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AUTHOR
Olivier Politano
showing 33 related works from this author
A 3D mesoscopic approach for discrete dislocation dynamics
2001
In recent years a noticeable renewed interest in modeling dislocations at the mesoscopic scale has been developed leading to significant advances in the field. This interest has arisen from a desire to link the atomistic and macroscopic length scales. In this context, we have recently developed a 3D-discrete dislocation dynamics model (DDD) based on a nodal discretization of the dislocations. We present here the basis of our DDD model and two examples of studies with single and multiple slip planes.
Numerical Studies of the Diffusion Processes and First Step Oxidation in Nickel-Oxygen Systems by Variable Charge Molecular Dynamics
2010
Variable charge molecular dynamic simulations have been performed to study the diffusion mechanisms of oxygen atoms (O) in nickel (Ni) in the temperature range 950-1600 K and the very first steps of oxidation of monocrystalline nickel surfaces at 300 K and 950 K. The oxygen diffusivity can be well described by an Arrhenius law over the temperature range considered. The oxygen diffusion coefficient has been analysed and values of Ea = 1.99 eV for the activation energy and D0 = 39 cm2.s-1 for the pre-exponential factor were obtained. The first steps growth of the oxide layer show that after the dissociative chemisorption of the oxygen molecules on nickel surface, the oxidation leads to an isl…
Grain size, stress and surface roughness
2008
In this article, we report molecular dynamics (MD) simulations on the formation of roughness at the surface of strained polycrystalline aluminum samples at 300 K. The computed roughness increases as a function of applied strain but does not follow a linear law for all applied strains. A linear relationship with a small slope is obtained in the elastic domain. Then, the roughness increases rapidly with the applied strain in the plastic domain studied. Moreover, the surface roughness increases as a function of grain size (between 5 and 20 nm) in the plastic domain (<6%).
On the dynamics of dislocation patterning
1997
Recent computer simulations on dislocation patterning have provided remarkable results in accordance with empirical laws. Moreover, several analytical models on dislocation dynamics have provided qualitative insight on dislocation patterning. However, a model, based on partial differential equations, which gives a dynamical evolution of dislocation patterns in function of measurable variables still missing. Here, we give a re-formulation of a model proposed some years ago. From this formulation, we obtained that the onset of a dislocation instability is related to the applied stress. The analytical and numerical results reported are partial and studies on this direction are under developmen…
Variable-charge method applied to study coupled grain boundary migration in the presence of oxygen
2009
International audience; One of the important differences between simulation and experiments in grain boundary (GB)-dominated metallic structures is the lack of impurities such as oxygen in computational samples. A modified variable-charge method [Elsener A, Politano O, Derlet PM, Van Swygenhoven H. Modell Simul Mater Sci Eng 2008;16:025006] based on the Streitz and Mintmire approach [Streitz FH, Mintmire JW. Phys Rev B 1994;50:11996] is used to study coupled GB motion in an Al bicrystal with a [1 1 2] symmetrical tilt GB in the presence of substitutional O, and compared with the stick–slip process identified by Cahn and Mishin [Cahn JW, Mishin Y, Suzuki A. Acta Mater 2006;54:4953]. It is found…
Formation of surface roughness on nanocrystalline aluminum samples under straining by molecular dynamics studies
2006
International audience; The surface roughening of nanocrystalline aluminum samples was investigated by molecular dynamics simulations. Attention was focused on the fact that roughness increases with the grain size and the strain. The elastic-plastic transition was found at around 3.5% strain and a reverse Hall-Petch effect was observed under straining conditions. Then, different strain distributions in grains and grain boundaries at the samples surface was highlighted, yielding to the formation of local roughness. Finally, a linear relationship between the magnitude of roughness and the out-of-plane strain component was found.
Numerical and theoretical considerations on the surface energy for pure solids under strain
2004
In this paper we developed a numerical analysis, by means of molecular dynamics (MD) simulations, for the surface energy of solids when a stress is applied parallel to the surface. Our MD simulations for Al showed that under these conditions; compression or an alternation of compression and tension, with respect to the bulk, of some atomic layers below the surface is present. Moreover, we quantified the surface energy variations that led us to propose an empirical model.
Structure evolution and reaction mechanism in the Ni/Al reactive multilayer nanofoils
2014
Abstract The extremely rapid gasless reactions in binary Ni/Al reactive multilayer nanofoils (RMNFs) are investigated both experimentally and theoretically. The quenching technique is used to study the dynamics of structural transformation on the micro- and nanoscales. The experimentally obtained patterns of structural evolution related to the heterogeneous reactions are compared with the results of molecular dynamics simulations carried out for the same reactive system. Based on the obtained data, the intrinsic mechanism of the reaction in Ni/Al RMNF is considered, which adequately explains the unusual parameters of a gasless combustion wave in such a system.
An empirical model for free surface energy of strained solids at different temperature regimes.
2006
Abstract We have developed an empirical formulation, based on the elastic theory, to calculate the variation of the surface free energy when a crystal is strained in the elastic regime. The model permits to obtain the variation of the surface energy at different strains and temperatures when are known the thermal dependence on the bulk and surface elastic constants. Molecular dynamics (MD) simulations were performed using the three low index surfaces of Al, to validate the accuracy of the model. The comparison between the empirical model and the MD simulations shows a good agreement for temperatures ranging between 0 and 900 K, and for deformation between −2% and 2%.
Nanoscale oxide growth on Al single crystals at low temperatures: Variable charge molecular dynamics simulations
2006
We investigate the oxidation of aluminum low-index surfaces [(100), (110), and (111)] at low temperatures (300-600 K) and three different gas pressure values. We use molecular dynamics (MD) simulations with dynamic charge transfer between atoms where the interaction between atoms is described by the Es+ potential composed of the embedded atom method (EAM) potential and an electrostatic contribution. In the considered temperature range and under different gas pressure conditions, the growth kinetics follow a direct logarithmic law where the oxide thickness is limited to a value of ∼3 nm. The fitted curves allow us to determine the temperature and the pressure dependencies of the parameters i…
Investigation of Solid State Diffusion Processes Involved in the Zinc Oxide Sulfidation Reaction
2016
Sulfidation of undoped and aluminum doped zinc oxide materials has been performed by TGA under a H2S atmosphere in order to evaluate the impact of the doping element on sulfidation reaction kinetics and mechanism. The presence of aluminum seems to slow-down the reaction kinetics. This phenomenon might be explained by a modification of the solid state diffusion processes involved in ZnO sulfidation reaction and the related ZnS outward growth, assuming the presence of aluminum atoms inside ZnO and ZnS phases. In order to determine solid state diffusion mechanisms controlling the reaction kinetics, molecular dynamics simulations were performed using a Coulomb-Buckingham potential. Firstly, the…
Numerical Determination of Intrinsic Diffusion Coefficient of Aluminide Coatings on Metals
2009
This paper presents a numerical method to determine the composition dependent diffusivities and to predict the concentration profile during the interdiffusion process. The intrinsic diffusion coefficients in diffusion aluminide coatings (Fe-Al) were determined at 1000oC. The obtained diffusion coefficient for iron in Fe3Al or FeAl is in the range 10-10 to 10-9 cm2.s-1. The aluminum diffusion coefficient varies from 10-11 to 10-7 cm2.s-1 in the same phases.The present approach also permits to model the reactive diffusion in the Fe-Al systems.
Explosive crystallization in amorphous CuTi thin films: a molecular dynamics study
2019
Abstract Molecular dynamic simulation was used to study mechanism of self-propagating waves of explosive crystallization (devitrification) in the CuTi metallic glass. Processes in thin rectangular samples composed of one to two million atoms were simulated and compared with experimental data. It was shown that the nucleation of primary crystalline clusters occurs homogeneously due to spontaneous fluctuations of atomic structure; the clusters not
Diffusion of oxygen in nickel: A variable charge molecular dynamics study
2010
Abstract Variable charge molecular dynamics have been performed to study the diffusion mechanisms of oxygen atoms (O) in nickel (Ni) in the temperature range 950 K–1600 K. It is observed that oxygen does not diffuse via jumps of oxygen through interstitial sites but via a vacancy diffusion mechanism. The oxygen diffusivity can be well described by an Arrhenius law over the temperature range considered. The oxygen diffusion coefficient has been analysed and indicates a value of E a = 1.99 eV for the activation energy and D 0 = 39.2 cm 2 s − 1 for the pre-exponential factor. Our numerical results were compared with a compilation of experimental and theoretical studies, and exhibit a good agre…
An empirical method to determine the free surface energy of solids at different deformations and temperatures regimes : An application to Al.
2005
Abstract We have performed molecular dynamics (MD) simulations using the three low index surfaces of Al to determine the variation of the surface energy as a function of deformation and temperature. We have also developed an empirical formulation for the surface free energy as a function of deformation. The observed difference between the numerical and analytical results has led us to divide the deformation into a mechanical and a thermal contribution. From this observation, we have obtained an expression for the surface free energy placing the temperature dependence on the bulk and surface elastic constants. Our simulations permitted us to analyze the multilayer relaxation for the particul…
Determination of the stress distribution at the interface metal-oxide: Numerical and theoretical considerations
2005
In this paper we give a brief presentation of the approaches we have recently developed on the oxidation of metals. Firstly, we present an analytical model based on non-equilibrium thermodynamics to describe the reaction kinetics present during the oxidation of a metal. Secondly, we present the molecular dynamics results obtained with a code specially tailored to study the oxidation and growth of an oxide film of aluminium. Our simulations present an excellent agreement with experimental results.
Molecular dynamics simulations of nanometric metallic multilayers: Reactivity of the Ni-Al system
2011
The reactivity of a layered Ni-Al-Ni system is studied by means of molecular dynamics simulations, using an embedded-atom method type potential. The system, made of an fcc-Al layer embedded in fcc-Ni, is initially thermalized at the fixed temperature of 600 K. The early interdiffusion of Ni and Al at interfaces is followed by the massive diffusion of Ni in the Al layer and by the spontaneous phase formation of $B2$-NiAl. The solid-state reaction is associated with a rapid system heating, which further enhances the diffusion processes. For longer times, the system may partly lose some its $B2$-NiAl microstructure in favor of the formation of $L{1}_{2}$-${\mathrm{Ni}}_{3}\mathrm{Al}$. This st…
Quenched molecular dynamics studies on the extraction energy of aluminum atoms
2007
The extraction energy of an aluminum atom is calculated at 0 K as a function of coordination number and defect depth for three surface orientations [(100), (110) and (111)]. For each orientation, atoms are selected and extracted one by one. A linear relationship is obtained between the extraction energy of surface atoms and their coordination numbers (with slight variations due to the geometrical configuration of the atoms). However, the study of the influence of the defect depth on the extraction energy highlights the role played by intrinsic stress on the extraction energy. Copyright © 2008 John Wiley & Sons, Ltd.
Oxidation of nanocrystalline aluminum by variable charge molecular dynamics
2010
International audience; We investigate the oxidation of nanocrystalline aluminum surfaces using molecular dynamics (MD) simulations with the variable charge model that allows charge dynamically transfer among atoms. The interaction potential between atoms is described by the electrostatic plus (Es+) potential model, which is composed of an embedded atom method potential and an electrostatic term. The simulations were performed from 300 to 750K on polycrystalline samples with a mean grain size of 5 nanometers. We mainly focused on the effect of the temperature parameter on the oxidation kinetic. The results show that, beyond a first linear regime, the kinetics follow a direct logarithmic law…
Preface for MMM 2016 focus issue
2017
International audience
ReaxFF molecular dynamics simulation study of nanoelectrode lithography oxidation process on silicon (100) surface
2019
Abstract The nanoelectrode lithography has been strengthened in recent years as one of the most promising methods due to its high reproducibility, low cost and ability to manufacture nano-sized structures. In this work, the mechanism and the parametric influence in nanoelectrode lithography have been studied qualitatively in atomic scale using ReaxFF MD simulation. This approach was originally developed by van Duin and co-workers to investigate hydrocarbon chemistry. We have investigated the water adsorption and dissociation processes on Si (100) surface as well as the characteristics (structure, chemical composition, morphology, charge distribution, etc.) of the oxide growth. The simulatio…
Molecular dynamics simulations of the nano-scale room-temperature oxidation of aluminum single crystals
2005
The oxidation of aluminum single crystals is studied using molecular dynamics (MD) simulations with dynamic charge transfer between atoms. The simulations are performed on three aluminum low-index surfaces ((1 0 0), (1 1 0) and (1 1 1)) at room temperature. The results show that the oxide film growth kinetics is independent of the crystallographic orientation under the present conditions. Beyond a transition regime (100 ps) the growth kinetics follow a direct logarithmic law and present a limiting thickness of 3 nm. The obtained amorphous structure of the oxide film has initially Al excess (compared to the composition of Al2O3) and evolves, during the oxidation process, to an Al percentage …
Electronic structure and energy decomposition analyses as a tool to interpret the redox potential ranking of naphtho-, biphenyl- and biphenylenequino…
2016
By calling on modelling approaches we have performed a comparative study on the redox properties of various naphtho-, biphenyl- and biphenylene-quinone isomers. These different compounds exhibit as a whole a redox potential range between 2.09 and 2.90 V vs. Li+/Li. A specific methodology was used to decrypt the interplay among isomerism, aromaticity and antiaromaticity modifications and the stabilization/destabilization effects due to other molecular components on this key electrochemical feature for electrode materials of batteries. In particular, energy decomposition analysis, within the Quantum Theory of Atoms in Molecules, along with the electron and electron spin population changes upo…
Study of the reactive dynamics of nanometric metallic multilayers using Molecular Dynamics: the Al−Ni system
2012
A molecular dynamics study of a layered Ni-Al-Ni system is developed using an embedded atom method potential. The specific geometry is designed to model a Ni-Al nanometric metallic multilayer. The system is initially thermalized at the fixed temperature of 600 K. We first observe the interdiffusion of Ni and Al at the interfaces, which is followed by the spontaneous phase formation of B2-NiAl in the Al layer. The solid-state reaction is associated with a rapid system's heating which further enhances the diffusion processes. NiAl phase is organized in small regions separated by grain boundaries. This study confirms the hypothesis of a layer-by-layer development of the new phase. For longer t…
Molecular dynamics study of high-pressure alumina polymorphs with a tight-binding variable-charge model
2016
Abstract A tight-binding variable-charge model aimed at performing large-scale realistic simulations of bulk, surfaces and interfaces of aluminum oxides have been developed. This model is based on the charge equilibration (QEq) method and explicitly takes into account the mixed iono–covalent character of the metal–oxygen bond by means of a tight-binding analytical approach in the second-moment approximation of the electronic structure. The parameters of the model were optimized to reproduce structural and energetic properties of the α-Al2O3 corundum structure at room temperature and pressure. The model exhibits a good transferability between five alumina polymorphs: corundum, Rh2O3(II)-type…
A local chemical potential approach within the variable charge method formalism
2008
A new and computationally efficient implementation of the variable charge method of Streitz and Mintmire (1994 Phys. Rev. B 50 11996) is presented. In particular a local chemical potential approach that optimizes the charge on only those atoms expected to be ionic is developed. By doing so, the charge fluctuation problem experienced in regions far from any oxygen is solved, leading to a linear minimization problem of the electrostatic energy. In the dilute oxygen limit, such an approach can lead to at least an order of magnitude saving in computation.
Numerical Determination of Intrinsic Diffusion in Fe-Cr-Al Systems
2010
The intrinsic diffusion coefficients in diffusion aluminide coatings based on Fe-30Cr were determined at 1000oC. The diffusion fluxes were given by the Nernst Planck formulae and the Darken method for multicomponent systems was applied. This paper summarizes some numerical results to determine the composition dependent diffusivities in Fe-Cr-Al systems. The method presented in this study to obtain average intrinsic diffusion coefficients is as an alternative to the Dayananda method. Our method based on empirical parameters allowed us to predict the concentration profile during the interdiffusion process.
Modeling self-sustaining waves of exothermic dissolution in nanometric Ni-Al multilayers
2016
Abstract The self-sustained propagating reaction occurring in nanometric metallic multilayers was studied by means of molecular dynamics (MD) and numerical modeling. We focused on the phenomenon of the exothermic dissolution of one metallic reactant into the less refractory one, such as Ni into liquid Al. The exothermic character is directly related to a negative enthalpy of mixing. An analytical model based on the diffusion-limited dissolution [1] coupled with heat transfer was derived to account for the main aspects of the process. Together, several microscopic simulations were carried out. The first series were set up to obtain all the parameters governing the process, including the heat…
Microstructure development during NiAl intermetallic synthesis in reactive Ni–Al nanolayers: Numerical investigations vs. TEM observations
2013
Abstract Heterogeneous reactions leading to the formation of intermetallic compounds in nanometric Ni–Al multilayer system are examined both numerically and experimentally. On the numerical side, the reactivity of a layered Ni–Al–Ni system is studied by means of molecular dynamics simulations, using an embedded-atom method (EAM) potential. The mechanism of nucleation and growth of the intermetallic phase is determined. Four main stages in the reactive process, which lead to the formation of rounded shape grains of intermetallic phase at the Ni–Al interface, are delineated. On the experimental side, TEM imaging of quenched samples revealed the behavior of the Ni–Al reactive foils and showed …
Dynamical features of forest interactions
2000
Abstract The 3D computer simulations presented here were developed to study at the mesoscopic scale the formation of junctions and their impact on hardening of crystals. The simulations consider the evolution of a dislocation interacting with immobile dislocations in a fcc single crystal of copper where we incorporate well known dislocation interaction mechanisms. From these studies, we deduced a `breaking angle' which characterize the strength of the junctions.
Electrochemical properties of crystallized dilithium squarate: insight from dispersion-corrected density functional theory.
2012
International audience; The stacking parameters, lattice constants, and bond lengths of solvent-free dilithium squarate (Li(2)C(4)O(4)) crystals were investigated using density functional theory with and without dispersion corrections. The shortcoming of the GGA (PBE) calculation with respect to the dispersive forces appears in the form of an overestimation of the unit cell volume up to 5.8%. The original Grimme method for dispersion corrections has been tested together with modified versions of this scheme by changing the damping function. One of the modified dispersion-corrected DFT schemes, related to a rescaling of van der Waals radii, provides significant improvements for the descripti…
A variable charge molecular dynamics study of the initial stage of nickel oxidation
2010
Abstract The oxidation of nickel single crystals is investigated by using variable charge molecular dynamics. The simulations are performed on three nickel low-index surfaces ((1 0 0), (1 1 0) and (1 1 1)) at temperatures between 300 K and 950 K. The results show that the shape of the oxidation kinetics is independent of the crystallographic orientation and the temperature under the present conditions. The oxide thin film grows according to an island growth mode, this initial stage of oxidation can be divided in three steps: (i) the dissociative chemisorption step (ii) the oxide island nucleation and (iii) the lateral growth of the island. The first step is slowdown/speedup by the surface o…
Numerical Simulations on the Growth of Thin Oxide Films on Aluminum Substrates
2010
We investigated the oxidation of nanocrystalline aluminum surfaces by using variable charge molecular dynamics at 600 K under three oxygen pressures: 1, 10 and 20 atm. The interaction potential was described by the electrostatic plus (Es+) model that allows dynamical charge transfer among atoms. We mainly focused on the effect of the oxygen pressure on the oxidation kinetic, the chemical composition and the microstructure of the oxide films formed. The results show that oxidation kinetics as well as chemical composition and microstructure depend on the applied oxygen pressure. The oxide film thickness tends to a limiting value equal to ~3 nm. Finally, we obtained a partially crystalline oxi…