The doping effects of boron on the atomic adsorption of hydrogen on graphene have been investigated using density functional theory calculations. The hydrogen adsorption energies and electronic structures have been considered for pristine and B-doped graphene with the adsorption of hydrogen on top of carbon or boron atom. It is found that the B-doping forms an electron-deficient structure and decreases the hydrogen adsorption energy dramatically. For the adsorption of hydrogen on top of other sites, similar results have also been found. These results indicate that the hydrogen storage capacity is improved by the doping of B atom.

First-principles pseudopotential plane-wave calculations based on density functional theory and the generalized-gradient approximation have been used to study the adsorption, dissociation, and diffusion of hydrogen on the α-U(001) surface. Weak molecular chemisorption was observed for H 2 approaching with its molecular axis parallel to the surface. The optimization of the adsorption geometries on the threefold hollow sites yields final configurations with H 2 molecules moving towards the top site at both coverages considered, 0.25 and 0.5 monolayers. A low dissociation barrier of 0.081 eV was determined for H 2 dissociated from the onefold top site with the H atoms falling into the two adjacent threefold hollow sites. The analysis of the density of states along the dissociation paths shows that the hybridization of U 5f and H 1s states only occurs when the H 2 molecule is dissociated.

As the macro behavior of the strength of exchange interaction, state of the art of Curie temperature Tc, which is directly proportional to the exchange integrals, makes sense to the high-frequency and high-reliability microwave devices. Challenge remains as finding a quantitative way to reveal the relationship between the Curie temperature and the exchange integrals for doped barium hexaferrites. Here in this report, for La-substituted barium hexaferrites, the electronic structure has been determined by the density functional theory (DFT) and generalized gradient approximation (GGA). By means of the comparison between the ground and relative state, thirteen exchange integrals have been calculated as a function of the effective value Ueff. Furthermore, based on the Heisenberg model, the molecular field approximation (MFA) and random phase approximation (RPA), which provide an upper and lower bound of the Curie temperature Tc, have been adopted to deduce the Curie temperature Tc. In addition, the Curie temperature Tc derived from the MFA are coincided well with the experimental data. Finally, the strength of superexchange interaction mainly depends on 2b-4f1, 4f2-12k, 2a-4f1, and 4f1-12k interactions.

No abstract

Using the first-principle density-functional theory, we study the structure and effects of vacancies in ⌺3 ͑112͒ grain boundary with the coincident-site lattice structure in Si. We find that the formation energy for a Si vacancy in the grain boundary is significantly lower than that in Si perfect region, indicating strong segregation of Si vacancy in grain boundary regions. The formation of Si vacancies in grain boundaries either cleans up the deep levels or facilitates complete passivation by H atoms. Our results suggest that vacancies in grain boundaries may play important role in determining grain boundary physics and passivation behavior.