X. Ju

Li-doped B2C graphene as potential hydrogen storage medium

Based on first-principles density functional theory, we show that Li-doped B2C graphene can serve as a high-capacity hydrogen storage medium with the gravimetric density of 7.54 wt %. The present results indicate that the strong binding of Li onto the substrate comes from the hybridizations of B 2p and C 2p orbitals with the partial occupancy of Li 2p orbitals. Both the polarization mechanism and the orbital hybridizations contribute to the adsorption of H2 molecules and the resulting adsorption energy is in the range of 0.12–0.22 eV/H2. The system reported here is favorable for the reversible hydrogen adsorption/desorption at the room temperature.

Theoretical realization of cluster-assembled hydrogen storage materials based on terminated carbon atomic chains

The capacity of carbon atomic chains with different terminations for hydrogen storage is studied using first-principles density functional theory calculations. Unlike the physisorption of H(2) on the H-terminated chain, we show that two Li (Na) atoms each capping one end of the odd- or even-numbered carbon chain can hold ten H(2) molecules with optimal binding energies for room temperature storage. The hybridization of the Li 2p states with the H(2)σ orbitals contributes to the H(2) adsorption. However, the binding mechanism of the H(2) molecules on Na arises only from the polarization interaction between the charged Na atom and the H(2). Interestingly, additional H(2) molecules can be bound to the carbon atoms at the chain ends due to the charge transfer between Li 2s2p (Na 3s) and C 2p states. More importantly, dimerization of these isolated metal-capped chains does not affect the hydrogen binding energy significantly. In addition, a single chain can be stabilized effectively by the C(60) fullerenes termination. With a hydrogen uptake of ∼10 wt.% on Li-coated C(60)-C(n)-C(60) (n = 5, 8), the Li(12)C(60)-C(n)-Li(12)C(60) complex, keeping the number of adsorbed H(2) molecules per Li and stabilizing the dispersion of individual Li atoms, can serve as better building blocks of polymers than the (Li(12)C(60))(2) dimer. These findings suggest a new route to design cluster-assembled hydrogen storage materials based on terminated sp carbon chains.

AN IMPROVED CLUSTER DYNAMICS MODEL FOR HYDROGEN RETENTION IN TUNGSTEN

An improved cluster dynamics model has been developed for studying the behaviors of hydrogen retention in tungsten under hydrogen ions irradiation. In addition to different types of objects, adopting up-to-date parameters and complex reaction processes, we newly introduce ion-induced and natural defects into our model. This improved model programmed in IRadMat2 could describe very well the depth distributions and the amounts of hydrogen retained in tungsten under different radiation conditions. The calculated results agree with the experimental ones much better than the previous model, especially for the depth-distribution of D retained in W, which imply that this model is applicable to the evolution of defects especially for low energy high flux ions irradiated on plasma-facing materials.

Helium-Implanted ODS-Fe Alloy Investigated by Positron-Annihilation Spectroscopy

At room temperature, oxide dispersion strengthened (ODS)-Fe (Fe-2%Al2O3) and pure Fe were implanted with 140-keV helium ions by doses up to 1e+15, 5e+15, and 1e+16/cm2. Vacancy-type defects induced by the implantation were investigated with positron-annihilation spectroscopy (PAS). Defect profiles of the S-parameter derived from PAS as a function of positron incident energy up to 20 KeV were analyzed. The S-parameter of the damaged layers in the ODS-Fe tend to be smaller than those in the pure Fe when the irradiation doses reach 1e+16/cm2. This finding indicates that the ODS-Fe sample exhibits a higher irradiation resistance than the pure Fe. The reason might be that helium irradiation-induced point defects in ODS-Fe are trapped and annihilated at interfaces between the Fe matrix and the Al2O3 nanoparticles. Vacancy clusters and helium-vacancy complexes were identified as the main defects. Through the S-W curves, however, we observed only one type of defect in the irradiated ODS-Fe. While for the pure Fe case, things seem to be more complicated, where the surface and the bulk have different types of defects.

Characterization of the defects in oxide dispersion-strengthened iron alloy after helium ion irradiation

Vacancy-type defects in Fe-2%Al2O3 (ODS-Fe) and pure Fe produced by helium ion irradiation at room temperature were investigated using positron beam Doppler broadening energy spectra (DBES). Defect profiles of the S parameter derived from DBES as a function of positron incident energy up to 20 KeV were analyzed. The S parameter values of the damaged layer for the ODS-Fe are smaller than those of pure Fe for irradiation dose of 1e+16/cm2. This finding indicates that ODS-Fe has a higher irradiation resistance than pure Fe. It also suggests that helium irradiation induced point defects in ODS-Fe were trapped and annihilated at the interfaces between the Fe matrix and the Al2O3 nanoparticles. The S-W curves indicate that only one type of defect was formed in the post-irradiated ODS-Fe. Vacancy clusters and helium-vacancy complexes were identified as the main defects. The defects are complicated for the irradiated pure Fe, for which the surface defects are different from the ones inside bulk.