88 research outputs found
Modelling diffusion in crystals under high internal stress gradients
Diffusion of vacancies and impurities in metals is important in many processes occurring in structural materials. This diffusion often takes place in the presence of spatially rapidly varying stresses. Diffusion under stress is frequently modelled by local approximations to the vacancy formation and diffusion activation enthalpies which are linear in the stress, in order to account for its dependence on the local stress state and its gradient. Here, more accurate local approximations to the vacancy formation and diffusion activation enthalpies, and the simulation methods needed to implement them, are introduced. The accuracy of both these approximations and the linear approximations are assessed via comparison to full atomistic studies for the problem of vacancies around a Lomer dislocation in Aluminium. Results show that the local and linear approximations for the vacancy formation enthalpy and diffusion activation enthalpy are accurate to within 0.05 eV outside a radius of about 13 Å (local) and 17 Å (linear) from the centre of the dislocation core or, more generally, for a strain gradient of roughly up to 6 × 10^6 m^-1 and 3 × 10^6 m^-1, respectively. These results provide a basis for the development of multiscale models of diffusion under highly non-uniform stress
Structural phase transformations in metallic grain boundaries
Structural transformations at interfaces are of profound fundamental interest
as complex examples of phase transitions in low-dimensional systems. Despite
decades of extensive research, no compelling evidence exists for structural
transformations in high-angle grain boundaries in elemental systems. Here we
show that the critical impediment to observations of such phase transformations
in atomistic modeling has been rooted in inadequate simulation methodology. The
proposed new methodology allows variations in atomic density inside the grain
boundary and reveals multiple grain boundary phases with different atomic
structures. Reversible first-order transformations between such phases are
observed by varying temperature or injecting point defects into the boundary
region. Due to the presence of multiple metastable phases, grain boundaries can
absorb significant amounts of point defects created inside the material by
processes such as irradiation. We propose a novel mechanism of radiation damage
healing in metals which may guide further improvements in radiation resistance
of metallic materials through grain boundary engineering.Comment: 25 pages, 11 figure
Evaluation of Constant Potential Method in Simulating Electric Double-Layer Capacitors
A major challenge in the molecular simulation of electric double layer
capacitors (EDLCs) is the choice of an appropriate model for the electrode.
Typically, in such simulations the electrode surface is modeled using a uniform
fixed charge on each of the electrode atoms, which ignores the electrode
response to local charge fluctuations induced by charge fluctuations in the
electrolyte. In this work, we evaluate and compare this Fixed Charge Method
(FCM) with the more realistic Constant Potential Method (CPM), [Reed, et al.,
J. Chem. Phys., 126, 084704 (2007)], in which the electrode charges fluctuate
in order to maintain constant electric potential in each electrode. For this
comparison, we utilize a simplified LiClO-acetonitrile/graphite EDLC. At
low potential difference (), the two methods yield
essentially identical results for ion and solvent density profiles; however,
significant differences appear at higher . At ,
the CPM ion density profiles show significant enhancement (over FCM) of
"partially electrode solvated" Li ions very close to the electrode surface.
The ability of the CPM electrode to respond to local charge fluctuations in the
electrolyte is seen to significantly lower the energy (and barrier) for the
approach of Li ions to the electrode surface.Comment: Corrected typo
Limnological, Ichthyological, and Parasitological Investigations on Arkansas Reservoris in Relation to Water Quality
Lake Fort Smith, a 525 acre (212 ha) reservoir, was impounded in 1936 as a water supply for the city of Fort Smith. The reservoir is located on Clear Creek (Frog Bayou), a tributary of the Arkansas River, in the Boston Mountains 28 miles (45 km) northeast of the city of Fort Smith in Crawford County, Arkansas. A map and morphometric characteristics of Lake Fort Smith are given in Fig. 1 and Table I (Hoffman, 1951; Nelson, 1952). In 1956 Lake Shepherd Springs, a 750 acre (304 ha) impoundment, was created one mile upstream of Lake Fort Smith (Rorie, 1961). Both lakes have a shale substrate and are subject to periods of high turbidity. The 2 two lakes have a water shed of 65 square mile area (168 km ) of mountainous oak-hickory forest. Lake Shepherd Springs has not acted as a settling basin for sediments; thus, the upper portion of Lake Fort Smith has numerous shallow areas with a mud bottom supporting various submergent and emergent aquatic plants. The lower portion of the lake has a rocky, shale substrate with only limited emergent vegetation
Structural disjoining potential for grain boundary premelting and grain coalescence from molecular-dynamics simulations
We describe a molecular dynamics framework for the direct calculation of the
short-ranged structural forces underlying grain-boundary premelting and
grain-coalescence in solidification. The method is applied in a comparative
study of (i) a Sigma 9 120 degress twist and (ii) a Sigma 9 {411}
symmetric tilt boundary in a classical embedded-atom model of elemental Ni.
Although both boundaries feature highly disordered structures near the melting
point, the nature of the temperature dependence of the width of the disordered
regions in these boundaries is qualitatively different. The former boundary
displays behavior consistent with a logarithmically diverging premelted layer
thickness as the melting temperature is approached from below, while the latter
displays behavior featuring a finite grain-boundary width at the melting point.
It is demonstrated that both types of behavior can be quantitatively described
within a sharp-interface thermodynamic formalism involving a width-dependent
interfacial free energy, referred to as the disjoining potential. The
disjoining potential for boundary (i) is calculated to display a monotonic
exponential dependence on width, while that of boundary (ii) features a weak
attractive minimum. The results of this work are discussed in relation to
recent simulation and theoretical studies of the thermodynamic forces
underlying grain-boundary premelting.Comment: 24 pages, 8 figures, 1 tabl
van der Waals density functional study of CO2 binding in zeolitic imidazolate frameworks
This is the publisher's version, also available electronically from http://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.085410The van der Waals density functional (vdW-DF) formalism is employed in a study of the binding energetics for CO2 in a set of five zeolitic imidazolate framework (ZIF) compounds. The ZIF structures investigated share the same RHO-type zeolite topology and metal atoms, but feature imidazolate linkers with different chemical functionalization. Three distinct binding sites are identified, for which the binding energies are found to show different dependencies on the functionalization of the linker molecules. The origin of the variations in the binding energies across the ZIF compounds is discussed through analyses of the binding geometries and charge-density distributions. A comparison of the vdW-DF results with those obtained by generalized-gradient-approximation calculations highlights the important contribution of the nonlocal correlation energy to the CO2 binding energies in these compounds
Lattice Resistance and Peierls Stress in Finite-size Atomistic Dislocation Simulations
Atomistic computations of the Peierls stress in fcc metals are relatively
scarce. By way of contrast, there are many more atomistic computations for bcc
metals, as well as mixed discrete-continuum computations of the Peierls-Nabarro
type for fcc metals. One of the reasons for this is the low Peierls stresses in
fcc metals. Because atomistic computations of the Peierls stress take place in
finite simulation cells, image forces caused by boundaries must either be
relaxed or corrected for if system size independent results are to be obtained.
One of the approaches that has been developed for treating such boundary forces
is by computing them directly and subsequently subtracting their effects, as
developed by V. B. Shenoy and R. Phillips [Phil. Mag. A, 76 (1997) 367]. That
work was primarily analytic, and limited to screw dislocations and special
symmetric geometries. We extend that work to edge and mixed dislocations, and
to arbitrary two-dimensional geometries, through a numerical finite element
computation. We also describe a method for estimating the boundary forces
directly on the basis of atomistic calculations. We apply these methods to the
numerical measurement of the Peierls stress and lattice resistance curves for a
model aluminum (fcc) system using an embedded-atom potential.Comment: LaTeX 47 pages including 20 figure
The Gauged Vector Model in Four-Dimensions: Resolution of an Old Problem?
A calculation of the renormalization group improved effective potential for
the gauged U(N) vector model, coupled to fermions in the fundamental
representation, computed to leading order in 1/N, all orders in the scalar
self-coupling , and lowest order in gauge coupling , with
of order , is presented. It is shown that the theory has two phases, one of
which is asymptotically free, and the other not, where the asymptotically free
phase occurs if , and
. In the asymptotically free phase, the effective
potential behaves qualitatively like the tree-level potential. In the other
phase, the theory exhibits all the difficulties of the ungauged
vector model. Therefore the theory appears to be consistent (only) in the
asymptotically free phase.Comment: Latex, 18 pages plus 3 figures using epsf. Substantially revised to
correct a factor of 2 error in the previous version of equation (2.5b). This
has significant effects on the results. The model has also been revised to
include fermion
Efficacy and Safety of Vancomycin Loading Doses in Critically Ill Patients with Methicillin-Resistant \u3ci\u3eStaphylococcus aureus\u3c/i\u3e Infection
Background: While vancomycin loading doses may facilitate earlier pharmacokinetic–pharmacodynamic target attainment, the impact of loading doses on clinical outcomes remains understudied. Critically ill patients are at highest risk of morbidity and mortality from methicillin resistant Staphylococcus aureus (MRSA) infection and hypothesized to most likely benefit from a loading dose. We sought to determine the association between receipt of a vancomycin loading dose and clinical outcomes in a cohort of critically ill adults.
Methods: Four hundred and forty-nine critically ill patients with MRSA cultures isolated from blood or respiratory specimens were eligible for the study. Cohorts were established by receipt of a loading dose (⩾20 mg/kg actual body weight) or not. The primary outcome was clinical failure, a composite outcome of death within 30 days of first MRSA culture, blood cultures positive ⩾7 days, white blood cell count up to 5 days from vancomycin initiation, temperature up to 5 days from vancomycin initiation, or substitution (or addition) of another MRSA agent.
Results: There was no difference in the percentage of patients experiencing clinical failure between the loading dose and no loading dose groups (74.8% versus 72.8%; p = 0.698). Secondary outcomes were also similar between groups, including mortality and acute kidney injury, as was subgroup analysis based on site of infection. Exploratory analyses, including assessment of loading dose based on quartiles and a multivariable logistic regression model showed no differences.
Conclusion: Use of vancomycin loading doses was not associated with improved clinical outcomes in critically ill patients with MRSA infection
- …