80 research outputs found
Diffusion of gold nanoclusters on graphite
We present a detailed molecular-dynamics study of the diffusion and
coalescence of large (249-atom) gold clusters on graphite surfaces. The
diffusivity of monoclusters is found to be comparable to that for single
adatoms. Likewise, and even more important, cluster dimers are also found to
diffuse at a rate which is comparable to that for adatoms and monoclusters. As
a consequence, large islands formed by cluster aggregation are also expected to
be mobile. Using kinetic Monte Carlo simulations, and assuming a proper scaling
law for the dependence on size of the diffusivity of large clusters, we find
that islands consisting of as many as 100 monoclusters should exhibit
significant mobility. This result has profound implications for the morphology
of cluster-assembled materials
Quantum Conductance in Silver Nanowires: correlation between atomic structure and transport properties
We have analyzed the atomic arrangements and quantum conductance of silver
nanowires generated by mechanical elongation. The surface properties of Ag
induce unexpected structural properties, as for example, predominance of high
aspect ratio rod-like wires. The structural behavior was used to understand the
Ag quantum conductance data and the proposed correlation was confirmed by means
of theoretical calculations. These results emphasize that the conductance of
metal point contacts is determined by the preferred atomic structures and, that
atomistic descriptions are essential to interpret the quantum transport
behavior of metal nanostructures.Comment: 4 pages, 4 figure
Ab initio Calculations of Multilayer Relaxations of Stepped Cu Surfaces
We present trends in the multilayer relaxations of several vicinals of
Cu(100) and Cu(111) of varying terrace widths and geometry. The electronic
structure calculations are based on density functional theory in the local
density approximation with norm-conserving, non-local pseudopotentials in the
mixed basis representation. While relaxations continue for several layers, the
major effect concentrates near the step and corner atoms. On all surfaces the
step atoms contract inwards, in agreement with experimental findings.
Additionally, the corner atoms move outwards and the atoms in the adjacent
chain undergo large inward relaxation. Correspondingly, the largest contraction
(4%) is in the bond length between the step atom and its bulk nearest neighbor
(BNN), while that between the corner atom and BNN is somewhat enlarged. The
surface atoms also display changes in registry of upto 1.5%. Our results are in
general in good agreement with LEED data including the controversial case of
Cu(511). Subtle differences are found with results obtained from semi-empirical
potentials.Comment: 21 pages and 3 figure
Model Hessian for accelerating first-principles structure optimizations
We present two methods to accelerate first-principles structural relaxations,
both based on the dynamical matrix obtained from a universal model of springs
for bond stretching and bending. Despite its simplicity, the normal modes of
this model Hessian represent excellent internal coordinates for molecules and
solids irrespective of coordination, capturing not only the long-wavelength
acoustic modes of large systems, but also the short-wavelength low-frequency
modes that appear in complex systems. In the first method, the model Hessian is
used to precondition a conjugate gradients minimization, thereby drastically
reducing the effective spectral width and thus obtaining a substantial
improvement of convergence. The same Hessian is used in the second method as a
starting point of a quasi-Newton algorithm (Broyden's method and modifications
thereof), reducing the number of steps needed to find the correct Hessian.
Results for both methods are presented for geometry optimizations of clusters,
slabs, and biomolecules, with speed-up factors between 2 and 8.Comment: 5 pages, 3 figures submites to Phys. Rev.
Interatomic potentials for atomistic simulations of the Ti-Al system
Semi-empirical interatomic potentials have been developed for Al, alpha-Ti,
and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large
database of experimental as well as ab-initio data. The ab-initio calculations
were performed by the linear augmented plane wave (LAPW) method within the
density functional theory to obtain the equations of state for a number of
crystal structures of the Ti-Al system. Some of the calculated LAPW energies
were used for fitting the potentials while others for examining their quality.
The potentials correctly predict the equilibrium crystal structures of the
phases and accurately reproduce their basic lattice properties. The potentials
are applied to calculate the energies of point defects, surfaces, planar faults
in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al
system, the proposed potentials provide reasonable description of the lattice
thermal expansion, demonstrating their usefulness in the molecular dynamics or
Monte Carlo studies at high temperatures. The energy along the tetragonal
deformation path (Bain transformation) in gamma-TiAl calculated with the EAM
potential is in a fairly good agreement with LAPW calculations. Equilibrium
point defect concentrations in gamma-TiAl are studied using the EAM potential.
It is found that antisite defects strongly dominate over vacancies at all
compositions around stoichiometry, indicating that gamm-TiAl is an antisite
disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press
Selective Growth of Low Stored Energy Grains During δ Sub-solvus Annealing in the Inconel 718 Nickel-Based Superalloy
The microstructure stability during δ sub-solvus annealing in Inconel 718 was investigated, focusing on the conditions that may lead to the development of very large grains (about 100 μm) in a recrystallized fine grained matrix (4 to 5 μm) despite the presence of second-phase particles. Microstructure evolution was analyzed by EBSD (grain size, intragranular misorientation) and SEM (δ phase particles). Results confirm that, in the absence of stored energy, the grain structure is controlled by the δ phase particles, as predicted by the Smith–Zener equation. If the initial microstructure is strained (ε < 0.1) before annealing, then low stored energy grains grow to a large extent, despite the Zener pinning forces exerted by the second-phase particles on the grain boundaries. Those selectively growing grains could be those of the initial microstructure that were the least deformed, or they could result from a nucleation process. The balance of three forces acting on boundary migration controls the growth process: if the sum of capillarity and stored energy driving forces exceeds the Zener pinning force, then selective grain growth occurs. Such phenomenon could be simulated, using a level set approach in a finite element context, by taking into account the three forces acting on boundary migration and by considering a realistic strain energy distribution (estimated from EBSD measurements). © 2015, The Minerals, Metals & Materials Society and ASM International
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