Field-ion imaging of a tungsten supertip

For the first time, a supertip on a<111>W base tip has been self-imaged by field-ion microscopy with atomic resolution. It is a tiny crystalline protrusion of a few nm in diameter.     

Field-ion microscopy of GaAs and GaP

Clean surfaces of GaAs and GaP were studied by field-ion microscope (FIM). Field-ion images with ordered surfaces were first obtained in pure hydrogen, neon-50% hydrogen and pure neon gases at 78 K, by using channeltron electron multiplier arrays (CEMA). The field-ion images of GaAs were quite similar to those of GaP with respect to the surface structure and the image contrast. They showed the anisotropies of the ion emission and the surface structure between the [111] and $\text{[}\text{1}\text{1}\text{1}\text{]}$ orientations. Ring steps expected from a spherical surface were observed on the (111) and {100} planes, but not on the $\text{[}\text{1}\text{1}\text{1}\text{]}$ and {110} planes. The regional brightness of the FIM patterns was discussed in terms of the Knor and Müller model and the atomic and electronic structures of the surface. The image field of these crystals was much lower than that of metals usually used in FIM. For example, the image field strength for the hydrogen and GaAs system was about 1.1 V/Å. The reduction of the field necessary to image was also discussed in terms of the field penetration effect.     

Structure and lattice dynamics of titanium hydrides due to thermobaric treatment

Abstract

An in situ X-ray diffraction study of the high-pressure ζ-phase evidenced a fct Ti sublattice. In the fco X-phase quenched under pressure, H atoms are displaced to octahedral sites, and the energy of H optic peak (at ～ 75 meV) is half that in other Ti-H phases where H occupy tetrahedral sites. Phase transformations on heating of x-TiH(D)～0.75 were studied by neutron diffraction, small angle and inelastic neutron scattering (INS). Bound multiphonons were observed in the INS spectra of ordered γ-TiH(D).     

Atomistic nitriding processes of titanium thin films due to nitrogen-implantation

Nitrogen ions (N₂⁺) with 62 keV were implanted into the as-deposited Ti film composed of mainly (1 1 0)-oriented TiH_x and (0 3 · 5)-oriented hcp-Ti at room temperature, which results in the epitaxial formation of (1 1 0)-oriented and (0 0 1)-oriented TiN_y, respectively. The electron energy loss spectroscopy experiments elucidate that in the early N-implanting stage the release of hydrogen constituting TiH_x gives rise to the shift of the loss peak due to plasmon excitation to lower loss energy side. On the other hand, the energy loss peaks due to plasmon excitation for nitriding of hcp-Ti gradually shifted to higher energy side with increasing dose. Through the N-invasion into the octahedral sites of hcp-Ti with larger space and lower electron density, the hcp-fcc transformation of Ti sublattices is induced by the shift of the (0 0 · 1)-plane in the 〈0 1 ⋅ 0〉 direction of hcp-Ti promoted by the forming of the strong Ti-N bonds including the π-type covalent bonds, and by the weakening of the Ti-Ti bonds. Furthermore, the inheritance of square atomic arrangement and the movement of the N atom to other neighboring O-site in the transformed fcc-Ti sublattice are responsible for the epitaxial growth of TiN_y. The atomistic processes of the epitaxial growth of TiN_y are discussed with the aid of the molecular orbital calculations.     

Field—ion microscopy observation of single—walled carbon nanotubes

Field-ion microscopy(FIM),a tool for surface analysis with atomic resolution,has been employed to observe the end structure of single-walled carbon nanotubes(SWCNTs).FIM images revealed the existence of open SWCNT ends,Amorphous carbon atoms were also observed to occur around SWCNTs and traditional field evaporation failed to remove them.Heat treatment was found to be efficacious in altering the end structures of SWCNT bundles.Carbon and oxygen atoms released from heated tungsten filament are believed to be responsible for the decoration imposed on the SWCNT ends.     

Atomic displacements due to interstitial hydrogen in Cu and Pd

The density functional theory (DFT) is used to study the atomic interactions in transition metal-based interstitial alloys. The strain field is calculated in the discrete lattice model using Kanzaki method. The total energy and hence atomic forces between interstitial hydrogen and transition metal hosts are calculated using DFT. The norm-conserving pseudopotentials for H, Cu and Pd are generated self-consistently. The dynamical matrices are evaluated considering interaction up to first nearest neighbors whereas impurity-induced forces are calculated with M₃₂H shell (where M = Cu and Pd). The atomic displacements produced by interstitial hydrogen at the octahedral site in Cu and Pd show displacements of 7.36% and 4.3% of the first nearest neighbors respectively. Both Cu and Pd lattices show lattice expansion due to the presence of hydrogen and the obtained average lattice expansion ΔV/V = 0.177 for Cu and 0.145 for Pd.      

Lattice expansion of niobium and tantalum due to dissolved hydrogen and deuterium

The volume change of niobium and tantalum due to interstitially dissolved hydrogen and deuterium has been determined by means of X-ray Bragg scattering. The volume increase in niobium is the same for hydrogen and deuterium $\text{δν=(0.174±0.005)Ω}$(Ω: atomic volume). A small isotope effect has been observed in the case of tantalum, $\text{Δν=(0.155±0.002) Ω}$ for hydrogen and $\text{Δν=(0.143±0.005)Ω}$ for deuterium.     

Temperature dependence of impurity resistivity due to hydrogen in metals

Using a self-consistent calculation, we show that the impurity resistivity of metals containing hydrogen or deuterium decreases as temperature increases. We also predict a significant isotope effect that is largest at O K and vanishes at high temperatures. These effects should be common to all metal-hydrogen systems and could be detected by precision experiments.     

Instabilities due to negative pressure effects

The presence of a negative pressure component in a plasma causes low frequency electrostatic ion waves to become unstable if the number density of that component is sufficiently high or its temperature sufficiently low.     

On the effects of dephasing due to local gradients in diffusion tensor imaging experiments: relevance for diffusion tensor imaging fiber phantoms

The effect of susceptibility differences between fluid and fibers on the properties of DTI fiber phantoms was investigated. Thereto, machine-made, easily producible and inexpensive DTI fiber phantoms were constructed by winding polyamide fibers of 15 microm diameter around a circular acrylic glass spindle. The achieved fractional anisotropy was 0.78+/-0.02. It is shown by phantom measurements and Monte Carlo simulations that the transversal relaxation time T(2) strongly depends on the angle between the fibers and the B(0) field if the susceptibilities of the fibers and fluid are not identical. In the phantoms, the measured T(2) time at 3 T decreased by 60% for fibers running perpendicular to B(0). Monte Carlo simulations confirmed this result and revealed that the exact relaxation time depends strongly on the exact packing of the fibers. In the phantoms, the measured diffusion was independent of fiber orientation. Monte Carlo simulations revealed that the measured diffusion strongly depends on the exact fiber packing and that field strength and -orientation dependencies of measured diffusion may be minimal for hexagonal packing while the diffusion can be underestimated by more than 50% for cubic packing at 3 T. To overcome these effects, the susceptibilities of fibers and fluid were matched using an aqueous sodium chloride solution (83 g NaCl per kilogram of water). This enables an orientation independent and reliable use of DTI phantoms for evaluation purposes.     

Penetration effects due to plane sources

Gamma penetration through the walls of cylindrical-hole collimators in the case of extended sources has been studied. Expressions are derived for the penetration fractions due to plane sources taking into consideration the diameter and effective thickness of the detector. The penetration factors for varying source position are presented for several absorption coefficients.     

Solvent effects in the conversion of anthraquinone to anthrahydroquinone and simultaneously hydrogen sulfide to hydrogen and sulfur

The conversion rate of hydrogen sulfide (H₂S) and t-butylanthraquinone (TBAQ) to a sulfur atom and t-butylanthrahydroquinone (HTBAQH) is determined by the strength of three complexes. The complexes are between the reaction solvent and the TBAQ diradical, the HTBAQ radical or the HS radical. The strength of these complexes was determined by calculating the highest value of the electrostatic potential surface overlap between the solvent and a given radical. These computations used either the TINDO2 or the Zindo/1 semi empirical method. The solvent/TBAQ diradical complex was confirmed by experimental proton NMR. The width of the NMR aromatic region is directly related to the strength of the solvent/TBAQ diradical complex and to the experimental overall TBAQ conversion level.     

Neutron diffraction study of hydrogen in α titanium

It has been determined, using neutron diffraction, that hydrogen dissolved in the α (h.c.p.) phase of titanium is distributed randomly among the tetrahedral sites.     

Reversed surface segregation in palladium-silver alloys due to hydrogen adsorption

It is well known that silver segregates to the surface of pure and ideal Pd–Ag alloy surfaces. By first-principles band-structure calculations it is shown in this paper how this may be changed when hydrogen is adsorbed on a Pd–Ag(1 1 1) surface. Due to hydrogen binding more strongly to palladium than to silver, there is a clear energy gain from a reversal of the surface segregation. Hydrogen-induced segregation may provide a fundamental explanation for the hydrogen or reducing treatments that are required to activate hydrogen-selective membrane or catalyst performance.     

Sites and diffusion for muons and hydrogen in titanium hydrides

Low temperature sites for muons implanted in TiHx have been found to be a mixture of interstitial and substitutional sites, with substitutional occupancy determined by the probability that a muon in an interstitial site will have a vacant nearest neighbor substitutional site. As with ZrHx, activation from the interstitial site is observed below 300 K. From the depolarization rate in the substitutional site, the muon likely displaces the neighboring H atoms by about 0.1 A. Diffusion for the substitutional muons occurs above room temperature with an activation of about 0.38 eV, which is less than the 0.505 eV for hydrogen vacancy motion observed by NMR. To explain this the muon transition rate to a vacancy must be less than that of hydrogen.     

Suppressed molecular ionization due to interferences effects

The phenomenon of suppressed molecular ionization denotes a number of experimental observations in recent years, in which, in general, the ion yields of a molecule are found to be suppressed as compared to those of its companion atom. The latter are often estimated by the popular tunneling formula given by Ammosov, Delone and Krainov (or ADK). Results of S-matrix calculations, obtained for a series of molecules, ranging from diatomics to fullerenes, show that interference effects arising due to the multi-center nature of the molecule contribute to the observed suppression.