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Author(s): X. Yuan [1, 2]; S. V. Sobolev [1]; R. Kind (corresponding author) [1, 2]; O. Oncken [1, 2]; G. Bock [1, 5]; G. Asch [1, 5]; B. Schurr [1, [...]

Author(s): X. Li [1, 2]; R. Kind (corresponding author) [1, 2]; K. Priestley [3]; S. V. Sobolev [1]; F. Tilmann [3]; X. Yuan [1, 2]; M. Weber [1, 4] The [...]

A study was conducted to determine the aerodynamic characteristics of actual, naturally grown frost-roughness samples that developed along aluminum plates exposed to the clear night sky. Boundary-layer measurements are taken, and results show that the effects of the frost roughness are accurately represented by the height and the spacing parameter. Moreover, effective height and spacing parameters can be determined from the higher frost formations, where the main aerodynamic effects reside. Finally, the ratio of equivalent sand roughness height to frost height varies widely between frost samples.

The performance of hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells is limited, as they contain a relatively high concentration of defects. The dark current voltage (JV) characteristics at low forward voltages of these devices are dominated by recombination processes. The recombination rate depends on the concentration of active recombination centers and the recombination efficacy of each of these centers. The first factor causes the ideality factor of the devices to be non-integer and to vary with voltage. The temperature dependence of the dark current can be expressed by its activation energy. For microcrystalline silicon solar cells the activation energy varies with voltage with a so-called thermal ideality factor of 2. This value was derived for devices with a spatially uniform defect distribution and reflects the recombination efficacy. Here we present results of a thickness series of a-Si:H p-i-n solar cells. We have matched the experimental curves with computer simulations, and show that the voltage-dependent ideality factor curve can be used to extract information on the cross sections for electron and hole capture. Also, the activation energy is used as a measure for the mobility gap, resulting in a mobility gap for a-Si:H of 1.69 eV. We find a thermal ideality factor close to 2 for all samples. This is explained with a theoretical derivation, followed by a comparison between the internal electric field strength and the spatial variation of the defect density in the intrinsic layer. The thermal ideality factor is shown to be insensitive to the defect distribution and the recombination profile in the device. It is, therefore, an appropriate parameter to characterize a-Si:H p-i-n devices, providing direct insight on the recombination efficacy. [ABSTRACT FROM AUTHOR]

We report positron depth-profiling studies on the hydrogen sorption behavior and phase evolution of Mg-based thin films. We show that the main changes in the depth profiles resulting from the hydrogenation to the respective metal hydrides are related to a clear broadening in the observed electron momentum densities in both Mg and Mg2Ni films. This shows that positron annihilation methods are capable of monitoring these metal-to-insulator transitions, which form the basis for important applications of these types of films in switchable mirror devices and hydrogen sensors in a depth-sensitive manner. Besides, some of the positrons trap at the boundaries of columnar grains in the otherwise nearly vacancy-free Mg films. The combination of positron annihilation and x-ray diffraction further shows that hydrogen loading at elevated temperatures, in the range of 480–600 K, leads to a clear Pd–Mg alloy formation of the Pd catalyst cap layer. At the highest temperatures, the hydrogenation induces a partial delamination of the ∼5 nm thin capping layer, as sensitively monitored by positron depth profiling of the fraction of ortho-positronium formed at interface with the cap layer. The delamination effectively blocks the hydrogen cycling. In Mg–Si bilayers, we investigated the reactivity upon hydrogen loading and heat treatments near 480 K, which shows that Mg2Si formation is fast relative to MgH2. The combination of positron depth profiling and transmission electron microscopy shows that hydrogenation promotes a complete conversion to Mg2Si for this destabilized metal hydride system, while a partially unreacted, Mg-rich amorphous prelayer remains on top of Mg2Si after a single heat treatment in an inert gas environment. Thin film studies indicate that the difficulty of rehydrogenation of Mg2Si is not primarily the result from slow hydrogen dissociation at surfaces, but is likely hindered by the presence of a barrier for removal of Mg from the readily formed Mg2Si. [ABSTRACT FROM AUTHOR]

In isotactic poly(methyl methacrylate) (PMMA), we investigate the dynamics of the ester methyl groups by means of deuteron magnetic resonance (DMR) in a deuterated sample. We find that the motion of the CD3-group affects the deuteron spin-lattice relaxation as well as the DMR line shape in a characteristic way. Quadrupolar order spin lattice relaxation measurements between T=291 K and T=70 K reveal a broad temperature dependent probability distribution of autocorrelation times τc for the 2π/3 reorientation. This broad distribution corresponds to a temperature independent Gaussian distribution of activation energies ρ(Ea) with variance σEa=13.8±0.5 meV (1.33 kJ/mol). The line shape transition between T=70 K and T=23 K is explained with the freezing in of the methyl group reorientation. By comparing our results in an 88% isotactic sample with results obtained from a 50% syndiotactic, 30% atactic, and 20% isotactic sample of a previous investigation, we demonstrate the higher local order of the 88% isotactic sample, which corresponds to a ratio of 1.6 in the relative width σEa/Ea of the Ea distribution. We show that different stereospecific forms of PMMA can be easily distinguished by the characteristics of their line shape transition between T=70 K and T=23 K. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Researches have been conducted on tip-clearance flows since these are major sources of losses in axial compressor and turbine blade rows. Models have also been developed to predict tip-clearance losses. Theoretical considerations of the retained lift concept are discussed. An alternative hypothesis on streamwise circulation downstream of blade rows with tip clearance is also presented.

Three plates roughened with regular arrays of cylindrical and hemispherical depressions were tested in a wind tunnel to gain insight into the aerodynamic effects of depression roughness. The term depression roughness refers to configurations where the roughness elements are depressions whose plan area constitutes less than about one-third of the total surface area. The effects of the three depression-roughness configurations on law-of-the-wall parameters were determined. They were found to be much less than that of protrusion roughness of corresponding cross section. Hemispherical depressions produced greater increases in skin friction than cylindrical depressions of the same diameter and depth. Flow development along surfaces with depression roughness can be computed using the same methods used for protrusion roughness, provided that an equivalent protrusion height can be established for the depressions.

Four turbulence models, Baldwin-Lomax (Baldwin, B., and Lomax, H., 'Thin-Layer Approximation and Algebraic Model for Separated Turbulent Flows,' AIAA Paper 78-257, 1978), Dash et al. k-[element of], (Dash, S. M., Beddini, R. A., Wolf, D. E., and Sinha, N., 'Viscous/Inviscid Analysis of Curved Sub- or Supersonic Wall Jets,' AIAA Paper 83-1679, 1983), Wilcox k-[Omega] (Wilcox, D. C., 'Reassessment of Scale-Determining Equation for Advanced Turbulence Models,' AIAA Journal, Vol. 26, No. 11, 1988, pp. 1299-1310), and Wilcox multiscale (Wilcox, D. C., 'Multiscale Model for Turbulent Flows,' AIAA Journal, Vol. 26, No. 11,1998, pp. 1311-1320), have been assessed for predicting development of plane and curved turbulent wall-jet flows. The k-[Omega] and multiscale models were also modified to improve their performance for wall jets. In their original form, all of the models exhibited significant deficiencies, although all performed reasonably well for wall jets in an external stream of relatively high velocity. The Dash et al. k-[element of] model performed reasonably well in an overall sense, particularly for curved wall jets in still air. The deficiencies of the other three models were particularly severe for these flows. After modification, the k-[Omega] model gave reasonably good predictions, except for [C.sub.f]. The multiscale model, even after modification, gave poor predictions of velocity profiles for both plane and curved still-air wall jets and also poor predictions of [C.sub.f] for curved still-air wall jets. Unfortunately, the modifications were not universal; that is, optimum values of the model 'constants' varied substantially from one flow to another. Either the Baldwin-Lomax model, with an outer-layer curvature correction, or a k-[element of] model similar to that of Dash et al. appears to be most suitable at this time for engineering calculations involving wall-jet flows.