2,067 research outputs found
Oxometalate-glass composites and thin films
New glass-composites with ion exchange properties have been developed.
Ammonium 12-molybdophosphate (AMP) (ΝΗ4)3ΡΜοΐ2θ4ο, and ammonium 12-tungstophosphate (AWP) (Nh4)3PW12O40, known for their ion exchange
capabilities, are included either in preformed aerogels with defined pore size, or are
added to sol-gel mixtures during the process of gel formation. Characterization is
carried out by FTIR, Raman and EXAFS spectroscopy. Ion exchange capacities for the
oxometalate precursors are determined for silver and rubidium and are compared to
those of the glass composites. Glass composites show high ion exchange capacity, but
some portion of the metalate complexes leaches from the glass during the procedure.
This is in contrast to thin composite films, which have almost no porosity and do not
show loss of metalate. EXAFS spectroscopy demostrates that the oxometalate
microstructure is maintained in glass composites and that rubidium ions after ion
exchange in glasses occupy similar cation positions as in the precursor compounds
Uniformity of the pseudomagnetic field in strained graphene
We present a study on the uniformity of the pseudomagnetic field in graphene
as a function of the relative orientation between the graphene lattice and
straining directions. For this, we strained a regular micron-sized graphene
hexagon by deforming it symmetrically by displacing three of its edges. By
simulations, we found that the pseudomagnetic field is strongest if the strain
is applied perpendicular to the armchair direction of graphene. For a hexagon
with a side length of 1 m, the pseudomagnetic field has a maximum of
1.2 T for an applied strain of 3.5% and it is uniform (variance %) within
a circle with a diameter of nm. This diameter is on the order of the
typical diameter of the laser spot in a state-of-the-art confocal Raman
spectroscopy setup, which suggests that observing the pseudomagnetic field in
measurements of shifted magneto-phonon resonance is feasible.Comment: 7 pages, 5 figure
New directions in InP solar cell research
Recent research efforts representing new directions in InP solar cell research are reviewed. These include heteroepitaxial growth on silicon and gallium arsenide substrates, V-grooved cells, large area high efficiency cells, and surface passivation. Improvements in heteroepitaxial cell efficiency are described together with processing of 19.1 percent, 4 sq cm cells. Recommendations are made for improvements in processing leading to increased efficiencies
Cross-Hedging Distillers Dried Grains: Exploring Corn and Soybean Meal Futures Contracts
Ethanol mandates and high fuel prices have led to an increase in the number of ethanol plants in the U.S. in recent years. In turn, this has led to an increase in the production of distillers dried grains (DDGs) as a co-product of ethanol production. DDG production in 2006 is estimated to be near 11 million tons. A sharp increase in ethanol production and thus DDGs is expected in 2007 with an increase with the number of ethanol plants. As with most competitive industries, there is some level of price risk in handling DDGs and no futures contract available for this co-product. Ethanol plants, as well as users of DDGs, may find cross-hedging DDGs with corn or soybean meal (SBM) futures as an effective means of managing risk. Traditionally, DDGs are hedged using only corn futures.
Radiation damage and annealing in large area n+/p/p+ GaAs shallow homojunction solar cells
Annealing of radiation damage was observed for the first time in VPE-grown, 2- by 2-cm, n+/p/p+ GaAs shallow homojunction solar cells. Electrical performance of several cells was determined as a function of 1-MeV electron fluence in the range of 10 to the 13th power to 10 to the 15th power e-/sq cm and as a function of thermal annealing time at various temperatures. Degradation of normalized power output after a fluence of 10 to the 15th power 1-MeV electrons/sq cm ranged from a low of 24 to 31 percent of initial maximum power. Normalized short circuit current degradation was limited to the range from 10 to 19 percent of preirradiated values. Thermal annealing was carried out in a flowing nitrogen gas ambient, with annealing temperatures spanning the range from 125 to 200 C. Substantial recovery of short circuit current was observed at temperatures as low as 175 C. In one case improvement by as much as 10 percent of the postirradiated value was observed. The key features of these cells are their extremely thin emitter layers (approxmately 0.05 micrometers), the absence of any Al sub xGd sub 1-x As passivating window layer, and their fabrication by vapor phase epitaxy
Cross-Hedging Distillers Dried Grains Using Corn and Soybean Meal Futures Contracts
Ethanol mandates have led to an increase in the production of distillers dried grains (DDGs), a co-product of ethanol production that is incorporated into livestock rations. As with most competitive industries, there is some level of price risk in handling DDGs, and there is no DDG futures contract available for managing price risk. Commonly, DDGs are hedged using only corn futures. Our results suggest that cross-hedge risk may be reduced by including soybean meal futures in an encompassing cross-hedge strategy. Further, we also conclude soybean meal futures currently may be slightly more effective at reducing risk than in the past.cross-hedge, distillers dried grains, ethanol, price risk, Agribusiness, Demand and Price Analysis,
Viscous coalescence of droplets: a Lattice Boltzmann study
The coalescence of two resting liquid droplets in a saturated vapor phase is
investigated by Lattice Boltzmann simulations in two and three dimensions. We
find that, in the viscous regime, the bridge radius obeys a t^{1/2}-scaling law
in time with the characteristic time scale given by the viscous time. Our
results differ significantly from the predictions of existing analytical
theories of viscous coalescence as well as from experimental observations.
While the underlying reason for these deviations is presently unknown, a simple
scaling argument is given that describes our results well.Comment: 12 pages, 10 figures; as published in Phys. Fluid
Effect of dislocations on properties of heteroepitaxial InP solar cells
The apparently unrelated phenomena of temperature dependency, carrier removal and photoluminescence are shown to be affected by the high dislocation densities present in heteroepitaxial InP solar cells. Using homoepitaxial InP cells as a baseline, it is found that the relatively high dislocation densities present in heteroepitaxial InP/GaAs cells lead to increased volumes of dVoc/dt and carrier removal rate and substantial decreases in photoluminescence spectral intensities. With respect to dVoc/dt, the observed effect is attributed to the tendency of dislocations to reduce Voc. Although the basic cause for the observed increase in carrier removal rate is unclear, it is speculated that the decreased photoluminescence intensity is attributable to defect levels introduced by dislocations in the heteroepitaxial cells
A comparative study of p(+)n and n(+)p InP solar cells made by a closed ampoule diffusion
The purpose was to demonstrate the possibility of fabricating thermally diffused p(+)n InP solar cells having high open-circuit voltage without sacrificing the short circuit current. The p(+)n junctions were formed by closed-ampoule diffusion of Cd through a 3 to 5 nm thick anodic or chemical phosphorus-rich oxide cap layer grown on n-InP:S Czochralski LEC grown substrates. For solar cells made by thermal diffusion the p(+)n configuration is expected to have a higher efficiency than the n(+)p configuration. It is predicted that the AM0, BOL efficiencies approaching 19 percent should be readily achieved providing that good ohmic front contacts could be realized on the p(+) emitters of thickness lower than 1 micron
Active Sampling-based Binary Verification of Dynamical Systems
Nonlinear, adaptive, or otherwise complex control techniques are increasingly
relied upon to ensure the safety of systems operating in uncertain
environments. However, the nonlinearity of the resulting closed-loop system
complicates verification that the system does in fact satisfy those
requirements at all possible operating conditions. While analytical proof-based
techniques and finite abstractions can be used to provably verify the
closed-loop system's response at different operating conditions, they often
produce conservative approximations due to restrictive assumptions and are
difficult to construct in many applications. In contrast, popular statistical
verification techniques relax the restrictions and instead rely upon
simulations to construct statistical or probabilistic guarantees. This work
presents a data-driven statistical verification procedure that instead
constructs statistical learning models from simulated training data to separate
the set of possible perturbations into "safe" and "unsafe" subsets. Binary
evaluations of closed-loop system requirement satisfaction at various
realizations of the uncertainties are obtained through temporal logic
robustness metrics, which are then used to construct predictive models of
requirement satisfaction over the full set of possible uncertainties. As the
accuracy of these predictive statistical models is inherently coupled to the
quality of the training data, an active learning algorithm selects additional
sample points in order to maximize the expected change in the data-driven model
and thus, indirectly, minimize the prediction error. Various case studies
demonstrate the closed-loop verification procedure and highlight improvements
in prediction error over both existing analytical and statistical verification
techniques.Comment: 23 page
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