P15
Cu-Fe 合金融体の表面張力測定:
ISS での Thermal Storage プロジェクトに向けて
Surface Tension Measurement of Molten Cu-Fe Alloys:
A Key Step for succeeding the Thermal Storage Project atthe ISS
菅原陸 1, 殿岡和己 1, 清宮優作 1, 小畠秀和 2, 石川毅彦 3,4, 杉岡健一 5, 白鳥英 6, 小澤俊平 1
Riku SUGAHARA1, Kazuki TONO-OKA,1 Yusaku SEIMIYA,1 Hidekazu KOBATAKE2,
Takehiko ISHIKAWA3,4, Ken-ichi SUGIOKA5, Suguru SHIRATORI6, and Shumpei OZAWA1
千葉工業大学，Department of Advanced Materials Science and Engineering, Chiba Institute of Technology
同志社大学，Organization for Research Initiatives & Development, Doshisha University
3JAXA，Institute of Space and Astronautical Science, JAXA
4 総合大学院大学，SOKENDAI (The Graduate University for Advanced Studies)
5 同富山県立大学，Department of Mechanical Systems Engineering, Toyama Prefectural University
6 東京首都大，Department of Mechanical Systems Engineering, Tokyo City University
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1. Introduction
Utilizing industrial waste heat can result in significant reductions in energy consumption and greenhousegas emissions by repurposing it in diverse applications. A promising approach to amplify these benefits is thedevelopment of high-temperature latent heat energy storage, which can enhance energy density andconversion efficiency. Fe-Cu binary alloy has potential as high-temperature latent heat storage materials. 1,2)This alloy exhibits a metastable miscibility gap over a wide composition range in the undercooled liquid state.
If small Cu-rich droplets can be dispersed within the Fe-rich liquid phase during the undercooled immiscible
liquid region and then solidified as they are, it could utilize the latent heat by melting and solidifying only
those Cu-rich particles that are encased in the outer shell of the Fe-rich solid. Accurate and precise
measurements of the thermophysical properties of molten Fe-Cu alloys are currently underway in themicrogravity environment using the electrostatic levitation furnace (ELF) installed on the Japanese science
module ‘KIBO’ of the International Space Station (ISS), named ‘ELF-Thermal Storage’.3)
In the present study, we investigated the variation in sample composition due to the evaporation of molten
Fe-Cu alloys at high temperature was investigated using electromagnetic levitation (EML) on the ground.
Furthermore, we measured the surface tension of molten Fe–Cu alloys as functions of sample composition,
temperature, and oxygen partial pressure (Po2) of measurement atmosphere. The purpose of this study was toobtain preliminary data to ensure the success of the microgravity experiment in the ELF-Thermal Storage
project.

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2. Experimental Procedure
A piece of high purity electrolytic iron was electromagnetically levitated with a high purity oxygen-freecopper simultaneously. These are then uniformly melted under an Ar-He mixed gas with a Po2 of 10-2 Pa,
flowing at 2L/min. The temperature of the droplet was controlled by adjusting the partial pressures of argonand helium gases. After maintaining the levitated droplet at a target temperature for various periods, it wasrapidly solidified by blowing high-purity helium gas at 20 L/min. The chemical composition of the solidified
sample was analyzed by using inductively coupled plasma optical emission spectrometry (ICP-OES).
For measuring the surface tension, the oscillation behavior of the droplet was captured from the top using
a high-speed video (HSV) camera. The Po2 was controlled using a gas phase equilibrium between H2 and CO2,
which were mixed with the Ar-He gas. The frequencies of the surface oscillations and translational oscillations
were analyzed from the HSV images using fast Fourier transformation (FFT). The surface tension of molten
Fe–Cu alloys was calculated from these frequencies using the Rayleigh equation4) and the Cummings andBlackburn calibration.5)

3. Results and discussion
Figure 1 shows the variations in the copper composition for the levitated samples with initial compositionsof Fe–25at%Cu and Fe–33at%Cu, after being maintained in the liquid state at high temperature. The copper
composition of the samples decreases almost linearly with time. This decline becomes more pronounced as
the sample temperature increases. This trend suggests that copper in the molten alloy evaporates more readily
than iron, especially at higher temperature.
Figure 2 exhibits the surface tension of molten Fe-Cu alloys measured at the Po2 of 10-4 Pa, as functions oftemperature and sample composition. Our previous results on the surface tension of liquid iron and copper
are also shown with solid lines. The surface tension of liquid iron measured at the Po2 of 10-4 Pa increases withelevating temperature. This is due to that although oxygen adsorption lowers the surface tension of liquid ironat lower temperatures, oxygen is desorbed from the melt surface due to a decrease in the equilibrium oxygen
adsorption reaction. As a result, the surface tension approaches the pure state value of the surface tension freefrom oxygen adsorption at elevated temperatures. A similar trend of increasing surface tension increases withtemperature is observed for molten Fe-Cu alloys. However, at higher temperatures, the surface tension ofmolten Fe-Cu alloys decreases as the copper composition increases. This would be attributed to the fact thatless effect of oxygen adsorption on the surface tension of liquid copper compared to that of liquid iron.
35
initial composition of Fe–33at%Cu
1900K

33at%Cu

31

2000K

292100K

272300K 2200K
initial composition of Fe–25at%Cu

25

1900K

23212300K

190100

2000K
2100K

2200K
200
Time t, s

300

400Fig.1 Variations in the copper composition for the levitated molten Fe-Cu alloys with initial compositionsof Fe–25at%Cu and Fe–33at%Cu, maintained at high temperatures.
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Surface tension σ, 10-3N∙m-1

2000
1900

Fe, Pure state value

1800
1700

1600
Fe
Fe-10%Cu

1500
1400

Fe-20%Cu

1300

Fe-30%Cu
Fe-40%Cu

1200

Cu, Pure state value
1100
1700 1800 1900 2000 2100 2200 2300 2400 2500
Temperature T, KFig.2 The surface tension of molten Fe-Cu alloys measured at the Po2 of 10-4 Pa.

4. Summary
To obtain preliminary data for the ELF-Thermal Storage project, the variation in sample composition formolten Fe-Cu alloys due to evaporation was examined by EML. Furthemore, the surface tension of moltenFe-Cu alloy was measured. As a result, copper in the molten alloys evaporated more preferentially than iron,which was more pronounced at higher temperatures. The surface tension of the molten Fe-Cu alloys,
measued at the Po2 of 10-4 Pa, increased with rising temperature due to oxygen adsorption/desorption
reaction similar to the behavior observed for liquid iron. An incrase in the copper composition of the alloy
decreased the surface tension at elevated temperatures.
Acknowledgement
Part of this work was financially support by JSPS KAKENHI under Grant No. P20H02453, and a research
grant from the Japan Institute of Copper. The authors are grateful to the ISS crew members and the ground
operation staff for their support.
References
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5)

H. Sugo, E. Kisi and D. Cuskelly: Miscibility Gap Alloys with Inverse Microstructures and High Thermal
Conductivity for High Energy Density Thermal Storage Applications. Appl. Therm. Eng, 51 (2013) 1345, DOI:
https://doi.org/10.1016/j.applthermaleng.2012.11.029.
A. Rawson, E. Kisi, H. Sugo and T. Fiedler: Effective Conductivity of Cu-Fe and Sn-Al Miscibility Gap Alloys. J. Heat
Mass Transf, 77 (2014) 395, DOI: https://doi.org/10.1016/j.ijheatmasstransfer.2014.05.024.
H. Kobatake (2021), Design of Thermal storage material from the aspect of nucleation and their thermophysicalproperties, https://humans-in-space.jaxa.jp/kibouser/subject/science/73004.html.
Load Rayleigh: On the Capillary Phenomena of Jets. The Royal Soc, 29 (1879) 71, DOI:
https://doi.org/10.1098/rspl.1879.0015.
D.L. Cummings and D.A. Blackburn: Oscillations of Magnetically Levitated Aspherical Droplets. J. Fluid. Mech, 224(1991) 395, DOI: https://doi.org/10.1017/S0022112091001817.
© 2023 by the authors. Submitted for possible open access publication under theterms and conditions of the Creative Commons Attribution (CC BY) license(http://creativecommons.org/li censes/by/4.0/).

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