Y. Song

Mechanical alloying and electronic simulations of (MgH2+M) systems (M=Al, Ti, Fe, Ni, Cu and Nb) for hydrogen storage

Abstract Mg-based alloys are promising candidates for hydrogen storage applications. Here, mechanical alloying (MA) was used to process powder mixtures of MgH2 with 8 mol % M (M=Al, Ti, Fe, Ni, Cu and Nb) in order to modify hydrogen storage properties of the Mg hydride. Electronic simulations of the systems were carried out to clarify the mechanisms of the alloy effects. X-ray diffraction (XRD) of the milled samples revealed the formation of new phases: a bcc solid solution phase for the (MgH2+Nb) mixture; TiH2 phase for the (MgH2+Ti); and MgCu2 phase for the (MgH2+Cu). For all the mixtures, a high-pressure phase, γ-MgH2, was also identified after mechanical alloying. Further qualitative and quantitative phase analyses were carried out using the Rietveld method. Scanning electron microscopy (SEM) of the milled powder clearly showed substantial particle size reduction after milling. Dehydrogenation at 300°C under vacuum shows that the (MgH2+Ni) mixture gives the highest level of hydrogen desorption and the most rapid kinetics, followed by MgH2 with Al, Fe, Nb, Ti and Cu. Theoretical predictions show that the (MgH2+Cu) system is the most unstable, followed by (MgH2+Ni), (MgH2+Fe), (MgH2+Al), (MgH2+Nb), (MgH2+Ti). The predicted alloying effects on the stability of MgH2 generally agree with the experimentally observed change in the hydrogen desorption capacity. The differences were discussed in the text.

First principles study of site substitution of ternary elements in NiAl

Site substitution of ternary elements in ordered compounds influences the electronic structure and hence the properties of compounds at the continuous level. The electronic structure and binding energy of a number of NiAl-X alloy systems (X=Ti, V, Cr, Mn, Fe, Co, Zr, Nb, Mo, Hf, Ta, W, Si, Ga, or Gel were calculated using the discrete variational cluster method based on the local density approximation of the density functional theory. The site preference of the ternary additions to NiAl was investigated by employing the Bra,og-Williams model to analyse the calculated binding energy. The results show that all the considered ternary elements possess stronger preference to the Al sublattice sites than a Ni atom does. A new method of identifying sublattice substitution of ternary additions in NiAl was proposed by comparison of the binding energies per atom of the ternary and the binary clusters involving the fourth nearest neighbours. The analysis suggests that Fe and Co atoms occupy the Ni sublattice sites, whereas Si, Ga and Ti atoms occupy the Al sublattice sites. The remaining elements may substitute for both sublattices: Mn is most likely to go for the Ni sublattice; V, Cr, Zr, Nb, Mo, Hf, Ta, W and Ge have a larger preference for the Al sublattice, but Cr and W do not show significant preference to any sublattice. The densities of states involving alloying additions of Co, Si and Cr were further investigated to clarify the site preference of the alloying additions

First principles studies of TiAl-based alloys

Abstract As potential candidates for high-temperature applications, TiAl-based intermetallic alloys have been under extensive experimental and theoretical studies in the past two decades. Particularly, simulations from the first principles have offered great insight into the electronic structure and the effects of alloying additions on the intrinsic properties of the materials. This article attempts to highlight the current activities in the field of first principles simulations for the prediction of mechanical properties of TiAl. The preference of sublattice sites of various alloying elements and their effects on lattice distortion were investigated and discussed in relation to experimental findings.

Bonding characteristics of micro-alloyed B2 NiAl in relation to site occupancies and phase stability

A recently developed mean-field model has been combined with first principles calculations of binding energy to investigate the site occupancies of micro-alloying elements and vacancies in NiAl as well as the stability of the micro-alloyed B2 phase with respect to disordering and second-phase formation. The theoretical results suggest that the transition metal elements in the same row of the periodic table increasingly tend to the Ni sublattice with increasing atomic number. Micro-alloying addition tends to decrease the vacancy concentration of NiAl alloys. Alloying with X that substitutes for Ni is predicted to have the sides of its solubility lobe parallel to the Ni-X side of the isotherm, but parallel to the Al-X side if X substitutes for Al. Micro-alloying was shown to raise the ordering temperature of the B2 phase over the corresponding binary alloy, in contrast with the effect of vacancies. Alloying effects on ordering temperature and the formation of point defects appear independent of the site substitution behaviour, and are less significant for 3d than for 4d and 5d transition metal elements

Metastable MgH2 phase predicted by first principles calculations

MgH2 is a promising compound for hydrogen storage. Its relatively high stability has been the main obstacle for practical applications. Here, the authors report a first principles prediction of a metastable phase of MgH2. The predicted phase is characterized by tetragonal symmetry (I41∕amd, group 141) with the lattice parameters a=0.3813nm and c=0.9416nm and meets all the mechanical stability criteria. Its heat of formation is −58.03kJ∕molH2, which is much less negative than that of the α-MgH2 phase (−76kJ∕molH2). Hence, the tetragonal structure is more desirable than the α-MgH2 for practical hydrogen storage applications.

Influence of interstitial elements on the bulk modulus and theoretical strength of alpha-titanium: a first-principles study

Abstract The influence of the interstitial elements H, B, C, N and O on the bulk modulus and the theoretical strength of α-Ti was investigated using the discrete variational method under the local density approximation from first principles. The electronic structure and the total and binding energies were first evaluated with and without lattice relaxation and then used to assess the theoretical values of the bulk modulus and the theoretical strength under hydrostatic tension. It was shown that the binding energy was increased when H was in a solution of α-Ti but reduced when B, C, N or O was in solution. The bulk modulus and the theoretical strength were increased by C, O and N but reduced by H and B. A charge-density analysis showed that the bonds between Ti atoms were enhanced by the interstitial elements N, C and O but weakened, to some extent, by the incorporation of H.

A first-principles study of the theoretical strength and bulk modulus of hcp metals

Abstract A first-principles method based on the local-density approximation using discrete variational clusters has been used to study the electronic structure of the hcp metals, Be, Mg, Sc, Y, Ti, Zr, Co, Zn and Cd. The binding energy of these metals was calculated in relation to the volume of a unit cell. The variation in the binding energy with the unit cell volume was obtained by means of a polynomial fit. The theoretical tensile strength and bulk modulus of these metals were estimated from the electronic structure and binding energy calculations. The predicted bulk moduli for these metals are in good agreement with experimental findings and other available theoretical data. A linear relationship between the calculated and the experimental strengths is observed.