237 research outputs found
Geological realism control in the process of oil and gas field models automatic history matching
On the role of confinement on solidification in pure materials and binary alloys
We use a phase-field model to study the effect of confinement on dendritic
growth, in a pure material solidifying in an undercooled melt, and in the
directional solidification of a dilute binary alloy. Specifically, we observe
the effect of varying the vertical domain extent () on tip selection,
by quantifying the dendrite tip velocity and curvature as a function of
, and other process parameters. As decreases, we find that the
operating state of the dendrite tips becomes significantly affected by the
presence of finite boundaries. For particular boundary conditions, we observe a
switching of the growth state from 3-D to 2-D at very small , in both
the pure material and alloy. We demonstrate that results from the alloy model
compare favorably with those from an experimental study investigating this
effect.Comment: 13 pages, 9 figures, 3 table
Influence of external flows on crystal growth: numerical investigation
We use a combined phase-field/lattice-Boltzmann scheme [D. Medvedev, K.
Kassner, Phys. Rev. E {\bf 72}, 056703 (2005)] to simulate non-facetted crystal
growth from an undercooled melt in external flows. Selected growth parameters
are determined numerically.
For growth patterns at moderate to high undercooling and relatively large
anisotropy, the values of the tip radius and selection parameter plotted as a
function of the Peclet number fall approximately on single curves. Hence, it
may be argued that a parallel flow changes the selected tip radius and growth
velocity solely by modifying (increasing) the Peclet number. This has
interesting implications for the availability of current selection theories as
predictors of growth characteristics under flow.
At smaller anisotropy, a modification of the morphology diagram in the plane
undercooling versus anisotropy is observed. The transition line from dendrites
to doublons is shifted in favour of dendritic patterns, which become faster
than doublons as the flow speed is increased, thus rendering the basin of
attraction of dendritic structures larger.
For small anisotropy and Prandtl number, we find oscillations of the tip
velocity in the presence of flow. On increasing the fluid viscosity or
decreasing the flow velocity, we observe a reduction in the amplitude of these
oscillations.Comment: 10 pages, 7 figures, accepted for Physical Review E; size of some
images had to be substantially reduced in comparison to original, resulting
in low qualit
Position observations of NEAs at the RTT150
In 2004-2007, 560 observations of 17 Near-Earth Asteroids of 15-21 magnitudes were made at the Russian-Turkish telescope (RTT150). The reduction was made using reference stars from the UCAC2 and USNO-B1 catalogues. The comparison of the observed and calculated positions of the asteroids using the HORIZONS system gave standard errors of a single position in the range 0.05÷H 0.50″. Analysis of the (O - C) is given in this paper. © 2008 American Institute of Physics
Synthesis and characterization of Dy3Fe5O12 nanoparticles fabricated with the anion resin exchange precipitation method
Dysprosium-iron garnet (DyIG) nanoparticles were synthesized with the new modification of the anion resin
exchange precipitation method. Fourier transform infrared spectroscopy, X-ray diffraction, and transmission
electron microscopy showed nanoparticles to be of the garnet structure with an excellent crystallinity. Magnetic
properties were studied by using QUANTUM Design MPMS-XL system and the visible magnetic circular dichroism
(MCD). Nanoparticles magnetic properties were close to those of bulk DyIG crystals. Dependence of the
nanoparticles magnetization (M) on the external magnetic field (H) is described by a narrow hysteresis loop in
relatively low fields and the strong linear M increase with the further H increase. The visible MCD of DyIG was
studied in this work for the first time. The MCD spectra consisted of several peaks associated with electron
transitions in iron and dysprosium ions located in different spectral intervals. Dependences of the MCD peak
intensities on temperature and magnetic field were studied
Effects of processing parameters on the morphology, structure, and magnetic properties of Cu1−xFexCr2Se4 nanoparticles synthesized with chemical methods
Cu1−xFexCr2Se4 nanoparticles with x = 0, 0.2, and 0.4 were synthesized via thermal decomposition of metal nitrate or chloride salts and selenium powder using different precursor compositions and processing details. Single crystalline nano-belts or nano-rods coexist in the synthesized powder samples with hexagon-shaped plates in dependence on the precursor composition. The belts gathered into conglomerates forming “hierarchical” particles. Visible magnetic circular dichroism (MCD) of Cu1−xFexCr2Se4 nanoparticles embedded into a transparent matrix was investigated for the first time. The similarity of the MCD spectra of all samples showed the similarity of the nanoparticles electronic structure independent of their morphology. Basing on the MCD spectral maxima characteristics, electron transitions from the ground to the excited states were identified with the help of the conventional band theory and the multi-electron approach
Microstructure and magnetooptics of silicon oxide with implanted nickel nanoparticles
Metallic nickel nanoparticles of various sizes are formed in a thin near-surface layer in an amorphous SiO 2 matrix during 40-keV Ni + ion implantation at a dose of (0.25-1.0) × 10 17 ions/cm 2. The microstructure of the irradiated layer and the crystal structure, morphology, and sizes of nickel particles formed at various irradiation doses are studied by transmission electron microscopy and electron diffraction. The magnetooptical Faraday effect and the magnetic circular dichroism in an ensemble of nickel nanoparticles are studied in the optical range. The permittivity ε∧ tensor components are calculated for the implanted samples using an effective medium model with allowance for the results of magnetooptical measurements. The spectral dependences of the tensor ε∧ components are found to be strongly different from those of a continuous metallic nickel film. These differences are related to a disperse structure of the magnetic nickel phase and to a surface plasma resonance in the metal nanoparticles. © Pleiades Publishing, Inc., 2011
Study of morphology, magnetic properties, and visible magnetic circular dichroism of Ni nanoparticles synthesized in SiO2 by ion implantation
A systematic study of ensembles of nickel nanoparticles fabricated by Ni+-ion implantation at a dose of (0.5-1.0)×1017 ions/cm2 in a thin near-surface layer of an amorphous SiO2 matrix by means of transmission electron microscopy (TEM), dc magnetic measurements, and magneto-optical technique is presented. TEM characterization of Ni nanoparticles proves the formation of isolated spherical nickel nanoparticles with diameters from 2 to 16 nm. The crystal structure and lattice constant of the nanoparticles correspond to face-centered-cubic Ni. The larger size nanoparticles are shown to have core-shell structure, which is unusual for the implantation conditions used. The shell of these nanoparticles consists of Ni, while the core has supposedly the composition coinciding with the matrix, i.e., SiO2. The core-shell nanoparticles in the investigated sample coexist with ordinary pure Ni nanoparticles, which strongly affects the magnetic and especially magneto-optic properties of the samples. For all three doses, the nanoparticles are in the superparamagnetic state at room temperature passing to the "frozen" state at lower temperatures. However, only the sample implanted with the lowest dose demonstrates the classic superparamagnetic behavior according to the shape of the experimental magnetization temperature dependencies for the zero-field-cooled (ZFC) and field-cooled regimes. This shape deviation from that characteristic of the pure superparamagnetic ensembles is ascribed mainly to the particle core-shell structure. The Ni nanoparticles' anisotropy constant estimated with the help of ZFC curves appears to exceed the bulk Ni anisotropy second constant approximately by two orders of magnitude. Magnetic circular dichroism (MCD) is characterized by spectral dependence modified strongly as compared to the MCD spectra of a continuous Ni film. In the spectral range 1.1-4.2 eV, the MCD spectrum consists of two broad maxima of opposite sign with the characteristics depending on the implantation dose and the measurement temperature. The MCD spectra analysis allows one to show that the higher-energy maximum (at 3.34-3.48 eV depending on the dose) is related to the surface plasmon resonance (SPR) excitation in pure Ni nanoparticles, while the lower-energy maximum (at 2.19-2.73 eV depending on the dose) should be associated with the SPR excitation in core-shell nanoparticles. © 2013 American Physical Society
The astrometric Gaia-FUN-SSO observation campaign of 99 942 Apophis
Astrometric observations performed by the Gaia Follow-Up Network for Solar
System Objects (Gaia-FUN-SSO) play a key role in ensuring that moving objects
first detected by ESA's Gaia mission remain recoverable after their discovery.
An observation campaign on the potentially hazardous asteroid (99 942) Apophis
was conducted during the asteroid's latest period of visibility, from
12/21/2012 to 5/2/2013, to test the coordination and evaluate the overall
performance of the Gaia-FUN-SSO . The 2732 high quality astrometric
observations acquired during the Gaia-FUN-SSO campaign were reduced with the
Platform for Reduction of Astronomical Images Automatically (PRAIA), using the
USNO CCD Astrograph Catalogue 4 (UCAC4) as a reference. The astrometric
reduction process and the precision of the newly obtained measurements are
discussed. We compare the residuals of astrometric observations that we
obtained using this reduction process to data sets that were individually
reduced by observers and accepted by the Minor Planet Center. We obtained 2103
previously unpublished astrometric positions and provide these to the
scientific community. Using these data we show that our reduction of this
astrometric campaign with a reliable stellar catalog substantially improves the
quality of the astrometric results. We present evidence that the new data will
help to reduce the orbit uncertainty of Apophis during its close approach in
2029. We show that uncertainties due to geolocations of observing stations, as
well as rounding of astrometric data can introduce an unnecessary degradation
in the quality of the resulting astrometric positions. Finally, we discuss the
impact of our campaign reduction on the recovery process of newly discovered
asteroids.Comment: Accepted for publication in A&
Selection of stable growth conditions for the parabolic dendrite tip in crystallization of multicomponent melts
- …