Cavitation induced by ultrasonic vibrations can cause nucleation of ice in supercooled water. In this study, the time required for ultrasonic-induced nucleation of ice was measured for water containing two different size distributions of air bubbles. When the water was supersaturated with air bubbles, there was a time lag of about 0.5 s between the onset of ultrasonic irradiation and the onset of ice nucleation, and the probability of ice nucleation was unusually high within 0.5-1.1 s after the onset of ultrasonic irradiation. These results cannot be explained by conventional models alone, in which the collapse of a cavitation bubble triggers the nucleation of ice. Secondary effects appear to also influence ice nucleation.

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Hydrogen hopping paths in LaNi 5 H solid solution were analyzed via first-principles calculations. Potential energy surfaces were determined for hydrogen on the plane with hydrogen sites 6m, 12o, and 4h and on the plane with hydrogen sites 12n, 3f, and 6i. From the zero-point vibration energy along the hopping path, it was found that hydrogen locations are grouped only at three regions; quasi-m site ͑o-m-o͒, h site, and quasi-f site ͑i-f-i͒. By applying the nudged elastic band method to hydrogen hopping paths between all the possible two sites, the most probable diffusion route was determined as quasi-f-quasi-m-quasi-f in the c direction and quasi-f-quasi-m-quasi-f-quasi-m-quasi-f in the a and b directions with the same saddle point energy of 0.37 eV, which is in good agreement with the measured activation energy, 0.3-0.5 eV, of the hydrogen diffusion in the solid solution phase of LaNi 5 -H system.

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Hydrogen storage in a model b.c.c. metallic nanoparticle was simulated by molecular dynamics method by changing length and energy parameters of metal-H bonds. A global image of hydrogen storage from the gas phase into the metallic nanoparticle was successfully reproduced by a single simulation. In case of weak metal-H bonds, hydrogen atoms rapidly diffuse into the particle and distribute homogeneously. The amount of absorbed hydrogen is maximized at optimized bond length, and decreases for both longer and shorter bonds. In case of strong metal-H bonds, hydrogen atoms localize in a shell-like layer near the particle surface and their inward diffusive motions are suppressed. Such a trapping phenomenon of hydrogen atoms near the surface is caused by low hydrogen diffusivity and lattice deformation due to the hydrogen absorption.