184 research outputs found
Calculation of Elastic Green's Functions for Lattices with Cavities
In this Brief Report, we present an algorithm for calculating the elastic
Lattice Greens Function of a regular lattice, in which defects are created by
removing lattice points. The method is computationally efficient, since the
required matrix operations are on matrices that scale with the size of the
defect subspace, and not with the size of the full lattice. This method allows
the treatment of force fields with multi-atom interactions.Comment: 3 pages. RevTeX, using epsfig.sty. One figur
Avalanche Size Scaling in Sheared Three-Dimensional Amorphous Solid
We have studied the statistics of plastic rearrangement events in a simulated
amorphous solid at T=0. Events are characterized by the energy release and the
``slip volume'', the product of plastic strain and system volume. Their
distributions for a given system size appear to be exponential, but a
characteristic event size cannot be inferred, because the mean values of these
quantities increase as with . In contrast to
results obtained in 2D models, we do not see simply connected avalanches. The
exponent suggests a fractal shape of the avalanches, which is also evidenced by
the mean fractal dimension and participation ratio.Comment: Accepted for publication in Physical Review Letter
Long time scale simulation of a grain boundary in copper
doi:10.1088/1367-2630/11/7/073034 Abstract. A general, twisted and tilted, grain boundary in copper has been simulated using the adaptive kinetic Monte Carlo method to study the atomistic structure of the non-crystalline region and the mechanism of annealing events that occur at low temperature. The simulated time interval spanned 67µs at 135 K. Similar final configurations were obtained starting from different initial structures: (i) by bringing the two grains into contact without any intermediate layer, and (ii) by inserting an amorphous region between the grains. The results obtained were analyzed with a radial distribution function and a common neighbor analysis. Annealing events leading to lowering of the energy typically involved concerted displacement of several atoms—even as many as 10 atoms displaced by more than half an Ångström. Increased local icosahedral ordering is observed in the boundary layer, but local HCP coordination was also observed. In the final low-energy configurations, the thickness of the region separating the crystalline grains corresponds to just one atomic layer, in good agreemen
Barriers to adherence in adolescents and young adults with cystic fibrosis: a questionnaire study in young patients and their parents
Vibeke Bregnballe1, Peter Oluf Schiøtz1, Kirsten A Boisen2, Tacjana Pressler3, Mikael Thastum4 1Department of Paediatrics, Aarhus University Hospital, Aarhus, Denmark; 2Centre of Adolescent Medicine, University Hospital of Copenhagen, Rigshospitalet, Copenhagen, Denmark; 3Cystic Fibrosis Centre, University Hospital of Copenhagen, Rigshospitalet, Copenhagen, Denmark; 4Department of Psychology, University of Aarhus, Aarhus, Denmark Background: Treatment adherence is crucial in patients with cystic fibrosis, but poor adherence is a problem, especially during adolescence. Identification of barriers to treatment adherence and a better understanding of how context shapes barriers is of great importance in the disease. Adolescent reports of barriers to adherence have been studied, but studies of their parents' experience of such barriers have not yet been carried out. The aim of the present study was to explore barriers to treatment adherence identified by young patients with cystic fibrosis and by their parents. Methods: A questionnaire survey of a cohort of young Danish patients with cystic fibrosis aged 14–25 years and their parents was undertaken. Results: Barriers to treatment adherence were reported by 60% of the patients and by 62% of their parents. Patients and parents agreed that the three most common barriers encountered were lack of time, forgetfulness, and unwillingness to take medication in public. We found a significant positive correlation between reported number of barriers and perceived treatment burden. We also found a statistically significant relationship between the reported number of barriers and treatment adherence. A significant association was found between the number of barriers and the reactions of adolescents/young adults and those of their mothers and fathers, and between the number of barriers and the way the family communicated about cystic fibrosis. Conclusion: The present study showed that the majority of adolescents with cystic fibrosis and their parents experienced barriers to treatment adherence. Agreement between adolescents and their parents regarding the level and types of barriers indicates an opportunity for close cooperation between adolescents, their parents, and health care professionals in overcoming adolescent adherence problems. Keywords: cystic fibrosis, adolescents, parents, barriers, adherenc
Room temperature structure and energetics of water-hydroxyl layers on Pt(111)
The interactions between water and hydroxyl species on Pt(111) surfaces have
been intensely investigated due to their importance to fuel cell
electrocatalysis. Here we present a room temperature molecular dynamics study
of their structure and energetics using an ensemble of neural network
potentials, which allow us to obtain unprecedented statistical sampling. We
first study the energetics of hydroxyl formation, where we find a near-linear
adsorption energy profile, which exhibits a soft and gradual increase in the
differential adsorption energy at high hydroxyl coverages. This is strikingly
different from the predictions of the conventional bilayer model, which
displays a kink at 1/3ML OH coverage indicating a sizeable jump in differential
adsorption energy, but within the statistical uncertainty of previously
reported ab initio molecular dynamics studies. We then analyze the structure of
the interface, where we provide evidence for the water-OH/Pt(111) interface
being hydrophobic at high hydroxyl coverages. We furthermore explain the
observed adsorption energetics by analyzing the hydrogen bonding in the
water-hydroxyl adlayers, where we argue that the increase in differential
adsorption energy at high OH coverage can be explained by a reduction in the
number of hydrogen bonds from the adsorbed water molecules to the hydroxyls
Effects of crack tip geometry on dislocation emission and cleavage: A possible path to enhanced ductility
We present a systematic study of the effect of crack blunting on subsequent
crack propagation and dislocation emission. We show that the stress intensity
factor required to propagate the crack is increased as the crack is blunted by
up to thirteen atomic layers, but only by a relatively modest amount for a
crack with a sharp 60 corner. The effect of the blunting is far less
than would be expected from a smoothly blunted crack; the sharp corners
preserve the stress concentration, reducing the effect of the blunting.
However, for some material parameters blunting changes the preferred
deformation mode from brittle cleavage to dislocation emission. In such
materials, the absorption of preexisting dislocations by the crack tip can
cause the crack tip to be locally arrested, causing a significant increase in
the microscopic toughness of the crack tip. Continuum plasticity models have
shown that even a moderate increase in the microscopic toughness can lead to an
increase in the macroscopic fracture toughness of the material by several
orders of magnitude. We thus propose an atomic-scale mechanism at the crack
tip, that ultimately may lead to a high fracture toughness in some materials
where a sharp crack would seem to be able to propagate in a brittle manner.
Results for blunt cracks loaded in mode II are also presented.Comment: 12 pages, REVTeX using epsfig.sty. 13 PostScript figures. Final
version to appear in Phys. Rev. B. Main changes: Discussion slightly
shortened, one figure remove
Hardness of porous nanocrystalline Co-Ni electrodeposits
The Hall-Petch relationship can fail when the grain size is below a critical value of tens of nanometres. This occurs particularly for coatings having porous surfaces. In this study, electrodeposited nanostructured Co-Ni coatings from four different nickel electroplating baths having grain sizes in the range of 11-23 nm have been investigated. The finest grain size, approximately 11 nm, was obtained from a coating developed from the nickel sulphate bath. The Co-Ni coatings have a mixed face centred cubic and hexagonal close-packed structures with varying surface morphologies and different porosities. A cluster-pore mixture model has been proposed by considering no contribution from pores to the hardness. As the porosity effect was taken into consideration, the calculated pore-free hardness is in agreement with the ordinary Hall-Petch relationship even when the grain size is reduced to 11 nm for the Co-Ni coatings with 77±2 at% cobalt. The present model was applied to other porous nanocrystalline coatings, and the Hall-Petch relationship was maintained. © 2013 The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht. © KIM and Springer
Characterization of defect structures in nanocrystalline materials by X-ray line profile analysis
X-ray line profile analysis is a powerful alternative tool for determining dislocation densities, dislocation type, crystallite and subgrain size and size-distributions, and planar defects, especially the frequency of twin boundaries and stacking faults. The method is especially useful in the case of submicron grain size or nanocrystalline materials, where X-ray line broadening is a well pronounced effect, and the observation of defects with very large density is often not easy by transmission electron microscopy. The fundamentals of X-ray line broadening are summarized in terms of the different qualitative breadth methods, and the more sophisticated and more quantitative whole pattern fitting procedures. The efficiency and practical use of X-ray line profile analysis is shown by discussing its applications to metallic, ceramic, diamond-like and polymer nanomaterials
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