SEARCH · Engineering Papers

Results for “amorphization”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,009 records · Page 56

All sources NASA NTRS DOE OSTI

NASA NTRS1992

Evidence for crystalline hematite as an accessory phase in Martian soils

Based on visible and near-IR spectral images of Mars obtained by us in 1988, we have confirmed the occurrence of bulk crystalline hematite as a minor or accessory phase on the surface. This hematite is not uniformly distributed, being less abundant or absent in certain northern low-albedo regions. Even where most abundant, bulk hematite is not the primary coloring agent of heavily weathered materials on Mars. The primary coloring agent is apparently an amorphous or nanophase Fe3(+) phase, analogous to the coloring agent in certain Hawaiian palagonites (e.g. Adams and Evans, 1980; Singer, 1982; Morris et al, 1989). Based on these results, we can say with considerable certainty that bulk crystalline hematite exists on Mars in small amounts, in addition to the more ubiquitous nanophase or poorly crystalline ferric oxide phase(s). We conclude that Type 2 low-albedo regions (typified by Acidalia Planitia) contain less of this hematite than Type 1 regions (typified by Meridiani Sinus). Oxia Palus shows indication of slightly more oxidation and bulk hematite than Acidalia. This may be a real compositional difference, indicating that Oxia is in some sense transitional between Type 2 and Type 1 regions, but we cannot rule out the possibility of some optical mixing with surrounding higher-albedo regions. We are currently investigating the relationship of ferric-oxides in high-albedo regions to both types of low-albedo regions. As expected, spectral ratios of data from western Arabia to dark regions show considerably greater Fe3(+) in Arabia. There are not obvious spectral indications of a difference in ferric-oxide composition between Arabia and Type 1 low-albedo regions, implying that Arabia also contains minor bulk hematite.

Singer, R. B.↗

NASA NTRS1991

Response of composite materials to low velocity impact

Orthotropic and quasi-isotropic laminates clamped at the edges were impacted by an instrumented falling weight. Five matrix materials, comprising a baseline epoxy, two toughened thermosets, and amorphous and crystalline thermoplastics were studied. For each material, the projected damage area and residual compression strengths and strains were determined as a function of impact energy. Principal conclusions are: (1) incipient damage associated with a prominent load drop during the impact test seems to be decisive for residual properties; (2) subtle rate effects are present with some materials; and (3) results from the small scale tests parallel those in standard tests and may be adequate for screening purposes.

Srinivasan, K.↗

NASA NTRS1991

Spectral signature of oxidized pyroxenes - Implications to remote sensing of terrestrial planets

The effect of the structural Fe(3+) or disseminated ferric oxides in oxidized ferromagnesian silicates on the reflectance spectral profiles of the remnant Fe(2+) ions in oxidized silicates was investigated. Results of spectral measurements in the visible NIR region, performed on several well-characterized pyroxenes representative of different structural types and compositions, were correlated with data obtained from the Mossbauer spectra of the oxidation products. The measurements made it possible to determine the relative proportions of Fe(2+) and Fe(3+) ions present in the oxidized pyroxenes and provided information on the amorphous ferric oxide phases present in the oxidation products. The results also suggest a mechanism for producing the nanophase hematite popularly considered to be responsible for the features observed around 0.62 and 0.87 micron in the remotely sensed spectra of Mars.

Straub, D'arcy W.↗

NASA NTRS1991

Sublimation and reformation of icy grains in the primitive solar nebula

The quantity of water ice that sublimates during the free fall of grains into the solar nebula from a surrounding interstellar cloud varies from over 90 percent of the grain mass as 30 AU from the nebular center to less than 10 percent at more than 100 AU. Virtually all the water that is sublimated ultimately recondenses, since the cold nebular gas lying beyond 10 AU is unable to hold more than a small portion as vapor. The return of most of the gas to solid phase near the nebular ambient temperature, of about 50 K, may result in at least two grain populations consisting, in one case, of unaltered interstellar grains which did not undergo sublimation, and in the other of water ice which cocondensed with more volatile gases at nebular ambient temperatures to yield volatile-rich amorphous phases.

Lunine, Jonathan I.↗

NASA NTRS1991

Equation of state of heated glassy carbon

New Hugoniot data are presented for glassy carbon preheated to 1550 K and shocked to 20 GPa. The high-temperature Hugoniot is very similar to the principal Hugoniot. This results argues against the diffusional mechanism for the shock-induced transformaton of amorphous carbon to diamond, although the present results are obviously limited to below 20 GPa. This study provides the first Higoniot data for carbon preheated to significantly high temperatures.

Sekine, Toshimori↗

NASA NTRS1992

Electronic structure of light-emitting porous Si

Characterization of light-emitting porous Si films with X-ray photoelectron spectroscopy is reported. Only traces of O are detected on HF-etched samples, in contradiction to an earlier report that oxides are a significant component of porous Si. Si 2p and valence-band measurements demonstrate that the near-surface region of high porosity films which exhibit visible luminescence consists of amorphous Si.

Vasquez, R. P.↗

NASA NTRS1992

Mineralogy of 12 large 'chondritic' interplanetary dust particles

Mineralogical analyses of 12 large 'chondritic' interplanetary dust particles (IDPs) are reported. It is found that nine of these particles belong to the smectite class of IDPs, and the remainder to the pyroxene class. In comparing these results to previous studies of generally smaller IDPs, a significantly higher percentage of smectite-class IDPs is found, which may therefore be a size-related phenomenon. The probable origin of these large IDPs is discussed, comparing hydrous to anhydrous particles. On the basis of mineralogical and compositional similarities it is suggested that the bulk (at least) of pyroxene- and smectite-class IDPs have experienced similar or identical parent body processes, and are possibly derived from the same parent bodies. It is noted that it is commonly difficult to classify certain IDPs by the current scheme because of the presence of abundant amorphous materials and coexistence of both hydrous and anhydrous silicates. It is suggested that the degree of compositional heterogeneity exhibited by anhydrous silicates could form the basis of an alternate IDP classification scheme.

Zolensky, Michael E.↗

NASA NTRS1990

Studies of mechanochemical interactions in the tribological behavior of materials

Mechanochemical interaction studies can contribute to the understanding of wear and friction of materials. Specific examples of experimental results relative to the subject are discussed. There are two parts: one describes the synergistic effect of corrosion and wear of iron sliding on sapphire in sulfuric acid, and the other describes the effect of surface films on the wear and friction of plasma-deposited diamondlike carbon (amorphous hydrogenated carbon) films in sliding contact with silicon nitride. The concentration of acid (pH) is an important factor in controlling the iron loss caused by wear-corrosion processes in sulfuric acid. The mechanical action can cause chemical reactions to proceed much faster than they would otherwise. The diamondlike carbon (DLC) films are shown to behave tribologically much like bulk diamond. In a dry nitrogen environment, a mechanochemical reaction produces a substance which greatly decreases the coefficient of friction. In a moist air environment, mechanochemical interactions drastically reduce the wear life of DLC films and water vapor greatly increases friction.

Miyoshi, Kazuhisa↗

NASA NTRS1992

An interplanetary dust particle with links to CI chondrites

W7013F5 is a chondritic, hydrated interplanetary dust particle whose composition and mineralogy is nearly identical to that found in the CI chondrites. Transmission electron microscope observations show that the phyllosilicates in W7013F5 consist largely of a coherent undergrowth of Mg-Fe serpentine and Fe-bearing saponite on the unitcell scale. This distinctive intergrowth of phyllosilicates has only been observed previously in the CI chondrites. Other secondary minerals in W7013F5 include Mg-Fe carbonates, magnetite, and pentlandite. The mineral assemblage in W7013F5 is generally not as oxidized as that in the CI chondrites. The presence of kamacite in W7013F5 indicates that the particle is extraterrestrial, and a thin amorphous rim surrounding the particle provides evidence that it is not a piece of a meteorite that fragmented during transit through the atmosphere. The apparent lack of hydrated IDPs with CI mineralogy and chemistry may indicate that CI-type dust-producing asteroids are uncommon in the asteroid belt.

Keller, Lindsay P.↗

NASA NTRS1992

Microstructural investigations of light-emitting porous Si layers

The structural and morphological characteristics of visible-light-emitting porous Si layers produced by anodic and stain etching of single-crystal Si substrates are compared using transmission electron microscopy and atomic force microscopy (AFM). AFM of conventionally anodized, laterally anodized and stain-etched Si layers show that the layers have a fractal-type surface morphology. The anodized layers are rougher than the stain-etched films. At higher magnification 10 nm sized hillocks are visible on the surface. Transmission electron diffraction patterns indicate an amorphous structure with no evidence for the presence of crystalline Si in the near-surface regions of the porous Si layers.

George, T.↗

NASA NTRS1991

Imide/arylene ether copolymers. I

The preparation of a series of novel imide/arylene ether copolymers is described together with the results of viscosity and DSC Tg(Tm) measurements. The copolymers were synthesized from an arylene ether block and either an amorphous or semicrystalline imide block. One block copolymer was end-capped, and the molecular weight was controlled to improve compression moldability. The paper also presents results of mechanical properties tests on copolymer samples.

Jensen, B. J.↗

NASA NTRS1992

Comments on 'Gas release from comets' and related trapped-gas experiments

The numerous variables influencing trapped-gas experiment results based on cometary ice analogs are presently discussed in view of the Notesco et al. (1991) data on Kr-trapping and release by H2O ice. Attention is drawn to the questionable tennability of a linear relation proposed between the thickness of the ice and the gas retained beyond the amorphous-to-cubic phase change of H2O. Exacting care is required in the application of laboratory results to cometary phenomena, rendering definitive conclusions extremely elusive.

Hudson, R. L.↗

NASA NTRS1989

Optical dispersion relations for 'diamondlike' carbon films

Ellipsometric measurements on plasma deposited diamondlike amorphous carbon (a-C:H) films were taken in the visible, (E = 1.75 to 3.5 eV). The films were deposited on Si and their properties were varied using high temperature (up to 750 C) anneals. The real (n) and imaginary (k) parts of the complex index of refraction N were obtained simultaneously. Following the theory of Forouhi and Bloomer, a least squares fit was used to find the dispersion relations n(E) and k(E). Reasonably good fits were obtained, showing that the theory can be used for a-C:H films. Moreover, the value of the energy gap (Eg) obtained in this way was compared to the Eg value using conventional Tauc plots and reasonably good agreement was obtained.

Alterovitz, Samuel A.↗

NASA NTRS1993

Polyimides Made From 4,4'-Isophthaloyldiphthalic Anhydride

High-performance, thermoplastic, amorphous, and semicrystalline materials obtained. In experiments, twelve high-temperature, high-performance polyimide compositions based on 4,4'-isophthaloyldiphthalic anhydride (IDPA) prepared and characterized. Tough, film-forming, polyimides insoluble in organic solvents obtained in most cases. Three materials semicrystalline. Materials offer potential for development of semicrystalline polyimides having enhanced resistances to solvents and caustic chemicals, higher tensile strengths and moduli, enhanced thermo-oxidative stabilities below their melting temperatures, and enhanced melt processability, especially when heated to melting temperatures. Several exhibited excellent long-term thermo-oxidative stability when subjected to isothermal thermogravimetric analysis (ITGA) at temperatures of 300 degrees C and 350 degrees C in air. One area of application, manufacturing of lighter and stronger aircraft and spacecraft structures.

Pratt, J. Richard↗

NASA NTRS1993

Processible Polyazomethines

Processibility enhanced by incorporating trifluoromethylbenzene units into molecules. Results of two unexpected properties: high solubility in common organic solvents and lack of crystallinity. Properties coupled with ability of this class of polymers to undergo exchange reactions at elevated temperatures allow these materials to moderate their molecular weights during processing, which affects resulting mechanical properties after processing. Fluorinated polyazomethines form amorphous films that have excellent mechanical properties. Used in moisture-free environments as electronic semiconducting films and adhesives, and as matrix resins for advanced structural composites.

Bryant, Robert G.↗

NASA NTRS1992

Optical and TEM study of shock metamorphism from the Sedan test site

Thus far, detailed petrologic studies of shock metamorphism have been performed on samples recovered from laboratory experiments and on a few natural impactites. The loading history of these samples is quite different: In particular, laboratory experiments spend only a short time (less than 1 microsec) at peak pressure, whereas natural impactites may have stress pulses from 0.1 - 1 ms. On the other hand, laboratory experiments have known stress histories; natural impactites do not. Natural samples are also subjected to thousands or millions of years of postshock annealing and/or weathering. A useful intermediate case is that of nuclear detonation. Stress pulses for these events can reach 0.1 ms or higher, and samples are obtained in pristine condition. All three types of loading produce stresses of hundreds of kilobars. Samples studied were taken from the Sedan nulcear test site, and consist of a coarse-grained granodiorite containing quartz, K-feldspar, cordierite, and hornblende. Samples were studied optically in this section, then were thinned with an ion mill and studied by transmission electron microscopy (TEM). Optically, quartz and K-feldspar displayed numerous sets of planar deformation features (PDF's) identical to the nondecorated PDF's seen in laboratory samples and many natural impactites. TEM study showed that the PDF's in quartz and feldspar corresponded to densely packed wide transformation lamellae identical to those described in laboratory studies. The transformation lamellae in both minerals were amorphous, with no sign of high-pressure phases. In the case of K-feldspar only, narrow sublamellae extended outward from some wide lamellae. Quartz, which was more abundant and studied more extensively, contained no shock-induced dislocations. Some planar features were also seen in cordierite, but could not be identified due to rapid beam damage. No shock defects were seen in hornblende in TEM. The shock-induced defects present at the Sedan site are very similar to those seen in shock recovery experiments, and also to those present at certain natural events (e.g., Meteor Crater). This suggests that shock deformation in quartz is not strongly dependent on shock pulse duration, and that laboratory recovery experiments are useful simulations of natural impact events.

Gratz, A. J.↗

NASA NTRS1992

Simulated meteorite impacts and volcanic explosions: Ejecta analyses and planetary implications

Past cratering studies have focused primarily on crater morphology. However, important questions remain about the nature of crater deposits. Phenomena that need to be studied include the distribution of shock effects in crater deposits and crater walls; the origin of mono- and polymict breccia; differences between local and distal ejecta; deformation induced by explosive volcanism; and the production of unshocked, high-speed ejecta that could form the lunar and martian meteorites found on the Earth. To study these phenomena, one must characterize ejecta and crater wall materials from impacts produced under controlled conditions. New efforts at LLNL simulate impacts and volcanism and study resultant deformation. All experiments use the two-stage light-gas gun facility at LLNL to accelerate projectiles to velocities of 0.2 to 4.3 km/s, including shock pressures of 0.9 to 50 GPa. We use granite targets and novel experimental geometries to unravel cratering processes in crystalline rocks. We have thus far conducted three types of simulations: soft recovery of ejecta, 'frozen crater' experiments, and an 'artificial volcano. Our ejecta recovery experiments produced a useful separation of impactites. Material originally below the projectile remained trapped there, embedded in the soft metal of the flyer plate. In contrast, material directly adjacent to the projectile was jetted away from the impact, producing an ejecta cone that was trapped in the foam recovery fixture. We find that a significant component of crater ejecta shows no signs of strong shock; this material comes from the near-surface 'interference zone' surrounding the impact site. This phenomenon explains the existence of unshocked meteorites on the Earth of lunar and martian origin. Impact of a large bolide on neighboring planets will produce high-speed, weakly shocked ejecta, which may be trapped by the Earth's gravitational field. 'Frozen crater' experiments show that the interference zone is highly localized; indeed, disaggregation does not extend beyond approx. 1.5 crater radii. A cone-shaped region extending downward from the impact site is completely disaggregated, including powdered rock that escaped into the projectile tube. Petrographic analysis of crater ejecta and wall material will be presented. Finally, study of ejecta from 0.9- and 1.3-GPa simulations of volcanic explosions reveal a complete lack of shock metamorphism. The ejecta shows no evidence of PDF's, amorphization, high-pressure phases, or mosaicism. Instead, all deformation was brittle, with fractures irregular (not planar) and most intergranular. The extent of fracturing was remarkable, with the entire sample reduced to fragments of gravel size and smaller.

Gratz, A. J.↗

NASA NTRS1992

Sol-gel synthesis and densification of aluminoborosilicate powders. Part 1: Synthesis

Aluminoborosilicate powders high in alumina content were synthesized by the sol-gel process utilizing various methods of preparation. Properties and microstructural effects related to these syntheses were examined. After heating to 600 C for 2 h in flowing air, the powders were amorphous with the metal oxides comprising 87 percent of the weight and uncombusted organics the remainder. DTA of dried powders revealed a T(sub g) at approximately 835 C and an exotherm near 900 C due to crystallization. Powders derived from aluminum secbutoxide consisted of particles with a mean diameter 5 microns less than those from aluminum isopropoxide. Powders synthesized with aluminum isopropoxide produced agglomerates comprised of rod shaped particulates while powders made with the secbutoxide precursor produced irregular glassy shards. Compacts formed from these powders required different loadings for equivalent densities according to the method of synthesis.

Bull, Jeffrey↗