718 research outputs found
A Note on the Efficiency of an Educational Voucher System
David E. R. Gay is an Associate Professor of Economics at the University of Arkansas
Adam Smith as A Moral Philosopher: A Comment
David E. R. Gay is a Visiting Associate Professor of Economics at Texas A&M University and Associate Professor of Economics at the University of Arkansas
Monopoles in the Higgs Phase
We describe new solutions of Yang-Mills-Higgs theories consisting of magnetic
monopoles in a phase with fully broken gauge symmetry. Rather than spreading
out radially, the magnetic field lines form flux tubes. The solution is
topologically stable and, when embedded in N=2 SQCD, preserves 1/4 of the
supercharges. From the perspective of the flux-tube the monopole appears as a
kink. Many monopoles may be threaded onto a single flux tube and placed at
arbitrary separation to create a stable, BPS necklace of solitons.Comment: 8 Pages, 1 Figure. v2: Added references and comments on 3He. v3:
Another reference and corrected term in Lagrangia
On finite-difference approximations for normalized Bellman equations
A class of stochastic optimal control problems involving optimal stopping is
considered. Methods of Krylov are adapted to investigate the numerical
solutions of the corresponding normalized Bellman equations and to estimate the
rate of convergence of finite difference approximations for the optimal reward
functions.Comment: 36 pages, ArXiv version updated to the version accepted in Appl.
Math. Opti
Calculation of the emission power distribution of microstructured OLEDs using the reciprocity theorem
S. Zhang, E.R. Martins, G.A. Turnbull and I.D.W. Samuel are grateful to the Scottish Universities Physics Alliance (SUPA) and the Engineering and Physical Sciences Research Council (EPSRC) for financial support.Integrating photonic microstructures into organic light-emitting diodes (OLEDs) has been a widely used strategy to improve their light out-coupling efficiency. However, there is still a need for optical modelling methods which quantitatively characterise the spatial emission pattern of microstructured OLEDs. In this paper, we demonstrate such rigorous calculation using the reciprocity theorem. The calculation of the emission intensity at each direction in the far field can be simplified into only two simple calculations of an incident plane wave propagating from the far field into a single cell of the periodic structure. The emission from microstructured OLED devices with three different grating periods was calculated as a test of the approach, and the calculated results were in good agreement with experiment. This optical modelling method is a useful calculation tool to investigate and control the spatial emission pattern of microstructured OLEDs.PostprintPeer reviewe
Stability and electrochemical performance of nanostructured La2CuO4þd cathodes
La2CuO4þd cathode layers are prepared by spray-pyrolysis deposition and their structural, microstructural
and electrical properties are compared with those of submicrometric powders obtained from
freeze-dried precursors. In order to improve the cathode performance, three different electrode architectures
have been proposed: (i) powder cathodes obtained by conventional screen-printing and sintering,
and cathodes deposited by spray-pyrolysis on: (ii) as-prepared electrolyte surfaces and (iii) porous
electrolyte backbones. The cathode activity for the oxygen reduction reaction has been investigated as a
function of the microstructure and the sintering temperature. The microstructural optimization of the
cathodes and the low fabrication temperature minimize the instability problems between the electrolyte
and the cathode materials, leading to polarization resistances as low as 0.14 U cm2 at 600 C.MINECO (Ministerio de EconomÃa y Competitividad), MAT2016-77648-R y EC2014-53906-
Durability and performance of CGO barriers and LSCF cathode deposited by spray-pyrolysis
Ce0.9Gd0.1O1.95 (CGO) protective layers are prepared by two different methods to prevent the reaction between
the Zr0.84Y0.16O1.92 (YSZ) electrolyte and the La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode. In the first method, the
CGO layers are deposited by an airbrushing technique from an ink containing CGO particles without and with
cobalt as sintering aids. The second strategy consists in preparing both a dense CGO barrier layer and a porous
LSCF cathode by spray-pyrolysis deposition, in order to further reduce the fabrication temperature and minimize
the reaction between the cell components. The samples prepared by spray-pyrolysis exhibit better performance
and durability than those obtained by conventional sintering methods. The results suggest that the interfacial
reactivity between YSZ and LSCF as well as the Sr-enrichment at the cathode surface can be avoided by using
low-temperature fabrication methods and by operating at temperatures lower than 650 °C.Ministerio de Ciencia e Innovación, EC2014-53906-R y MAT2016-77648-
Effect of Zn addition on the structure and electrochemical properties of codoped BaCe0.6Zr0.2Ln0.2O3-δ (Ln=Y, Gd, Yb) proton conductors
In this work, BaCe0.6Zr0.2Y0.2-xYbxO3-δ and BaCe0.6Zr0.2Gd0.2-xYbxO3-δ (x=0–0.20), proton conducting materials
are prepared by the freeze-drying precursor method. The sintering conditions were optimized by adding Zn
(NO3)2·6H2O as sintering additive. The materials are thoroughly characterized by different structural and microstructural
techniques, including X-ray diffraction, scanning and transmission electron microscopy, and
thermogravimetric-differential thermal analysis. The addition of Zn favours the phase formation and densification
at lower sintering temperatures; however, it leads to the segregation of a Zn-rich secondary phase, with
general formula BaLn2ZnO5 (LnËY, Gd and Yb), which is identified and quantified for the first time. All samples
with Zn as sintering aid exhibit cubic structure; however, the samples without Zn crystallize with orthorhombic
or cubic structure, depending on the composition and thermal treatment. The electrical properties are studied by
impedance spectroscopy. A deep analysis of the bulk and grain boundary contributions to the conductivity has
revealed that the bulk conductivity remains almost unchanged along both series over Yb-doping; however, the
grain boundary resistance decreases. The highest conductivity values are found for the intermediate members of
both series, BaCe0.6Zr0.2Y0.1Yb0.1O3-δ and BaCe0.6Zr0.2Gd0.1Yb1O3-δ, with 33 and 28 mS cm−1 at 750 °C, respectively.Ministerio de EconomÃa y Competitividad), MAT2016-77648-
Highly efficient La0.8Sr0.2MnO3-δ - Ce0.9Gd0.1O1.95 nanocomposite cathodes for solid oxide fuel cells
La0.8Sr0.2MnO3-δ-Ce0.9Gd0.1O1.95 (LSM-CGO) nanostructured cathodes are successfully prepared in a single
process by a chemical spray-pyrolysis deposition method. The cathode is composed of nanometric particles of
approximately 15 nm of diameter, providing high triple-phase boundary sites for the oxygen reduction reactions.
A low polarization resistance of 0.046 Ω cm2 is obtained at 700 °C, which is comparable to the most efficient
cobaltite-based perovskite cathodes. A NiO-YSZ anode supported fuel cell with the nanostructured cathode
generates a power output of 1.4 W cm−2 at 800 °C, significantly higher than 0.75 W cm−2 for a cell with conventional
LSM-CGO cathode. The results suggest that this is a promising strategy to achieve high efficiency
electrodes for Solid Oxide Fuel Cells in a single preparation step, simplifying notably the fabrication process
compared to traditional methods.Ministerio de Ciencia e Innovació
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