The thermophysical property measurements have been performed on the International Space Station (ISS) by the authors’ groups[1, 2]. In these experiments, accurate temperature measurement is a key issue.
In the present study, a novel method to determine the emissivity of the molten oxide was developed. The obtained emissivity value was utilized to investigate the temperature dependence of the density of the molten oxide.
The normal spectral radiance of the sample (molten oxide), at the temperature $T_\mathrm{ref}$ and at the wavelength $\lambda_0$, can be expressed as $\epsilon_\mathrm{S}(\lambda_0, T_\mathrm{ref}) = \epsilon_\mathrm{ref} (\lambda_0, T_\mathrm{ref}) \cdot R^\mathrm{exp}_\mathrm{S} (\lambda_0, T_\mathrm{ref}) / R^\mathrm{exp}_\mathrm{ref} (\lambda_0, T_\mathrm{ref})$ , where $\epsilon_\mathrm{S}$, $\epsilon_\mathrm{ref}$, $R^\mathrm{exp}_\mathrm{S}$ and $R^\mathrm{exp}_\mathrm{ref}$ are the emissivity of the sample (molten oxide), the emissivity of the reference material (pure molten iron), the experimentally measured normal spectral radiance of the sample and the experimentally measured normal spectral radiance of the reference material, respectively. The pure iron and oxide were levitated in an electromagnetic levitation system (EML) and melted, and the molten iron droplet which is partly covered by molten oxide was obtained.
The surface of the molten iron and molten oxide was scanned by a pyrometer. By setting the emissivity as 1, the normal spectral radiance of the iron and oxide was obtained. For the emissivity of the Al₂O₃, which emissivity is known, a reasonable value, 0.80 ± 0.03 was obtained using the method developed in the present study. The emissivity of a molten welding flux type oxide (SiO₂:CaO:Mn₃O₄:TiO₂=27:7:13:53 mass%) was measured in the same way and found that the emissivity is 0.76 ± 0.01.
The density of the molten welding flux type oxide was measured on the ISS in a wider temperature range (1200–2000 K), including the undercooled region, and the above-mentioned emissivity value was used to correct the temperature. The density was also measured by a ground experiment using an aerodynamic levitation (ADL) method, and the results were compared.

References
[1] M. Watanabe, K. Onodera, K. Tanaka, S. Taguchi, R. Serizawa, S. Hakamada, A. Nakamura, A. Mizuno, S. Ueno, T. Tsukada, H. Gotoh, T. Tanaka, H. Tamaru and T. Ishikawa, Int. J. Microgravity Sci. Appl., 33 (2016) 330212, DOI: 10.15011/jasma.33.330212.
[2] M. Watanabe, K. Onodera, S. Ueno, T. Tsukada, T. Tanaka, H. Tamaru and T. Ishikawa, Proceedings of
the 10^th International Conference on Molten Slags, Fluxes and Salts 2016, R.G. Reddy, P. Chaubal, P.C. Pistorius, U. Pal, Eds.; Springer, Cham., 2016, 1245, DOI: 10.1007/978-3-319-48769-4_134.