Diffusion of hydrogen in metals

A Study of the Key Factors on Production of Graphene Materials from Fe-Lignin Nanocomposites through a Molecular Cracking and Welding (MCW) Method

In this work, few-layer graphene materials were produced from Fe-lignin nanocomposites through a molecular cracking and welding (MCW) method. MCW process is a low-cost, scalable technique to fabricate few-layer graphene materials. It involves preparing metal (M)-lignin nanocomposites from kraft lignin and a transition metal catalyst, pretreating the M-lignin composites, and forming of the graphene-encapsulated metal structures by catalytic graphitization the M-lignin composites. Then, these graphene-encapsulated metal structures are opened by the molecule cracking reagents. The graphene shells are peeled off the metal core and simultaneously welded and reconstructed to graphene materials under a selected welding reagent. The critical parameters, including heating temperature, heating time, and particle sizes of the Fe-lignin composites, have been explored to understand the graphene formation mechanism and to obtain the optimized process parameters to improve the yield and selectivity of graphene materials.

Embedded-atom method potential for modeling hydrogen and hydrogen-defect interaction in tungsten

An embedded-atom method potential has been developed for modeling hydrogen in body-centered-cubic (bcc) tungsten by fitting to an extensive database of density functional theory (DFT) calculations. Comprehensive evaluations of the new potential are conducted by comparing various hydrogen properties with DFT calculations and available experimental data, as well as all the other tungsten-hydrogen potentials. The new potential accurately reproduces the point defect properties of hydrogen, the interaction among hydrogen atoms, the interplay between hydrogen and a monovacancy, and the thermal diffusion of hydrogen in tungsten. The successful validation of the new potential confirms its good reliability and transferability, which enables large-scale atomistic simulations of tungsten-hydrogen system. The new potential is afterward employed to investigate the interplay between hydrogen and other defects, including [1 1 1] self-interstitial atoms (SIAs) and vacancy clusters in tungsten. It is found that both the [1 1 1] SIAs and the vacancy clusters exhibit considerable attraction for hydrogen. Hydrogen solution and diffusion in strained tungsten are also studied using the present potential, which demonstrates that tensile (compressive) stress facilitates (impedes) hydrogen solution, and isotropic tensile (compressive) stress impedes (facilitates) hydrogen diffusion while anisotropic tensile (compressive) stress facilitates (impedes) hydrogen diffusion.

Experimental neutron scattering evidence for proton polaron in hydrated metal oxide proton conductors

Hydration of oxygen-deficient metal oxides causes filling of oxygen vacancies and formation of hydroxyl groups with interstitial structural protons, rotating around the oxygen in localized motion. Thermal activation from 500 to 800 K triggers delocalization of the protons by jumping to adjacent oxygen ions, constituting proton conductivity. We report quantitative analyses of proton and lattice dynamics by neutron-scattering data, which reveal the interaction of protons with the crystal lattice and proton–phonon coupling. The motion for the proton trapped in the elastic crystal field yields Eigen frequencies and coupling constants, which satisfy Holstein’s polaron model for electrons and thus constitutes first experimental evidence for a proton polaron at high temperature. Proton jump rates follow a polaron model for cerium-oxygen and hydroxyl stretching modes, which are thus vehicles for proton conductivity. This confirms that the polaron mechanism is not restricted to electrons, but a universal charge carrier transport process.

Hydration of oxygen vacancies could form hydroxyl groups with interstitial structural protons. Here a quasi-elastic neutron scattering study reveals proton polarons in proton-conducting ceramic electrolytes, and the proton transport turns out to be a cooperative process.

Methods for quantifying the influences of pressure and temperature variation on metal hydride reaction rates measured under isochoric conditions

Analysis techniques for determining gas-solid reaction rates from gas sorption measurements obtained under non-constant pressure and temperature conditions often neglect temporal variations in these quantities. Depending on the materials in question, this can lead to significant variations in the measured reaction rates. In this work, we present two new analysis techniques for comparison between various kinetic models and isochoric gas measurement data obtained under varying temperature and pressure conditions in a high pressure Sievert system. We introduce the integral pressure dependence method and the temperature dependence factor as means of correcting for experimental variations, improving model-measurement fidelity, and quantifying the effect that such variations can have on measured reaction rates. We use measurements of hydrogen absorption in LaNi5 and TiCrMn to demonstrate the effect of each of these methods and show that their use can provide quantitative improvements in interpretation of kinetics measurements.

Significance of self-trapping on hydrogen diffusion

The diffusion rate of hydrogen in Nb was calculated using ab initio molecular dynamics simulations. At low temperatures the hydrogen is strongly trapped in a local strain field which is caused by the elastic response of the lattice. At elevated temperatures, the residence time (τ) of hydrogen in an interstitial site is not sufficient for fully developing the local strain field. This unbinding of the interstitial hydrogen and the strain field increases the hopping rate (1/τ) at elevated temperatures (>400  K). These results call for a revision of the conceptual framework of diffusion of hydrogen in transition metals at elevated temperatures.