196 research outputs found
Tunable Hydrogen Storage in Magnesium - Transition Metal Compounds
Magnesium dihydride (\mgh) stores 7.7 weight % hydrogen, but it suffers
from a high thermodynamic stability and slow (de)hydrogenation kinetics.
Alloying Mg with lightweight transition metals (TM = Sc, Ti, V, Cr) aims at
improving the thermodynamic and kinetic properties. We study the structure and
stability of MgTMH compounds, -1], by first-principles
calculations at the level of density functional theory. We find that the
experimentally observed sharp decrease in hydrogenation rates for
correlates with a phase transition of MgTMH from a fluorite to
a rutile phase. The stability of these compounds decreases along the series Sc,
Ti, V, Cr. Varying the transition metal (TM) and the composition , the
formation enthalpy of MgTMH can be tuned over the substantial
range 0-2 eV/f.u. Assuming however that the alloy MgTM does not
decompose upon dehydrogenation, the enthalpy associated with reversible
hydrogenation of compounds with a high magnesium content () is close to
that of pure Mg.Comment: 8 pages, 5 figure
Half-metallicity and efficient spin injection in AlN/GaN:Cr (0001) heterostructure
First-principles investigations of the structural, electronic and magnetic
properties of Cr-doped AlN/GaN (0001) heterostructures reveal that Cr
segregates into the GaN region, that these interfaces retain their important
half-metallic character and thus yield efficient (100 %) spin polarized
injection from a ferromagnetic GaN:Cr electrode through an AlN tunnel barrier -
whose height and width can be controlled by adjusting the Al concentration in
the graded bandgap engineered Al(1-x)Ga(x)N (0001) layers.Comment: submitted for publicatio
Onzekerheid over de baten van de Betuwelijn
Model estimates of the pay-off of the Betuwe railway have played an important role for politicians to decide to build this railway. This has already been discussed extensively, especially with respect to the macro-economic effects of this investment. A deeper analysis shows that these effects are very difficult to assess. The NIJFER institute for example calculated a long term profit of 52.1 biljon Dutch guilders, but our analysis of this model shows that the profitlays with 50 reliability between 15 and 30 biljon Dutch guilders. Our conclusion is that this political investment decision is not sufficiently supported by the expected macro-economic pay-off. A prudent use of quantitative research requires that the involved uncertainties in the model outcomes is properly taken into account.investment;cross-country analysis;regression;infrastructure;policy design
Onzekerheid over de baten van de Betuwelijn
Model estimates of the pay-off of the Betuwe railway have played an important role for politicians to decide to build this railway. This has already been discussed extensively, especially with respect to the macro-economic effects of this investment. A deeper analysis shows that these effects are very difficult to assess. The NIJFER institute for example calculated a long term profit of 52.1 biljon Dutch guilders, but our analysis of this model shows that the profit
lays with 50 reliability between 15 and 30 biljon Dutch guilders. Our conclusion is that this political investment decision is not sufficiently supported by the expected macro-economic pay-off. A prudent use of quantitative research requires that the involved uncertainties in the model outcomes is properly taken into account
First-principles study of the optical properties of MgxTi(1-x)H2
The optical and electronic properties of Mg-Ti hydrides are studied using
first-principles density functional theory. Dielectric functions are calculated
for MgxTi(1-x)H2 with compositions x = 0.5, 0.75, and 0.875. The structure is
that of fluorite TiH2 where both Mg and Ti atoms reside at the Ti positions of
the lattice. In order to assess the effect of randomness in the Mg and Ti
occupations we consider both highly ordered structures, modeled with simple
unit cells of minimal size, and models of random alloys. These are simulated by
super cells containing up to 64 formula units (Z = 64). All compositions and
structural models turn out metallic, hence the dielectric functions contain
interband and intraband free electron contributions. The former are calculated
in the independent particle random phase approximation. The latter are modeled
based upon the intraband plasma frequencies, which are also calculated from
first-principles. Only for the models of the random alloys we obtain a black
state, i.e. low reflection and transmission in the energy range from 1 to 6 eV.Comment: 7 pages, 8 figure
Surface Half-Metallicity of CrAs in the Zinc-Blende Structure
The development of new techniques such as the molecular beam epitaxy have
enabled the growth of thin films of materials presenting novel properties.
Recently it was made possible to grow a CrAs thin-film in the zinc-blende
structure. In this contribution, the full-potential screened KKR method is used
to study the electronic and magnetic properties of bulk CrAs in this novel
phase as well as the Cr and As terminated (001) surfaces. Bulk CrAs is found to
be half-ferromagnetic for all three GaAs, AlAs and InAs experimental lattice
constants with a total spin magnetic moment of 3 . The Cr-terminated
surface retains the half-ferromagnetic character of the bulk, while in the case
of the As-termination the surface states destroy the gap in the minority-spin
band.Comment: 4 pages, 2 figures, new text, new titl
A model for the formation energies of alanates and boranates
We develop a simple model for the formation energies (FEs) of alkali and
lkaline earth alanates and boranates, based upon ionic bonding between metal
cations and (AlH4)- or (BH4)- anions. The FEs agree well with values obtained
from first principles calculations and with experimental FEs. The model shows
that details of the crystal structure are relatively unimportant. The small
size of the (BH4)- anion causes a strong bonding in the crystal, which makes
boranates more stable than alanates. Smaller alkali or alkaline earth cations
do not give an increased FE. They involve a larger ionization potential that
compensates for the increased crystal bonding.Comment: 3 pages, 2 figure
Probing Ion-Ion and Electron-Ion Correlations in Liquid Metals within the Quantum Hypernetted Chain Approximation
We use the Quantum Hypernetted Chain Approximation (QHNC) to calculate the
ion-ion and electron-ion correlations for liquid metallic Li, Be, Na, Mg, Al,
K, Ca, and Ga. We discuss trends in electron-ion structure factors and radial
distribution functions, and also calculate the free-atom and metallic-atom
form-factors, focusing on how bonding effects affect the interpretation of
X-ray scattering experiments, especially experimental measurements of the
ion-ion structure factor in the liquid metallic phase.Comment: RevTeX, 19 pages, 7 figure
Slater-Pauling Behavior of the Half-Ferromagnetic Full-Heusler Alloys
Using the full-potential screened Korringa-Kohn-Rostoker method we study the
full-Heusler alloys based on Co, Fe, Rh and Ru. We show that many of these
compounds show a half-metallic behavior, however in contrast to the
half-Heusler alloys the energy gap in the minority band is extremely small.
These full-Heusler compounds show a Slater-Pauling behavior and the total
spin-magnetic moment per unit cell (M_t) scales with the total number of
valence electrons (Z_t) following the rule: M_t=Z_t-24. We explain why the
spin-down band contains exactly 12 electrons using arguments based on the group
theory and show that this rule holds also for compounds with less than 24
valence electrons. Finally we discuss the deviations from this rule and the
differences compared to the half-Heusler alloys.Comment: 10 pages, 8 figures, revised figure 3, new text adde
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