Hydrogen Transportation Behaviour of V–Ni Solid Solution: A First-Principles Investigation

Hydrogen embrittlement causes deterioration of materials used in metal–hydrogen systems. Alloying is a good option for overcoming this issue. In the present work, first-principles calculations were performed to systematically study the effects of adding Ni on the stability, dissolution, trapping, and diffusion behaviour of interstitial/vacancy H atoms of pure V. The results of lattice dynamics and solution energy analyses showed that the V–Ni solid solutions are dynamically and thermodynamically stable, and adding Ni to pure V can reduce the structural stability of various VHx phases and enhance their resistance to H embrittlement. H atoms preferentially occupy the characteristic tetrahedral interstitial site (TIS) and the octahedral interstitial site (OIS), which are composed by different metal atoms, and rapidly diffuse along both the energetically favourable TIS → TIS and OIS → OIS paths. The trapping energy of monovacancy H atoms revealed that Ni addition could help minimise the H trapping ability of the vacancies and suppress the retention of H in V. Monovacancy defects block the diffusion of H atoms more than the interstitials, as determined from the calculated H-diffusion barrier energy data, whereas Ni doping contributes negligibly toward improving the H-diffusion coefficient.

Influence of Nickel Addition on the Stability, Trapping, and Diffusion Behaviour of Hydrogen in Vanadium: A First-Principles Investigation

Hydrogen embrittlement causes deterioration of materials used in hydrogen energy systems. Alloying is an effective means for overcoming this issue. In this study, the first-principles calculation method was used to investigate the effects of alloying Ni on the stability, dissolution, trapping, and diffusion behaviour of interstitial/vacancy H atoms in V. The calculated phonon spectra and solution energies of the vacancy/interstitial H atoms revealed that the V–Ni phase was dynamically and thermodynamically stable, and Ni addition could reduce the stability of V hydrides and improve their resistance to H embrittlement. H atoms in the interstitials and vacancies preferentially occupied the tetrahedral interstitial site (TIS) and octahedral interstitial site (OIS) with the lowest solution energies and diffused along the TIS → TIS and OIS → OIS paths with the minimum diffusion barrier energies. The trapping energy of the vacancy H atoms indicated that the addition of Ni could reduce the H trapping capability of the vacancies and suppress the retention of H in V. Detailed analysis of the calculated H diffusion barriers indicated that the presence of monovacancy defects blocked the diffusion of H atoms more than the presence of interstitials, and Ni doping did not enhance the H diffusion coefficient.

Magnetic Volume Effect of Hydrogen Diffusion in Palladium

The electronic and magnetic properties of palladium hydrogen are investigated using first-principles spin-polarized density functional theory. By studying the magnetic moments and electronic structures of hydrogen atoms diffusing in face-centered cubic structure of transition metal Pd, we found that the results of magnetic moments are exactly the same in the two direct octahedral interstitial site-octahedral interstitial site diffusion paths-i.e. the magnetic moments are the largest in the octahedral interstitial site, and the magnetic moments are the lowest in saddle point positions. We also studied on the density of states of some special points, with the result that the density of states near the Fermi level is mainly contributed by 4d electrons of Pd and the change of magnetic moments with the cell volume in the unit cell of transition metal Pd with a hydrogen atom.

First-Principles Investigation of Atomic Hydrogen Adsorption and Diffusion on/into Mo-doped Nb (100) Surface

To investigate Mo doping effects on the hydrogen permeation performance of Nb membranes, we study the most likely process of atomic hydrogen adsorption and diffusion on/into Mo-doped Nb (100) surface/subsurface (in the Nb12Mo4 case) via first-principles calculations. Our results reveal that the (100) surface is the most stable Mo-doped Nb surface with the smallest surface energy (2.75 J/m2). Hollow sites (HSs) in the Mo-doped Nb (100) surface are H-adsorption-favorable mainly due to their large adsorption energy (−4.27 eV), and the H-diffusion path should preferentially be HS→TIS (tetrahedral interstitial site) over HS→OIS (octahedral interstitial site) because of the correspondingly lower H-diffusion energy barrier. With respect to a pure Nb (100) surface, the Mo-doped Nb (100) surface has a smaller energy barrier along the HS→TIS pathway (0.31 eV).

First-principles study on the mechanism of lithium intercalation in cubic CoN

Intercalation mechanism of Li into cubic Co4N4 has been investigated by the first-principles calculations. Lattice constants, ratio of volume expansion, and formation energies of Li[Formula: see text]Co4N4 (x = 0, 1, 2, 3, 4) were calculated. Results indicate that Li prefers to fill the octahedral interstitial site [Formula: see text] rather than the tetrahedral interstitial site [Formula: see text]. With the increase in intercalation Li, the ratio of volume expansion increases from 8.29% (x = 1) to 31.58% (x = 4). Ternary phase Li4Co4N4 has the most stability with the negative intercalation energy, and the corresponding theoretical specific capacity reaches 367 mA/g. Furthermore, the analysis of density of states, valence electron density distribution maps, and electron localization function (ELF) of Co4N4 and Li4Co4N4 indicates that Li intercalation enhances the electrical conductivity of Co4N4 and weakens the bonding of Co and N. Finally, Li-ion migration dynamics in the Co4N4 bulk were investigated with nudged elastic band (NEB) methods. Results show that the migration path of Li-ion is along [Formula: see text] with the energy barrier of 0.44 eV.

Diffusion of Fission Gas in Uranium Dioxide: A First-Principles Study

Uranium dioxide (UO2) is the typical fuel that is used in nuclear fission reactor, fission gas are produced during and after the reactor operation, and the fission gas have a significant impact on the performance of UO2 in reactor. In this paper, we investigated the effects of the fission gas on the performance of UO2 by using the first-principles calculation method based on the density functional theory. The results are that, the volume of UO2 increased when there is a fission gas atom enter in UO2 supercell; fission gas prefer to occupy the octahedral interstitial site over the uranium vacancy site and the oxygen vacancy site, and the oxygen vacancy site is the most difficult occupied site due to the formation of an oxygen vacancy is more difficult than that of the uranium vacancy; our results of the UO2 elastic constants are in good agreement with other simulation results and experimental data, and the fission gas atoms make the ductility of UO2 decreased. Our works may shed some light on the development of the UO2 fuel and the spent fuel reprocessing.

Effects of boron on the mechanical properties of the TiAl–Ti3Al alloy: A first-principles investigation

Employing a first-principles method in combination with the empirical criterions, we have investigated the site preference of boron (B) and its effect on the mechanical properties of the binary-phase TiAl–Ti3Al alloy. It is found that B energetically prefers to occupy the Ti-rich octahedral interstitial site, because B is more favorable to bond with Ti in comparison with Al. The occupancy tendency of B in the TiAl–Ti3Al alloy is the TiAl/Ti3Al interface [Formula: see text] Ti3Al [Formula: see text] TiAl, thus B tends to segregate into the binary-phase interface in the TiAl–Ti3Al alloy. The charge density difference shows that B at the TiAl–Ti3Al interface will form strong B–Ti bonds and weak B–Al bonds, leading to the significant increasing of the cleavage energy [Formula: see text] and the unstable stacking fault energy [Formula: see text]. This indicates that the presence of B will strengthen the TiAl/Ti3Al interface, but block its mobility. Further, the ratio of [Formula: see text]/[Formula: see text] of the B-doped system is 4.63%, 8.19% lower than that of the clean system. Based on the empirical criterions, B will have a negative effect on the ductility of the TiAl–Ti3Al alloy.

Lithium and boron as interstitial palladium dopants for catalytic partial hydrogenation of acetylene

It is demonstrated that light elements, including lithium and boron atoms, can take residence in the octahedral (interstitial) site of a Pd lattice by modifying the electronic properties of the metal nanoparticles, and hence the adsorptive strength of a reactant.

Studies of the EPR Parameters and Local Structure for the Cu2+ Center in Lanthanum Magnesium Nitrate

The electron paramagnetic resonance parameters (g-factors and the hyperfine structure constants) for the Cu2+ center in lanthanum magnesium nitrate (LMN) are theoretically studied from the conventional perturbation formulas of these parameters for a 3d9 ion in tetragonally elongated octahedra. The studied complex is found to exhibit the slight tetragonal elongation (characterized by the relative elongation ratio ρ  4%) due to the Jahn-Teller effect, which may entirely conceal the original trigonal distortion of the host Mg2+ site in LMN. The conventional formulas containing only the metal orbital and spin-orbit coupling contributions are proved to be valid for the Cu2+ center in view of the weak covalency and ligand spin-orbit coupling interactions. This defect is also compared with the similar Cu2+ center of the Jahn-Teller nature on the octahedral interstitial site in the CdSe nanocrystals.