3,191 research outputs found
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
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
Actions speak louder than words: designing transdisciplinary approaches to enact solutions
Sustainability science uses a transdisciplinary research process in which academic and non-academic partners collaborate to identify a common problem and co-produce knowledge to develop more sustainable solutions. Sustainability scientists have advanced the theory and practice of facilitating collaborative efforts such that the knowledge created is usable. There has been less emphasis, however, on the last step of the transdisciplinary process: enacting solutions. We analyzed a case study of a transdisciplinary research effort in which co-produced policy simulation information shaped the creation of a new policy mechanism. More specifically, by studying the development of a mechanism for conserving vernal pool ecosystems, we found that four factors helped overcome common challenges to acting upon new information: creating a culture of learning, co-producing policy simulations that acted as boundary objects, integrating research into solution development, and employing an adaptive management approach. With an increased focus on these four factors that enable action, we can better develop the same level of nuanced theoretical concepts currently characterizing the earlier phases of transdisciplinary research, and the practical advice for deliberately designing these efforts
Alien Registration- Levesque, Edward P. (Sanford, York County)
https://digitalmaine.com/alien_docs/2796/thumbnail.jp
Alien Registration- Levesque, Joseph P. (Millinocket, Penobscot County)
https://digitalmaine.com/alien_docs/8056/thumbnail.jp
Low energy electron scattering from DNA and RNA bases: shape resonances and radiation damage
Calculations are carried out to determine elastic scattering cross sections
and resonance energies for low energy electron impact on uracil and on each of
the DNA bases (thymine, cytosine, adenine, guanine), for isolated molecules in
their equilibrium geometry. Our calculations are compared with available theory
and experiment. We also attempt to correlate this information with experimental
dissociation patterns through an analysis of the temporary anion structures
that are formed by electron capture in shape resonances.Comment: 20 pages, 12 figures, submitted to J. Chem. Phy
Turning Contention into Collaboration: Engaging Power, Trust, and Learning in Collaborative Networks
Given the complexity and multiplicity of goals in natural resource governance, it is not surprising that policy debates are often characterized by contention and competition. Yet at times adversaries join together to collaborate to find creative solutions not easily achieved in polarizing forums. We employed qualitative interviews and a quantitative network analysis to investigate a collaborative network that formed to develop a resolution to a challenging natural resource management problem, the conservation of vernal pools. We found that power had become distributed among members, trust had formed across core interests, and social learning had resulted in shared understanding and joint solutions. Furthermore, institutions such as who and when new members joined, norms of inclusion and openness, and the use of small working groups helped create the observed patterns of power, trust, and learning
The impact of mass-loss on the evolution and pre-supernova properties of red supergiants
The post main-sequence evolution of massive stars is very sensitive to many
parameters of the stellar models. Key parameters are the mixing processes, the
metallicity, the mass-loss rate and the effect of a close companion. We study
how the red supergiant lifetimes, the tracks in the Hertzsprung-Russel diagram
(HRD), the positions in this diagram of the pre-supernova progenitor as well as
the structure of the stars at that time change for various mass-loss rates
during the red supergiant phase (RSG), and for two different initial rotation
velocities. The surface abundances of RSGs are much more sensitive to rotation
than to the mass-loss rates during that phase. A change of the RSG mass-loss
rate has a strong impact on the RSG lifetimes and therefore on the luminosity
function of RSGs. At solar metallicity, the enhanced mass-loss rate models do
produce significant changes on the populations of blue, yellow and red
supergiants. When extended blue loops or blue ward excursions are produced by
enhanced mass-loss, the models predict that a majority of blue (yellow)
supergiants are post RSG objects. These post RSG stars are predicted to show
much smaller surface rotational velocities than similar blue supergiants on
their first crossing of the HR gap. The position in the HRD of the end point of
the evolution depends on the mass of the hydrogen envelope. More precisely,
whenever, at the pre-supernova stage, the H-rich envelope contains more than
about 5\% of the initial mass, the star is a red supergiant, and whenever the
H-rich envelope contains less than 1\% of the total mass the star is a blue
supergiant. For intermediate situations, intermediate colors/effective
temperatures are obtained. Yellow progenitors for core collapse supernovae can
be explained by the enhanced mass-loss rate models, while the red progenitors
are better fitted by the standard mass-loss rate models.Comment: 19 pages, 11 figures, 6 tables, accepted for publication in Astronomy
and Astrophysic
Non destructive evaluation of absorbing materials using microwave stimulated infrared thermography
The electromagnetic wave nondestructive evaluation methods that appeared some years ago are attractive because many materials can absorb hyperfrequency energy. Nevertheless, generally the detections are achieved point by point which is highly time consuming for NDE of extended structures. We developed a global method which associates electromagnetic stimulation and a detection by an IR camera (EMIR method). For others applications, this method has been used over the past several years [1,2]. The aim of this paper is to give the first results about the possibilities of this new method for NDE
Effective interactions in the colloidal suspensions from HNC theory
The HNC Ornstein-Zernike integral equations are used to determine the
properties of simple models of colloidal solutions where the colloids and ions
are immersed in a solvent considered as a dielectric continuum and have a size
ratio equal to 80 and a charge ratio varying between 1 and 4000. At an infinite
dilution of colloids, the effective interactions between colloids and ions are
determined for ionic concentrations ranging from 0.001 to 0.1 mol/l and
compared to those derived from the Poisson-Boltzmann theory. At finite
concentrations, we discuss on the basis of the HNC results the possibility of
an unambiguous definition of the effective interactions between the colloidal
molecules.Comment: 26 pages, 15 figure
- …