Kazuhiko Kuribayashi

Ohmic Resistance Measurement of Bubble Froth Layer in Water Electrolysis under Microgravity

Galvanostatic water electrolysis was conducted in 2 wt % KOH and 0.1 N H 2 SO 4 solutions under microgravity. The corresponding terrestrial experiments employed two kinds of electrode configurations: a vertical cathode and downward-facing horizontal cathode-over-anode (C/A) arrangement. The latter configuration was designed to simulate the microgravity (μ-G) condition. The ohmic resistance of the gas bubble dispersion zone near Pt electrodes was measured by the current interrupter method and compared for these three different cases. The transient variation of resistance for C/A configuration behaved similarly to that under μ-G in H 2 SO 4 , but the resistance varied more slowly in KOH. When water electrolysis was conducted with a vertical plane cathode under 1-G, the resistance reached an essentially constant value within a few 100 ms in KOH, whereas the resistance increased linearly for a few seconds, followed by a zig-zag variation in H 2 SO 4 . Water electrolysis under μ-G resulted in stable froth layer formation, and the accompanying ohmic resistance increased in linear proportion to the froth layer thickness. The contributions of electrode surface coverage by bubbles and electrolyte-phase bubble void fraction to the ohmic drop were also assessed. These parameters were compared with corresponding values in terrestrial experiments analyzed by Balzer and Vogt.

Morphological transition in crystallization of Si from undercooled melt

Using CO2 laser equipped electro-magnetic levitator, we carried out the crystallization of Si at undercoolings from 0 K to 200 K. From the point of the interface morphologies, the relationship between growth velocities and undercoolings was classified into two regions, I and II, respectively. In region I where the undercooling is approximately less than 100 K, needle-like thin plate crystals whose interface consists of faceted plane were observed. In region II, the morphology of growing crystals changed to massive dendrites. Although the interface morphologies look quite different between region I and II, the growth velocities are expressed by two dimensional (2D) nucleation-controlled growth model, and at undercoolings larger than 150 K, the growth velocities asymptotically close to the analysis of the mono-parametric linear kinetics growth model. In this stage, the kinetic coefficient of 0.1 m/sK is equivalent to that derived by the diffusion-controlled growth model. This result means that with increase of undercooling, the rate-determining factor changes from 2D nucleation on the faceted interface to random incorporation of atoms on the rough interface.

Metastable phase formation from undercooled melt of oxide material

Undercooling a melt often facilitates a metastable phase to nucleate preferentially. Although the classical nucleation theory shows that the most critical factor for forming a metastable phase is the interface free energy, the crystallographic stability is also indispensable for the phase to be frozen at ambient temperature. In compound materials such as oxides, authors have suggested that the decisive factors for forming a critical nucleus are not only the free energy difference but also the difference of the entropy of fusion between stable and metastable phases. In the present study, using REFeO3 (RE: rare-earth element) as a model material, we investigate the formation of a metastable phase from undercooled melts with respect to the competitive nucleation and crystallographical stabilities of both phases.

Real-time x-ray diffraction of metastable phases during solidification from the undercooled LuFeO3 melt by two-dimensional detector at 1 kHz

In-situ identification of metastable phases formed from the undercooled LuFeO3 melt under controlled oxygen partial pressure Po2 was studied by x-ray diffraction measurements at a synchrotron radiation source. Real-time observation of the formation and growth of individual phases during the single recalescence of Lu3Fe2O7 and LuFe2O4 phases at Po2 of 1u2009×u2009103u2009Pa has been revealed by a high speed imaging system at 1u2009kHz. The obtained diffraction pattern of the metastable phase in the LuFeO3 system was consistent with that of the metastable and stable phases reported in the Lu-Fe-O system.