3,179 research outputs found
Improved silicon nitride for advanced heat engines
The results of a four year program to improve the strength and reliability of injection-molded silicon nitride are summarized. Statistically designed processing experiments were performed to identify and optimize critical processing parameters and compositions. Process improvements were monitored by strength testing at room and elevated temperatures, and microstructural characterization by optical, scanning electron microscopes, and scanning transmission electron microscope. Processing modifications resulted in a 20 percent strength and 72 percent Weibull slope improvement of the baseline material. Additional sintering aids screening and optimization experiments succeeded in developing a new composition (GN-10) capable of 581.2 MPa at 1399 C. A SiC whisker toughened composite using this material as a matrix achieved a room temperature toughness of 6.9 MPa m(exp .5) by the Chevron notched bar technique. Exploratory experiments were conducted on injection molding of turbocharger rotors
Improved silicon nitride for advanced heat engines
The technology base required to fabricate silicon nitride components with the strength, reliability, and reproducibility necessary for actual heat engine applications is presented. Task 2 was set up to develop test bars with high Weibull slope and greater high temperature strength, and to conduct an initial net shape component fabrication evaluation. Screening experiments were performed in Task 7 on advanced materials and processing for input to Task 2. The technical efforts performed in the second year of a 5-yr program are covered. The first iteration of Task 2 was completed as planned. Two half-replicated, fractional factorial (2 sup 5), statistically designed matrix experiments were conducted. These experiments have identified Denka 9FW Si3N4 as an alternate raw material to GTE SN502 Si3N4 for subsequent process evaluation. A detailed statistical analysis was conducted to correlate processing conditions with as-processed test bar properties. One processing condition produced a material with a 97 ksi average room temperature MOR (100 percent of goal) with 13.2 Weibull slope (83 percent of goal); another condition produced 86 ksi (6 percent over baseline) room temperature strength with a Weibull slope of 20 (125 percent of goal)
Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. III. Detection of Fe II Reverberation in Nine Narrow-Line Seyfert 1 Galaxies
This is the third in a series of papers reporting on a large
reverberation-mapping campaign aimed to study the properties of active galactic
nuclei (AGNs) with high accretion rates. We present new results on the
variability of the optical Fe II emission lines in 10 AGNs observed by the
Yunnan Observatory 2.4m telescope during 2012--2013. We detect statistically
significant time lags, relative to the AGN continuum, in nine of the sources.
This accurate measurement is achieved by using a sophisticated spectral fitting
scheme that allows for apparent flux variations of the host galaxy, and several
narrow lines, due to the changing observing conditions. Six of the newly
detected lags are indistinguishable from the Hbeta lags measured in the same
sources. Two are significantly longer and one is slightly shorter. Combining
with Fe II lags reported in previous studies, we find a Fe II
radius--luminosity relationship similar to the one for Hbeta, although our
sample by itself shows no clear correlation. The results support the idea that
Fe II emission lines originate in photoionized gas which, for the majority of
the newly reported objects, is indistinguishable from the Hbeta-emitting gas.
We also present a tentative correlation between the lag and intensity of Fe II
and Hbeta and comment on its possible origin.Comment: 14 pages, 10 figures, accepted for publication in The Astrophysical
Journa
III–V Nanowires: Synthesis, Property Manipulations, and Device Applications
III–V semiconductor nanowire (NW) materials possess a combination of fascinating properties, including their tunable direct bandgap, high carrier mobility, excellent mechanical flexibility, and extraordinarily large surface-to-volume ratio, making them superior candidates for next generation electronics, photonics, and sensors, even possibly on flexible substrates. Understanding the synthesis, property manipulation, and device integration of these III–V NW materials is therefore crucial for their practical implementations. In this review, we present a comprehensive overview of the recent development in III–V NWs with the focus on their cost-effective synthesis, corresponding property control, and the relevant low-operating-power device applications. We will first introduce the synthesis methods and growth mechanisms of III–V NWs, emphasizing the low-cost solid-source chemical vapor deposition (SSCVD) technique, and then discuss the physical properties of III–V NWs with special attention on their dependences on several typical factors including the choice of catalysts, NW diameters, surface roughness, and surface decorations. After that, we present several different examples in the area of high-performance photovoltaics and low-power electronic circuit prototypes to further demonstrate the potential applications of these NW materials. Towards the end, we also make some remarks on the progress made and challenges remaining in the III–V NW research field
SDSS J013127.34032100.1: A newly discovered radio-loud quasar at with extremely high luminosity
Only very few z>5 quasars discovered to date are radio-loud, with a
radio-to-optical flux ratio (radio-loudness parameter) higher than 10. Here we
report the discovery of an optically luminous radio-loud quasar, SDSS
J013127.34-032100.1 (J0131-0321 in short), at z=5.18+-0.01 using the Lijiang
2.4m and Magellan telescopes. J0131-0321 has a spectral energy distribution
consistent with that of radio-loud quasars. With an i-band magnitude of 18.47
and radio flux density of 33 mJy, its radio-loudness parameter is ~100. The
optical and near-infrared spectra taken by Magellan enable us to estimate its
bolometric luminosity to be L_bol ~ 1.1E48 erg/s, approximately 4.5 times
greater than that of the most distant quasar known to date. The black hole mass
of J0131-0321 is estimated to be 2.7E9 solar masses, with an uncertainty up to
0.4 dex. Detailed physical properties of this high-redshift, radio-loud,
potentially super-Eddington quasar can be probed in the future with more
dedicated and intensive follow-up observations using multi-wavelength
facilities.Comment: 5 pages, 3 figures, accepted to ApJ
Remarkable Disk and Off-nuclear Starburst Activity in the "Tadpole Galaxy" as revealed by the Spitzer Space Telescope
We present ground-based optical and Spitzer infrared imaging observations of
the interacting galaxy UGC 10214, the "Tadpole Galaxy" (z = 0.0310), focusing
on the star formation activity in the nuclear, disk, spiral arms and tidal tail
regions. The major findings of this study are that the Tadpole is actively
forming stars in the main disk outside of the nucleus and in the tidal plume,
with an estimated mean star formation rate of ~2 to 4 M_sun/yr. The most
prominent sites of mid-infrared emission define a "ring" morphology that,
combined with the overall morphology of the system, suggest the interaction may
belong to the rare class of off-center collisional ring systems that form both
shock-induced rings of star formation and tidal plumes. The nuclear emission is
solely powered by older stars, with little evidence for ongoing star formation
at the center of the Tadpole. Extra-nuclear star formation accounts for >50% of
the total star formation in the disk and spiral arms, featuring infrared-bright
'hot spots' that exhibit strong PAH emission, whose band strength is comparable
to that of late-type star-forming disk galaxies. The tidal tail, which extends
2 arcmin (~75 kpc) into the intergalactic medium, is populated by super massive
star clusters likely triggered by the galaxy-galaxy interaction that has
distorted UGC 10214 into its current "tadpole" shape.Comment: to appear in the January 2006 (vol 131) issue of the Astronomical
Journal; high quality graphics are located here:
http://spider.ipac.caltech.edu/staff/jarrett/tadpole.htm
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