Optical properties of hydrogenated amorphous silicon

A detailed study of the optical properties of sputtered hydrogenated amorphous silicon films with varying hydrogen concentration is presented here. The energy dependence of the absorption coefficient is looked into, in detail, from a point of view of understanding the well known Tauc rule and the alternate relations being proposed in recent years. Spectroscopic and band‐structural models like Wemple-Didomenico and Penn are then utilized to analyze the optical parameters near the band‐gap region of the wavelength spectra. Extensive comparisons of our results are made with those of sputtered a‐Si:H films of other workers, glow discharge prepared a‐Si:H, chemically vapor deposited and evaporated a‐Si, and also crystalline silicon. The similarities in the variation of the optical properties of a‐Si:H with increasing hydrogen concentration (or decreasing measurement temperature) to that of crystalline silicon with decreasing measurement temperature lead us to interesting conclusions. Thus, it seems that decreasing disorder (topological or thermal) in a‐Si:H is equivalent to decreasing thermal disorder in c‐Si, at least as far as the disorder‐optical property relationships are concerned

Toward evaluating the effect of climate change on investments in the water resources sector: insights from the forecast and analysis of hydrological indicators in developing countries

The World Bank has recently developed a method to evaluate the effects of climate change on six hydrological indicators across 8951 basins of the world. The indicators are designed for decision-makers and stakeholders to consider climate risk when planning water resources and related infrastructure investments. Analysis of these hydrological indicators shows that, on average, mean annual runoff will decline in southern Europe; most of Africa; and in southern North America and most of Central and South America. Mean reference crop water deficit, on the other hand, combines temperature and precipitation and is anticipated to increase in nearly all locations globally due to rising global temperatures, with the most dramatic increases projected to occur in southern Europe, southeastern Asia, and parts of South America. These results suggest overall guidance on which regions to focus water infrastructure solutions that could address future runoff flow uncertainty. Most important, we find that uncertainty in projections of mean annual runoff and high runoff events is higher in poorer countries, and increases over time. Uncertainty increases over time for all income categories, but basins in the lower and lower-middle income categories are forecast to experience dramatically higher increases in uncertainty relative to those in the upper-middle and upper income categories. The enhanced understanding of the uncertainty of climate projections for the water sector that this work provides strongly support the adoption of rigorous approaches to infrastructure design under uncertainty, as well as design that incorporates a high degree of flexibility, in response to both risk of damage and opportunity to exploit water supply 'windfalls' that might result, but would require smart infrastructure investments to manage to the greatest benefit

Feedback Control of the National Airspace System

This paper proposes a general modeling framework adapted to the feedback control of traffic flows in Eulerian models of the National Airspace System. It is shown that the problems of scheduling and routing aircraft flows in the National Airspace System can be posed as the control of a network of queues with load-dependent service rates. Focus can then shift to developing techniques to ensure that the aircraft queues in each airspace sector, which are an indicator of the air traffic controller workloads, are kept small. This paper uses the proposed framework to develop control laws that help prepare the National Airspace System for fast recovery from a weather event, given a probabilistic forecast of capacities. In particular, the model includes the management of airport arrivals and departures subject to runway capacity constraints, which are highly sensitive to weather disruptions.National Science Foundation (U.S.) (Contract ECCS-0745237)United States. National Aeronautics and Space Administration (Contract NNA06CN24A

Scaling Constraints for Urban Air Mobility Operations: Air Traffic Control, Ground Infrastructure, and Noise

The scalability of the current air traffic control system, the availability of aviation ground infrastructure, and the acceptability of aircraft noise to local communities have been identified as three key operational constraints that may limit the implementation or growth of Urban Air Mobility (UAM) systems. This paper identifies the primary mechanisms through which each constraint emerges to limit the number of UAM operations in an area (i.e. the scale of the service). Technical, ecosystem, or operational factors that influence each of the mechanisms are also identified. Interdependencies between the constraints are shown. Potential approaches to reduce constraint severity through adjustments to the mechanisms are introduced. Finally, an effort is made to characterize the severity of each operational constraint as a function of the density of UAM operations in a region of interest. To this end, a measure of severity is proposed for each constraint. This measure is used to notionally display how the severity of the constraint responds to UAM scaling, and to identify scenarios where efforts to relieve the constraint are most effective. The overall purpose of this paper is to provide an abstraction of the workings of the key UAM operational constraints so that researchers, developers, and practitioners may guide their efforts to mitigation pathways that are most likely to increase achievable UAM system scale

MÖSSBAUER EFFECT STUDIES IN AMORPHOUS AsxTe1-x

Nous avons étudié la structure des films minces ainsi que des échantillons verreux de AsxTe1-x [l, 2] en fonction de leur composition et du traitement thermique. Les spectres de 125Te dans le AsxTe1-x amorphe pour les compositions correspondantes à 0.10 ≤ x ≤ 0.57 présentent un doublet quadrupolaire caractéristique d'un écartement bien défini. Les valeurs obtenues pour un ensemble de six compositions dans le domaine mentionné plus liant montrent que l'écartement quadrupolaire augmente de façon régulière lorsque x décroît. Ces résultats suggèrent l'absence de tout ordre chimique dans le système amorphe. Les valeurs de l'interaction quadrupolaire dans les verres sont identiques à celles des films minces pour les compositions correspondantes à x = 0.3 et x = 0.5 ; ceci corrobore les résultats de Cornet et Rossier [3] obtenus par RDF X et électronique. Nous faisons également la synthèse des indications que l'interaction quadrupolaire et le déplacement isomérique donnent sur l'ordre atomique dans la phase amorphe. Nous avons effectué une étude systématique de la cinétique de cristallisation des verres As30 Te70 et As50 Te50 au moyen des isotopes Mössbauer l25Te et 129I ainsi que par diffraction des rayons X. En général, la transition de la phase amorphe à la phase cristalline dans le AsxTe1-x s'accompagne d'une réduction spectaculaire de l'interaction quadrupolaire du 125Te et 129I. Dans la phase riche en Te (x = 0.3), la cristallisation produit deux environnements chimiques distincts pour le Te, à savoir le Te métallique et le As2Te3 monochimique. Pour la phase riche en As, la cristallisation comporte plusieurs phases intermédiaires que nous discuterons plus en détail.Structural investigations on amorphous AsxTe1-x [l, 2] thin films and bulk glass samples have been performed as a function of composition x and heat treatment. 125Te spectra of amorphous AsxTe1-x in the composition range 0.10 ≤ x ≤ 0.57 show quadrupole doublets, characterized by fairly well defined splittings. Data obtained for a total of six compositions in the specified range, show that the quadrupole splitting (QS) increases smoothly as x decreases, suggesting the lack of chemical ordering in this amorphous system. QS values for bulk glasses and thin films examined at each of the two compositions x = 0.30 and 0.50 were found to be identical, thus corroborating the X-ray and Electron RDF measurements of Cornet and Rossier [3] who found the local order parameters in AsxTe1-x thin films and bulk glasses to be identical. Implications of the QS and isomer shift (IS) data on the atomic order in the amorphous phase will be reviewed. Kinetics of crystallization of As30Te70 and As50Te50 bulk glasses were systematically investigated using l25Te and 129I Mössbauer spectroscopies in conjunction with X-ray diffraction. Generally speaking, the amorphous to crystalline transformation in the AsxTe1-x system is accompanied by a dramatic reduction in the 125Te and 129I quadrupole interactions. In the Te rich phase (x = 0.30) crystallization leads to Te in two chemical environments, Te metal and monoclinic As2Te3. In the As rich phase (x = 0.50), the process of crystallization reveals intermediate phases, the details of which will be elaborated