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At least 613 records · Page 34

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NASA NTRS1992

Crystalline orientations of Tl2Ba2Ca2Cu3O(x) grains on MgO, SrTiO3, and LaAlO3 substrates

Crystalline orientations of Tl2Ba2Ca2Cu3O(x) grains in magnetron sputtered films on MgO (001), SrTiO3 (001), and LaAlO3 (001) substrates were investigated by scanning electron microscopy. In contrast to the nearly single crystalline films on the lattice matched substrates SrTiO3 and LaAlO3, films on the MgO (001) substrate, being polycrystalline in nature, exhibit several preferred in-plane grain orientations. These orientations agree well with a simplified theory of near-coincidence site lattices between Tl2Ba2Ca2Cu3O(x) and MgO.

Liou, S. H.↗

NASA NTRS1990

Millimeter wave surface resistance of grain-aligned Y1Ba2Cu3O(x) bulk material

Measurements are reported of the millimeter-wave surface resistance of grain-aligned YBa2Cu3O(x) bulk material grown by a liquid-phase process. The measurements were performed by replacing the endplate of a TE(011) cylindrical copper cavity with the superconducting sample. Surface resistance was measured for samples with surfaces oriented perpendicular and parallel to the c-axis of the grains. For the parallel configuration, the surface resistance at 77 K and 80 GHz is given. For a very well-aligned sample with a very low density of Y2BaCuO(y) precipitates, measured in the perpendicular configuration, the transition width (10-90 percent) is about 2 K and the surface resistance is derived at 88 K. The effect of microstructure on surface resistance is discussed.

Wosik, J.↗

NASA NTRS1992

Synthesis and characterization of fine grain diamond films

A fine grain diamond film has been developed by microwave plasma assisted chemical vapor deposition. Various analytical techniques, including Rutherford backscattering, proton recoil analysis, Raman spectroscopy, and X-ray diffraction, were utilized to characterize the diamond films. The grain size of the film was determined from bright and dark field electron micrographs, and found to be 200-1000 A. The films exhibited good optical transmission between 2.5 and 10 microns, with a calculated absorption coefficient of 490/cm. The friction coefficients of this film were found to be 0.035 and 0.030 at dry nitrogen and humid air environments, respectively, and the films had low wear rates.

Wu, Richard L. C.↗

NASA NTRS1993

Gas-Grain Simulation Facility (GGSF). Volume 1: Stage 1 facility definition studies

The Gas-Grain Simulation Facility (GGSF) is a facility-type payload to be included in the Space Station Freedom (SSF). The GGSF is a multidisciplinary facility that will accommodate several classes of experiments, including exobiology, planetary science, atmospheric science, and astrophysics. The physical mechanisms envisioned to be investigated include crystal growth, aggregation, nucleation, coagulation, condensation, collisions, fractal growth, cycles of freezing and evaporation, scavenging, longevity of bacteria, and more. TRW performed a Phase A study that included analyses of the science and technical (S&T) requirements, the development of facility functional requirements, and a conceptual design of the facility. The work that was performed under Stage 1 of the Phase A study and the results to date are summarized. In this stage, facility definition studies were conducted in sufficient detail to establish the technical feasibility of the candidate strawman experiments. The studies identified technical difficulties, identified required facility subsystems, surveyed existing technology studies and established preliminary facility weight, volume, power consumption, data systems, interface definition, and crew time requirements. The results of this study served as the basis for Stage 2 of the Phase A study in which a conceptual design and a reference design were performed. The results also served as a basis for a related study for a Gas-Grain Simulation Experiment Module (GGSEM), which is an apparatus intended to perform a subset of the GGSF experiments on board a low-Earth-orbiting platform.

Gat, Nahum↗

NASA NTRS1993

The role of grain boundaries in hydrogen diffusion in metals at 25 C

The effect of grain size on hydrogen diffusion at 25 C was examined for 4340 steel (body-centered cubic) and for Inconel 718 (face-centered cubic). It was found that the effect of grain size is important for body-centered cubic structures, but plays a much less important role in face centered cubic structures. Accurate measurements of hydrogen desorption coefficients during hydrogen desorption show that these are not greatly different for both types of structures.

Danford, M. D.↗

NASA NTRS1992

On the molecular complexity of the hot cores in Orion A - Grain surface chemistry as 'The last refuge of the scoundrel'

We have modeled the gas phase chemistry of warm molecular material around protostars that is seeded with evaporating grain mantles. We show that the release of simple molecules into the gas drives ion-molecule and neutral chemistries which can account for many of the complex O-bearing and N-bearing molecules observed in hot cores. Initial grain mantle components and secondary product molecules are identified, and the observational consequences are discussed.

Charnley, S. B.↗

NASA NTRS1993

Laboratory simulation of organic grain mantles

Laboratory simulations have been conducted of interstellar space conditions conducive to grain mantle evolution, including the photoproduction of a material which resembles the organic refractory mantle on interstellar grains. Upon analysis by various methods these organic refractory samples are noted to consist of a complex mixture of long, cross-linked chains, with a high probability of being aromatic carbon molecules.

Mendoza-Gomez, Celia X.↗

NASA NTRS1993

Optical response of an ultrathin film and a large-angle grain-boundary in superconducting YBa2Cu3O(7-delta)

We have investigated the optical response of epitaxial ultrathin (3 nm) YBa2Cu3O(7-delta) film on an NdGaO3 substrate and thin (50 nm) film of YBa2Cu3O(7-delta) on a bicrystal SrTiO3 substrate. While the current-voltage characteristics of the ultrathin film indicate a Kosterlitz-Thouless transition (associated with vortex-antivortex unbinding) at a temperature T(KT), we do not observe any optical response at or below T(KT). An enhanced optical response with nonbolometric characteristics was observed from the grain-boundary as compared to the bolometric optical response of the epitaxial film at the resistive transition. These results indicate that the grain-boundary weak links play a significant role in the enhanced optical response near Tc.

Bhattacharya, S.↗

NASA NTRS1993

Measuring track densities in lunar grains by image analysis

We have developed techniques to use digitized scanning electron micrographs and computer image analysis programs to measure track densities in lunar soil grains. Tracks were formed by highly ionizing solar energetic particles and cosmic rays during near surface exposure on the Moon. The track densities are related to the exposure conditions (depth and time). Distributions of the number of grains as a function of their track densities can reveal the modality of soil maturation. We used a sample that had already been etched in 6 N NaOH at 118 C for 15 h to reveal tracks. We determined that back-scattered electron images taken at 50 percent contrast and approximately 49.8 percent brightness produced suitable high contrast images for analysis. We ascertained gray-scale thresholds of interest: 0-230 for tracks, 231 for masked regions, and 232-255 for background. We found no need to set an upper size limit for distinguishing tracks. We did use lower limits to exclude noise: 16 pixels at 15000x, 4 pixels at 10000x, 2 pixels at 6800x, and 0 pixels at 4600x. We used computer counting and measurement of area to obtain track densities. We found an excellent correlation with manual measurements for track densities below 1x10(exp 8) sq cm. For track densities between 1x10(exp 8) sq cm to 1x10(exp 9) sq cm, we found that a regression formula using the percentage area covered by tracks gave good agreement with manual measurements. Finally we used these new techniques to obtain a track density distribution that gave more detail and was more rapidly obtained than using manual techniques 15 years ago.

Blanford, George E.↗

NASA NTRS1994

Measuring track densities in lunar grains using image analysis

We have used digitized scanning electron micrographs and computer image analysis programs to measure track densities in lunar soil grains. Tracks were formed by highly ionizing solar energetic particles and cosmic rays. Back-scattered electron images produced suitable high contrast images for analysis. We used computer counting and measurement of area to obtain track densities. We found an excellent correlation with manual measurements for track densities below 1x10(exp 8) cm(exp -2). For track densities between 1x10(exp 8) to 1x10(exp 9) cm(exp -2) we found that a regression formula using the percentage area covered by tracks gave good agreement with manual measurements. Measurement of tract densities in lunar samples has been a very rewarding technique for measuring exposure ages and soil maturation processes. We have shown that we can reliably measure track densities in lunar grains using image analysis techniques. Automating track counting may allow application of this technique to important problems in regolith dynamics including the ratio of radiation exposure to reworking in various surface and core samples and in regolith breccias.

Blanford, G. E.↗

NASA NTRS1994

Velocity distribution of Earth-crossing asteroid grains

We have systematically studied the evolution of 4- and 9-micrometer-diameter dust particles from three prominent asteroid families, Eos, Koronis, and Themis, until they reach the Earth. We have found that they are quite distinguishable in their (I cos omega, I sin omega) phase space. The different distributions in the inclination space have several important observational consequences, which can be used to identify the origins of these dust particles. We convert the orbital elements of these Earth-crossing dust grains to distinguish them by using any one single velocity component. We conclude that in order to identify the origins of the Earth-crossing dust grains we need all three velocity components to get their orbital elements and find their positions in the inclination space.

Liou, Jer-Chyi↗

NASA NTRS1995

Charging of interplanetary grains

The objective of this program is to quantify, by laboratory experiments, the charging of ices and other insulators subject to irradiation with electrons, ions and ultraviolet photons and to model special conditions based on the data. The system and conditions to be studied are those relevant for charging of dust in magnetospheric plasmas. The measurements are supplemented by computer simulations of charging or grains under a variety of conditions. Our work for this period involved experiments on water ice, improved models of charging of ice grains for Saturn's E-ring, and the construction of apparatus for electron impact studies and measurements of electron energy distributions.

Baragiola, R. A.↗

NASA NTRS1994

Extreme Mg-26 and O-17 enrichments in an Orgueil corundum: Identification of a presolar oxide grain

A corundum (Al2O3) grain from the Orgueil meteorite is greatly enriched in O-17 and (Mg-26)(sup *). The measured O-16/O-17 is 1028 +/- 11 compared to solar(O-16/O-17) = 2610. This is the largest O-17 excess so far observed in any meteoritic material. The Mg-26 excess ((Mg-26)(sup *)) is most plausibly due to in situ decay of Al-26. The inferred (Al-26/Al-27)(sub 0) ratio of 8.9 x 10(exp -4) is approximately 18 times larger than the 5 x 10(exp -5) value commonly observed in refractory inclusions formed in the solar system. The large O-17 excess and high (Mg-26)(sup *) Al-27 ratio unambiguously identify this corundum as a presolar oxide grain. Enrichments in O-17 and Al-26 are characteristic of H-burning and point to red giant or asymptotic giant branch (AGB) stars as likely sources.

Hutcheon, I. D.↗

NASA NTRS1994

Variable extinction in HD 45677 and the evolution of dust grains in pre-main-sequence disks

Changes in the UV extinction and IR emission were sought in the Herbig Ae/Be star candidate HD 45677 (= FS CMa) by comparing UV, optical, and IR observations made approximately 10 yr apart. HD 45677 varied significantly, becoming more than 50% brighter in the UV and optical than it was a decade ago. A comparison of the observations between epochs indicates that if the variations are due to changes in dust obscuration, the dust acts as a gray absorber into the near-IR and must be depleted in grains smaller than 1 micron. This is similar to the results obtained on the circumstellar disks of stars like Vega and Beta Pic, and suggests that radiation pressure may be responsible for the small-grain depletion. In addition, the total IR flux seems to have declined, indicating a decrease in the total mass of the dust envelope that contributes to the IR emission in this part of the spectrum. Due to the anomalous nature of the extinction, the use of normal extinction curves to deredden the spectral energy distributions of stars with circumstellar dust may lead to significant errors and should be used with great caution.

Sitko, Michael L.↗

NASA NTRS1994

Near-infrared absorption spectroscopy of interstellar hydrocarbon grains

We present new 3600 - 2700/cm (2.8 - 3.7 micrometer) spectra of objects whose extinction is dominated by dust in the diffuse interstellar medium. The observations presented here augment an ongoing study of the organic component of the diffuse interstellar medium. These spectra contain a broad feature centered near 3300/cm (3.0 micrometers) and/or a feature with a more complex profile near 2950/cm (3.4 micrometers), the latter of which is attributed to saturated aliphatic hydrocarbons in interstellar grains and is the primary interest of this paper. As in our earlier work, the similarity of the absorption bands near 2950/cm (3.4 micrometers) along different lines of sight and the correlation of these features with interstellar extinction reveal that the carrier of this band lies in the dust in the diffuse interstellar medium (DISM). At least 2.5% of the cosmic carbon in the local interstellar medium and 4% toward the Galactic center is tied up in the carrier of the 2950/cm (3.4 micrometer) band. The spectral structure of the diffuse dust hydrocarbon C-H stretch absorption features is reasonably similar to UV photolyzed laboratory ice residues and is quite similar to the carbonaceous component of the Murchison meteorite. The similarity between the DISM and the meteoritic spectrum suggests that some of the interstellar material originally incorporated into the solar nebula may have survived relatively untouched in primitive solar system bodies. Comparisons of the DISM spectrum to hydrogenated amorphous carbon and quenched carbonaceous composite are also presented. The A(sub V)/tau ratio for the 2950/cm (3.4 micrometer) feature is lower toward the Galactic center than toward sources in the local solar neighborhood (approximately 150 for the Galactic center sources vs. approximately 250 for the local ISM sources). A similar trend has been observed previously for silicates in the diffuse medium by Roche & Aitken, suggesting that (1) the silicate and carbonaceous materials in the DISM may be physically correlated and (2) there is either dust compositional variation in the galaxy or galactic variation in the grain population density distribution. We also note a possible absorption feature near 3050/cm (3.28 micrometers), a wavelength position that is characteristic of polycyclic aromatic hydrocarbons (PAHs).

Pendleton, Y. J.↗

NASA NTRS1995

Evidence for chemical processing of precometary icy grains in circumstellar environments of pre-main-sequence stars

We report the detection of a broad absorption feature near 2166/cm in the spectrum of the Taurus cloud cource Elias 18. This pre-main-sequence source is the second in Taurus, the third in our survey, and the fifth known in the sky to show the broad 2166/cm absorption feature. Of equal importance, this feature is not seen toward several other embedded sources in our survey, nor is it seen toward the source Elias 16, located behind the Taurus cloud. Laboratory experiments with interstellar ice analogs show that such a feature is associated with a complex C triple bonded to N containing compound (called X(C triple bonded to N)) that results from high-energy processing (ultraviolet irradiation or ion bombardment) of simple ice components into more complex, organic components. We find a nonlinear anticorrelation between the abundance of X(C triple bonded to N) and frozen CO components in nonpolar lattices. We find no correlation between the abundance of X(C triple bonded to N) and frozen CO in polar lattices. Because the abundances of frozen CO and H2O are strongly correlated with each other and with visual extinction toward sources embedded in and located behind the Taurus molecular cloud, these ice components usually are associated with intracloud material. Our results indicate that X(C triple bonded to N) molecules result from chemical processing of dust grains dominated by nonpolar icy mantles in the local environments of pre-main-sequence stars. Such processing of icy grains in the early solar system may be an important source of organic compounds observed in minor solar system bodies. The delivery of these organic compounds to the surface of the primitive Earth through comet impacts may have provided the raw materials for prebiotic chemistry.

Tegler, Stephen C.↗

NASA NTRS1995

Effect of microwave-enhanced superconductivity in YBa2Cu3O7 Bi-crystalline grain bounda ry weak-links

We have studied systematically the effect of microwave irradiation on the temperature dependent resistivity (R(I) and the current-voltage (I-V) characteristics of YBa2Gu3O(7 - x) (YBCO) bicrystalline grain boundary weak-links (GBWL's), with grain boundary of three different tilt angles. The superconducting transition temperature, T(sub c), has significant enhancement upon microwave irradiation. The microwave enhanced T(sub c) is increased as a function of incident microwave power, but limited to an optimum power level. The GBWL's of 45 deg tilt boundary has shown to be most sensitive to the microwave irradiation power, and the GBWL's of 36.8 deg tilt boundary has displayed a moderate response. In contrast, no enhancement of T(sub c) was observed in the GBWL's of 24 deg tilt boundary, as well as in the uniform films. Under the microwave irradiation, the R(T) dependent is hystertic as the transition taken from superconducting state to normal state and vice versa. Mechanisms associated with the redistribution of nonequilibrium quasiparticles under microwave irradiation are discussed.

Fu, C. M.↗

NASA NTRS1996

The Role of Hydrogen in Small Amorphous Carbon Grains: The IR Spectrum

Preliminary results on the evolution of the IR spectrum of hydrogenated carbon grains as a function of heat treatment are presented. The transformation of C-H and C-C bonding configurations is considered and the correlation with other properties of grains, such as their microstructure, is analyzed.

Mennella, V.↗