1,327 research outputs found
Io: IUE observations of its atmosphere and the plasma torus
Two of the main components of the atmosphere of Io, neutral oxygen and sulfur, were detected with the IUE. Four observations yield brightnesses that are similar, regardless of whether the upstream or the downstream sides of the torus plasma flow around Io is observed. A simple model requires the emissions to be produced by the interaction of O and S columns in the exospheric range with 2 eV electrons. Cooling of the 5 eV torus electrons is required prior to their interaction with the atmosphere of Io. Inconsistencies in the characteristics of the spectra that cannot be accounted for in this model require further analysis with improved atomic data. The Io plasma torus was monitored with the IUE. The long-term stability of the warm torus is established. The observed brightnesses were analyzed using a model of the torus, and variations of less than 30 percent in the composition are observed, the quantitative results being model dependent
Comparative behaviour in terms of wear and corrosion resistance of galvanized and zinc-iron coated steels
Materials degradation due to simultaneous chemical and mechanical effects may occur under a variety of conditions. Galvanized and zinc-iron electrodeposited steels are widely used to protect automobile bodies against corrosion. In such applications, it is important to investigate the electrochemical and tribological behaviour of the coatings, in order to understand the behaviour of those surfaces in a tribological contact exposed to a corrosive environment. In the present paper, the evolution of open circuit potential, for both surfaces, was monitored during reciprocating sliding against an alumina pin in a 3% NaCl solution. The coatings are compared in terms of electrochemical behaviour by polarization tests, open circuit potential and friction coefficient during sliding; and loss of mass after sliding. The surface morphology, after sliding, was examined by SEM. Under the experimental conditions of the present study, the results found in terms of corrosion and wear resistance could be correlated to the differences in morphology, rugosity and composition of the two analysed surfaces.Capes Brasil.CNPq Brasil
The effect of the presence of silicone films on the corrosion resistance of Zn and ZnFe coated steel
Chromatation pre-treatments have been widely used to improve galvanized steel corrosion resistance. However, due to the high toxicity of chromate ions, chromatation pre-treatments tend to be banned and, in last years, alternative coating systems are under investigation.
Recently, polysiloxanes have been developed for application as coatings. Among them, and due to their specific properties, such as hardness, chemical resistance and hydrophobicity, silicone resins may be considered as promising substitutes for chromatation pre-treatments.
In this work silicone films, obtained from the hydrolysis of a methoxy functional silicone
reactive intermediate, were applied on galvanized steel and on steel electroplated with a ZnFe alloy. Electrochemical techniques were used to characterise the degradation behaviour of the samples. These consisted on the monitoring of the open circuit potential (OCP), and on the potentiodynamic polarization of the samples, which was performed in a 3% NaCl aqueous
solution. Additionally, electrochemical impedance spectroscopy (EIS) was used as a complementary technique for the evaluation of the corrosion mechanisms of the coating
system. SEM and EDS were employed to inspect the surface of the samples before and after the electrochemical tests. EIS data was fitted to an equivalent circuit from which the electrochemical parameters were obtained.
Results show the protective character of the resin films, when compared with uncovered
specimens. The capacitance of the films increased with the immersion time, in accordance to the behaviour expected for an organic film. The overall performance of the coating systems appears to be highly dependent on the type of metallic coating applied to the steel. During the first three days of immersion the coatings applied upon galvanized steel showed larger |Z|
values when compared with those applied to the electroplated steel, indicating a superior
corrosion resistance of the former. However, after that time, an abrupt drop of |Z| is observed in the film applied on galvanized steel. In comparison, the coating system involving ZnFe alloy evidences a better stability throughout the immersion time.CAPES - Brasil
CNPq - Brasi
Can we identify non-stationary dynamics of trial-to-trial variability?"
Identifying sources of the apparent variability in non-stationary scenarios is a fundamental problem in many biological data analysis settings. For instance, neurophysiological responses to the same task often vary from each repetition of the same experiment (trial) to the next. The origin and functional role of this observed variability is one of the fundamental questions in neuroscience. The nature of such trial-to-trial dynamics however remains largely elusive to current data analysis approaches. A range of strategies have been proposed in modalities such as electro-encephalography but gaining a fundamental insight into latent sources of trial-to-trial variability in neural recordings is still a major challenge. In this paper, we present a proof-of-concept study to the analysis of trial-to-trial variability dynamics founded on non-autonomous dynamical systems. At this initial stage, we evaluate the capacity of a simple statistic based on the behaviour of trajectories in classification settings, the trajectory coherence, in order to identify trial-to-trial dynamics. First, we derive the conditions leading to observable changes in datasets generated by a compact dynamical system (the Duffing equation). This canonical system plays the role of a ubiquitous model of non-stationary supervised classification problems. Second, we estimate the coherence of class-trajectories in empirically reconstructed space of system states. We show how this analysis can discern variations attributable to non-autonomous deterministic processes from stochastic fluctuations. The analyses are benchmarked using simulated and two different real datasets which have been shown to exhibit attractor dynamics. As an illustrative example, we focused on the analysis of the rat's frontal cortex ensemble dynamics during a decision-making task. Results suggest that, in line with recent hypotheses, rather than internal noise, it is the deterministic trend which most likely underlies the observed trial-to-trial variability. Thus, the empirical tool developed within this study potentially allows us to infer the source of variability in in-vivo neural recordings
Resistência à corrosão de aço galvanizado e de aço eletrodepositado com znfe revestido por resinas de silicone
As caracterĂsticas protectoras das superfĂcies de aços galvanizados sĂŁo geralmente intensificadas atravĂ©s da passivação por crĂłmio hexavalente (Cr+6). PorĂ©m, por ser este um elemento tĂłxico, vários estudos visando a substituição do procedimento de cromatização vĂŞm sendo realizados. As resinas de silicone, devido a propriedades como dureza, resistĂŞncia quĂmica e aderĂŞncia, podem vir a ser um excelente substituinte ao processo de cromatização. Neste trabalho, filmes de silicone obtidos pela hidrĂłlise e subsequente condensação do tetraetoxisilano (TEOS) em presença de Polidimetilsiloxano (PDMS - OH) e de uma resina de silicone comercial foram aplicados em aço galvanizado e aço electrodepositado com ZnFe. A composição estudada foi 45% PDMS – 40% TEOS
– 15 % Resina. O revestimento foi então avaliado através de Espectroscopia de
Impedância ElectroquĂmica, em solução de NaCl 3%. A evolução do comportamento
frente Ă corrosĂŁo do sistema de revestimento foi investigada de forma temporal, verificando-se a influĂŞncia dos diferentes componentes na resposta electroquĂmica do filme. Os dados obtidos pela Espectroscopia foram modelados atravĂ©s de um circuito equivalente, permitindo a obtenção dos parâmetros electroquĂmicos para o sistema
avaliado.
Os resultados obtidos mostram que o comportamento dos filmes de silicone é dependente o tipo de revestimento metálico que foi aplicado no aço. Maiores valores para |Z| foram obtidos para os filmes aplicados sobre o aço galvanizado, sendo este valor pouco alterado com o tempo de imersão.Corrosion properties of galvanized steels are generally increased through a passivation
processes with Cr+6 resulting in the formation of a passive layer. However, these kinds of chromates are toxic and environmentally legislation prohibits pre-treatments with chromates. As a consequence alternative pre-treatments have been studied along of last decades. Thus, alternative coatings of silicone resins due to their specific properties, such as hardness, chemical resistance and hydrofobicity, can be a viable substitute. In the present work silicon films obtained by hydrolysis and posterior condensation of tetraethoxysilane (TEOS) in presence of Poli(dimetilsiloxane) (PDMS - OH) and a commercial silicone resin, in a sol-gel process conditions, were applied on galvanized steel and on steel electroplated with a ZnFe alloy. The composition studied was 45%PDMS - 40% TEOS - 15% Resin. The film corrosion protection properties were
evaluated by Electrochemical Impedance Spectroscopy (EIS) in a NaCl 3% solution at
different immersion times. EIS experimental data were fitted to an equivalent circuit from
which the electrochemical parameters were obtained.
Results show that the overall performance of the coatings system appears to be highly
dependent on the metallic coating kind applied to the steel. The better results in terms of
impedance |Z| were obtained in coatings applied over galvanized steels, these
impedance values showing a stable behaviour with immersion time increasing.Coordenação de Aperfeiçoamento de Pessoal de NĂvel Superior (CAPES)
Conselho Nacional de Desenvolvimento CientĂfico e TecnolĂłgico (CNPq)
Fundação para a Ciência e a Tecnologia (FCT
Organic-inorganic hybrid coatings for corrosion protection of galvanized steel and electroplated znfe steel
The development of hybrids materials by the sol-gel process has been extensively investigated in recent years. The combination a wide variety of compositions and production
processes had permit the use of these aterials in different applications like coatings for corrosion protection of metals and abrasion resistant coatings. In this work organic-inorganic hybrid materials
have been prepared from the hydrolysis of tetraethylorthosilicate (TEOS) and silanol-terminated polidymetilmetoxysilane (PDMS) using a sol-gel process. The composition evaluated was 50% TEOS- 50% PDMS. These materials have been applied on galvanized steel and on steel electroplated with a ZnFe alloy. In order to evaluate the degradation behaviour of these coatings, electrochemical techniques were used. They consisting in the monitoring of the open-circuit potential (Ecorr), and in the realization of potentiodynamic polarization measurements of the samples, which was performed in a
3% NaCl aqueous solution. Additionally, electrochemical impedance spectroscopy (EIS) was used as a complementary technique for the evaluation of the corrosion mechanisms of the coating system. The
surface of the samples, before and after the electrochemical tests, have been inspected by SEM and EDS. EIS data was fitted to an equivalent circuit from which the lectrochemical parameters were obtained. Results show a good protective character of the hybrid films, when compared with
uncovered specimens. The capacitance of the films increased with the immersion time, in accordance to the behaviour expected for an organic film. The overall performance of the coating systems appears to be highly dependent on the kind of metallic coating applied to the steel. Coatings applied upon
galvanized steel showed larger |Z| values when compared with those applied to the electroplated steel, indicating a superior corrosion resistance of the former and a better stability throughout the immersion time
Collective and static properties of model two-component plasmas
Classical MD data on the charge-charge dynamic structure factor of
two-component plasmas (TCP) modeled in Phys. Rev. A 23, 2041 (1981) are
analyzed using the sum rules and other exact relations. The convergent power
moments of the imaginary part of the model system dielectric function are
expressed in terms of its partial static structure factors, which are computed
by the method of hypernetted chains using the Deutsch effective potential.
High-frequency asymptotic behavior of the dielectric function is specified to
include the effects of inverse bremsstrahlung. The agreement with the MD data
is improved, and important statistical characteristics of the model TCP, such
as the probability to find both electron and ion at one point, are determined.Comment: 25 pages, 6 figures, 5 tables. Published in Physical Review E
http://link.aps.org/abstract/PRE/v76/e02640
Camarão-da-malásia.
O camarĂŁo da espĂ©cie Macrobrachium rosenbergii, conhecido como camarĂŁo-da-malásia, Ă© um dos mais cultivados no mundo. De acordo com a Organização das Nações Unidas para a Alimentação e a Agricultura (FAO), a produção mundial de camarĂŁo-da-malásia pela carcinicultura (como Ă© chamada a criação de camarões) foi de 234 mil toneladas em 2016. Os camarões dessa espĂ©cie sĂŁo grandes e podem atingir 32 centĂmetros de comprimento no ambiente natural. Em cativeiro, geralmente sĂŁo cultivados e despescados para comercialização quando atingem de 20 gramas a 50 gramas, dependendo da exigĂŞncia do consumidor.bitstream/item/205222/1/Camarao-da-Malasia.pd
Oscillatory Modes of a Prominence-PCTR-Corona Slab Model
Oscillations of magnetic structures in the solar corona have often been
interpreted in terms of magnetohydrodynamic waves. We study the adiabatic
magnetoacoustic modes of a prominence plasma slab with a uniform longitudinal
magnetic field, surrounded by a prominence-corona transition region (PCTR) and
a coronal medium. Considering linear small-amplitude oscillations, the
dispersion relation for the magnetoacoustic slow and fast modes is deduced
assuming evanescent-like perturbations in the coronal medium. In the system
without PCTR, a classification of the oscillatory modes according to the
polarisation of their eigenfunctions is made in order to distinguish modes with
fast-like or slow-like properties. Internal and external slow modes are
governed by the prominence and coronal properties respectively, and fast modes
are mostly dominated by prominence conditions for the observed wavelengths. In
addition, the inclusion of an isothermal PCTR does not substantially influence
the mode frequencies, but new solutions (PCTR slow modes) are present.Comment: Accepted for publication in Solar Physic
First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A
We present the abundance analyses of three isolated A-type supergiant stars
in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES
spectrograph at the VLT. Detailed model atmosphere analyses have been used to
determine the stellar atmospheric parameters and the elemental abundances of
the stars. The mean iron group abundance was determined from these three stars
to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of
the present-day iron group abundances in Sextans A. These three stars now
represent the most metal-poor massive stars for which detailed abundance
analyses have been carried out. The mean stellar alpha element abundance was
determined from the alpha element magnesium as
[alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the
nebular alpha element abundances as determined from oxygen in the H II regions.
These results are consistent from star-to-star with no significant spatial
variations over a length of 0.8 kpc in Sextans A. This supports the nebular
abundance studies of dwarf irregular galaxies, where homogeneous oxygen
abundances are found throughout, and argues against in situ enrichment. The
alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which
is consistent with the solar ratio. This is consistent with the results from
A-supergiant analyses in other Local Group dwarf irregular galaxies but in
stark contrast with the high [alpha/Fe] results from metal-poor stars in the
Galaxy, and is most clearly seen from these three stars in Sextans A because of
their lower metallicities. The low [alpha/Fe] ratios are consistent with the
slow chemical evolution expected for dwarf galaxies from analyses of their
stellar populations.Comment: 40 pages, 8 figures, accepted for publication in A
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