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At least 127 records · Page 7

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NASA NTRS1984

New phosphorus-containing bisimide resins

Phosphorus-based flame retardants have been effectively used in a wide variety of polymeric materials. Such additives, however, may either influence the decomposition reaction in polymers or lack durability due to a tendency to be leached out by solvents. Attention is given to the synthesis, characterization, thermal stability and degradation mechanisms of bisimide resins, and an evaluation is conducted of the flammability and mechanical properties of graphite cloth-reinforced laminates fabricated from one of the six phosphorus-containing bisimide resins considered.

Varma, I. K.↗

NASA NTRS1985

High-Purity Silicon Seeds for Silane Pyrolysis

Seed particles for fluidized-bed production of silicon made by new contamination-free, economical method. In new method, large particles of semiconductor-grade silicon fired at each other by high-speed streams of gas and thereby break up into particles of suitable size for fluidized bed. No foreign materials introduced, and leaching unnecessary. Method used to feed fluidized-bed reactor for continuous production of high-purity silicon.

Hsu, G. C.↗

NASA NTRS1985

Nonyellowing Polycarbonate for Outdoor Use

Two-step photochemical process prevents yellowing of polycarbonate plastics. Use of polycarbonate plastics growing in windows, aircraft glazing, and streetlight globes. New photochemical process causes formation of stabilizers bound to polymer chains and therefore cannot be leached out.

Gupta, A.↗

NASA NTRS1986

Refining of metallurgical-grade silicon

A basic requirement of large scale solar cell fabrication is to provide low cost base material. Unconventional refining of metallurical grade silicon represents one of the most promising ways of silicon meltstock processing. The refining concept is based on an optimized combination of metallurgical treatments. Commercially available crude silicon, in this sequence, requires a first pyrometallurgical step by slagging, or, alternatively, solvent extraction by aluminum. After grinding and leaching, high purity qualtiy is gained as an advanced stage of refinement. To reach solar grade quality a final pyrometallurgical step is needed: liquid-gas extraction.

Dietl, J.↗

NASA NTRS1986

Solar silicon from directional solidification of MG silicon produced via the silicon carbide route

A process of metallurgical grade (MG) silicon production is presented which appears particularly suitable for photovoltaic (PV) applications. The MG silicon is prepared in a 240 KVA, three electrode submerged arc furnace, starting from high grade quartz and high purity silicon carbide. The silicon smelted from the arc furnace was shown to be sufficiently pure to be directionally solidified to 10 to 15 kg. After grinding and acid leaching, had a material yield larger than 90%. With a MG silicon feedstock containing 3 ppmw B, 290 ppmw Fe, 190 ppmw Ti, and 170 ppmw Al, blended with 50% of off grade electronic grade (EG) silicon to reconduct the boron content to a concentration acceptable for solar cell fabrication, the 99% of deep level impurities were concentrated in the last 5% of the ingot. Quite remarkably this material has OCV values higher tham 540 mV and no appreciable shorts due to SiC particles.

Rustioni, M.↗

NASA NTRS1985

Overview of processing activities aimed at higher efficiencies and economical production

An overview of processing activities aimed at higher efficiencies and economical production were presented. Present focus is on low-cost process technology for higher-efficiency cells of up to 18% or higher. Process development concerns center on the use of less than optimum silicon sheet, the control of production yields, and making uniformly efficient large-area cells. High-efficiency cell factors that require process development are bulk material perfection, very shallow junction formation, front-surface passivation, and finely detailed metallization. Better bulk properties of the silicon sheet and the keeping of those qualities throughout large areas during cell processing are required so that minority carrier lifetimes are maintained and cell performance is not degraded by high doping levels. When very shallow junctions are formed, the process must be sensitive to metallizatin punch-through, series resisitance in the cell, and control of dopant leaching during surface passivation. There is a need to determine the sensitivity to processing by mathematical modeling and experimental activities.

Bickler, D. B.↗

NASA NTRS1986

Chemical systematics of the Shergotty meteorite and the composition of its parent body (Mars)

Sixty elements in two bulk samples of Shergotty meteorite and 30 elements in various mineral separates of Shergotty were identified, using mainly INAA and RNAA techniques. In addition, elements leached out from powdered samples of Shergotty and EETA 79001 meteorites by 0.1 N HCl, as well as the elements of their residues, were analyzed. The results have indicated that Shergotty meteorite is homogeneous in its major element composition, but heterogeneous with respect to large-ion lithophile elements, such as K, Ba, Sr, Zr, Hf, Ta, Th, and rare-earth elements (REEs). It is even more heterogeneous with respect to volatile elements, such as Cd, Te, Tl, and Bi, and the siderophiles Au and Ag. The REE patterns of the Shergotty and EETA 79001 residues are identical, indicating that the parent magmas of both meteorites are compositionally similar. However, their leachate (phosphate) patterns are different, suggesting two components for the Shergotty, one of which is similar to the EETA 79001 leachate.

Laul, J. C.↗

NASA NTRS1987

Fluidized-Bed Cleaning of Silicon Particles

Fluidized-bed chemical cleaning process developed to remove metallic impurities from small silicon particles. Particles (250 micrometer in size) utilized as seed material in silane pyrolysis process for production of 1-mm-size silicon. Product silicon (1 mm in size) used as raw material for fabrication of solar cells and other semiconductor devices. Principal cleaning step is wash in mixture of hydrochloric and nitric acids, leaching out metals and carrying them away as soluble chlorides. Particles fluidized by cleaning solution to assure good mixing and uniform wetting.

Rohatgi, Naresh K.↗

NASA NTRS1987

Rare earth patterns in shergottite phosphates and residues

Leaching experiments with 1M HCl on ALHA 77005 powder show that rare earth elements (REE) are concentrated in accessory phosphate phases (whitlockite, apatite) that govern the REE patterns of bulk shergottites. The REE patterns of whitlockite are typically light REE-depleted with a negative Eu anomaly and show a hump at the heavy REE side, while the REE pattern of apatite (in Shergotty) is light REE-enriched. Parent magmas are calculated from the modal compositions of residues of ALHA 77005, Shergotty, and EETA 79001. The parent magmas lack a Eu anomaly, indicating that plagioclase was a late-stage crystallizing phase and that it probably crystallized before the phosphates. The parent magmas of ALHA 77005 and Shergotty have similar REE patterns, with a subchondritic Nd/Sm ratio. However, the Sm/Nd isotopoics require a light REE-depleted source for ALHA 77005 (if the crystallization age is less than 600 Myr) and a light REE-enriched source for Shergotty. Distant Nd and Sr isotopic signatures may suggest different source regions for shergottites.

Laul, J. C.↗

NASA NTRS1987

Microbial trace-fossil formation, biogenous, and abiotic weathering in the Antarctic cold desert

In the Antarctic cold desert (Ross Desert), the survival of the cryptoendolithic microorganisms that colonize the near-surface layer of porous sandstone rocks depends on a precarious equilibrium of biological and geological factors. An unfavorable shift of this equilibrium results in death, and this may be followed by formation of trace fossils that preserve the characteristic iron-leaching pattern caused by microbial activity. Similar microbial trace fossils may exist in the geological record. If life ever arose on early Mars, similar processes may have occurred there and left recognizable traces.

Friedmann, E. Imre↗

NASA NTRS1988

Geochronological and lead-isotope evidences for rapid crust formation in middle-proterozoic time: The Labrador example

Extensive U-Pb geochronological studies in the Grenville and Makkovik provinces have shown that eastern Labrador is underlain by two distinct crustal blocks. In order to substantiate the juvenile character of the middle-Proterozoic crustal block, the isotopic compositon of lead in leached k-feldspars from the same rocks were analyzed. The results of the analysis are briefly discussed.

Schaerer, Urs↗

NASA NTRS1989

Glass corrosion in natural environment

A series of studies of the effects of solutes which appear in natural aqueous environments, specifically Mg and Al, under controlled conditions, permit characterization of the retardation of silicate glass leaching in water containing such solutes. In the case of Mg the interaction with the glass appears to consist of exchange with alkali ions present in the glass to a depth of several microns. The effect of Al can be observed at much lower levels, indicating that the mechanism in the case of Al involves irreversible formation of aluminosilicate species at the glass surface.

Thorpe, Arthur N.↗

NASA NTRS1989

Soil developments in polar deserts: Implications for exobiology and future Mars missions

Chemical alterations, weathering, and diagenesis of soil profiles from the dry valleys of Antarctica were studied as analogs of regolith development for the Martian regolith. Chemical weathering processes play an important part in soil development within the dry valleys of Antarctica. A suite of core samples were studied which were taken within the valley floors in addition to samples taken in the vicinity of evaporite and brine ponds. Analysis of water soluable cations and anions from core samples were performed along with petrographic analysis of selected samples. It was shown that ionic transport processes operate primarily above the permafrost zone. Abundances of the water soluable ions reflect the nature of secondary minerals produced by evaporation and weathering. Chloride, calcium, and sodium abundances for soils from the cores within the North and South Forks of Wright Valley, reflect the secondary mineralogy of the soil columns. Calculations for Na, Ca, and Cl abundances reflect the appearance of halite and antarcticite. In areas where excess Ca is present, X-ray diffraction studies show the presence of gypsum. It is well known that the Martian surface conditions may be favorable for chemical weathering. Primary silicates would be expected to be reactive with any ground water. It seems likely that Martian subsurface water is available to assist in the weathering of the primary minerals. Such weathering could result in the formation of clays, sulfates, carbonates, hydrates, halides, and zeolites. The dry valley cores have shown that they maybe excellent analogs to weathering processes on the near-surface of Mars. Since movement of water within the near-surface region clearly results in chemical weathering, leaching, and salt formation in the dry valleys, similar processes are probably operating within the Martian regolith.

Gibson, Everett K., Jr.↗

NASA NTRS1988

Interactions of silicate glasses with aqueous environments under conditions of prolonged contact and flow

This paper discusses mechanisms involving saturation and reactions that lead to the formation of altered phases in silicate glasses considered for use in geologic repositories for nuclear waste. It is shown that the rate of dissolution of silicate glasses exposed to a broad range of contact times, leachant compositions, and surface-to-volume ratios is strongly affected by the presence of reactive species such as Al, Mg, and Fe. The reactive materials may originate in the leachant or, under conditions of high surface-to-volume ratio, in the glass itself. The effects of glass composition on the course of the corrosion process can be viewed in terms of the formation of a surface layer on the leached glass; the type, composition, and structure of this layer control the dissolution behavior of the glass.

Barkatt, Aaron↗

NASA NTRS1989

Pb isotopes in anorthositic breccias 67075 and 62237 - A search for primitive lunar lead

The lunar Pb isotopic evolution is explored based on an investigation of the U-Th-Pb systematics of the primitive Apollo 16 anorthositic breccias 76075 and 62237. The isotope data for acid-leached residues from 67075 lie within or on a triangle formed by plagioclase, opaques, and a pyroxen-olivine mixture, and the isotope data for 62237 lie within or on a triangle formed by plagioclase, olivine, and pyroxene. The results suggest that the moon had high first-stage values for the Pb evolution equations from the onset.

Premo, W. R.↗

NASA NTRS1989

Silicification of holocene soils in northern Monitor Valley, Nevada

Chemical, physical, and microscopic data for three soils in the northern Monitor Valley are analyzed. The soils ranked in order of increasing age are: Mule, Rotinom, and Nayped. The procedures and techniques used to obtain and study that data are described. It is observed that: (1) redistribution of carbonate is detectable in all soils; (2) clay illuviation is insignificant in the Mule soil, weak but identifiable in the Rotinom soil, and significant in the Nayped soil; and (3) the maximum sodium adsorption ratio (SAR) and electrical conductivity (EC) for the Mule soil is between 64-89 cm, for the Rotinom soil the values are below 100 cm, and for Nayped the maximum SAR values range from 51-117 cm and maximum EC values are between 117-152 cm. The relationship between volcanic glass weathering and the amount of silica cementation in the soils is studied. It is noted that silicification of Monitor Valley holocene soils is due to there being enough moisture to release silica from volcanic glass, but not enough to leach the weathering products from the profile.

Chadwick, O. A.↗

NASA NTRS1990

U-Th-Pb systematics of the Estherville mesosiderite

Results are presented on a detailed U-Th-Pb systematics of the Estherville mososiderite, which was performed in a study involving stepwise leaching experiments. The Pb-Pb internal 'isochrons' for Estherville yielded ages of 4556 + or - 35 Ma, 4506 + or - 75 Ma, and 4422 + or - 50 Ma, indicating that the silicate fraction of the Estherville mesosiderite is very heterogeneous and was formed early in the solar system history. Results clearly identifies the Pb-Pb isochron as a mixing line. The U-Pb lower-intercept ages could be divided into two groups: (1) around 3 Ga, and likely related to the 3.6 Ga heating event, and (2) close to 0 Ma and to 62 Ma.

Brouxel, M.↗

NASA NTRS1991

Oxygen production processes on the Moon: An overview

The production of oxygen on the Moon utilizing indigenous material is paramount to a successful lunar colonization. Several processes were put forth to accomplish this. The lunar liquid oxygen (LLOX) generation schemes which have received the most study to date are those involving: (1) the reduction of ilmenite (FeTiO3) by H2, C, CO, CH4, CO-Cl2 plasma; (2) magma electrolysis, both unadulterated and fluoride-fluxed, and (3) several others, including carbo-chlorination, HF acid leaching, fluorine extraction, magma oxidation, and vapor pyrolysis. The H2 reduction of ilmenite and magma electrolysis processes have received the most study to date. At this stage of development, they both appear feasible schemes with various pros and cons. However, all processes should be addressed at least at the onset of the considerations. It is ultimatley the energy requirements of the entire process, including the acquisition of feedstock, which will determine the mode of oxygen productions. There is an obvious need for considerably more experimentation and study. Some of these requisite studies are in progress, and several of the most studied and feasible processes for winning oxygen from lunar materials are reviewed.

Taylor, Lawrence A.↗