2,027 research outputs found
Thermoelectric power factors of nanocarbon ensembles as a function of temperature
Thermoelectric power factors of nanocarbon ensembles have been determined as a function of temperature from 400 to 1200 K. The ensembles, composed of mixtures of nanographite or disperse ultrananocrystalline diamond with B 4 C
B4C
, are formed into mechanically rigid compacts by reaction at 1200 K with methane gas and subsequently annealed in an argon atmosphere at temperatures up to 2500 K. The ensembles were characterized using scanning electron microscopy, Raman, x-ray diffraction, and high resolution transmission electron microscopy techniques and found to undergo profound nanostructural changes as a function of temperature while largely preserving their nanometer sizes. The power factors increase strongly both as a function of annealing temperature and of the temperature at which the measurements are carried out reaching 1 µW/K 2 ¿cm
1 µW/K2¿cm
at 1200 K without showing evidence of a plateau. Density functional “molecular analog” calculations on systems based on stacked graphene sheets show that boron substitutional doping results in a lowering of the Fermi level and the creation of a large number of hole states within thermal energies of the Fermi level [P. C. Redfern, D. M. Greun, and L. A. Curtiss, Chem. Phys. Lett. 471, 264 (2009)]. We propose that enhancement of electronic configurational entropy due to the large number of boron configurations in the graphite lattice contributes to the observed thermoelectric properties of the ensembles
Modifications of comet materials by the sublimation process: Results from simulation experiments
An active comet like comet Halley loses by sublimation a surface layer of the order of 1 m thickness per perihelion passage. In situ measurements show that water ice is the main constituent which contributes to the gas emission although even more volatile species (CO, NH3, CH4, CO2 etc.) have been identified. Dust particles which were embedded in the ices are carried by the sublimating gases. Measurements of the chemical composition of cometary grains indicate that they are composed of silicates of approximate chondritic composition and refractory carbonaceous material. Comet simulation experiments show that significant modifications of cometary materials occur due to sublimation process in near surface layers which have to be taken into account in order to derive the original state of the material
Obesity: A Biobehavioral Point of View
Excerpt: If you ask an overweight person, “Why are you fat?’, you will, almost invariably, get the answer, “Because 1 eat too much.” You will get this answer in spite of the fact that of thirteen studies, six find no significant differences in the caloric intake of obese versus nonobese subjects, five report that the obese eat significantly less than the nonobese, and only two report that they eat significantly more
Calculation of the Phase Behavior of Lipids
The self-assembly of monoacyl lipids in solution is studied employing a model
in which the lipid's hydrocarbon tail is described within the Rotational
Isomeric State framework and is attached to a simple hydrophilic head.
Mean-field theory is employed, and the necessary partition function of a single
lipid is obtained via a partial enumeration over a large sample of molecular
conformations. The influence of the lipid architecture on the transition
between the lamellar and inverted-hexagonal phases is calculated, and
qualitative agreement with experiment is found.Comment: to appear in Phys.Rev.
A stacking-fault based microscopic model for platelets in diamond
We propose a new microscopic model for the planar defects in
diamond commonly called platelets. This model is based on the formation of a
metastable stacking fault, which can occur because of the ability of carbon to
stabilize in different bonding configurations. In our model the core of the
planar defect is basically a double layer of three-fold coordinated
carbon atoms embedded in the common diamond structure. The properties of
the model were determined using {\it ab initio} total energy calculations. All
significant experimental signatures attributed to the platelets, namely, the
lattice displacement along the direction, the asymmetry between the
and the directions, the infrared absorption peak
, and broad luminescence lines that indicate the introduction of
levels in the band gap, are naturally accounted for in our model. The model is
also very appealing from the point of view of kinetics, since naturally
occurring shearing processes will lead to the formation of the metastable
fault.Comment: 5 pages, 4 figures. Submitted for publication on August 2nd, 200
Fatigue testing of a proximal femoral hip component
Published versio
Exploring attitudes to edgy urban destinations: the case of Deptford, London
The role of tourists and tourism in urban development is not fully understood. Research has focused on tourism districts within city centres, but less is known about tourism in peripheral, less affluent urban districts. These areas can appeal to visitors as edgy alternatives to mainstream destinations. This study establishes who is interested in visiting and why, and it explores the underlying rationale for negative attitudes. The aims are addressed by an in-depth analysis of Deptford in South East London. This area is a relatively deprived part of a world city, albeit one that has long been earmarked as London's next cool district. The study uses a mix of different sources to analyse the case. Responses to a New York Times article on Deptford are analysed and the attitudes of actual visitors and key stakeholders are explored. The discussion includes an examination of different interpretations and attitudes towards the notion of edginess. Edginess is deemed attractive by certain audiences; something linked to a reverence for working-class life in the arts. The study concludes that, whilst edginess is a noted characteristic, what people appreciate about Deptford is its ‘distinctive ordinariness’ – its contrast with more polished and contrived urban districts
Unveiling covariate inclusion structures in economic growth regressions using latent class analysis
We propose the use of Latent Class Analysis methods to analyze the covariate inclusion patterns across specifications resulting from Bayesian Model Averaging exercises. Using Dirichlet Process clustering, we are able to identify and describe dependency structures among variables in terms of inclusion in the specifications that compose the model space. We apply the method to two datasets of potential determinants of economic growth. Clustering the posterior covariate inclusion structure of the model pace formed by linear regression models reveals interesting patterns of complementarity and substitutabiliy across economic growth determinants
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