2,312 research outputs found
Restoring observed classical behavior of the carbon nanotube field emission enhancement factor from the electronic structure
Experimental Fowler-Nordheim plots taken from orthodoxly behaving carbon
nanotube (CNT) field electron emitters are known to be linear. This shows that,
for such emitters, there exists a characteristic field enhancement factor (FEF)
that is constant for a range of applied voltages and applied macroscopic fields
. A constant FEF of this kind can be evaluated for classical CNT
emitter models by finite-element and other methods, but (apparently contrary to
experiment) several past quantum-mechanical (QM) CNT calculations find
FEF-values that vary with . A common feature of most such
calculations is that they focus only on deriving the CNT real-charge
distributions. Here we report on calculations that use density functional
theory (DFT) to derive real-charge distributions, and then use these to
generate the related induced-charge distributions and related fields and FEFs.
We have analysed three carbon nanostructures involving CNT-like nanoprotrusions
of various lengths, and have also simulated geometrically equivalent classical
emitter models, using finite-element methods. We find that when the
DFT-generated local induced FEFs (LIFEFs) are used, the resulting values are
effectively independent of macroscopic field, and behave in the same
qualitative manner as the classical FEF-values. Further, there is fair to good
quantitative agreement between a characteristic FEF determined classically and
the equivalent characteristic LIFEF generated via DFT approaches. Although many
issues of detail remain to be explored, this appears to be a significant step
forwards in linking classical and QM theories of CNT electrostatics. It also
shows clearly that, for ideal CNTs, the known experimental constancy of the FEF
value for a range of macroscopic fields can also be found in appropriately
developed QM theory.Comment: A slightly revised version has been published - citation below -
under a title different from that originally used. The new title is:
"Restoring observed classical behavior of the carbon nanotube field emission
enhancement factor from the electronic structure
Modeling the Field Emission Enhancement Factor for Capped Carbon Nanotubes using the Induced Electron Density
In many field electron emission experiments on single-walled carbon nanotubes
(SWCNTs), the SWCNT stands on one of two well-separated parallel plane plates,
with a macroscopic field FM applied between them. For any given location "L" on
the SWCNT surface, a field enhancement factor (FEF) is defined as
/, where is a local field defined at "L".
The best emission measurements from small-radii capped SWCNTs exhibit
characteristic FEFs that are constant (i.e., independent of ). This
paper discusses how to retrieve this result in quantum-mechanical (as opposed
to classical electrostatic) calculations. Density functional theory (DFT) is
used to analyze the properties of two short, floating SWCNTS, capped at both
ends, namely a (6,6) and a (10,0) structure. Both have effectively the same
height ( nm) and radius ( nm). It is found that apex
values of local induced FEF are similar for the two SWCNTs, are independent of
, and are similar to FEF-values found from classical conductor
models. It is suggested that these induced-FEF values relate to the SWCNT
longitudinal system polarizabilities, which are presumed similar. The DFT
calculations also generate "real", as opposed to ``induced", potential-energy
(PE) barriers for the two SWCNTs, for FM-values from 3 V/m to 2 V/nm. PE
profiles along the SWCNT axis and along a parallel ``observation line" through
one of the topmost atoms are similar. At low macroscopic fields the details of
barrier shape differ for the two SWCNT types. Even for , there
are distinct PE structures present at the emitter apex (different for the two
SWCNTs); this suggests the presence of structure-specific chemically induced
charge transfers and related patch-field distributions
CD-62°1346: An extreme halo or hypervelocity CH star?
High-velocity halo stars provide important information about the properties of the extreme Galactic halo. The study of unbound and bound Population II stars permits us to better estimate the mass of the halo. Aims: We carried out a detailed spectroscopic and kinematic study and have significantly refined the distance and the evolutionary state of the star. Methods: Its atmospheric parameters, chemical abundances and kinematical properties were determined using high-resolution optical spectroscopy and employing the local-thermodynamic-equilibrium model atmospheres of Kurucz and the spectral analysis code moog. Results: We found that CD-62°1346 is a metal-poor ([Fe/H] = -1.6) evolved giant star with Teff = 5300 K and log g = 1.7. The star exhibits high carbon and s-element abundances typical of CH stars. It is also a lead star. Our kinematic analysis of its 3D space motions shows that this star has a highly eccentric (e = 0.91) retrograde orbit with an apogalactic distance of ~100 kpc, exceeding by a factor of two the distance of the Magellanic Clouds. The star travels with very high velocity relative to the Galactocentric reference frame (VGRF = 570 km s-1). Conclusions: CD-62°1346 is an evolved giant star and not a subgiant star, as was considered earlier. Whether it is bound or unbound to the Galaxy depends on the assumed mass and on the adopted Galactic potential. We also show that the star HD 5223 is another example of a high-velocity CH star that exceeds the Galactic escape velocity. Possible origins of these two high-velocity stars are briefly discussed. CD-62°1346 and HD 5223 are the first red giant stars to join the restricted group of hypervelocity stars.Fil: Pereira, C. B.. Ministério de Ciencia, Tecnologia e Innovacao. Observatorio Nacional; BrasilFil: Jilinski, E.. Ministério de Ciencia, Tecnologia e Innovacao. Observatorio Nacional; Brasil. Universidade do Estado de Rio do Janeiro; Brasil. Russian Academy of Sciences. Pulkovo Observatory; RusiaFil: Drake, N. A.. Ministério de Ciencia, Tecnologia e Innovacao. Observatorio Nacional; Brasil. Russian Academy of Sciences. Pulkovo Observatory; RusiaFil: de Castro, D. B.. Ministério de Ciencia, Tecnologia e Innovacao. Observatorio Nacional; BrasilFil: Ortega, V. G.. Ministério de Ciencia, Tecnologia e Innovacao. Observatorio Nacional; BrasilFil: Chavero, Carolina Andrea. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Roig, Fernando Virgilio. Ministério de Ciencia, Tecnologia e Innovacao. Observatorio Nacional; Brasi
On the quantum mechanics of how an ideal carbon nanotube field emitter can exhibit a constant field enhancement factor
Measurements of current-voltage characteristics from ideal carbon nanotube
(CNT) field electron emitters of small apex radius have shown that these
emitters can exhibit a linear Fowler-Nordheim (FN) plot [e.g., Dean and
Chalamala, Appl. Phys. Lett., 76, 375, 2000]. From such a plot, a constant
(voltage-independent) characteristic field enhancement factor (FEF) can be
deduced. Over fifteen years later, this experimental result has not yet been
convincingly retrieved from first-principles electronic structure calculations,
or more generally from quantum mechanics (QM). On the contrary, several QM
calculations have deduced that the characteristic FEF should be a function of
the macroscopic field applied to the CNT. This apparent contradiction between
experiment and QM theory has been an unexplained feature of CNT emission
science, and has raised doubts about the ability of existing QM models to
satisfactorily describe experimental CNT emission behavior. In this work we
demonstrate, by means of a density functional theory analysis of single-walled
CNTs "floating" in an applied macroscopic field, the following significant
result. This is that agreement between experiment, classical-conductor CNT
models and QM calculations can be achieved if the latter are used to calculate
(from the "real" total-charge-density distributions initially obtained) the
distributions of charge-density, induced local fields and
induced local FEFs. The present work confirms, more reliably and in
significantly greater detail than in earlier work on a different system, that
this finding applies to the common "post-on-a-conducing plane" situation of CNT
field electron emission. This finding also brings out various further
theoretical questions that need to be explored
Valor nutritivo de las fracciones solubles e insolubles del follaje de Gliricidia sepium (Jacq.) Kunth ex Walp
Nutritional potential of water-soluble and water non-soluble leafage of Gliricidia sepium, tested by ruminant degradability and in vitro gas production techniques, was studied.
Ruminant degradability of dry matter and nitrogen showed high values, i.e., 54,5 to
81,9 % and 73,2 to 92,1 %respectively. Lower values were registered in leafage with an older rebudding age or stem inclusion. Both water-soluble and water-nonsoluble leafage are nutritionally relevant. Water-soluble leafage nutritional contribution was higher during the first twelve-hour fermentation process. G. sepium leafage nutritional potential derives from its gradual nitrogen and energy supply to rumen micro-organisms.Se caracterizó la contribución nutritiva de las fracciones solubles e insolubles de diferentes follajes de G. sepium, medidos con las técnicas de degradabilidad ruminal y de producción de gas in vitro. Los potenciales de degradabilidad ruminal de la materia seca (entre 54,5 y 81,9 %) y el nitrógeno (entre 73,2 y 92,1 %) se pueden considerar altos. Los valores menores se alcanzan en follajes con mayores edades de rebrote y con inclusión de tallos. Tanto la fracción soluble como insoluble en agua realizan una importante contribución nutritiva; en las primeras 12 horas de fermentación la fracción soluble hace su mayor aporte. El valor nutritivo del follaje de G. sepium radica, fundamentalmente, en su aporte gradual de nitrógeno y energía a los microorganismos del rumen
A Rapid Screening Psychometric Test
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66666/2/10.1177_000992286900800506.pd
A novel multivariate STeady-state index during general ANesthesia (STAN)
The assessment of the adequacy of general anesthesia for surgery, namely the nociception/anti-nociception balance, has received wide attention from the scientific community. Monitoring systems based on the frontal EEG/EMG, or autonomic state reactions (e.g. heart rate and blood pressure) have been developed aiming to objectively assess this balance. In this study a new multivariate indicator of patients' steady-state during anesthesia (STAN) is proposed, based on wavelet analysis of signals linked to noxious activation. A clinical protocol was designed to analyze precise noxious stimuli (laryngoscopy/intubation, tetanic, and incision), under three different analgesic doses; patients were randomized to receive either remifentanil 2.0, 3.0 or 4.0 ng/ml. ECG, PPG, BP, BIS, EMG and [Formula: see text] were continuously recorded. ECG, PPG and BP were processed to extract beat-to-beat information, and [Formula: see text] curve used to estimate the respiration rate. A combined steady-state index based on wavelet analysis of these variables, was applied and compared between the three study groups and stimuli (Wilcoxon signed ranks, Kruskal-Wallis and Mann-Whitney tests). Following institutional approval and signing the informed consent thirty four patients were enrolled in this study (3 excluded due to signal loss during data collection). The BIS index of the EEG, frontal EMG, heart rate, BP, and PPG wave amplitude changed in response to different noxious stimuli. Laryngoscopy/intubation was the stimulus with the more pronounced response [Formula: see text]. These variables were used in the construction of the combined index STAN; STAN responded adequately to noxious stimuli, with a more pronounced response to laryngoscopy/intubation (18.5-43.1 %, [Formula: see text]), and the attenuation provided by the analgesic, detecting steady-state periods in the different physiological signals analyzed (approximately 50 % of the total study time). A new multivariate approach for the assessment of the patient steady-state during general anesthesia was developed. The proposed wavelet based multivariate index responds adequately to different noxious stimuli, and attenuation provided by the analgesic in a dose-dependent manner for each stimulus analyzed in this study.The first author was supported by a scholarship from the Portuguese Foundation for Science and Technology (FCT SFRH/BD/35879/2007). The authors would also like to acknowledge the support of UISPA—System Integration and Process Automation Unit—Part of the LAETA (Associated Laboratory of Energy,
Transports and Aeronautics) a I&D Unit of the Foundation for Science and Technology (FCT), Portugal. FCT support under project PEst-OE/EME/LA0022/2013.info:eu-repo/semantics/publishedVersio
Bacterioplankton Community Shifts during a Spring Bloom of Aphanizomenon gracile and Sphaerospermopsis aphanizomenoides at a Temperate Shallow Lake
Climate change is enhancing the frequency of cyanobacterial blooms not only during summer but also in spring and autumn, leading to increased ecological impacts. The bacterioplankton community composition (BCC), in particular, is deeply affected by these blooms, although at the same time BCC can also play important roles in blooms’ dynamics. However, more information is still needed regarding BCC during species-specific cyanobacterial blooms. The goal of this study was to assess BCC succession in a hypereutrophic shallow lake (Vela Lake, Portugal) during a warm spring using a metagenomic approach to provide a glimpse of the changes these communities experience during the dominance of Aphanizomenon-like bloom-forming species. BCC shifts were studied using 16S rRNA gene metabarcoding and multivariate analyses. A total of 875 operational taxonomic units (OTUs) were retrieved from samples. In early spring, the dominant taxa belonged to Proteobacteria (mainly Alphaproteobacteria—Rickettsiales) and Bacteroidetes (Saprospirales, Flavobacteriales and Sphingobacteriales). However, at the end of May, a bloom co-dominated by cyanobacterial populations of Aphanizomenon gracile, Sphaerospermopsis aphanizomenoides and Synechococcus sp. developed and persisted until the end of spring. This led to a major BCC shift favouring the prevalence of Alphaproteobacteria (Rickettsiales and also Rhizobiales, Caulobacteriales and Rhodospirillales) and Bacteroidetes (Saprospirales, followed by Flavobacteriales and Sphingobacteriales). These results contribute to the knowledge of BCC dynamics during species-specific cyanobacterial blooms, showing that BCC is strongly affected (directly or indirectly) by Aphanizomenon-Sphaerospermopsis blooms.publishe
Handmade clay bricks: chemical, physical and mechanical properties
The clay brick masonry that is much used in historical structures often is in a rather poor state of conservation. In order to intervene correctly in these buildings, it is convenient to characterize the old material. For this purpose, a large sample of clay brick specimens from the 12th to 19th century were collected from six Portuguese monasteries, and were characterized chemically, physically and mechanically. A large variability of the properties was found. Additionally, a sample of handmade new bricks, which are commonly used as replacing material, was also analysed. The results were compared to the old bricks and could be possibly adequate as substitution bricks. Still, significant differences were found in chemical composition, and in water absorption and porosity, which are much lower in modern handmade bricks. With respect to mechanical properties, the range of values found in old bricks was rather high and the degree of deterioration exhibited a large scatter, meaning that a conclusion is hardly possible.The authors gratefully acknowledge the Instituto de Gestao do Patrimonio Arquitectonico e Arqueologico (IGESPAR) for providing the old clay bricks used in the present work. The first author acknowledges the partial funding of this work by the FCT through the following scholarships POCTI SFRH/BD/6409/2001 and POCTI SFRH/BPD/26706/2005
Gene therapy with mesenchymal stem cells expressing IFN-ß ameliorates neuroinflammation in experimental models of multiple sclerosis
[Background and Purpose]: Recombinant IFN‐ß is one of the first‐line treatments in multiple sclerosis (MS), despite its lack of efficacy in some patients. In this context, mesenchymal stem cells (MSCs) represent a promising therapeutic alternative due to their immunomodulatory properties and multipotency. Moreover, by taking advantage of their pathotropism, these cells can be genetically modified to be used as carriers for delivering or secreting therapeutic drugs into injured tissues. Here, we report the therapeutic effect of systemic delivery of adipose‐derived MSCs (AdMSCs), transduced with the IFN‐β gene, into mice with experimental autoimmune encephalomyelitis (EAE).[Experimental Approach]: Relapsing–remitting and chronic progressive EAE were induced in mice. Cells were injected i.v. Disease severity, inflammation and tissue damage were assessed clinically, by flow cytometry of spleens and histopathological evaluation of the CNS respectively.[Key Results]: Genetic engineering did not modify the biological characteristics of these AdMSCs (morphology, growth rate, immunophenotype and multipotency). Furthermore, the transduction of IFN‐ß to AdMSCs maintained and, in some cases, enhanced the functional properties of AdMSCs by ameliorating the symptoms of MS in EAE models and by decreasing indications of peripheral and central neuro‐inflammation.[Conclusion and Implications]: Gene therapy was found to be more effective than cell therapy in ameliorating several clinical parameters in both EAE models, presumably due to the continuous expression of IFN‐β. Furthermore, it has significant advantages over AdMSC therapy, and also over systemic IFN‐ß treatment, by providing long‐term expression of the cytokine at therapeutic concentrations and reducing the frequency of injections, while minimizing dose‐limiting side effects.This work was supported by Fondo de Investigaciones Sanitarias ISCIII (Spain) and Fondo Europeo de Desarrollo Regional (FEDER) from the European Union through the research grants PI12/01097 and PI15/00963 and ISCIII Red de Terapia Celular TerCel RD12/0019/0006 to F.M., by the Consejería de Economía, Innovación, Ciencia y Empleo, Junta de Andalucía‐FEDER/Fondo de Cohesion Europeo (FSE) de Andalucía through the research grants P09‐CTS‐04532, PI‐57069 and PAIDI‐Bio‐326 to F.M. and PI‐0160/2012 to K.B. M.J.P.‐M. has been supported by grants from Red Temática de Investigación Cooperativa Red Española de Esclerosis Múltiple REEM (RD07/0060 and RD12/0032). B.O. is financed by a contract from Excelent Project CTS‐7670/11 from Consejería de Economía, Innovación, Ciencia y Empleo (Junta de Andalucía)
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