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At least 1,423 records · Page 79

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NASA NTRS1985

Excimer laser annealing for fabrication of low-cost solar cells

Pulsed excimer laser annealing was successfully performed using a 50 w laser. Both polished and texturized cells were tried, however, there are serious problems with nonuniformity on texturized cells. A number of cells were produced and compared to diffusion furnace annealed cells. There was no clear economic advantage in using an excimer laser and there was a small penalty on average efficiency. The conclusion was that the excimer laser anneal process must be able to produce superior cells to be considered as a viable process option.

Spitzer, M. B.↗

NASA NTRS1985

Preliminary geologic analyses of SIR-B radar data for Hawaii

A number of distinct topographic and textural units were discerned on SIR-B radar images of the Kilauea/Kau Desert area. The SIR-B enhanced large-scale topographic features, such as the Kahuku Pali and the Koae Faults (both oriented towards the radar antenna), while it was unable to detect features facing away from the radar antenna such as the Hilina and Kaoiki Pali. Textural units which were identified on the SIR-B images include aa flows of various ages, and several pyroclastic units. The extreme surface roughness of aa flows produced high radar returns on both the 28 deg and 48 deg SIR-B data sets. In contrast, pahoehoe flows produced smooth, low return units which were difficult to distinguish from surrounding radar-dark materials. It is suggested that multilook direction data, acquired at steeper incidence angles during a reflight of SIR-B, may aid in the further identification of pahoehoe flows and subtle topographic features.

Gaddis, L. R.↗

NASA NTRS1986

Alkali norite, troctolites, and VHK mare basalts from breccia 14304

Six pristine rocks, two mare basalts, and four nonpristine highlands rocks were separated from breccia 14304 for consortium study. The pristine highlands rocks include representatives of the Mg troctolite-anorthosite and alkali suites of the Apollo 14 site. Two troctolite clasts have olivine and plagioclase compositions similar to one group of Apollo 14 troctolites and one also contains spinel. Incompatible element abundances in one are similar to those of 14305 troctolites, although the heavy rare earth elements pattern is distinct among Apollo 14 troctolites. Alkali lithologies include an alkali anorthosite and an alkali norite, the latter having a pristine igneous texture and resembling alkali gabbronites from Apollo 14 and 67975 in mineralogy and mineral compositions. It is suggested that Apollo 14 alkali lithologies and PO4-bearing Mg anorthosites formed from Mg-rich magmas that assimilated various amounts of material rich in P and REE. Another pristine clast from 14304 is an Mg-gabbronorite. The two mare basalt clasts are very high potassium basalts, whose parent magmas could have formed from a typical low-Ti, high-Al basaltic magmas by assimilation of K-rich material. Nonpristine 14304 clasts include melt-textured anorthosites and an augite-rich poikilitic melt rock.

Goodrich, C. A.↗

NASA NTRS1986

Relation between L-band soil emittance and soil water content

An experimental relation between soil emittance (E) at L-band and soil surface moisture content (M) is compared with a theoretical one. The latter depends on the soil dielectric constant, which is a function of both soil moisture content and of soil texture. It appears that a difference of 10 percent in the surface clay content causes a change in the estimate of M on the order of 0.02 cu m/cu m. This is based on calculations with a model that simulates the flow of water and energy, in combination with a radiative transfer model. It is concluded that an experimental determination of the E-M relation for each soil type is not required, and that a rough estimate of the soil texture will lead to a sufficiently accurate estimate of soil moisture from a general, theoretical relationship obtained by numerical simulation.

Stroosnijder, L.↗

NASA NTRS1985

An integrated Landsat/ancillary data classification of desert rangeland

Range inventorying methods using Landsat MSS data, coupled with ancillary data were examined. The study area encompassed nearly 20,000 acres in Rush Valley, UT. The vegetation is predominately desert shrub and annual grasses, with same annual forbs. Three Landsat scenes were evaluated using a Kauth-Thomas brightness/greenness data transformation (May, June, and August dates). The data was classified using a four-band maximum-likelihood classifier. A print map was taken into the field to determine the relationship between print symbols and vegetation. It was determined that classification confusion could be greatly reduced by incorporating geomorphic units and soil texture (coarse vs fine) into the classification. Spectral data, geomorphic units, and soil texture were combined in a GIS format to produce a final vegetation map identifying 12 vegetation types.

Price, K. P.↗

NASA NTRS1987

Design, fabrication and performance of small, graphite electrode, multistage depressed collectors with 200-W, CW, 8- to 18-GHz traveling-wave tubes

Small multistage depressed collectors (MDC's) which used pyrolytic graphite, ion-beam-textured pyrolytic graphite, and isotropic graphite electrodes were designed, fabricated, and evaluated in conjuntion with 200-W, continuous wave (CW), 8- to 18-GHz traveling-wave tubes (TWT's). The design, construction, and performance of the MDC's are described. The bakeout performance of the collectors, in terms of gas evolution, was indistinguishable from that of typical production tubes with copper collectors. However, preliminary results indicate that some additional radiofrequency (RF) and dc beam processing time (and/or longer or higher temperature bakeouts) may be needed beyond that of typical copper electrode collectors. This is particularly true for pyrolytic graphite electrodes and for TWT's without appendage ion pumps. Extended testing indicated good long-term stability of the textured pyrolytic graphite and isotropic graphite electrode surfaces. The isotropic graphite in particular showed considerable promise as an MDC electrode material because of its high purity, low cost, simple construction, potential for very compact overall size, and relatively low secondary electron emission yield characteristics in the as-machined state. However, considerably more testing experience is required before definitive conclusions on its suitability for electronic countermeasure systems and space TWT's can be made.

Ebihara, Ben T.↗

NASA NTRS1987

An evaluation of candidate oxidation resistant materials

Ground based testing of materials considered for Kapton solar array blanket protection, graphite epoxy structural member protection, and high temperature radiators was performed in an RF plasma asher. Ashing rates for Kapton were correlated with rates measured on STS-8 to determine the exposure time equivalent to one year in low Earth orbit (LEO) at a constant density space station orbital flux. Protective coatings on Kapton from Tekmat, Andus Corporation, and LeRC were evaluated in the plasma asher and mass loss rates per unit area were measured for each sample. All samples evaluated provided some protection to the underlying surface but ion beam sputter deposited samples of SiO2 and SiO2 with 8% polytetrafluoroethylene (PTFE) showed no evidence of degradation after 47 hours of exposure. Mica paint was evaluated as a protective coating for graphite epoxy structural members. Mica appears to be resistant to attack by atomic oxygen but only offers some limited protection as a paint because the paint vehicles evaluated to date were not resistant to atomic oxygen. Four materials were selected for evaluation as candidate radiator materials: stainless steel, copper, niobium-1% zirconium, and titanium-6% aluminum-4% vanadium. These materials were surface textured by various means to improve their emittance. Emittances as high as 0.93 at 2.5 microns for stainless steel and 0.89 at 2.5 microns for Nb-1 Zr were obtained from surface texturing. There were no significant changes in emittance after asher exposure.

Rutledge, Sharon↗

NASA NTRS1988

Part 1: Aspects of lithospheric evolution on Venus. Part 2: Thermal and collisional histories of chondrite parent bodies

The geological evolution of distinctly different kinds of solar system objects is addressed. Venus has been observed over the past decade by orbital radars on both American and Soviet spacecraft. These surface measurements provide clues to the structure and evolution of the lithosphere. The parent bodies of chondritic meteorites, thought to resemble asteroids, represent the other end of the size spectrum of terrestrial objects. Their early thermal and collisional histories may be constrained by the chemical and textural record preserved in meteorite samples. Impact craters on Venus have been observed by the Soviet Venera 15/16 spacecraft. A formalism is presented by which the size-frequency distribution of impact craters may be used to estimate upper bounds on the mean global rates of volcanic resurfacing and lithospheric recycling on that planet over the past several hundred million years. The impact crater density reported from Venera observations, if valid for the entire Venus surface, indicates a mean volcanic flux no greater than 2 cu km/y, corresponding to a maximum average rate of resurfacing of about 4 km/b.y. For the lowest estimated mean crater retention age of the surface of Venus imaged by Venera 15/16, the rate of lithospheric recycling on Venus does not exceed 1.5 sq km/y. Ordinary chondrite meteorites show textural and chemical patterns indicative of varying intensities of thermal metamorphism. The conventional onion-shell model, which envisions highly metamorphosed material in the core and less intensely heated rocks near the surface, predicts an inverse relation between peak temperature and cooking rate, but none has been observed. A metamorphosed-planetesimal model is devised to explain this discrepancy, whereby heating occurs in planetesimals a few kilometers in radius which then accrete to form 100-km-radius parent bodies. Cooling rates are then randomly controlled by burial depth. Thermal and collisional constraints are examined, and the model is found to be applicable only to highly insulating Al-26-rich planetesimals that remain closely aggregated upon accretion. An alternative model is presented, in which onion-shell parent bodies are collisionally fragmented during metamorphism and then gravitationally reassembled. If reassembly times are short, then cooling rates would be determined by burial depth in the reaccreted parent body. This model, unlike previous ones, can explain both coherent and incoherent cooling of Breccia clasts by collisions during or after metamorphism, respectively.

Grimm, Robert E.↗

NASA NTRS1988

The emittance of space radiator materials measured at elevated temperatures

The spectral emittances of textured space radiator materials between 1.7 and 14.7 micrometer have been evaluated at room temperature and elevated temperature (630 C) in air. Heating in air caused a permanent increase in spectral emittance for all materials tested: HCl/ion beam textured 304 stainless steel, untextured Ti (6 percent Al, 4 percent V), and sandblasted Ti (6 percent Al, 4 percent V). Changes in the surface chemistry and/or surface morphology of these materials were also observed. Elevated temperature spectral emittance was measured in an argon atmosphere and compared to the measurements in air. Similarity between the room temperature and elevated temperature spectral emittance measurements was also investigated, and limited agreement was found.

Mirtich, Michael J.↗

NASA NTRS1988

Impact and extinction signatures in complete Cretaceous-Tertiary (K-T) boundary sections

The Zumaya, Caravaca and Agost sections in Spain, the El Kef section in Tunisia and the Negev (Nahal Avdat) sections in Israel are among the most continuous, expanded and complete K-T boundary sections. The distribution patterns of the planktic faunas were quantitatively analyzed in closely spaced samples across the K-T boundary in these sections, in conjuction with the geochemistry, stable isotopes, mineralogy and magnetostratigraphy. Three hundred foraminiferal specimens were randomly selected and determined. Reliable estimates for the foraminiferal productivity changes across the K-T boundary and for the 1 to 2 Ma interval preceding the K-T boundary were made from the numbers of individuals/gram of sediment corrected for the sedimentation rates (calculated from magnetic reversals and lithology). No gradual or stepwise extinction is seen below the K-T boundary nor any productivity decrease. Stable isotope analyses show a warming just after deposition of the ejecta layer, not cooling as predicted by nuclear winter scenarios, although the duration of such cooling may be too short to be observed even in these complete sections. Low REE values and cpx spherules with quench textures idential to quench-textures in diagenetically altered spherules, strongly indicate an oceanic site of (one of) the impactor(s).

Smit, J.↗

NASA NTRS1988

The geology and petrogenesis of the southern closepet granite

The Archaean Closepet Granite is a polyphase body intruding the Peninsular Gneiss Complex and the associated supracrustal rocks. The granite out-crop runs for nearly 500 km with an approximate width of 20 to 25 km and cut across the regional metamorphic structure passing from granulite facies in the South and green schist facies in the north. In the amphibolite-granulite facies transition zone the granite is intimately mixed with migmatites and charnockite. Field observations suggests that anatexis of Peninsular gneisses led to the formation of granite melt, and there is a space relationship between migmatite formation, charnockite development and production and emplacement of granite magma. Based on texture and cross cutting relationships four major granite phases are recognized: (1) Pyroxene bearing dark grey granite; (2) Porphyritec granite; (3) Equigranular grey granite; and (4) Equigranular pink granite. The granite is medium to coarse grained and exhibit hypidiomorphic granular to porphyritic texture. The modal composition varies from granite granodiorite to quartz monzonite. Geochemical variation of the granite suite is consistent with either fractional crystallization or partial melting, but in both the cases biotite plus feldspar must be involved as fractionating or residual phases during melting to account trace element chemistry. The trace element data has been plotted on discriminant diagrams, where majority of samples plot in volcanic arc and within plate, tectonic environments. The granite show distinct REE patterns with variable total REE content. The REE patterns and overall abundances suggests that the granite suite represents a product of partial melting of crustal source in which fractional crystallization operated in a limited number of cases.

Jayananda, M.↗

NASA NTRS1989

River Valley pluton, Ontario - A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

This paper presents Nd, Sr, and Pb isotopic data indicating a late-Archean/early-Proterozoic age for the River Valley anorthositic pluton of the southwestern Grenville Province of Sudbury, Ontario. Pb-Pb isotopic data on 10 whole-rock samples ranging in composition from anorthosite to gabbro yield an age of 2560 + or - 155 Ma. The River Valley pluton is thus the oldest anorthositic intrusive yet recognized within the Grenville Province. The Sm-Nd isotopic system records an age of 2377 + or - 68 Ma. High Pb-208/Pb-204 of deformed samples relative to igneous-textured rocks implies Th introduction and/or U loss during metamorphism in the River Valley area. Rb-Sr data from igneous-textured and deformed samples and from mineral separates give an age of 2185 + or - 105 Ma, indicating substantial disturbance of the Rb-Sr isotopic system.

Ashwal, Lewis D.↗

NASA NTRS1989

High compression image and image sequence coding

The digital representation of an image requires a very large number of bits. This number is even larger for an image sequence. The goal of image coding is to reduce this number, as much as possible, and reconstruct a faithful duplicate of the original picture or image sequence. Early efforts in image coding, solely guided by information theory, led to a plethora of methods. The compression ratio reached a plateau around 10:1 a couple of years ago. Recent progress in the study of the brain mechanism of vision and scene analysis has opened new vistas in picture coding. Directional sensitivity of the neurones in the visual pathway combined with the separate processing of contours and textures has led to a new class of coding methods capable of achieving compression ratios as high as 100:1 for images and around 300:1 for image sequences. Recent progress on some of the main avenues of object-based methods is presented. These second generation techniques make use of contour-texture modeling, new results in neurophysiology and psychophysics and scene analysis.

Kunt, Murat↗

NASA NTRS1989

What if chondritic porous interplanetary dust particles are not the real McCoy

To select a target comet for a Comet Nucleus Sample Return Mission (CNSRM) it is necessary to have an experimental data base to evaluate the extent of diversity and similarity of comets. For example, the physical properties (e.g., low density) of chondritic porous (CP) interplanetary dust particles (IDPs) are believed to resemble these properties of cometary dust although it is yet to be demonstrated that the porous structure of CP IDPs is inherent to presolar dust particles stored in comet nuclei. Porous structures of IDPs could conceivably form during sublimation at the surface of active comet nuclei. Porous structures are also obtained during annealing of amorphous Mg-SiO smokes which initially forms porous aggregates of olivine + platey tridymite and which, upon continued annealing, react to fluffy enstatite aggregates. It is therefore uncertain that CP IDPs are entirely composed of unmetamorphosed presolar dust. Conceivably, new minerals and textures may form in situ in nuclei of active comets as a function of their individual thermal history. Unmetamorphosed comet dust is probably structurally amorphous. Thermal annealing of this dust can produce ultra fine-grained minerals and this ultrafine grain size of CP IDPs should be considered in assessments of aqueous alterations that could affect presolar dust in comet nuclei between 200 and 400 K. Devitrification and hydration may occur in situ in ice-dust mixtures and the mantle of active comet nuclei. Devitrification, or uncontrolled crystallization, of amorphous precursor dust can produce a range of chemical compositions of ultrafine-grained minerals and (non-equilibrium) mineral assemblages and textures in dust contained in comet nuclei as a function of period and trajectory of orbit and number of perihelion passages (not considering internal heating). Thus, experimental data on relevant processes and reaction rates between 200 and 400 K are needed in order to evaluate comet selection, penetration depth for sampling device and curation of samples for CNSRM.

Rietmeijer, Frans J. M.↗

NASA NTRS1989

The effects of atomic oxygen on the thermal emittance of high temperature radiator surfaces

Radiator surfaces on high temperature space power systems such as SP-100 space nuclear power system must maintain a high emittance level in order to reject waste heat effectively. One of the primary materials under consideration for the radiators is carbon-carbon composite. Since carbon is susceptible to attack by atomic oxygen in the low earth orbital environment, it is important to determine the durability of carbon composites in this environment as well as the effect atomic oxygen has on the thermal emittance of the surface if it is to be considered for use as a radiator. Results indicate that the thermal emittance of carbon-carbon composite (as low as 0.42) can be enhanced by exposure to a directed beam of atomic oxygen to levels above 0.85 at 800 K. This emittance enhancement is due to a change in the surface morphology as a result of oxidation. High aspect ratio cones are formed on the surface which allow more efficient trapping of incident radiation. Erosion of the surface due to oxidation is similar to that for carbon, so that at altitudes less than approximately 600 km, thickness loss of the radiator could be significant (as much as 0.1 cm/year). A protective coating or oxidation barrier forming additive may be needed to prevent atomic oxygen attack after the initial high emittance surface is formed. Textured surfaces can be formed in ground based facilities or possibly in space if emittance is not sensitive to the orientation of the atomic oxygen arrival that forms the texture.

Rutledge, Sharon K.↗

NASA NTRS1990

Pristine moon rocks - An alkali anorthosite with coarse augite exsolution from plagioclase, a magnesian harzburgite, and other oddities

Results are presented on the analyses of 18 samples of pristine rocks obtained from the primarily mare Apollo 12 site and from the primarily highland Apollo 14 site, as well as samples from the nonmare Apollo 15 site. It was found that, while two of anorthosites from Apollo 12 were similar in composition to most other anorthosites from the west-central near region, the texture of an alkali anorthosite featured a long and narrow crystal of augite surrounded by a single crystal of plagioclase, clearly suggesting that the augite formed by exsolution out of the plagioclase. Another Apollo 12 rocklet was an unusual magnesian harzburite, with subequal amounts of enstatite and olivine, traces of Cr-Fe spinel, and FeNi metal, but no plagioclase; the bulk composition was found to be remarkably Ir-rich (53 percent) for a pristine rock, and the texture was also unusual. Apollo 14 samples included several uncommonly Al-rich and REE-poor impact melt breccias.

Warren, P. H.↗

NASA NTRS1991

Effects of dust accumulation and removal on radiators surfaces on Mars

Tests were carried out to assess the impact of wind blown dust accumulation and abrasion on radiator surfaces on Mars. High emittance arc-textured copper and niobium-1 percent-zirconium samples were subjected to basaltic dust laden wind at Martian pressure (1000 Pa) at speeds varying from 19 to 97 m/s in the Martian Surface Wind Tunnel. The effect of accumulated dust was also observed by pre-dusting some of the samples before the test. Radiator degradation was determined by measuring the change in the emittance after dust was deposited and/or removed. The principle mode of degradation was abrasion. Arc textured Nb-1 percent-Zr proved to be more susceptible to degradation than Cu, and pre-dusting appeared to have lessened the abrasion.

Gaier, James R.↗

NASA NTRS1991

LDEF contaminants analysis data

Modification of surface texture for Ram Facing FEP Teflon on the Long Duration Exposure Facility (LDEF) satellite is documented. Photos of changes in surface texture if thermal blanket material with position on tray C-08 are presented. Infrared spectra and photographic documentation of molecular film deposits on LDEF surfaces by location is reported. Results of a survey of the functional groups and elemental composition of the brown film found widely distributed on the surface of LDEF is shown. A partial photographic catalog of LDEF particulate contaminants is included. A set of photos of particles collected or photographed in place on the surface of LDEF is shown. Particle counts and surface obscuration data for the LDEF satellite are reported. The surface data in tables and charts is presented for the LDEF satellite by location.

Crutcher, E. Russ↗