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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS2009

Observations of Screw Dislocation Driven Growth and Faceting During CVD Homoepitaxy on 4H-SiC On-Axis Mesa Arrays

Previous studies of (0001) homoepitaxial growth carried out on arrays of small-area mesas etched into on-axis silicon-face 4H-SiC wafers have demonstrated that spiral growth emanating from at least one screw dislocation threading the mesa is necessary in order for a mesa to grow taller in the <0001> (c-axis vertical) direction while maintaining 4H stacking sequence [1]. However, even amongst mesas containing the screw dislocation step source necessary for vertical c-axis growth, we have observed striking differences in the height and faceting that evolve during prolonged homoepitaxial growths. This paper summarizes Atomic Force Microscopy (AFM), Electron Channeling Contrast Imaging (ECCI), Scanning Electron Microscopy (SEM), and optical microscopy observations of this phenomenon. These observations support our initially proposed model [2] that the observed large variation (for mesas where 3C-SiC nucleation has not occurred) is related to the lateral positioning of a screw dislocation step source within each etched mesa. When the screw dislocation step source is located close enough to the developing edge/sidewall facet of a mesa, the c-axis growth rate and facet angle are affected by the resulting interaction. In particular, the intersection (or near intersection) of the inward-sloping mesa sidewall facet with the screw dislocation appears to impede the rate at which the spiral provides new steps required for c-axis growth. Also, the inward slope of the sidewall facet during growth (relative to other sidewalls of the same mesa not near the screw dislocation) seems to be impeded by the screw dislocation. In contrast, mesas whose screw dislocations are centrally located grow vertically, but inward sloping sidewall facets shrink the area of the top (0001) growth surface almost to the point of vanishing.

Neudeck, Philip G.↗
NASA NTRS2005

N-Type delta Doping of High-Purity Silicon Imaging Arrays

A process for n-type (electron-donor) delta doping has shown promise as a means of modifying back-illuminated image detectors made from n-doped high-purity silicon to enable them to detect high-energy photons (ultraviolet and x-rays) and low-energy charged particles (electrons and ions). This process is applicable to imaging detectors of several types, including charge-coupled devices, hybrid devices, and complementary metal oxide/semiconductor detector arrays. Delta doping is so named because its density-vs.-depth characteristic is reminiscent of the Dirac delta function (impulse function): the dopant is highly concentrated in a very thin layer. Preferably, the dopant is concentrated in one or at most two atomic layers in a crystal plane and, therefore, delta doping is also known as atomic-plane doping. The use of doping to enable detection of high-energy photons and low-energy particles was reported in several prior NASA Tech Briefs articles. As described in more detail in those articles, the main benefit afforded by delta doping of a back-illuminated silicon detector is to eliminate a "dead" layer at the back surface of the silicon wherein high-energy photons and low-energy particles are absorbed without detection. An additional benefit is that the delta-doped layer can serve as a back-side electrical contact. Delta doping of p-type silicon detectors is well established. The development of the present process addresses concerns specific to the delta doping of high-purity silicon detectors, which are typically n-type. The present process involves relatively low temperatures, is fully compatible with other processes used to fabricate the detectors, and does not entail interruption of those processes. Indeed, this process can be the last stage in the fabrication of an imaging detector that has, in all other respects, already been fully processed, including metallized. This process includes molecular-beam epitaxy (MBE) for deposition of three layers, including metallization. The success of the process depends on accurate temperature control, surface treatment, growth of high-quality crystalline silicon, and precise control of thicknesses of layers. MBE affords the necessary nanometer- scale control of the placement of atoms for delta doping. More specifically, the process consists of MBE deposition of a thin silicon buffer layer, the n-type delta doping layer, and a thin silicon cap layer. The n dopant selected for initial experiments was antimony, but other n dopants as (phosphorus or arsenic) could be used. All n-type dopants in silicon tend to surface-segregate during growth, leading to a broadened dopant-concentration- versus-depth profile. In order to keep the profile as narrow as possible, the substrate temperature is held below 300 C during deposition of the silicon cap layer onto the antimony delta layer. The deposition of silicon includes a silicon- surface-preparation step, involving H-termination, that enables the growth of high-quality crystalline silicon at the relatively low temperature with close to full electrical activation of donors in the surface layer.

Blacksberg, Jordana↗
NASA NTRS1980

Calculations and measurements in support of the triglycine sulfate zero gravity growth experiment

Two areas of investigation which are required for maximum utilization of the flight experiment results from the solution growth of crystals of triglycine sulfate (TGS) are addressed. The use of an ultramicroscope to characterize structural defects in the grown TGS crystals is discussed. Mie theory is applied to the light scattering expected from idealized crystal defects in order to predict the sensitivity of ultramicroscopy in the detection of small defects. Secondly, the measurement of optical properties of TGS solutions in water is discussed. This data is needed in order to interpret interferometric studies of mass transport through the TGS solution during the growth phase of the flight experiment. In particular, refractive index data is obtained for a wide range of temperatures and concentrations of TGS solutions. Absorbance measurements in the ultraviolet have also been made for the same samples.

Springer, J. M.↗
NASA NTRS1972

The kinetic and dynamic aspects of corrosion fatigue in a gaseous hydrogen environment.

The stable, subcritical crack growth stage of fracture under conditions of corrosion fatigue was studied experimentally in order to demonstrate the importance of the kinetic and dynamic aspects of environment-sensitive behavior. The cyclic loading of a titanium alloy in a low-pressure gaseous hydrogen environment is compared to that in a vacuum environment. The influence of the hydrogen environment on the rate of subcritical crack growth is discussed.

Nelson, H. G.↗
NASA NTRS1989

A critical comparison of turbulence models for homogeneous shear flows in a rotating frame

A variety of turbulence models, including five second-order closure models and four two equation models, are tested for the problem of homogeneous turbulent shear flow in a rotating frame. The model predictions for the time evolution of the turbulent kinetic energy and dissipation rate, as well as those for the equilibrium states, are compared with the results of physical and numerical experiments. Most of the two-equation models predict the same results for all rotation rates (omega/S) in which there is an exponential time growth of the turbulent kinetic energy and dissipation rate. The second-order closures are qualitatively superior since, consistent with physical and numerical experiments, they only predict this type of unstable flow for intermediate rotation rates in the range -0.1 less than or equal to omega/S less than or equal to 1.6. For rotation rates outside this range, there is an exchange of stabilities with a solution whose kinetic energy and dissipation rate decay with time. Although the second-order closures are superior to the two-equation models, there are still problems with the quantitative accuracy of their predictions.

Speziale, Charles G.↗
NASA NTRS1989

A critical comparison of turbulence models for homogeneous shear flows in a rotating frame

A variety of turbulence models, including five second-order closure models and four two equation models, are tested for the problem of homogeneous turbulent shear flow in a rotating frame. The model predictions for the time evolution of the turbulent kinetic energy and dissipation rate, as well as those for the equilibrium states, are compared with the results of physical and numerical experiments. Most of the two-equation models predict the same results for all rotation rates (omega/S) in which there is an exponential time growth of the turbulent kinetic energy and dissipation rate. The second-order closures are qualitatively superior since, consistent with physical and numerical experiments, they only predict this type of unstable flow for intermediate rotation rates in the range -0.1 less than or equal to omega/S less than or equal to 1.6. For rotation rates outside this range, there is an exchange of stabilities with a solution whose kinetic energy and dissipation rate decay with time. Although the second-order closures are superior to the two-equation models, there are still problems with the quantitative accuracy of thier predictions.

Speziale, Charles G.↗
NASA NTRS1990

A critical comparison of turbulence models for homogeneous shear flows in a rotating frame

A variety of turbulence models, including five second-order closure models and four two equation models, are tested for the problem of homogeneous turbulent shear flow in a rotating frame. The model predictions for the time evolution of the turbulent kinetic energy and dissipation rate, as well as those for the equilibrium states, are compared with the results of physical and numerical experiments. Most of the two-equation models predict the same results for all rotation rates (omega/S) in which there is an exponential time growth of the turbulent kinetic energy and dissipation rate. The second-order closures are qualitatively superior since, consistent with physical and numerical experiments, they only predict this type of unstable flow for intermediate rotation rates in the range -0.1 less than or equal to omega/S less than or equal to 1.6. For rotation rates outside this range, there is an exchange of stabilities with a solution whose kinetic energy and dissipation rate decay with time. Although the second-order closures are superior to the two-equation models, there are still problems with the quantitative accuracy of their predictions.

Speziale, Charles G.↗
NASA NTRS1999

Growth and Characterization of Bulk GeSi Solid Solutions

In this work we have grown and characterized several GeSi samples in order to investigate the effects that Silicon concentration, applied magnetic field, and liquid encapsulation have on crystalline quality. Characterization techniques include NDIC microscopy and microprobe spectroscopy. Two samples were grown with a Silicon concentration of approximately 3% and are compared to previous growths having a Silicon fraction of approximately 5%. Growth conditions for one of these samples was varied with the presence of an external applied magnetic field to investigate the possibility of magnetic field damping. A comparison between these two ingots, and with previously grown material, revealed no clear improvement in sample crystalline quality. Three additional samples were grown using a CaCl2 liquid encapsulation technique that produced GeSi material with improved structural quality over previous samples. Comparisons to prior non-encapsulation grown material, details of our methodology, and suggestions for further improvements are discussed.

Ritter, Timothy M.↗
NASA NTRS2001

Cell and Molecular Biology of Ataxia Telangiectasia Heterozygous Human Mammary Epithelial Cells Irradiated in Culture

Autologous isolates of cell types from obligate heterozygotes with the autosomal disorder ataxia-telangiectasia (A-T)were used to begin a tissue culture model for assessing pathways of radiation-induced cancer formation in this target tissue. This was done by establishing cultures of stromal fibroblasts and long-term growth human mammary epithelial cells (HMEC) in standard 2-dimensional tissue culture in order to establish expression of markers detailing early steps of carcinogenesis. The presumptive breast cancer susceptibility of A-T heterozygotes as a sequel to damage caused by ionizing radiation provided reason to study expression of markers in irradiated HMEC. Findings from our study with HMEC have included determination of differences in specific protein expression amongst growth phase (e.g., log vs stationary) and growth progression (e.g., pass 7 vs pass 9), as well as differences in morphologic markers within populations of irradiated HMEC (e.g., development of multinucleated cells).

Richmond, Robert C.↗
NASA NTRS1999

Design of Ceramic Springs for Use in Semiconductor Crystal Growth in Microgravity

Segregation studies can be done in microgravity to reduce buoyancy driven convection and investigate diffusion-controlled growth during the growth of semiconductor crystals. During these experiments, it is necessary to prevent free surface formation in order to avoid surface tension driven convection (Marangoni convection). Semiconductor materials such as gallium arsenide and germanium shrink upon melting, so a spring is necessary to reduce the volume of the growth chamber and prevent the formation of a free surface when the sample melts. A spring used in this application must be able to withstand both the high temperature and the processing atmosphere. During the growth of gallium arsenide crystals during the GTE Labs/USAF/NASA GaAs GAS Program and during the CWRU GaAs programs aboard the First and Second United States microgravity Laboratories, springs made of pyrolytic boron nitride (PBN) leaves were used. The mechanical properties of these PBN springs have been investigated and springs having spring constants ranging from 0.25 N/mm to 25 N/mm were measured. With this improved understanding comes the ability to design springs for more general applications, and guidelines are given for optimizing the design of PBN springs for crystal growth applications.

Kaforey, M. F.↗
NASA NTRS1989

The effect of Mach number on the stability of a plane supersonic wave

The influence of compressibility on the mechanisms governing the various stages of transition in a supersonic wake is investigated. Results from linear stability theory are used to provide physical insights into the observed reduction in growth rate at high Mach numbers. A newly developed hybrid algorithm is used to solve the compressible inviscid linear disturbance equations. Growth rates for both antisymmetric and symmetric modes of two-dimensional and oblique waves are computed for a wide range of Mach numbers. Results from two and three-dimensional direct numerical simulations of a forced compressible time-developing wake are presented in order to understand the nonlinear stages of transition at high Mach numbers. Observed nonlinear growth rate comparisons are made for wakes at two different Mach numbers. The reduction in growth rate at high Mach numbers is explained by examining contour plots of baroclinic torques and the product of dilatation and vorticity.

Chen, Jacqueline H.↗
NASA NTRS1990

Interface demarcation in GaAs by current pulsing

GTE Laboratories is currently conducting a program to investigate the effect of convection in the melt on the properties of bulk grown gallium arsenide (GaAs). In addition to extensive ground based experimentation, a Get Away Special growth system has been developed to grow two GaAs crystals aboard the Space Shuttle, each with a one inch diameter. In order to perform a complete segregation analysis of the crystals grown in space, it is necessary to measure the interface shape and growth rate as well as the spatial distribution of the selenium dopant. The techniques for interface demarcation in selenium doped GaAs by current pulsing have been developed at GTE Laboratories and successful interface demarcation has been achieved for current pulses ranging from 20 to 90 amps, in both single crystal and polycrystalline regions.

Matthiesen, D. H.↗
NASA NTRS1993

Kinetic dissection of individual steps in the poly(C)-directed oligoguanylate synthesis from guanosine 5'-monophosphate 2-methylimidazolide

A kinetic study of oligoguanylate synthesis on a polycytidylate template, poly(C), as a function of the concentration of the activated monomer, guanosine 5'-monophosphate 2-methylimidazolide, 2-MeImpG, is reported. Reactions were run with 0.005-0.045 M 2-MeImpG in the presence of 0.05 M poly(C) at 23 degrees C. The kinetic results are consistent with a reaction scheme (eq 1) that consists of a series of consecutive steps, each step representing the addition of one molecule of 2-MeImpG to the growing oligomer. This scheme allows the calculation of second-order rate constants for every step by analyzing the time-dependent growth of each oligomer. Computer simulations of the course of reaction based on the determined rate constants and eq 1 are in excellent agreement with the product distributions seen in the HPLC profiles. In accord with an earlier study (Fakhrai, H.; Inoue, T.; Orgel, L. E. Tetrahedron 1984, 40, 39), rate constants, ki, for the formation of the tetramer and longer oligomers up to the 16-mer were found to be independent of length and somewhat higher than k3 (formation of trimer), which in turn is much higher than k2 (formation of dimer). The ki (i > or = 4), k3, and k2 values are not true second-order rate constants but vary with monomer concentration. Mechanistic models for the dimerization (Scheme I) and elongation reactions (Scheme II) are proposed that are consistent with our results. These models take into account that the monomer associates with the template in a cooperative manner. Our kinetic analysis allowed the determination of rate constants for the elementary processes of covalent bond formation between two monomers (dimerization) and between an oligomer and a monomer (elongation) on the template. A major conclusion from our study is that bond formation between two monomer units or between a primer and a monomer is assisted by the presence of additional next-neighbor monomer units. This is consistent with recent findings with hairpin oligonucleotides (Wu, T.; Orgel, L. E. J. Am. Chem. Soc. 1992, 114, 317). Our study is the first of its kind that shows the feasibility of a thorough kinetic analysis of a template-directed oligomerization and provides a detailed mechanistic model of these reactions.

Non-NASA Center↗
NASA NTRS1979

Shock induced magnetic effects in fine particle iron dispersions

Magnetic effects associated with shock induced transformation of fcc antiferromagnetic iron precipitates in polycrystalline copper disks at levels up to 5 GPa in weak magnetic fields (H not greater than 0.5 Oe) were investigated. The demagnetization and anisotropy associated with second order transition, the effects of plastic deformation in imparting magnetic anisotropy and magnetic hardening, and the influence of post shock thermal transients on magnetization associated with recovery, recrystallization and grain growth were studied. It was found that on the microsecond time scale of the shock induced first order transformation, the field sense is recorded in the transformed iron particles. For a given particle size the degree of transformation of fcc iron depends on the level of the shock. For a given shock level the resultant magnetic properties depend on the particle size distribution, with maximum effects noted in specimens with 400 to 600 A particles.

Wasilewski, P. J.↗
NASA NTRS2011

Preliminary Results of the Effect of Microgravity on Seated Height

The new vehicle for future space travel to the International Space Station (ISS) and beyond will be highly dependent on the seat layout. A primary concern with the seat layout design of the new vehicle is the amount of seated height growth that occurs in space; this could cause a major accommodation issue. The design of the seats, seat layout, suit fit, and crew accommodation are all critically affected due to the increase in height that occurs in microgravity. The increase in height due to spinal elongation caused by the absence of gravity could lead to inadequate clearances that would have implications for the ability of crewmembers to return safely or to conduct nominal operations during the mission. This study was designed to reduce the risk of inadequate design of the vehicle, environment, tools, equipment, etc. (SHFE risk 2.3.1.1) and safely return crewmembers to earth from low-earth orbit travel, ISS, and beyond. In order to safely return the crewmembers, the design requirements must anticipate microgravity growth, elongation of the spine, bone and muscle loss, fluid shifts, etc. Thus, this study is to determine the amount of torso growth (spinal elongation) for a seated posture during Shuttle and ISS missions. Crewmembers seated heights were collected before, during, and after spaceflight to quantify the amount of growth that occurred as a result of microgravity. The changes in seated height will provide the designers with a design requirement which allows for change in spinal growth for a seated posture. Preliminary results have shown that , during flight, seated height increases by a range of approximately 2-6 percent compared to pre-launch seated height.

Rajulu, Sudhakar↗
NASA NTRS1996

Self-Consistent Simulation of the Brownian Stage of Dust Growth

It is a widely accepted view that in proto-planetary accretion disks the collision and following sticking of dust particles embedded in the gas eventually leads to the formation of planetesimals (coagulation). For the smallest dust grains, Brownian motion is assumed to be the dominant source of their relative velocities leading to collisions between these dust grains. As the dust grains grow they eventually couple to the turbulent motion of the gas which then drives the coagulation much more efficiently. Many numerical coagulation simulations have been carried out to calculate the fractal dimension of the aggregates, which determines the duration of the ineffective Brownian stage of growth. Predominantly on-lattice and off-lattice methods were used. However, both methods require simplification of the astrophysical conditions. The aggregates found by those methods had a fractal dimension of approximately 2 which is equivalent to a constant, mass-independent friction time. If this value were valid for the conditions in an accretion disk, this would mean that the coagulation process would finally 'freeze out' and the growth of a planetesimal would be impossible within the lifetime of an accretion disk. In order to investigate whether this fractal dimension is model independent, we simulate self-consistently the Brownian stage of the coagulation by an N-particle code. This method has the advantage that no further assumptions about homogeneity of the dust have to be made. In our model, the dust grains are considered as aggregates built up of spheres. The equation of motion of the dust grains is based on the probability density for the diffusive transport within the gas atmosphere. Because of the very low number density of the dust grains, only 2-body-collisions have to be considered. As the Brownian stage of growth is very inefficient, the system is to be simulated over long periods of time. In order to find close particle pairs of the system which are most likely to undergo a collision, we use a particle-in-cell (PIC) method for the early stages of the simulation where the system is still very homogeneous and a tree method later when the particles are more clustered.

Kempf, S.↗
NASA NTRS1973

GaN for LED applications

In order to improve the synthesis of GaN the effect of various growth and doping parameters has been studied. Although Be, Li, Mg, and Dy can be used to overcompensate native donors, the most interesting acceptor element is Zn. The emission spectrum and the luminescence efficiency depend on the growth temperature (below 800 C), on the partial pressure of the doping impurity, and on the duration of growth. Blue-green electroluminescence with a power efficiency of 0.1 percent and a brightness of 850 fL (at 0.6 mA and 22.5 V) was obtained. Some diodes allow the color of the emitted light to change by reversing the polarity of the bias. Continuous operation of a diode over a period of 5 months showed no evidence of degradation. The luminescence properties of ion-implanted GaN were studied. Delay effects were found in the electroluminescence of diodes, although, with a dc bias, a 70-MHz modulation was possible.

Pankove, J. I.↗
NASA NTRS1979

Fatigue crack propagation behaviour of unidirectionally solidified gamma/gamma-prime-delta eutectic alloys

Fatigue crack propagation studies were carried out on unidirectionally solidified gamma/gamma-prime-delta (Ni-Nb-Al) alloys over an aluminum content range of 1.5-2.5% by weight. The variation of Al content of as-grown alloys did not significantly affect the crack growth behavior of these eutectic composites. The results indicate that the addition of Al to the eutectic dramatically improved the FCP behavior. The gamma/gamma-prime-delta alloy exhibited crack growth rates for a given stress intensity range that are an order of magnitude lower than those for the gamma-delta alloy. It is suggested that this difference in FCP behavior can be explained on the basis of stacking fault energy considerations. Extensive delaminations at the crack tip were also revealed, which contributed to the superior fatigue response. Delamination was predominantly intergranular in nature.

Bretz, P. E.↗