3,669 research outputs found
Structure and magnetic properties of nanocrystalline PrCo3
The structure and magnetic properties of nanocrystalline PrCo prepared by
high-energy milling technique have been investigated by means of X-ray
diffraction using the Rietveld method coupled to Curie temperature and magnetic
measurements. The as-milled samples were subsequently annealed in temperature
range from 750 to 1050 {\deg}C for 30 min to optimize the extrinsic properties.
From x-ray studies of magnetic aligned samples, the magnetic anisotropy of this
compounds is found uniaxial. The Curie temperature is 349 {\deg}K and no
saturation reached at room temperature for applied field of 90 kOe. The
coercive field of 55 kOe and 12 kOe measured at 10 and 293 K respectively is
obtained after annealing at 750 {\deg}C for 30 min suggests that
nanocrystalline PrCo are interesting candidates in the field of permanent
magnets. We have completed this experimental study by simulations in the
micromagnetic framework in order to get a qualitative picture of the
microstructure effect on the macroscopic magnetization curve. From this simple
model calculation, we can suggest that the after annealing the system behaves
as magnetically hard crystallites embedded in a weakly magnetized amorphous
matrix. PACS : 75.50.Bb, 75.50.Tt, 76.80.+yComment: Published in Journal of Applied Physics, 107, 083916 (2010). To be
found at: http://jap.aip.or
Magnetic and structural properties of nanocrystalline PrCo
The structure and magnetic properties of nanocrystalline PrCo obtained
from high energy milling technique are investigated by X-ray diffraction, Curie
temperature determination and magnetic properties measurements are reported.
The as-milled samples have been annealed in a temperature range of 1023 K to
1273 K for 30 mn to optimize the extrinsic properties. The Curie temperature is
349\,K and coercive fields of 55\,kOe at 10\,K and 12\,kOe at 293\,K are
obtained on the samples annealed at 1023\,K. A simulation of the magnetic
properties in the framework of micromagnetism has been performed in order to
investigate the influence of the nanoscale structure. A composite model with
hard crystallites embedded in an amorphous matrix, corresponding to the
as-milled material, leads to satisfying agreement with the experimental
magnetization curve. [ K. Younsi, V. Russier and L. Bessais, J. Appl. Phys.
{\bf 107}, 083916 (2010)]. The microscopic scale will also be considered from
DFT based calculations of the electronic structure of Co compounds,
where = (Y, Pr) and = 2,3 and 5.Comment: To be published in J. Phys.: Conference Series in the JEMS 2010
special issue. To be found once published at
http://iopscience.iop.org/1742-659
Huxley-type cross-bridge models in largeish-scale musculoskeletal models; an evaluation of computational cost
A Huxley-type cross-bridge model is attractive because it is inspired by our current understanding of the processes underlying muscle contraction, and because it provides a unified description of muscle's mechanical behavior and metabolic energy expenditure. In this study, we determined the computational cost for task optimization of a largeish-scale musculoskeletal model in which muscles are represented by a 2-state Huxley-type cross-bridge model. Parameter values defining the rate functions of the Huxley-type cross-bridge model could be chosen such that the steady-state force-velocity relation resembled that of a Hill-type model. Using these parameter values, maximum-height squat jumping was used as the example task to evaluate the computational cost of task optimization for a skeletal model driven by a Huxley-type cross-bridge model. The optimal solutions for the Huxley- and Hill-type muscle models were similar for all mechanical variables considered. Computational cost of the Huxley-type cross-bridge model was much higher than that of the Hill-type model. Compared to the Hill-type model, the number of state variables per muscle was large (2 vs about 18,000), the integration step size had to be about 100 times smaller, and the computational cost per integration step was about 100 times higher
Improving Money’s Worth Ratio Calculations: The Case of Singapore’s Pension Annuities
This paper contributes to a better understanding of the risks involved in a life annuity investment. We study the full distribution of weighted annuity benefits and quantify risk measures such as dispersion and skewness, thereby extending the usefulness of the popular money’s worth valuation framework for life annuities. Using data from pension annuities in Singapore, we also introduce several risk measures that might appeal to less financially sophisticated retirees. A more detailed and accurate picture of the risk of investing in life annuities emerges, enabling prospective annuitants to differentiate among products that may appear seemingly uniform in terms of money’s worth, but vary widely in terms of their risk attributes
Phonons and Colossal Thermal Expansion Behavior of Ag3Co(CN)6 and Ag3Fe(CN)6
Recently colossal positive volume thermal expansion has been found in the
framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. Phonon spectra have been
measured using the inelastic neutron scattering technique as a function of
temperature and pressure. The data has been analyzed using ab-initio
calculations. We find that the bonding is very similar in both compounds. At
ambient pressure modes in the intermediate frequency part of the vibrational
spectra in the Co compound are shifted to slightly higher energies as compared
to the Fe compound. The temperature dependence of the phonon spectra gives
evidence for large explicit anharmonic contribution to the total anharmonicity
for low-energy modes below 5 meV. We found that modes are mainly affected by
the change in the size of unit cell, which in turn changes the bond lengths and
vibrational frequencies. Thermal expansion has been calculated via the volume
dependence of phonon spectra. Our analysis indicates that Ag phonon modes in
the energy range from 2 to 5 meV are strongly anharmonic and major contributors
to thermal expansion in both compounds. The application of pressure hardens the
low-energy part of the phonon spectra involving Ag vibrations and confirms the
highly anharmonic nature of these modes.Comment: 19 pages, 14 figures and one tabl
Early Childhood Education in Ghana: Perceptions of Stakeholders in the Western Region of Ghana
The purpose of the study was to determine the perceptions of the stakeholders in early childhood education and their perceptions affect the performance of their roles in the implementation of early childhood education programmes. Parents, community members, early childhood education teachers, members of religious organisations and Non-Governmental Organisations formed population of the study. Simple random sampling technique was used to sample the early childhood schools and stratified sampling technique participants in each district. The instruments used for data collection were questionnaire and structured interview guide. Data collected were summarised using percentages, means, standard deviations and Pearson Moment Correlation Coefficient The Stakeholder in the Western Region of Ghana have low perception of early childhood education and this adversely affects the performance of their roles to ensure effective implementation of early childhood programmes and policies in the region. There is a relationship between their perceptions and the level of performance of their roles. It is recommended that Religious organizations, District and Municipal Assemblies as well as non-governmental organizations should be encouraged to participate actively in the implementation of early childhood programmes and policies in the regio
Dust Emission Features in NGC 7023 between 0.35 and 2.5 micron: Extended Red Emission (0.7 micron) and Two New Emission Features (1.15 and 1.5 micron)
We present 0.35 to 2.5 micron spectra of the south and northwest filaments in
the reflection nebula NGC 7023. These spectra were used to test the theory of
Seahra & Duley that carbon nanoparticles are responsible for Extended Red
Emission (ERE). Our spectra fail to show their predicted second emission band
at 1.0 micron even though both filaments exhibit strong emission in the
familiar 0.7 micron ERE band. The northwest filament spectrum does show one,
and possibly two, new dust emission features in the near-infrared. We clearly
detect a strong emission band at 1.5 micron which we tentatively attribute to
beta-FeSi_2 grains. We tentatively detect a weaker emission band at 1.15 micron
which coincides with the location expected for transitions from the conduction
band to mid-gap defect states of silicon nanoparticles. This is added evidence
that silicon nanoparticles are responsible for ERE as they already can explain
the observed behavior of the main visible ERE band.Comment: 9 pages, color figures, accepted to the ApJ, color and b/w versions
available at http://dirty.as.arizona.edu/~kgordon/papers/ere_1um.htm
SUMER: Solar Ultraviolet Measurements of Emitted Radiation
The SUMER (solar ultraviolet measurements of emitted radiation) experiment is described. It will study flows, turbulent motions, waves, temperatures and densities of the plasma in the upper atmosphere of the Sun. Structures and events associated with solar magnetic activity will be observed on various spatial and temporal scales. This will contribute to the understanding of coronal heating processes and the solar wind expansion. The instrument will take images of the Sun in EUV (extreme ultra violet) light with high resolution in space, wavelength and time. The spatial resolution and spectral resolving power of the instrument are described. Spectral shifts can be determined with subpixel accuracy. The wavelength range extends from 500 to 1600 angstroms. The integration time can be as short as one second. Line profiles, shifts and broadenings are studied. Ratios of temperature and density sensitive EUV emission lines are established
Near-Infrared Spectroscopy of Molecular Filaments in the Reflection Nebula NGC 7023
We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2)
emission from molecular filaments in the reflection nebula NGC 7023. We derive
the relative column densities of H_2 rotational-vibrational states from the
measured line emission and compare these results with several model
photodissociation regions covering a range of densities, incident UV-fields,
and excitation mechanisms. Our best-fit models for one filament suggest, but do
not require, either a combination of different densities, suggesting clumps of
10^6 cm^{-3} in a 10^4 - 10^5 cm^{-3} filament, or a combination of fluorescent
excitation and thermally-excited gas, perhaps due to a shock from a bipolar
outflow. We derive densities and UV fields for these molecular filaments that
are in agreement with previous determinations.Comment: ApJ accepted, 26 pages including 5 embedded figures, uses AASTEX.
Also available at http://www-astronomy.mps.ohio-state.edu/~martini/pubs.htm
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