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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Design of experiments to observe radiation stabilized Rayleigh-Taylor instability growth at an embedded decelerating interface

Using a hohlraum produced thermal x-ray drive at the National Ignition Facility (NIF) to create pressure by material ablation, a shock exceeding 200 Mbar can be driven through a planar, solid-density target and into a lower-density foam material. The shock driven through the foam is strongly radiative, and this radiation significantly alters the dynamics of the system, including those of the Rayleigh-Taylor (RT) fluid instability at the interface between the two materials. We discuss here the design of experiments that can produce such radiative conditions. One will be able to compare the observed growth rates with an extensive body of hydrodynamic experiments performed previously. In this paper, we describe a set of 1D simulations performed to understand the mechanisms of stabilization in a strongly radiative Rayleigh-Taylor unstable system. Simulation results are used to calculate modified analytic RT growth rates which have been proposed in the literature. Calculations predict reduced RT spike growth as a result of increases in density gradient scale length and mass ablation from the unstable interface. This work has direct applicability to the observable features in upcoming NIF experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Macroscopic and microscopic defects and nonlinear optical properties of KH{sub 2}PO{sub 4} crystals with embedded TiO{sub 2} nanoparticles

Results from the successful growth of high quality KH{sub 2}PO{sub 4} (KDP) crystals with incorporated TiO{sub 2} anatase nanoparticles and the characterization of these crystals using several complementary methods are presented. The study allowed the nature and distribution of macroscopic and microscopic defects in the KDP:TiO{sub 2} crystals to be clarified. The relationship between these defects and the distribution of TiO{sub 2} nanoparticles, and the influence of incorporated nanoparticles on the nonlinear optical properties of composite crystals in comparison with pure crystals were also elucidated. Visual observations, transmission and scanning electron microscopy have shown that the anatase nanoparticles were captured mainly by the pyramidal growth sector and, to a considerably lesser extent, by the prismatic growth sector. Energy dispersive x-ray analysis was able to confirm that the growth layer stacks contain the TiO{sub 2} particles. Fourier transformation infrared spectra have clearly shown the presence of an absorption band at about 800 cm{sup -1} in both KDP:TiO{sub 2} and TiO{sub 2}, and the disappearance of the band, associated with hydroxyl OH{sup -} groups on the TiO{sub 2} surface in KDP:TiO{sub 2}. Significant variation in the imaginary and real parts of the cubic nonlinear optical susceptibilities and refractive index changes at continuous wave excitation were found in prism and pyramid parts of pure KDP and KDP:TiO{sub 2} samples. Deciphering complicated electron paramagnetic resonance spectra in KDP:TiO{sub 2} and comparison with published data permitted the identification of paramagnetic defects along with their associated g-factors and zero-field splitting parameters (in some cases for the first time). It was found that the dominant lines belong to four different centers Fe{sub A}{sup 3+}, Fe{sub B}{sup 3+}, Cr{sub R}{sup 3+}, and Cr{sub GB}{sup 3+}. From analysis of line intensities it was concluded that the concentration of intrinsic defects like potassium and hydrogen vacancies in KDP:TiO{sub 2} is comparatively small, that the concentration of non-controlled impurities in nominally pure KDP samples is several times larger than in KDP:TiO{sub 2}, and that the concentration of non-controlled impurities in the prismatic part of the KDP:TiO{sub 2} boule is approximately twice as large as in the pyramid part.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Dynamics of positronium formation in positron-hydrogen collisions embedded in weakly coupled plasmas

Dynamics of positronium (Ps) formation in an arbitrary s-state from an arbitrary s-state of the hydrogen atom in weakly coupled plasma has been investigated within the framework of a distorted-wave theory. The interactions among the charged particles in the plasma have been represented by Debye-Huckel potentials. Using simple variational hydrogenic wave functions and arbitrary order derivatives of the general three-denominator Lewis integral partial-wave scattering amplitudes have been obtained in closed forms. These distorted-wave scattering amplitudes have been used to make a detailed study on differential and total cross sections. It has been found that screening of the interaction potentials has significant effect on the scattering dynamics. Some notable features of cross sections have also been revealed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Positron scattering from hydrogen atom embedded in weakly coupled plasma

The positron-hydrogen collision problem in weakly coupled plasma environment has been investigated by applying a formulation of the three-body collision problem in the form of coupled multi-channel two-body Lippmann-Schwinger equations. The interactions among the charged particles in the plasma have been represented by Debye-Huckel potentials. A simple variational hydrogenic wave function has been employed to calculate the partial-wave scattering amplitude. Plasma screening effects on various possible mode of fragmentation of the system e{sup +}+H(1s) during the collision, such as 1s{yields}1s and 2s{yields}2s elastic collisions, 1s{yields}2s excitation, positronium formation, elastic proton-positronium collisions, have been reported. Furthermore, a detailed study has been made on differential and total cross sections of the above processes in the energy range 13.6-350 eV of the incident positron.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Photoluminescence spectra of InAs quantum dots embedded in GaAs heterostructure

Self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE) were subjected to growth ripening pause of comparatively shorter durations (0-50s) at the growth temperature (520 Degree-Sign C). Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) studies indicate that the QD islands increase in size with the growth pause and correspondingly their density decreases. Photoluminescence (PL) studies were carried out on these QDs. The temperature dependence of the PL is studied and models to explain it are explored.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Control of silicon nanoparticle size embedded in silicon oxynitride dielectric matrix

In this study, silicon rich silicon oxynitride layers containing more than 15% nitrogen were deposited by electron cyclotron resonance assisted plasma enhanced vapor deposition in order to form silicon nanoparticles after a high temperature thermal annealing. The effect of the flows of the precursor gases on the composition and the structural properties of the layers was assessed by Rutherford backscattering spectroscopy, elastic recoil detection analysis, and infrared spectroscopic measurements. The morphological and crystallinity properties were investigated by energy filtered transmission electron microscopy and Raman spectroscopy. We show that the excess of silicon in the silicon oxynitride layer controls the silicon nanoparticles size. On the other hand, the crystalline fraction of particles is found to be strongly correlated to the nanoparticle size. Finally, the photoluminescence measurements show that it is also possible to tune the photoluminescence peak position between 400 and 800 nm and its intensity by changing the silicon excess in the silicon rich silicon oxynitride matrix.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Ultrasonic wave-based structural health monitoring embedded instrument

Piezoelectric sensors and actuators are the bridge between electronic and mechanical systems in structures. This type of sensor is a key element in the integrity monitoring of aeronautic structures, bridges, pressure vessels, wind turbine blades, and gas pipelines. In this paper, an all-in-one system for Structural Health Monitoring (SHM) based on ultrasonic waves is presented, called Phased Array Monitoring for Enhanced Life Assessment. This integrated instrument is able to generate excitation signals that are sent through piezoelectric actuators, acquire the received signals in the piezoelectric sensors, and carry out signal processing to check the health of structures. To accomplish this task, the instrument uses a piezoelectric phased-array transducer that performs the actuation and sensing of the signals. The flexibility and strength of the instrument allow the user to develop and implement a substantial part of the SHM technique using Lamb waves. The entire system is controlled using configuration software and has been validated through functional, electrical loading, mechanical loading, and thermal loading resistance tests.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Tailoring the surface density of silicon nanocrystals embedded in SiO{sub x} single layers

In this article, we explore the possibility of modifying the silicon nanocrystal areal density in SiO{sub x} single layers, while keeping constant their size. For this purpose, a set of SiO{sub x} monolayers with controlled thickness between two thick SiO{sub 2} layers has been fabricated, for four different compositions (x = 1, 1.25, 1.5, or 1.75). The structural properties of the SiO{sub x} single layers have been analyzed by transmission electron microscopy (TEM) in planar view geometry. Energy-filtered TEM images revealed an almost constant Si-cluster size and a slight increase in the cluster areal density as the silicon content increases in the layers, while high resolution TEM images show that the size of the Si crystalline precipitates largely decreases as the SiO{sub x} stoichiometry approaches that of SiO{sub 2}. The crystalline fraction was evaluated by combining the results from both techniques, finding a crystallinity reduction from 75% to 40%, for x = 1 and 1.75, respectively. Complementary photoluminescence measurements corroborate the precipitation of Si-nanocrystals with excellent emission properties for layers with the largest amount of excess silicon. The integrated emission from the nanoaggregates perfectly scales with their crystalline state, with no detectable emission for crystalline fractions below 40%. The combination of the structural and luminescence observations suggests that small Si precipitates are submitted to a higher compressive local stress applied by the SiO{sub 2} matrix that could inhibit the phase separation and, in turn, promotes the creation of nonradiative paths.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

GaAs single quantum dot embedded into AlGaAs nanowire

We report on a study of the photoluminescence spectra taken from quasi one-dimensional and quasi zero-dimensional semiconductor heterostructures. The structures were grown by molecular-beam epitaxy in (111) direction and were cylindrical nanowires based on AlGaAs, of 20 - 50 nm in diameter and 0.5 - 1 μm in length. Inside the nanowires contain one or two GaAs quantum dots, of 2 nm thick and 15 - 45 nm in diameter. We studied a single nanowire. The photoluminescence and photoluminescence excitation spectra were registered as a function of the intensity of optical excitation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Embedded ultrasound sensor in a silicon-on-insulator photonic platform

A miniaturized ultrasound sensor is demonstrated in a silicon-on-insulator platform. The sensor is based on a π-phase-shifted Bragg grating formed by waveguide corrugation. Ultrasound detection is performed by monitoring shifts in the resonance frequency of the grating using pulse interferometry. The device is characterized by measuring its response to a wideband acoustic point source generated using the optoacoustic effect. Experimental results show that the sensor's response is dominated by the formation of surface acoustic waves.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗