Journal of The Less Common Metals最新文献

The influence of the microstructure on the relaxation behaviour of hydrogen in partially crystallized nano- and microcrystalline Fe₉₀Zr₁₀ as well as in single-phase Fe₂Zr was investigated in the temperature range between 4.2 and 500 K by means of magnetic after-effect measurements. Reorientation processes of hydrogen in amorphous, grain boundary and crystalline phases led to characteristic relaxation maxima and activation enthalpies which were assigned to hydrogen jumps between anisotropic interstitial sites within the different structures. In addition, the degassing kinetics above 300 K were studied and allow a more detailed correlation between relaxation processes and microstructures.

Hydrogenated FeTi superlattices have been studied by hydrogen depth profiling, using the ¹⁵N method, conversion electron Mössbauer spectroscopy and X-ray diffraction. The modulation wavelength of the sample was 40 nm and the ratio of the titanium to iron thicknesses was equal to three. It was found that mixing of the iron and titanium layers occurred during the annealing and hydrogenation procedure but that the superlattice structure was preserved. The latter result was explained by the negligible solubility of iron in titanium.

The hydrogen permeation characteristics of alloy membranes based on V-15 at.%Ni have been investigated in the temperature range 373–673 K. It has been found that palladium-coated V-15 at.%Ni alloy membranes exhibit good resistance to hydrogen embrittlement with a hydrogen permeability larger than that of a palladium membrane. The VNi alloy membrane did not exhibit any cracks even under an upstream hydrogen pressure of 0.2 MPa during the permeation test at 473 K. This revealed a conspicuous hydrogen trapping phenomenon below 473 K. The hydrogen permeability of the VNi alloy membrane increased as a result of adding 0.05 at.% of titanium, zirconium or yttrium, which was nearly equal to the amount of oxygen in the alloy; however, the hydrogen trapping phenomenon could not be eliminated by adding such deoxidizers. The trapping phenomenon could be markedly improved by removing the deformed layer of the alloy membrane before plating with palladium.

Computer simulations of the diffusion of hydrogen in structurally disordered alloys have been made. Calculations of the dipolar nuclear magnetic relaxation rate in the simulated alloys are reported.

Nuclear electric quadrupolar relaxation of deuterium or quadrupolar metal nuclei, due to diffusing hydrogen or deuterium, is estimated to dominate magnetic dipolar relaxation in metal-hydrogen systems. The longitudinal magnetization recoveries for quadrupolar relaxation can be linear combinations of several exponentials and expressions for the exponents depend on the spin I, whether a spin-temperature exists in the system and whether a static quadrupole interaction exists. Some limiting cases and examples are discussed.

Tracer diffusion coefficients D_t of hydrogen in niobium and some Nb_1−y V_yH_x alloys have been measured and are compared with chemical diffusion coefficients D_c from the literature. Nuclear magnetic resonance pulsed field gradient techniques were used to measure D_t. Comparison of the two diffusion coefficients is made taking into consideration the thermodynamic factor $\text{(k}{}_{\mathrm{B}}\text{T)}{}^{\mathrm{-1}}\text{[}\text{∂μ}\text{∂(ln x}\text{)]}$ which is obtained from pressure-composition isotherms for the alloys. The comparisons indicate satisfactory agreement between the two measures of the diffusion coefficient.

An overview of the recently discovered binding of molecular hydrogen (H₂) to transition metal complexes is given. Emphasis is placed on the features which distinguish non-classical metal-dihydrogen bonding from classical atomic metal-hydride systems. Crystallographic and spectroscopic data (IR, nuclear magnetic resonance (NMR), inelastic neutron scattering) and theoretical aspects are discussed. The subtle factors which influence cleavage of bound H₂ to hydride are identified and examples are given demonstrating that a compound containing a dihydrogen “pair” can exist in facile equilibrium with the “isomeric” compound derived from it containing two separated hydrides. Metal-H₂ systems show a uniquely rich dynamic behavior, including rotation of the side-on bonded H₂. The relatively low energy barrier to rotation is studied experimentally by inelastic neutron scattering and theoretically by ab initio methods, in relation to the bonding model for M-H₂. Quantum mechanical tunneling and exchange processes are identified for bound dihydrogen and certain trihydride systems.