Abstract:

Prof. Toshihiro Tanaka has been conducting research on the surface and interface properties of high-temperature materials and elucidating the mechanism of various surface and interface phenomena that occur in high-temperature materials with the aim of developing various new material processes. This talk handles a wide range of objects, from nanoparticles to high-temperature melts using the space station, and some research work on elucidation of the mechanism of phenomena on various material surfaces. The following three topics will be given in his lecture :

1) Study on estimation of solid-liquid equilibrium phase diagram of nanoparticle alloy:
Thermodynamic databases for calculating the equilibrium phase diagram of alloy systems have been developed for a long time, and are currently used for analyzing various phase equilibria. As for the surface free energy of the alloy, a thermodynamic function in the surface phase is assumed, and the temperature / composition dependence of the surface free energy can be calculated using the thermodynamic data for the bulk. We found that by combining the calculation of the phase equilibrium and the calculation of the surface free energy, it was possible to calculate the solid-liquid phase equilibrium phase diagram of the nanoparticles, in which the influence of the surface appears strongly. It has been clarified that the melting point and liquidus temperature of nanoparticles decrease. In addition, we directly observed the melting of the nanoparticle alloy in a high-resolution electron microscope, and found that the calculated results of the phase equilibrium in the nanoparticles obtained by the above calculation agree with the experimental results.

2) Super -spreading wetting behavior on surface micro crevice stricture:It is generally known that molten metal spontaneously penetrates into a capillary having a small radius. We discovered that when the surface of a metal was oxidized and reduced, oxygen was released from the surface and a microporous structure was formed in the surface layer. It has been found that a metal spontaneously wets and spreads on a metal surface over a wide area by capillary action. This wetting phenomenon is called super-spreading wetting. However, in this method, the liquid metal cannot cause super-spreading wetting only locally because a microporous structure is formed on the entire surface of the target metal. Therefore, using laser processing, we succeeded in fabricating a surface structure that causes local wetting by capillary action. We found that fine folds were randomly formed on the surface of the metal by this processing, and this surface structure was called a surface fine crevice structure. Furthermore, we have found that when a liquid metal is brought into contact with the fine crevice structure on the surface, the liquid metal exhibits super-spreading wetting only in the surface processed portion. In addition, we have found that the metal bonding is possible by forming the surface fine crevice structure on the surface of two plates, making them face each other, and infiltrating the liquid metal into the gap. Utilizing special processing of metal surfaces and surface physical properties, we are developing new metal bonding methods.

3) Trial of in-situ observation of the interface between liquid iron alloy and molten slag and simultaneous measurement of the interfacial tension of those two phases.

Finally, I explain our recent trial related to the surface science in metallurgical processing in a short talk. Since the interface between the molten steel and the molten slag is usually generated in a crucible at laboratory-scale experiments, the interface cannot be directly observed. If the droplets with the molten slag coated on the surface of the molten steel can be floated in gas or vacuum state, the oxide melt is transparent, so the interface can be seen directly. The interfacial tension can also be measured. To conduct such an experiment, we try to suspend the above-mentioned molten steel surface with molten slag and floated double-structure droplets using an electrostatic levitation furnace equipped in the International Space Station. We are currently conducting experiments to simultaneously observe the interface and measure the interfacial tension. I will talk on the outline of this experiment.