Temperature-dependent internal quantum efficiency of blue high-brightness light-emitting diodes

Internal quantum efficiency (IQE) of a blue high-brightness InGaN/GaN light-emitting diode (LED) was evaluated from the external quantum efficiency measured as a function of current at various temperatures ranged between 13 and 440 K. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined the temperature-dependent IQE of the LED structure and light extraction efficiency of the LED chip. Separate evaluation of these parameters is helpful for further optimization of the heterostructure and chip designs. The data obtained enable making a guess on the temperature dependence of the radiative and Auger recombination coefficients, which may be important for identification of dominant mechanisms responsible for the efficiency droop in III-nitride LEDs. Thermal degradation of the LED performance in terms of the emission efficiency is also considered.

Planar solid oxide fuel cell modeling and optimization targeting the stack's temperature gradient minimization

Minimization of undesirable temperature gradients in all dimensions of a planar solid oxide fuel cell (SOFC) is central to the thermal management and commercialization of this electrochemical reactor. This article explores the effective operating variables on the temperature gradient in a multilayer SOFC stack and presents a trade-off optimization. Three promising approaches are numerically tested via a model-based sensitivity analysis. The numerically efficient thermo-chemical model that had already been developed by the authors for the cell scale investigations (Tang et al. Chem. Eng. J. 2016, 290, 252-262) is integrated and extended in this work to allow further thermal studies at commercial scales. Initially, the most common approach for the minimization of stack's thermal inhomogeneity, i.e., usage of the excess air, is critically assessed. Subsequently, the adjustment of inlet gas temperatures is introduced as a complementary methodology to reduce the efficiency loss due to application of excess air. As another practical approach, regulation of the oxygen fraction in the cathode coolant stream is examined from both technical and economic viewpoints. Finally, a multiobjective optimization calculation is conducted to find an operating condition in which stack's efficiency and temperature gradient are maximum and minimum, respectively.

Sensitivity of LPGs in PCFs fabricated by an electric arc to temperature, strain, and external refractive index

Sensing properties of long-period gratings (LPGs) fabricated in photonic crystal fibers by an electric arc are explained and quantified by semianalytical and numerical models. In particular, the grating's insensitivity to temperature and simultaneous sensitivity to strain and refractive index are simulated. The modeling procedure is generalized so that it can be applied to a wide range of LPGs in various fibers.

Design, modeling and construction of a novel ablative fast pyrolysis reactor and product collection system

The objective of this work was to design, construct and commission a new ablative pyrolysis reactor and a high efficiency product collection system. The reactor was to have a nominal throughput of 10 kg/11r of dry biomass and be inherently scalable up to an industrial scale application of 10 tones/hr. The whole process consists of a bladed ablative pyrolysis reactor, two high efficiency cyclones for char removal and a disk and doughnut quench column combined with a wet walled electrostatic precipitator, which is directly mounted on top, for liquids collection. In order to aid design and scale-up calculations, detailed mathematical modelling was undertaken of the reaction system enabling sizes, efficiencies and operating conditions to be determined. Specifically, a modular approach was taken due to the iterative nature of some of the design methodologies, with the output from one module being the input to the next. Separate modules were developed for the determination of the biomass ablation rate, specification of the reactor capacity, cyclone design, quench column design and electrostatic precipitator design. These models enabled a rigorous design protocol to be developed capable of specifying the required reactor and product collection system size for specified biomass throughputs, operating conditions and collection efficiencies. The reactor proved capable of generating an ablation rate of 0.63 mm/s for pine wood at a temperature of 525 'DC with a relative velocity between the heated surface and reacting biomass particle of 12.1 m/s. The reactor achieved a maximum throughput of 2.3 kg/hr, which was the maximum the biomass feeder could supply. The reactor is capable of being operated at a far higher throughput but this would require a new feeder and drive motor to be purchased. Modelling showed that the reactor is capable of achieving a reactor throughput of approximately 30 kg/hr. This is an area that should be considered for the future as the reactor is currently operating well below its theoretical maximum. Calculations show that the current product collection system could operate efficiently up to a maximum feed rate of 10 kg/Fir, provided the inert gas supply was adjusted accordingly to keep the vapour residence time in the electrostatic precipitator above one second. Operation above 10 kg/hr would require some modifications to the product collection system. Eight experimental runs were documented and considered successful, more were attempted but due to equipment failure had to be abandoned. This does not detract from the fact that the reactor and product collection system design was extremely efficient. The maximum total liquid yield was 64.9 % liquid yields on a dry wood fed basis. It is considered that the liquid yield would have been higher had there been sufficient development time to overcome certain operational difficulties and if longer operating runs had been attempted to offset product losses occurring due to the difficulties in collecting all available product from a large scale collection unit. The liquids collection system was highly efficient and modeling determined a liquid collection efficiency of above 99% on a mass basis. This was validated due to the fact that a dry ice/acetone condenser and a cotton wool filter downstream of the collection unit enabled mass measurements of the amount of condensable product exiting the product collection unit. This showed that the collection efficiency was in excess of 99% on a mass basis.

Sensitivity of LPGs in PCFs fabricated by an electric arc to temperature, strain, and external refractive index

Sensing properties of long-period gratings (LPGs) fabricated in photonic crystal fibers by an electric arc are explained and quantified by semianalytical and numerical models. In particular, the grating's insensitivity to temperature and simultaneous sensitivity to strain and refractive index are simulated. The modeling procedure is generalized so that it can be applied to a wide range of LPGs in various fibers. © 2007 IEEE.

Modeling and optimization of cascaded erbium and holmium doped fluoride fiber lasers

Numerical modeling of cascade erbium-doped and holmium-doped fluoride fiber lasers is presented. Fiber lengths were optimized for cascade lasers that had fixed or free-running wavelengths using all known spectroscopic parameters. The performance of the cascade laser was tested against dopant concentration, energy transfer process, heat generation, output coupling, and pump schemes. The results suggest that the slope efficiencies and thresholds for both transitions increase with increasing Ho3+ or Er3+ concentration with the slope efficiency stabilizing after 1 mol% rare earth doping. The heat generation in the Ho3+-based system is lower compared to the Er 3+-based system at low dopant concentration as a result of the lower rates of multiphonon relaxation. Decreasing the output coupling for the upper (∼3 μm) transition decreases the threshold of the lower transition and the upper transition benefits from decreasing the output coupling for the lower transition for both cascade systems. The highest slope efficiency was achieved under counter-propagating pump conditions. Saturation of the output power occurs at comparatively higher pump power with dilute Er3+ doping compared with heavier doping. Overall, we show that the cascade Ho3+ -doped fluoride laser is the best candidate for high power output because of its higher slope efficiency and lower temperature excursion of the core and no saturation of the output. © 2013 IEEE.