PS21
国際宇宙ステーションでの Thermal Storage ミッションにおいて測定した Fe-Cu 合金融体の熱物性値
Measurement of Thermophysical properties of molten
Fe–Cu alloys during the “Thermal Storage” Mission on
the International Space Station
黒澤修也 1，清宮優作 1，菅原陸 1，殿岡和己 1，伊藤輝 1，小澤俊平 1，杉岡健一 2，白鳥英 3，石川毅彦 4，小畠秀和 5First name LASTNAME1, Second AUTHOR1, Third AUTHOR1,2 and Last AUTHOR2
千葉工業大学，Chiba Institute of Technology,
富山県立大学，Toyama Prefectural University
3 東京都市大学，Tokyo City University
4 宇宙航空研究開発機構宇宙化学研究所，Institute of Astronautical Science, JAXA
5 同志社大学，Doshisha University
121.

Introduction
The Fe–Cu binary alloy, known for exhibiting a metastable miscibility gap over a wide composition range

in the undercooled liquid state, presents a unique microstructural behavior. When small Cu-rich droplets are
dispersed within an Fe-rich liquid matrix in the undercooled immiscible liquid region and subsequently
solidified in this state, the alloy emerges as a promising candidate for high-temperature latent heat thermalenergy storage applications.1)2) In such alloys, the Cu-rich phase, encased by an outer shell of Fe-rich solid, canbe selectively melted and solidified to utilize latent heat effectively.
Understanding and controlling which phase—Cu-rich or Fe-rich—becomes dispersed or continuous, as wellas managing their replacement, shapes, and the number of droplets, require precise measurements of surface
tension, interfacial tension, density and viscosity of the liquid phases. To address this, our study focuses on
accurately and precisely measuring the thermophysical properties of molten Fe-Cu alloys using electrostatic
levitation aboard the International Space Station (ELF-ISS).
2.

Experimental procedure
Small spheres of Fe-25at%Cu and Fe-50at%Cu alloys, each approximately 2 mm in diameter, were preparedby arc melting. A positively charged spherical sample was suspended between three pairs of electrodes using
electrostatic forces under microgravity conditions using ELF-ISS. The sample was heated and melted by theradiation from semiconductor laser beams from four directions. To minimize the effects of oxygen adsorption
on the surface tension of the molten sample, an Ar-H2 pressurized atmosphere at 2×105 Pa was used to lower

the oxygen partial pressure of the atmosphere.
The density of each droplet was calculated by dividing the mass of the sample by its volume as determinedfrom the sample image, assuming the molten sample was axisymmetric.
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Surface and interfacial tensions were calculated from the frequencies of the surface oscillations of the droplet,based on methodologies proposed by Rayleigh and further developed by Egry et al. These ossifications wereinduced by superimposing a sinusoidal excitation voltage onto the levitation electric field.
The viscosity of the molten samples was calculated from the damping time of the surface oscillation.
3.

Results and discussion
We successfully measured the density, surface tension, and viscosity of molten samples under microgravity

conditions. The density of molten Fe-25at%Cu and Fe-50at%Cu alloys linearly decreased with increasing
temperatures. An increase in the copper composition led to higher densities, a trend that aligned well with
The temperature dependence of thesurface tension for molten Fe-25at%Cu
alloy is slightly convex upward curve, as
shown in Fig. 1. A similar temperaturedependent trend was observed for the Fe50at%Cu alloy sample; however, its
surface tension is lower than that ofmolten

Fe-25at%Cu

alloy.

These

measurement results align well withterrestrial

studies

using

the

electromagnetic levitation technique. At
lower temperatures, copper atoms would
tendtosegregate

atthesurface,

Surface tension σ, 10-3N·m-1

data reported in the literature by Kobatake et al., Watanabe et al., and Vyukhim et al.

1900
1800
1700
1600

Fe-25at%Cu●ISS-ELFFeEML

△

1500
1400
1300
1200
1100
1500

Fig.1

Cu
1700

1900
2100
Temperature T, K

2300

2500

The temperature dependence of the surface
tension for molten Fe-25at%Cu alloy

decreasing the surface tension of molten alloy. As the temperature increases, the melt would experienceenhanced mixing. In this scenario, the surface tension may increase with rising temperature, given that thesurface tension of pure liquid iron is higher than that of liquid copper.
The viscosity of molten Fe-25at%Cu and Fe-50at%Cu alloys are decreased with elevating temperatures aswell as those for pure liquid iron and copper. The increase in copper composition lowers the viscosity.
Despite successful measurements of surface tension, density, and viscosity, the interfacial tension between
the separated Fe-rich and Cu-rich liquid phases could not be measured due to challenges in exciting surface
oscillations at the low temperatures necessary for observing the metastable separation of these phases. The
natural frequencies of the two liquid phases, as predicted from ground experiments, may not have been
accurately estimated. We hope to further investigate this issue and consider an attempt to measure theinterfacial tension under microgravity conditions in the future.
Acknowledgements
This research was partially funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI underGrant No. P20H02453. Additionally, part of this work was supported by a research grant from the Japan
Institute of Copper. The authors wish to express their gratitude to the ISS crew members and the ground
operation staff for their invaluable support during the onboard experiments.

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