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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

2200 C oxidizing atmosphere furnace for space manufacturing

The design of a high temperature furnace is presented which uses electrically conducting ceramic oxide heating elements. The heating elements are made of either thoria or stabilized zirconia which become conductive when preheated to 700 to 1000 C. These heater elements can be operated to 2200 C in oxidizing or inert atmospheres. The furnace is being designed to have a temperature which can be controlled to within 11 C. By a replacement of the heater element, the working cavity can be changed from isothermal to a cavity with a selected specific axial temperature gradient of up to 200 C per centimeter. This furnace concept is appropriate for the growth of crystals which might be grown best in an oxidizing atmosphere such as sapphire (melting point 2040 C), yttrium aluminum garnet (1970 C) or yttrium orthoaluminate (1875 C).

Halbach, C. R.↗
NASA NTRS2000

Measuring the spectrum of mutation induced by nitrogen ions and protons in the human-hamster hybrid cell line A(L)C

Astronauts can be exposed to charged particles, including protons, alpha particles and heavier ions, during space flights. Therefore, studying the biological effectiveness of these sparsely and densely ionizing radiations is important to understanding the potential health effects for astronauts. We evaluated the mutagenic effectiveness of sparsely ionizing 55 MeV protons and densely ionizing 32 MeV/nucleon nitrogen ions using cells of two human-hamster cell lines, A(L) and A(L)C. We have previously characterized a spectrum of mutations, including megabase deletions, in human chromosome 11, the sole human chromosome in the human-hamster hybrid cell lines A(L)C and A(L). CD59(-) mutants have lost expression of a human cell surface antigen encoded by the CD59 gene located at 11p13. Deletion of genes located on the tip of the short arm of 11 (11p15.5) is lethal to the A(L) hybrid, so that CD59 mutants that lose the entire chromosome 11 die and escape detection. In contrast, deletion of the 11p15.5 region is not lethal in the hybrid A(L)C, allowing for the detection of chromosome loss or other chromosomal mutations involving 11p15.5. The 55 MeV protons and 32 MeV/nucleon nitrogen ions were each about 10 times more mutagenic per unit dose at the CD59 locus in A(L)C cells than in A(L) cells. In the case of nitrogen ions, the mutations observed in A(L)C cells were predominantly due to chromosome loss events or 11p deletions, often containing a breakpoint in the pericentromeric region. The increase in the CD59(-) mutant fraction for A(L)C cells exposed to protons was associated with either translocation of portions of 11q onto a hamster chromosome, or discontinuous or "skipping" mutations. We demonstrate here that A(L)C cells are a powerful tool that will aid in the understanding of the mutagenic effects of different types of ionizing radiation.

Non-NASA Center↗
NASA NTRS2006

Spherulitic (c-axis) Growth for Terrestrial (Mauna Kea, Hawaii) and Martian Hematite "blueberries"

Hematite concentrations observed by Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor were considered a possible indicator for aqueous processes on Mars. Observations made by Opportunity show that the hematite at Meridiani Planum is present as spherules ( blueberries) and their fragments. The internal structure of the hematite spherules is not discernable at the resolution limit (approx.30 m/pixel) of Opportunity s Microscopic Imager (MI). A terrestrial analog for martian hematite spherules are spherules from hydrothermally altered and sulfate-rich tephra from the summit region of Mauna Kea volcano, Hawaii. The objective of this study is to determine the crystal growth fabric of the Mauna Kea hematite spherules using transmission electron microscopy (TEM) techniques and to relate that crystalline fabric to the observed TES signature of Meridiani Planum "blueberries." TEM analysis of Mauna Kea spherules exhibited a radial growth pattern consisting of "fibrous" hematite with the c-axis of hematite particles aligned along the elongation direction of the hematite fibers. The individual fibers appear to be made of coalesced nano-particles of hematite arranged with their c-axis oriented radially to form a spherical structure. Lattice fringes suggest long-range order across particles and along fibers. According to interpretations of thermal emission spectra for Meridian Planum hematite, the absence of a band at approx. 390/cm implies a geometry where c-face emission dominates. Because the c-face is perpendicular to the c-axis, this is precisely the geometry for the Mauna Kea spherules because the c-axis is aligned parallel to their radial growth direction. Therefore, we conclude as a working hypothesis that the martian spherules also have radial, c-axis growth pattern on a scale that is too small to be detected by the MER MI. Furthermore, by analogy with the Mauna Kea spherules, the martian blueberries could have formed during hydrothermal alteration of basaltic precursors under acid-sulfate conditions.

Golden, D. C.↗
NASA NTRS1993

(abstract) The SIR-C/X-SAR Mission

The Shuttle Imaging Radar-C and X-Band Synthetic Aperture Radar (SIR-C/X-SAR) is a cooperative experiment between the National Aeronautics and Space Administration (NASA), the German Space Agency (DARA), and the Italian Space Agency (ASI). The experiment is the next evolutionary step in NASA's Spaceborne Imaging Radar (SIR) program that began with the Seasat SAR in 1978, and continued with SAR -A in 1981 and SAR-B in 1984. The program will eventually lead to the Earth Observing System (EOS) SAR later in this decade. SIR-C will provide increased capability over Seasat, SIR-A, and SIR-B by acquiring polarimetric images simultaneously at two microwave frequencies: L-band (wavelength 24 cm) and C-band (wavelength 6 cm). X-SAR will operate at X-band (wavelength 3 cm) with VV polarization, resulting in a three-frequency capability. Because radar backscatter is most strongly influenced by objects comparable in size to the radar wavelength, this multifrequency capability will provide information about the Earth's surface over a wide range of scales not discernable with single-frequency experiments. The polarimetric data will yield quantitative information about the surface geometric structure, vegetation dielectric properties, and surface discontinuities. The first SIR-C/X-SAR flight is planned for early in 1994, with two subsequent flights in different seasons to enable data aquisition under different environmental conditions. SIR-C/X-SAR will collect data for a period of 6 to 9 days during each flight. The instrument will be flown in a 57(deg) inclination orbit with a nomimal orbit altitude of 215 km. This altitude will result in a slightly drifting orbit. The latest status of the SIR-C/X-SAR mission and instrument design, science activities, and expected results will be presented.

SIR-C/XSAR radar backscatter surface geometric str↗
NASA NTRS1972

High temperature (1200 C) ceramic-to-metal seal development

Two phases have been completed, of a program whose ultimate objective is the development of an alkali metal resistant, thermal shock resistant, leak-tight, and neutron radiation resistant ceramic-to-metal seal capable of operation at 1200 C for three to five years. The first phase involved the screening of platinum-base, vanadium-base and vanadium-niobium base brazes for the joining of Cb-1Zr or T-111 alloys to high purity alumina. The second phase involved studies of the performance of sealed capsule samples during 5000-hour aging tests at 800, 1000, and 1200 C in high vacuum. Sealed capsules which were made using pure vanadium braze, and were brazed at 1850 C for one minute, survived 64 thermal cycles to 1200 C at the heating/cooling rate of 100 C/minute. Vanadium braze samples survived 5000-hour aging tests at 800, 1000, and 1200 C. One thermally cycled sample survived a subsequent 5000-hour aging period at 1000 C, but another, at 1200 C, did not survive. It was concluded that a pure vanadium braze used to bond high purity alumina to Cb-1Zr alloy is the best of the systems studied, but that additional studies must be performed to establish its service temperature limitations for the desired three to five years' service.

Mckisson, R. L.↗
NASA NTRS1982

Surface chemistry and wear behavior of single-crystal silicon carbide sliding against iron at temperatures to 1500 C in vacuum

X-ray photoelectron and Auger electron spectroscopy analyses and morphological studies of wear and metal transfer were conducted with a single-crystal silicon carbide 0001 surface in contact with iron at various temperatures to 1500 C in a vacuum of 10 to the minus 8th power pascal. The results indicate that below 800 C, carbide-carbon and silicon are primarily seen on the silicon carbide surface. Above 800 C the graphite increases rapidly with increase in temperature. The outermost surficial layer, which consists mostly of graphite and little silicon at temperatures above 1200 C is about 2 nm thick. A thicker layer, which consists of a mixture of graphite, carbide, and silicon is approximately 100 nm thick. The closer the surface sliding temperature is to 800 C, the more the metal transfer produced. Above 800 C, there was a transfer of rough, discontinuous, and thin iron debris instead of smooth, continuous and thin iron film which was observed to transfer below 800 C. Two kinds of fracture pits were observed on the silicon carbide surface: (1) a pit with a spherical asperity; and (2) multiangular shaped pits.

Miyoshi, K.↗
NASA NTRS1987

In situ cosmogenic H-3, C-14, and Be-10 for determining the net accumulation and ablation rates of ice sheets

The usefulness of the in situ cosmogenic H-3, C-14, and Be-10 produced by spallation of oxygen nuclei in ice, as tracers to determine net accumulation/ablation rates of ice sheets is explored. The application of the in situ H-3 and Be-10 is severely constrained because at deposition, ice contains appreciable amounts of these isotopes from the atmosphere. The case is much more favorable for C-14, which is not carried with wet precipitations; atmospheric C-14 gets mechanically trapped in the ice during deposition. It is pointed out that cosmogenic C-14 would probably exist as (C-14)O in ice. This seems to be supported by the published results of Fireman and Norris (1981). Considering their inherent amounts in the ice and the expected in situ production rates, conditions under which these isotopes can be used to study net accumulation and ablation rates are discussed. Available data on C-14 and Be-10 on polar ice from accumulation and ablation zones is also discussed. It is concluded that H-3 and C-14 should find wide applications in studying ice dynamics and Be-10 in very special circumstances.

Lal, D.↗
NASA NTRS1988

Friction and Wear of Monolithic and Fiber Reinforced Silicon-Ceramics Sliding Against IN-718 Alloy at 25 to 800 C in Atmospheric Air at Ambient Pressure

The friction and wear of monolithic and fiber reinforced Si-ceramics sliding against the nickel base alloy IN-718 at 25 to 800 C was measured. The monolithic materials tested were silicon carbide (SiC), fused silica (SiO2), syalon, silicon nitride (Si3N4) with W and Mg additives, and Si3N4 with Y2O3 additive. At 25 C fused silica had the lowest friction while Si3N4 (W,Mg type) had the lowest wear. At 800 C syalon had the lowest friction while Si3N4 (W,Mg type) and syalon had the lowest wear. The SiC/IN-718 couple had the lowest total wear at 25 C. At 800 C the fused silica/IN-718 couple exhibited the least total wear. SiC fiber reinforced reaction bonded silicon nitride (RBSN) composite material with a porosity of 32 percent and a fiber content of 23 vol percent had a lower coefficient of friction and wear when sliding parallel to the fiber direction than in the perpendicular at 25 C. The coefficient of friction for the carbon fiber reinforced borosilicate composite was 0.18 at 25 C. This is the lowest of all the couples tested. Wear of this material was about two decades smaller than that of the monolithic fused silica. This illustrates the large improvement in tribological properties which can be achieved in ceramic materials by fiber reinforcement. At higher temperatures the oxidation products formed on the IN-718 alloy are transferred to the ceramic by sliding action and forms a thin, solid lubricant layer which decreases friction and wear for both the monolithic and fiber reinforced composites.

Deadmore, Daniel L.↗
NASA NTRS1988

Air-broadened Lorentz halfwidths and pressure-induced line shifts in the nu(4) band of C-13H4

Air-broadened halfwidths and pressure-induced line shifts in the nu(4) fundamental of C-13H4 were determined from spectra recorded at room temperature and at 0.01/cm resolution using a Fourier transform spectrometer. Halfwidths and pressure shifts were determined for over 180 transitions belonging to J-double prime values of less than or = to 16. Comparisons of air-broadened halfwidths and pressure-induced line shifts made for identical transitions in the nu(4) bands of C-12H4 and C-13H4 have shown that C-13H4 air-broadened halfwidths are about 5 percent smaller than the corresponding C-12H4 halfwidths, and the pressure shifts for C-13H4 lines are about 5-15 percent larger than those for C-12H4.

Devi, V. Malathy↗
NASA NTRS1990

Flux flow and flux dynamics in high-T(sub c) superconductors

Because high temperature superconductors, including BYCO and BSCCO, are type 2 superconductors with relatively low H(sub c 1) values and high H(sub c 2) values, they will be in a critical state for many of their applications. In the critical state, with the applied field between H(sub c 1) and H(sub c 2), flux lines have penetrated the material and can form a flux lattice and can be pinned by structural defects, chemical inhomogeneities, and impurities. A detailed knowledge of how flux penetrates the material and its behavior under the influence of applied fields and current flow, and the effect of material processing on these properties, is required in order to apply, and to improve the properties of, these superconductors. When the applied field is changed rapidly, the time dependence of flux change can be divided into three regions, an initial region which occurs very rapidly, a second region in which the magnetization has a 1n(t) behavior, and a saturation region at very long times. A critical field is defined for depinning, H(sub c,p) as that field at which the hysteresis loop changes from irreversible to reversible. As a function of temperature it is found that H(sub c,p) is well described by a power law with an exponent between 1.5 and 2.5. The behavior of H(sub c,p) for various materials and its relationship to flux flow and flux dynamics are discussed.

Bennett, L. H.↗
NASA NTRS1990

Laser ablated high T(sub c) superconducting thin YBa2Cu3O(7-x) films on substrates suitable for microwave applications

The development of high temperature superconducting YBa2Cu3O(7-x) thin films on substrates suitable for microwave applications is of great interest for evaluating their applications for space radar, communication, and sensor systems. Thin films of YBa2Cu3O(7-x) were formed on SrTiO3, ZrO2, MgO, and LaAlO3 substrates by laser ablation. The wavelength used was 248 nm from a KrF excimer laser. During deposition the films were heated to 600 C in a flowing oxygen environment, and required no post annealing. The low substrate temperature during deposition with no post annealing gave films which were smooth, which had their c-axis aligned to the substrates, and which had grains ranging from 0.2 to 0.5 microns in size. The films being c-axis aligned gave excellent surface resistance at 35 GHz which was lower than that of copper at 77 K. At present, LaAlO3 substrates with a dielectric constant of 22, appears suitable as a substrate for microwave and electronic applications. The films were characterized by resistance-temperature measurements, scanning electron microscopy, and x ray diffraction. The highest critical transition temperatures (T sub c) are above 89 K for films on SrTiO3 and LaAlO3, above 88 K for ZrO2, and above 86 K for MgO. The critical current density (J sub c) of the films on SrTiO3 is above 2 x 10(exp 6) amperes/sq cm at 77 K. The T(sub c) and J(sub c) are reported as a function of laser power, composition of the substrate, and temperature of the substrate during deposition.

Warner, J. D.↗
NASA NTRS1990

On determining ice accumulation rates in the past 40,000 years using in situ cosmogenic C-14

Radiocarbon is produced in situ in ice by nuclear spallations of oxygen by cosmic ray neutrons. As the firn accumulates, it acquires a predictable concentration of in situ C-14, inversely proportional to the rate of accumulation. Most of this production occurs when the amount of overlying ice is less than (2-3) Lambda, where lambda is the absorption mean free path for cosmic radiation in ice, about 150 g/sq cm, i.e. within the top 10 m. In most accumulation areas, this is firn. In situ produced C-14 is added to the firn as it accumulates, and is not expected to be lost by diffusion. During the firn-ice transition, atmospheric CO2 is trapped, adding (C-14)02 to the ice. The signature of in situ C-14 is however not obliterated since about 60 percent of in situ C-14 is instantly oxidized to (C-14)O in the ice. The results available to date are discussed, and it is proposed that this in situ (C-14)O can be used to determine ice accumulation rates back to 40,000 yrs in the past.

Lal, D.↗
NASA NTRS1991

The 158 micron forbidden C II line - A measure of global star formation activity in galaxies

Some 158 micron forbidden C II fine structure line observations from a sample of fourteen gas rich galaxies are reported. These measurements confirm and generalize previous basic results that the C II line is bright amounting to approximately 0.1 to 1 percent of the FIR luminosity of the nuclear regions of galaxies; the C II line is formed in the warm (temperature of the gas is greater than 200 K), dense (n sub H greater than 1000/cu cm) photodissociated gas at the interfaces between giant molecular clouds and ionized gas regions and is therefore associated with the molecular gas component in spiral galaxies; the C II line tracks the FIR continuum in a manner consistent with the PDR models; the integrated C II to isotope (C-12)D (transition 1 to 0) line ratio is large (greater than or equal to 1000) in all galaxies studied, and is similarly large for galactic molecular clouds; the C II line is therefore energetically very important for the study of giant molecular clouds. Conclusions obtained from these results are given.

Stacey, G. J.↗
NASA NTRS1993

Stress corrosion evaluation of HP 9Ni-4Co-0.30C steel plate welds

A stress corrosion cracking (SCC) investigation was conducted on HP 9Ni-4Co-0.30C steel plate welds (welded by using straight polarity plasma arc and HP 9Ni-4Co-0.20C weld wire) since this material is being considered for use in the Advanced Solid Rocket Motor (ASRM) program. Prior to the welding, the material was double tempered at 538 C (1,000 F). After welding, only part of the material was stress relieved at 510 C (950 F) for 3 h. Round tensile specimens obtained from nonstress-relieved material were tested in 100-percent relative humidity at 38 C (100 F), in 3.5-percent NaCl alternate immersion, and in 5-percent salt spray at 35 C (95 F). Specimens obtained from stress-relieved material were tested in alternate immersion. The stress levels were 50, 75, and 90 percent of the corresponding 0.2-percent yield strength (YS). All the nonstress-relieved specimens exposed to salt spray and alternate immersion failed. Stress-relieved specimens (exposed to alternate immersion) failed at 75 and 90 percent of YS. No failures occurred at 50 percent of YS in the stress-relieved specimens which indicates a beneficial effect of the stress relief on the SCC resistance of these welds. The stress relief also had a positive effect on the mechanical properties of the welds (the most important being an increase of 21 percent on the YS). Under the conditions of these tests, the straight polarity plasma are welded HP 9Ni4Co-0.30C steel plate was found highly susceptible to SCC in the nonstress-relieved condition. This susceptibility to SCC was reduced by stress relieving.

Torres, Pablo D.↗
NASA NTRS1994

Upper limits on metals in quasar Lyman-alpha forest clouds: Absence of C 5 lines in echelle spectra

Recently Lu presented tentative evidence for C IV lines in Quasi-Stellar Object (QSO) Ly-alpha forest systems with strong lines. We have performed a similar search for C IV in our 10 km/s resolution echelle spectra of the bright QSO HS 1946+7658. We shifted the spectra to align the expected positions of the C IV lines in 65 Ly-alpha systems and then added them. The resulting composite spectrum, equivalent to 390 hr of exposure time on the Lick 3 m telescope, has a signal-to-noise ratio of 80 per 0.025 A in the rest frame of the absorbers. We do not see any C IV lines down to a 2 sigma limit of W(1548) less than or equal to 1.4 mA, about one fifth of the strength of the lines seen by Lu. The C IV lines that Lu saw must be restricted to rare Ly-alpha systems with large H I column densities greater than or equal to 10(exp 14)/sq cm, which are too rare to show C IV in our sample. More common Ly-alpha systems with H I column densities of 10(exp 13)-10(exp 14)/sq cm do not show C IV lines. If their ionization is H/H I = 10(exp 4) then they have (C/H) less than or equal to -2.0.

Tytler, David↗
NASA NTRS1996

Effects of thermomechanical processing on the microstructure and mechanical properties of Nb-1Zr-C alloys

A systematic study to evaluate the effects of thermomechanical processing on the microstructure and mechanical properties of Nb-1Zr alloy sheet containing 0.06 and 0.1 wt.%C (PWC-11) was conducted and compared to the results of Nb-1Zr. Coarse orthorhombic Nb2C precipitates were present in all the cast, extruded and cold rolled Nb-Zr samples containing C. After high temperature (greater than 0.5 T(sub m)) exposure (with or without applied stress), the Nb2C transforms to very fine and extremely stable FCC (Zr, Nb)C dispersoid, resulting in a highly creep resistant material. Only ZrO2 precipitates were found in Nb-1Zr. The creep strength of the 0.06C and the 0.1C carbide strengthened alloys were much superior to Nb-1Zr. At 1350 K the strength of the 0.06C alloy was about three times that of Nb-1Zr, while the 0.1C alloy had about five times the creep stress capability of Nb-1Zr. The tensile strength, long term creep strength, and stability of the microstructure of the PWC-11 sheet appear to be independent of the number of 1900 K extrusions performed prior to cold rolling. The microhardness of these single, double and triple extnided PWC-11 sheets also were comparable. The tensile strength of PWC-11 and Nb-1Zr at room temperature and 1350 K were comparable.

Titran, Robert H.↗
NASA NTRS2002

Measurements of [C I] 9850 A Emission from Comet Hale-Bopp

We present quantitative measurements of cometary [C I] 9850 A, emission obtained during observations of comet Hale-Bopp (C/1995 O1) in 1997 March and April. The observations were carried out using a high-resolution (lambda/Delta lambda approx. 40,000) Fabry-Perot/CCD spectrometer at the McMath-Pierce solar telescope on Kitt Peak. This forbidden line, the carbon analog of [O I] 6300 A, is emitted in the radiative decay of C(1D) atoms. In the absence of other sources and sinks, [C I] 9850 A emission may be used as a direct tracer of CO photodissociation in comets. However, in Hale-Bopp's large, dense coma, other processes, such as collisional excitation of ground-state C(3P), dissociative recombination of CO+, and collisional dissociation of CO and CO2 may produce significant amounts of C(1D). The long C(1D) radiative lifetime (approx. 4000 s) makes collisional de-excitation (quenching) the primary loss mechanism in the inner coma. Thus, a detailed, self-consistent global model of collisional and photochemical interactions is necessary to fully account for [C I] 9850 A emission in comet Hale-Bopp.

Oliversen, R. J.↗
NASA NTRS2006

Switching Characteristics of a 4H-SiC Based Bipolar Junction Transistor to 200 C

Static curves and resistive load switching characteristics of a 600 V, 4 A rated, SiC-based NPN bipolar power transistor (BJT) were observed at selected temperatures from room to 200 C. All testing was done in a pulse mode at low duty cycle (approx.0.1 percent). Turn-on was driven by an adjustable base current pulse and turn-off was accelerated by a negative base voltage pulse of 7 V. These base drive signals were implemented by 850 V, gated power pulsers, having rise-times of roughly 10 ns, or less. Base charge sweep-out with a 7 V negative pulse did not produce the large reverse base current pulse seen in a comparably rated Si-based BJT. This may be due to a very low charge storage time. The decay of the collector current was more linear than its exponential-like rise. Switching observations were done at base drive currents (I(sub B)) up to 400 mA and collector currents (I(sub C)) up to 4 A, using a 100 Omega non-inductive load. At I(sub B) = 400 mA and I(sub C) = 4 A, turn-on times typically varied from 80 to 94 ns, over temperatures from 23 to 200 C. As expected, lowering the base drive greatly extended the turn-on time. Similarly, decreasing the load current to I(sub C) = 1 A with I(sub B) = 400 mA produced turn-on times as short as 34 ns. Over the 23 to 200 C range, with I(sub B) = 400 mA and I(sub C) = 4 A, turn-off times were in the range of 72 to 84 ns with the 7 V sweep-out.

Niedra, Janis M.↗