Position:     Associate Professor
Degree:       Ph.D., Science
Laboratory: Energy and Environmental Materials
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Thermophysical properties of molten materials

In the Fukushima Daiichi (1F) nuclear power plant accident, molten core materials containing ZrO₂ and UO₂ were formed, which resulted in the relocation of the core materials. To understand the progression of the accident, it is important to obtain accurate thermophysical data on molten core materials. However, this is a highly difficult task because of their ultrahigh melting points (around 3,000°C). We have thus adapted various levitation techniques, such as electrostatic levitation and aerodynamic levitation, to measure the thermophysical properties of ZrO₂ and UO₂. In addition, we developed a new method that allowed us to obtain the thermophysical data in a very short time. Using the adapted methods and the newly developed method, we successfully evaluated the thermophysical properties of ZrO₂ and are now attempting to discover the thermophysical properties of UO₂-containing melts.

Development of Si-based environmentally friendly thermoelectric materials

Thermoelectric (TE) energy conversion from heat to electricity has attracted a great deal of attention in terms of waste heat recovery. Various candidate materials have been explored, but today’s TE materials, such as PbTe- and Bi₂Te₃-based alloys, are still not widely used. The reasons for this include the high cost of the raw elements and the toxicity of the TE materials. These issues can be avoided if silicon (Si)-based high-performance TE materials are developed, since Si is a highly representative environmentally friendly material. While bulk Si is a poor TE material, it has been proven that nanostructures can improve TE performance. This motivated us to develop Si-based high-performance TE materials via nanostructuring. Recently, we identified a novel approach to introducing dense dislocations into Si and demonstrated that these dislocations worked to improve the TE performance of Si.

Publication

1. J. Xie, Y. Ohishi, S. Ichikawa, H. Muta, K. Kurosaki and S. Yamanaka, “Naturally decorated dislocations capable of enhancing multiple phonon scattering in Si-based thermoelectric composites”, J. Appl. Phys. 123, 115114, 2018.
2. T. Kondo, H. Muta, K. Kurosaki, F. Kargl, A. Yamaji, M. Furuya and Y. Ohishi, “Density and viscosity of liquid ZrO₂ measured by aerodynamic levitation technique”, Heliyon 5, e02049, 2019.
3. Y. Ohishi, K. Kurokawa, Y. Sun, and H. Muta, “Thermophysical properties of molten Zr_1-xO_x(x=0.1,0.2) measured by electrostatic levitation”, J. Nucl. Mater. 528, 151873, 2020.
4. T. Kondo, H. Muta, and Y. Ohishi, “Droplet impingement method to measure the surface tension of molten zirconium oxide”, J. Nucl. Sci. Technol. DOI: 10.1080/00223131.2020.1736681