663 research outputs found
Immersed nano-sized Al dispersoids in an Al matrix; effects on the structural and mechanical properties by Molecular Dynamics simulations
We used molecular dynamics simulations based on a potential model in analogy
to the Tight Binding scheme in the Second Moment Approximation to simulate the
effects of aluminum icosahedral grains (dispersoids) on the structure and the
mechanical properties of an aluminum matrix. First we validated our model by
calculating several thermodynamic properties referring to the bulk Al case and
we found good agreement with available experimental and theoretical data.
Afterwards, we simulated Al systems containing Al clusters of various sizes. We
found that the structure of the Al matrix is affected by the presence of the
dispersoids resulting in well ordered domains of different symmetries that were
identified using suitable Voronoi analysis. In addition, we found that the
increase of the grain size has negative effect on the mechanical properties of
the nanocomposite as manifested by the lowering of the calculated bulk moduli.
The obtained results are in line with available experimental data.Comment: 15 pages, 8 figures. Submitted to J. Phys: Condens. Matte
Experiments on higher-order and degenerate Akhmediev breather-type rogue water waves
A possible mechanism that is responsible for the occurrence of rogue waves in the ocean is the BenjaminâFeir instability or modulation instability. The deterministic framework that describes this latter instability of Stokes waves in deep water is provided by the family of Akhmediev breather (AB) solutions of the nonlinear Schrödinger equation (NLS). It is indeed very convenient to use these exact pulsating envelopes particularly for laboratory experiments, since they allow to generate extreme waves at any location in space at any instant of time. As such, using this framework is more advantageous compared to the classical initialization of the unstable wave dynamics from a three wave system (main wave frequency and one pair of unstable sidebands). In this work, we report an experimental study on higher-order AB hydrodynamics that describe a higher-order stage of modulation instability, namely, starting from five wave systems (main wave frequency and two pairs of unstable sidebands). The corresponding laboratory experiments, that have been conducted in a large water wave facility, confirm the NLS wave dynamics forecast while boundary element method-based numerical wave tank simulations show a very good agreement with the experimental data.A.C. acknowledges the support from the Japan Society for the Promotion of Science (JSPS) and the Burgundy Region
(France). A.C. and T.W. acknowledges the support from the Japan Society for the Promotion of Science (JSPS) through Grants-in-Aid for Scientific Research (KAKENHI). B.K. is thankful for support from French project PIA2/ISITE-BFC. N.A. acknowledges the support of the Australian Research Council (Discovery Project numbers DP140100265 and DP150102057) and support from the Volkswagen Stiftung
An accurate determination of the Avogadro constant by counting the atoms in a 28Si crystal
The Avogadro constant links the atomic and the macroscopic properties of
matter. Since the molar Planck constant is well known via the measurement of
the Rydberg constant, it is also closely related to the Planck constant. In
addition, its accurate determination is of paramount importance for a
definition of the kilogram in terms of a fundamental constant. We describe a
new approach for its determination by "counting" the atoms in 1 kg
single-crystal spheres, which are highly enriched with the 28Si isotope. It
enabled isotope dilution mass spectroscopy to determine the molar mass of the
silicon crystal with unprecedented accuracy. The value obtained, 6.02214084(18)
x 10^23 mol^-1, is the most accurate input datum for a new definition of the
kilogram.Comment: 4 pages, 5 figures, 3 table
Exact limiting relation between the structure factors in neutron and x-ray scattering
The ratio of the static matter structure factor measured in experiments on
coherent X-ray scattering to the static structure factor measured in
experiments on neutron scattering is considered. It is shown theoretically that
this ratio in the long-wavelength limit is equal to the nucleus charge at
arbitrary thermodynamic parameters of a pure substance (the system of nuclei
and electrons, where interaction between particles is pure Coulomb) in a
disordered equilibrium state. This result is the exact relation of the quantum
statistical mechanics. The experimental verification of this relation can be
done in the long wavelength X-ray and neutron experiments.Comment: 7 pages, no figure
Structural Stability of Amorphous Metals
Effects of annealing and plastic deformation on structure of several amorphous metals (Pd-Si, Fe-Si-B, Ni-Si-B, Co-Si-B, etc.) were examined by using dark-field electron microscopy, x-ray and electron diffraction, hardness, specific gravity and electrical resistivity. In T-T-T diagrams, distinct differences in transformation sequence were observed beyond and below the critical temperature. Above this temperature, crystallization proceeds through two metastable phases and finally to the stable phase by nucleation and growth mechanisms. Below the temperature, however, progressive aging gradually changes the structure through two stages ; the first stage is due to release of internal strain in the rapidly quenched specimens and the second stage due to transformation from amorphous to single phase with the same structure as the major element. Plastic deformation at room temperature produces a much more disordered atomic structure than is present in the as-quenched state perhaps by introducing additional irregularities
Persistent Oscillations of X-ray Speckles: Pt (001) Step Flow
We have performed coherent x-ray scattering experiments on the hexagonally
reconstructed Pt (001) surface to study the temperature-dependent surface
dynamics. By correlating speckle patterns collected at the (001) anti-Bragg
position we are able to measure surface dynamics when the averaged incoherent
x-ray scattering appears static. In the temperature range above the rotational
epitaxy transition and below the roughening transition (1750 K - 1830 K), we
have observed well-defined oscillatory autocorrelations of speckles that
persist for tens of minutes, in addition to the expected thermal decorrelation.
The observed oscillations indicate surface dynamics due to "step-flow" motion.
This is shown with a simple model in which the phase of the scattered x-rays
from the steps within the illumination area is retained in the coherent x-ray
scattering. This demonstrates a possibility that x-ray speckles can be used to
monitor the real-space real-time evolution of surfaces in addition to the
traditional decorrelation measurements.Comment: 12 pages, 3 figure
Analysis of Granular Packing Structure by Scattering of THz Radiation
Scattering methods are widespread used to characterize the structure and
constituents of matter on small length scales. This motivates this introductory
text on identifying prospective approaches to scattering-based methods for
granular media. A survey to light scattering by particles and particle
ensembles is given. It is elaborated why the established scattering methods
using X-rays and visible light cannot in general be transferred to granular
media. Spectroscopic measurements using Terahertz radiation are highlighted as
they to probe the scattering properties of granular media, which are sensitive
to the packing structure. Experimental details to optimize spectrometer for
measurements on granular media are discussed. We perform transmission
measurements on static and agitated granular media using Fourier-transform
spectroscopy at the THz beamline of the BessyII storage ring. The measurements
demonstrate the potential to evaluate degrees of order in the media and to
track transient structural states in agitated bulk granular media.Comment: 12 Pages, 9 Figures, 56 Reference
Peak positions and shapes in neutron pair correlation functions from powders of highly anisotropic crystals
The effect of the powder average on the peak shapes and positions in neutron
pair distribution functions of polycrystalline materials is examined. It is
shown that for highly anisotropic crystals, the powder average leads to shifts
in peak positions and to non-Gaussian peak shapes. The peak shifts can be as
large as several percent of the lattice spacing
Structural and dynamical properties of liquid Si. An orbital-free molecular dynamics study
Several static and dynamic properties of liquid silicon near melting have
been determined from an orbital free {\em ab-initio} molecular dynamics
simulation. The calculated static structure is in good agreement with the
available X-ray and neutron diffraction data. The dynamical structure shows
collective density excitations with an associated dispersion relation which
closely follows recent experimental data. It is found that liquid silicon can
not sustain the propagation of shear waves which can be related to the power
spectrum of the velocity autocorrelation function. Accurate estimates have also
been obtained for several transport coefficients. The overall picture is that
the dynamic properties have many characteristics of the simple liquid metals
although some conspicuous differences have been found.Comment: 12 pages, 11 figure
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