1,649 research outputs found
First-principles study of ternary fcc solution phases from special quasirandom structures
In the present work, ternary Special Quasirandom Structures (SQSs) for a fcc
solid solution phase are generated at different compositions,
and , ,
whose correlation functions are satisfactorily close to those of a random fcc
solution. The generated SQSs are used to calculate the mixing enthalpy of the
fcc phase in the Ca-Sr-Yb system. It is observed that first-principles
calculations of all the binary and ternary SQSs in the Ca-Sr-Yb system exhibit
very small local relaxation. It is concluded that the fcc ternary SQSs can
provide valuable information about the mixing behavior of the fcc ternary solid
solution phase. The SQSs presented in this work can be widely used to study the
behavior of ternary fcc solid solutions.Comment: 20 pages, 7 figure
Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia
The high-temperature cubic-tetragonal phase transition of pure stoichiometric
zirconia is studied by molecular dynamics (MD) simulations and within the
framework of the Landau theory of phase transformations. The interatomic forces
are calculated using an empirical, self-consistent, orthogonal tight-binding
(SC-TB) model, which includes atomic polarizabilities up to the quadrupolar
level. A first set of standard MD calculations shows that, on increasing
temperature, one particular vibrational frequency softens. The temperature
evolution of the free energy surfaces around the phase transition is then
studied with a second set of calculations. These combine the thermodynamic
integration technique with constrained MD simulations. The results seem to
support the thesis of a second-order phase transition but with unusual, very
anharmonic behaviour above the transition temperature
Microstructural Characterization of Graphite Spheroids in Ductile Iron
The present work brings new insights by transmission electron microscopy allowing disregarding or supporting some of the models proposed for spheroidal growth of graphite in cast irons. Nodules consist of sectors made of graphite plates elongated along a hai direction and stack on each other with their c axis aligned with the radial direction. These plates are the elementary units for spheroidal growth and a calculation supports the idea that new units continuously nucleate at the ledge between sectors
Eutectic Colony Formation: A Stability Analysis
Experiments have widely shown that a steady-state lamellar eutectic
solidification front is destabilized on a scale much larger than the lamellar
spacing by the rejection of a dilute ternary impurity and forms two-phase cells
commonly referred to as `eutectic colonies'. We extend the stability analysis
of Datye and Langer for a binary eutectic to include the effect of a ternary
impurity. We find that the expressions for the critical onset velocity and
morphological instability wavelength are analogous to those for the classic
Mullins-Sekerka instability of a monophase planar interface, albeit with an
effective surface tension that depends on the geometry of the lamellar
interface and, non-trivially, on interlamellar diffusion. A qualitatively new
aspect of this instability is the occurence of oscillatory modes due to the
interplay between the destabilizing effect of the ternary impurity and the
dynamical feedback of the local change in lamellar spacing on the front motion.
In a transient regime, these modes lead to the formation of large scale
oscillatory microstructures for which there is recent experimental evidence in
a transparent organic system. Moreover, it is shown that the eutectic front
dynamics on a scale larger than the lamellar spacing can be formulated as an
effective monophase interface free boundary problem with a modified
Gibbs-Thomson condition that is coupled to a slow evolution equation for the
lamellar spacing. This formulation provides additional physical insights into
the nature of the instability and a simple means to calculate an approximate
stability spectrum. Finally, we investigate the influence of the ternary
impurity on a short wavelength oscillatory instability that is already present
at off-eutectic compositions in binary eutectics.Comment: 26 pages RevTex, 14 figures (28 EPS files); some minor changes;
references adde
State of the Art and Future Challenges in Multiple Sclerosis Research and Medical Management: An Insight into the 5th International Porto Congress of Multiple Sclerosis
The 5th International Porto Congress of Multiple Sclerosis took place between the 14th and 16th of February 2019 in Porto, Portugal. Its intensive programme covered a wide-range of themes—including many of the hot topics, challenges, pitfalls and yet unmet needs in the field of multiple sclerosis (MS)—led by a number of well-acknowledged world experts. This meeting review summarizes the talks that took place during the congress, which focussed on issues in MS as diverse as the development and challenges of progressive MS, epidemiology, differential diagnosis, medical management, molecular research and imaging tools
The {\eta}'-carbon potential at low meson momenta
The production of mesons in coincidence with forward-going
protons has been studied in photon-induced reactions on C and on a
liquid hydrogen (LH) target for incoming photon energies of 1.3-2.6 GeV at
the electron accelerator ELSA. The mesons have been identified
via the decay
registered with the CBELSA/TAPS detector system. Coincident protons have been
identified in the MiniTAPS BaF array at polar angles of . Under these kinematic constraints the
mesons are produced with relatively low kinetic energy (
150 MeV) since the coincident protons take over most of the momentum of the
incident-photon beam. For the C-target this allows the determination of the
real part of the -carbon potential at low meson momenta by
comparing with collision model calculations of the kinetic energy
distribution and excitation function. Fitting the latter data for
mesons going backwards in the center-of-mass system yields a potential depth of
V = (44 16(stat)15(syst)) MeV, consistent with earlier
determinations of the potential depth in inclusive measurements for average
momenta of 1.1 GeV/. Within the experimental
uncertainties, there is no indication of a momentum dependence of the
-carbon potential. The LH data, taken as a reference to check
the data analysis and the model calculations, provide differential and integral
cross sections in good agreement with previous results for
photoproduction off the free proton.Comment: 9 pages, 13 figures. arXiv admin note: text overlap with
arXiv:1608.0607
Photoproduction of meson pairs: First measurement of the polarization observable I^s
The polarization observable I^s, a feature exclusive to the acoplanar
kinematics of multi-meson final states produced via linearly polarized photons,
has been measured for the first time. Results for the reaction g p -> p pi0 eta
are presented for incoming photon energies between 970 MeV and 1650 MeV along
with the beam asymmetry I^c. The comparably large asymmetries demonstrate a
high sensitivity of I^s to the dynamics of the reaction. Fits using
Bonn-Gatchina partial wave analysis demonstrate that the new polarization
observables carry significant information on the contributing partial waves.Comment: 11 pages, 6 figures, v2 to appear in Phys. Lett.
Measurement of polarisation observables in photoproduction off the proton
The reaction is studied in the
photon energy range from threshold. Linearly polarised photon beams from
coherent bremsstrahlung enabled the first measurement of photon beam
asymmetries in this reaction up to MeV. In addition, the
recoil hyperon polarisation was determined through the asymmetry in the weak
decay up to MeV. The data are
compared to partial wave analyses, and the possible impact on the
interpretation of a recently observed cusp-like structure near the
thresholds is discussed.Comment: 6 pages, 5 figures. References [8,9,10,11] which were not on the
original submission are now include
Measurement of the Beam Asymmetry in the Forward Direction for pi0 Photoproduction
Photoproduction of neutral pions has been studied with the CBELSA/TAPS
detector for photon energies between 0.92 and 1.68~GeV at the electron
accelerator ELSA. The beam asymmetry~ has been extracted for of the ~meson and for . The new beam asymmetry data improve the world database for
photon energies above 1.5~GeV and, by covering the very forward region, extend
previously published data for the same reaction by our collaboration. The
angular dependence of shows overall good agreement with the SAID
parameterization.Comment: 11 pages, 10 figures, published in Phys. Rev. C, included LEPS data
and MAID 2007 predictions for comparison with our dat
Photoproduction of Neutral Pions off Protons
Photoproduction of neutral pions has been studied with the CBELSA/TAPS
detector in the reaction for photon energies between 0.85
and 2.50 GeV. The mesons are observed in their dominant neutral decay
mode: . For the first time, the differential cross
sections cover the very forward region, . A
partial-wave analysis of these data within the Bonn-Gatchina framework observes
the high-mass resonances (2190), (2080), and (2070).Comment: 12 pages, 9 figures, minor modifications to PWA sectio
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