463 research outputs found
Influence of Sintering Conditions on Specific Electrical Conductivity in Aluminum-Graphene Composite
Dependence of specific electrical resistance on temperature (20 - 1600 ∘C) and processing method in an aluminum-graphene (up to 2wt.%) composite is investigated. It is established that spark plasma sintering (SPS) under pressure 40 MPа does not influence on electrical resistance, whereas SPS at low pressure (<10 MPa) reduces electrical resistance at a room temperature on 6 orders. Lower values of electrical resistance (up to 90 Ω *mm) received at sintering in hot pressing set at radiating heating. It is supposed that the reason of sharp decrease in electrical resistance at the lowered pressure is presence of current pulsations during SPS. They induces magnetic fields in graphene flake which lead to their moving and forming of particles to electroconductive chains or their capture in arched cells at applied pressure.
Keywords: composite, aluminum, graphene, electrical resistance, temperature dependence
CMB component separation by parameter estimation
We propose a solution to the CMB component separation problem based on
standard parameter estimation techniques. We assume a parametric spectral model
for each signal component, and fit the corresponding parameters pixel by pixel
in a two-stage process. First we fit for the full parameter set (e.g.,
component amplitudes and spectral indices) in low-resolution and high
signal-to-noise ratio maps using MCMC, obtaining both best-fit values for each
parameter, and the associated uncertainty. The goodness-of-fit is evaluated by
a chi^2 statistic. Then we fix all non-linear parameters at their
low-resolution best-fit values, and solve analytically for high-resolution
component amplitude maps. This likelihood approach has many advantages: The
fitted model may be chosen freely, and the method is therefore completely
general; all assumptions are transparent; no restrictions on spatial variations
of foreground properties are imposed; the results may be rigorously monitored
by goodness-of-fit tests; and, most importantly, we obtain reliable error
estimates on all estimated quantities. We apply the method to simulated Planck
and six-year WMAP data based on realistic models, and show that separation at
the muK level is indeed possible in these cases. We also outline how the
foreground uncertainties may be rigorously propagated through to the CMB power
spectrum and cosmological parameters using a Gibbs sampling technique.Comment: 20 pages, 10 figures, submitted to ApJ. For a high-resolution
version, see http://www.astro.uio.no/~hke/docs/eriksen_et_al_fgfit.p
Hydroperoxide oxidation: unexpected C–C bond cleavage in branched alkanes and oxidation of molecular nitrogen
Foreground removal from CMB temperature maps using an MLP neural network
One of the main obstacles in extracting the Cosmic Microwave Background (CMB)
signal from observations in the mm-submm range is the foreground contamination
by emission from galactic components: mainly synchrotron, free-free and thermal
dust emission. Due to the statistical nature of the intrinsic CMB signal it is
essential to minimize the systematic errors in the CMB temperature
determinations. Following the available knowledge of the spectral behavior of
the galactic foregrounds simple, power law-like spectra have been assumed. The
feasibility of using a simple neural network for extracting the CMB temperature
signal from the combined CMB and foreground signals has been investigated. As a
specific example, we have analysed simulated data, like that expected from the
ESA Planck Surveyor mission. A simple multilayer perceptron neural network with
2 hidden layers can provide temperature estimates, over more than 80 percent of
the sky, that are to a high degree uncorrelated with the foreground signals. A
single network will be able to cover the dynamic range of the Planck noise
level over the entire sky.Comment: Accepted for publication in Astrophysics and Space Scienc
Harmonic analysis of cosmic microwave background data I: ring reductions and point-source catalogue
We present a harmonic model for the data analysis of an all-sky cosmic
microwave background survey, such as Planck, where the survey is obtained
through ring-scans of the sky. In this model, resampling and pixelisation of
the data are avoided. The spherical transforms of the sky at each frequency, in
total intensity and polarization, as well as the bright-point-source catalogue,
are derived directly from the data reduced onto the rings. Formal errors and
the most significant correlation coefficients for the spherical transforms of
the frequency maps are preserved. A clean and transparent path from the
original samplings in the time domain to the final scientific products is thus
obtained. The data analysis is largely based on Fourier analysis of rings; the
positional stability of the instrument's spin axis during these scans is a
requirement for the data model and is investigated here for the Planck
satellite. Brighter point sources are recognised and extracted as part of the
ring reductions and, on the basis of accumulated data, used to build a
catalogue. The analysis of the rings is performed iteratively, involving a
range of geometric and detector response calibrations. The reconstructed
spherical transforms of the sky form the input to the subsequent analysis
stages. Although the methods in this paper were developed with the data
processing for Planck in mind, many aspects should have wider application, such
as in the construction of real-space pixelised maps. (Abridged)Comment: 20 pages, 9 figures. To appear in MNRA
Component separation methods for the Planck mission
The Planck satellite will map the full sky at nine frequencies from 30 to 857
GHz. The CMB intensity and polarization that are its prime targets are
contaminated by foreground emission. The goal of this paper is to compare
proposed methods for separating CMB from foregrounds based on their different
spectral and spatial characteristics, and to separate the foregrounds into
components of different physical origin. A component separation challenge has
been organized, based on a set of realistically complex simulations of sky
emission. Several methods including those based on internal template
subtraction, maximum entropy method, parametric method, spatial and harmonic
cross correlation methods, and independent component analysis have been tested.
Different methods proved to be effective in cleaning the CMB maps from
foreground contamination, in reconstructing maps of diffuse Galactic emissions,
and in detecting point sources and thermal Sunyaev-Zeldovich signals. The power
spectrum of the residuals is, on the largest scales, four orders of magnitude
lower than that of the input Galaxy power spectrum at the foreground minimum.
The CMB power spectrum was accurately recovered up to the sixth acoustic peak.
The point source detection limit reaches 100 mJy, and about 2300 clusters are
detected via the thermal SZ effect on two thirds of the sky. We have found that
no single method performs best for all scientific objectives. We foresee that
the final component separation pipeline for Planck will involve a combination
of methods and iterations between processing steps targeted at different
objectives such as diffuse component separation, spectral estimation and
compact source extraction.Comment: Matches version accepted by A&A. A version with high resolution
figures is available at http://people.sissa.it/~leach/compsepcomp.pd
Magnetic resonances in EuSnAs single crystal
In this work, we report the broad-band ferromagnetic resonance spectroscopy
of EuSnAs single crystals at different temperatures in combination with
magnetization measurements and structural characterization. We observe
conventional collective acoustic resonance mode of the A-type antiferromagnetic
spin-flop phase in the Eu sub-lattice, and its transition to the paramagnetic
resonance above the ordering temperature. Furthermore, we observe reproducibly
additional well-defined spectral line. The origin of the additional line
remains unclear. However, its temperature dependence attributes it to magnetism
in the Eu sub-lattice.Comment: 6 pages, 4 figures, 34 reference
Limits on the detectability of the CMB B-mode polarization imposed by foregrounds
We investigate which practical constraints are imposed by foregrounds to the
detection of the B-mode polarization generated by gravitational waves in the
case of experiments of the type currently being planned. Because the B-mode
signal is probably dominated by foregrounds at all frequencies, the detection
of the cosmological component depends drastically on our ability for removing
foregrounds. We provide an analytical expression to estimate the level of the
residual polarization for Galactic foregrounds, according to the method
employed for their subtraction. We interpret this result in terms of the lower
limit of the tensor-to-scalar ratio r that allows to disentangle the
cosmological B-mode polarization from the foregrounds contribution. Polarized
emission from extragalactic radio sources and gravitational lensing is also
taken into account. As a first approach, we consider the ideal limit of an
instrumental noise--free experiment: for a full--sky coverage and a degree
resolution, we obtain a limit of r~10^(-4). This value can be improved by
high--resolution experiments and, in principle, no clear fundamental limit on
the detectability of gravitational waves polarization is found. Our analysis is
also applied to planned or hypothetical future polarization experiments, taking
into account expected noise levels.Comment: 15 pages, 9 figures, version accepted for publication in MNRA
Planck intermediate results. XXIX. All-sky dust modelling with Planck, IRAS, and WISE observations
We present all-sky modelling of the high resolution Planck, IRAS, and WISE
infrared (IR) observations using the physical dust model presented by Draine
and Li in 2007 (DL). We study the performance and results of this model, and
discuss implications for future dust modelling. The present work extends the DL
dust modelling carried out on nearby galaxies using Herschel and Spitzer data
to Galactic dust emission. We employ the DL dust model to generate maps of the
dust mass surface density, the optical extinction Av, and the starlight
intensity parametrized by Umin. The DL model reproduces the observed spectral
energy distribution (SED) satisfactorily over most of the sky, with small
deviations in the inner Galactic disk and in low ecliptic latitude areas. We
compare the DL optical extinction Av for the diffuse interstellar medium with
optical estimates for 2 10^5 quasi-stellar objects (QSOs) observed in the Sloan
digital sky survey. The DL Av estimates are larger than those determined
towards QSOs by a factor of about 2, which depends on Umin. The DL fitting
parameter Umin, effectively determined by the wavelength where the SED peaks,
appears to trace variations in the far-IR opacity of the dust grains per unit
Av, and not only in the starlight intensity. To circumvent the model
deficiency, we propose an empirical renormalization of the DL Av estimate,
dependent of Umin, which compensates for the systematic differences found with
QSO observations. This renormalization also brings into agreement the DL Av
estimates with those derived for molecular clouds from the near-IR colours of
stars in the 2 micron all sky survey. The DL model and the QSOs data are used
to compress the spectral information in the Planck and IRAS observations for
the diffuse ISM to a family of 20 SEDs normalized per Av, parameterized by
Umin, which may be used to test and empirically calibrate dust models.Comment: Final version that has appeared in A&
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