1,755 research outputs found
Asymptotic solution of a model for bilayer organic diodes and solar cells
Organic diodes and solar cells are constructed by placing together two organic semiconducting materials with dissimilar electron affinities and ionization potentials. The electrical behavior of such devices has been successfully modeled numerically using conventional drift diffusion together with recombination (which is usually assumed to be bimolecular) and thermal generation. Here a particular model is considered and the dark current-voltage curve and the spatial structure of the solution across the device is extracted analytically using asymptotic methods. We concentrate on the case of Shockley-Read-Hall recombination but note the extension to other recombination mechanisms. We find that there are three regimes of behavior, dependent on the total current. For small currents-i.e., at reverse bias or moderate forward bias-the structure of the solution is independent of the total current. For large currents-i.e., at strong forward bias-the current varies linearly with the voltage and is primarily controlled by drift of charges in the organic layers. There is then a narrow range of currents where the behavior undergoes a transition between the two regimes. The magnitude of the parameter that quantifies the interfacial recombination rate is critical in determining where the transition occurs. The extension of the theory to organic solar cells generating current under illumination is discussed as is the analogous current-voltage curves derived where the photo current is small. Finally, by comparing the analytic results to real experimental data, we show how the model parameters can be extracted from the shape of current-voltage curves measured in the dark. © 2012 Society for Industrial and Applied Mathematics
MEMS based hair flow-sensors as model systems for acoustic perception studies
Arrays of MEMS fabricated flow sensors inspired by the acoustic flow-sensitive hairs found on the cerci of crickets, have been designed, fabricated and characterized. The hairs consist of up to 1 mm long SU-8 structures mounted on suspended membranes with normal translational and rotational degrees of freedom. Electrodes on the membrane and on the substrate form variable capacitors allowing for capacitive read-out. Capacitance versus voltage, frequency dependency and directional sensitivity measurements have been successfully carried out on fabricated sensor arrays, showing the viability of the concept. The sensors form a model-system allowing for investigations on sensory acoustics by their arrayed nature, their adaptivity via electrostatic interaction (frequency tuning and parametric amplifica- tion) and their susceptibility to noise (stochastic resonance
A Model to Estimate Sediment Yield from Field-Sized Areas: Development of Model
A tool for evaluating sediment yield from field-sized areas is needed for planning management practices to control sediment yield. We developed a reasonably simple simulation model which incorporates fundamental principles of erosion, deposition, and sediment transport mechanics. The model summarizes the state-of-the-art in erosion and sediment yield modeling with appropriate simplifications required to couple the governing equations.
Limited testing showed that the procedures developed here give improved estimates over the Universal Soil Loss Equation. Specific components of the model were tested using experimental data from overland flow, erodible channel, and impoundment studies. These results suggest that the model produces reasonable estimates of erosion, sediment transport, and deposition under a variety of circumstances common to field-scale areas.
Alternative management practices such as conservation tillage, terracing, and contouring can be evaluated separately or in combination to determine their influence on sediment yield. Given a particular location with specified characteristics for climate, soils, topography, and crops, the model provides a means of evaluating alternative management practices to suit a particular farming operation
Calibration of the Neutron Detection Efficiency of the COMPTEL NE213 Detector
This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440
Minimum charge-recovery time control with parallel connected buck converters
Optimal-time control to minimise a converter’s recovery time
has thus far been reported only for single power module
converters. This paper adapts the optimal-time control
problem and applies it to converters based on multiple power
modules. Additionally, a novel minimum charge-recovery
time control is also proposed for the multiple power module
converter which produces a recovery time shorter than that in
the optimal-time control. A 20 W converter is used to
demonstrate the improved characteristics under primary
regions of operation. Results show that the transient recovery
time during a load step change is improved by 75% compared
to traditional optimal time control
Differential localization of LTA synthesis proteins and their interaction with the cell division machinery in Staphylococcus aureus
Lipoteichoic acid (LTA) is an important cell wall component of Gram-positive bacteria. In Staphylococcus aureus it consists of a polyglycerolphosphate-chain that is retained within the membrane via a glycolipid. Using an immunofluorescence approach, we show here that the LTA polymer is not surface exposed in S. aureus, as it can only be detected after digestion of the peptidoglycan layer. S. aureus mutants lacking LTA are enlarged and show aberrant positioning of septa, suggesting a link between LTA synthesis and the cell division process. Using a bacterial two-hybrid approach, we show that the three key LTA synthesis proteins, YpfP and LtaA, involved in glycolipid production, and LtaS, required for LTA backbone synthesis, interact with one another. All three proteins also interacted with numerous cell division and peptidoglycan synthesis proteins, suggesting the formation of a multi-enzyme complex and providing further evidence for the co-ordination of these processes. When assessed by fluorescence microscopy, YpfP and LtaA fluorescent protein fusions localized to the membrane while the LtaS enzyme accumulated at the cell division site. These data support a model whereby LTA backbone synthesis proceeds in S. aureus at the division site in co-ordination with cell division, while glycolipid synthesis takes place throughout the membrane
Bifurcations of self-similar solutions for reversing interfaces in the slow diffusion equation with strong absorption
Bifurcations of self-similar solutions for reversing interfaces are studied in the slow diffusion equation with strong absorption. The self-similar solutions bifurcate from the time-independent solutions for standing interfaces. We show that such bifurcations occur at particular points in parameter space (characterizing the exponents in the diffusion and absorption terms) where the confluent hypergeometric functions satisfying Kummer's differential equation truncate to finite polynomials. A two-scale asymptotic method is employed to obtain the local dependencies of the self-similar reversing interfaces near the bifurcation points. The asymptotic results are shown to be in excellent agreement with numerical approximations of the self-similar solutions
Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors
We present results from new experiments to study the dynamics of radiative
shocks, reverse shocks and radiative precursors. Laser ablation of a solid
piston by the Orion high-power laser at AWE Aldermaston UK was used to drive
radiative shocks into a gas cell initially pressurised between and $1.0 \
bar with different noble gases. Shocks propagated at {80 \pm 10 \ km/s} and
experienced strong radiative cooling resulting in post-shock compressions of {
\times 25 \pm 2}. A combination of X-ray backlighting, optical self-emission
streak imaging and interferometry (multi-frame and streak imaging) were used to
simultaneously study both the shock front and the radiative precursor. These
experiments present a new configuration to produce counter-propagating
radiative shocks, allowing for the study of reverse shocks and providing a
unique platform for numerical validation. In addition, the radiative shocks
were able to expand freely into a large gas volume without being confined by
the walls of the gas cell. This allows for 3-D effects of the shocks to be
studied which, in principle, could lead to a more direct comparison to
astrophysical phenomena. By maintaining a constant mass density between
different gas fills the shocks evolved with similar hydrodynamics but the
radiative precursor was found to extend significantly further in higher atomic
number gases (\sim4$ times further in xenon than neon). Finally, 1-D and 2-D
radiative-hydrodynamic simulations are presented showing good agreement with
the experimental data.Comment: HEDLA 2016 conference proceeding
Absolute Calibration of a 200 MeV Proton Polarimeter for Use with the Brookhaven Linac
This work was supported by the National Science Foundation Grant NSF PHY 81-14339 and by Indiana Universit
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