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At least 577 records · Page 32

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NASA NTRS1993

Thermal control paints on LDEF: Results of M0003 sub-experiment 18

Several thermal control paints were flown on the Long Duration Exposure Facility (LDEF), including the white paints Chemglaze A276, S13GLO, and YB-71, and the black paint D-111. The effects of low earth orbit, which includes those induced by UV radiation and atomic oxygen, varied significantly with each paint and its location on LDEF. For example, samples of Chemglaze A276 located on the trailing edge of LDEF darkened significantly due to UV-induced degradation of the paint's binder, while leading edge samples remained white but exhibited severe atomic oxygen erosion of the binder. Although the response of S13GLO to low earth orbit is much more complicated, it also exhibited greater darkening on trailing edge samples as compared to leading edge samples. In contrast, YB-71 and D-111 remained relatively stable and showed minimal degradation. The performance of these paints as determined by changes in their optical and physical properties, including solar absorptance as well as surface chemical changes and changes in surface morphology is examined. It will also provide a correlation of these optical and physical property changes to the physical phenomena that occurred in these materials during the LDEF mission.

Jaggers, C. H.↗

NASA NTRS1993

Solid propellant exhausted aluminum oxide and hydrogen chloride - Environmental considerations

Measurements of gaseous hydrogen chloride (HCl) and particulate aluminum oxide (Al2O3) were made during penetrations of five Space Shuttle exhaust clouds and one static ground test firing of a shuttle booster. Instrumented aircraft were used to penetrate exhaust clouds and to measure and/or collect samples of exhaust for subsequent analyses. The focus was on the primary solid rocket motor exhaust products, HCl and Al2O3, from the Space Shuttle's solid boosters. Time-dependent behavior of HCl was determined for the exhaust clouds. Composition, morphology, surface chemistry, and particle size distributions were determined for the exhausted Al2O3. Results determined for the exhaust cloud from the static test firing were complicated by having large amounts of entrained alkaline ground debris (soil) in the lofted cloud. The entrained debris may have contributed to neutralization of in-cloud HCl.

Cofer, W. R., III↗

NASA NTRS1993

Low earth simulation and materials characterization

Oxygen plasma ashers and an electron cyclotron resonance (ECR) sources are currently being used for low Earth orbit (LEO) simulation. The suitability of each of these simulation techniques is considered. Thin film coatings are characterized by optical techniques, including variable-angle spectroscopic ellipsometry, optical spectrophotometry, and laser light scatterometry. Atomic force microscopy (AFM) has been used to characterize the surface morphology of thin aluminum films as a function of substrate temperature during deposition. Results on diamondlike carbon (DLC) films show that DLC degrades with simulated atomic oxygen (AO) exposure at a rate comparable to Kapton polyimide. Since DLC is not as susceptible as Kapton to environmental factors such as moisture absorption, it could potentially provide more accurate measurements of AO fluence on short space flights.

Synowicki, R. A.↗

NASA NTRS1993

Continued improvment of large area, in situ sputter deposition of superconducting YBCO thin films

The deposition of thin films of superconducting YBa2Cu3O7-x onto substrates of up to 3-in diameter by an integrated off-axis sputtering is reported. The substrate is located above the center of an 8-in-diameter YBCO planar target, and, in conjunction with a negative ion shield, negative ion effects are avoided. A large radiant heater provides backside, noncontact heating of the bare substrates. YBCO films have been grown on polished 1-cm2 MgO and LaAlO3 substrates with Tc = 90 K or greater, Jc = 2.5 x 10 exp 6 A/sq cm or greater at 77 K, and microwave surface resistance Rs less than 0.4 micro-ohm at 77 K and 10 GHz. The films have a very smooth surface morphology. Uniformity data for LaAlO3 substrates are less than +/-5 percent in Rs. Thickness uniformity results for 3-in substrates indicate less than 10 percent variation. The growth of epitaxial insulating films for use with YBCO films and application of the YBCO films in microwave devices are briefly discussed.

Truman, J. K.↗

NASA NTRS1991

Organometallic chemical vapor deposition and characterization of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates

In this paper, we report the epitaxial growth of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates by open tube OMCVD. The chemical composition of the alloys characterized by energy dispersive X-ray spectroscopy shows that alloys with x up to 0.13 can be deposited on (001) GaP. Epitaxial growth with mirror smooth surface morphology has been achieved for x less than or equals to 0.05. Selected area electron diffraction pattern of the alloy shows that the epitaxial layer crystallizes in the chalcopyrite structure with relatively weak superlattice reflections indicating certain degree of randomness in the cation sublattice. Hall measurements show that the alloys are p-type, like the unalloyed films; the carrier concentration, however, dropped about 10 times from 2 x 10 exp 18 to 2 x 10 exp 17/cu cm. Absorption measurements indicate that the band tailing in the absorption spectra of the alloy has been shifted about 0.04 eV towards shorter wavelength as compared to the unalloyed material.

Xing, G. C.↗

NASA NTRS1993

Synthesis of polycrystalline chalcopyrite semiconductors by microwave irradiation

Polycrystalline samples of the chalcopyrites CuInS2, CuInSe2, and CuInSSe were prepared from stoichiometric mixtures of the pure elements by microwave irradiation. The reactions were performed in sealed quartz tubes in as few as 3 minutes. The products were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy. The surface morphology and shape of the particles produced by this method suggest that the products are formed from liquid melts. This method could be applied to the production of bulk chalcopyrite as sources for thin film growth.

Landry, Christopher C.↗

NASA NTRS1992

Geologic remote sensing - New technology, new information

Results of geologic studies using data collected by the NASA/JPL Thermal Infrared Imaging Spectrometer (TIMS), Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), and the Airborne Synthetic Aperture Radar (AIRSAR) are discussed. These instruments represent prototypes for the Earth Observing System (EOS) satellite instruments ASTER, High Resolution Imaging Spectrometer (HIRIS), and EOS SAR. Integrated analysis of this data type is one of the keys to successful geologic research using EOS. TIMS links the physical properties of surface materials in the 8-12-*mm region to their composition. Calibrated aircraft data make direct lithological mapping possible. AVIRIS, an analog for HIRIS, provides quantitative information about the surface composition of materials based on their detailed visible and infrared spectral signatures (0.4-2.45 mm). Calibrated AVIRIS data make direct identification of minerals possible. The AIRSAR provides additional complementary information about the surface morphology of rocks and soils.

Kruse, F. A.↗

NASA NTRS1992

Growth and characterizaton of 3C-SiC and 6H-SiC films on 6H-SiC wafers

Single crystal SiC films were grown by CVD on vicinal (0001) SiC wafers cut from boules produced by the modified sublimation method. Wafers with tilt angles less than 0.5 deg yielded 3C-SiC; tilt angles of 3 to 4 deg resulted in 6H-SiC films. The surface morphology of these films, up to 24 microns thick, were investigated as a function of growth parameters such as the Si/C ratio in the input gases and the presence of dopant materials such as nitrogen and trimethylaluminum.

Powell, J. A.↗

NASA NTRS1991

Physical studies of planetary and satellite surfaces

The publications and doctoral dissertations generated by this NASA grant are presented in bibliographic form. The topics include but are not limited to the photometric properties and surface morphology of planets and their satellites.

Veverka, Joseph↗

NASA NTRS1993

Organometallic chemical vapor deposition and characterization of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates

The epitaxial growth of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates by open tube organometallic chemical vapor deposition (OMCVD) is reported. The chemical composition of the alloys characterized by energy dispersive X-ray spectroscopy shows that alloys with x up to 0.13 can be deposited on (001) GaP. Epitaxial growth with mirror smooth surface morphology was achieved for x less than or equal to 0.05. Transmission electron microscopy (TEM) micrographs of these alloys show specular epitaxy and the absence of microstructural defects indicating a defect density of less than 10(exp 7) cm(sup -2). Selected area electron diffraction pattern of the alloy shows that the epitaxial layer crystallizes in the chalcopyrite structure with relatively weak superlattice reflections indicating certain degree of randomness in the cation sublattice. Hall measurements show that the alloys are p-type, like the unalloyed films; the carrier concentration, however, dropped about 10 times from 2 x 10(exp 18) to 2 x 10(exp 17) cm(sup -3). Absorption measurements indicate that the band tailing in the absorption spectra of the alloy was shifted about 0.04 eV towards shorter wavelength as compared to the unalloyed material. Diodes fabricated from the n(+)-GaP/p-ZnSiP2-ZnGeP2-Ge heterostructure at x = 0.05 have a reverse break-down voltage of -10.8 V and a reverse saturation current density of approximately 6 x 10(exp -8) A/sq cm.

Xing, G. C.↗

NASA NTRS1993

Geology and surface characteristics of Bell Regio, Venus

Bell Regio is a broad topographic rise located at approximately 30 deg N, 45 deg E which extends approximately 1500 km in the N-S direction and is characterized by extensive volcanism. The geology and surface characteristics of Bell Regio are examined in order to understand the geologic evolution of the region and the surface characteristics of the major volcanic units. Relationships between Magellan SAR backscatter values and altimeter-derived ring slope data are analyzed, and terrestrial SAR data are used to suggest possible surface morphologies.

Campbell, Bruce A.↗

NASA NTRS1993

Modes of formation of lunar light plains and the detection of cryptomaria deposits

The early volcanic and impact histories of the Moon are closely linked and the record preserved in surface morphology and samples is tightly convolved because of the interaction of the two processes. The deconvolution of the record is an important goal in order to assess early volcanic flux and the mode of emplacement of large crater and basin deposits. For example, lunar light plains have been variously interpreted as volcanic, impact, and volcanic covered by impact deposits. The development of criteria for the determination of the origin of light plains and the detection of cryptomaria is a key to the deconvolution of this early record. We outline the various hypotheses for the origin of and potential modes of occurrence of light plains and cryptomaria, and develop criteria for their recognition and documentation. We use the example of the Schiller-Schickard and Balmer cryptomaria to illustrate the application of these techniques to the problem of light plains interpretation and cryptomaria documentation.

Head, James W.↗

NASA NTRS1992

Extraction of quantitative surface characteristics from AIRSAR data for Death Valley, California

Polarimetric Airborne Synthetic Aperture Radar (AIRSAR) data were collected for the Geologic Remote Sensing Field Experiment (GRSFE) over Death Valley, California, USA, in Sep. 1989. AIRSAR is a four-look, quad-polarization, three frequency instrument. It collects measurements at C-band (5.66 cm), L-band (23.98 cm), and P-band (68.13 cm), and has a GIFOV of 10 meters and a swath width of 12 kilometers. Because the radar measures at three wavelengths, different scales of surface roughness are measured. Also, dielectric constants can be calculated from the data. The AIRSAR data were calibrated using in-scene trihedral corner reflectors to remove cross-talk; and to calibrate the phase, amplitude, and co-channel gain imbalance. The calibration allows for the extraction of accurate values of rms surface roughness, dielectric constants, sigma(sub 0) backscatter, and polarization information. The radar data sets allow quantitative characterization of small scale surface structure of geologic units, providing information about the physical and chemical processes that control the surface morphology. Combining the quantitative information extracted from the radar data with other remotely sensed data sets allows discrimination, identification and mapping of geologic units that may be difficult to discern using conventional techniques.

Kierein-Young, K. S.↗

NASA NTRS1993

Contributions of the Clementine mission to our understanding of the processes and history of the Moon

The Clementine mission will provide us with an abundance of information about lunar surface morphology, topography, and composition, and it will permit us to infer the history of the Moon and the processes that have shaped that history. This information can be used to address fundamental questions in lunar science and allow us to make significant advances towards deciphering the complex story of the Moon. The Clementine mission will also permit a first-order global assessment of the resources of the Moon and provide a strategic base of knowledge upon which future robotic and human missions to the Moon can build.

Spudis, Paul D.↗

NASA NTRS1993

Magellan Mission

The Magellan spacecraft was launched from Cape Kennedy on 4 May 1989 and was inserted into orbit around Venus on 10 Aug. 1990. The Magellan spacecraft carries a radar instrument that makes synthetic aperture radar (SAR) images of the surface, measures the altitude of the Venusian surface directly below the spacecraft, and obtains radiometric observations of the surface. Radar and radiometric observations of the Venusian surface commenced on 15 Sep. 1990 and continued until 15 Sep. 1992. Gravity observations began on 24 Sep. 1992 and will continue until late May 1993. The radar observations produced SAR images and surface topography for 99 percent of the surface. These radar observations support the objective of improving the knowledge of the geological history of Venus by analysis of surface morphology and the processes that control them. The gravity observations that are being conducted now support the Magellan objective of improving the knowledge of the geophysics of Venus, principally its density distribution and dynamics. Also, Magellan generated more digital planetary image data than all previous planetary missions.

Thompson, Thomas W.↗

NASA NTRS1994

A New Tribological Test for Candidate Brush Seal Materials Evaluation

A new tribological test for candidate brush seal materials evaluation has been developed. The sliding contact between the brush seal wires and their mating counterface journal is simulated by testing a small tuft of wire against the outside diameter of a high speed rotating shaft. The test configuration is similar to a standard block on ring geometry. The new tester provides the capability to measure both the friction and wear of candidate wire and counterface materials under controlled loading conditions in the gram to kilogram range. A wide test condition latitude of speeds (1 to 27 m/s), temperatures (25 to 700 C), and loads (0.5 to 10 N) enables the simulation of many of the important tribological parameters found in turbine engine brush seals. This paper describes the new test rig and specimen configuration and presents initial data for candidate seal materials comparing tuft test results and wear surface morphology to field tested seal components.

Fellenstein, James A.↗

NASA NTRS1994

Ignition and combustion of bulk metals in a microgravity environment

Knowledge of the oxidation, ignition, and combustion of bulk metals is important for fire safety in the production, management, and utilization of liquid and gaseous oxygen for ground based and space applications. This report summarizes research under NASA support to investigate the ignition and combustion characteristics of bulk metals under varying gravity conditions. Metal ignition and combustion have not been studied previously under these conditions and the results are important not only for improved fire safety but also to increase knowledge of basic ignition and combustion mechanisms. The studies completed to date have led to the development of a clean and reproducible ignition source and diagnostic techniques for combustion measurements and have provided normal gravity combustion data on ten different pure metals. Metal specimens were ignited using a xenon short-arc lamp and measurements were made of the radiant energy flux, surface temperature history, spectroscopy of surface and gas products, and surface morphology and chemistry. Elevated gravity was provided by the University of Colorado Geotechnical Centrifuge.

Branch, Melvyn C.↗

NASA NTRS1995

Further investigations of experiment A0034 atomic oxygen stimulated outgassing

Thermal control coatings within the recessed compartments of LDEF Experiment A0034 experienced the maximum leading edge fluence of atomic oxygen with considerably less solar UV radiation exposure than top-surface mounted materials of other LDEF experiments on either the leading or the trailing edge. This combination of exposure within A0034 resulted in generally lower levels of darkening attributable to solar UV radiation than for similar materials on other LDEF experiments exposed to greater cumulative solar UV radiation levels. Changes in solar absorptance and infrared thermal emittance of the exposed coatings are thus unique to this exposure. Analytical results for other applications have been found for environmentally induced changes in fluorescence, surface morphology, light scattering, and the effects of coating outgassing products on adjacent mirrors and windows of the A0034 experiment. Some atmospheric bleaching of the thermal control coatings, in addition to that presumably experience during reentry and recovery operations, has been found since initial post-flight observations and measurements.

Linton, Roger C.↗