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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Structural Characterization of Solar Silicon

Compared to single crystal silicon, solar silicon generally contains large residual stresses and numerous structural defects. During subsequent processing, the defect structure can undergo further changes since, at the high temperatures required for diffusion, dislocations are sufficiently mobile to rearrange themselves in patterns which reduce long range residual stresses. This process, which is similar to the polygonization of strained metals, has no counterpart in single crystal silicon. Many of the solar silicon materials are grown from graphite dies and crucibles and therefore contain carbon concentrations in excess of 1E18. The interactions between carbon, crystal defects, intrinsic point defects, oxygen and diffusing dopants are discussed.

Ast, D. G.↗
NASA NTRS1987

Theoretical approach to oxygen atom degradation of silver

Based on available Rutherford backscattering spectrometry (RBS), proton induced X-ray emission (PIXE) and ellipsometry data obtained on silver specimens subjected to atomic oxygen attack in low Earth orbit STS flight 41-G, a theory was developed to model the oxygen atom degradation of silver. The diffusion of atomic oxygen in a microscopically nonuniform medium is an essential constituent of the theory. The driving force for diffusion is the macroscopic electrochemical potential gradient developed between the specimen surface exposed to the ambient and the bulk of the silver specimen. The longitudinal electric effect developed parallel to the gradient is modified by space charge of the diffusing charged species. Lateral electric fields and concentration differences also exist due to the nonuniform nature of the medium. The lateral concentration differences are found to be more important than the lateral electric fields in modifying the diffusion rate. The model was evaluated numerically. Qualitative agreement exists between the kinetics predicted by the theory and kinetic data taken in ground-based experiments utilizing a plasma asher.

Fromhold, Albert T., Jr.↗
NASA NTRS1993

Collision cross sections and diffusion parameters for H and D in atomic oxygen

Modeling the behavior of H and D in planetary exospheres requires detailed knowledge of the differential scattering cross sections for all of the important neutral-neutral and ion-neutral collision processes affecting these species over their entire ranges of interaction energies. In the upper atmospheres of Earth, Venus, and other planets as well, the interactions of H and D with atomic oxygen determine the rates of diffusion of escaping hydrogen isotopes through the thermosphere, the velocity distributions of exospheric atoms that encounter the upper thermosphere, the lifetimes of exospheric orbiters with periapsides near the exobase, and the transfer of momentum in collisions with hot O. The nature of H-O and D-O collisions and the derivation of a data base consisting of phase shifts and the differential, total, and momentum transfer cross sections for these interactions in the energy range 0.001 - 10 eV are discussed. Coefficients of mutual diffusion and thermal diffusion factors are calculated for temperatures of planetary interest.

Hodges, R. R., Jr.↗
NASA NTRS2020

Unexpected Source and Transport of Iron from the Deep Peru Margin

Iron is the most important micronutrient in the ocean, but the nature and magnitude of its sources and sinks to the ocean are poorly constrained. Here we assess our understanding of the sources and sinks of iron in margin environments by synthesizing observations from the U.S. GEOTRACES GP16 Eastern Tropical Pacific Zonal Transect (EPZT) cruise near the Peru margin. GP16 observations showed elevated dissolved iron (dFe) concentrations along the margin, but a larger westward plume of dFe at slope depths (1000-3000 m) in oxygenated waters, rather than at shelf depths (100-300 m) in oxygen deficient waters. We examine the potential explanations for this unexpected observation. Multiple tracers from GP16 suggest that sediment resuspension was important at slope depths, which would lead to enhanced benthic flux of dFe above what was previously measured. The difference in the apparent persistence and penetration of shelf vs slope plumes of dFe into the interior of the ocean likely results from faster removal rates of the shelf dFe compared to slope dFe. DFe sourced from the shelf was almost entirely in the dFe(II) form, whereas dFe sourced from the slope was almost entirely in the dFe(III) form. Although benthic dFe(II) diffuses into oxygen deficient overlying waters, there is still oxidation of dFe(II), which precipitates to particulate Fe(III). In contrast, the slope plume appears to persist in a stabilized dFe(III) form. We hypothesize that sediment porewaters with moderate organic carbon delivery to sediments and shallow oxygen penetration are especially good sources of persistent dFe to the water column.

iron↗
NASA NTRS2004

Chondrites and the Protoplanetary Disk, Part 1

The papers discussed the following: The Formation Process of Adhering and Consorting Compound Chondrules Inferred Their Petrology and Major-Element Composition. The Prospect of High-Precision Pb Isotopic Dating of Meteorites. Evolution of UV-Irradiated Protoplanetary Disks. A Model for the Formation of E Chondrites. Oxygen Isotopic Diffusion and Exchange Experiments on Olivine and Chondrule Melts: Preliminary Results. Shock Heating: Origin of Shock Waves in the Protoplanetary Disk. Thermal Structures of Protoplanetary Disks. Meteoritical Astrophysics: A New Subdiscipline. Origin and Thermal History of FeNi-Metal in Primitive Chondrites. The Collisions of Chondrules Behind Shock Waves. Primary Signatures of the Nebular Dust Preserved in Accretionary Rims and Matrices of CV Chondrites. History of Thermally Processed Solids in the Protoplanetary Disk: Reconciling Theoretical Models and Meteoritical. Evidence Evaporation and Condensation During CAI and Chondrule Formation. Shock Heating: Effects on Chondritic Material. Rhounite-bearing Inclusions E201 and E202 from Efremovka: Constraints from Trace. Element Measurements Element Mapping in Anhydrous IDPs: Identification of the Host Phases of Major/Minor Elements as a Test of Nebula Condensation Models. Theoretical Studies of Disk Evolution Around Solar Mass Stars. Chemical Effects of High-Temperature Processing of Silicates. I-Xe and the Chronology of the Early Solar System. The Effects of X-Rays on the Gas and Dust in Young Stellar Objects. Origin of Short-lived Radionuclides in the Early Solar System. On Early Solar System Chronology: Implications of an Initially Heterogeneous Distribution of Short-lived Radionuclides. The Origin of Short-lived Radionuclides and Early Solar System Irradiation. Disequilibrium Melting and Oxygen Isotope Exchange of CAIs and Chondrules in the Solar Nebula. Mineralogy and Chemistry of Fine-grained Matrices, Rims, and Dark Inclusions in the CR Carbonaceous Chondrites Acfer/El Djouf 001 and the Ungrouped Carbonaceous Chondrites Acfer 094 and Adelaide. Oxygen Isotopes of Aluminum-rich Chondrules from Unequilibrated Enstatite Chondrites.

Source record↗
NASA NTRS1975

Diurnal variations in the thermosphere. I - Theoretical formulation

A nonlinear perturbation theory is formulated for the solution of the multicomponent equations of energy, mass, and momentum conservation in the atmosphere. The theory is three-dimensional and includes the effects of heat conduction and advection, viscosity, ion drag, and diffusion. The theory is described as a superposition of mathematical modes obtained by expanding the physical quantities into vector and spherical harmonics. The coupling between the various modes, both linear and nonlinear, is included. The theory provides a basis for the treatment of the thermosphere and its interaction with the lower atmosphere, where 'mode coupling' is most important. As an example, a comparison is presented between one-dimensional and three-dimensional calculations of the fundamental mode of the diurnal component in the thermosphere. Coupling between the lowest modes is considered to describe the physical conditions of the lower thermosphere where inertia and Coriolis forces become dominant over the ion-drag and viscous forces. In this region, the latitude structures of the temperature, wind field, and diffusively controlled oxygen are shown to change significantly.

Harris, I.↗
NASA NTRS1985

Alkaline battery containing a separator of a cross-linked copolymer of vinyl alcohol and unsaturated carboxylic acid

A battery separator for an alkaline battery is described. The separator comprises a cross linked copolymer of vinyl alcohol units and unsaturated carboxylic acid units. The cross linked copolymer is insoluble in water, has excellent zincate diffusion and oxygen gas barrier properties and a low electrical resistivity. Cross linking with a polyaldehyde cross linking agent is preferred.

Hsu, L. C.↗
NASA NTRS1987

High temperature (900-1300 C) mechanical behaviour of dendritic web grown silicon ribbons - Strain rate and temperature dependence of the yield stress

The mechanical behavior of dendritic web Si ribbons close the melting point was studied experimentally. The goal of the study was to generate data for modeling the generation of stresses and dislocation structures during growth of dendritic web Si ribbons, thereby permitting modifications to the production process, i.e., the temperature profile, to lower production costs for the photovoltaic ribbons. A laser was used to cut specimens in the direction of growth of sample ribbons, which were then subjected to tensile tests at temperatures up to 1300 C in an Ar atmosphere. The tensile strengths of the samples increased when the temperature rose above 1200 C, a phenomena which was attributed to the diffusion of oxygen atoms to the quasi-dislocation sites. The migration to the potential dislocations sites effectively locked the dislocations.

Mathews, V. K.↗
NASA NTRS1995

Flame-Resistant Composite Materials For Structural Members

Matrix-fiber composite materials developed for structural members occasionally exposed to hot, corrosive gases. Integral ceramic fabric surface layer essential for resistance to flames and chemicals. Endures high temperature, impedes flame from penetrating to interior, inhibits diffusion of oxygen to interior where it degrades matrix resin, resists attack by chemicals, helps resist erosion, and provides additional strength. In original intended application, composite members replace steel structural members of rocket-launching structures that deteriorate under combined influences of atmosphere, spilled propellants, and rocket exhaust. Composites also attractive for other applications in which corrosion- and fire-resistant structural members needed.

Spears, Richard K.↗
NASA NTRS1995

Thermo-Oxidative Degradation Of SiC/Si3N4 Composites

Experimental study conducted on thermo-oxidative degradation of composite-material specimens made of silicon carbide fibers in matrices of reaction-bonded silicon nitride. In SiC/Si3N4 composites of study, interphase is 3-micrometers-thick carbon-rich coat on surface of each SiC fiber. Thermo-oxidative degradation of these composites involves diffusion of oxygen through pores of composites to interphases damaged by oxidation. Nondestructive tests reveal critical exposure times.

Baaklini, George Y.↗
NASA NTRS2000

Rhenium/Oxygen Interactions at Elevated Temperatures

The oxidation of pure rhenium is examined from 600-1400 C in oxygen/argon mixtures. Linear weight loss kinetics are observed. Gas pressures, flow rates, and temperatures are methodically varied to determine the rate controlling steps. The reaction at 600 and 800 C appears to be controlled by a chemical reaction step at the surface; whereas the higher temperature reactions appear to be controlled by gas phase diffusion of oxygen to the rhenium surface. Attack of the rhenium appears to be along grain boundaries and crystallographic planes.

Jacobson, Nathan↗
NASA NTRS2001

A Rocket Investigation of Mesospheric Eddy Diffusion Effects on Airglow and Oxygen Chemistry

A Terrier Orion rocket was launched at 0750 Z on 02/25/98 about seven minutes after the Clemson University chemical release rocket. Measurements made of the electron density by a dc probe calibrated by a capacitance probe showed several layers of electron density on a rocket ascent in the altitude range from 90 to 110 km. Rocket descent results showed several but not all of the ascent structure. From power spectral analysis of the measured electron densities, turbulent parameters are derived Measurements were made on rocket ascent and descent by an infrared radiometer of the OH Meinel (3-1) band and O2 singlet delta emissions. Profiles of the emissions are presented and discussed on both rocket ascent and descent an enhancement of the OH emission monitored by the OH radiometer was observed above 90 km. The glow was not defected by the O2 radiometer and was significantly reduced on rocket descent. Using these data and a mechanistic analysis, a profile proportional to atomic oxygen is obtained. This profile is compared to one from the ATOX probe on the rocket. A one-dimensional (1-D) photochemical model that solves the time-dependent continuity equations is used with the rocket data to investigate the odd-oxygen concentration in the near equatorial mesosphere.

Ulwick, James C.↗
NASA NTRS2001

Optimized suspension culture: the rotating-wall vessel

Suspension culture remains a popular modality, which manipulates mechanical culture conditions to maintain the specialized features of cultured cells. The rotating-wall vessel is a suspension culture vessel optimized to produce laminar flow and minimize the mechanical stresses on cell aggregates in culture. This review summarizes the engineering principles, which allow optimal suspension culture conditions to be established, and the boundary conditions, which limit this process. We suggest that to minimize mechanical damage and optimize differentiation of cultured cells, suspension culture should be performed in a solid-body rotation Couette-flow, zero-headspace culture vessel such as the rotating-wall vessel. This provides fluid dynamic operating principles characterized by 1) solid body rotation about a horizontal axis, characterized by colocalization of cells and aggregates of different sedimentation rates, optimally reduced fluid shear and turbulence, and three-dimensional spatial freedom; and 2) oxygenation by diffusion. Optimization of suspension culture is achieved by applying three tradeoffs. First, terminal velocity should be minimized by choosing microcarrier beads and culture media as close in density as possible. Next, rotation in the rotating-wall vessel induces both Coriolis and centrifugal forces, directly dependent on terminal velocity and minimized as terminal velocity is minimized. Last, mass transport of nutrients to a cell in suspension culture depends on both terminal velocity and diffusion of nutrients. In the transduction of mechanical culture conditions into cellular effects, several lines of evidence support a role for multiple molecular mechanisms. These include effects of shear stress, changes in cell cycle and cell death pathways, and upstream regulation of secondary messengers such as protein kinase C. The discipline of suspension culture needs a systematic analysis of the relationship between mechanical culture conditions and biological effects, emphasizing cellular processes important for the industrial production of biological pharmaceuticals and devices.

Non-NASA Center↗
NASA NTRS2003

Polymer/Silicate Nanocomposites Developed for Improved Strength and Thermal Stability

Over the past decade, polymer-silicate nanocomposites have been attracting considerable attention as a method of enhancing polymer properties. The nanometer dimensions of the dispersed silicate reinforcement can greatly improve the mechanical, thermal, and gas barrier properties of a polymer matrix. In a study at the NASA Glenn Research Center, the dispersion of small amounts (less than 5 wt%) of an organically modified layered silicate (OLS) into the polymer matrix of a carbon-fiber-reinforced composite has improved the thermal stability of the composite. The enhanced barrier properties of the polymer-clay hybrid are believed to slow the diffusion of oxygen into the bulk polymer, thereby slowing oxidative degradation of the polymer. Electron-backscattering images show cracking of a nanocomposite matrix composite in comparison to a neat resin matrix composite. The images show that dispersion of an OLS into the matrix resin reduces polymer oxidation during aging and reduces the amount of cracking in the matrix significantly. Improvements in composite flexural strength, flexural modulus, and interlaminar shear strength were also obtained with the addition of OLS. An increase of up to 15 percent in these mechanical properties was observed in composites tested at room temperature and 288 C. The best properties were seen with low silicate levels, 1 to 3 wt%, because of the better dispersion of the silicate in the polymer matrix.

Campbell, Sandi G.↗
NASA NTRS2005

The current development status of the Orbiting Carbon Observatory (OCO) instrument optical design

The status of the OCO instrument optical design is presented in this paper. The optical bench assembly comprises three cooled grating spectrometers coupled to an all-reflective telescope/relay system. Dichroic beam splitters are used to separate the light from a common telescope into the three spectral bands. The three bore sighted spectrometers allow the total column CO2 absorption path to be corrected for optical path and surface pressure uncertainties, aerosols, and water vapor. The design of the instrument is based on classic flight proven technologies.

global carb↗
NASA NTRS2006

pH Sensitive Microcapsules for Delivery of Corrosion Inhibitors

A considerable number of corrosion problems can be solved by coatings. However, even the best protective coatings can fail by allowing the slow diffusion of oxygen and moisture to the metal surface. Corrosion accelerates when a coating delaminates. Often, the problems start when microscopic nicks or pits on the surface develop during manufacturing or through wear and tear. This problem can be solved by the incorporation of a self-healing function into the coating. Several new concepts are currently under development to incorporate this function into a coating. Conductive polymers, nanoparticles, and microcapsules are used to release corrosion-inhibiting ions at a defect site. The objective of this investigation is to develop a smart coating for the early detection and inhibition of corrosion. The dual function of this new smart coating system is performed by pH-triggered release microcapsules. The microcapsules can be used to deliver healing agents to terminate the corrosion process at its early stage or as corrosion indicators by releasing dyes at the localized corrosion sites. The dyes can be color dyes or fluorescent dyes, with or without pH sensitivity. Microcapsules were formed through the interfacial polymerization process. The average size of the microcapsules can be adjusted from 1 to 100 micron by adjusting the emulsion formula and the microcapsule forming conditions. A typical microcapsule size is around 10 microns with a narrow size distribution. The pH sensitivity of the microcapsule can also be controlled by adjusting the emulsion formula and the polymerization reaction time. Both corrosion indicator (pH indicator) and corrosion inhibitor containing microcapsules were formed and incorporated into paint systems. Test panels of selected steels and aluminum alloys were painted using these paints. Testing of compatibility between the microcapsule system and different paint systems are in progress. Initial experiments with the microcapsule containing paint show visible color changes at induced corrosion sites and improvement of corrosion protection. Further investigation of the performance of the coating using electrochemical techniques and long term exposure are currently underway.

Li, Wenyan↗