430 research outputs found
Submonolayer growth with decorated island edges
We study the dynamics of island nucleation in the presence of adsorbates
using kinetic Monte Carlo simulations of a two-species growth model. Adatoms
(A-atoms) and impurities (B-atoms) are codeposited, diffuse and aggregate
subject to attractive AA- and AB-interactions. Activated exchange of adatoms
with impurities is identified as the key process to maintain decoration of
island edges by impurities during growth. While the presence of impurities
strongly increases the island density, a change in the scaling of island
density with flux, predicted by a rate equation theory for attachment-limited
growth [D. Kandel, Phys. Rev. Lett. 78, 499 (1997)], is not observed. We argue
that, within the present model, even completely covered island edges do not
provide efficient barriers to attachment.Comment: 7 pages, 2 postscript figure
Re-entrant Layer-by-Layer Etching of GaAs(001)
We report the first observation of re-entrant layer-by-layer etching based on
{\it in situ\/} reflection high-energy electron-diffraction measurements. With
AsBr used to etch GaAs(001), sustained specular-beam intensity oscillations
are seen at high substrate temperatures, a decaying intensity with no
oscillations at intermediate temperatures, but oscillations reappearing at
still lower temperatures. Simulations of an atomistic model for the etching
kinetics reproduce the temperature ranges of these three regimes and support an
interpretation of the origin of this phenomenon as the site-selectivity of the
etching process combined with activation barriers to interlayer adatom
migration.Comment: 11 pages, REVTeX 3.0. Physical Review Letters, in press
Modeling sublimation by computer simulation: morphology dependent effective energies
Solid-On-Solid (SOS) computer simulations are employed to investigate the
sublimation of surfaces. We distinguish three sublimation regimes:
layer-by-layer sublimation, free step flow and hindered step flow. The
sublimation regime is selected by the morphology i.e. the terrace width. To
each regime corresponds another effective energy. We propose a systematic way
to derive microscopic parameters from effective energies and apply this
microscopical analysis to the layer-by-layer and the free step flow regime. We
adopt analytical calculations from Pimpinelli and Villain and apply them to our
model. Key-Words: Computer simulations; Models of surface kinetics; Evaporation
and Sublimation; Growth; Surface Diffusion; Surface structure, morphology,
roughness, and topography; Cadmium tellurideComment: 12 pages, 6 Postscript figures, uses psfig.st
Actes de la conférence conjointe JEP-TALN-RECITAL 2016, volume 09 : ELTAL
National audienceELTAL is a workshop organized within the JEP-TALN-RECITAL 2016 conference. This workshop brings together papers investigating metdods and applications in NLP applied to language teaching.ELTAL est un atelier organisé au sein de la conférence JEP-TALN-RECITAL 2016 et regroupe des contributions qui étudient les méthodes et les applications en TAL dans le domaine de l'enseignement des langues
Hygro-mechanical model for concrete pavement with long-term drying analysis
Concrete pavements are subjected to the combination of moisture transport, heat transport and traffic loading. A hygro-mechanical 3D finite element model was created in OOFEM software in order to analyse the stress field and deformed shape from a long-term non-uniform drying. The model uses a staggered approach, solving moisture transfer weakly coupled with MPS viscoelastic model for ageing concrete creep and shrinkage. Moisture transport and mechanical sub-models are calibrated with lab experiments, long-term monitoring on D1 highway and data from 40 year old highway pavement. The slab geometry is 3.5×5.0×0.29 m, resting on elastic Winkler-Pasternak foundation. The validation covers autogenous and drying strain on the slab. The models predict drying-induced tensile stress up to 3.3 MPa, inducing additional loading on the slab, uncaptured by current design methods
THERMO-MECHANICAL MODEL FOR CONCRETE PAVEMENT
This paper describes a numerical thermo-mechanical model for concrete pavement, implemented in OOFEM software. The thermal part is a heat transfer problem with appropriate initial and boundary conditions (sun irradiation, radiation and convection), calibrated from experimental data. Heat release from cement hydration is also included, calibrated for commonly used cements to demonstrate the difference that can be achieved with the binder selection. The mechanical part of the problem is composed of a 3D elastic concrete slab, subsoil Winkler-Pasternak elements and 1D interface elements, allowing separation in tension. The Winkler-Pasternak constants C1 and C2 were firstly determined from TP170 document and refined later from static load tests on the highway. The model validates well temperature field, static load test and provide several useful insight such as feasible time for summer casting, stress/strain fields and slab separation from the base
Dynamic Bridge Response for a Bridge-friendly Truck
A truck with controlled semi-active suspensions traversing a bridge is examined for benefits to the bridge structure. The original concept of a road-friendly truck was extended to a bridge-friendly vehicle, using the same optimization tools. A half-car model with two independently driven axles is coupled with simply supported bridges (beam, slab model) with the span range from 5 m to 50 m. Surface profile of the bridge deck is either stochastic or in the shape of a bump or a pot in the mid-span. Numerical integration in the MATLAB/SIMULINK environment solves coupled dynamic equations of motion with optimized truck suspensions. The rear axle generates the prevailing load and to a great extent determines the bridge response. A significant decrease in contact road-tire forces is observed and the mid-span bridge deflections are on average smaller, when compared to commercial passive suspensions.
Unstable Growth and Coarsening in Molecular-Beam Epitaxy
The coarsening dynamics of three-dimensional islands on a growing film is
discussed. It is assumed that the origin of the initial instability of a planar
surface is the Ehrlich-Schwoebel step-edge barrier for adatom diffusion. Two
mechanisms of coarsening are identified: (i) surface diffusion driven by an
uneven distribution of bonding energies, and (ii) mound coalescence driven by
random deposition. Semiquantitative estimates of the coarsening time are given
in each case. When the surface slope saturates, an asymptotic dynamical
exponent is obtained.Comment: 12 pages, 4 figure
Behavior of predried mature concrete beams subject to partial wetting and drying cycles
The presented research focuses on the behavior of predried concrete beams with 2.5 m span subjected to cycles of nonsymmetric wetting and drying. Wetting is induced by partially immersing the specimens in a water basin. The submerged portion of the specimens was relatively low (1/5 to 1/10 of their height) leading to a highly nonuniform and nonsymmetric distribution of eigenstrains due to concrete swelling. Unlike conventional experiments on the volume changes of concrete, the measured quantity is not the axial deformation but the vertical displacement instead. This paper presents the experimental data obtained within two wetting and drying cycles, running over 1 year
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