JASMAC

1 of 4the core-shell drop using X-ray radiography. From the X-ray radiograph images shown in Fig. 2, it was foundthat the radius of the core iron changed from 0.76 mm before the experiment (Fig. 2(a)) to 0.67 mm after theexperiment (Fig. 2(b)). We found that the core iron size was reduced, but the sample mass of the core-shell
droplet remained unchanged. These results showed that the volume increase of core-shall droplets by liquid
iron and molten oxide was attributed to the dissolution of Fe from core iron to shell oxides. The shell oxides
incorporated Fe volume would increase with Fe amount. From the assumptions, we can estimate the volume
increase of shell oxide using the density of molten oxide incorporated Fe. Therefore, we investigated the densitychange of Flux-9 oxides with the change in the Fe amount using the aerodynamic levitation on the ground 6).
Since the volume after solidification changes from its molten state, the volume at the molten state mustbe determined from the density and mass of the molten sample. The mass of reduced Fe δmFe is known fromthe x-ray radiography results. Moreover, since the mass of the entire samplemox , mfe is unchanged, it can beassumed that all the reduced Fe is dissolved into the shell oxide. Therefore, the volume of the shell oxide in a
molten state Vshell can be determined from the following equation if the density of the molten oxide in which Fe
is dissolved ρox+Fe is known.
Vshell =(mox + δmFe )ρox+Fe(1)The density of Flux-9 oxide alone obtained by the aerodynamic levitation method on the ground is 3.24 ×
103 kg/m3 , which is consistent with the value obtained by Thermo-calc simulation and ELF measurements
under microgravity. Therefore, the density measurement by the aerodynamic levitation method is accurate,and it is possible to estimate the change in shell oxide volume from the result of density measurement by theaerodynamic levitation method. The density of the oxide melt was 3.85 × 103 kg/m3 based on the volume of thecore-shell droplet after 20 min of melting, using the reduced mass of Fe δmFe to calculate the shell oxide volume.

Figure 1. Volume change of core-shell droplet with time at 1605◦ C. The large deviation was sample position change byapplying an electrical field to excite the drop oscillations.

Figure 2. X-ray radiograph images of solidified core-shell drop samples: (a) before and (b) after being processed by ELF inthe ISS.

Proceedings of the JASMAC

2 of 4

4. Dissolution of Fe into shell oxide from core iron
From the above, Fe dissolves from core iron to shell oxide in the core-shell droplet, and the density of shelloxide increases from 3.24 × 103 kg/m3 to 3.85 × 103 kg/m3 . ThermoCalc simulations show that the density ofFlux-9 oxide at equilibrium with liquid iron is 3.4 × 103 kg/m3 at 1605 ◦ C. The dissolution of Fe into shell oxide
is larger than the thermal equilibrium. The dissolution of Fe for this density increase is thought to be due tooxidation of liquid iron. Since the sample’s mass did not change before and after melting, it is unlikely that theoxygen used for oxidation was supplied from the outside. The oxygen source for the oxidation of liquid ironat the interface is attributed to the oxygen included in core iron 7) and/or to the oxygen from the reaction inthe molten oxide in which Fe2 O3 decomposed to Fe3 O4 and O2 (6Fe2 O3 →4Fe3 O4 +O2 )8). Decomposed oxygenfrom Fe2 O3 near the interface is considered to be adsorbed on liquid iron and diffused as FeO into the shelloxide, where it decomposes into Fe and O. The Fe decomposed from FeO diffused into the shell oxide expands
in volume because the Fe in the oxide breaks the oxide network structure. Moreover, we should consider theeffect of an electrical double layer 9) at the interface between liquid iron and molten oxide because the core-shell
droplet stays in the electrical field to maintain its position in the electrode of ELF. Because of the electricalcharge at the interface would affect the change of interfacial properties. This phenomenon of dissolution anddiffusion of Fe from liquid iron into the oxide and the increase in the volume of the molten oxide is difficult toobserve on the ground, and we believe that it was first extracted in the levitation experiment in a microgravity
environment.
The change in the core-shell radius ratio and the density of the shell oxide affect the initial purpose of
the interfacial tension measurement. However, since the density of the oxide melt change was obtained fromthis measurement, these two can also be inferred if the change in the core-shell droplet volume is known.
Therefore, it is possible to measure the interfacial tension even when Fe is dissolved. Furthermore, the value ofthe interfacial tension decreases if there is oxygen adsorption on liquid iron, necessitating the assumption thatthe interfacial tension is lower than in the equilibrium state during the measurement. In the future, we will
elucidate the source of oxygen causing the oxidation of liquid iron and explore the behavior of liquid iron andmolten oxide at the core-shell droplets’ interface.
5. conclusion
Core-shell droplets of liquid iron and molten oxide were successfully formed using the ELF on board theISS. The volume of the core-shell droplet sample was observed while it was held at a constant temperature at1605 ◦ C for oscillating drop experiments, and it was found that the volume increased with time. This is thought
to be due to the diffusion of Fe atoms into the shell oxide by oxidation of liquid iron; the dissolution of Featoms disrupts the network structure of the molten oxide, causing the volume to increase. This phenomenon
was picked up by observation in microgravity conditions. In the future, the oxidation phenomenon of liquidiron will be clarified, and the interface phenomenon between liquid iron and molten oxide will be clarified inconjunction with oscillating drop experiments of core-shell droplets.
References1) K. Ogino: Interfacial Tension between Molten Iron Alloys and Molten Slags (in Japanease), Tetsu-to-Hagane, 61 (1975)
2118, DOI: https://doi.org/10.2355/tetsutohagane1955.61.8_2118.
2) H. Tamaru, T. Ishikawa, J. Okada, Y. Nakamura, Y. Ohkuma, S. Yukizono, Y. Sakai and T. Takada: Overview of theElectrostatic Levitation Furnace (ELF) for the International Space Station (ISS), Int. J. Microgravity Sci. Appl., 32 (2015)
320104, DOI: https://doi.org/10.15011/jasma.32.1.320104.
3) T. Ishikawa, C. Koyama, H. Oda, H. Saruwatari and P. .-F. Paradis: Status of the Electrostatic Levitation Furnacein the ISS -Surface Tension and Viscosity Measurements, Int. J. Microgravity Sci. Appl., 39 (2022) 390101, DOI:
https://doi.org/10.15011/jasma.39.390101.
4) S. Taguchi, H. Hasome, S. Shimizu, R. Ishiwata, R. Inoue, M. Yamada, M. Watanabe, T. Matsushita, T. Ishikawa, H. Oda,C. Koyama and T. Ito: Proposal of Temperature Correction of Molten Oxide Based on Its Emissivity for Measurementof Temperature Dependence of Its Density Using ELF in ISS, Int. J. Microgravity Sci. Appl., 40 (2023) 400101, DOI:
https://doi.org/10.15011/jasma.40.400101.
5) R. Sato, R. Ishiwata, S. Taguchi, and M. Watanabe: Measurement of the Normal Spectral Emissivity of Molten Oxide
Using an Electromagnetically Levitated Complex Droplet of Molten Oxide and Liquid Fe, High Temp. High Press., 52(2023) 249, DOI: https://doi.org/10.32908/hthp.v52.1433.
6) A. Nakamura, S. Hakamada, A. Mizuno and M. Watanabe: Measurement of the Normal Spectral Emissivity of Molten
Oxide Using an Electromagnetically Levitated Complex Droplet of Molten Oxide and Liquid Fe, Int. J. Microgravity
Sci. Appl., 34 (2017) 340404, DOI: https://doi.org/10.15011/ijmsa.34.4_340404.
7) K. Ogino, S. Hara, T. Miwa and S. Kimoto: The Effect of Oxygen Content in Molten Steel on the Interfacial Tension
between Molten Steel and Slag(in Japanease), Tetsu-to-Hagane, 65 (1979) 2012
, DOI: https://doi.org/10.2355/tetsutohagane1955.65.14_2012.

Proceedings of the JASMAC

3 of 48) J. Wiencke, H. Lavelaine, P. .-J. Panteix, C. Petitjean and C. Rapin: Electrolysis of iron in a molten oxide electrolyte, J.
Appl. Electrochem., 48 (2018) 115, DOI: https://doi.org/10.1007/s10800-017-1143-5.
9) E. Karimi-Sibaki, A. Kharicha, M. Wu, A. Ludwig and J. Bohacek: Modeling electrochemical transport of ionsin the molten CaF2 -FeO slag operating under a DC voltage, Appl. Math. Comput., 357 (2019) 357, DOI:
https://doi.org/10.1016/j.amc.2018.01.008.
© 2024 by the authors.
This article is an open access article distributed under
the terms and conditions of the Creative Commons Attribution (CC BY) license(https://creativecommons.org/licenses/by/ 4.0/).

Proceedings of the JASMAC

4 of 4