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At least 1,441 records · Page 80
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Microstructures and mechanical properties of the welded AA5182/HC340LA joint by magnetic pulse welding

Highlights: • The magnetic pulse welding of 5× aluminum alloy with high strength steel is performed. • The microstructure evolution of welding interface under different parameters is investigated. • The intermediate layer is characterized by high-resolution transmission electron microscope and electron diffraction pattern. - Abstract: Magnetic pulse welding (MPW) is an innovative joining technology for dissimilar materials. The MPW with high joining qualities could be realized between high strength steel and 5× aluminum alloys. In this work, the MPW joints between aluminum alloy (AA 5182) sheet and high-strength low alloy steel (HC340LA) sheet were prepared under various discharge energies. The mechanical properties and microstructure observations were employed to evaluate the joint performance. Results showed that a lower discharge voltage tended to cause joints with poor quality. When the discharge energy increased, the failure position gradually changed from the weld zone to the weaker base metal (Al) during quasi-static tensile tests. In addition, the effect of discharge energies on microstructure evolution in the intermediate layer was revealed by high-resolution transmission electron microscopy (HRTEM) and scanning electron microscope (SEM). Atomic-scale diffusion was found to be the mainly welding mechanism of the Al/Fe joint. The intermetallic compound (IMC) FeAl{sub 2} and ultra-fine grains (UFG) were found in the intermediate layer under high discharge energy.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanoscale Spatially Resolved Mapping of Uranium Enrichment

Abstract Spatially resolved analysis of uranium (U) isotopes in small volumes of actinide-bearing materials is critical for a variety of technical disciplines, including earth and planetary sciences, environmental monitoring, bioremediation, and the nuclear fuel cycle. However, achieving subnanometer-scale spatial resolution for such isotopic analysis is currently a challenge. By using atom probe tomography—a three-dimensional nanoscale characterisation technique—we demonstrate unprecedented nanoscale mapping of U isotopic enrichment with high sensitivity across various microstructural interfaces within small volumes (~100 nm 3 ) of depleted and low-enriched U alloyed with 10 wt% molybdenum that has different nominal enrichments of 0.20 and 19.75% 235 U, respectively. We map enrichment in various morphologies of a U carbide phase, the adjacent γ-UMo matrix, and across interfaces (e.g., carbide/matrix, grain boundary). Results indicate the U carbides were formed during casting, rather than retained from either highly enriched or depleted U feedstock materials. The approach presented here can be applied to study nanoscale variations of isotopic abundances in the broad class of actinide-bearing materials, providing unique insights into their origins and thermomechanical processing routes.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Unsupervised data mining in nanoscale x-ray spectro-microscopic study of NdFeB magnet

Novel developments in X-ray based spectro-microscopic characterization techniques have increased the rate of acquisition of spatially resolved spectroscopic data by several orders of magnitude over what was possible a few years ago. This accelerated data acquisition, with high spatial resolution at nanoscale and sensitivity to subtle differences in chemistry and atomic structure, provides a unique opportunity to investigate hierarchically complex and structurally heterogeneous systems found in functional devices and materials systems. However, handling and analyzing the large volume data generated poses significant challenges. Here we apply an unsupervised data-mining algorithm known as DBSCAN to study a rare-earth element based permanent magnet material, Nd 2 Fe 14 B. We are able to reduce a large spectro-microscopic dataset of over 300,000 spectra to 3, preserving much of the underlying information. Scientists can easily and quickly analyze in detail three characteristic spectra. Our approach can rapidly provide a concise representation of a large and complex dataset to materials scientists and chemists. For instance, it shows that the surface of common Nd 2 Fe 14 B magnet is chemically and structurally very different from the bulk, suggesting a possible surface alteration effect possibly due to the corrosion, which could affect the material’s overall properties.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effects of Co layer thickness and annealing temperature on the magnetic properties of inverted [Pt/Co] multilayers

The effects of Co layer thickness and annealing temperature on the perpendicular magnetic anisotropy (PMA) properties of inverted [Pt (0.2 nm)/Co (t{sub Co})]{sub 6} multilayers (where t{sub Co} indicates the thickness of the Co layer) have been investigated. The cross-sectional microstructure, as observed from the high-resolution transmission electron microscope images, shows a clear layered structure with atomically flat interfaces both in the as-deposited state as well as after annealing, indicating the interface effects for PMA. The effective PMA energy density (K{sub eff}) increases significantly with an increase in t{sub Co} from 0.2 to 0.28 nm and then becomes almost saturated with further increases in t{sub Co}, followed by a slight reduction at the highest Co thickness, t{sub Co} = 0.6 nm. In order to explain the t{sub Co} dependence on K{sub eff}, the intrinsic PMA energy density (K{sub i}) is calculated by additionally measuring a similar set of results for the saturation magnetization. The K{sub i} value increases nearly linearly with the increase in t{sub Co} from 0.2 to 0.5 nm, followed by saturation at a higher t{sub Co} value of 0.6 nm. Owing to a close relationship between K{sub i} and the quality of the interfaces, these results indicate a similar t{sub Co} dependence on the quality of the interfaces. This is further supported from the magnetic measurements of the samples annealed at the highest temperature of 500 °C, where a second phase is formed, which show a similar t{sub Co} dependence on the amount of the second phase. The K{sub i} value is nearly independent of the annealing temperature at t{sub Co} ≤ 0.4 nm, above which a substantial reduction is observed, when the annealing temperature exceeds 500 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Low temperature epitaxy of Ge-Sb-Te films on BaF{sub 2} (111) by pulsed laser deposition

Pulsed laser deposition was employed to deposit epitaxial Ge{sub 2}Sb{sub 2}Te{sub 5}-layers on the (111) plane of BaF{sub 2} single crystal substrates. X-ray diffraction measurements show a process temperature window for epitaxial growth between 85 °C and 295 °C. No crystalline growth is observed for lower temperatures, whereas higher temperatures lead to strong desorption of the film constituents. The films are of hexagonal structure with lattice parameters consistent with existing models. X-ray pole figure measurements reveal that the films grow with one single out-of-plane crystal orientation, but rotational twin domains are present. The out-of-plane epitaxial relationship is determined to be Ge{sub 2}Sb{sub 2}Te{sub 5}(0001) || BaF{sub 2}(111), whereas the in-plane relationship is characterized by two directions, i.e., Ge{sub 2}Sb{sub 2}Te{sub 5} [-12-10] || BaF{sub 2}[1-10] and Ge{sub 2}Sb{sub 2}Te{sub 5}[1-210] || BaF{sub 2}[1-10]. Aberration-corrected high-resolution scanning transmission electron microscopy was used to resolve the local atomic structure and confirm the hexagonal structure of the films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Interfacial structure in epitaxial perovskite oxides on (001) Ge crystal

We investigated the interfacial structure of hetero-epitaxial SrZr{sub 0.68}Ti{sub 0.32}O{sub 3} thin film deposited on (001) Ge single crystal via transmission electron microscopy (TEM). The results from high-resolution scanning TEM and electron energy-loss spectroscopy show an atomically abrupt interface without secondary phase. We found misfit dislocations with Burgers vector of 1/2a 〈111〉 and threading dislocations with Burgers vector of a 〈100〉. Furthermore, we observed the coupling between dislocation half-loop and anti-phase boundary induced by the lattice terrace of Ge along 〈100〉 direction and their decoupling after annealing. We proposed models based on half-loop theory to interpret the coupling and the dislocation reactions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Plasma mass separation

Here, this tutorial describes mechanisms for separating ions in a plasma device with respect to their atomic or molecular mass for practical applications. The focus here is not on separating isotopes of a single atomic species but rather on systems with a much lower mass resolution and a higher throughput. These separation mechanisms include ion gyro-orbit separation, drift-orbit separation, vacuum arc centrifugation, steady-state rotating plasmas, and several other geometries. Generic physics issues are discussed such as the ion charge state, neutrals and molecules, collisions, radiation loss, and electric fields and fluctuations. Generic technology issues are also discussed such as plasma sources and ion heating, and suggestions are made for future research.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Ambidextrous Helical Nanotubes from Self-Assembly of Designed Helical Hairpin Motifs

Tandem repeat proteins exhibit native designability and represent potentially useful scaffolds for the construction of synthetic biomimetic assemblies. We have designed two synthetic peptides, HEAT R1 and LRV M3Δ1, based on the consensus sequences of single repeats of thermophilic HEAT (PBS HEAT) and Leucine- Rich Variant (LRV) structural motifs, respectively. Self-assembly of the peptides afforded high aspect-ratio helical helical nanotubes. Cryo-electron microscopy with direct electron detection was employed to analyze the structures of the solvated filaments. Three dimensional reconstructions from the cryo-EM maps led to atomic models for the HEAT R1 and LRV M3Δ1 filaments at resolutions of 6.0 Å and 4.4 Å, respectively. Surprisingly, despite sequence similarity at the lateral packing interface, HEAT R1 and LRV M3Δ1 filaments adopt the opposite helical hand and differ significantly in helical geometry, while retaining a local conformation similar to previously characterized repeat proteins of the same class. The differences in the two filaments could be rationalized on the basis of differences in cohesive interactions at the lateral and axial interfaces. These structural data reinforce previous observations regarding the structural plasticity of helical protein assemblies and the need for high resolution structural analysis. Despite these observations, the native designability of tandem repeat proteins offers the opportunity to engineer novel helical nanotubes. Furthermore, the resultant nanotubes have independently addressable and chemically distinguishable interior and exterior surfaces that would facilitate applications in selective recognition, transport, and release.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites

Femtosecond resolution electron scattering techniques are applied to resolve the first atomic-scale steps following absorption of a photon in the prototypical hybrid perovskite methylammonium lead iodide. Following above-gap photoexcitation, we directly resolve the transfer of energy from hot carriers to the lattice by recording changes in the mean square atomic displacements on 10-ps time scales. Measurements of the time-dependent pair distribution function show an unexpected broadening of the iodine-iodine correlation function while preserving the Pb-I distance. This indicates the formation of a rotationally disordered halide octahedral structure developing on picosecond time scales. Here, this work shows the important role of light-induced structural deformations within the inorganic sublattice in elucidating the unique optoelectronic functionality exhibited by hybrid perovskites and provides new understanding of hot carrier-lattice interactions, which fundamentally determine solar cell efficiencies.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Activity and Stability of Nanoscale Oxygen Reduction Catalysts

Design of highly active and stable nanoscale catalysts for electro-oxidation of small organic molecules is of great importance to the development of efficient fuel cells. The amount and instability of Pt-based catalysts in the cathode limits the cost, efficiency and lifetime of proton exchange membrane fuel cells. We developed a microscopic understanding of the factors governing activity and stability in Pt and PtM alloys. Experimental efforts were focused on probing the size and shape dependence of ORR activity of Pt-based nanoparticles supported on carbon nanotubes. A microscopic understanding of the activity was achieved by correlating voltammetry and rotating ring disk electrodes to surface atomic and electronic structures, which were elucidated predominantly by high-resolution transmission electron microscopy (HRTEM), Scanning transmission electron microscopy energy dispersive X-ray Spectroscopy (STEM-EDS) and synchrotron X-ray absorption spectroscopy (XAS).

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2017

Survey of Water and Ammonia in Nearby Galaxies (SWAN): Resolved Ammonia Thermometry, and Water and Methanol Masers in the Nuclear Starburst of NGC 253

We present Karl G. Jansky Very Large Array molecular line observations of the nearby starburst galaxy NGC 253, from SWAN, the Survey of Water and Ammonia in Nearby galaxies. SWAN is a molecular line survey at centimeter wavelengths designed to reveal the physical conditions of star-forming gas over a range of star-forming galaxies. NGC 253 has been observed in four 1 GHz bands from 21 to 36 GHz at 6″ ∼ 100 pc) spatial and 3.5 km s{sup −1} spectral resolution. In total we detect 19 transitions from 7 molecular and atomic species. We have targeted the metastable inversion transitions of ammonia (NH{sub 3}) from (1, 1) to (5, 5) and the (9, 9) line, the 22.2 GHz water (H{sub 2}O) (6{sub 16}--5{sub 23}) maser, and the 36.1 GHz methanol (CH{sub 3}OH) (4{sub −1}--3{sub 0}) maser. Using NH{sub 3} as a thermometer, we present evidence for uniform heating over the central kpc of NGC 253. The molecular gas is best described by a two kinetic temperature model with a warm 130 K and a cooler 57 K component. A comparison of these observations with previous ALMA results suggests that the molecular gas is not heated in photon-dominated regions or shocks. It is possible that the gas is heated by turbulence or cosmic rays. In the galaxy center we find evidence for NH{sub 3}(3, 3) masers. Furthermore, we present velocities and luminosities of three water maser features related to the nuclear starburst. We partially resolve CH{sub 3}OH masers seen at the edges of the bright molecular emission, which coincides with expanding molecular superbubbles. This suggests that the masers are pumped by weak shocks in the bubble surfaces.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

The potential role of methanesulfonic acid (MSA) in aerosol formation and growth and the associated radiative forcings

Atmospheric marine aerosol particles impact Earth's albedo and climate. These particles can be primary or secondary and come from a variety of sources, including sea salt, dissolved organic matter, volatile organic compounds, and sulfur-containing compounds. Dimethylsulfide (DMS) marine emissions contribute greatly to the global biogenic sulfur budget, and its oxidation products can contribute to aerosol mass, specifically as sulfuric acid and methanesulfonic acid (MSA). Further, sulfuric acid is a known nucleating compound, and MSA may be able to participate in nucleation when bases are available. As DMS emissions, and thus MSA and sulfuric acid from DMS oxidation, may have changed since pre-industrial times and may change in a warming climate, it is important to characterize and constrain the climate impacts of both species. Currently, global models that simulate aerosol size distributions include contributions of sulfate and sulfuric acid from DMS oxidation, but to our knowledge, global models typically neglect the impact of MSA on size distributions. In this study, we use the GEOS-Chem-TOMAS (GC-TOMAS) global aerosol microphysics model to determine the impact on aerosol size distributions and subsequent aerosol radiative effects from including MSA in the size-resolved portion of the model. The effective equilibrium vapor pressure of MSA is currently uncertain, and we use the Extended Aerosol Inorganics Model (E-AIM) to build a parameterization for GC-TOMAS of MSA's effective volatility as a function of temperature, relative humidity, and available gas-phase bases, allowing MSA to condense as an ideally nonvolatile or semivolatile species or too volatile to condense. We also present two limiting cases for MSA's volatility, assuming that MSA is always ideally nonvolatile (irreversible condensation) or that MSA is always ideally semivolatile (quasi-equilibrium condensation but still irreversible condensation). We further present simulations in which MSA participates in binary and ternary nucleation with the same efficacy as sulfuric acid whenever MSA is treated as ideally nonvolatile. When using the volatility parameterization described above (both with and without nucleation), including MSA in the model changes the global annual averages at 900 hPa of submicron aerosol mass by 1.2 %, N3 (number concentration of particles greater than 3 nm in diameter) by -3.9 % (non-nucleating) or 112.5 % (nucleating), N80 by 0.8 % (non-nucleating) or 2.1 % (nucleating), the cloud-albedo aerosol indirect effect (AIE) by -8.6 mW m -2 (non-nucleating) or -26 mW m -2 (nucleating), and the direct radiative effect (DRE) by -15 mW m -2 (non-nucleating) or -14  mW m -2 (nucleating). The sulfate and sulfuric acid from DMS oxidation produces 4–6 times more submicron mass than MSA does, leading to an ~10 times stronger cooling effect in the DRE. But the changes in N80 are comparable between the contributions from MSA and from DMS-derived sulfate/sulfuric acid, leading to comparable changes in the cloud-albedo AIE. Model–measurement comparisons with the Heintzenberg et al. (2000) dataset over the Southern Ocean indicate that the default model has a missing source or sources of ultrafine particles: the cases in which MSA participates in nucleation (thus increasing ultrafine number) most closely match the Heintzenberg distributions, but we cannot conclude nucleation from MSA is the correct reason for improvement. Model–measurement comparisons with particle-phase MSA observed with a customized Aerodyne high-resolution time-of-flight aerosol mass spectrometer (AMS) from the ATom campaign show that cases with the MSA volatility parameterizations (both with and without nucleation) tend to fit the measurements the best (as this is the first use of MSA measurements from ATom, we provide a detailed description of these measurements and their calibration). However, no one model sensitivity case shows the best model–measurement agreement for both Heintzenberg and the ATom campaigns. As there are uncertainties in both MSA's behavior (nucleation and condensation) and the DMS emissions inventory, further studies on both fronts are needed to better constrain MSA's past, current, and future impacts upon the global aerosol size distribution and radiative forcing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1983

Studies on lasers and laser devices

The goal of this grant was to study lasers, laser devices, and uses of lasers for investigating physical phenomena are studied. The active projects included the development of a tunable, narrowband XUV light source and its application to the spectroscopy of core excited atomic states, and the development of a technique for picosecond time resolution spectroscopy of fast photophysical processes.

Harris, S. E.↗
NASA NTRS1984

Detection of shocked atomic gas in the Kleinmann-Low nebula

The 63 micrometer (3)p(1)-(3)P(2) fine structure line emission of neutral atomic oxygen at the center of the Orion nebula with a resolution of 30" is presented. There are three main emission peaks. One is associated with the region of strongest thermal radio continuum radiation close to the Trapezium cluster, and probably arises at the interface between the HII region and the dense Orion molecular cloud. The other two line emission peaks, associated with the Kleinmann Low nebula, are similar in both distribution and velocity to those of the 2 micrometer S(1) line of molecular hydrogen and of the high velocity wings of rotational CO emission. The OI emission from the KL nebula can be produced in the shocked gas associated with the mass outflows in this region and is an important coolant of the shocked gas.

Werner, M.↗
NASA NTRS1984

Non-Imaging Remote Sensing Data

Spectrometric, radiometric and polarimetric remote sensing observations of wavelengths from gamma-rays to microwaves are addressed. The basic form of the data is one dimensional arrays. At the high energy end of the spectrum, data are typically presented as pulse count versus energy, and at lower energies, as intensity versus wavelength. High spectral resolution measurements (better 1% of wavelength) are particularly useful for identifying atomic, molecular, and ionic species while broader band measurements are adequate for identifying minerals and for determining total energy fluxes. Polarization data permit the study of finely divided material such as clouds and surfaces.

Source record↗
NASA NTRS1984

Detection of shocked atomic gas in the Kleinmann-Low nebula

The 63 micrometer (3)P(1)-(3)P(2) fine structure line emission of neutral atomic oxygen at the center of the Orion nebula with a resolution of 30'' is presented. There are three main emission peaks. One is associated with the region of strongest thermal radio continuum radiation close to the Trapezium cluster, and probably arises at the interface between the H II region and the dense Orion molecular cloud. The other two line emission peaks, associated with the Kleinmann-Low nebula, are similar in both distribution and velocity to those of the 2 micrometer S(1) line of molecular hydrogen and of the high velocity wings of rotational CO emission. The OI emission from the KL nebula can be produced in the shocked gas associated with the mass outflows in this region and is an important coolant of the shocked gas.

Werner, M. W.↗
NASA NTRS1991

Observations of Comet Levy (1990c) with the Hopkins Ultraviolet Telescope

Observations of Comet Levy (1990c) were made with the Hopkins Ultraviolet Telescope during the Astro-1 Space Shuttle mission on December 10, 1990. The spectrum, covering the wavelength range 415-1850 A at a spectral resolution of 3 A, shows the presence of carbon monoxide and atomic hydrogen, carbon, and sulfur in the coma. Aside from H I Lyman-beta, no cometary features are detected below 1200 A, although cometary O I and O II would be masked by the same emissions present in the day airglow spectrum. The 9.4 x 116 arcsecond aperture corresponds to 12,000 x 148,000 km at the comet. The derived production rate of CO relative to water is 0.11 + or - 0.02, compared with 0.04 + or - 0.01 derived from IUE observations (made in September 1990) which sample a much smaller region of the coma. This suggests the presence of an extended source of CO, as was found in comet Halley. Upper limits on Ne and Ar abundance are within one order of magnitude of solar abundances.

Feldman, P. D.↗
NASA NTRS1993

Density wave triggered star formation in grand design spirals

In normal spiral galaxies the arms are the main sites for star formation. This is the cause of their optical contrast compared with the rest of the disc. The spiral structure can be observed as a higher concentration of H2 regions, neutral gas (both atomic and molecular via CO), dust and stars than in the interarm disc. It seens generally accepted that, at least in grand design spirals, there are density waves in the discs. However, several questions are not clear yet and still under discussion. An important question could be termed the triggering dilemma (by analogy with the 'winding dilemma' raised in the forties): Is the enhanced star formation in the spiral arms triggered by the passage of a system of density waves or is it simply due to the presence of a higher column density of gas there? In the present work, we use triggering in the same sense as the moderate to strong triggering defined by Elmegreen (1992), that is to say that star formation in the arms occurs at a rate faster than that in the interarm zone, relative to the available placental gas. Our group has designed several tests to elucidate whether or not star formation is triggered in the arms with respect to the interarm region and we summarize one of them, that of the ratio of the star formation efficiency in the arms divided by that of the interarm zone at the same galactocentric distance which we may call the relative massive star formation efficiency, where the efficiency is defined using the ratio of the mass of stars (evaluated via the H alpha flux) to the mass of neutral gas, atomic plus molecular (which must be measured with the adequate angular resolution). If the relative efficiency is of order unity, the star formation is proportional to the mass of gas, if some kind of induced star formation is present, the relative efficiency should be considerably larger than unity.

Cepa, J.↗