292 research outputs found
Reactions and clustering of water with silica surface
The interaction between silicasurface and water is an important topic in geophysics and materials science, yet little is known about the reaction process. In this study we use first-principles molecular dynamics to simulate the hydrolysis process of silicasurface using large cluster models. We find that a single water molecule is stable near the surface but can easily dissociate at three-coordinated silicon atom defect sites in the presence of other water molecules. These extra molecules provide a mechanism for hydrogen transfer from the original water molecule, hence catalyzing the reaction. The two-coordinated silicon atom is inert to the water molecule, and water clusters up to pentamer could be stably adsorbed at this site at room temperature.Peer reviewe
Tailoring negative pressure by crystal defects: Crack induced hydride formation in Al alloys
Climate change motivates the search for non-carbon-emitting energy generation
and storage solutions. Metal hydrides show promising characteristics for this
purpose. They can be further stabilized by tailoring the negative pressure of
microstructural and structural defects. Using systematic ab initio and
atomistic simulations, we demonstrate that an enhancement in the formation of
hydrides at the negatively pressurized crack tip region is feasible by
increasing the mechanical tensile load on the specimen. The theoretical
predictions have been used to reassess and interpret atom probe tomography
experiments for a high-strength 7XXX-aluminium alloy that show a substantial
enhancement of hydrogen concentration at structural defects near a
stress-corrosion crack tip. These results contain important implications for
enhancing the capability of metals as H-storage materials.Comment: 22 pages, 9 figure
Lattice dynamics and structural stability of ordered Fe3Ni, Fe3Pd and Fe3Pt alloys
We investigate the binding surface along the Bain path and phonon dispersion
relations for the cubic phase of the ferromagnetic binary alloys Fe3X (X = Ni,
Pd, Pt) for L12 and DO22 ordered phases from first principles by means of
density functional theory. The phonon dispersion relations exhibit a softening
of the transverse acoustic mode at the M-point in the L12-phase in accordance
with experiments for ordered Fe3Pt. This instability can be associated with a
rotational movement of the Fe-atoms around the Ni-group element in the
neighboring layers and is accompanied by an extensive reconstruction of the
Fermi surface. In addition, we find an incomplete softening in [111] direction
which is strongest for Fe3 Ni. We conclude that besides the valence electron
density also the specific Fe-content and the masses of the alloying partners
should be considered as parameters for the design of Fe-based functional
magnetic materials.Comment: Revised version, accepted for publication in Physical Review
Surface Grafting of Poly(L-glutamates). 3. Block Copolymerization
This paper describes for the first time the synthesis of surface-grafted AB-block copolypeptides, consisting of poly(γ-benzyl L-glutamate) (PBLG) as the A-block and poly(γ-methyl L-glutamate) (PMLG) as the B-block. Immobilized primary amine groups of (γ-aminopropyl)triethoxysilane (APS) on silicon wafers initiated the ring-opening polymerization of N-carboxyanhydrides of glutamic acid esters (NCAs). After removal of the BLG-NCA monomer solution after a certain reaction time, the amine end groups of the formed PBLG blocks acted as initiators for the second monomers. This method provides the possibility of making layered structures of surface-grafted block copolymers with tuned properties. Ellipsometry and small-angle X-ray reflection (SAXR) measurements revealed the thickness of the polypeptide layers ranging from 45-100 Å of the first block to 140-270 Å for the total block copolypeptides. The chemical composition of the blocks was determined by X-ray photoelectron spectroscopy (XPS). In addition, Fourier transform infrared transmission spectroscopy (FT-IR) revealed that the polypeptide main chains of both blocks consisted of pure R-helices. The average orientation of the helices ranging from 22-42° with respect to the substrate within the first block to 31-35° in the second block could be derived with FT-IR as well.
He II 4686 emission from the massive binary system in Car: constraints to the orbital elements and the nature of the periodic minima
{\eta} Carinae is an extremely massive binary system in which rapid spectrum
variations occur near periastron. Most notably, near periastron the He II
line increases rapidly in strength, drops to a minimum value,
then increases briefly before fading away. To understand this behavior, we
conducted an intense spectroscopic monitoring of the He II
emission line across the 2014.6 periastron passage using ground- and
space-based telescopes. Comparison with previous data confirmed the overall
repeatability of EW(He II ), the line radial velocities, and the
timing of the minimum, though the strongest peak was systematically larger in
2014 than in 2009 by 26%. The EW(He II ) variations, combined
with other measurements, yield an orbital period d. The observed
variability of the EW(He II ) was reproduced by a model in which
the line flux primarily arises at the apex of the wind-wind collision and
scales inversely with the square of the stellar separation, if we account for
the excess emission as the companion star plunges into the hot inner layers of
the primary's atmosphere, and including absorption from the disturbed primary
wind between the source and the observer. This model constrains the orbital
inclination to -, and the longitude of periastron to
-. It also suggests that periastron passage occurred on
d. Our model also reproduced EW(He II )
variations from a polar view of the primary star as determined from the
observed He II emission scattered off the Homunculus nebula.Comment: The article contains 23 pages and 17 figures. It has been accepted
for publication in Ap
Workflow Engineering in Materials Design within the BATTERY 2030+ Project
In recent years, modeling and simulation of materials have become indispensable to complement experiments in materials design. High-throughput simulations increasingly aid researchers in selecting the most promising materials for experimental studies or by providing insights inaccessible by experiment. However, this often requires multiple simulation tools to meet the modeling goal. As a result, methods and tools are needed to enable extensive-scale simulations with streamlined execution of all tasks within a complex simulation protocol, including the transfer and adaptation of data between calculations. These methods should allow rapid prototyping of new protocols and proper documentation of the process. Here an overview of the benefits and challenges of workflow engineering in virtual material design is presented. Furthermore, a selection of prominent scientific workflow frameworks used for the research in the BATTERY 2030+ project is presented. Their strengths and weaknesses as well as a selection of use cases in which workflow frameworks significantly contributed to the respective studies are discussed
Proper weak-coupling approach to the periodic s-d(f) exchange model
The periodic s-d(f) exchange model is characterized by a wide variety of
interesting applications, a simple mathematical structure and a limited number
of reliable approximations which take care of the quantum nature of the
participating spins. We suggest the use of a projection-operator method for
getting information perturbationally, which are not accessible via diagrammatic
approaches. In this paper we present in particular results beyond perturbation
theory, which are obtained such that almost all exactly known limiting cases
are incorporated correctly. We discuss a variety of possible methods and
evaluate their consequences for one-particle properties. These considerations
serve as a guideline for a more effective approach to the model.Comment: 11 pages, 6 figures; accepted by Phys. Rev.
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