35,086 research outputs found
Development of thermally stable phosphonitrile elastomers for advanced aerospace structures
Attempts to prepare low molecular weight, curable poly-(fluoroalkoxyphosphazenes) have been successful. Derivatization of /Cl2PN/n polymer with alkoxides gave functionally reactive terpolymers. These terpolymers could be crosslinked with polyisocyanates at room temperature. Attempts to control molecular weight have not been as successful. The effects of (Cl2PN)3 monomer purity, use of (Cl2PN)3,4 mixture, and early termination of the bulk polymerization of (Cl2PN)3 were studied briefly. Both low and high molecular weight polymers were obtained. Reaction of NH4Cl with PCl5 with subsequent heating to give chain extension gave either gels of oils with molecular weights of several thousand. The stabilization of poly-(fluoroalkoxyphosphazene) was investigated. The results generally were inconclusive, but acids were found to be deleterious while bases had little discernible effect. Improvements in stability by modification of end groups was inconclusive
Persistent currents in mesoscopic rings and boundary conformal field theory
A tight-binding model of electron dynamics in mesoscopic normal rings is
studied using boundary conformal field theory. The partition function is
calculated in the low energy limit and the persistent current generated as a
function of an external magnetic flux threading the ring is found. We study the
cases where there are defects and electron-electron interactions separately.
The same temperature scaling for the persistent current is found in each case,
and the functional form can be fitted, with a high degree of accuracy, to
experimental data.Comment: 6 pages, 4 enclosed postscript figure
Spin projection and spin current density within relativistic electronic transport calculations
A spin projection scheme is presented which allows the decomposition of the
electric conductivity into two different spin channels within fully
relativistic transport calculations that account for the impact
of spin-orbit coupling. This is demonstrated by calculations of the
spin-resolved conductivity of FeCr and CoPt disordered
alloys on the basis of the corresponding Kubo-Greenwood equation implemented
using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA)
band structure method. In addition, results for the residual resistivity of
diluted Ni-based alloys are presented that are compared to theoretical and
experimental ones that rely on Mott's two-current model for spin-polarized
systems. The application of the scheme to deal with the spin-orbit induced spin
Hall effect is discussed in addition
Flatfish herding behavior in response to trawl sweeps: a comparison of diel responses to conventional sweeps and elevated sweeps
Commercial bottom trawls often have sweeps to herd fish into
the net. Elevation of the sweeps off the seaf loor may reduce seafloor disturbance, but also reduce herding
effectiveness. In both field and laboratory experiments, we examined the behavior of flatfish in response to sweeps. We tested the hypotheses that 1) sweeps are more effective at
herding flatfish during the day than at night, when fish are unable to see approaching gear, and that 2) elevation
of sweeps off the seafloor reduces herding during the day, but not at night. In sea trials, day catches were greater than night catches for four out of six flatfish species examined. The elevation of sweeps 10 cm significantly
decreased catches during the day, but not at night. Laboratory experiments revealed northern rock sole (Lepidopsetta polyxystra) and Pacific halibut (Hippoglossus stenolepis) were more likely to be herded\ud
by the sweep in the light, whereas in the dark they tended to pass under or over the sweep. In the light, elevation
of the sweep reduced herding, and more fish passed under the sweep. In contrast, in the dark, sweep elevation
had little effect upon the number of fish that exhibited herding behavior. The results of both field and laboratory
experiments were consistent with the premise that vision is the principle sensory input that controls fish behavior and orientation to trawl gear, and gear performance will differ
between conditions where flatfish can see, in contrast to where they cannot see, the approaching gear
Development of thermally stable phosphonitrile elastomers for advanced aerospace structures
Both high and low molecular weight, curable poly(fluoroalkoxy phosphazene) terpolymers were prepared. These terpolymers resulted from reaction of (Cl2PNn) polymer with alkoxides derived from CF3CH2OH and C3F7CH2OH, and an alkoxide derived from CH3CH(OH)C2H4OH. The terpolymers were crosslinked with polyisocyanates at room temperature. High molecular weight materials were converted into isocyanate prepolymers which as films underwent moisture cures at room temperature. Prepolymer solutions were stable for several days, and showed good adhesion. Also the effects of polymerization of (Cl2PN)3 were studied. Purified octachlorophosphazene, thiocyanate salts, or hydrogen chloride were employed in attempts to decrease molecular weight. Hydrogen chloride was found to be a good agent for preparation of low molecular weight poly(dichloro phosphazene)
Coherent description of the intrinsic and extrinsic anomalous Hall effect in disordered alloys on an level
A coherent description of the anomalous Hall effect (AHE) is presented that
is applicable to pure as well as disordered alloy systems by treating all
sources of the AHE on equal footing. This is achieved by an implementation of
the Kubo-St\v{r}eda equation using the fully relativistic
Korringa-Kohn-Rostoker (KKR) Green's function method in combination with the
Coherent Potential Approximation (CPA) alloy theory. Applications to the pure
elemental ferromagnets bcc-Fe and fcc-Ni led to results in full accordance with
previous work. For the alloy systems fcc-FePd and
fcc-NiPd very satisfying agreement with experiment could be
achieved for the anomalous Hall conductivity (AHC) over the whole range of
concentration. To interpret these results an extension of the definition for
the intrinsic AHC is suggested. Plotting the corresponding extrinsic AHC versus
the longitudinal conductivity a linear relation is found in the dilute regimes,
that allows a detailed discussion of the role of the skew and side-jump
scattering processes.Comment: * shortened manuscript * slight rewordings * changed line style in
Fig 1 * corrected misprinted S (skewness) factor * merged Fig. 3 with Fig. 1
* new citation introduce
Ab-initio calculation of the Gilbert damping parameter via linear response formalism
A Kubo-Greenwood-like equation for the Gilbert damping parameter is
presented that is based on the linear response formalism. Its implementation
using the fully relativistic Korringa-Kohn-Rostoker (KKR) band structure method
in combination with Coherent Potential Approximation (CPA) alloy theory allows
it to be applied to a wide range of situations. This is demonstrated with
results obtained for the bcc alloy system FeCo as well as for a
series of alloys of permalloy with 5d transition metals.
To account for the thermal displacements of atoms as a scattering mechanism,
an alloy-analogy model is introduced. The corresponding calculations for Ni
correctly describe the rapid change of when small amounts of
substitutional Cu are introduced
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