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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

A note on dust grain charging in space plasmas

Central to the study of dust-plasma interactions in the solar system is the electrostatic charging of dust grains. While previous calculations have generally assumed that the distributions of electrons and ions in the plasma are Maxwellian, most space plasmas are observed to have non-Maxwellian tails and can often be fit by a generalized Lorentzian (kappa) distribution. Here we use such a distribution to reevaluate the grain potential, under the condition that the dominant currents to the grain are due to electron and ion collection, as is the case in certain regions of space. The magnitude of the grain potential is found to be larger than that in a Maxwellian plasma as long as the electrons are described by a kappa distribution: this enhancement increased with ion mass and decreasing electron kappa. The modification of the grain potential in generalized Lorentzian plasmas has implications for both the physics (e.g., grain growth and disruption) and the dynamics of dust in space plasmas. These are also briefly discussed.

Rosenberg, M.↗
NASA NTRS1993

Effect of metamorphism on isolated olivine grains in CO3 chondrites

The presence of a metamorphic sequence in the CO3 chondrite group has been shown previously to result in changes in properties of chondrule silicates. However, the role of isolated olivine grains during metamorphism of these chondrites has not been addressed. Isolated olivine grains in two metamorphosed CO3 chondrites, Lance and Isna, have been investigated in this study in order to assess the compositional properties of isolated olivine grains that may be attributable to metamorphism. Compositional changes in isolated olivines with increasing petrologic subtype are very similar to changes in chondrule olivines in the same chondrites. Olivine compositions from all occurrences (chondrules, isolated grains, and matrix) converge with increasing petrologic subtype. The degree of equilibration of minor elements is qualitatively related to the diffusion rate of each element in olivine, suggesting that diffusion-controlled processes are the most important processes responsible for compositional changes within the metamorphic sequence. The data are consistent with metamorphism taking place in a closed system on the CO3 chondrite parent body. Fe-poor olivine grains in metamorphosed chondrites are characterized by an Fe-rich rim, which is the result of diffusion of Fe into the grains from Fe-rich matrix. In some instances, 'complex', Fe-rich rims have been identified, which appear to have originated as igneous overgrowths and subsequently to have been overprinted by diffusion processes during metamorphism. Processes experienced by CO3 chondrites are more similar to those experienced by the ordinary chondrites than to those encountered by other carbonaceous chondrites, such as the CV3 group.

Jones, Rhian H.↗
NASA NTRS1994

Formation of dust grains with impurities in red giant winds

Among the several proposed carriers of diffuse interstellar bands (DIB's) are impurities in small dust grains, especially in iron oxide grains (Huffman 1977) and silicate grains (Huffman 1970). Most promising are single ion impurities since they can reproduce the observed band widths (Whittet 1992). These oxygen-rich grains are believed to originate mostly in the mass flows from red giants and in supernovae ejecta (e.g. Gehrz 1989). A question of considerable impact for the origin of DIB's is therefore, whether these grains are produced as mainly clean crystals or as some dirty materials. A formalism has been developed that allows tracking of the heterogeneous growth of a dust grain and its internal structure during the dust formation process. This formalism has been applied to the dust formation in the outflow from a red giant star.

Dominik, Carsten↗
NASA NTRS1994

Chondrule remelting: Evidence from coarse-grained chondrule rims and compound chondrules

The meteorites that best preserve the nebular record are the type 3 ordinary (OC) and carbonaceous chondrites; their major components include chondrules and chondrule fragments, refractory and mafic inclusions, FeO-rich silicate matrix material. Many chondrules are surrounded by nms; these can be divided into two major types: (1) fine-grained rims, which are composed of matrix material; and (2) igneous or coarse-grained rims, which have igneous textures and larger, less-ferroan mafic silicate grains. Fine-grained rims surround most of the chondrules in the least-equilibrated type 3 ordinary and carbonaceous chondrites. They were most likely derived via collapse of clumps of nebular dust that accreted around coarse objects such as chondrules and inclusions. Coarse-grined or igneous rims surround approximately 10% of the chondrules in type 3 OC and approximately 50% of the chondrules in CV3 chondrites as well as some chondrule fragments and isolated mineral grains. They probably formed by partly melting finer-grained dust-rich precursors and admixed chondrule fragments.

Rubin, A. E.↗
NASA NTRS1994

Oxygen isotopes in circumstellar Al203 grains from meteorites and stellar nucleosynthesis

We have found a approximately 3 micrometer Al2O3 grain (B39) in the Bishunpur LL3.1 chondrite that is enriched in 0-17 by a factor of approximately 6.8 (0-16/0-17 = 385 +/- 9) and depleted in 0-18 by almost 60% (0-16/0-18 = 853 +/- 30) relative to solar system oxygen and has an initial Al-26/Al-27 = 1.7 +/- 0.2 x 10 (exp 3), approximately 34 times greater than the initial solar system value. The isotopic compositions of B39 and two other Al2O3 grains previously reported from the Orgueil and Murchison meteorites show that these grains formed directly from the ejecta of low-mass AGB stars with C/O less than 1. A simple theoretical analysis is presented showing that the oxygen systematics of the grains are a natural consequence of main-sequence evolution followed by first dredge-up Al-26 is the result of third dredge-up. Circumstellar Al2O3 grains provide very precise isotopic data for stellar ejecta that complement spectroscopic observations of oxygen-rich stars. Isotopic differences indicate that the Al2O3 grains come from separate stars of different mass and intital oxygen composition that originated in molecular clouds different from the one in which the solar system formed.

Huss, Gary R.↗
NASA NTRS1994

Composition and radiative properties of grains in molecular clouds and accretion disks

We define a model of the compositon and abundances of grains and gases in molecular cloud cores and accretion disks around young stars by employing a wide range of astronomical data and theory, the composition of primitive bodies in the solar system, and solar elemental abundances. In the coldest portions of these objects, we propose that the major grain species include olivine (Fe, Mg, 2SiO4), orthopyroxene (Fe, Mg, SiO3), volatile and refractory organics, water ice, troilite (FeS), and metallic iron. This compositional model differs from almost all previous models of the interstellar medium (ISM) by having organics as the major condensed C species, rather than graphite; by including troilite as a major grain species; and by specifying the mineralogical composition of the condensed silicates. Using a combination of laboratory measurements of optical constants and asymptotic theory, we derive values of the real and imaginary indices of refraction of these grain species over a wavelength range that runs from the vacuum ultraviolet (UV) to the radio domain. The above information on grain properties is used to estimate the Rosseland mean opacity of the grains and their monochromatic opacity.

Pollack, James B.↗
NASA NTRS1995

Interstellar grains in elliptical galaxies

We consider the evolution and heating of dust embedded in the hot interstellar medium of isolated elliptical galaxies. We first construct a new set of galaxy models spanning a decade in luminosity which we use to study the evolution of dust as it is ejected by stellar sources, merges and flows with the interstellar gas, and is sputtered away due to the presence of the hot gas. We find that although grains can flow a considerable distance from the parent star in some cases before being sputtering away, the grain size distribution at a given location is accurately determined by assuming in situ sputtering of dust grains as they are ejected by the parent star. We find that dust heating is dominated by absorption of ambient starlight. Heating due to collisions with energetic electrons in the hot gas and absorption of thermal X-rays is smaller by orders of magnitude. We also find that for the largest galaxy considered (L(sub B) approximately 10 (exp 11) solar luminosity) the energy lost from the hot gas due to electronic collisions with the dust is about an order of magnitude less than that lost due to thermal emission in X-rays. The gas in smaller galaxies loses even less of its energy via heating of grains. In deriving this result, we find that the inclusion of grain sputtering is a crucial ingredient of the calculation. The presence of dust grains is therefore energetically unimportant for elliptical galaxy cooling flow models.

Tsai, John C.↗
NASA NTRS1996

Observations of Two Non-Standard Grain Components

We discuss evidence for two non-standard grain components: small conglomerates and astronomical iron. The small conglomerates are small, temperature fluctuating grains made up of loosely bound 12 micron emitters (either PAH's or dielectrics). When these grains are exposed to strong UV radiation fields they break apart into smaller pieces. For high UV radiation regimes, such as H II regions, conglomerate grains best explain the reduction of 25 micron emission relative to 12 micron. Astronomical iron was originally proposed for isolated cirrus clouds in order to account for the strength of the 60 micron emission. Within the diffuse ISM, these grains emit at a relatively high equilibrium temperature of 50 K. Our analysis of IRAS data shows the presence of a strong 60 micron emitter in the Lambda Orionis H II region and a component within the Rosette Nebula emitting primarily at 25 micron. Both of these observations can be accommodated by the presence of iron grains within the H II regions.

Shipman, R. F.↗
NASA NTRS2001

Microprobe Evaluations of Grain Boundary Segregation in KM4 and IN100

Turbine disk alloys subjected to fatigue cycles with dwells at high temperatures and stresses can fail by cracking along grain boundaries. This could be due to concentrated creep deformation or environmental attack at grain boundaries. It would be important to identify any chemical segregation along grain boundaries to aid understanding of this intergranular failure mode. The objective of this study was to evaluate the degree of chemical segregation present at the grain boundaries of two disk alloys, KM4 and IN 100. An electron microprobe employing wavelength dispersive x-ray chemical analyses was used to characterize the chemistry along multiple grain boundaries in metallographically prepared samples of each alloy. Some degrees of boron, chromium, and cobalt enrichment of grain boundaries were observed in each alloy.

Gabb, T. P.↗
NASA NTRS2001

Grain Properties of Comet C/1995 O1 (Hale-Bopp) Deduced Through Computational Techniques

We present the computational analysis of the 7.6 - 13.2 micrometer infrared (IR) spectrophotometry (R approximately equal to 120) of comet C/1995 O1 (Hale-Bopp) in conjunction with concurrent observations which extend the spectral energy distribution from the near-infrared to far-infrared wavelengths. The observations include temporal epochs pre-perihelion, (1996 October UT and 1997 February UT), near perihelion (1997 April UT), and postperihelion (1997 June UT). Through the computational modeling of small amorphous carbon, and crystalline and amorphous silicate grains in Hale-Bopp's coma, we find that as the comet approached perihelion, the grain size distribution (the Hanner modified power law) steepened (N = 3.4 pre-perihelion, to N = 3.7 near and post-perihelion) along with an increase in the fractal porosity of larger (greater than 1 micrometer) grains. The peak of the grain size distribution remained constant (ap = 0.2 micrometer) at each epoch. We attribute the emergence of the 9.3 micrometer peak near perihelion to crystalline orthopyroxeno grains released from inside the nucleus. Crystalline silicates (olivine and orthopyroxene) make up about 30% (by mass) of the submicron sized (less than 1 micrometer) dust grains in Hale-Bopp's coma during each epoch.

Harker, David E.↗
NASA NTRS2002

Toward an Improved Single-Particle Model for Large Irregular Grains

To interpret remote spectral observations, scattering and absorption in a particulate surface are simulated via radiative transfer models. The standard model for this purpose among the planetary science community is the Hapke model. This model (like many others) uses two parameters to characterize the optical behavior of individual grains in a particulate surface, the single-scattering albedo omega and phase function p(g). These terms describe, respectively, the quantity and the angular distribution of light scattered by an individual grain. Unfortunately, these parameters are strictly optical. They can be rather difficult to interpret in terms of more interesting particle properties such as grain sizes, shapes, and compositions, that a remote sensing experiment might seek to discover. An equivalent slab approximation is typically used to relate omega to the grain size and optical constants of the material. This approach can mimic the wavelength-dependent absorption behavior of irregular grains, as long as the imaginary index kappa is much less than 1, the shape is equant, and the grain size D is much larger than the wavelength lambda. Unfortunately, the equivalent slab approach provides no information about p(g), which also has a strong dependence on optical constants and particle form.

Grundy, W. M.↗
NASA NTRS2003

Mechanical Properties of a Superalloy Disk with a Dual Grain Structure

Mechanical properties from an advanced, nickel-base superalloy disk, with a dual grain structure consisting of a fine grain bore and coarse grain rim, were evaluated. The dual grain structure was produced using NASA's low cost Dual Microstructure Heat Treatment (DMHT) process. The results showed the DMHT disk to have a high strength, fatigue resistant bore comparable to a subsolvus (fine grain) heat treated disk, and a creep resistant rim comparable to a supersolvus (coarse grain) heat treated disk. Additional work on subsolvus solutioning before or after the DMHT conversion appears to be a viable avenue for further improvement in disk properties.

Gayda, John↗
NASA NTRS2003

Impact Melting of Ordinary Chondrite Regoliths and the Production of Fine-grained Fe(sup 0)

The detailed study of individual lunar soil grains provides evidence that the major optical properties of the lunar surface are primarily related to the production of fine-grained (< 20 nm, super-paramagnetic) Fe-particles in agglutinitic impact melts and to iron-rich vapor deposits on the surfaces of individual grains. These Fe-rich materials are derived from oxidized species due to high post-shock temperatures in the presence of solar-wind derived H2; part of the Fe-rich grain surfaces may also be due to sputtering processes. Identical processes were recently suggested for the optical maturation of S-type asteroid surfaces, the parent objects of ordinary chondrites (OCs). OCs, however, do not contain impact-produced soil melts, and should thus also be devoid of impact-triggered vapor condensates. The seeming disparity can only be understood if all OCs resemble relatively immature impact debris, akin to numerous lunar highland breccias. It is possible to assess this scenario by evaluating experimentally whether impact velocities of 5- 6 km/s, typical for the present day asteroid belt, suffice to produce both impact melts and fine-grained metallic iron. We used 125-250 m powders of the L6 chondrite ALH85017. These powders were aliquots from fines that were produced by collisionally disrupting a single, large (461g) chunk of this meteorite during nine impacts and by subjecting the resulting rubble to an additional 50 impacts. As a consequence, the present shock-recovery experiments employ target materials of exceptional fidelity (i.e., a real chondrite that was impact pulverized). The target powders were packed into tungsten-alloy containers to allow for the potential investigation of freshly produced, fine-grained iron and impacted by stainless-steel and tungsten flyer plates; the packing density varied between 38 and 45% porosity. Peak pressures ranged from 14.5 to 67 GPa and were attained after multiple reverberations of the shock wave at the interface of the silicate powder and metal container. Pressures in the 50 to 70 GPa range should be fairly typical for asteroid impacts at approx. 5-6 km/s, yet we note that these pressures refer to those at the projectile/target interface only and that most crater ejecta on OC parent-bodies will have experienced much lower stresses.

Hoerz, Friedrich↗
NASA NTRS2004

Organics on Fe-Silicate Grains: Potential Mimicry of Meteoritic Processes?

Currently, it is unknown what exact process or combination of processes produced organics that are found in meteorites or are detected in comets and nebulas. One particular process that forms organics involves Fischer-Tropsch type (FTT) reactions. Fischer-Tropsch type reactions produce hydrocarbons by hydrogenating carbon monoxide via catalytic reactions. The overall reaction is shown. The products of these reactions have been studied using natural catalysts and calculations of the efficiency of FTT synthesis in the Solar Nebula suggest that these types of reactions could make significant contributions to the composition of material near three AU. We use FTT synthesis to coat amorphous Fesilicate grains with organic material to simulate the chemistry in the early Solar Nebula. We used lab-synthesized amorphous Fe-silicate grains for the catalyst because they might better simulate the starting materials found in protostellar nebulas. A brief description of the synthesis of these grains is given in Experiments. This work is different from previous studies because we focus here on the carbonaceous material deposited on the grains. In our experiments, we roughly simulate a model of the nebular environment where grains are successively transported from hot to cold regions of the nebula. In other words, the starting cold regions of the nebula. In other words, the starting gases and FTT products are continuously circulated through the grains at high temperature with intervals of cooling. Overall, organics generated in this manner could represent the carbonaceous material incorporated into comets and meteorites. We present the analyses of the organics produced using pyrolysis gas chromatography mass spectrometry (GCMS) and compare the results with those organics found in the Murchison meteorite.

Johnson, N. M.↗
NASA NTRS2004

Laboratory Experiments on Rotation and Alignment of the Analogs of Interstellar Dust Grains by Radiation

The processes and mechanisms involved in the rotation and alignment of interstellar dust grains have been of great interest in astrophysics ever since the surprising discovery of the polarization of starlight more than half a century ago. Numerous theories, detailed mathematical models, and numerical studies of grain rotation and alignment with respect to the Galactic magnetic field have been presented in the literature. In particular, the subject of grain rotation and alignment by radiative torques has been shown to be of particular interest in recent years. However, despite many investigations, a satisfactory theoretical understanding of the processes involved in subject, we have carried out some unique experiments to illuminate the processes involved in the rotation of dust grains in the interstellar medium. In this paper we present the results of some preliminary laboratory experiments on the rotation of individual micron/submicron-sized, nonspherical dust grains levitated in an electrodynamic balance evacuated to pressures of approximately 10(exp -3) to 10(exp -5) torr. The particles are illuminated by laser light at 5320 A, and the grain rotation rates are obtained by analyzing the low-frequency (approximately 0 - 100 kHz) signal of the scattered light detected by a photodiode detector. The rotation rates are compared with simple theoretical models to retrieve some basic rotational parameters. The results are examined in light of the current theories of alignment.

Abbas, M. M.↗
NASA NTRS2005

Cyclic Spin Testing of Superalloy Disks With a Dual Grain Microstructure

An aggressive cyclic spin test program was run to verify the reliability of superalloy disks with a dual grain structure, fine grain bore and coarse grain rim, utilizing a disk design with web holes bisecting the grain size transition zone. Results of these tests were compared with conventional disks with uniform grain structures. Analysis of the test results indicated the cyclic performance of disks with a dual grain structure could be estimated to a level of accuracy which does not appear to prohibit the use of this technology in advanced gas turbine engines, although further refinement of lifing methodology is clearly warranted.

Gayda, John↗
NASA NTRS2003

Superalloy Disk With Dual-Grain Structure Spin Tested

Advanced nickel-base disk alloys for future gas turbine engines will require greater temperature capability than current alloys, but they must also continue to deliver safe, reliable operation. An advanced, nickel-base disk alloy, designated Alloy 10, was selected for evaluation in NASA s Ultra Safe Propulsion Project. Early studies on small test specimens showed that heat treatments that produced a fine grain microstructure promoted high strength and long fatigue life in the bore of a disk, whereas heat treatments that produced a coarse grain microstructure promoted optimal creep and crack growth resistance in the rim of a disk. On the basis of these results, the optimal combination of performance and safety might be achieved by utilizing a heat-treatment technology that could produce a fine grain bore and coarse grain rim in a nickel-base disk. Alloy 10 disks that were given a dual microstructure heat treatment (DMHT) were obtained from NASA s Ultra-Efficient Engine Technology (UEET) Program for preliminary evaluation. Data on small test specimens machined from a DMHT disk were encouraging. However, the benefit of the dual grain structure on the performance and reliability of the entire disk still needed to be demonstrated. For this reason, a high temperature spin test of a DMHT disk was run at 20 000 rpm and 1500 F at the Balancing Company of Dayton, Ohio, under the direction of NASA Glenn Research Center personnel. The results of that test showed that the DMHT disk exhibited significantly lower crack growth than a disk with a fine grain microstructure. In addition, the results of these tests could be accurately predicted using a two-dimensional, axisymmetric finite element analysis of the DMHT disk. Although the first spin test demonstrated a significant performance advantage associated with the DMHT technology, a second spin test on the DMHT disk was run to determine burst margin. The disk burst in the web at a very high speed, over 39 000 rpm, in line with the predicted location and speed. Furthermore, significant growth of the disk was observed before failure, in line with predictions, clearly demonstrating the reliability and safety of the DMHT technology. Although successful spin testing in Ultra Safe's Nickel Disk Program represents a significant milestone for DMHT technology, realistic engine operation will require repeated loading of a DMHT nickel disk. For this reason, a cyclic spin test study of DMHT nickel disk technology has been proposed to start in fiscal year 2003 under NASA's Aviation Safety Program. The goal of this program will be to determine the fatigue failure mechanism in DMHT nickel disks, thereby demonstrating the reliability and safety of DMHT technology under repetitive loading conditions encountered in realistic engine operation.

Gayda, John↗
NASA NTRS2006

Photoelectric Emission Measurements on the Analogs of Individual Cosmic Dust Grains

The photoelectric emission process is considered to be the dominant mechanism for charging of cosmic dust grains in many astrophysical environments. The grain charge and equilibrium potentials play an important role in the dynamical and physical processes that include heating of the neutral gas in the interstellar medium, coagulation processes in the dust clouds, and levitation and dynamical processes in the interplanetary medium and planetary surfaces and rings. An accurate evaluation of photoelectric emission processes requires knowledge of the photoelectric yields of individual dust grains of astrophysical composition as opposed to the values obtained from measurements on flat surfaces of bulk materials, as it is generally assumed on theoretical considerations that the yields for the small grains are much different from the bulk values. We present laboratory measurements of the photoelectric yields of individual dust grains of silica, olivine, and graphite of approx. 0.09-5 micrometer radii levitated in an electrodynamic balance and illuminated with ultraviolet radiation at 120-160 nm wavelengths. The measured yields are found to be substantially higher than the bulk values given in the literature and indicate a size dependence with larger particles having order-of-magnitude higher values than for submicron-size grains.

Abbas, M. M.↗