Somei Ohnuki

Stress-induced amorphization at moving crack tips in NiTi

In situ fracture studies have been carried out on thin films of the NiTi intermetallic compound under plane stress, tensile loading conditions in the high-voltage electron microscope. Local stress-induced amorphization of regions directly in front of moving crack tips has been observed. The upper cutoff temperature, TC–Amax, for the stress-induced crystalline-to-amorphous transformation was found to be 600 K, identical to that for heavy ion-induced amorphization of NiTi and for ion-beam mixing-induced amorphization of Ni and Ti multilayer specimens. 600 K is also both the lower cutoff temperature, TA–Cmin, for radiation-induced crystallization of initially-unrelaxed amorphous NiTi and the lowest isothermal annealing temperature, TXmin, at which stress-induced amorphous NiTi crystallizes. Since TXmin should be TK, the ideal glass transition temperature, the discovery that TC–Amax=TA–Cmin=TXmin=TK implies that disorder-driven crystalline-to-amorphous transformations result in the formation of the ideal glas...

Study on reaction mechanism of dehydrogenation of magnesium hydride by in situ transmission electron microscopy

In situ observation on dehydrogenation of MgH2 was performed by using transmission electron microscope (TEM). The dehydrogenation of MgH2 with 1 mol % Nb2O5 and formation of nanosized Mg particles were observed at 150 °C. Nb2O5 was not confirmed in diffraction patterns and TEM images probably due to wide dispersion. On MgH2 with 10 mol % Nb2O5, the high resolution TEM could recognize the dehydrogenation at the interface between MgH2 and Nb2O5, proceeding with increasing temperature. This suggests that hydrogen atoms could diffuse from MgH2 phase to the interface between Mg and Nb2O5, resulting in formation of hydrogen molecules at the interface.

Effect of mechanical alloying parameters on irradiation damage in oxide dispersion strengthened ferritic steels

Abstract Issues for developing oxide dispersion strengthened (ODS) steel are anisotropic mechanical properties due to the bamboo-like structure, impurity pick up during the mechanical alloying (MA) process, stability of oxide particles, heat-treatment condition and chemical composition. Several ODS steels were fabricated with a changing gas environment during MA, heat-treatment condition and chemical composition, and were electron-irradiated to 12 dpa at 673–748 K in a high-voltage electron microscope. An ODS martensitic steel (M–Ar) with high dislocation density showed very good swelling resistance. Swelling levels of ODS ferritic steels depended on the gas environment during MA and the recrystallization condition. These indicated that a helium gas environment during MA was more effective to suppress swelling than an argon gas environment and that cold working after recrystallization reduced void formation and swelling. The effect of MA parameters, such as the gas environment, heat-treat condition and cold working on the swelling behavior was evaluated.

Amorphization and solid-phase epitaxial growth in tin-ion-implanted gallium arsenide

The amorphization and recrystallization of tin‐ion‐implanted gallium arsenide were studied by cross‐sectional transmission electron microscopy. Amorphization occurred in the sample implanted at a dose of 1014 ions/cm2. The interface between the amorphous region and the crystalline matrix is not flat. The amorphous region recrystallizes epitaxially with microtwin formation at 673 K. The amorphous‐crystalline interface in the sample implanted at a dose of 1016 ions/cm2 is flat. In the deep region of this sample a solid‐phase epitaxial growth without microtwin formation is observed after annealing at 673 K. These structural changes were compared with the nuclear energy loss (damage energy) distribution simulated by the trim code. It is concluded that the amorphization of the sample implanted at a dose of 1014 ions/cm2 is induced by the accumulation of damage energy; on the other hand, the amorphization of the sample implanted at a dose of 1016 ions/cm2 cannot be explained only by this process. The contributi...

Low temperature hydrogenation of iron nanoparticles on graphene.

Hydrogenation of iron nanoparticles was performed both computationally and experimentally where previously chemically-bonded iron hydride is considered to be unachievable under ordinary conditions. Density functional theory (DFT) calculations predict that hydrogenated iron nanoparticles are stabilized on a single-layer graphene/Cu substrate. Experimentally, iron nanoparticles were deposited onto a graphene/Cu substrate by vacuum deposition. Hydrogenation was done at 1atm of hydrogen gas and under liquid nitrogen. Mass spectrometry peak confirmed the hydrogen release from hydrogenated iron nanoparticles while a scanning transmission electron microscopy is used in order to link a geometrical shape of iron hydride nanoparticles between experimental and theoretical treatments. The hydrogenated iron nanoparticles were successfully synthesized where hydrogenated iron nanoparticles are stable under ordinary conditions.

Chemisorption of hydrogen on Fe clusters through hybrid bonding mechanisms

The interaction of H and Fe clusters of up to nine atoms were investigated within a density functional theory. Calculations indicate that one gas-phase Fe atom can absorb ten H atoms, an amount 2.5 times more than methane (CH4). The magnetic state of Fe atoms non-uniformly decrease by increasing the number of H. The bonding of Fe-H in FeH clusters consists of charge transfer and electron pairing. Thus, two types of bondings are involved. The bond mechanism is general in nature within transition metal clusters, bringing insight for the development of heterogeneous catalyst and hydrogen storage materials.

Plastic bag method for active sample loading into transmission electron microscope

A plastic bag method was developed to observe air-sensitive samples on microstructure and phase distribution without exposure to air during the holder transfer process into the transmission electron microscope (TEM). As an example, a type of lithium aluminum hydride (Li(3)AlH(6)) was observed in the TEM to demonstrate the effectiveness of this method. Results show that the plastic bag method is a simple and practical TEM transfer method utilized to reduce air contact for a series of air-sensitive materials.

Accelerated helium and hydrogen production in 54Fe doped alloys – measurements and calculations for the FIST experiment

Abstract F-82H alloys isotopically enriched in 54 Fe up to 86% were irradiated in the high flux isotope reactor (HFIR) to determine the accelerated production of helium and hydrogen due to isotopic effects. Results are compared to calculations using isotopic helium production cross-sections from ENDF/B-VI or GNASH and measured neutron spectra. Helium measurements demonstrated an accelerated helium (appm)/dpa ratio of 2.3 after a 1.25-year irradiation, an increase of a factor of 4.3 over natural iron. The accelerated helium production is due to higher helium production cross-sections for 54 Fe and 55 Fe. Alloys doped with 55 Fe could achieve helium/dpa ratios up to about 20, well above the fusion reactor ratio of 10. Hydrogen measurements were performed using a newly developed quadrupole mass spectrometer system at PNNL. Calculations predict that hydrogen production will be accelerated by about a factor of 13 over natural iron. However, measurements show that most of this hydrogen is not retained in the samples.

A Homogeneous Metal Oxide Catalyst Enhanced Solid–Solid Reaction in the Hydrogen Desorption of a Lithium–Hydrogen–Nitrogen System

In this study, LiTi2O4 was synthesized as a possible catalyst in the Li‐N‐H system. The properties of hydrogen desorption in the Li‐N‐H system with a homogeneous catalyst have been investigated. The X‐ray diffraction and X‐ray photoelectron spectroscopy results indicated that the single phase of LiTi2O4 was successfully synthesized and it was stable in the sample after high energy ball‐milling and heat treatment. LiTi2O4 exhibited a catalytic effect in the Li‐N‐H system according to the thermogravimetry differential thermal analysis results. During dehydrogenation, a storage capacity of 5.7 wt % was obtained under moderate temperature. A sharp peak of thermal gas desorption mass spectrometry curve occurred at 227 °C. Furthermore, the catalytic mechanism of LiTi2O4 in the Li‐N‐H system was discussed in accordance with the experimental results.

H2 dissociation over NbO: the first step toward hydrogenation of Mg.

Niobium-based oxide nanoparticles have proven to be catalytically effective toward hydrogenation of Mg where H2 dissociation over the niobium-oxides is considered to be a crucial reaction step. However, the role of niobium oxides toward H2 dissociation still remains unclear as to what atomic configurations are responsible for the catalytic activity. H2 dissociation over different surface planes of Nb, NbO, and Nb2O5 as well as small NbO clusters is performed by using a density functional theory. The calculations reveal that H2 dissociation, adsorption energy, and the bond type between H and surfaces (clusters) depend on the atomic configurations of Nb and O. In particular, H2 adsorption on NbO(111) is enhanced by O atoms without forming O-H bond where the bond type of H and surface is found to be an electron pairing. Thus, NbO(111) could not only be a effective catalyst but also potentially prevent the formation of MgO during the hydrogenation of Mg. The results should be helpful in developing and tailoring the efficient catalyst toward H2 dissociation and hydrogenation of Mg.