A kinetic and equilibrium analysis of silicon carbide chemical vapor deposition on monofilaments

Chemical kinetics of atmospheric pressure silicon carbide (SiC) chemical vapor deposition (CVD) from dilute silane and propane source gases in hydrogen is numerically analyzed in a cylindrical upflow reactor designed for CVD on monofilaments. The chemical composition of the SiC deposit is assessed both from the calculated total fluxes of carbon and silicon and from chemical equilibrium considerations for the prevailing temperatures and species concentrations at and along the filament surface. The effects of gas and surface chemistry on the evolution of major gas phase species are considered in the analysis

Convection and chemistry effects in CVD: A 3-D analysis for silicon deposition

The computational fluid dynamics code FLUENT has been adopted to simulate the entire rectangular-channel-like (3-D) geometry of an experimental CVD reactor designed for Si deposition. The code incorporated the effects of both homogeneous (gas phase) and heterogeneous (surface) chemistry with finite reaction rates of important species existing in silane dissociation. The experiments were designed to elucidate the effects of gravitationally-induced buoyancy-driven convection flows on the quality of the grown Si films. This goal is accomplished by contrasting the results obtained from a carrier gas mixture of H2/Ar with the ones obtained from the same molar mixture ratio of H2/He, without any accompanying change in the chemistry. Computationally, these cases are simulated in the terrestrial gravitational field and in the absence of gravity. The numerical results compare favorably with experiments. Powerful computational tools provide invaluable insights into the complex physicochemical phenomena taking place in CVD reactors. Such information is essential for the improved design and optimization of future CVD reactors

CVD of silicon carbide on structural fibers: Microstructure and composition

Structural fibers are currently being considered as reinforcements for intermetallic and ceramic materials. Some of these fibers, however, are easily degraded in a high temperature oxidative environment. Therefore, coatings are needed to protect the fibers from environmental attack. Silicon carbide (SiC) was chemically vapor deposited (CVD) on Textron's SCS6 fibers. Fiber temperatures ranging from 1350 to 1500 C were studied. Silane (SiH4) and propane (C2H8) were used for the source gases and different concentrations of these source gases were studied. Deposition rates were determined for each group of fibers at different temperatures. Less variation in deposition rates were observed for the dilute source gas experiments than the concentrated source gas experiments. A careful analysis was performed on the stoichiometry of the CVD SiC coating using electron microprobe. Microstructures for the different conditions were compared. At 1350 C, the microstructures were similar; however, at higher temperatures, the microstructure for the more concentrated source gas group were porous and columnar in comparison to the cross sections taken from the same area for the dilute source gas group

First principles study of structural phase stability of wide-gap semiconductors MgTe, MgS and MgSe

Cataloged from PDF version of article.The structural stability of different crystallographic phases of three magnesium chalcogenides, MgTe, MgS and MgSe, are investigated from first principles calculations based on density functional theory (DFT) using projector augmented waves (PAW) potentials within both generalized gradient approximation (GGA) and local density approximation (LDA). Five different phases of these compounds, rock-salt (B1), cesium chloride (B2), zinc-blende (B3), wurtzite (B4) and NiAs (B8(1)), are considered in detail LDA. calculations of the ground state structure of all compounds considered are in accordance with previous studies, while calculations with GGA functionals resulted interesting perspectives. Ground state stable phases of MgS and MgSe are found as rock-salt structure, while the NiAs is the ground state of MgTe within the LDA scheme. But GGA calculations show that the ground states of MgTe and MgSe are wurtzite, while the ground state of MgS is same as in LDA calculations. There is also a structural transition observed for MgTe around similar to 1.2 GPa pressure from wurtzite to NiAs by GGA calculations. The calculated quantities show that the rock-salt-NiAs and zinc-blende-wurtzite pairs have similar energetics. The calculated structural quantities for all compounds under study in all phases agree well with the available experimental and theoretical values. (C) 2009 Elsevier B.V. All rights reserved

Sensitivity Analysis of the Change of Renal Stone Occurrence Rates in Astronauts Using Urine Chemistries

Changes in urine chemistry, during and post-flight, potentially alter the likelihood of renal stones in astronauts. Although much is known about the effects of space flight on urine chemistry, no inflight incidences of renal stones in US astronauts exist and the question How much does this risk change with space flight? remains difficult to accurately quantify. Previous work by our group has illustrated the application of multi-factor deterministic and probabilistic modeling to assess the change in predicted likelihood of renal stone. Utilizing 1517 astronaut urine chemistries to inform the renal stone occurrence rate forecasting model, we performed a sensitivity analysis on urine chemistry components for their influence on predictions of renal stone size and rate of renal stone occurrence

In-Space Propulsion, Logistics Reduction, and Evaluation of Steam Reformer Kinetics: Problems and Prospects

Human space missions generate waste materials. A 70-kg crewmember creates a waste stream of 1 kg per day, and a four-person crew on a deep space habitat for a 400+ day mission would create over 1600 kg of waste. Converted into methane, the carbon could be used as a fuel for propulsion or power. The NASA Advanced Exploration Systems (AES) Logistics Reduction and Repurposing (LRR) project is investing in space resource utilization with an emphasis on repurposing logistics materials for useful purposes and has selected steam reforming among many different competitive processes as the preferred method for repurposing organic waste into methane. Already demonstrated at the relevant processing rate of 5.4 kg of waste per day, high temperature oxygenated steam consumes waste and produces carbon dioxide, carbon monoxide, and hydrogen which can then be converted into methane catalytically. However, the steam reforming process has not been studied in microgravity. Data are critically needed to understand the mechanisms that allow use of steam reforming in a reduced gravity environment. This paper reviews the relevant literature, identifies gravity-dependent mechanisms within the steam gasification process, and describes an innovative experiment to acquire the crucial kinetic information in a small-scale reactor specifically designed to operate within the requirements of a reduced gravity aircraft flight. The experiment will determine if the steam reformer process is mass-transport limited, and if so, what level of forced convection will be needed to obtain performance comparable to that in 1-g

Global Perspectives on Task Shifting and Task Sharing in Neurosurgery.

BACKGROUND: Neurosurgical task shifting and task sharing (TS/S), delegating clinical care to non-neurosurgeons, is ongoing in many hospital systems in which neurosurgeons are scarce. Although TS/S can increase access to treatment, it remains highly controversial. This survey investigated perceptions of neurosurgical TS/S to elucidate whether it is a permissible temporary solution to the global workforce deficit. METHODS: The survey was distributed to a convenience sample of individuals providing neurosurgical care. A digital survey link was distributed through electronic mailing lists of continental neurosurgical societies and various collectives, conference announcements, and social media platforms (July 2018-January 2019). Data were analyzed by descriptive statistics and univariate regression of Likert Scale scores. RESULTS: Survey respondents represented 105 of 194 World Health Organization member countries (54.1%; 391 respondents, 162 from high-income countries and 229 from low- and middle-income countries [LMICs]). The most agreed on statement was that task sharing is preferred to task shifting. There was broad consensus that both task shifting and task sharing should require competency-based evaluation, standardized training endorsed by governing organizations, and maintenance of certification. When perspectives were stratified by income class, LMICs were significantly more likely to agree that task shifting is professionally disruptive to traditional training, task sharing should be a priority where human resources are scarce, and to call for additional TS/S regulation, such as certification and formal consultation with a neurosurgeon (in person or electronic/telemedicine). CONCLUSIONS: Both LMIC and high-income countries agreed that task sharing should be prioritized over task shifting and that additional recommendations and regulations could enhance care. These data invite future discussions on policy and training programs

New Mediterranean Biodiversity Records (April, 2019)

The Collective Article on “New Mediterranean Biodiversity Records” offers the means to publish biodiversity records in the Mediterranean Sea. The current article presents new biodiversity data information on 18 taxa (14 alien, 3 native and 1 cryptogenic species) belonging to 5 Phyla that are reported for 11 different countries listed according to their geographic position from the western to the eastern Mediterranean Sea. Algeria: the alien green alga Caulerpa taxifolia var. distichophylla and the heterobranchs Aplysia dactylomeda and Aplysia punctata are first reported from the entire country. France: a new record of the alien hydroid Porpita porpita is reported from the Corsican Sea, representing the second record of the species in the western Mediterranean Sea. Tunisia: the alien fish Scatophagus argus is reported from the Gulf of Gabès, probably representing an aquarium release. Italy: the well-established Mediterranean alien bivalve Malleus regula is first reported from the entire country. Slovenia: the alien marine bivalve Xenostrobus securis is reported for the first time from the entire country. Croatia: the alien gastropod Biuve fulvipunctata is reported for the first time from the country but also from the entire Adriatic Sea. Montenegro: the alien bivalve Rapana venosa is reported for the first time from the Montenegrin waters. Albania: the well-established alien fish Lagocephalus sceleratus and the rare native shark Rhizoprionodon acutus are reported for the first from the entire country. Greece: the rare native ray Leucoraja circularis is reported for the first time from the Argolikos Gulf and for the first time for the entire country its identification is confirmed morphologically and molecularly; additional records of the alien nomad jellyfish Rhopilema nomadic document its expanding distribution, while the occurrence and the alien fishes Sillago suezensis and Pomadasys stridens are reported for the first time from Hellenic waters. Turkey: additional records of the Egyptian Prawn Metapenaeopsis aegyptia are reported with some biological information. Cyprus: the alien fish Variola louti is reported for the first time for the entire Mediterranean Sea, probably released from an aquarium, while the alien moon crab Matuta victor is reported for the first time from the entire country