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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Boundary conditions at grain boundaries

Grain boundaries in polycrystalline semiconductor material are frequently idealized as surfaces possessing a characteristic surface recombination velocity. Minority carrier densities generated by external sources (electron beams, light, etc.) in polycrystalline p-n junctions must satisfy certain boundary conditions at these grain boundaries which will be derived. It will also be shown that a 'folding technique' introduced recently to deal with this problem is of limited value.

Von Roos, O.↗
NASA NTRS1984

Crystallization of the glassy phase of grain boundaries in silicon nitride

Three types of hot-pressed silicon nitride specimens (containing 5wt% Y2O3 and 2wt% Al2O3 additives) which were subjected to different temperature heat treatments were studied by X-ray diffraction, X-ray microanalysis and high resolution electron microscopy. The results indicated that there were phase changes in the grain boundaries after heat treatment and the glassy phase at the grain boundaries was crystallized by heat treatment.

Jefferson, D. A.↗
NASA NTRS1983

Theoretical studies of the effects of grain noise on photographic stellar astrometry and photometry

A method of incorporating the effects of photographic emulsion grain noise into digital image centering algorithms is presented which improves the accuracy of the derived stellar positions and magnitudes. Theoretical formulae are then derived for the limiting error of the center, and the photometric parameters. For IIIa-J, this error is 0.2-0.3 mu for bright unsaturated images, which agrees quite well with measurements made with the Yale PDS microdensitometer. It is expected that, with further improvements in the positional accuracy of the PDS, it should be possible to reach the emulsion grain noise limit, providing that emulsion shifts or other large scale errors do not dominate. It is also shown that, with appropriate trimming, marginal distribution image centering algorithms can yield an accuracy only slightly poorer than that obtained with two-dimensional distributions.

Lee, J.-F.↗
NASA NTRS1982

Enhanced diffusion of phosphorus at grain boundaries

Enhanced diffusion of phosphorus at grain boundaries in cast polycrystalline photovoltaic materials (Wacker, HEM, and Semix) was studied. It was found that the enhancements for the three materials were the same, indicating that the properties of boundaries are similar, even though they were grown by different techniques. In addition, it was observed that grain boundaries capable of enhancing the diffusion always have strong recombination activities. Both phenomena could be related to dangling bonds existing at the boundaries. The present study gives the first evidence that incoherent second order twins of 111/115-plane type are diffusion-active.

Cheng, L. J.↗
NASA NTRS1984

Comet Cernis - Icy grains at last?

An absorption feature at 2.9-3.0 microns has been detected for the first time in comet Cernis 19831 at 3.3 AU. The feature is interpreted as scattering by ice grains in the comet's coma. The thermal emission spectrum indicates that hot, micron-sized absorbing grains are also present in the coma.

Hanner, M. S.↗
NASA NTRS1983

Properties of ices and grains - An experimental study

Proton irradiation of a variety of ice mixtures was carried out to study the effect of galactic cosmic ray irradiation of comets in the Oort Cloud. Three significant effects were noted: (1) production of new molecules; (2) production of a more energetic ice at low temperatures; (3) production of a non-volatile, complex organic residue. These phenomena suggest various effects on new comets approaching the sun including enhanced activity. Experiments on the condensation of silicate grains provide information on the condensation mechanism and properties of grains. Controlled annealing of the amorphous condensates shows how crystallization occurs. Infrared spectra of different stages of crystallization contain features that may identify composition, structure and history of refractory material.

Donn, B.↗
NASA NTRS1985

Operational performance of the photovoltaic-powered grain mill and water pump at Tangaye, Burkina Faso (formerly Upper Volta)

A photovoltaic (PV) system powering a grain mill and water pump was installed in the remote African village of Tangaye, Burkina Faso (formerly Upper Volta) under the sponsorship of the U.S. Agency for International Development (AID) and by the National Aeronautics and Space Administration (NASA) Lewis Research Center (LeRC) in early 1979. The presence reports covers the second two years of operation from April 1981 through June 1983. During this time, the grain mill and water pump were operational 96 and 88 percent of the time respectively, and the PV system generated sufficient electricity to enable the grinding of about 111 metric tons of finely ground flow and the pumping of over 5000 cm sq of water from the 10 m deep well. The report includes a description of the current configuration of the system, a review of system performance, a discussion of the socioeconomic impact of the system on the villagers and a summary of results and conclusions covering the entire four-year period.

Martz, J. E.↗
NASA NTRS1983

Early season spring small grains direct proportion estimation - Development and evaluation of a Landsat based methodology

The Inventory Technology Development (ITD) project of the Agriculture and Resources Inventory Surveys Through Aerospace Remote Sensing (AgRISTARS) program has developed an accurate, automated technology for early season estimation of spring small grains areal proportion from Landsat MSS data. The design criteria for an early season procedure included estimates available within the first 30 days of the growing season, low data processing/preprocessing requirements and no need for scene-to-scene registration. The prototype estimator which meets the design goals is based on a constrained linear model in which the observed spectral response of an entire scene is modeled as a linear combination of the major constituent elements in the scene. The procedure was evaluated over 100 sample segments collected for crop years 1976 through 1979 in the U.S. Northern Great Plains. Analysis of the test results indicated accuracy that compare favorably with both the automated at-harvest technologies tested during the FY81-82 AgRISTARS Spring Small Grains Pilot experiments and earlier analyst-intensive at-harvest technologies.

Phinney, D. E.↗
NASA NTRS1984

Evolution of grains in a turbulent solar nebula

In the present computation of the evolution of grain size distribution in a turbulent disk for turbulent solar nebula models, using realistic collisional outcomes and strengths of grain aggregates, the half space above the central plane of the disk is divided into a series of levels. At each level, the size distribution is represented by the population of a series of discrete size bins, each spanning an order of magnitude in mass. Coagulation, erosion, or disruption, occur for various combinations of relative particle sizes and velocities, together with assumed material strength, based on experimental impact data. The calculation is one-dimensional, in that it is performed at a single radial location in the disk.

Weidenschilling, S. J.↗
NASA NTRS1985

Effects of multiquantum transitions on molecular populations in grain-forming circumstellar environments

Simplification of astrophysical calculations can be achieved by invoking the condition of local thermodynamic equilibrium; however, recent investigations suggest that this assumption may not be valid for certain astrophysical regions. To examine the effects of multiquantum translation to vibration transitions in expanding circumstellar envelopes, vibrational populations of the lowest 20 levels of CO have been calculated as a function of pressure and radiation density for H atom-CO collisions. Significant departure from local thermodynamic equilibrium is indicated, which implies lower dissociation rates for molecular components and a subsequent enhancement in the rate of grain formation by many orders of magnitude. Stabilization of intermediate species before they can dissociate may facilitate the formation of refractory grain cores in very hot, dilute outflows. As the present calculations indicate, laboratory measurements of state-to-state translation to vibration rates are needed for a more complete understanding of circumstellar chemistry.

Nuth, J. A.↗
NASA NTRS1986

Spring Small Grains Area Estimation

SSG3 automatically estimates acreage of spring small grains from Landsat data. Report describes development and testing of a computerized technique for using Landsat multispectral scanner (MSS) data to estimate acreage of spring small grains (wheat, barley, and oats). Application of technique to analysis of four years of data from United States and Canada yielded estimates of accuracy comparable to those obtained through procedures that rely on trained analysis.

Palmer, W. F.↗
NASA NTRS1986

Interstellar grains

There are few aspects of interstellar grains that can be unambiguously defined. Very little can be said that is independent of models or presuppositions; hence issues are raised and questions categorized, rather than providing definitive answers. The questions are issues fall into three general areas; the general physical and chemical nature of the grains; the processes by which they are formed and destroyed; and future observational approaches.

Snow, T. P.↗
NASA NTRS1986

Preparation, analysis and release of simulated interplanetary grains into low Earth orbit

Astronomical observations which reflect the optical and dynamical properties of interstellar and interplanetary grains are the primary means of identifying the shape, size, and the chemistry of extraterrestrial grain materials. Except for recent samplings of extraterrestrial particles in near-Earth orbit and in the stratosphere observations were the only method of deducing the properties of extraterrestrial particles. In order to elucidate the detailed characteristics of observed dust, the observations must be compared with theoretical studies, some of which are discussed in this volume, or compared with terrestrial laboratory experiments. The formation and optical characterization of simulated interstellar and interplanetary dust with particular emphasis on studying the properties on irregularly shaped particles were discussed. Efforts to develop the techniques to allow dust experiments to be carried out in low-Earth orbit were discussed, thus extending the conditions under which dust experiments may be performed.

Stephens, J. R.↗
NASA NTRS1986

Photolysis products of CO, NH3 aND H2O and their significance to reactions on interstellar grains

With the increase in evidence that interstellar grains are the basic building blocks of comets and with the realization that comet collisions with the earth have probably occured at a much higher frequency than earlier assumed it may be presumed that interstellar dust chemistry played an important role in the early chemistry of the earth. As a part of the study of the photochemical processes taking place on interstellar grains the photolysis of mixtures of CO, NH3 and H2O was performed at 10 K, 77K and 298K. The reaction products were determined by GC/MS and HPLC analysis to be lactic acid, glycolic acid, hydroxyacetamide, urea, biuret, oxamic acid, oxamide, glyceric acid and glyceramide. Ethylene glycol and glycerol were also detected but is is not clear at present whether these are true photoproducts or contaminants. The mechanism of formation of these molecules are discussed as well as their possible significance to the origins of life.

Ferris, J. P.↗
NASA NTRS1986

Solar wind helium, neon and argon released by oxidation of metal grains from the Weston chondrite

A set of experiments were carried out to test the feasibility of determining unfractionated elemental and isotopic ratios for the noble gases in the presumably ancient solar wind present in the gas rich meteorites. The problems of diffusive loss was avoided by analyzing metal rather than the usual silicates. In order to avoid chemical, and even harsh physical, treatment of the sample, which might have affected the surfaces of metal grains, a means of analyzing the metal in the presence of residual silicate not removed by gentle crushing and magnetic separation was devised. Preliminary results given were obtained by taking advantage of the differing properties of metal and silicates with regard to diffusion. The results suggests that, with some modifications in the choice of pyrolysis and combustion temperatures and in the amount of O2 used, it should be possible, by oxidizing the surfaces of metal grains from gas rich meteorites, to obtain data on solar wind that has not been fractionated by diffusive loss.

Becker, R. H.↗
NASA NTRS1986

Determination of grain size distribution function using two-dimensional Fourier transforms of tone pulse encoded images

Microstructural images may be tone pulse encoded and subsequently Fourier transformed to determine the two-dimensional density of frequency components. A theory is developed relating the density of frequency components to the density of length components. The density of length components corresponds directly to the actual grain size distribution function from which the mean grain shape, size, and orientation can be obtained.

Generazio, E. R.↗
NASA NTRS1985

Compatibility of grain-stabilized platinum with candidate propellants for resistojets

An examination is conducted into the suitability of grain-stabilized Pt as a resistojet thruster material for Space Station auxiliary propulsion. A series of 1000-hour tests was conducted in CO2, H2, and NH3 at 1400 C; another series was conducted at 500 C in CH4 for the same duration. SEM, Auger electron microscopy, and depth profiling analysis were used to determine the effects of propellants on the material surface as well as to evaluate possible material contamination and possible grain growth. Carbon deposition is noted on the surface of Pt/Y2O3 and Pt/ZrO2 in both the CO2 and CH4 environments.

Whalen, M. V.↗
NASA NTRS1985

Dark grains in comet Crommelin

Comet P/Crommelin (1983n) was observed at 1-20 microns at the NASA IRTF March 29-April 1, 1984. The thermal emission spectrum fits a black body of about 290 K, cooler than other comets observed at 1 AU. The ratio of thermal to scattered radiation indicates that these grains were extremely dark with a geometric albedo of about 0.015 at J(1.25 microns) and 0.02 at K(2.25 microns). The 10 micron flux distribution on April 1 was very asymmetric, decreasing rapidly on the sunward side, less than 6000 km from the nucleus, and extended in the north and east. The spatial distribution and photometric data suggest that large grains emitted from the nucleus at very low velocity were viewed.

Hanner, M. S.↗