Masamichi Ishikawa

Recent progress in environmental catalytic technology

Recent progress and trends in environmental catalytic technology in Japan are described with emphasis on the catalysts having hybridized functions. In addition to automobile exhaust cleaning, use of environmental catalysts such as titanium oxide photocatalysts is rapidly growing for the control of residential environments, e.g., antimicrobial activity and odor control, as life styles change gradually and the living environment is deteriorating considerably. Many new catalysts are evolving through hybridization of functions. The market of environmental catalyst products in 2005 is estimated 2,000 billion yen/year (photocatalyst: 1,100 billion yen/year).

Nucleation kinetics of polystyrene colloidal crystallization in highly deionized water

The effects of gravity on the growth processes of colloidal crystals were evaluated under low gravity using aircraft flights. Monodispersed colloidal particles, such as polystyrene and silica particles, form colloidal crystals in highly deionized water. Colloidal crystals offer an important opportunity to study the dynamics of crystallization. The purpose of this study is to observe the gravitational effects on the nucleation and post-nucleation stages of crystal growth in low gravity. The dynamics of colloidal crystallization was evaluated using the in situ optical reflection spectrum method. Although colloidal crystals obeyed the classical nucleation theory under low gravity, on-ground experiments showed an increase of the steady state nucleation rate and a remarkable reduction of the lag time predicted from the classical theory. To confirm the effects of gravitational field, DC electric field was applied based on the consideration that both fields affect the motion of colloidal particles as the directional body forces. The experiments with the electric field under low gravity showed the similar reduction of the lag time under normal gravity. The nucleation mechanism of colloidal crystals under gravity was explained by the enhanced particle adhesion to critical nuclei under the presence of the forced particle and cluster movements.

Particle ordering at the initial stage of colloidal crystallization: implication for non-classical dynamic behavior

Abstract The particle ordering dynamics of polystyrene colloids were studied using low-angle laser light scattering and Brownian dynamics (BD) simulation. The polystyrene particles were prepared in the highly deionized water and crystalline states were realized after shear melting due to vigorous circulation of fluids. The radius of gyration of crystalline clusters was evaluated by the laser light scattering and showed the rapid increase of cluster size similar to percolation dynamics with increasing crystalline fraction. The following dynamics were confirmed by BD simulation. The ordered domains were formed under the repulsive potential of latex particles and extended to all over the sample within a relatively short time. The domains were connected as in percolation transitions at a critical threshold. This means that the critical clusters are not compact as in the final crystal but form a rather loose structure in the nucleation stage of colloidal crystallization. The consistent results between experiments and simulation indicate a new ordering dynamics from metastable liquid, which is quite different from the standard nucleation minimizing the free energy of critical nucleus.

A diagnostic study of plasma CVD under microgravity and its application to diamond deposition

Abstract Diamond thin-film was synthesized under microgravity using Japanese free-flyer (SFU, Space Flyer Unit) launched in March 1995. This program achieved the space-based production of diamond by Chemical Vapor Deposition (CVD). The DC plasma excitation method was applied to hydrogen-methane mixed gas within the pressure range of 2.66∼6.55 × 10 3 Pa. Remarkable differences of the electron temperature of hydrogen plasma were confirmed between the microgravity and the on-ground experiments. The intensity ratio of CH/H α measured by emission spectroscopy was found as a sensitve indicator of gravity effects on gaseous chemistry and diamond quality.

Linear stability analysis of magnetic Rayleigh-Bénard convection

Abstract The instability of magnetic Rayleigh-Benard convection is studied by the linear stability theory. Nonlinear equations of magnetic Rayleigh-Benard convection are introduced and linearised. The linear equations are solved by the finite difference method and the critical magnetic Rayleigh number and critical wave number are obtained. It has been found that stationary rather than oscillatory convection occurs at the critical point and that the critical magnetic Rayleigh number decreases and the critical wave number increases as the aspect ratio increases. The result is compared to that obtained by a microgravity experiment. The agreement is very good.

Measurements of Feldspar Dissolution Rates Under Supercritical CO2-Water-Mineral System Based on Nanoscale Surface Observation

Publisher Summary As one option of the measures against global warming, R&D on the underground sequestration of CO 2 in a sandstone aquifer has just started, operated by RITE in Japan. For the assessment of the stability of injected CO 2 , the influences of long-term geochemical phenomena, such as dissolution of sandstone minerals in CO 2 -saturated groundwater, should be taken into account in addition to various physical effects. As a part of the assessment on the underground sequestration of CO 2 , a feldspar dissolution experiment was performed in a supercritical CO 2 -water-mineral system at 25, 50, 65 and 80°C, leaving the CO 2 pressure constant at 10MPa. An atomic force microscope (AFM) was applied for the nanoscale observations of the dissolved feldspar surface. The average dissolution rates of anorthite during the initial one week were estimated from surface retreat, based on vertical cross section profiles. Many experiments with regard to feldspar dissolution have been carried to date, under various environmental conditions. These were mainly focused on solution chemistry and required very long times to detect compositional changes sufficiently-large to be analyzed, as the feldspar dissolution rate is generally very slow. Therefore, it has been difficult to obtain precise dissolution rates, especially at relatively low temperatures below 100°C. On the other hand, nanoscale observation of the mineral surface is expected to become a powerful tool for the measurement of feldspar dissolution rate.

Diagnostic study of plasma CVD under microgravity

Diamond synthesis under microgravity was successfully executed using Japanese free-flyer (SFU, Space Flyer Unit) launched in 1995. This program achieved the space-based production of diamond by plasm-assisted Chemical Vapor Deposition (CVD). The results showed the better quality of diamond than that of earth-grown diamond and the microgravity effects on gaseous chemistry of CVD processes. To obtain a detailed understanding of plasma processes under microgravity, the post flight experiments of the SFU program were executed. A diagnostic study of hydrogen-methane plasma was applied to the CVD experiments using the 4.5s drop-shift facility in Toki, Japan. We found the direct effects of microgravity on the pattern formation of plasma discharge. The plasma instabilities affected the emission characteristics of plasma. Plasma temperature was estimated based on the emission spectra as a function of position from anode. Remarkable differences of the electron temperature of plasma were confirmed between the microgravity and the normal-gravity conditions.

Systematic evolution of secondary and tertiary side branches of dendrite under microgravity

The evolution of dendrites in solidifying succinonitrile was investigated under normal and microgravity using an airplane. The authors aimed to extend the observations of gravity effects to the later-stage formation of dendrites. Microgravity experiments realized higher degrees of supercooling than normal gravity experiments, and the systematic formation of sencodary and tertiary arms was observed. The morphology of dendrites at the later-stage of growth clearly showed the self-similar shape having a fractal characteristic and the self-similar shape correlates with the growth of high-order dendrite arms under microgravity.