Hydrogen Absorption of Palladium Thin Films Observed by in Situ Transmission Electron Microscopy with an Environmental Cell.

A window type of the environmental cell system for a high-voltage electron microscope was developed and applied to in situ observation of a palladium (Pd) thin film. For in situ hydrogenation of Pd thin films, the distances of the lattice fringes were 0.20 and 0.23 nm, which correspond to the lattice d spacings of ß-phase (200) and (111) planes. Expansion of the Pd lattice happened as a result of phase transformation from the α phase to the ß phase. In particular, the lattice fringes were clearly distinguished, and the dislocation behavior during Pd hydrogenation was easily recognized according to the corresponding inverse fast fourier transform images. Furthermore, significant growth in the number of dislocations was observed at the grain boundary during increasing hydrogen pressure in the cell.

Selective Growth of Noble Gases at Metal/Oxide Interface.

The locations and roles of noble gases at an oxide/metal interface in oxide dispersed metal are theoretically and experimentally investigated. Oxide dispersed metal consisting of FCC Fe and Y2Hf2O7 (Y2Ti2O7) is synthesized by mechanical alloying under a saturated Ar gas environment. Transmission electron microscopy and density functional theory observes the strain field at the interface of FCC Fe {111} and Y2Hf2O7 {111} whose physical origin emerges from surface reconstruction due to charge transfer. Noble gases are experimentally observed at the oxide (Y2Ti2O7) site and calculations reveal that the noble gases segregate the interface and grow toward the oxide site. In general, the interface is defined as the trapping site for noble gases; however, transmission electron microscopy and density functional theory found evidence which shows that noble gases grow toward the oxide, contrary to the generally held idea that the interface is the final trapping site for noble gases. Furthermore, calculations show that the inclusion of He/Ar hardens the oxide, suggesting that material fractures could begin from the noble gas bubble within the oxides. Thus, experimental and theoretical results demonstrate that noble gases grow from the interface toward the oxide and that oxides behave as a trapping site for noble gases.

The effect of point defects on diffusion pathway within α-Fe.

The diffusion mechanism of point defects within α-Fe with a single vacancy is investigated using the density functional theory. Calculation reveals that H has a slight effect towards Fe diffusion to a vacancy. He has a strong binding with a vacancy; therefore, Fe diffusion is unlikely to happen. The diffusion of C and N from a vacancy has a high barrier. However, Fe diffusion to a vacancy decreases if the C and N diffuse from a vacancy. Thus, the effect of interstitial atoms within α-Fe with a single vacancy towards diffusion and a possible diffusion pathway is discussed.

Interaction of electrons with light metal hydrides in the transmission electron microscope.

Transmission electron microscope (TEM) observation of light metal hydrides is complicated by the instability of these materials under electron irradiation. In this study, the electron kinetic energy dependences of the interactions of incident electrons with lithium, sodium and magnesium hydrides, as well as the constituting element effect on the interactions, were theoretically discussed, and electron irradiation damage to these hydrides was examined using in situ TEM. The results indicate that high incident electron kinetic energy helps alleviate the irradiation damage resulting from inelastic or elastic scattering of the incident electrons in the TEM. Therefore, observations and characterizations of these materials would benefit from increased, instead decreased, TEM operating voltage.

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.

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.

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.

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.

Phase transformation of zirconia ceramics by hydrothermal degradation.

Zirconia has found wide application in dentistry because of its high mechanical strength and superior esthetic properties. However, zirconia degradation caused by phase transformation occurring in a hydrothermal environment is of concern. In the present study, phase transformation and microstructure of tetragonal zirconia polycrystal partially stabilized with yttrium oxide (Y-TZP) and alumina-toughened zirconia (ATZ) sintered at different temperatures were estimated. On grazing angle X-ray diffraction analysis, ATZ showed less phase transformation to the monoclinic phase during hydrothermal treatment and this transformation appeared to occur within a few micrometers below the surface. At a higher sintering temperature the monoclinic phase content of ATZ was found to be lesser than that of Y-TZP, indicating that the alumina in ATZ was effective in suppressing hydrothermal degradation. Examination by transmission electron microscopy and studying of electron backscatter diffraction patterns indicated that grain growth in ATZ was slightly suppressed compared with that in Y-TZP at higher sintering temperatures. The present study demonstrated the effect of adding alumina to zirconia for suppressing hydrothermal degradation and studied the effect of this addition on grain growth in zirconia.