Página 1 dos resultados de 58 itens digitais encontrados em 0.009 segundos

## Broadband energy transfer to sensitizing dyes by mobile quantum dot mediators in solar cells

Adhyaksa, Gede Widia Pratama; Lee, Ga In; Baek, Se-Woong; Lee, Jung-Yong; Kang, Jeung Ku
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
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The efficiency of solar cells depends on absorption intensity of the photon collectors. Herein, mobile quantum dots (QDs) functionalized with thiol ligands in electrolyte are utilized into dye–sensitized solar cells. The QDs serve as mediators to receive and re–transmit energy to sensitized dyes, thus amplifying photon collection of sensitizing dyes in the visible range and enabling up–conversion of low-energy photons to higher-energy photons for dye absorption. The cell efficiency is boosted by dispersing QDs in electrolyte, thereby obviating the need for light scattering1 or plasmonic2 structures. Furthermore, optical spectroscopy and external quantum efficiency data reveal that resonance energy transfer due to the overlap between QD emission and dye absorption spectra becomes dominant when the QD bandgap is higher than the first excitonic peak of the dye, while co–sensitization resulting in a fast reduction of oxidized dyes is pronounced in the case of lower QD band gaps.

## A facile approach to prepare silicon-based Pt-Ag tubular dendritic nano-forests (tDNFs) for solar-light-enhanced methanol oxidation reaction

Lin, Chun-Ting; Shiao, Ming-Hua; Chang, Mao-Nan; Chu, Nancy; Chen, Yu-Wei; Peng, Yu-Hsuan; Liao, Bo-Huei; Huang, Hung Ji; Hsiao, Chien-Nan; Tseng, Fan-Gang
Fonte: Springer US Publicador: Springer US
Tipo: Artigo de Revista Científica
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In this paper, a facile two-step Galvanic replacement reaction (GRR) is proposed to prepare Pt-Ag tubular dendritic nano-forests (tDNFs) in ambient condition for enhancing methanol oxidation reaction (MOR) under solar illumination. In the first GRR, a homogeneous layer of silver dendritic nano-forests (DNFs) with 10 μm in thickness was grown on Si wafer in 5 min in silver nitride (AgNO3) and buffer oxide etchant (BOE) solution. In the second GRR, we utilized chloroplatinic acid (H2PtCl6) as the precursor for platinum (Pt) deposition to further transform the prepared Ag DNFs into Pt-Ag tDNFs. The catalytic performance and solar response of the Pt-Ag tDNFs toward methanol electro-oxidation are also studied by cyclic voltammetry (CV) and chronoamperometry (CA). The methanol oxidation current was boosted by 6.4% under solar illumination on the Pt-Ag tDNFs due to the induced localized surface plasmon resonance (LSPR) on the dendritic structure. Current results provide a cost-effective and facile approach to prepare solar-driven metallic electrodes potentially applicable to photo-electro-chemical fuel cells.

## Switching Patterns and Steady-State Analysis of Grid-Connected and Stand-Alone Single-Stage Boost-Inverters for PV Applications

Saghaleini, Mahdi
Fonte: FIU Digital Commons Publicador: FIU Digital Commons
Tipo: Artigo de Revista Científica Formato: application/pdf
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Renewable or sustainable energy (SE) sources have attracted the attention of many countries because the power generated is environmentally friendly, and the sources are not subject to the instability of price and availability. This dissertation presents new trends in the DC-AC converters (inverters) used in renewable energy sources, particularly for photovoltaic (PV) energy systems. A review of the existing technologies is performed for both single-phase and three-phase systems, and the pros and cons of the best candidates are investigated. In many modern energy conversion systems, a DC voltage, which is provided from a SE source or energy storage device, must be boosted and converted to an AC voltage with a fixed amplitude and frequency. A novel switching pattern based on the concept of the conventional space-vector pulse-width-modulated (SVPWM) technique is developed for single-stage, boost-inverters using the topology of current source inverters (CSI). The six main switching states, and two zeros, with three switches conducting at any given instant in conventional SVPWM techniques are modified herein into three charging states and six discharging states with only two switches conducting at any given instant. The charging states are necessary in order to boost the DC input voltage. It is demonstrated that the CSI topology in conjunction with the developed switching pattern is capable of providing the required residential AC voltage from a low DC voltage of one PV panel at its rated power for both linear and nonlinear loads. In a micro-grid...

## A Study of Machine Learning Techniques for Daily Solar Energy Forecasting using Numerical Weather Models

Aler, Ricardo; Martín, Ricardo; Valls, José M.; Galván, Inés M.
Fonte: Springer International Publishing Publicador: Springer International Publishing
Tipo: info:eu-repo/semantics/acceptedVersion; info:eu-repo/semantics/conferenceObject; info:eu-repo/semantics/bookPart
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Forecasting solar energy is becoming an important issue in the context of renewable energy sources and Machine Learning Algorithms play an important rule in this field. The prediction of solar energy can be addressed as a time series prediction problem using historical data. Also, solar energy forecasting can be derived from numerical weather prediction models (NWP). Our interest is focused on the latter approach.We focus on the problem of predicting solar energy from NWP computed from GEFS, the Global Ensemble Forecast System, which predicts meteorological variables for points in a grid. In this context, it can be useful to know how prediction accuracy improves depending on the number of grid nodes used as input for the machine learning techniques. However, using the variables from a large number of grid nodes can result in many attributes which might degrade the generalization performance of the learning algorithms. In this paper both issues are studied using data supplied by Kaggle for the State of Oklahoma comparing Support Vector Machines and Gradient Boosted Regression. Also, three different feature selection methods have been tested: Linear Correlation, the ReliefF algorithm and, a new method based on local information analysis.; Proceedings of: 8th International Symposium on Intelligent Distributed Computing (IDC'2014). Madrid...

## Detection of AIBO and Humanoid Robots using Cascades of Boosted Classifiers

Ruiz del Solar, Javier; Verschae, Rodrigo; Arenas, Matías
Tipo: Artículo de revista
EN
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In the present article a framework for the robust detection of mobile robots using nested cascades of boosted classifiers is proposed. The boosted classifiers are trained using Adaboost and domain-partitioning weak hypothesis. The most interesting aspect of this framework is its capability of building robot detection systems with high accuracy in dynamical environments (RoboCup scenario), which achieve, at the same time, high processing and training speed. Using the proposed framework we have built robust AIBO and humanoid robot detectors, which are analyzed and evaluated using real-world video sequences.; This research was partially supported by FONDECYT (Chile) under Project Number 1061158.

## Real-time Hand Gesture Detection and Recognition using Boosted Classifiers and Active Learning

Ruiz del Solar, Javier; Verschae, Rodrigo; Francke, Hardy
Tipo: Artículo de revista
EN
Relevância na Pesquisa
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In this article a robust and real-time hand gesture detection and recognition system for dynamic environments is proposed. The system is based on the use of boosted classifiers for the detection of hands and the recognition of gestures, together with the use of skin segmentation and hand tracking procedures. The main novelty of the proposed approach is the use of innovative training techniques - active learning and bootstrap -, which allow obtaining a much better performance than similar boosting-based systems, in terms of detection rate, number of false positives and processing time. In addition, the robustness of the system is increased due to the use of an adaptive skin model, a color-based hand tracking, and a multi-gesture classification tree. The system performance is validated in real video sequences.; This research was funded by Millenium Nucleus Center for Web Research, Grant P04-067-F, Chile.

## A unified learning framework for object detection and classification using nested cascades of boosted classifiers

Ruiz del Solar, Javier; Correa, Mauricio; Verschae, Rodrigo
Tipo: Artículo de revista
EN
Relevância na Pesquisa
36.13%
Artículo de publicación ISI; A unified learning framework for object detection and classification using nested cascades of boosted classifiers; This research was funded by Millenium Nucleus Center for Web Research, Grant P04-067-F, Chile. Part of the research in this paper use the FERET database of facial images collected under the FERET program. During part of this research work the authors took part of the Alfa project N◦AML//19.0902/97/06660/II-0366-FA and they would like to acknowledge its support.

## Object Detection Using Nested Cascades of Boosted Classifiers. A Learning Framework and Its Extension to The Multi-Class Case

Verschae Tannenbaum, Rodrigo
Tipo: Tesis
EN
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## Photo-generated carriers lose energy during extraction from polymer-fullerene solar cells

Melianas, Armantas; Etzold, Fabian; Savenije, Tom J.; Laquai, Frédéric; Inganäs, Olle; Kemerink, Martijn
Fonte: Nature Pub. Group Publicador: Nature Pub. Group
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
25.91%
In photovoltaic devices, the photo-generated charge carriers are typically assumed to be in thermal equilibrium with the lattice. In conventional materials, this assumption is experimentally justified as carrier thermalization completes before any significant carrier transport has occurred. Here, we demonstrate by unifying time-resolved optical and electrical experiments and Monte Carlo simulations over an exceptionally wide dynamic range that in the case of organic photovoltaic devices, this assumption is invalid. As the photo-generated carriers are transported to the electrodes, a substantial amount of their energy is lost by continuous thermalization in the disorder broadened density of states. Since thermalization occurs downward in energy, carrier motion is boosted by this process, leading to a time-dependent carrier mobility as confirmed by direct experiments. We identify the time and distance scales relevant for carrier extraction and show that the photo-generated carriers are extracted from the operating device before reaching thermal equilibrium.

## The effect of stellar evolution uncertainties on the rest-frame ultraviolet stellar lines of CIV and HeII in high-redshift Lyman-break galaxies

Eldridge, John J.; Stanway, Elizabeth R.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.05%
Young, massive stars dominate the rest-frame ultraviolet spectra of star-forming galaxies. At high redshifts (z>2), these rest-UV features are shifted into the observed-frame optical and a combination of gravitational lensing, deep spectroscopy and spectral stacking analysis allows the stellar population characteristics of these sources to be investigated. We use our stellar population synthesis code BPASS to fit two strong rest-UV spectral features in published Lyman-break galaxy spectra, taking into account the effects of binary evolution on the stellar spectrum. In particular, we consider the effects of quasi-homogeneous evolution (arising from the rotational mixing of rapidly-rotating stars), metallicity and the relative abundance of carbon and oxygen on the observed strengths of HeII (1640 Angstroms) and CIV (1548,1551 Angstroms) spectral lines. We find that Lyman-break galaxy spectra at z=2-3 are best fit with moderately sub-solar metallicities, and with a depleted carbon-to-oxygen ratio. We also find that the spectra of the lowest metallicity sources are best fit with model spectra in which the HeII emission line is boosted by the inclusion of the effect of massive stars being spun-up during binary mass-transfer so these rapidly-rotating stars experiencing quasi-homogeneous evolution.; Comment: Accepted for publication in MNRAS...

## Can the WIMP annihilation boost factor be boosted by the Sommerfeld enhancement?

Lattanzi, Massimiliano; Silk, Joseph
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
25.91%
We demonstrate that the Sommerfeld correction to CDM annihilations can be appreciable if even a small component of the dark matter is extremely cold. Subhalo substructure provides such a possibility given that the smallest clumps are relatively cold and contain even colder substructure due to incomplete phase space mixing. Leptonic channels can be enhanced for plausible models and the solar neighbourhood boost required to account for PAMELA/ATIC data is plausibly obtained, especially in the case of a few TeV mass neutralino for which the Sommerfeld-corrected boost is found to be $\sim10^4-10^5.$ Saturation of the Sommerfeld effect is shown to occur below $\beta\sim 10^{-4},$ thereby constraining the range of contributing substructures to be above $\sim 10^5\rm M_\odot.$ We find that the associated diffuse gamma ray signal from annihilations would exceed EGRET constraints unless the channels annihilating to heavy quarks or to gauge bosons are suppressed. The lepton channel gamma rays are potentially detectable by the FERMI satellite, not from the inner galaxy where substructures are tidally disrupted, but rather as a quasi-isotropic background from the outer halo, unless the outer substructures are much less concentrated than the inner substructures and/or the CDM density profile out to the virial radius steepens significantly.; Comment: 8 pages...

## RR Lyrae Studies with Kepler

Kolenberg, Katrien
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
25.91%
The spectacular data delivered by NASA's {\it Kepler} mission have not only boosted the discovery of planets orbiting other stars, but they have opened a window on the inner workings of the stars themselves. For the study of the RR Lyrae stars, Kepler has led to a breakthrough. To date, over 50 RR Lyrae stars are known in the Kepler field. They are studied within the RR Lyrae/Cepheid working group of the Kepler Asteroseismic Science Consortium (KASC). This paper highlights some of the most interesting results on RR Lyrae stars obtained through Kepler so far.; Comment: Presented at the "40 Years of Variable Stars: A Celebration of Contributions by Horace A. Smith" conference (arXiv:1310.0149). 7 pages, 1 figure

## Solar Corona Loop Studies with AIA: I. Cross-Sectional Temperature Structure

Aschwanden, Markus J.; Boerner, Paul
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.12%
We present a first systematic study on the cross-sectional temperature structure of coronal loops using the six coronal temperature filters of the Atmospheric Imaging Assembly (AIA) instrument on the Solar Dynamics Observatory (SDO). We analyze a sample of 100 loop snapshots measured at 10 different locations and 10 different times in active region NOAA 11089 on 2010 July 24, 21:00-22:00 UT. The cross-sectional flux profiles are measured and a cospatial background is subtracted in 6 filters in a temperature range of $T \approx 0.5-16$ MK, and 4 different parameterizations of differential emission measure (DEM) distributions are fitted. We find that the reconstructed DEMs consist predominantly of narrowband peak temperature components with a thermal width of $\sigma_{log(T)} \le 0.11\pm0.02$, close to the temperature resolution limit of the instrument, consistent with earlier triple-filter analysis from TRACE by Aschwanden and Nightingale (2005) and from EIS/Hinode by Warren et al. (2008) or Tripathi et al. (2009). We find that 66% of the loops could be fitted with a narrowband single-Gaussian DEM model, and 19% with a DEM consisting of two narrowband Gaussians (which mostly result from pairs of intersecting loops along the same line-of-sight). The mostly isothermal loop DEMs allow us also to derive an improved empirical response function of the AIA 94 \ang\ filter...

## Stable higher order finite-difference schemes for stellar pulsation calculations

Reese, D. R.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
25.91%
Context: Calculating stellar pulsations requires a sufficient accuracy to match the quality of the observations. Many current pulsation codes apply a second order finite-difference scheme, combined with Richardson extrapolation to reach fourth order accuracy on eigenfunctions. Although this is a simple and robust approach, a number of drawbacks exist thus making fourth order schemes desirable. A robust and simple finite-difference scheme, which can easily be implemented in either 1D or 2D stellar pulsation codes is therefore required. Aims: One of the difficulties in setting up higher order finite-difference schemes for stellar pulsations is the so-called mesh-drift instability. Current ways of dealing with this defect include introducing artificial viscosity or applying a staggered grids approach. However these remedies are not well-suited to eigenvalue problems, especially those involving non-dissipative systems, because they unduly change the spectrum of the operator, introduce supplementary free parameters, or lead to complications when applying boundary conditions. Methods: We propose here a new method, inspired from the staggered grids strategy, which removes this instability while bypassing the above difficulties. Furthermore...

## A NLTE analysis of the hot subdwarf O star BD+28 4211. II. The optical spectrum

Latour, M.; Fontaine, G.; Green, E. M.; Brassard, P.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.16%
We present the second part of our detailed analysis of the hot sdO and spectroscopic standard star BD+28 4211, in which we focus on the optical spectrum. In the first part of our study, we determined the abundances of some 11 metals detected in the atmosphere of BD+28 4211 using UV spectra of the star and corroborated the fundamental parameters estimated in past studies (Teff $\sim$ 82,000 K, log g $\sim$ 6.2, and solar N(He)/N(H)). In this work, we aim at rederiving these secured parameters on the sole basis of high-quality optical spectra. A first grid of non-LTE line-blanketed model atmospheres, including metals with the abundances derived from the UV spectrum, does not give satisfactory results when we apply a standard simultaneous fitting procedure to the observed H and He lines of our optical spectra. The line profiles are not finely reproduced and the resulting effective temperatures, in particular, are too low by $\sim$10,000 K. We next investigate the probable cause of this failure, that is, the importance of missing opacity sources on the atmospheric stratification. We compare line profiles computed from models with artificially boosted metallicities, from solar abundances to 15$\times$ these values. We find that the structural effects saturate for a metallicity of $\sim$10x solar...

## Cosmic-ray acceleration in young protostars

Padovani, Marco; Hennebelle, Patrick; Marcowith, Alexandre; Ferrière, Katia
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.05%
The main signature of the interaction between cosmic rays and molecular clouds is the high ionisation degree. This decreases towards the densest parts of a cloud, where star formation is expected, because of energy losses and magnetic effects. However recent observations hint to high levels of ionisation in protostellar systems, therefore leading to an apparent contradiction that could be explained by the presence of energetic particles accelerated within young protostars. Our modelling consists of a set of conditions that has to be satisfied in order to have an efficient particle acceleration through the diffusive shock acceleration mechanism. We find that jet shocks can be strong accelerators of protons which can be boosted up to relativistic energies. Another possibly efficient acceleration site is located at protostellar surfaces, where shocks caused by impacting material during the collapse phase are strong enough to accelerate protons. Our results demonstrate the possibility of accelerating particles during the early phase of a proto-Solar-like system and can be used as an argument to support available observations. The existence of an internal source of energetic particles can have a strong and unforeseen impact on the star and planet formation process as well as on the formation of pre-biotic molecules.; Comment: Accepted by Astronomy and Astrophysics

## Micrometer-Thin Crystalline-Silicon Solar Cells Integrating Numerically Optimized 2-D Photonic Crystals

Depauw, V.; Meng, X.; Daif, O. El; Gomard, G.; Lalouat, L.; Drouard, E.; Trompoukis, C.; Fave, A.; Seassal, C.; Gordon, I.
Tipo: Artigo de Revista Científica
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A 2-D photonic crystal was integrated experimentally into a thin-film crystalline-silicon solar cell of 1-{\mu}m thickness, after numerical optimization maximizing light absorption in the active material. The photonic crystal boosted the short-circuit current of the cell, but it also damaged its open-circuit voltage and fill factor, which led to an overall decrease in performances. Comparisons between modeled and actual optical behaviors of the cell, and between ideal and actual morphologies, show the global robustness of the nanostructure to experimental deviations, but its particular sensitivity to the conformality of the top coatings and the spread in pattern dimensions, which should not be neglected in the optical model. As for the electrical behavior, the measured internal quantum efficiency shows the strong parasitic absorptions from the transparent conductive oxide and from the back-reflector, as well as the negative impact of the nanopattern on surface passivation. Our exemplifying case, thus, illustrates and experimentally confirms two recommendations for future integration of surface nanostructures for light trapping purposes: 1) the necessity to optimize absorption not for the total stack but for the single active material...

## Zeeman-Doppler imaging of active young solar type stars

Hackman, Thomas; Lehtinen, Jyri; Rosén, Lisa; Kochukhov, Oleg; Käpylä, Maarit J.
Tipo: Artigo de Revista Científica
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36.12%
By studying young magnetically active late-type stars, i.e. analogues to the young Sun, one can draw conclusions on the evolution of the solar dynamo. We determine the topology of the surface magnetic field and study the relation between the magnetic field and cool photospheric spots in three young late-type stars. High-resolution spectropolarimetry of the targets were obtained with the HARPSpol instrument mounted at the ESO 3.6 m telescope. The signal-to-noise ratio of the Stokes IV measurements were boosted by combining the signal from a large number of spectroscopic absorption lines through the least squares deconvolution technique. Surface brightness and magnetic field maps were calculated using the Zeeman-Doppler imaging technique. All the three targets show clear signs of both magnetic fields and cool spots. Only one of the targets, namely V1358 Ori, shows evidence of the dominance of non-axisymmetric modes. In two of the targets, the poloidal field is significantly stronger than the toroidal one, indicative of an $\alpha^2$-type of a dynamo, in which convective turbulence effects dominate over the weak differential rotation. In two of the cases there is a slight anti-correlation between the cool spots and the strength of the radial magnetic field. However...

## Ipopv2: Photoionization of Ni XIV -- a test case

Delahaye, F.; Palmeri, P.; Quinet, P.; Zeippen, C. J.
Tipo: Artigo de Revista Científica