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At least 523 records · Page 29

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NASA NTRS1981

Metal and Bi/Pb microdistribution studies of an L3 chondrite - Their implications for a meteorite parent body

Strong localizations relative to bulk are reported of Bi, and to a lesser extent Pb, in some of the kamacite grains in Khohar, while other kamacite grains show no such enrichments. Distinctive and correlated differences in the Ni content are found in the two kamacite populations, with the Bi/Pb-rich kamacite grains having consistently lower Ni levels than the 6-7% typical of the Bi/Pb-poor kamacite. The Bi/Pb-rich kamacite grains also exhibit a highly reactive attack upon etching, which may be due to a finely polycrystaline microstructure. It is concluded that the trace element microdistributions were not established in the nebula, and that the enrichments were not likely to have occurred with slow cooling in the presence of a vapor phase during the kamacite-taenite phase transitions.

Woolum, D. S.↗

NASA NTRS1982

Solidification mechanism of highly undercooled metal alloys

Experiments were conducted on metal droplet undercooling, using Sn-25wt%Pb and Ni-34wt%Sn alloys. To achieve the high degree of undercooling, emulsification treatments were employed. Results show the fraction of supersaturated primary phase is a function of the amount of undercooling, as is the fineness of the structures. The solidification behavior of the tin-lead droplets during recalescence was analyzed using three different hypotheses; (1) solid forming throughout recalescence is of the maximum thermodynamically stable composition; (2) partitionless solidification below the T sub o temperature, and solid forming thereafter is of the maximum thermodynamically stable composition; and (3) partitionless solidification below the T sub o temperature with solid forming thereafter that is of the maximum thermodynamically metastable composition that is possible. The T sub o temperature is calculated from the equal molar free energies of the liquid solid using the regular solution approximation.

Shiohara, Y.↗

NASA NTRS1981

Lu-Hf total-rock age for the Amitsoq gneisses, West Greenland

Lu-Hf total-rock data for the Amitsoq gneisses of West Greenland yield an age of 3.55 + or - 0.22 billion years, based on the decay constant for Lu-176 of 1.96 x 10 to the -11th/year, and an initial Hf-176/Hf-177 ratio of 0.280482 + or - 33. The result is in good agreement with Rb-Sr total-rock and U-Pb zircon ages. In spite of severe metamorphism of the area at 2.9 billion years, zircons from two of the samples have remained on the total-rock line, and define points close to the initial Hf ratio. The initial Hf-176/Hf-177 lies close to a chondritic Hf isotopic evolution curve from 4.55 billion years to present. This is consistent with the igneous precursors to the Amitsoq gneisses having been derived from the mantle at or shortly before 3.6 billion years. Anomalous relationships between Hf concentration and the Lu-176/Hf-177 ratio may suggest that trace element abundances in the Amitsoq gneisses are partly controlled by processes related to metamorphism.

Pettingill, H. S.↗

NASA NTRS1981

Modeling the formation of ion clusters by applying classical nucleation theory

Experiments have been conducted to study the clustering of atmospheric trace gases around ion cores (Castleman and Tang, 1972; Searcy and Fenn, 1974; Castleman, 1978). The classical liquid-drop model is used to investigate this ion-induced formation mechanism. Results obtained from models of the distribution of Pb(+)-(H2O)n and H(+)-(H2O)n type clusters under various conditions are compared with experimental results. The distribution of water-ion clusters in the atmosphere as a function of altitude is calculated. In situ measurements of the water-ion cluster distributions in the upper atmosphere are then compared with present predictions. It is concluded that the classical nucleation theory can be used to predict rough estimates for ion cluster sizes under many conditions.

Yue, G. K.↗

NASA NTRS1981

Primitive ultrafine matrix in ordinary chondrites

Ultrafine matrix material has been concentrated by sieving and filtering disaggregated samples of six ordinary chondrites of different classes. This component(s), 'Holy Smoke' (HS), is enriched in both volatile, e.g. Na, K, Zn, Sb, and Pb, as well as refractory elements, e.g. W and REE; however, the element ratios vary greatly among the different chondrites. SEM studies show that HS contains fragile crystals, differing in composition, and apparently in gross disequilibrium not only among themselves but also with the major mineral phases and consequently thermodynamic equilibration did not occur. Thus HS must have originated from impacting bodies and/or was inherent in the 'primitive' regolith. Subsequent impact brecciation and reheating appears to have altered, to varying degrees, the original composition of this ultrafine matrix material. Recent 'cosmic dust' studies may indicate that HS still exists in the solar system. Survival of such delicate material must be considered in all theories for the origin of chondrites.

Rambaldi, E. R.↗

NASA NTRS1982

Surface diffusion activation energy determination using ion beam microtexturing

The activation energy for impurity atom (adatom) surface diffusion can be determined from the temperature dependence of the spacing of sputter cones. These cones are formed on the surface during sputtering while simultaneously adding impurities. The impurities form clusters by means of surface diffusion, and these clusters in turn initiate cone formation. Values are given for the surface diffusion activation energies for various materials on polycrystalline Cu, Al, Pb, Au, and Ni. The values for different impurity species on each of these substrates are approximately independent of impurity species within the experimental uncertainty, suggesting the absence of strong chemical bonding effects on the diffusion.

Rossnagel, S. M.↗

NASA NTRS1980

Redistribution of volatiles during lunar metamorphism

Thermal release profiles of Pb, Zn, and Cd in sample 66095 (highly shocked breccia with melt rock matrix) showed that these volatiles were mostly present on the surface of the grains. Zn in rusty grains from 66095 was also mostly surface Zn, probably from sphalerite in grain boundaries and cracks. Simulation experiments of volatile transfer showed that Fe, FeCl2, iron phosphide, and troilite (FeS) can be produced and transported during subsolidus reactions. These results suggest that volatiles, rust, schreibersite, and possible siderophiles which are observed in lunar highland samples might have been redistributed during disequilibrium thermal metamorphism in hot ejecta blankets, and were not necessarily introduced by volcanic activity or meteoritic addition.

Cirlin, E. H.↗

NASA NTRS1982

Vacuum tight quartz ampoule for Bridgman growth of crystals with interface demarcation

A growth ampoule for protecting the furnace lining and to prevent a change in the composition of the melt during growth of Pb-T-Te or Ge semiconductors doped with Ga is described. Mo foil is inserted into a quartz tube and sealed at the ends. A vacuum pumping system collapses the tube onto the foil. Spotwelded Mo wires lead in two directions from the collapsed section through quartz capillaries, one of which is also collapsed, on the wire, which provides a contact point for seeded growth of the semiconductor. The ampoule has been used for Bridgeman growth of crystals using a hot zone of 1150 C, with a temperature gradient of 240 C/m, and with current pulses up to 40 A/sq cm.

Crouch, R. K.↗

NASA NTRS1982

The effect of grooves in amorphous substrates on the orientation of metal deposits. I - Carbon substrates

The graphoepitaxial alignment of vapor-deposited discrete metal crystallites is investigated in the nucleation and growth stages and during annealing by in situ UHV/TEM techniques. Various stages of nucleation, growth and coalescence of vapor deposits of Au, Ag, Pb, Sn, and Bi on amorphous, topographically structured C substrates are analyzed by advanced dark-field techniques to detect preferred local orientations. It is found that the topography-induced orientation of metal crystallites depends strongly on their mobility and their respective tendency to develop pronounced crystallographic shapes. Lowering of the average surface free energies and increasing the crystallographic surface energy anisotropies cause generally improved graphoepitaxial alignments.

Anton, R.↗

NASA NTRS1982

High resolution atmospheric spectroscopy using a diode laser heterodyne spectrometer

Ultrahigh resolution (0.007 per cm) atmospheric solar absorption spectra that have been obtained from a tunable infrared heterodyne radiometer are discussed. The radiometer was developed for ground based observations in the 8-12 micron region, and tunability is achieved through the use of Pb-salt semiconductor laser local oscillators. Spectra have been obtained in a piecewise fashion from 9.1 to 11.1 microns using laser emission modes that exhibit characteristics suitable for local-oscillator operation. Spectra showing absorption features of HNO3, O3, CO2, and H2O are presented along with comparisons of experimental and synthetic spectra calculated using a line-by-line atmospheric transmission model.

Hoell, J. M., Jr.↗

NASA NTRS1982

Quasi-liquid states observed on ion beam microtextured surfaces

Liquid-like properties have been observed on surface structures developed by means of ion beam microtexturing. The structures include cones, pyramids, or wavelike formations. The observed liquid-like effects are drips and ripples on the sides of cones, droplet formation, the apparent flow and coalescence of closely packed structures, wetting angle and other surface tension effects, and the bending of cones by additional heating. The bulk temperatures are in the range of 50-600 C. These effects are seen to some extent on Cu, Al, Au, Pb, and Ni substrates.

Rossnagel, S. M.↗

NASA NTRS1982

Ultraheavy cosmic rays - HEAO-3 results

The instrumentation and results from the Heavy Nuclei experiment on the HEAO 3 satellite are described. Six independently analyzed dual-gap ionization chambers measured the energy loss of the cosmic rays while a Cerenkov counter with 8 independently analyzed photomultipliers viewed two sheets of Pilot 425 plastic in a white box. Trajectories of the cosmic ray nuclei were determined in multiwire ionization hodoscopes. Variations in abundances were observed to be imperfectly ordered in terms of the first ionization potential, and volatility was also ruled out as the controlling factor. A predicted drop in abundance after Ba-56 was found, along with another sharp fall above Pb-82. Only one actinide-type event was detected during the 14 mos of viewing, a result consistent with other findings but which testifies against r-process formation.

Israel, M. H.↗

NASA NTRS1982

Petrology and petrogenesis of lunar breccia 12013

Sample 12013 is extremely rich in incompatible elements (K, P, U, Th, Pb, Ba, Rb, Zr, Nb, and rare earths) relative to other lunar samples, and it contains abundant granitic material composed of silica minerals and K-feldspar. The present investigation provides detailed petrologic and mineralogic descriptions of the rock and a discussion of its genesis and history. The sample is made up of two lithologies, including a mottled gray and white lithology, and a black lithology. A detailed petrology of the gray breccia is presented, taking into account lithic clasts, mineral clasts, deformation textures in clasts, the matrix, and felsite. A detailed petrology of the black breccia is also provided. Attention is given to a model for the formation of the breccia, an evaluation of the possible genetic relationships between the various components, and some constraints on the nature of the source area.

Quick, J. E.↗

NASA NTRS1982

Spheres of the metallic glass Au55 Pb22.5 Sb22.5 and their surface characteristics

Spheres of the metallic glass Au55 Pb22.5 Sb22.5 have been formed up to a size of approximately 1.5 mm in diameter. X-ray diffraction was used to establish the glassy nature of the samples and to provide evidence of two phase-separated glass regions. Scanning electron microscopy provided a direct visual observation of the two-phase amorphous network on the surface of the sphere. The physical dimensions of the phase-separated regions were observed to be cooling-rate sensitive. Energy dispersive spectroscopy indicated that the compositions of these two glassy phases were Au-rich and Pb-rich, respectively, confirming the results of Kim and Johnson (1981). In addition, the spheres exhibited an unusual surface smoothness of better than + or - 250 A

Lee, M. C.↗

NASA NTRS1982

Oligonucleotide formation catalyzed by mononucleotide matrices

Pb(2+)-containing precipitates of mononucleotides form matrices which catalyze the self-condensation of nucleotide 5-prime-phosphorimidazolides and their condensation with nucleosides. The reactions exhibit base-pairing specificity between matrix nucleotide and substrate, and usually follow the Watson-Crick pairing rules. Although purine polynucleotides do not facilitate the oligomerization of pyrimidine nucleotide monomers in solution, it is interesting that purine-containing matrices do catalyze such a reaction. The significance of the results in the context of the prebiotic evolution of polynucleotides is discussed.

Lohrmann, R.↗

NASA NTRS1983

Ion exchange selectivity for cross-linked polyacrylic acid

The ion separation factors for 21 common metal ions with cross-linked polyacrylic acid were determined as a function of pH and the percent of the cross-linked polyacrylic acid neutralized. The calcium ion was used as a reference. At a pH of 5 the decreasing order of affinity of the ions for the cross-linked polyacrylic acid was found to be: Hg++, Fe+++, Pb++, Cr+++, Cu++, Cd++, Al+++, Ag+, Zn++, Ni++, Mn++, Co++, Ca++, Sr++, Ba++, Mg++, K+, Rb+, Cs+, Na+, and Li+. Members of a chemical family exhibited similar selectivities. The Hg++ ion appeared to be about a million times more strongly bound than the alkali metal ions. The relative binding of most of the metal ions varied with pH; the very tightly and very weakly bound ions showed the largest variations with pH. The calcium ion-hydrogen ion equilibrium was perturbed very little by the presence of the other ions. The separation factors and selectivity coefficients are discussed in terms of equilibrium and thermodynamic significance.

May, C. E.↗

NASA NTRS1982

Linewidth measurements of tunable diode lasers using heterodyne and etalon techniques

Measurements of the linewidths of Pb-salt diode lasers operating in the 8- and 9-micron region are reported. The linewidths of the 9-micron lasers were determined by conventional heterodyne techniques, while for the 8-micron lasers a new technique based on a Fabry-Perot etalon was used. The new technique avoids the complexity and limited wavelength range of the heterodyne measurements and can be used for any tunable laser. The linewidths observed varied from 0.6 to more than 500-MHz FWHM. The linewidth was found to vary dramatically from device to device, to depend strongly on junction temperature and injection current, and to be correlated with vibrations caused by operation of a closed-cycle refrigerator.

Reid, J.↗

NASA NTRS1981

High-resolution atmospheric spectroscopy using a diode laser heterodyne spectrometer

A tunable-diode-laser radiometer with a spectral resolution of 0.007/cm was used to obtain IR solar absorption spectra in the 9.1-11.1 micron region. Spectra exhibiting absorption by HNO3, CO2, O3, and H2O were obtained in a piecewise manner using emission modes from a single Pb-salt laser local oscillator. The measured spectra were compared to synthetic atmospheric spectra calculated using a line-by-line model. The results demonstrate that high-resolution IR solar absorption spectroscopy and atmospheric extinction measurements can be routinely performed over wide spectral regions using semiconductor lasers.

Hoell, J. M., Jr.↗