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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Geochemistry of grain-size fractions of soils from the Taurus-Littrow valley floor

Results are presented for a study in which high-precision instrumental neutron activation analysis was applied to determine the abundances of seven rare-earth and nine other elements in two grain-size fractions (90 to 150 microns and less than 20 microns) of eight soils from the Taurus-Littrow Valley floor and one Apollo 11 bulk-soil fraction with grain sizes of less than 1 mm. Compositional differences between the two size fractions of two valley-floor soils are examined, and mixing of soil components is investigated. It is found that a five-component mixing model describes very adequately the chemical composition of bulk soils with grain sizes of less than 1 mm as mixtures of local Apollo 17 rock types (basalt, anorthositic gabbro, noritic breccia), orange glass, and meteorites, but does not describe well the chemical compositions of the other two size fractions. A ten-component model is used to show that the compositions of those two size fractions can be well represented as mixtures of the five components if the mineralogy and chemical composition of the basalt component are allowed to vary in the size fractions.

Korotev, R. L.↗
NASA NTRS1977

An upper limit for the flux of interstellar dust grains in the solar system

Data from the LEAM (Lunar Ejecta and Meteorite experiment, a micrometeorite detector at the Apollo 17 landing site) have been examined for evidence of interstellar (IS) dust grains traversing the solar system. The analysis technique considers IS grains approaching the solar system from the local solar apex. A model calculates the grains' hyperbolic orbits into the solar system and predicts the impact directions on the moon. The observations are then compared with the predicted impact directions to measure the IS dust flux. No evidence has been found (at the 97.5% confidence level) for a flux greater than about 6 hundred-thousandths per sq m/sec for particles of at least 2 by 10 to the -14th power g.

Tomandl, D. A.↗
NASA NTRS1978

On the interactions between large-scale structure and fine-grained turbulence in a free shear flow. II - The development of spatial interactions in the mean

The interactions between large-scale structure and fine-grained turbulence in a spatially developing mixing layer are investigated. Both a conditioned average and a time average are employed to separate the nonrandom and random fluctuations in the kinetic energy treatment of the interactions. Shape assumptions applying to the interaction integrals are adopted to obtain a solution to the closure problem. Energy-transfer mechanisms among the mean flow, the fine-grained turbulence and the large-scale structure are discussed. The magnitude of the large-scale structure and its streamwise lifetime both prove to be important in accounting for fine-grained turbulence enhancement.

Alper, A.↗
NASA NTRS1978

An experimental investigation of the condensation of silicate grains

Results are presented for a series of laboratory experiments designed to investigate the nucleation of small silicate grains from a vapor of astrophysically significant elements and compounds. In the experiments, magnesium silicate grains were condensed by simultaneously evaporating Mg and SiO solids into an atmosphere of argon or hydrogen at a pressure of a few torr. The results show that at low temperatures (up to a few hundred degrees C) the condensates are amorphous grains and have widely varying stoichiometries. The thermodynamically most stable compounds (Mg2SiO4, MgSiO3, SiO2) do not form readily, but all initial condensates can be converted to crystalline forsterite (Mg2SiO4) by heating to 1000 C in vacuum. At higher temperatures (above 700 K) it becomes more difficult to nucleate any silicates, and those that do form are amorphous, indicating that surface energies and kinetic effects are very important in determining under what conditions condensation will occur. The IR spectra of the experimentally produced magnesium silicates are found to have a strong resemblance to those observed in many astronomical clouds.

Day, K. L.↗
NASA NTRS1977

C-14 in lunar soil - Temperature-release and grain-size dependence

C-14 is released from surface soils by heatings between 600 and 1000 C. The amounts of 600-1000-C C-14 obtained increase inversely with grain diameter for the 10-30 and 74-125-micron size fractions. The inverse relation shows that the 600-1000-C C-14 is located on the surface of grains and probably is of solar-wind origin. The amount released at these temperatures gives 0.002 C-14 ions per sq cm/sec for the implantation rate and corresponds to a C-14/H ratio of the order of 4 hundred-billionths for the solar wind. This result from the grain-size dependence is in accord with C-14 depth-variation studies. The C-14 released above melting temperatures can be attributed to spallation production.

Fireman, E. L.↗
NASA NTRS1978

Dust from periodic comet Encke - Large grains in short supply

A photographic observation of periodic comet Encke is analyzed which reveals the emission of large dust grains from the comet. Photometric reduction of the photographic plate is described, and a two-dimensional isodensitometer scan of P/Encke is presented. An analytical expression is obtained for the average mass-loss rate of the dust over the revolution period of about 1200 days. A comparison of the results for P/Encke with previous results for P/d'Arrest indicates that both comets shed dust at just about equal average rates. It is concluded that the current contribution of P/Encke to the interplanetary dust is quite negligible if the dust particles reflect more than 1% of light and that smaller dust grains probably tend to contribute more mass than larger grains.

Sekanina, Z.↗
NASA NTRS1978

Effect of grain boundaries in silicon on minority-carrier diffusion length and solar-cell efficiency

The spatial variation of minority-carrier diffusion length in the vicinity of a grain boundary for a polycrystalline silicon sheet has been measured by the use of the EBIC technique. The effect of such a variation on solar-cell output has then been computed as a function of grain size. Calculations show that the cell output drops considerably for grain size smaller than three times the bulk diffusion length.

Daud, T.↗
NASA NTRS1980

On the interactions between large-scale structure and fine-grained turbulence in a free shear flow. III - A numerical solution

The results of a numerical computation of the interactions between the horizontally periodic monochromatic component of a large-scale coherent structure and the fine-grained turbulence in a mixing layer are presented. In the numerical calculations, the appropriate dependent variable is one which comprises both the mean and the large-scale coherent structure. The dynamical equations obtained for such a total coherent structure quantity are identical to the unsteady equations for the mean quantities in the Reynolds sense, except that the fine-grained turbulent stresses are interpreted as being conditionally averaged. It is shown how three dimensional fine-grained turbulence can be produced indirectly from the two dimensional large-scale structure via the isotropizing process of the approximated pressure-strain correlation.

Gatski, T. B.↗
NASA NTRS1979

Ultraviolet observations of interstellar molecules and grains from Spacelab

The paper presents observational results obtained on interstellar grains and molecules and suggests promising areas for Spacelab research. Regarding grains, useful data can be expected on the shape of the ultraviolet extinction curve for new interstellar regions, on the nature of UV extinction at short wavelengths, the presence of structure in the UV extinction curve comparable to the visible-wavelength diffuse bands, and the polarization properties of grains in UV wavelengths. The advances in observations of molecules will include the ability to prove more heavily-obscured regions, coverage of wavelength regions inadequately studied with previous instruments, and the capability of observing in optical absorption species detected in the same line of sight in radio emission.

Snow, T. P., Jr.↗
NASA NTRS1979

Grain quality inspection system

A review of grain quality indicators and measurement methods was conducted in order to assess the feasibility of using remote sensing technology to develop a continuous monitoring system for use during grain transfer operations. Most detection methods were found to be too slow or too expensive to be incorporated into the normal inspection procedure of a grain elevator on a continuous basis. Two indicators, moisture content and broken corn and foreign material, show potential for automation and are of an economic value. A microprocessor based system which utilizes commercially available electronic moisture meter was developed and tested. A method for automating BCFM measurement is described. A complete system description is presented along with performance test results.

Flood, C. A., Jr.↗
NASA NTRS1980

Behavior of molecules on interstellar grains - Application of the Langevin equation and iterative extended Hueckel

The Langevin equation was used to explore an adsorbate desorption mechanism. Calculations were performed using iterative extended Hueckel on a silica model site with various small adsorbates, e.g., H, CH, OH, NO, CO. It was found that barriers to free traversal from one site to another are substantial (about 3-10 eV). A bootstrap desorption mechanism for some molecules in the process of forming at a site also became apparent from the calculations. The desorption mechanisms appear to be somewhat balanced by a counterforce - the attraction of sites for the newly desorbed molecule. The order of attraction to a silica grain site for the diatomic molecules considered was OH greater than CH greater than CO greater than NO, when these entities were sufficiently distant. The nature of the silica grain and that of the 'cold' desorption mechanism, when considered together, suggest that the abundance of very small grains might be less common than anticipated.

Aronowitz, S.↗
NASA NTRS1981

A once-molten, coarse-grained, Ca-rich inclusion in Allende

A compact, spheroidal Type B inclusion in Allende contains melilite laths that project radially inward from the inclusion edge which show interference growth textures. The combined textural and chemical features of this object cannot be explained by independent vapor-solid condensation of grains in space, followed by random aggregation of these grains into an inclusion. Instead, it probably formed from a once-molten droplet that crystallized in response to radiative cooling from its outer surface. The crystallization sequence in this and another similar inclusion in which oxygen isotopes have been measured is melilite-spinel-anorthite-fassaite; this sequence shows that oxygen isotopic heterogeneities in coarse-grained inclusions were formed after complete solidification of these objects by partial exchange with a less O-16-rich gas, and not during or before a melting event.

Macpherson, G. J.↗
NASA NTRS1980

Grain size dependence of silicon solar cell parameters

Measurements of the non-uniform diffusion length of the minority carriers near grain boundaries in polycrystalline silicon have been used to develop an analytical model for the calculation of solar cell output as a function of grain size. Experimental results are presented which verify the theoretical analysis. Variation of open circuit voltage and fill factor with grain size is discussed.

Koliwad, K. M.↗
NASA NTRS1982

Convection in grain refining

The relationship between fluid flow phenomena, nucleation, and grain refinement in solidifying metals both in the presence and in the absence of a gravitational field was investigated. The reduction of grain size in hard-to-process melts; the effects of undercooling on structure in solidification processes, including rapid solidification processing; and control of this undercooling to improve structures of solidified melts are considered. Grain refining and supercooling thermal modeling of the solidification process, and heat and fluid flow phenomena in the levitated metal droplets are described.

Flemings, M. C.↗
NASA NTRS1980

Lunar core 15010/11 - Grain size, petrology, and implications for regolith dynamics

Petrographic observations of six samples from various depths show that core 15010/11 is relatively homogeneous and consists mostly of basalt fragments and minerals derived from basalts. However, it also contains an appreciable highland component nearly equally distributed among the fine-grained fraction, dark matrix breccia and identifiable highland grains, and agglutinates. Grain size data and agglutinates show that this core is not mature and does not contain a well developed reworking profile. The unusual properties of the core are thought to be a direct result of its close proximity to the rim of Hadley Rille. This location favors relatively rapid regolith erosion into the rille, produces a thin regolith, and provides an abundant supply of bedrock-derived ejecta.

Mckay, D. S.↗
NASA NTRS1982

Stability of negatively charged dust grains in Saturn's ring plane

The boundary between the inner and outer parts of Saturn's B ring is located at the theoretical limit of stability of dust grains with large negative charge to mass ratio. A grain inside of this stability limit will move along magnetic field lines and strike Saturn if given a slight velocity component normal to the ring plane. Outside of this marginal stability radius, a perturbed grain merely oscillates back and forth through the ring plane. The theoretical location of the marginal stability radius is at 1.625 Saturn radius. Observations by Pioneer 11 and Voyager 2 in the infared see the boundary as a prominent change in ring brightness at this radius. The occultation of delta-Scorpii by the rings in the ultraviolet seen by Voyager 2 shows about a factor of two change in optical depth beginning very close to this radius.

Northrop, T. G.↗
NASA NTRS1982

Recrystallization and grain growth in NiAl

Aluminide intermetallics, because of their strength, microstructural stability, and oxidation resistance at elevated temperatures, represent potential structural materials for use in advanced energy conversion systems. This inherent potential of the intermetallics can currently not be realized in connection with the general brittleness of the materials under ambient conditions. It is pointed out, however, that brittleness is not an inherent characteristic. Single crystals are ductile and polycrystals may be, too, if their grains are fine enough. The present investigation is concerned with an approach for reducing material brittleness, taking into account thermal-mechanically induced grain refinement in NiAl, a B2 aluminide which melts at 1638 C and which retains complete order to its melting point. Attention is given to the kinetics of recrystallization and grain growth of warm-worked, nickel-rich material.

Haff, G. R.↗
NASA NTRS1982

Mechanisms of devitrification of grain boundary glassy phases in Si3N4 materials

Changes in the grain boundary (g.b.) phases of Si3N4 are analyzed, the effects of composition and thermal history on devitrification of the g.b. phases are determined, devitrification of the g.b. phases of Si3N are related to mechanical behavior and oxidation sensitivity of the material. The phase relationships that occur within the grain boundaries of Si3N4 containing various densification aids are reviewed. Comparisons of the effects of MgO, Y2O3, CeO2, and Y2O3 + AL2O3 are made in terms of the phase equilibria of the Si3N4 + SiO2 + additive compositional system. Two new equilibrium phase diagrams for the Si3N4-SiO2 and Y2O3 and Si3N4-SiO2-Ce2O3 systems are preented. The effects of Y2O3 vs CeO2 densification aids on the fracture surfaces of Si3N4 are compared. Auger electron spectroscopy shows that both oxides are concentrated within the fracture surface. Scanning electron microscopy shows evidence that Si3N4 with CeO2 formed an intergranular structure of fine grained oxynitride reaction products, as predicted by phase quilibria, whereas the Y2O3 containing sample shows evidence of an intergranular glassy phase.

Hench, L. L.↗