5,280 research outputs found
Flux-cutting and flux-transport effects in type-II superconductor slabs in a parallel rotating magnetic field
The magnetic response of irreversible type-II superconductor slabs subjected
to in-plane rotating magnetic field is investigated by applying the circular,
elliptic, extended-elliptic, and rectangular flux-line-cutting critical-state
models. Specifically, the models have been applied to explain experiments on a
PbBi rotating disk in a fixed magnetic field , parallel to the flat
surfaces. Here, we have exploited the equivalency of the experimental situation
with that of a fixed disk under the action of a parallel magnetic field,
rotating in the opposite sense. The effect of both the magnitude of the
applied magnetic field and its angle of rotation upon the
magnetization of the superconductor sample is analyzed. When is smaller
than the penetration field , the magnetization components, parallel and
perpendicular to , oscillate with increasing the rotation angle. On
the other hand, if the magnitude of the applied field, , is larger than
, both magnetization components become constant functions of at
large rotation angles. The evolution of the magnetic induction profiles inside
the superconductor is also studied.Comment: 12 pages, 29 figure
Development and test of advanced composite components. Center Directors discretionary fund program
This report describes the design, analysis, fabrication, and test of a complex bathtub fitting. Graphite fibers in an epoxy matrix were utilized in manufacturing of 11 components representing four different design and layup concepts. Design allowables were developed for use in the final stress analysis. Strain gage measurements were taken throughout the static load test and correlation of test and analysis data were performed, yielding good understanding of the material behavior and instrumentation requirements for future applications
Low level of virological failure and drug resistance among patients receiving antiretroviral treatment under programme conditions in Maputo, Mozambique
Mexico AIDS Conference 200
Fermion dynamics in torsion theories
In this work we have studied the non-geodesical behaviour of particles with
spin 1/2 in Poincar\'e gauge theories of gravity, via the WKB method and the
Mathisson-Papapetrou equation. We have analysed the relation between the two
approaches and we have argued the different advantages associated with the WKB
approximation. Within this approach, we have calculated the trajectories in a
particular Poincar\'e gauge theory, discussing the viability of measuring such
a motion.Comment: 17 pages, 2 figures, minor changes, conclusions unchanged. It matches
the version published in JCA
Strain balanced quantum posts
Quantum posts are assembled by epitaxial growth of closely spaced quantum dot
layers, modulating the composition of a semiconductor alloy, typically InGaAs.
In contrast with most self-assembled nanostructures, the height of quantum
posts can be controlled with nanometer precision, up to a maximum value limited
by the accumulated stress due to the lattice mismatch. Here we present a strain
compensation technique based on the controlled incorporation of phosphorous,
which substantially increases the maximum attainable quantum post height. The
luminescence from the resulting nanostructures presents giant linear
polarization anisotropy.Comment: Submitted to Applied Physics Letters (7th March 2011). 4 pages, 4
figure
Quasienergy spectrum and tunneling current in ac-driven triple quantum dot shuttles
The dynamics of electrons in ac driven double quantum dots have been
extensively analyzed by means of Floquet theory. In these systems, coherent
destruction of tunneling has been shown to occur for certain ac field
parameters. In the present work we analyze, by means of Floquet theory, the
electron dynamics of a triple quantum dot in series attached to electric
contacts, where the central dot position oscillates. In particular, we analyze
the quasienergy spectrum of this ac driven nanoelectromechanical system, as a
function of the intensity and frequency of the ac field and of external dc
voltages. For strong driving fields, we derive, by means of perturbation
theory, analytical expressions for the quasienergies of the driven oscillator
system. From this analysis we discuss the conditions for coherent destruction
of tunneling (CDT) to occur as a function of detuning and field parameters. For
zero detuning, and from the invariance of the Floquet Hamiltonian under a
generalized parity transformation, we find analytical expressions describing
the symmetry properties of the Fourier components of the Floquet states under
such transformation. By using these expressions, we show that in the vicinity
of the CDT condition, the quasienergy spectrum exhibits exact crossings which
can be characterized by the parity properties of the corresponding
eigenvectors
Mixed optimization of power transmission structures: An application of the simulated annealing algorithm
En el presente artículo se expone una metodología que encara una optimización general del peso de las estructuras de transporte de energía. Esta metodología se basa en la algorítmica del recocido simulado enunciada por Kirkpatrick a principios de los años ochenta, que consiste en un proceso estocástico de pruebas basado en la analogía con el recocido de los metales. El método enunciado por Kirkpatrick permite adoptar soluciones que puedan empeorar la función objetivo, con la finalidad de mejorar la exploración del entorno factible y posibilitar una mejora final de la solución obtenida. El algoritmo propuesto compatibiliza la naturaleza discreta de las secciones de las barras con la naturaleza continua de las variables que definen la geometría y la forma global de la estructura. Así se desarrolla una metodología capaz de obtener la solución a un problema de optimización mixto, evitando, a la vez, posibles explosiones combinatorias derivadas del proceso estocástico. Por otra parte, también se ha complementado el algoritmo de Kirkpatrick con un análisis de sensibilidad de primer orden que proporciona un considerable ahorro en el coste computacional del método y se implementan funciones de penalización exterior para mejorar el tratamiento de las restricciones del diseño. Con todo ello se consigue una metodología general que permite la optimización de estructuras reales de transporte de energía en tiempos de computación asumibles.A general methodology to optimize the weight of power transmission structures is presented in this article. This methodology is based on the simulated annealing algorithm defined by Kirkpatrick in the early ‘80s. This algorithm consists of a stochastic approach that allows to explore and analyze solutions that do not improve the objective function in order to develop a better exploration of the design region and to obtain the global optimum. The proposed algorithm allows to consider the discrete behavior of the sectional variables for each element and the continuous behavior of the general geometry variables. Thus, an optimization methodology that can deal with a mixed optimization problem and includes both continuum and discrete design variables is developed. In addition, it does not require to study all the possible design combinations defined by discrete design variables. The algorithm proposed usually requires to develop a large number of simulations (structural analysis in this case) in practical applications. Thus, the authors have developed first order Taylor expansions and the first order sensitivity analysis involved in order to reduce the CPU time required. Exterior penalty functions have been also included to deal with the design constraints. Thus, the general methodology proposed allows to optimize real power transmission structures in acceptable CPU time.Peer Reviewe
Alkaline cement mortars. Chemical resistance to sulfate and seawater attack
The durability and chemical resistance of alkali activated slag and fly ash/slag mortars in contact with sulfates and
seawater media have been studied. Two methods were used in the evaluation of such durability: Kock-Steinegger and ASTM CI012. A mineralogical and a microstructural
characterization of mortars were done at different ages of their conservation in aggressive media through XRD, SEM/EDX and mercury porosimetry.
Results showed a high durability of activated cement mortars in sulfates and seawater media. NaOH activated mortars are the most sensitive to environment attack with
formation of expansive products as gypsum and ettringite, although in very low proportion.Se ha estudiado la estabilidad química en medios sulfáticos y de agua de mar de morteros de escorias activadas alcalinamente y morteros de mezclas de escoria y cenizas
volantes activadas alcalinamente. Se han empleado dos métodos para evaluar dicha estabilidad: Kock-Steinegger y la norma ASTM CIO 12. Se ha realizado una caracterización
mineralógica y micro estructural de los morteros (a diferentes edades de permanencia en los medios agresivos) a través de
DRX, SEM/EDXy porosimetría de mercurio.
Los resultados obtenidos han demostrado la elevada durabilidad de todos los morteros de cementos activados estudiados frente a la agresividad de los sulfatos y del agua
de mar Los morteros de escoria activada con NaOH son los más susceptibles al ataque por esos medios, conformación de productos expansivos como el yeso y la etringita, aunque en proporciones muy bajas.Consejo Superior de investigaciones Cientificas (CSIC)Peer reviewe
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