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Sökning: LAR1:lu > Chalmers tekniska högskola

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  • Aarnio, H, et al. (författare)
  • Recombination studies in a polyfluorene copolymer for photovoltaic applications
  • 2005
  • Ingår i: Synthetic Metals. - Elsevier. - 0379-6779. ; 155:2, s. 299-302
  • Tidskriftsartikel (refereegranskat)abstract
    • We present detailed continuous wave (cw) and transient photoinduced absorption (PA) measurements in thin films of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3-benzo-thiadiazole)] (DiO-PFDTBT), and its blends with the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in weight ratios of 1:0, 4:1 and 1:4. We measure the frequency, intensity and temperature dependence of the PA signal in the frequency domain, and compare with the results obtained from the transient PA decay measurements in the time domain. In all blends, the PA spectrum shows a broad high energy PA band ranging from 1 eV to 2 eV as well as a low energy band peaking at 0.35 eV. We attribute the low energy band to the P1 transition of polarons and part of the high energy band to the correlated P2 transition of polarons. Both frequency and time domain measurements show that the high energy band has two decay components, a faster component in the microsecond time regime and a slower component in the millisecond time regime. The slow component is strongly dispersive, whereas the fast component is practically non-dispersive.
  • Aas, W., et al. (författare)
  • Lessons learnt from the first EMEP intensive measurement periods
  • 2012
  • Ingår i: Atmospheric Chemistry and Physics. - 1680-7316. ; 12:17, s. 8073-8094
  • Tidskriftsartikel (refereegranskat)abstract
    • The first EMEP intensive measurement periods were held in June 2006 and January 2007. The measurements aimed to characterize the aerosol chemical compositions, including the gas/aerosol partitioning of inorganic compounds. The measurement program during these periods included daily or hourly measurements of the secondary inorganic components, with additional measurements of elemental- and organic carbon (EC and OC) and mineral dust in PM1, PM2.5 and PM10. These measurements have provided extended knowledge regarding the composition of particulate matter and the temporal and spatial variability of PM, as well as an extended database for the assessment of chemical transport models. This paper summarise the first experiences of making use of measurements from the first EMEP intensive measurement periods along with EMEP model results from the updated model version to characterise aerosol composition. We investigated how the PM chemical composition varies between the summer and the winter month and geographically. The observation and model data are in general agreement regarding the main features of PM10 and PM2.5 composition and the relative contribution of different components, though the EMEP model tends to give slightly lower estimates of PM10 and PM2.5 compared to measurements. The intensive measurement data has identified areas where improvements are needed. Hourly concurrent measurements of gaseous and particulate components for the first time facilitated testing of modelled diurnal variability of the gas/aerosol partitioning of nitrogen species. In general, the modelled diurnal cycles of nitrate and ammonium aerosols are in fair agreement with the measurements, but the diurnal variability of ammonia is not well captured. The largest differences between model and observations of aerosol mass are seen in Italy during winter, which to a large extent may be explained by an underestimation of residential wood burning sources. It should be noted that both primary and secondary OC has been included in the calculations for the first time, showing promising results. Mineral dust is important, especially in southern Europe, and the model seems to capture the dust episodes well. The lack of measurements of mineral dust hampers the possibility for model evaluation for this highly uncertain PM component. There are also lessons learnt regarding improved measurements for future intensive periods. There is a need for increased comparability between the measurements at different sites. For the nitrogen compounds it is clear that more measurements using artefact free methods based on continuous measurement methods and/or denuders are needed. For EC/OC, a reference methodology (both in field and laboratory) was lacking during these periods giving problems with comparability, though measurement protocols have recently been established and these should be followed by the Parties to the EMEP Protocol. For measurements with no defined protocols, it might be a good solution to use centralised laboratories to ensure comparability across the network. To cope with the introduction of these new measurements, new reporting guidelines have been developed to ensure that all proper information about the methodologies and data quality is given.
  • Abay, Simon, 1980-, et al. (författare)
  • Charge transport in InAs nanowire Josephson junctions
  • 2014
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 89
  • Tidskriftsartikel (refereegranskat)abstract
    • We present an extensive experimental and theoretical study of the proximity effect in InAs nanowires connected to superconducting electrodes. We fabricate and investigate devices with suspended gate-controlled nanowires and nonsuspended nanowires, with a broad range of lengths and normal-state resistances. We analyze the main features of the current-voltage characteristics: the Josephson current, excess current, and subgap current as functions of length, temperature, magnetic field, and gate voltage, and compare them with theory. The Josephson critical current for a short-length device, L = 30 nm, exhibits a record high magnitude of 800 nA at low temperature that comes close to the theoretically expected value. The critical current in all other devices is typically reduced compared to the theoretical values. The excess current is consistent with the normal resistance data and agrees well with the theory. The subgap current shows a large number of structures; some of them are identified as subharmonic gap structures generated by multiple Andreev reflection. The other structures, detected in both suspended and nonsuspended devices, have the form of voltage steps at voltages that are independent of either the superconducting gap or length of the wire. By varying the gate voltage in suspended devices, we are able to observe a crossover from typical tunneling transport at large negative gate voltage, with suppressed subgap current and negative excess current, to pronounced proximity junction behavior at large positive gate voltage, with enhanced Josephson current and subgap conductance as well as a large positive excess current.
  • Abay, Simon, et al. (författare)
  • High Critical-Current Superconductor-InAs Nanowire-Superconductor Junctions
  • 2012
  • Ingår i: Nano letters. - 1530-6984. ; 12:11, s. 5622-5625
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the fabrication of InAs nanowires coupled to superconducting leads with high critical current and widely tunable conductance. We implemented a double lift off nanofabrication Method to get very short nanowire devices with Ohmic contacts. We observe very high critical. currents of up to 800 nA in a wire with a diameter of 80 nm. The current- voltage characteristics of longer and suspended nanowires display,either. Coulomb blockade or supercurrent depending on a local gate voltage, combining different regimes of transport in a single device.
  • Abay, Simon, 1980-, et al. (författare)
  • Quantized Conductance and Its Correlation to the Supercurrent in a Nanowire Connected to Superconductors
  • 2013
  • Ingår i: Nano Letters. - 1530-6984. ; 13:8, s. 3614-3617
  • Tidskriftsartikel (refereegranskat)abstract
    • We report conductance and supercurrent of InAs nano wires coupled to Al-superconducting electrodes with short channel lengths and good Ohmic contacts. The nanowires are suspended 15 nm above a local gate electrode. The charge density in the nanowires can be controlled by a small change in the gate voltage. For large negative gate voltages, the number of conducting channels is reduced gradually, and we observe a stepwise decrease of both conductance and critical current before the conductance vanishes completely.
  • Abou-Taouk, Abdallah, et al. (författare)
  • A Four-Step Global Reaction Mechanism for CFD Simulations of Flexi-Fuel Burner for Gas Turbines
  • 2011
  • Ingår i: Turbulence, Heat and Mass Transfer 7. - ICHMT. - 9781567003017 - 978-1-56700-301-7 ; s. 785-788
  • Konferensbidrag (refereegranskat)abstract
    • A reduced four-step scheme for Computational Fluid Dynamics (CFD) analysis is presented here in the context of industrial type laboratory combustor. The present scheme has been optimized for a syngas mixture consisting of 10% CH4, 22.5% CO and 67.5% H2 by volume, and for a methane-air mixture. The optimization of the global scheme is done by comparing with the detailed San Diego mechanism using perfectly stirred reactor (PSR) and laminar flame speed calculations. The four-step global scheme has been applied to the CFD analysis of a swirl-stabilized flexi-fuel burner. Both reacting and non-reacting cases has been computed, using a hybrid Unsteady RANS/Large Eddy Simulation (URANS/LES) technique. Comparisons between CFD results and experimental data in the form of Particle Image Velocimetry (PIV) data, Planar Laser Induced Fluorescence (pLIF) and Proper Orthogonal Decomposition (POD) analysis from an atmospheric burner test rig at Lund University are presented. The CFD results scheme show good agreement with the experimental data.
  • Abrahmsen-Alami, S., et al. (författare)
  • New release cell for NMR microimaging of tablets Swelling and erosion of poly(ethylene oxide)
  • 2007
  • Ingår i: International Journal of Pharmaceutics. - 0378-5173. ; 342:1-2, s. 105-114
  • Tidskriftsartikel (refereegranskat)abstract
    • A small release cell, in the form of a rotating disc, has been constructed to fit into the MRI equipment. The present work show that both qualitative and quantitative information of the swelling and erosion behavior of hydrophilic extended release (ER) matrix tablets may be obtained using this release cell and non-invasive magnetic resonance imaging (MRI) studies at different time-points during matrix dissolution. The tablet size, core size and the gel layer thickness of ER matrix formulations based on poly(ethylene oxide) have been determined. The dimensional changes as a function of time were found to correspond well to observations made with texture analysis (TA) methodology. Most importantly, the results of the present study show that both the erosion (displacement of the gel-dissolution media interface) and the swelling (decrease of dry tablet core size) proceed with a faster rate in radial than in axial direction using the rotating disk set-up. This behavior was attributed to the higher shear forces experienced in the radial direction. The results also indicate that front synchronization (constant gel layer thickness) is associated with the formation of an almost constant polymer concentration profile through the gel layer at different time-points. (c) 2007 Elsevier B.V. All rights reserved.
  • Adalbjornsson, S. I., et al. (författare)
  • Estimating Periodicities in Symbolic Sequences Using Sparse Modeling
  • 2015
  • Ingår i: IEEE Transactions on Signal Processing. - IEEE. - 1053-587X. ; 63:8, s. 2142-2150
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, we propose a method for estimating statistical periodicities in symbolic sequences. Different from other common approaches used for the estimation of periodicities of sequences of arbitrary, finite, symbol sets, that often map the symbolic sequence to a numerical representation, we here exploit a likelihood-based formulation in a sparse modeling framework to represent the periodic behavior of the sequence. The resulting criterion includes a restriction on the cardinality of the solution; two approximate solutions are suggested-one greedy and one using an iterative convex relaxation strategy to ease the cardinality restriction. The performance of the proposed methods are illustrated using both simulated and real DNA data, showing a notable performance gain as compared to other common estimators.
  • Adebahr, Josefina, 1973-, et al. (författare)
  • Cation coordination in ion conducting gels based on PEO-grafted polymers
  • 2000
  • Ingår i: Solid State Ionics. - 0167-2738. ; 135:1-4, s. 149-154
  • Tidskriftsartikel (refereegranskat)abstract
    • Ionic conducting polymer gels prepared from PEG-grafted acrylates, ethylene carbonate (EC), dimethyl carbonate (DMC), and LiPF6 are studied by means of infrared and Raman spectroscopy. It is found that the presence of grafted PEO chains substantially changes the coordination of lithium cations from 'cation-solvent' to 'cation-polymer'. Spectroscopic studies show that Li+-PEO coordination is strongly favored in competition with the solvent molecules and dominates completely at ether oxygen to Li-cation ratios (O:Li) of 10:1 and 5:1. At an O:M of 4:1 a solvent-cation interaction arises, in agreement with previously reported MD calculations of a preferred coordination number of 5 in similar systems. The results suggest that the complex interactions which determine long time stability and ionic transport properties can be designed by functionalization of the polymer backbone in PMMA-based gel electrolytes.
  • Adebahr, J, et al. (författare)
  • Ion and solvent dynamics in gel electrolytes based on ethylene oxide grafted acrylate polymers
  • 2002
  • Ingår i: Journal of Physical Chemistry B. - 1089-5647. ; 106:47, s. 12119-12123
  • Tidskriftsartikel (refereegranskat)abstract
    • Multinuclear pulsed field gradient NMR measurements and theological viscosity measurements were performed on three series of polymer gel electrolytes. The gels were based on a lithium salt electrolyte swollen into a copolymer matrix comprising an acrylate backbone and ethylene oxide side chains. In each series the side chains differed in length and number, but the acrylate-to-ethylene oxide ratio was kept constant. It was found that the self-diffusion coefficient of the cations was much lower than that of the anions, and that it decreased rapidly when the side chains got longer. In contrast, the self-diffusion coefficient of the anions was found to be independent of chain length. In the gel electrolytes, the diffusion coefficients of the solvent molecules are relatively constant despite an increased viscosity with increasing length of the side chains. However, in saltfree gels made for comparison, the diffusion coefficients of the solvent molecules decreased with, increasing length of the side chains, which is consistent with an increased viscosity.
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