Luminescence coupling effects on multijunction solar cell external quantum efficiency measurement

The effects of luminescence coupling on the external quantum efficiency (EQE) measurement of an InGaP/InGaAs/Ge triple junction solar cell were investigated. A small signal model was used to study the interaction of the subcells during EQE measurement. It was found that an optical–electrical feedback mechanism results in EQE measurement artifacts. Measurements of luminescence from the InGaP p–n junction are also performed to quantitatively determine the strength of the luminescence coupling. These results offer a more thorough and comprehensive interpretation of EQE measurement results that are needed for the design and accurate performance evaluation of high‐efficiency multijunction solar cells. The effects demonstrated here can also be used to measure the spontaneous emission efficiency of the subcells, which is useful for nondestructive assessment of multijunction solar cell material quality. Copyright © 2011 John Wiley & Sons, Ltd.     

AC-driven,color- and brightness-tunable organic light-emitting diodes constructed from an electron only device

In this paper, a color- and brightness-tunable organic light-emitting diode (OLED) is reported. This OLED was realized by inserting a charge generation layer into an electron only device to form an n-i-p-i-n structure. It is shown that, by changing the polarity of applied voltage, only the p-i-n junction operated under positive bias can emit light and, by applying an AC voltage, emission from both junctions was realized. It is also shown that, by using a combination of blue- and red-emiting layers in two p-i-n junctions, both the color and brightness of the resulting white OLED can be tuned independently by changing the positive and negative amplitudes of the AC voltage.     

Tunable Chirped Fiber Grating Based Variable Time-Delay Network for Phased-Array Antenna Beamforming

In this paper, we present a photonics continuous beamforming system for optically controlled phased-array antenna based on a tunable fiber ring laser source and a novel variable true-time delay network consisting of one single-mode fiber delay line and four tunable chirped grating delay lines. The chirp rate of each chirped grating is tuned by bending a uniform fiber Bragg grating (FBG) bonded at a slant onto the lateral surface of a simply supported beam. This kind of variable true-time delay network can provide five-channel time-delayed signals that can be fed to an antenna array to achieve continuously beam scanning. The proposed true-time-delay unit with 5-element phased array steerer is suitable for phased-array antenna beamforming at frequencies up to 12 GHz.     

Sulfonated poly(ether ether ketone)/poly(ether sulfone) composite membranes as an alternative proton exchange membrane in microbial fuel cells

Custom-made proton exchange membranes (PEM) are synthesized by incorporating sulfonated poly(ether ether ketone) (SPEEK) in poly(ether sulfone) (PES) for electricity generation in microbial fuel cells (MFCs). The composite PES/SPEEK membranes at various composition of SPEEK are prepared by the phase inversion method. The membranes are characterized by measuring roughness, proton conductivity, oxygen diffusion, water crossover and electrochemical impedance. The conductivity of hydrophobic PES membrane increases when a small amount (3–5%) of hydrophilic SPEEK is added. The electrochemical impedance spectra shows that the conductivity and capacitance of PES/SPEEK composite membranes during MFC operation are reduced from 6.15 × 10⁻⁷ to 6.93 × 10⁻⁵ (3197 Ω–162 Ω) and from 3.00 × 10⁻⁷ to 1.56 × 10⁻³ F, respectively when 5% of SPEEK added into PES membrane. The PES/SPEEK 5% membrane has the highest performance compared to other membranes with a maximum power density of 170 mW m⁻² at the maximum current density of 340 mA m⁻². However, the interfacial reaction between the membrane and the cathode with Pt catalyst indicates moderate reaction efficiency compared to other membranes. The COD removal efficiency of MFCs with composite membrane PES/SPEEK 5% is nearly 26-fold and 2-fold higher than that of MFCs with Nafion 112 and Nafion 117 membranes respectively. The results suggest that the PES/SPEEK composite membrane is a promising alternative to the costly perfluorosulfonate membranes presently used as separators in MFC systems.     

A panel method computation for oscillating aerofoil in compressible flow

A panel method using source and doublet singularity has been proposed to solve for subcritical aerodynamics of a two dimensional steady and unsteady aerofoil. The source singularities are placed on the aerofoil surface. The doublet singularity is distributed by a function along the chordline of the aerofoil; this distribution is further projected downstream into infinity. The aerodynamics of an oscillating aerofoil is investigated. The governing unsteady linearized potential equation has a Hankel function as its fundamental solution, which is a source type function. A combination of source and doublet singularity is therefore used for solving the unsteady compressible problem by means of the panel method, this methodology being an extension of a steady aerofoil formulation. Incremental effects of profile change in aerofoil and wake geometry are accounted for. A surface boundary condition is applied on the stationary mean aerofoil surface with time dependent geometrical changes accounted for. An unsteady Kutta condition of equal pressure across the trailing edge is assumed. Results are presented on the aerodynamic influence of Mach number, oscillating frequency parameter, angle of incidence and change of pivoting point. Results are also compared with linear theory, a subsonic experimental result and a subcritical solution of a transonic model.     

Impact of Solute Properties and Water Matrix on Nanofiltration of Pesticides

The application of different nanofiltration membranes for the separation of pesticides, i.e., atrazine and dimethoate, from aqueous solutions is described. The nanoflitration membranes DK, NF270, NF200, and NF90 were tested for the pesticide retention performance in a stirred dead‐end filtration system. NF90 demonstrated the best pesticide retention with over 95 % for atrazine and approximately 80 % for dimethoate. All membranes consistently showed better retention of atrazine than of dimethoate. Dissolving the pesticides in river or tap water amplified the overall pesticide retention performance, indicating that filtration in water treatment plants could render superior pesticide retention. However, a lower flux was obtained for the filtration of tap and river water.     

Characterization of magnetic nanoparticle by dynamic light scattering

Here we provide a complete review on the use of dynamic light scattering (DLS) to study the size distribution and colloidal stability of magnetic nanoparticles (MNPs). The mathematical analysis involved in obtaining size information from the correlation function and the calculation of Z-average are introduced. Contributions from various variables, such as surface coating, size differences, and concentration of particles, are elaborated within the context of measurement data. Comparison with other sizing techniques, such as transmission electron microscopy and dark-field microscopy, revealed both the advantages and disadvantages of DLS in measuring the size of magnetic nanoparticles. The self-assembly process of MNP with anisotropic structure can also be monitored effectively by DLS.     

Combined power and rate adaptation for wireless cellular systems

We extend the throughput optimization technique of Qiu and Chawla (1999) for adaptive modulation, to combine power and rate adaptation in wireless cellular systems. We develop new combined power and rate control algorithms for wireless multimedia systems, in which the transmitted powers and rates of different media users are adapted based on the signal-to-interference power ratio. Using simulations, we show that with appropriately chosen power and rate limits, our proposed combined power and rate control algorithms can achieve a higher throughput when compared to previously proposed algorithms with power control only.     

Side-projected fiber-optic velocity sensors

Side-projected dual-fiber velocity sensors with various fiber core separations for in vivo measurements were developed and tested. Good linearity between the flow velocity and the Doppler shift frequency is obtained with this sensor in both the forward (toward the sensor tip) and reverse (away from the sensor tip) flow directions. Experimental and numerical analysis of the data showed that the boundary layer along the side wall of the fiber sensor varies with the flow velocity