542 research outputs found
Measuring The Co2 Flux At The Air/water Interface In Lakes Using Flow Injection Analysis.
The carbon dioxide flux at the air/water interface in lakes was calculated after the determination of H2CO3* (free CO2) and atmospheric CO2 using flow injection analysis (FIA) coupled to a conductometric detector. The method is based on the diffusion of CO2 through a hydrophobic membrane into a flow of deionized water, generating a gradient of conductivity proportional to the concentration of CO2 in the sample. Using one experimental set-up, the speciation of the inorganic carbon (H2CO3* and dissolved inorganic carbon) was accomplished by simply adjusting the sample pH. The determination of CO2 in the atmosphere was carried out by direct injection of the gaseous samples. The FIA apparatus was taken into the field and CO2 fluxes were evaluated in several Brazilian lakes. In these lakes, representing different eutrophic stages, the CO2 flux varied from -242 (invasive) up to 3227 (evasive) mumol CO2 m-2 h-1.3317-2
Magnetic properties of Fe3O4 nanoparticles coated with oleic and dodecanoic acids
Magnetic nanoparticles (NP) of magnetite (Fe3O4) coated with oleic acid (OA)
and dodecanoic acid (DA) were synthesized and investigated through Transmission
Electron Microscopy (TEM),magnetization M, and ac magnetic susceptibility
measurements. The OA coated samples were produced with different magnetic
concentrations (78, 76, and 65%) and the DA sample with 63% of Fe3O4. Images
from TEM indicate that the NP have a nearly spherical geometry and mean
diameter ~ 5.5 nm. Magnetization measurements, performed in zero field cooled
(ZFC) and field cooled (FC) processes under different external magnetic fields
H, exhibited a maximum at a given temperature TB in the ZFC curves, which
depends on the NP coating (OA or DA), magnetite concentration, and H. The
temperature TB decreases monotonically with increasing H and, for a given H,
the increase in the magnetite concentration results in an increase of TB. The
observed behavior is related to the dipolar interaction (DI) between NP which
seems to be an important mechanism in all samples studied. This is supported by
the results of the ac magnetic susceptibility Xac measurements, where the
temperature in which X' peaks for different frequencies follows the
Vogel-Fulcher model, a feature commonly found in systems with dipolar
interactions. Curves of H vs. TB/TB(H=0) for samples with different coatings
and magnetite concentrations collapse into a universal curve, indicating that
the qualitative magnetic behavior of the samples may be described by the NP
themselves, instead of the coating or the strength of the dipolar interaction.
Below TB, M vs. H curves show a coercive field (HC) that increases
monotonically with decreasing temperature. The saturation magnetization (MS)
follows the Bloch's law and values of MS at room temperature as high as 78
emu/g were estimated, a result corresponding to ~80% of the bulk value. The
overlap of M/MS vs. H/T curves for a given sample and the low HC at high
temperatures suggest superparamagnetic behavior in all samples studied. The
overlap of M/MS vs. H curves at constant temperature for different samples
indicates that the NP magnetization behavior is preserved, independently of the
coating and magnetite concentration.Comment: 8 pages and 9 figure
Heterotic Compactification, An Algorithmic Approach
We approach string phenomenology from the perspective of computational
algebraic geometry, by providing new and efficient techniques for proving
stability and calculating particle spectra in heterotic compactifications. This
is done in the context of complete intersection Calabi-Yau manifolds in a
single projective space where we classify positive monad bundles. Using a
combination of analytic methods and computer algebra we prove stability for all
such bundles and compute the complete particle spectrum, including gauge
singlets. In particular, we find that the number of anti-generations vanishes
for all our bundles and that the spectrum is manifestly moduli-dependent.Comment: 36 pages, Late
Metal-insulator transition in NdEuNiO compounds
Polycrystalline NdEuNiO () compounds
were synthesized in order to investigate the character of the metal-insulator
(MI) phase transition in this series. Samples were prepared through the sol-gel
route and subjected to heat treatments at 1000 C under oxygen
pressures as high as 80 bar. X-ray Diffraction (XRD) and Neutron Powder
Diffraction (NPD), electrical resistivity , and Magnetization
measurements were performed on these compounds. The results of NPD and XRD
indicated that the samples crystallize in an orthorhombic distorted perovskite
structure, space group . The analysis of the structural parameters
revealed a sudden and small expansion of 0.2% of the unit cell volume
when electronic localization occurs. This expansion was attributed to a small
increase of 0.003 \AA{} of the average Ni-O distance and a simultaneous
decrease of of the Ni-O-Ni superexchange angle. The
measurements revealed a MI transition occurring at temperatures
ranging from to 336 K for samples with and 0.50,
respectively. These measurements also show a large thermal hysteresis in
NdNiO during heating and cooling processes suggesting a first-order
character of the phase transition at . The width of this thermal
hysteresis was found to decrease appreciably for the sample
NdEuNiO. The results indicate that cation disorder
associated with increasing substitution of Nd by Eu is responsible for changing
the first order character of the transition in NdNiO.Comment: 19 pages, 9 figure
Instability of scalar perturbation in a phantomic cosmological scenario
Scalar perturbations can grow during a phantomic cosmological phase as the
big rip is approached, in spite of the high accelerated expansion regime, if
the equation of state is such that . It is
shown that such result is independent of the spatial curvature. The perturbed
equations are exactly solved for any value of the curvature parameter and
of the equation of state parameter . Growing modes are found
asymptotically under the condition . Since the Hubble radius
decreases in a phantom universe, such result indicates that a phantom scenario
may not survive longtime due to gravitational instability.Comment: Latex file, 6 page
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