31,996 research outputs found
Tailored plasma sprayed MCrAlY coatings for aircraft gas turbine applications
Eighteen plasma sprayed coating systems, nine based on the NiCoCrAly chemistry and nine based on the CoCrAly composition, were evaluated to identify coating systems which provide equivalent or superior life to that shown by the electron beam physical vapor deposited NiCoCrAly and CoCrAly coatings respectively. NiCoCrAly type coatings were examined on a single crystal alloy and the CoCrAly based coatings were optimized on the B1900+ Hf alloy. Cyclic burner rig oxidation and hot corrosion and tensile ductility tests used to evaluate the various coating candidates. For the single crystal alloy, a low pressure chamber plasma sprayed NiCoCrAly + Si coating exhibited a 2x oxidation life improvement at 1394 K (2050 F) over the vapor deposited NiCoCrAly material while showing equivalent tensile ductility. A silicon modified low pressure chamber plasma sprayed CoCrAly coating was found to be more durable than the baseline vapor deposited CoCrAly coating on the B1900+ Hf alloy
Coulomb screening in linear coasting nucleosynthesis
We investigate the impact of coulomb screening on primordial nucleosynthesis
in a universe having scale factor that evolves linearly with time. Coulomb
screening affects primordial nucleosynthesis via enhancement of thermonuclear
reaction rates. This enhancement is determined by the solving Poisson equation
within the context of mean field theory (under appropriate conditions during
the primordial nucleosynthesis). Using these results, we claim that the mean
field estimates of coulomb screening hardly affect the predicted element
abundances and nucleosynthesis parameters. The deviations
from mean field estimates are also studied in detail by boosting genuine
screening results with the screening parameter (). These deviations
show negligible effect on the element abundances and on nucleosynthesis
parameters. This work thus rules out the coulomb screening effects on
primordial nucleosynthesis in slow evolving models and confirms that
constraints in ref.[7] on nucleosynthesis parameters remain unaltered.Comment: 14 pages,11 figur
Ion stopping in dense plasma target for high energy density physics
The basic physics of nonrelativistic and electromagnetic ion stopping in hot and ionized plasma targets is thoroughly updated. Corresponding projectile-target interactions involve enhanced projectile ionization and coupling with target free electrons leading to significantly larger energy losses in hot targets when contrasted to their cold homologues. Standard stoppping formalism is framed around the most economical extrapolation of high velocity stopping in cold matter. Further elaborations pay attention to target electron coupling and nonlinearities due to enhanced projectile charge state, as well. Scaling rules are then used to optimize the enhanced stopping of MeV/amu ions in plasmas with electron linear densities nel ~ 10 18 -10 20 cm -2 . The synchronous firing of dense and strongly ionized plasmas with the time structure of bunched and energetic multicharged ion beam then allow to probe, for the first time, the long searched enhanced plasma stopping and projectile charge at target exit. Laser ablated plasmas (SPQR1) and dense linear plasma columns (SPQR2) show up as targets of choice in providing accurate and on line measurements of plasma parameters. Corresponding stopping results are of a central significance in asserting the validity of intense ion beam scenarios for driving thermonuclear pellets. Other applications of note feature thorium induced fission, novel ion sources and specific material processing through low energy ion beams. Last but not least, the given ion beam-plasma target interaction physics is likely to pave a way to the production and diagnostics of warm dense matter (WDM)
Si-SiO2 interface behavior in n-MOSFETs with screening potential during high-field injection
This work investigates the screening of hot carrier stress degradation in n-channel MOSFETs when the devices were exposed to plasma processing. Devices with various antenna ratios were subjected to current stress (both gate injection and substrate injection) while the source and drain terminals were reverse biased by a screening potential followed by hot carrier stress. It was observed that screening of the drain edge was effective for both gate injection and substrate injection at different screening potentials. The hot carrier lifetime is directly related to interface state density (Dit), measured by charge pumping method. The results suggest that hot electron degradation could be severe or mild for devices affected by plasma damage depending on their exposure to the level of screening potential.
This work also investigates the screening of Si-H bond concentration for polarity-dependent high field electron injection under effective screening potentials. It was observed that Si-H bond concentration varies based on the screening of the source and the drain edges during current stress when a reverse bias potential is applied to the source and drain terminals. The interface state density (Dit), measured by charge pumping method, is found to have strong dependence on the concentration of the Si-H bonds. Hot carrier stress that significantly contributes to Si-H bond breaking confirmed the effective screening. The results also indicate that Si-H bond breaking mechanism during screening is dependent on the polarity of the current stress and the screening potential applied
Coatings for directional eutectics
Eleven coating systems based on MCrAlY overlay and diffusion aluminide prototypes were evaluated to determine their capability for protecting the gamma/gamma prime-delta directionally solidified eutectic alloy (Ni-20Cb-6Cr-2.5Al) in gas turbine engine applications. Furnace oxidation and hot corrosion, Mach 0.37 burner-rig, tensile ductility, stress-rupture and thermomechanical fatigue tests were used to evaluate the coated gamma/gamma prime-delta alloy. The diffusion aluminide coatings provided adequate oxidation resistance at 1144 K (1600 F) but offered very limited protection in 114 K (1600 F) hot corrosion and 1366 K (2000 F) oxidation tests. A platinum modified NiCrAlY overlay coating exhibited excellent performance in oxidation testing and had no adverse effects upon the eutectic alloy
First principles calculation of the effect of Coulomb collisions in partially ionized gases
Coulomb collisions, at appreciable ratios (\eta) of the electron to the
neutral particle density, influence significantly the electron kinetics in
particle swarms and in plasmas of gas discharges. This paper introduces a
combination of Molecular Dynamics and Monte Carlo simulation techniques, to
provide a novel, approximation free, first principles calculation method for
the velocity distribution function of electrons, and related swarm
characteristics, at arbitrary \eta. Simulation results are presented for
electrons in argon gas, for density ratios between zero and 0.1, representing
the limits of a negligible electron density and an almost complete
Maxwellization of the velocity distribution function, respectively
The interaction of spacecraft high voltage power systems with the space plasma environment
The development of spacecraft with electrical loads that require high voltage power is discussed. The high voltage solar array has been considered for supplying d.c. power directly to high voltage loads such as ion thrusters and communication tubes without intermediate power processing. Space power stations for transferring solar power to earth are being studied in the 40 kilovolt, multikilowatt regime. Analytical and experimental studies have determined that with the advent of high voltage power, new problems will arise through the interaction of the high voltage surfaces with the charged particle environment of space. The interactive environment has been identified and duplicated to some extent in simulation facilities at NASA-Lewis Research Center and at several contractor locations
Tunability and Losses of Mid-infrared Plasmonics in Heavily Doped Germanium Thin Films
Heavily-doped semiconductor films are very promising for application in
mid-infrared plasmonic devices because the real part of their dielectric
function is negative and broadly tunable in this wavelength range. In this work
we investigate heavily n-type doped germanium epilayers grown on different
substrates, in-situ doped in the to cm range, by
infrared spectroscopy, first principle calculations, pump-probe spectroscopy
and dc transport measurements to determine the relation between plasma edge and
carrier density and to quantify mid-infrared plasmon losses. We demonstrate
that the unscreened plasma frequency can be tuned in the 400 - 4800 cm
range and that the average electron scattering rate, dominated by scattering
with optical phonons and charged impurities, increases almost linearly with
frequency. We also found weak dependence of losses and tunability on the
crystal defect density, on the inactivated dopant density and on the
temperature down to 10 K. In films where the plasma was optically activated by
pumping in the near-infrared, we found weak but significant dependence of
relaxation times on the static doping level of the film. Our results suggest
that plasmon decay times in the several-picosecond range can be obtained in
n-type germanium thin films grown on silicon substrates hence allowing for
underdamped mid-infrared plasma oscillations at room temperature.Comment: 18 pages, 10 figure
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