High temperature glass coatings for superalloys and refractory metals

New glasses are used as protective coatings on metals and alloys susceptible to oxidation at high temperatures in oxidizing atmospheres. Glasses are stable and solid at temperatures up to 1000 deg C, adhere well to metal surfaces, and are usable for metals with broad range of expansion coefficients

High Energy Gamma-Ray Emission From Blazars: EGRET Observations

We will present a summary of the observations of blazars by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO). EGRET has detected high energy gamma-ray emission at energies greater than 100 MeV from more that 50 blazars. These sources show inferred isotropic luminosities as large as $3\times 10^{49}$ ergs s$^{-1}$. One of the most remarkable characteristics of the EGRET observations is that the gamma-ray luminosity often dominates the bolometric power of the blazar. A few of the blazars are seen to exhibit variability on very short time-scales of one day or less. The combination of high luminosities and time variations seen in the gamma-ray data indicate that gamma-rays are an important component of the relativistic jet thought to characterize blazars. Currently most models for blazars involve a beaming scenario. In leptonic models, where electrons are the primary accelerated particles, gamma-ray emission is believed to be due to inverse Compton scattering of low energy photons, although opinions differ as to the source of the soft photons. Hardronic models involve secondary production or photomeson production followed by pair cascades, and predict associated neutrino production.Comment: 16 pages, 7 figures, style files included. Invited review paper in "Observational Evidence for Black Holes in the Universe," 1999, ed. S. K. Chakrabarti (Dordrecht: Kluwer), 215-23

A Survey for Transient Astronomical Radio Emission at 611 MHz

We have constructed and operated the Survey for Transient Astronomical Radio Emission (STARE) to detect transient astronomical radio emission at 611 MHz originating from the sky over the northeastern United States. The system is sensitive to transient events on timescales of 0.125 s to a few minutes, with a typical zenith flux density detection threshold of approximately 27 kJy. During 18 months of around-the-clock observing with three geographically separated instruments, we detected a total of 4,318,486 radio bursts. 99.9% of these events were rejected as locally generated interference, determined by requiring the simultaneous observation of an event at all three sites for it to be identified as having an astronomical origin. The remaining 3,898 events have been found to be associated with 99 solar radio bursts. These results demonstrate the remarkably effective RFI rejection achieved by a coincidence technique using precision timing (such as GPS clocks) at geographically separated sites. The non-detection of extra-solar bursting or flaring radio sources has improved the flux density sensitivity and timescale sensitivity limits set by several similar experiments in the 1970s. We discuss the consequences of these limits for the immediate solar neighborhood and the discovery of previously unknown classes of sources. We also discuss other possible uses for the large collection of 611 MHz monitoring data assembled by STARE.Comment: 24 pages, 6 figures; to appear in PAS

The EGRET data products

We describe the Energetic Gamma Ray Experiment Telescope (EGRET) data products which we anticipate will suffice for virtually all guest and archival investigations. The production process, content, availability, format, and the associated software of each product is described. Supplied here is sufficient detail for each researcher to do analysis which is not supported by extant software

The EGRET high energy gamma ray telescope

The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (GRO) is sensitive in the energy range from about 20 MeV to about 30,000 MeV. Electron-positron pair production by incident gamma photons is utilized as the detection mechanism. The pair production occurs in tantalum foils interleaved with the layers of a digital spark chamber system; the spark chamber records the tracks of the electron and positron, allowing the reconstruction of the arrival direction of the gamma ray. If there is no signal from the charged particle anticoincidence detector which surrounds the upper part of the detector, the spark chamber array is triggered by two hodoscopes of plastic scintillators. A time of flight requirement is included to reject events moving backward through the telescope. The energy of the gamma ray is primarily determined by absorption of the energies of the electron and positron in a 20 cm deep NaI(Tl) scintillator

Design Rules for Self-Assembly of 2D Nanocrystal/Metal-Organic Framework Superstructures.

We demonstrate the guiding principles behind simple two dimensional self-assembly of MOF nanoparticles (NPs) and oleic acid capped iron oxide (Fe3 O4 ) NCs into a uniform two-dimensional bi-layered superstructure. This self-assembly process can be controlled by the energy of ligand-ligand interactions between surface ligands on Fe3 O4 NCs and Zr6 O4 (OH)4 (fumarate)6 MOF NPs. Scanning transmission electron microscopy (TEM)/energy-dispersive X-ray spectroscopy and TEM tomography confirm the hierarchical co-assembly of Fe3 O4 NCs with MOF NPs as ligand energies are manipulated to promote facile diffusion of the smaller NCs. First-principles calculations and event-driven molecular dynamics simulations indicate that the observed patterns are dictated by combination of ligand-surface and ligand-ligand interactions. This study opens a new avenue for design and self-assembly of MOFs and NCs into high surface area assemblies, mimicking the structure of supported catalyst architectures, and provides a thorough fundamental understanding of the self-assembly process, which could be a guide for designing functional materials with desired structure

On the Properties of Plastic Ablators in Laser-Driven Material Dynamics Experiments

Radiation hydrodynamics simulations were used to study the effect of plastic ablators in laser-driven shock experiments. The sensitivity to composition and equation of state was found to be 5-10% in ablation pressure. As was found for metals, a laser pulse of constant irradiance gave a pressure history which decreased by several percent per nanosecond. The pressure history could be made more constant by adjusting the irradiance history. The impedance mismatch with the sample gave an increase o(100%) in the pressure transmitted into the sample, for a reduction of several tens of percent in the duration of the peak load applied to the sample, and structured the release history by adding a release step to a pressure close to the ablation pressure. Algebraic relations were found between the laser pulse duration, the ablator thickness, and the duration of the peak pressure applied to the sample, involving quantities calculated from the equations of state of the ablator and sample using shock dynamics.Comment: Typos fixe

Development of Carbon Dioxide Removal Systems for Advanced Exploration Systems

"NASA's Advanced Exploration Systems (AES) program is pioneering new approaches for rapidly developing prototype systems, demonstrating key capabilities, and validating operational concepts for future human missions beyond Earth orbit" (NASA 2012). These forays beyond the confines of earth's gravity will place unprecedented demands on launch systems. They must not only blast out of earth's gravity well as during the Apollo moon missions, but also launch the supplies needed to sustain a crew over longer periods for exploration missions beyond earth's moon. Thus all spacecraft systems, including those for the separation of metabolic carbon dioxide and water from a crewed vehicle, must be minimized with respect to mass, power, and volume. Emphasis is also placed on system robustness both to minimize replacement parts and ensure crew safety when a quick return to earth is not possible. Current efforts are focused on improving the current state-of-the-art systems utilizing fixed beds of sorbent pellets by seeking more robust pelletized sorbents, evaluating structured sorbents, and examining alternate bed configurations to improve system efficiency and reliability. These development efforts combine testing of sub-scale systems and multi-physics computer simulations to evaluate candidate approaches, select the best performing options, and optimize the configuration of the selected approach, which is then implemented in a full-scale integrated atmosphere revitalization test. This paper describes the carbon dioxide (CO2) removal hardware design and sorbent screening and characterization effort in support of the Atmosphere Resource Recovery and Environmental Monitoring (ARREM) project within the AES program. A companion paper discusses development of atmosphere revitalization models and simulations for this project

Optical and radio variability of the BL Lac object AO 0235+16: a possible 5-6 year periodicity

New optical and radio data on the BL Lacertae object AO 0235+16 have been collected in the last four years by a wide international collaboration, which confirm the intense activity of this source. The optical data also include the results of the Whole Earth Blazar Telescope (WEBT) first-light campaign organized in November 1997. The optical spectrum is observed to basically steepen when the source gets fainter. We have investigated the existence of typical variability time scales and of possible correlations between the optical and radio emissions by means of visual inspection, Discrete Correlation Function analysis, and Discrete Fourier Transform technique. The major radio outbursts are found to repeat quasi-regularly with a periodicity of about 5.7 years; this period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them.Comment: to be published in A&

Multiwavelength study of TeV Blazar Mrk421 during giant flare

Context: The nearby (z=0.031) TeV blazar Mrk421 was reported to be in a high state of flux activity since November, 2009. Aims: To investigate possible changes in the physical parameters of Mrk421 during its high state of activity using multiwavelength data. Methods: We have observed this source in bright state using High Altitude GAmma Ray (HAGAR) telescope array at energies above 250 GeV during February 13 - 19, 2010. Optical, X-ray and gamma-ray archival data are also used to obtain the SEDs and light curves. Results: Mrk421 was found to undergo one of its brightest flaring episodes on February 17, 2010 by various observations in X-rays and gamma-rays. HAGAR observations during February 13 - 19, 2010 at the energies above 250 GeV show an enhancement in the flux level, with a maximum flux of ~ 7 Crab units being detected on February 17, 2010. We present the spectral energy distributions during this flaring episode and investigate the correlation of the variability in X-ray and gamma-ray bands. Conclusions: Our multiwavelength study suggests that the flare detected during February 16 and 17, 2010 could arise due to a passing shock in the jet.Comment: 9 pages, 10 figures, Accepted for publication in Astronomy and Astrophysics (A & A