Naoyuki Hashimoto

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.

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.

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.

Radiation-induced segregation accompanied by grain boundary migration in austenitic stainless steel

Abstract A computer simulation and an electron irradiation in a high voltage electron microscope (1000 kV) were used to study radiation-induced solute segregation and point defect flow in typical austenitic Fe Cr Ni alloys. The calculation was conducted by solving the coupled rate equations for solute and defect concentrations considering the Kirkendall effect at a moving grain boundary sink. The experimental solute redistribution profiles were explained qualitatively. Redistribution of nickel and chromium solutes near the grain boundaries and simultaneously grain boundary migration occurred during irradiation. The amount of nickel enrichment at a grain boundary was especially remarkable, comparing to the amount of chromium depletion. It is suggested that grain boundary migration may contribute to the flow of under-sized nickel solute toward the boundary. The influence of the probe size on EDS analysis of compositional profiles was investigated, with some experimental data.

Three-level picture for chirp-dependent fluorescence yields under femtosecond optical pulse irradiation

We propose a simple model to account for chirp-dependent fluorescence yields from a substance under femtosecond optical pulse irradiation. The model is simple, consisting of a three-level system, and yet it explains the essential feature of the chirp-dependent fluorescence yields experimentally observed, for example, with cyanine dye molecules. Based on the model, the dependence of the fluorescence on the excitation pulse properties such as the chirp rate and pulse intensity has been examined in detail. The results indicate that chirp-dependent fluorescence can be utilized as a convenient means for characterizing phase distortions in optical pulses such as those in optical fiber communication systems.

Lithium metatitanate enhanced solid–solid reaction in a lithium–nitrogen–hydrogen system

We have decreased the end temperature of the Li–N–H system, a hydrogen storage material developed in 2002, to below 260 °C, and obtained a lowest peak temperature of 223 °C.

Catalytic efficiency of Nb and Nb oxides for hydrogen dissociation

In this letter, catalytic efficiency of Nb, NbO, Nb2O3, NbO2, and Nb2O5 for dissociation and recombination of hydrogen were experimentally investigated. On the surface of Nb and Nb oxides in a gas mixture of H2 and D2, H2 and D2 molecules can be dissociated to H and D atoms; then, H2, D2, and HD molecules can be produced according to the law of probability. With increase of frequency of the dissociation and recombination, HD ratio increases. The ratio of H2 and HD gas was analyzed by quadrupole mass spectrometry. As a result, NbO showed the highest catalytic activity towards hydrogen dissociation and recombination.

Hydrogen-induced change in core structures of {110}[111] edge and {110}[111] screw dislocations in iron

Employing the empirical embedded-atom method potentials, the evolution of edge and screw dislocation core structure is calculated at different hydrogen concentrations. With hydrogen, the core energy and Peierls potential are reduced for all dislocations. A broaden-core and a quasi-split core structure are observed for edge and screw dislocation respectively. The screw dislocation and hydrogen interaction in body-centred cubic iron is found to be not mainly due to the change of elastic modulus, but the variation of dislocation core structure.

A solid–solid reaction enhanced by an inhomogeneous catalyst in the (de)hydrogenation of a lithium–hydrogen–nitrogen system

The reaction mechanism of the (de)hydrogenation of a LiNH2 + LiH mixture with a nanoscale catalyst has been investigated. In this research, the position of each solid phase was examined by transmission electron microscopy (TEM). The observation showed that LiH particles, around 100 nm in size, formed around LiNH2 in the hydrogenation process. According to in situ TEM observation of the dehydrogenation process, the LiH particles became smaller as they reacted with LiNH2. Fine particles of Li2NH with crystallites of size 30–40 nm formed on the surface of the LiNH2. It indicated that H+ from the LiNH2 moved to the interface and combined with H− from the LiH in the dehydrogenation process, consequently H2 was released. At the same time, Li+ ions diffused from the LiH to the LiNH2 through the interface and Li2NH formed. On the other hand, it was confirmed that the catalyst was located at the interface between the LiH and LiNH2. It was found that the catalyst had the effect of improving the migration of Li+ from LiH to LiNH2.

Vacancy effects on one-dimensional migration of interstitial clusters in iron under electron irradiation at low temperatures

Abstract We performed in situ observation of one-dimensional (1D) migration of self-interstitial atom (SIA) clusters in iron under electron irradiation at 110–300 K using high-voltage electron microscopy. Most 1D migration was stepwise positional changes of SIA clusters at irregular time intervals at all temperatures. The frequency of 1D migration did not depend on the irradiation temperature. It was directly proportional to the damage rate, suggesting that 1D migration was induced by electron irradiation. In contrast, the 1D migration distance depended on the temperature: distribution of the distance ranged over 100 nm above 250 K, decreased steeply between 250 and 150 K and was less than 20 nm below 150 K. The distance was independent of the damage rate at all temperatures. Next, we examined fluctuation in the interaction energy between an SIA cluster and vacancies of random distribution at concentrations 10−4–10−2, using molecular statics simulations. The fluctuation was found to trap SIA clusters of 4 nm diameter at vacancy concentrations higher than 10−3. We proposed that 1D migration was interrupted by impurity atoms at temperatures higher than 250 K, and by vacancies accumulated at high concentration under electron irradiation at low temperatures where vacancies are not thermally mobile.