324 research outputs found
Heat recovery subsystem and overall system integration of fuel cell on-site integrated energy systems
The best HVAC (heating, ventilating and air conditioning) subsystem to interface with the Engelhard fuel cell system for application in commercial buildings was determined. To accomplish this objective, the effects of several system and site specific parameters on the economic feasibility of fuel cell/HVAC systems were investigated. An energy flow diagram of a fuel cell/HVAC system is shown. The fuel cell system provides electricity for an electric water chiller and for domestic electric needs. Supplemental electricity is purchased from the utility if needed. An excess of electricity generated by the fuel cell system can be sold to the utility. The fuel cell system also provides thermal energy which can be used for absorption cooling, space heating and domestic hot water. Thermal storage can be incorporated into the system. Thermal energy is also provided by an auxiliary boiler if needed to supplement the fuel cell system output. Fuel cell/HVAC systems were analyzed with the TRACE computer program
Domain wall structure in magnetic bilayers with perpendicular anisotropy
We study the magnetic domain wall structure in magnetic bilayers (two
ultrathin ferromagnetic layers separated by a non magnetic spacer) with
perpendicular magnetization. Combining magnetic force and ballistic electron
emission microscopies, we are able to reveal the details of the magnetic
structure of the wall with a high spatial accuracy. In these layers, we show
that the classical Bloch wall observed in single layers transforms into
superposed N\'eel walls due to the magnetic coupling between the ferromagnetic
layers. Quantitative agreement with micromagnetic calculations is achieved.Comment: Author adresses AB, SR, JM and AT: Laboratoire de Physique des
Solides, CNRS, Universit\'e Paris Sud, UMR 8502, 91405 Orsay Cedex, France ML
: Laboratoire PMTM, Institut Galil\'ee, CNRS, Universit\'e Paris-13, UPR
9001, 93430 Villetaneuse, Franc
Les stratégies adaptatives des oiseaux de mer
The striking convergences in morphology, behaviour and
physiology existing between various species of sea birds are discussed,
both on the basis of personal observations, mostly carried
out in the Antarctic and Sub Antarctic zones, and from data in the literature.
The morphological adaptations are reviewed first. The body
weight of species nesting underground is shown to be lighter than that of species nesting above ground. Plankton-feeders usually
have a broad bill, while fish-feeders have a long and narrow one.
There is a broad and continuons range of flight capabilities, ranging
from frigate-birds and albatrosses at one extreme to flightless
penguins at the other. The difficulty of a proper comparison of
flight and diving abilities between different categories of sea birds
is pointed out, and the results given by various measurements and
indices are compared.
The adaptive value of cryptic coloration of both eggs and
nestlings to their background is discussed, as well as the prolective
role of some immature and adult plumages. The adaptive function
of sea bird ritualized displays and vocalizations is suggested.
Further data supporting David Lack's (1968) conclusions on
the adaptive value of variations in egg weight and incubation
length are provided. Most sea birds have a low reproductive rate
which is counter-balanced by a relatively high survival rate. They
can be considered K-strategists.
Some estimates of the resting metabolic rate of adult sea birds are given.
The interdependence of the various adaptations is emphasized.
To better understand the variety of adaptive strategies displayed
by sea birds, the factors which are responsible for the heterogeneity
of the oceanic environment are reviewed. Further, the variations
in foraging distances observed in the various bird groups during
the breeding season in emphasized (fig. 11). Foraging distance is
significantly correlated with the average number of eggs produced
per year per female, and with the aspect ratio of the win
Probing the interface magnetism in the FeMn/NiFe exchange bias system using magnetic second harmonic generation
Second harmonic generation magneto-optic Kerr effect (SHMOKE) experiments,
sensitive to buried interfaces, were performed on a polycrystalline NiFe/FeMn
bilayer in which areas with different exchange bias fields were prepared using
5 KeV He ion irradiation. Both reversible and irreversible uncompensated spins
are found in the antiferromagnetic layer close to the interface with the
ferromagnetic layer. The SHMOKE hysteresis loop shows the same exchange bias
field as obtained from standard magnetometry. We demonstrate that the exchange
bias effect is controlled by pinned uncompensated spins in the
antiferromagnetic layer.Comment: submitted to Phys. Rev. Let
Aging dynamics of non-linear elastic interfaces: the Kardar-Parisi-Zhang equation
In this work, the out-of-equilibrium dynamics of the Kardar-Parisi-Zhang
equation in (1+1) dimensions is studied by means of numerical simulations,
focussing on the two-times evolution of an interface in the absence of any
disordered environment. This work shows that even in this simple case, a rich
aging behavior develops. A multiplicative aging scenario for the two-times
roughness of the system is observed, characterized by the same growth exponent
as in the stationary regime. The analysis permits the identification of the
relevant growing correlation length, accounting for the important scaling
variables in the system. The distribution function of the two-times roughness
is also computed and described in terms of a generalized scaling relation.
These results give good insight into the glassy dynamics of the important case
of a non-linear elastic line in a disordered medium.Comment: 14 pages, 6 figure
Spatially periodic domain wall pinning potentials: Asymmetric pinning and dipolar biasing
Domain wall propagation has been measured in continuous, weakly disordered,
quasi-two-dimensional, Ising-like magnetic layers that are subject to spatially
periodic domain wall pinning potentials. The potentials are generated
non-destructively using the stray magnetic field of ordered arrays of
magnetically hard [Co/Pt] nanoplatelets which are patterned above and are
physically separated from the continuous magnetic layer. The effect of the
periodic pinning potentials on thermally activated domain wall creep dynamics
is shown to be equivalent, at first approximation, to that of a uniform,
effective retardation field, , which acts against the applied field,
. We show that depends not only on the array geometry but also on
the relative orientation of and the magnetization of the nanoplatelets. A
result of the latter dependence is that wall-mediated hysteresis loops obtained
for a set nanoplatelet magnetization exhibit many properties that are normally
associated with ferromagnet/antiferromagnet exchange bias systems. These
include a switchable bias, coercivity enhancement and domain wall roughness
that is dependent on the applied field polarity.Comment: 12 pages, 9 figure
Foliar trait contrasts between African forest and savanna trees: Genetic versus environmental effects
Journal ArticleVariations in leaf mass per unit area (Ma) and foliar concentrations of N, P, C, K, Mg and Ca were determined for 365 trees growing in 23 plots along a West African precipitation gradient ranging from 0.29 to 1.62m a-1. Contrary to previous studies, no marked increase in Ma with declining precipitation was observed, but savanna tree foliar [N] tended to be higher at the drier sites (mass basis). Generally, Ma was slightly higher and [N] slightly lower for forest vs savanna trees with most of this difference attributable to differences in soil chemistry. No systematic variations in [P], [Mg] and [Ca] with precipitation or between trees of forest vs savanna stands were observed. We did, however, find a marked increase in foliar [K] of savanna trees as precipitation declined, with savanna trees also having a significantly lower [K] than those of nearby forest. These differences were not related to differences in soil nutrient status and were accompanied by systematic changes in [C] of opposite sign. We suggest an important but as yet unidentified role for K in the adaption of savanna species to periods of limited water availability; with foliar [K] being also an important factor differentiating tree species adapted to forest vs savanna soils within the 'zone of transition' of Western Africa.Natural Environment Research Council TROBIT Consortium projectRoyal Society - University Research Fellowshi
Bioactive Hydrogel Substrates: Probing Leukocyte Receptor–Ligand Interactions in Parallel Plate Flow Chamber Studies
The binding of activated integrins on the surface of leukocytes facilitates the adhesion of leukocytes to vascular endothelium during inflammation. Interactions between selectins and their ligands mediate rolling, and are believed to play an important role in leukocyte adhesion, though the minimal recognition motif required for physiologic interactions is not known. We have developed a novel system using poly(ethylene glycol) (PEG) hydrogels modified with either integrin-binding peptide sequences or the selectin ligand sialyl Lewis X (SLe(X)) within a parallel plate flow chamber to examine the dynamics of leukocyte adhesion to specific ligands. The adhesive peptide sequences arginine–glycine–aspartic acid–serine (RGDS) and leucine–aspartic acid–valine (LDV) as well as sialyl Lewis X were bound to the surface of photopolymerized PEG diacrylate hydrogels. Leukocytes perfused over these gels in a parallel plate flow chamber at physiological shear rates demonstrate both rolling and firm adhesion, depending on the identity and concentration of ligand bound to the hydrogel substrate. This new system provides a unique polymer-based model for the study of interactions between leukocytes and endothelium as well as a platform to develop improved scaffolds for cardiovascular tissue engineering
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