Electronic density-of-states from pressure-composition isotherms in metalhydrides

Using the semi-empirical model of Griessen and Driessen for the heat of formation of metal hydrogen systems it is shown that quantitative information on the electronic density-of-states n(E_F) and the hydrogen concentration dependence of the Fermi energy may be calculated directly from experimentally determined pressure-composition isotherms. In the limit of low hydrogen concentrations the method makes it possible to determine n₀(E_F) for the pure host metal. Results obtained by means of this new method are compared to band structure calculations for Y, V, Nb, Ta and Pd.     

Competition between hydrogen and oxygen dissociation on palladium surfaces at atmospheric pressures

A large change in the surface and interface potential of Pd-films on SiO₂ is observed at a constant ratio between hydrogen and oxygen pressure in the ambient. This observation, which is most probably due to a competition between hydrogen and oxygen for the same dissociative adsorption sites, is described in this communication. Furthermore the potential measurements indicate the presence of another type of hydrogen adsorption sites on the oxygen covered surface.     

The motions of hydrogen impurities in α-Palladium-hydride

The intensity distribution of inelastically scattered thermal and hot neutrons on hydrogen impurities in α-palladium hydride has been studied as a function of concentration, temperature, momentum transfer and different annealing procedures. In up-scattering experiments the first and second harmonics appear at E₁^H = (66 ± 4) me V and E₂^H = (135 ± 15) meV respectively, and the hydrogen band modes have a frequency distribution as expected from measured dispersion curves for pure palladium. For deuterium the first harmonic appears at E₁^D = (48 ± 4) meV. The width of the hydrogen local mode E₁^H changes from 30 to 20 me V and its position from 63 to 66 meV, when the hydrogen concentration is altered from 2.7 to 0.2 at.%. After an extended annealing procedure and at the lowest concentration of 0.2 at.%, the local mode appears in down-scattering experiments at E₁^H = (68.5 ± 2) meV with a full width at half height ΔE₁^H = 4 meV, which is purely instrumental. For higher concentrations and insufficient annealing of the sample, cluster of hydrogen atoms are formed even in the α-phase, as indicated by the increasing width of the local mode. The peak intensity of the E₁^H mode decreases upon changing the temperature from 423 to 673°K. Upon changing the direction of the k-vector from the [1,0,0]- to the [1,1,0]-direction, the peak intensity of the local mode decreases by a factor of ten. These observations indicate the existence of anhannonic effects along the [l,l,0]-direction.     

Analysis of hydrogen adsorption in the bulk and on the surface of magnesium nanoparticles

The stability of magnesium hydride (MgH _x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ～ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH _x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH₂. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH₂ are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH₂. To attain high diffusion rates, the structures of Mg₅₃Sc and Mg₅₃Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg₅₃ScH₁₀₈ near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH₂, while the frequency of jumps in Mg₅₃TiH₁₀₈ near Ti atoms is higher by approximately a factor of 2.5 × 10⁶. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles.     

Effect of substrate temperature on the structural and optical properties of ZnO and Al-doped ZnO thin films prepared by dc magnetron sputtering

ZnO and Al-doped ZnO(ZAO) thin films have been prepared on glass substrates by direct current (dc) magnetron sputtering from 99.99% pure Zn metallic and ZnO:3 wt%Al₂O₃ ceramic targets, the effects of substrate temperature on the crystallization behavior and optical properties of the films have been studied. It shows that the surface morphologies of ZAO films exhibit difference from that of ZnO films, while their preferential crystalline growth orientation revealed by X-ray diffraction remains always the (0 0 2). The optical transmittance and photoluminescence (PL) spectra of both ZnO and ZAO films are obviously influenced by the substrate temperature. All films exhibit a transmittance higher than 86% in the visible region, while the optical transmittance of ZAO films is slightly smaller than that of ZnO films. More significantly, Al-doping leads to a larger optical band gap (E_g) of the films. It is found from the PL measurement that near-band-edge (NBE) emission and deep-level (DL) emission are observed in pure ZnO thin films. However, when Al was doped into thin films, the DL emission of the thin films is depressed. As the substrate temperature increases, the peak of NBE emission has a blueshift to region of higher photon energy, which shows a trend similar to the E_g in optical transmittance measurement.     

Excitons and linear optical spectra of E33 and E44 in single wall carbon nanotubes under axial magnetic field

The E₃₃ and E₄₄ optical spectra of semiconducting single-walled carbon nanotubes under axial magnetic field (B) are studied using the tight-binding model, which also takes into account the exciton effect. It is found that the E₃₃ and E₄₄ splitting, induced by the axial magnetic field, is line increased with magnetic field, which can be described by the splitting rate. Also by investigation of the dependence of splitting rate, we found that it shows a clear (2n+m) family behavior besides the diameter dependence, which can be used as a supplemental tool to identify the tubes used in the experiment, and is expected to be detected by the future experiment.     

Hydrogen permeation of Pd-coated V90Al10 alloy membranes at different pressures in the presence and absence of carbon dioxide

The hydrogen permeation characteristics of alloy membranes based on Pd-coated V₉₀Al₁₀ alloy membrane have been investigated in the pressure range 1-3 atm under pure hydrogen and hydrogen-carbon dioxide gas mixture at 450 °C. Hydrogen permeation experiments have been confirmed that hydrogen flux was 21.1 ml/min/cm² for a Pd-coated V₉₀Al₁₀ alloy membrane (thickness: 0.5 mm) using pure hydrogen as the feed gas. It has been found that Pd-coated V₉₀Al₁₀ alloy membranes exhibit good resistance to hydrogen embrittlement in pure hydrogen atmosphere. After different hydrogen permeation flux tests under different pressure condition in presence of hydrogen-carbon dioxide gas mixture, the characteristics of the Pd-coated V₉₀Al₁₀ alloy membranes were examined by ex-situ analysis techniques. The loss of cell performance observed in the presence of hydrogen-carbon dioxide gas mixture is mainly attributed to both physical and chemical degradations of membrane, which led to structural changes in the Pd-coated V₉₀Al₁₀ alloy membrane.     

Nanoindentation and nanoscratch behaviors of Ag/Ni multilayers

The hardness, elastic modulus and scratch behaviors of Ag/Ni mulitlayers deposited by evaporation have been carried out by nanoindentation and nanoscratch. It has been found that the hardness (H) increases, while the modulus (E) decreases, that is to say an increase of H/E as the periodicity decreases. Many mechanisms are included in nanoscratch, including initial elastic contact, plowing and fracture stage, in each multilayer. Coefficient of friction during plowing decreases with the decrease of the periodicity, which can be ascribed to decreasing material pile-up due to the increase of H/E. Elastic recovery after scratching also increases as the periodicity decreases because of the increase of H/E, which leads to improved wear resistance. The fracture stage will be postponed with decreasing periodicity, which also leads to better wear behavior.     

Electron paramagnetic resonance and 1H nuclear magnetic resonance study of Y-doping effect on the hydrogen shallow donors in ZnO nanoparticles

Hydrogen shallow donors in sol-gel-derived pristine and rare-earth Y-doped ZnO nanoparticles have been investigated by electron paramagnetic resonance (EPR) and high-resolution ¹H nuclear magnetic resonance (NMR). It is shown by EPR measurements that the energy level of the hydrogen shallow donors in the Y-doped ZnO is much deeper (E ～ 174 meV) than in the pristine ZnO (E ～ 75 meV). The temperature-dependent ¹H NMR chemical shift and linewidth measurements of the pristine and the Y-doped ZnO systems indicated that Y-doping effectively modifies the lattice environment and hinders the hydrogen motions in the ZnO nanoparticles.     

Negative differential conductivity of metal-insulator-metal tunneling structures

The tunneling characteristics of a metal-insulator-metal junction are calculated in the framework of the two-band model, which takes into account the presence of the valence band for the insulator layer. It is demonstrated that, in the case where the Fermi level E _F of the structure under investigation lies below the middle of the band gap of the insulator, the dependence of the tunneling current on the bias voltage across the junction contains portions with a negative differential resistance at V > E _F/e. The iron-aluminum oxide-iron magnetoresistive junctions are considered as samples in which the effect under discussion can be observed. It is shown that, in the given case, the appearance of a portion with a negative differential resistance should be expected at voltages exceeding the Fermi energy \(E_{F_1 } \) for the spin-up electron band.     

Green function theory of chemisorption on sp-hybrid crystals

A self-consistent Green function calculation of the adatom charge transfer (Δq) and the chemisorption energy (ΔE) is performed for hydrogen and the halogens on Si and Ge substrates, which are modeled by sp-hybrid orbital chains. The effect of Shockley surface states on the hydrogen chemisorption process is studied in some detail. From the chemisorption point of view, the behaviour of Si resembles that of Ge, the (111) surfaces having a smaller value of |ΔE| than the (100) ones for all the adsorbates considered. In the case of the halogens, the strong reactivity of fluorine is confirmed by the large values obtained for Δq and |ΔF|.     

Hydrogen-induced reconstruction of the W(100) surface,studied by surface core level spectroscopy

We report W(4?) surface core level shifts which yield new information on the energetics of the W(100) (1 × 1) → C(2 × 2)H phase transition. At small hydrogen coverages we find two co-existing surface core levels from atoms on normal lattice sites and from atoms in reconstructed domains. These surface levels are shifted to smaller binding energy (toward E_F) by 0.35 eV and 0.13 eV relative to the bulk level, respectively. The most stable configuration is obtained at a fractional coverage θ_H ? 0.2, at which all surface atoms are shown to be paired with neighboring atoms in the surface plane.     

The solution of Hydrogen in TaV2

TaV₂ may be prepared as a random b.c.c. alloy or as a (C-15) Laves phase. The behavior of hydrogen dissolved in b.c.c. TaV₂ is significantly different than that exhibited by hydrogen dissolved in (C-15) TaV₂. The b.c.c. TaV₂/H₂; system is similar to pure vanadium/H₂; in the region of hydrogen solid solution, the electronic contribution to the thermodynamics of solution is negligible and the system exhibits apparent H-H attractive interactions. A non-stoichiometric f.c.c. dihydride is formed at p_H2 = 10 atm, T = 273 K. (C-15) TaV₂ does not form a hydride phase; hydrogen is continuously absorbed into solid solution to a maximum hydrogen content H/M ? 0.60. The solid solution of hydrogen in (C-15) TaV₂ exhibits trends which are typical of intermetallie/H₂ systems. The dilute hydrogen region is marked by a strong positive deviation from Sieverts' Law, and the relative partial molar enthalpy of solution becomes less exothermic as the hydrogen content is increased. ΔV? vs H/M relationships and ΔS_H vs H/M relationships indicate that hydrogen segregation at the metal surface or at abnormally stable trapping sites does not contribute significantly to the thermodynamic behavior exhibited by hydrogen in (C-15) TaV₂. Analysis of the excess free energy of hydrogen suggests that electronic effects may exert a significant influence on the behavior of hydrogen in (C-15) TaV₂, even at dilute hydrogen contents. It is suggested that the radom substitution of tantalum into the vanadium lattice exerts a significantly greater effect on the vibrational entropy of dissolved hydrogen than does the structural transformation b.c.c. TaV₂ → (C-15) TaV₂.     

Memory switching of germanium tellurium amorphous semiconductor

The dc conductivity and switching properties of amorphous GeTe thin film of thickness 262 nm are investigated in the temperature range 303-373 K. The activation energy ΔE_σ, the room temperature electrical conductivity σ_RT and the pre-exponential factor σ₀ were measured and validated for the tested sample. The conduction activation energy ΔE_σ is calculated. The I-V characteristic curves of the thin film samples showing a memory switching at the turnover point (TOP) from high resistance state (OFF state) to the negative differential resistance state (NDRS) (ON state). It is found that the mean values of the threshold electrical field E_th decreased exponentially with increasing temperatures in the investigated range. The switching activation energy ΔE_th is calculated. Measurements of the dissipated threshold power P_th and the threshold resistance R_th were carried out at TOP point at different temperatures of the samples. The activation energies ΔE_R and ΔE_P caused by resistance and power respectively are deduced. The results obtained support thermal model for initiating switching process in this system.     

Vibrational symmetry classification and torsional tunneling splitting patterns in G6(EM), G12, and G36(EM) molecules

It is shown that the torsional splitting patterns in methanol-like molecules, with the excitation of small amplitude vibrational modes in the methyl group, are determined by mechanisms that can be formulated in an almost identical fashion to that for ethane-like molecules. This is achieved by treating ethane-like molecules by the internal axis method (IAM) and methanol-like molecules by the principal axis method (PAM) or rho-axis method (RAM). Using the extended molecular groups G₆(EM) or C_6v(M) for methanol and G₃₆(EM) for ethane, vibrations perpendicular to the internal rotation axis are conveniently described by modes of higher degeneracy (E for methanol and G_s for ethane) in the absence of coupling of top and frame. Head–tail coupling operators, except the cos-type barrier terms, lower the degeneracy, causing vibrational splittings. Coupled vibrational pairs with torsional splitting patterns that we call ‘regular’ (pure A₁, A₂ pairs for methanol and pure E_1d, E_2d pairs for ethane) or ‘inverted’ (pure B₁, B₂ pairs for methanol and pure E_1s, E_2s pairs for ethane) can be formed as limit cases. Actual splitting patterns occur between the above limits, and are basically determined by torsional Coriolis coupling, which can tune more or less to resonance pairs of uncoupled basis levels linked by specific head-tail coupling operators. The inversion of torsional splitting patterns, observed in perpendicular vibrational modes of the methyl group of methanol, can be predicted by these theoretical considerations. Similar considerations apply to molecules of G₁₂ symmetry.     

Role of hydrogen in the absorption of ionizing radiation energy by a metal-hydrogen system

Ab initio calculations of the electronic structure of pure Pd, pure Ti, and PdH_x and TiH_x (x = 1, 2, 3) systems are performed within the local density approximation. It is found that the electronic subsystem of metals containing dissolved hydrogen increases their capacity to absorb the energy of electromagnetic radiation and accumulate it for a longer time than pure metals. These two factors promote the nonequilibrium migration of hydrogen atoms and their release from metals upon exposure to ionizing radiation.     

NMR study of hydrogen diffusion in uranium hydride

The diffusion of hydrogen in uranium hydride is studied employing the NMR technique. From measurements of spin-spin relaxation time T₂, the activation energy for hydrogen diffusion in β-UH₃ is determined to be $\text{E}{}_{\mathrm{a}}\text{= (19.25 ± 0.4)}\text{kcal}\text{mole}$ and the preexponential factor to be A₀ ≈ 5 × 10¹⁴ Hz. It is shown that these results are in fair agreement with spin-lattice relaxation time T₁ data. Assuming that hydrogen diffusion proceeds via vacancies whose concentration is temperature dependent, it is concluded that E_a is the sum of the energies of vacancy formation and barrier height, and that A₀ contains an entropy change factor. Using vacancy concentration data calculated by Libowitz, we estimate the barrier height energy to be E_b ≈7 kcal/mole. Using a value for the frequency of hydrogen vibration v₀ determined from inelastic neutron scattering by Rush et al., we estimate the entropy change due to vacancy formation and the hydrogen atom jump to be about $\text{S}\text{k}{}_{\mathrm{B}}\text{≈3}$. Similar measurements on samples containing less hydrogen than is needed to compose stoichiometric UH₃, show that the rate of diffusion is enhanced by the presence of excess metal in the sample. The jump frequency at 500°K in UH₃ is found to be approximately 10⁶ Hz while for the two-phase samples of H/U = 2.8 and 2.5, it is larger by a factor of about 3 and 3.5, respectively.     

Hydrogen-induced low temperature CO displacement from the Pt(111) surface

A new low temperature displacement mechanism for CO on the Pt(111) surface has been observed in the presence of high pressures of hydrogen (0.001 to 0.1 Torr H₂). Temperature-programmed fluorescence yield near-edge spectroscopy (TP FYNES) was used to continuously monitor the CO coverage as a function of temperature both with and without hydrogen. For hydrogen pressures above 0.01 Torr, removal of CO begins at 130 K (E_d = 10.6 kcal/mol) instead of near the desorption temperature of 400 K (E_d = 26 kcal/mol). The large decrease in CO desorption energy appears to be caused by substantial repulsive interactions in the compressed monolayer induced by coadsorbed hydrogen. The new low temperature CO desorption channel appears to be caused by displacement of the compressed CO adlayer by coadsorbed hydrogen. In addition, the desorption activation energy for the main desorption channel of CO near 400 K is lowered by ~ 1 kcal/mol for hydrogen pressures in the 0.001 to 0.1 Torr range. These new results clearly emphasize the importance of in-situ methods capable of performing kinetic experiments at high pressures on well characterized adsorbed monolayers on single crystal surfaces. High coverages of coadsorbed hydrogen resulting from substantial overpressures may substantially modify desorption activation energies and thus coverages and kinetic pathways available even for strongly chemisorbed species. These phenomena may play an important role in surface reactions which occur at high pressure.     

Specific features of hydrogenation of chromium-doped polycrystalline thin vanadium dioxide films

It has been found that hydrogen penetration into chromium-doped polycrystalline thin vanadium dioxide films occurs with a lower rate than in the case of pure vanadium dioxide films. It has been shown that hydrogenation of films with low chromium concentrations is accompanied by a decrease in the phase transition temperature below T _c = 340 K. However, at room temperature in these hydrogenated films, no traces of M1 monoclinic phase have been observed. As the chromium concentration increases, hydrogenation ceases to be accompanied by the decrease in the phase transition temperature.     

Evidence for an alkali-like conduction band in alkali graphite intercalation compounds

The valence bands of pure graphite and several alkali graphite intercalation compounds (AGIC's) were studied by UPS (hv = 21.2 eV). The most significant observation is an intensity peak at the Fermi energy E_F in the intecalation compounds. This peak is mainly due to alkali-like s-states. The density of states at E_F is enhanced by a factor of 30 compared to pure graphite. The alkali-like conduction bands in the first stage AGIC's are similar to those of pure alkali metals.