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Murina E.L.,CAC CNEA | Murina E.L.,CONICET | Fernandez-Prini R.,CAC CNEA | Fernandez-Prini R.,CONICET | Pastorino C.,CONICET
Journal of Chemical Physics | Year: 2017

We studied the behavior of long chain alkanes (LCAs) as they were transferred from gas to bulk water, through the liquid-vapor interface. These systems were studied using umbrella sampling molecular dynamics simulation and we have calculated properties like free energy profiles, molecular orientation, and radius of gyration of the LCA molecules. The results show changes in conformation of the solutes along the path. LCAs adopt pronounced molecular orientations and the larger ones extend appreciably when partially immersed in the interface. In bulk water, their conformations up to dodecane are mainly extended. However, larger alkanes like eicosane present a more stable collapsed conformation as they approach bulk water. We have characterized the more probable configurations in all interface and bulk regions. The results obtained are of interest for the study of biomatter processes requiring the transfer of hydrophobic matter, especially chain-like molecules like LCAs, from gas to bulk aqueous systems through the interface. © 2017 Author(s).

Baque L.,CONICET | Lamas D.G.,CONICET | Arico S.F.,CAC CNEA | Craievich A.F.,University of Sao Paulo | Sommadossi S.,CONICET
Calphad: Computer Coupling of Phase Diagrams and Thermochemistry | Year: 2013

The Cu-In-Sn system is one of the Pb-free options to replace conventional Pb-Sn alloys in electronic industry. However, controversies still exist regarding some regions of the equilibrium phase diagram of the Cu-In-Sn ternary and also of the Cu-In and Cu-Sn binary systems. One of the most controversial fields of the Cu-In binary phase diagram lies between ~33 and 38 at% In and temperatures ranging from 100 up to 500 C. In this work, binary Cu-In alloys, with 30-37 at% In nominal compositions and annealed at two different temperatures (i.e. 300 C and 500 C) for a long period (i.e. 7 months) were characterized by scanning electron microscopy (SEM), wavelength dispersive spectroscopy (WDS) and X-ray diffraction (XRD). Three phases exist over the 33-38 at% In composition range, namely the phase A at high temperatures and the B and C phases at low temperatures. These three phases can be described as superstructures of the hexagonal phase h (Cu2In) and differ, although slightly, from those previously reported in the literature. In addition, it has been demonstrated that even conventional XRD allows to unequivocally distinguishing between these phases despite their similar crystal structure. © 2013 Published by Elsevier Ltd.

Di Napoli S.,CAC CNEA | Di Napoli S.,CONICET | Thiess A.,Jülich Research Center | Blugel S.,Jülich Research Center | Mokrousov Y.,Jülich Research Center
Journal of Physics Condensed Matter | Year: 2012

In this work we present the generalization of the model for chain formation in break junctions, introduced by Thiess etal (2008 Nano Lett. 8 2144), to zigzag transition-metal chains with s and p impurities. We apply this extended model to study the producibility trends for noble-metal chains with impurities, often present in break junction experiments, namely, Cu, Ag and Au chains with H, C, O and N adatoms. Providing the material-specific parameters for our model from systematic full-potential linearized augmented plane-wave first-principles calculations, we find that the presence of such impurities crucially affects the binding properties of the noble-metal chains. We reveal that both the impurity-induced bond strengthening and the formation of zigzag bonds can lead to a significantly enhanced probability for chain formation in break junctions. © 2012 IOP Publishing Ltd.

Bonny G.,Belgian Institute for Nuclear Sciences | Pasianot R.C.,CAC CNEA | Pasianot R.C.,CONICET | Zhurkin E.E.,Saint Petersburg State University | Hou M.,Free University of Colombia
Computational Materials Science | Year: 2011

Prior to applying any interatomic potential, it is important to know the stability of the different phases it describes. In the literature many methods to determine the phase diagram from an interatomic potential are described. Although for pure elements the procedure to obtain the thermodynamic functions is well established, for alloys it is not. In this work a method is developed to determine the phase diagram, i.e., solubility limits and spinodal gap, for the case of miscibility gaps. The method combines Monte Carlo simulations in the isobaric semi-grand canonical ensemble, full thermodynamic integration and Redlich-Kister expansions to parameterize the Gibbs free energy. Besides numerical inaccuracies, this method does not rely on any physical approximations to determine the phase diagram of a given interatomic potential. The method is applied to two different Fe-Cr potentials that are widely used in the literature. The resulting phase diagrams are discussed by comparing them to the experimental one and ones obtained in other works from the same potentials. © 2011 Elsevier B.V. All rights reserved.

Terentyev D.,Belgian Institute for Nuclear Sciences | Bonny G.,Belgian Institute for Nuclear Sciences | Castin N.,Belgian Institute for Nuclear Sciences | Domain C.,Électricité de France | And 5 more authors.
Journal of Nuclear Materials | Year: 2011

In this paper we review the current status of our efforts to model the Fe-Cr system, which is a model alloy for high-Cr ferritic-martensitic steels, using large-scale atomistic methods. The core of such methods are semi-empirical interatomic potentials. Here we discuss their performance with respect to the features that are important for an accurate description of radiation effects in Fe-Cr alloys. We describe their most recent improvements regarding macroscopic thermodynamic properties as well as microscopic point-defect properties. Furthermore we describe a new type of large-scale atomistic kinetic Monte Carlo (AKMC) approach driven by an artificial neural network (ANN) regression method to generate the local migration barrier for a defect accounting for the local chemistry around it. The results of the thermal annealing of the Fe-20Cr alloy modelled using this AKMC approach, parameterized by our newly developed potential, were found to be in very good agreement with experimental data. Furthermore the interaction of a 1/2 〈1 1 1〉 screw dislocation with Cr precipitates as obtained from the AKMC simulations was studied using the same potential. In summary, we critically discuss our current achievements, findings and outline issues to be addressed in the near future development. © 2010 Elsevier B.V. All rights reserved.

Djurabekova F.,Belgian Institute for Nuclear Sciences | Djurabekova F.,Helsinki Institute of Physics | Malerba L.,Belgian Institute for Nuclear Sciences | Pasianot R.C.,CAC CNEA | And 3 more authors.
Philosophical Magazine | Year: 2010

Monte Carlo models are widely used for the study of microstructural and microchemical evolution of materials under irradiation. However, they often link explicitly the relevant activation energies to the energy difference between local equilibrium states. We provide a simple example (di-vacancy migration in iron) in which a rigorous activation energy calculation, by means of both empirical interatomic potentials and density functional theory methods, clearly shows that such a link is not granted, revealing a migration mechanism that a thermodynamics-linked activation energy model cannot predict. Such a mechanism is, however, fully consistent with thermodynamics. This example emphasizes the importance of basing Monte Carlo methods on models where the activation energies are rigorously calculated, rather than deduced from widespread heuristic equations. © 2010 Taylor & Francis.

Bonny G.,Belgian Institute for Nuclear Sciences | Pasianot R.C.,CAC CNEA | Pasianot R.C.,CONICET | Pasianot R.C.,Instituto Sabato
Philosophical Magazine Letters | Year: 2010

Many-body interatomic potentials play an important role in atomistic modelling of materials. For pure elements it is known that there exist gauge transformations that can change the form of the potential functions without modifying its properties. These same transformations, however, fail when applied to alloys. Even though different research groups may use the same potentials to describe pure elements, the gauges employed for fitting alloys will generally be different. In this scenario, it is a priori impossible to merge them into one potential describing the combined system, and thus no advantage is taken from state-of-the-art developments in the literature. Here, we generalise the gauge transformations applied to pure species in order to leave the properties of alloys invariant. Based on these transformations, a strategy to merge potentials developed within different gauges is presented, aiming at the description of the combined system. Advantage of existing state-of-the-art potentials is so taken, thus focusing the efforts on fitting only the missing interactions. Such a procedure constitutes a helpful tool for the development of potentials targeted to alloys of increased complexity, while maintaining the description quality of their constituents. © 2010 Taylor and Francis.

Bonny G.,Belgian Institute for Nuclear Sciences | Pasianot R.C.,CAC CNEA | Pasianot R.C.,CONICET | Terentyev D.,Belgian Institute for Nuclear Sciences | Malerba L.,Belgian Institute for Nuclear Sciences
Philosophical Magazine | Year: 2011

We present an Fe-Cr interatomic potential to model high-Cr ferritic alloys. The potential is fitted to thermodynamic and point-defect properties obtained from density functional theory (DFT) calculations and experiments. The developed potential is also benchmarked against other potentials available in literature. It shows particularly good agreement with the DFT obtained mixing enthalpy of the random alloy, the formation energy of intermetallics and experimental excess vibrational entropy and phase diagram. In addition, DFT calculated point-defect properties, both interstitial and substitutional, are well reproduced, as is the screw dislocation core structure. As a first validation of the potential, we study the precipitation hardening of Fe-Cr alloys via static simulations of the interaction between Cr precipitates and screw dislocations. It is concluded that the description of the dislocation core modification near a precipitate might have a significant influence on the interaction mechanisms observed in dynamic simulations. © 2011 Taylor & Francis.

Ramunni V.P.,CONICET | Hurtado-Norena C.,CONICET | Bruzzoni P.,CAC CNEA
Physica B: Condensed Matter | Year: 2012

We calculate the equilibrium energies and migration barriers of Fe, Cr and H interstitial defects in α-FeX(X=Cr). We use the ab-initio electronic structure code, SIESTA, coupled to the monomer method to find activated states (or migration barriers), in order to study atomic defects diffusion. Ab-initio calculations reveal that in the presence of Cr the H migration barriers are higher than in pure α-Fe. On the other hand, our permeation tests on 9%Cr-91%Fe alloys reveal a permeation coefficient 10 times lower and a diffusion coefficient 200 times lower than in pure, annealed iron. Focusing on our experimental results, we explore very simple model of new H trapping sites and possible migration paths that can explain the experimental observations. © 2011 Elsevier B.V. All rights reserved.

Type: Journal Article | Journal: Physical review. E | Year: 2016

We characterize the fluctuation properties of a polymer chain under external tension and the fluctuation-induced forces between two ring molecules threaded around the chain. The problem is relevant in the context of fluctuation-induced forces in soft-matter systems, features of liquid interfaces, and to describe the properties of polyrotaxanes and slide-ring materials. We perform molecular-dynamics simulations of the Kremer-Grest bead-spring model for the polymer and a simple ring-molecule model in the canonical ensemble. We study transverse fluctuations of the stretched chain as a function of chain stretching and in the presence of ring-shaped threaded molecules. The fluctuation spectra of the chains are analyzed in equilibrium at constant temperature, and the differences in the presence of two-ring molecules are compared. For the rings located at fixed distances, we find an attractive fluctuation-induced force between the rings, proportional to the temperature and decaying with the ring distance. We characterize this force as a function of ring distance, chain stretching, and ring radius, and we measure the differences between the free chain spectrum and the fluctuations of the chain constrained by the rings. We also compare the dependence and range of the force found in the simulations with theoretical models coming from different fields.

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