Seiichi Watanabe

Size-Controlled Ni Nanoparticles Formation by Solution Glow Discharge

We report the size control of Ni nanoparticles generated via solution glow discharge and focus on the effect of electrolyte concentration on Ni nanoparticles. In our experiments, voltage was applied to generate a plasma in NaOH electrolytes with concentrations ranging from 1.0 to 0.001 kmol m -3 . The applied voltage strongly depended on the electrolyte concentration, and interestingly, product size decreased with electrolyte concentration; for example, (mean diameter, applied voltage, electrolyte concentration) = (148 nm, 90 V, 0.5 kmol m -3 ), and (70 nm, 590 V, 0.001 kmol m -3 ). These results suggested the possibility of using plasma electrolysis for synthesizing size-controlled nanoparticles by changing only electrolyte concentration.

Dislocation Loop Formation and Growth under In Situ Laser and/or Electron Irradiation

Vacancies and interstitial atoms are primary lattice (point) defects that cause observable microstructural changes, such as the formation of dislocation loops and voids in crystalline solids. These defects diffusion properties determine the phase stability and environmental resistibility of macroscopic materials under ambient conditions. Although in situ methods have been proposed for measuring the diffusion energy of point defects, direct measurement has been limited. In this study, we propose an alternative in situ method to measure the activation energy for vacancy migration under laser irradiation using a pulsed laser beam from a laser-equipped high-voltage electron microscope (laser-HVEM). We made in situ observations that revealed the formation and growth of vacancy dislocation loops in an austenitic stainless steel during laser irradiation. These loops continued to grow when thermal annealing was performed after laser irradiation at the same temperature. We anticipate that laser-HVEM will provide a new method for investigating lattice defects.

Self-Organized Two-Dimensional Vidro-Nanodot Array on Laser-Irradiated Si Surface

We report a periodic two-dimensional (2D) array of uniquely shaped dotlike nanoprotrusions (NPs), which simultaneously self-organize on a Si surface under pulsed laser irradiation. The shape of the dotlike NPs can be controlled by adjusting the number of laser pulses. The flask-shaped dotlike NP array is named a vidro-nanodot (VND) array. We present a detailed analysis of the internal structure of VND using high-resolution electron microscopy.

Numerical Simulation of Solidified Structure Formation of Al-Si Alloy Casting Using Cellular Automaton Method

For the purpose of the prediction of casting structures, heterogeneous nucleation rate in the undercooled melt of solififying Al-Si alloys were evaluated by comparing experimentally observed macrostructures of solidified ingots with numerically simulated ones. Molten alloys were unidirectionally solidified in an adiabatic mold from a steel chill block located at the bottom of the mold. In the experiment, columnar to equiaxed transition (CET) was observed. A numerical simulation for grain structure formation of the sample ingots was carried out using a cellular automaton (CA) method, and heterogeneous nucleation rate in the solidifying alloys were evaluated by producing the similar structures to experimental ones. An attempt was made to predict the grain structure of conventionally cast ingots using the evaluated heterogeneous nucleation rate. However, the simulation could not predict the structure of ingot with low superheat due to crystal multiplication near the mold wall. The crystal multiplication mechanism, so-called Big Bang mechanism, was introduced into the simulation and the simulation could predict the grain macrostructure composed of columnar and equiaxed crystals that were similar to experimentally observed one.

Direct Observation of Nanostructural Fluctuation during Radiation-Induced Amorphization

An atomistic study of radiation-induced amorphization in the NiTi intermetallic compound was performed by using in-situ high-resolution high-voltage electron microscopy and molecular dynamics in conjunction with image simulations. Both theoretical and experimental results show that metastable nanometer-size inherent atomic clusters form and disappear during irradiation, so that a spatiotemporal fluctuation under amorphization is induced. The random formation and annihilation of such inherent nanoclusters are believed to be responsible for these fluctuations, which appear to be related to transitions between the ideal glass state and metastable, unrelaxed states in an energy-dissipative system under irradiation.