58 research outputs found
Anomalous Surface Segregation Profiles in Ferritic FeCr Stainless Steel
The iron-chromium alloy and its derivatives are widely used for their
remarkable resistance to corrosion, which only occurs in a narrow concentration
range around 9 to 13 atomic percent chromium. Although known to be due to
chromium enrichment of a few atoms thick layer at the surfaces, the
understanding of its complex atomistic origin has been a remaining challenge.
We report an investigation of the thermodynamics of such surfaces at the atomic
scale by means of Monte Carlo simulations. We use a Hamiltonian which provides
a parameterization of previous ab initio results and successfully describes the
alloy's unusual thermodynamics. We report a strong enrichment in Cr of the
surfaces for low bulk concentrations, with a narrow optimum around 12 atomic
percent chromium, beyond which the surface composition decreases drastically.
This behavior is explained by a synergy between (i) the complex phase
separation in the bulk alloy, (ii) local phase transitions that tune the layers
closest to the surface to an iron-rich state and inhibit the bulk phase
separation in this region, and (iii) its compensation by a strong and
non-linear enrichment in Cr of the next few layers. Implications with respect
to the design of prospective nanomaterials are briefly discussed.Comment: 6 pages, 4 figure
Molecular diffusion between walls with adsorption and desorption
The time dependency of the diffusion coefficient of particles in porous media
is an efficient probe of their geometry. The analysis of this quantity,
measured e.g. by nuclear magnetic resonance (PGSE-NMR), can provide rich
information pertaining to porosity, pore size distribution, permeability and
surface-to-volume ratio of porous materials. Nevertheless, in numerous if not
all practical situations, transport is confined by walls where adsorption and
desorption processes may occur. In this article, we derive explicitly the
expression of the time-dependent diffusion coefficient between two confining
walls in the presence of adsorption and desorption. We show that they strongly
modify the time-dependency of the diffusion coefficient, even in this simple
geometry. We finally propose several applications, from sorption rates
measurements to the use as a reference for numerical implementations for more
complex geometries.Comment: 4 pages, 2 figures, 1 supplementary material of 3 page
Effect of surface hydrogen on the anomalous surface segregation behavior of Cr in Fe-rich Fe-Cr alloys
The segregation behavior of Cr in dilute Fe-Cr alloys is known to be
anomalous since the main barrier for surface segregation of Cr in these alloys
arises not from the topmost surface layer but from the subsurface layer where
the solution energy of Cr is much more endothermic as compared to the topmost
surface layer. The Fe-Cr alloys are candidate structural materials for the new
generation of nuclear reactors. The surfaces of these alloys will be exposed to
hydrogen or its isotopes in these reactors, and although hydrogen is soluble
neither in Fe nor in Fe-Cr alloys, it is known that the adsorption energy of
hydrogen on the surface of iron is not only exothermic but relatively large.
This clearly raises the question of the effect of the hydrogen adsorbed on the
surface of iron on the segregation behavior of chromium towards the surface of
iron. In this paper we show, on the basis of our ab initio density functional
theory calculations, that the presence of hydrogen on the surface of iron leads
to a considerably reduced barrier for Cr segregation to both the topmost
surface layer and the subsurface layer, but the subsurface layer still controls
the barrier for surface segregation. This reduction in the barrier for surface
segregation is due to the nature of the Cr-H couple that acts in a complex and
synergistic manner. The presence of Cr enhances the exothermic nature of
hydrogen adsorption that in turn leads to a reduced barrier for surface
segregation. These results should be included in the multiscale modeling of
Fe-Cr alloys
Taylor Dispersion with Adsorption and Desorption
We use a stochastic approach to show how Taylor dispersion is affected by
kinetic processes of adsorption and desorption onto surfaces. A general theory
is developed, from which we derive explicitly the dispersion coefficients of
canonical examples like Poiseuille flows in planar and cylindrical geometries,
both in constant and sinusoidal velocity fields. These results open the way for
the measurement of adsorption and desorption rate constants using stationary
flows and molecular sorting using the stochastic resonance of the adsorption
and desorption processes with the oscillatory velocity field.Comment: 6 pages, 4 figure
Fast Computation of Solvation Free Energies with Molecular Density Functional Theory: Thermodynamic-Ensemble Partial Molar Volume Corrections
Molecular Density Functional Theory (MDFT) offers an efficient implicit-
solvent method to estimate molecule solvation free-energies whereas conserving
a fully molecular representation of the solvent. Even within a second order ap-
proximation for the free-energy functional, the so-called homogeneous reference
uid approximation, we show that the hydration free-energies computed for a
dataset of 500 organic compounds are of similar quality as those obtained from
molecular dynamics free-energy perturbation simulations, with a computer cost
reduced by two to three orders of magnitude. This requires to introduce the
proper partial volume correction to transform the results from the grand
canoni- cal to the isobaric-isotherm ensemble that is pertinent to experiments.
We show that this correction can be extended to 3D-RISM calculations, giving a
sound theoretical justifcation to empirical partial molar volume corrections
that have been proposed recently.Comment: Version with correct equation numbers is here:
http://compchemmpi.wikispaces.com/file/view/sergiievskyi_et_al.pdf/513575848/sergiievskyi_et_al.pdf
Supporting information available online at:
http://compchemmpi.wikispaces.com/file/view/SuppInf_sergiievskyi_et_al_07-04-2014.pdf/513576008/SuppInf_sergiievskyi_et_al_07-04-2014.pd
Hydration of Clays at the Molecular Scale: The Promising Perspective of Classical Density Functional Theory
We report here how the hydration of complex surfaces can be efficiently
studied thanks to recent advances in classical molecular density functional
theory. This is illustrated on the example of the pyrophylite clay. After
presenting the most recent advances, we show that the strength of this implicit
method is that (i) it is in quantitative or semi-quantitative agreement with
reference all-atoms simulations (molecular dynamics here) for both the
solvation structure and energetics, and that (ii) the computational cost is two
to three orders of magnitude less than in explicit methods. The method remains
imperfect, in that it locally overestimates the polarization of water close to
hydrophylic sites of the clay. The high numerical efficiency of the method is
illustrated and exploited to carry a systematic study of the electrostatic and
van der Waals components of the surface-solvant interactions within the most
popular force field for clays, CLAYFF. Hydration structure and energetics are
found to weakly depend upon the electrostatics. We conclude on the consequences
of such findings in future force-field development.Comment: 24 pages, 8 figures. Molecular Physics (2014
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