32 research outputs found
Bent surface free energy differences from simulation
We present a calculation of the change of free energy of a solid surface upon
bending of the solid. It is based on extracting the surface stress through a
molecular dynamics simulation of a bent slab by using a generalized stress
theorem formula, and subsequent integration of the stress with respect to
strain as a function of bending curvature. The method is exemplified by
obtaining and comparing free energy changes with curvature of various
reconstructed Au(001) surfaces.Comment: 14 pages, 2 figures, accepted for publication in Surface Science
(ECOSS-19
Magic structures of helical multi-shell zirconium nanowires
The structures of free-standing zirconium nanowires with 0.62.8 nm in
diameter are systematically studied by using genetic algorithm simulations with
a tight-binding many body potential. Several multi-shell growth sequences with
cylindrical structures are obtained. These multi-shell structures are composed
of coaxial atomic shells with the three- and four-strands helical, centered
pentagonal and hexagonal, and parallel double-chain-core curved surface
epitaxy. Under the same growth sequence, the numbers of atomic strands in
inner- and outer-shell show even-odd coupling and usually differ by five. The
size and structure dependence of angular correlation functions and vibrational
properties of zirconium nanowire are also discussed.Comment: 14 pages, 4 figure
Electronic Properties of Ultra-Thin Aluminum Nanowires
We have carried out first principles electronic structure and total energy
calculations for a series of ultrathin aluminum nanowires, based on structures
obtained by relaxing the model wires of Gulseren et al. The number of
conducting channels is followed as the wires radius is increased. The results
suggest that pentagonal wires should be detectable, as the only ones who can
yield a channel number between 8 and 10.Comment: 9 pages + 3 figures, to appear on Surface Scienc
The Puzzling Stability of Monatomic Gold Wires
We have examined theoretically the spontaneous thinning process of
tip-suspended nanowires, and subsequently studied the structure and stability
of the monatomic gold wires recently observed by Transmission Electron
Microscopy (TEM). The methods used include thermodynamics, classical many-body
force simulations, Local Density (LDA) and Generalized Gradient (GGA)
electronic structure calculations as well as ab-initio simulations including
the two tips. The wire thinning is well explained in terms of a thermodynamic
tip suction driving migration of surface atoms from the wire to the tips. For
the same reason the monatomic wire becomes progressively stretched.
Surprisingly, however, all calculations so far indicate that the stretched
monatomic gold wire should be unstable against breaking, contrary to the
apparent experimental stability. The possible reasons for the observed
stability are discussed.Comment: 4 figure
Roughening of close-packed singular surfaces
An upper bound to the roughening temperature of a close-packed singular
surface, fcc Al (111), is obtained via free energy calculations based on
thermodynamic integration using the embedded-atom interaction model. Roughening
of Al (111) is predicted to occur at around 890 K, well below bulk melting (933
K), and it should therefore be observable, save for possible kinetic hindering.Comment: RevTeX 4 pages, embedded figure
Melting behavior of ultrathin titanium nanowires
The thermal stability and melting behavior of ultrathin titanium nanowires
with multi-shell cylindrical structures are studied using molecular dynamic
simulation. The melting temperatures of titanium nanowires show remarkable
dependence on wire sizes and structures. For the nanowire thinner than 1.2 nm,
there is no clear characteristic of first-order phase transition during the
melting, implying a coexistence of solid and liquid phases due to finite size
effect. An interesting structural transformation from helical multi-shell
cylindrical to bulk-like rectangular is observed in the melting process of a
thicker hexagonal nanowire with 1.7 nm diameter.Comment: 4 pages, 4 figure
Self-organization and annealed disorder in a fracturing process
We show that a vectorial model for inhomogeneous elastic media self-organizes under external stress. An onset of crack avalanches of every duration and length scale compatible with the lattice size is observed. The behavior is driven by the introduction of annealed disorder, i.e., by lowering the breaking threshold in the neighborhood of a bond broken by the stress, with a process similar to self-organized criticality. A further comparison with experimental results of acoustic emission (AE), shows that the stability of the elastic potential energy of the system in the AE regime is a sufficient condition for reproducing the algebraic distribution of the energy released during cracks formation
Atomic-scale modeling of the deformation of nanocrystalline metals
Nanocrystalline metals, i.e. metals with grain sizes from 5 to 50 nm, display
technologically interesting properties, such as dramatically increased
hardness, increasing with decreasing grain size. Due to the small grain size,
direct atomic-scale simulations of plastic deformation of these materials are
possible, as such a polycrystalline system can be modeled with the
computational resources available today.
We present molecular dynamics simulations of nanocrystalline copper with
grain sizes up to 13 nm. Two different deformation mechanisms are active, one
is deformation through the motion of dislocations, the other is sliding in the
grain boundaries. At the grain sizes studied here the latter dominates, leading
to a softening as the grain size is reduced. This implies that there is an
``optimal'' grain size, where the hardness is maximal.
Since the grain boundaries participate actively in the deformation, it is
interesting to study the effects of introducing impurity atoms in the grain
boundaries. We study how silver atoms in the grain boundaries influence the
mechanical properties of nanocrystalline copper.Comment: 10 pages, LaTeX2e, PS figures and sty files included. To appear in
Mater. Res. Soc. Symp. Proc. vol 538 (invited paper). For related papers, see
http://www.fysik.dtu.dk/~schiotz/publist.htm