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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 703 records · Page 39
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NASA NTRS1988

The interaction of gold with gallium arsenide

Gold and gold-based alloys, commonly used as solar-cell contact materials, are known to react readily with gallium arsenide. Experiments designed to identify the mechanisms involved in these GaAs-metal interactions have yielded several interesting results. It is shown that the reaction of GaAs with gold takes place via a dissociative diffusion process. It is shown further that the GaAs-metal reaction rate is controlled to a very great extent by the condition of the free surface of the contact metal, an interesting example of which is the previously unexplained increase in the reaction rate that has been observed for samples annealed in a vacuum environment as compared to those annealed in a gaseous ambient. A number of other hard-to-explain observations, such as the low-temperature formation of voids in the gold lattice and crystallite growth on the gold surface, are also explained by invoking this mechanism.

Weizer, Victor G.↗
NASA NTRS1988

Cosmic strings and the large-scale structure

A possible problem for cosmic string models of galaxy formation is presented. If very large voids are common and if loop fragmentation is not much more efficient than presently believed, then it may be impossible for string scenarios to produce the observed large-scale structure with Omega sub 0 = 1 and without strong environmental biasing.

Stebbins, Albert↗
NASA NTRS1989

Superclustering in the explosion scenario

A simple toy model is used to study the spatial distribution of rich clusters in a generic type of explosion scenario. The model, parameterized by the distribution of shell radii and the filling factor, places spherical shells at random and identifies each 'knot' as a cluster. The resulting cluster correlation function is close to a power law extending to the diameter of the largest spheres. Richer clusters form at the intersections of bigger shells and so have stronger correlations. Typical shell radii and filling factors are required to produce the observed number density of clusters. Models with a power-law radius distribution also reproduce the richness distribution of clusters in the Abell catalog. Supercluster multiplicity functions, void probabilities, number counts, topology statistics, and velocity correlations confirm the presence of strong superclustering and quantify the non-Gaussian nature of the model.

Weinberg, David H.↗
NASA NTRS1988

Measuring the topology of large-scale structure in the universe

An algorithm for quantitatively measuring the topology of large-scale structure has now been applied to a large number of observational data sets. The present paper summarizes and provides an overview of some of these observational results. On scales significantly larger than the correlation length, larger than about 1200 km/s, the cluster and galaxy data are fully consistent with a sponge-like random phase topology. At a smoothing length of about 600 km/s, however, the observed genus curves show a small shift in the direction of a meatball topology. Cold dark matter (CDM) models show similar shifts at these scales but not generally as large as those seen in the data. Bubble models, with voids completely surrounded on all sides by wall of galaxies, show shifts in the opposite direction. The CDM model is overall the most successful in explaining the data.

Gott, J. Richard, III↗
NASA NTRS1988

The distribution of emission-line galaxies in selected areas of the sky

The spatial distribution of emission-line galaxies (ELGs) relative to normal galaxies in several areas of the sky is discussed. Current evidence supports the notion that ELGs trace a low-density population in all the surveyed areas with the possible exception of the CfA 'Slice of the Universe' survey. Based on this and other survey data in the north galactic cap, it is suggested that the ELGs inside the Bootes void may actually define the edge of a totally empty volume within an underdense distribution of normal galaxies.

Moody, J. Ward↗
NASA NTRS1988

Resurrecting hot dark matter - Large-scale structure from cosmic strings and massive neutrinos

These are the results of a numerical simulation of the formation of large-scale structure from cosmic-string loops in a universe dominated by massive neutrinos (hot dark matter). This model has several desirable features. The final matter distribution contains isolated density peaks embedded in a smooth background, producing a natural bias in the distribution of luminous matter. Because baryons can accrete onto the cosmic strings before the neutrinos, the galaxies will have baryon cores and dark neutrino halos. Galaxy formation in this model begins much earlier than in random-phase models. On large scales the distribution of clustered matter visually resembles the CfA survey, with large voids and filaments.

Scherrer, Robert J.↗
NASA NTRS1989

Formation of large-scale structure from cosmic strings and massive neutrinos

Numerical simulations of large-scale structure formation from cosmic strings and massive neutrinos are described. The linear power spectrum in this model resembles the cold-dark-matter power spectrum. Galaxy formation begins early, and the final distribution consists of isolated density peaks embedded in a smooth background, leading to a natural bias in the distribution of luminous matter. The distribution of clustered matter has a filamentary appearance with large voids.

Scherrer, Robert J.↗
NASA NTRS1988

A new synthetic route to a family of non-classical addition-type thermoplastics. I - Concept and demonstration

A synthesis scheme was developed for a host of nonclassical addition-type thermoplastics (ATT) that can be cured by an addition reaction, leading to a linear polymer structure. The synthesis involves the reaction of an acetylene-terminated prepolymer with either a bismaleimide (BMI) or a biscitraconimide. A new polymer, designated LaRC-RP80, synthesized using this scheme, was found to exhibit several significantly improved properties over the commercial BMI, Kerimid 601, including an eight-fold increase in toughness while maintaining a high Tg, a higher (by 167 C) heat stability, and a 50-percent increase in moisture resistance. In addition, LaRC-RP80 has good hot/wet lap shear strength and processes easily at 288 C without voids in the finished product.

Pater, Ruth H.↗
NASA NTRS1988

Ignition of nonmetallic materials by impact of high-pressure gaseous oxygen

The sources of variability in the pneumatic impact test commonly used to screen nonmetallic materials for oxygen service and to rank batches or lots of particular materials were investigated together with the mechanism of ignition. Tests were conducted at the NASA/White Sands Test Facility in which the standard test chamber was replaced by an instrumented chamber to obtain information on the rates of pressurization, specimen heating, and time to ignition. Results showed that much of the variability in test data could be traced to the variability in system cleanliness and in the opening time of the high-speed valve. The prinicipal mechanism of ignition in the test is considered to be the heating of the gas initially in the test chamber by adiabatic compression; the presence of voids in the test materials may also be important.

Moffett, Gary E.↗
NASA NTRS1989

Cosmic structure from radiation-blown bubbles

Absorbing material in an expanding universe filled with sources of radiation is subject to an instability driven by radiation pressure. In the optically thick limit, this instability takes the form of rapidly growing cavities or bubbles in the absorbing material. No matter how small they are when they begin, such bubbles grow to approach a limiting size which depends only on the ratio of photon pressure to absorber inertia. For a nonprimordial submillimeter radiation background as strong as that recently detected by the Berkely-Nagoya group, the bubbles grow to a comoving radius of about 20 Mpc, comparable in scale to the voids seen in the large-scale galaxy distribution.

Hogan, Craig J.↗
NASA NTRS1989

Effects of various consolidation techniques on microstructure, strength, and reliability of alpha-SiC

The effects of slurry pressing, HIP, and sinter-HIP on the strength, microstructure, and critical processing flaws in alpha-SiC are examined. Also, the feasibility of glass encapsulation is determined. Baseline strength was improved by about 25 percent by using slurry pressing instead of dry pressing. HIP further improved average strength to as high as 655 MPa. Sinter-HIP did not noticeably improve average strength. Although process-related flaws such as shrinkage cracks and large voids were not seen in HIPed alpha-SiC, surface-related flaws were observed. It is suggested that surface finishing by lapping, polishing, and heat treating might reduce the effect of surface-related flaws, improving the Weibull modulus in HIPed alpha-SiC.

Dutta, Sunil↗
NASA NTRS1989

TEM studies of oxidized NiAl and Ni3Al cross sections

Cross sections of oxide scale/(Ni-Al) intermetallics were prepared by a new method and studied using primarily TEM. The cross sections were prepared by encasing an oxidized metal specimen sandwich in a low-melting-temperature zinc alloy. Observations of oxidized zirconium-doped beta-NiAl cross sections revealed crystallographic voids beneath an adherent Al2O3 scale. The oxide-metal interface was incoherent, but a high dislocation density in the metal near the interface suggested that a large tensile stress was induced by the attached oxide scale. A duplex Al2O3-NiAl2O4 scale formed on zirconium-doped and zirconium/boron-doped gamma-prime-Ni3Al alloys. Additional results are presented involving oxidation mechanisms and oxide-metal interface structures.

Doychak, J.↗
NASA NTRS1989

Deformation and fracture of single-crystal and sintered polycrystalline silicon carbide produced by cavitation

An investigation was conducted to examine the deformation and fracture behavior of single-crystal and sintered polycrystalline SiC surfaces exposed to cavitation. Cavitation erosion experiments were conducted in distilled water at 25 C by using a magnetostrictive oscillator in close proximity (1 mm) to the surface of SiC. The horn frequency was 20 kHz, and the double amplitude of the vibrating disk was 50 microns. The results of the investigation indicate that the SiC (0001) surface could be deformed in a plastic manner during cavitation. Dislocation etch pits were formed when the surface was chemically etched. The number of defects, including dislocations in SiC (0001) surface, increased with increasing exposure time to cavitation. The presence of intrinsic defects such as voids in the surficial layers of the sintered polycrystalline SiC determined the zones at which fractured grains and fracture pits (pores) were generated. Single-crystal SiC had superior erosion resistance to that of sintered polycrystalline SiC.

Miyoshi, Kazuhisa↗
NASA NTRS1990

Filter Enhances Fluorescent-Penetrant-Inspecting Borescope

Slip-on eyepiece for commercial ultraviolet-light borescope reduces both amount of short-wave ultraviolet light that reaches viewer's eye and apparent intensity of unwanted reflections of white light from surfaces undergoing inspection. Fits on stock eyepiece of borescope, which illuminates surface inspected with intense ultraviolet light. Surface, which is treated with fluorescent dye, emits bright-green visible light wherever dye penetrates - in cracks and voids. Eyepiece contains deep-yellow Wratten 15 (G) filter, which attenuates unwanted light strongly but passes yellow-green fluorescence so defects seen clearly.

Molina, Orlando G.↗
NASA NTRS1990

Enhancement Of Penetrant-Inspection Images

Proposed computerized video system processes images of fluorescent dyes absorbed in flaws in welds. Video camera, held by operator or by remote manipulator, views weld illuminated by visible white and ultraviolet light. Images of penetrating dye in cracks and voids in weld joint appear on video monitor. Fluorescent features enhanced by software to facilitate identification of true flaws and record important data.

Wilson, Rhonda C.↗
NASA NTRS1989

Mean stress and the exhaustion of fatigue-damage resistance

Mean-stress effects on fatigue life are critical in isothermal and thermomechanically loaded materials and composites. Unfortunately, existing mean-stress life-prediction methods do not incorporate physical fatigue damage mechanisms. An objective is to examine the relation between mean-stress induced damage (as measured by acoustic emission) and existing life-prediction methods. Acoustic emission instrumentation has indicated that, as with static yielding, fatigue damage results from dislocation buildup and motion until dislocation saturation is reached, after which void formation and coalescence predominate. Correlation of damage processes with similar mechanisms under monotonic loading led to a reinterpretation of Goodman diagrams for 40 alloys and a modification of Morrow's formulation for life prediction under mean stresses. Further testing, using acoustic emission to monitor dislocation dynamics, can generate data for developing a more general model for fatigue under mean stress.

Berkovits, Avraham↗
NASA NTRS1989

Wet spinning of solid polyamic acid fibers

The invention is a process for the production of solid aromatic polyamic acid and polyimide fibers from a wet gel or coagulation bath wet gel using N,N-dimethylacetamide (DMAc) solution of the polyamic acid derived from aromatic dianhydrides such as 3,3',4,4'-benzo phenone tetracarboxylic dianhydride (BTDA) and aromatic diamines such as 4,4'oxydianiline (4,4'-ODA). By utilizing the interrelationship between coagulation medium and concentration, resin inherent viscosity, resin percent solids, filament diameter, and fiber void content, it is possible to make improved polyamic acid fibers. Solid polyimide fibers, obtained by the thermal cyclization of the polyamic acid precursor, have increased tensile properties compared to fibers containing macropores from the same resin system.

Dorogy, William E., Jr.↗
NASA NTRS1990

Investigation of methods to produce a uniform cloud of fuel particles in a flame tube

The combustion of a uniform, quiescent cloud of 30-micron fuel particles in a flame tube was proposed as a space-based, low-gravity experiment. The subject is the normal- and low-gravity testing of several methods to produce such a cloud, including telescoping propeller fans, air pumps, axial and quadrature acoustical speakers, and combinations of these devices. When operated in steady state, none of the methods produced an acceptably uniform cloud (+ or - 5 percent of the mean concentration), and voids in the cloud were clearly visible. In some cases, severe particle agglomeration was observed; however, these clusters could be broken apart by a short acoustic burst from an axially in-line speaker. Analyses and experiments reported elsewhere suggest that transient, acoustic mixing methods can enhance cloud uniformity while minimizing particle agglomeration.

Siegert, Clifford E.↗