19,629 research outputs found
Synthesis, Crystal Structure and Photophysical Properties of Lanthanide Coordination Polymers of 4- 4-(9H-Carbazol-9-Yl)Butoxy Benzoate: The Effect of Bidentate Nitrogen Donors on Luminescence
A new aromatic carboxylate ligand, 4-[4-(9H-carbazol-9-yl)butoxy]benzoic acid (HL), has been synthesized by the replacement of the hydroxyl hydrogen of 4-hydroxy benzoic acid with a 9-butyl-9H-carbazole moiety. The anion derived from HL has been used for the support of a series of lanthanide coordination compounds [Ln = Eu (1), Gd (2) and Tb (3)]. The new lanthanide complexes have been characterized by a variety of spectroscopic techniques. Complex 3 was structurally authenticated by single-crystal X-ray diffraction and found to exist as a solvent-free 1D coordination polymer with the formula [Tb(L)(3)](n). The structural data reveal that the terbium atoms in compound 3 reside in an octahedral ligand environment that is somewhat unusual for a lanthanide. It is interesting to note that each carboxylate group exhibits only a bridging-bidentate mode, with a complete lack of more complex connectivities that are commonly observed for extended lanthanide-containing solid-state structures. Examination of the packing diagram for 3 revealed the existence of two-dimensional molecular arrays held together by means of CH-pi interactions. Aromatic carboxylates of the lanthanides are known to exhibit highly efficient luminescence, thus offering the promise of applicability as optical devices. However, due to difficulties that arise on account of their polymeric nature, their practical application is somewhat limited. Accordingly, synthetic routes to discrete molecular species are highly desirable. For this purpose, a series of ternary lanthanide complexes was designed, synthesized and characterized, namely [Eu(L)(3)(phen)] (4), [Eu(L)(3)(tmphen)] (5), [Tb(L)(3)(phen)] (6) and [Tb(L)(3)(tmphen)] (7) (phen = 1,10-phenanthroline and tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline). The photophysical properties of the foregoing complexes in the solid state at room temperature have been investigated. The quantum yields of the ternary complexes 4 (9.65%), 5 (21.00%), 6 (14.07%) and 7 (32.42%), were found to be significantly enhanced in the presence of bidentate nitrogen donors when compared with those of the corresponding binary compounds 1 (0.11%) and 3 (1.45%). Presumably this is due to effective energy transfer from the ancillary ligands.Council of Scientific and Industrial Research (CSIR-TAPSUN Project) SSL, NWP-55CSIR, New DelhiRobert A. Welch Foundation F-0003Chemistr
Mineralogy and Surface Composition of Asteroids
Methods to constrain the surface mineralogy of asteroids have seen
considerable development during the last decade with advancement in laboratory
spectral calibrations and validation of our interpretive methodologies by
spacecraft rendezvous missions. This has enabled the accurate identification of
several meteorite parent bodies in the main asteroid belt and helped constrain
the mineral chemistries and abundances in ordinary chondrites and basaltic
achondrites. With better quantification of spectral effects due to temperature,
phase angle, and grain size, systematic discrepancies due to non-compositional
factors can now be virtually eliminated for mafic silicate-bearing asteroids.
Interpretation of spectrally featureless asteroids remains a challenge. This
paper presents a review of all mineralogical interpretive tools currently in
use and outlines procedures for their application.Comment: Chapter to appear in the Space Science Series Book: Asteroids IV, 51
pages, 7 figures, 2 table
Towards Benchmarking Scene Background Initialization
Given a set of images of a scene taken at different times, the availability
of an initial background model that describes the scene without foreground
objects is the prerequisite for a wide range of applications, ranging from
video surveillance to computational photography. Even though several methods
have been proposed for scene background initialization, the lack of a common
groundtruthed dataset and of a common set of metrics makes it difficult to
compare their performance. To move first steps towards an easy and fair
comparison of these methods, we assembled a dataset of sequences frequently
adopted for background initialization, selected or created ground truths for
quantitative evaluation through a selected suite of metrics, and compared
results obtained by some existing methods, making all the material publicly
available.Comment: 6 pages, SBI dataset, SBMI2015 Worksho
Dynamic analysis of a pre-and-post ice impacted blade
The dynamic characteristics of an engine blade are evaluated under pre-and-post ice impact conditions using the NASA in-house computer code BLASIM. The ice impacts the leading edge of the blade causing severe local damage. The local structural response of the blade due to the ice impact is predicted via a transient response analysis by modeling only a local patch around the impact region. After ice impact, the global geometry of the blade is updated using deformations of the local patch and a free vibration analysis is performed. The effects of ice impact location, size and ice velocity on the blade mode shapes and natural frequencies are investigated. The results indicate that basic nature of the mode shapes remains unchanged after impact and that the maximum variation in natural frequencies occurs for the twisting mode of the blade
Structural tailoring of aircraft engine blade subject to ice impact constraints
Results are presented for the minimum weight design of SR2 unswept blade made of (titanium/graphite-epoxy/titanium) sub s fiber composite. The blade which is rotating at high RPM is subject to ice impact. The root chord length, blade thicknesses at five stations, and graphite-epoxy ply orientation are chosen as design variables. Design constraints are placed on the behavior variables: local leading edge strain and root damage parameter (combined stress failure criteria) as a function due to ice impact, maximum spanwise centrifugal stress at the root of the deformed blade due to local damage, first three natural frequencies, and resonance margin after impact. The method of feasible directions is employed to solve the inequality constrained minimization problem. The effect of ice speed and the ice impact location on the final design are discussed
Root damage analysis of aircraft engine blade subject to ice impact
The blade root response due to ice impact on an engine blade is simulated using the NASA in-house code BLASIM. The ice piece is modeled as an equivalent spherical object impacting on the leading edge of the blade and has the velocity opposite to that of the aircraft with direction parallel to the engine axis. The effect of ice impact is considered to be an impulse load on the blade with its amplitude computed based on the momentum transfer principle. The blade response due to the impact is carried out by modal superposition using the first three modes. The maximum dynamic stresses at the blade root are computed at the quarter cycle of the first natural frequency. A combined stress failure function based on modified distortion energy is used to study the spanwise bending damage response at the blade root. That damage function reaches maximum value for very low ice speeds and increases steeply with increases in engine speed
Association of Breakfast Intake with Obesity, Dietary and Physical Activity Behavior Among Urban School-Aged Adolescents in Delhi, India: Results of a Cross-Sectional Study
In developed countries, regular breakfast consumption is inversely associated with excess weight and directly associated with better dietary and improved physical activity behaviors. Our objective was to describe the frequency of breakfast consumption among school-going adolescents in Delhi and evaluate its association with overweight and obesity as well as other dietary, physical activity, and sedentary behaviors. Methods: Design: Cross-sectional study. Setting: Eight schools (Private and Government) of Delhi in the year 2006. Participants: 1814 students from 8th and 10th grades; response rate was 87.2%; 55% were 8th graders, 60% were boys and 52% attended Private schools. Main outcome measures: Body mass index, self-reported breakfast consumption, diet and physical activity related behaviors, and psychosocial factors. Data analysis: Mixed effects regression models were employed, adjusting for age, gender, grade level and school type (SES). Results: Significantly more Government school (lower SES) students consumed breakfast daily as compared to Private school (higher SES) students (73.8% vs. 66.3%; p<0.01). More 8th graders consumed breakfast daily vs. 10th graders (72.3% vs. 67.0%; p<0.05). A dose-response relationship was observed such that overall prevalence of overweight and obesity among adolescents who consumed breakfast daily (14.6%) was significantly lower vs. those who only sometimes (15.2%) or never (22.9%) consumed breakfast (p<0.05 for trend). This relationship was statistically significant for boys (15.4 % vs. 16.5% vs. 26.0; p<0.05 for trend) but not for girls. Intake of dairy products, fruits and vegetables was 5.5 (95% CI 2.4-12.5), 1.7 (95% CI 1.1-2.5) and 2.2 (95% CI 1.3-3.5) times higher among those who consumed breakfast daily vs. those who never consumed breakfast. Breakfast consumption was associated with greater physical activity vs. those who never consumed breakfast. Positive values and beliefs about healthy eating; body image satisfaction; and positive peer and parental influence were positively associated with daily breakfast consumption, while depression was negatively associated. Conclusion: Daily breakfast consumption is associated with less overweight and obesity and with healthier dietary-and physical activity-related behaviors among urban Indian students. Although prospective studies should confirm the present results, intervention programs to prevent or treat childhood obesity in India should consider emphasizing regular breakfast consumption.Obesity Prevention Center, University of MinnesotaPRIME program of the University of Texas, School of Public Health (Stigler, PI)Center for Health Promotion and Disease Prevention Research in Underserved Population
Electronic screening and damping in magnetars
We calculate the screening of the ion-ion potential due to electrons in the
presence of a large background magnetic field, at densities of relevance to
neutron star crusts. Using the standard approach to incorporate electron
screening through the one-loop polarization function, we show that the magnetic
field produces important corrections both at short and long distances. In
extreme fields, realized in highly magnetized neutron stars called magnetars,
electrons occupy only the lowest Landau levels in the relatively low density
region of the crust. Here our results show that the screening length for
Coulomb interactions between ions can be smaller than the inter-ion spacing.
More interestingly, we find that the screening is anisotropic and the screened
potential between two static charges exhibits long range Friedel oscillations
parallel to the magnetic field. This long-range oscillatory behavior is likely
to affect the lattice structure of ions, and can possibly create rod-like
structures in the magnetar crusts. We also calculate the imaginary part of the
electron polarization function which determines the spectrum of electron-hole
excitations and plays a role in damping lattice phonon excitations. We
demonstrate that even for modest magnetic fields this damping is highly
anisotropic and will likely lead to anisotropic phonon heat transport in the
outer neutron star crust.Comment: 14 pages, 5 Figure
Reinterpreting Shock Wave Structure Predictions using the Navier-Stokes Equations
Classical Navier-Stokes equations fail to predict shock wave profiles
accurately. In this paper, the Navier-Stokes system is fully transformed using
a velocity variable transformation. The transformed equations termed the
re-casted Navier-Stokes equations display physics not initially included. We
then analyse the stationary shock structure problem in a monatomic gas by
solving both the classical and the re-casted Navier-Stokes equations
numerically using a finite difference global solution (FDGS) scheme. The
numerical results are presented for different upstream Mach numbers ranging
from supersonic to hypersonic flows. We found that the re-casted Navier-Stokes
equations show better agreements with the experimentally measured density and
reciprocal shock thickness profiles
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