2,003 research outputs found
Mechanical properties of wood-derived silicon carbide aluminum-alloy composites as a function of temperature
The mechanical behavior [i.e., stiffness, strength, and toughness (K_(IC))] of SiC AlâSiâMg metalâceramic composites (50:50 by volume) was studied at temperatures ranging from 25 to 500 °C. The SiC phase was derived from wood precursors, which resulted in an interconnected anisotropic ceramic that constrained the pressure melt-infiltrated aluminum alloy. The composites were made using SiC derived from two woods (sapele and beech) and were studied in three orthogonal orientations. The mechanical properties and corresponding deformation micromechanisms were different in the longitudinal (LO) and transverse directions, but the influence of the precursor wood was small. The LO behavior was controlled by the rigid SiC preform and the load transfer from the metal to the ceramic. Moduli in this orientation were lower than the HalpinâTsai predictions due to the nonlinear and nonparallel nature of the Al-filled pores. The LO K_(IC) agreed with the Ashby model for the K_(IC) contribution of ductile inclusions in a brittle ceramic
Kinetics of the inner ring in the exciton emission pattern in GaAs coupled quantum wells
We report on the kinetics of the inner ring in the exciton emission pattern.
The formation time of the inner ring following the onset of the laser
excitation is found to be about 30 ns. The inner ring was also found to
disappear within 4 ns after the laser termination. The latter process is
accompanied by a jump in the photoluminescence (PL) intensity. The spatial
dependence of the PL-jump indicates that the excitons outside of the region of
laser excitation, including the inner ring region, are efficiently cooled to
the lattice temperature even during the laser excitation. The ring formation
and disappearance are explained in terms of exciton transport and cooling.Comment: 19 pages, 6 figure
Measurement techniques and instruments suitable for life-prediction testing of photovoltaic arrays
Array failure modes, relevant materials property changes, and primary degradation mechanisms are discussed as a prerequisite to identifying suitable measurement techniques and instruments. Candidate techniques and instruments are identified on the basis of extensive reviews of published and unpublished information. These methods are organized in six measurement categories - chemical, electrical, optical, thermal, mechanical, and other physicals. Using specified evaluation criteria, the most promising techniques and instruments for use in life prediction tests of arrays were selected
The Case for Optically-Thick High Velocity Broad Line Region Gas in Active Galactic Nuclei
A combined analysis of the profiles of the main broad quasar emission lines
in both Hubble Space Telescope and optical spectra shows that while the
profiles of the strong UV lines are quite similar, there is frequently a strong
increase in the Ly-alpha/H-alpha ratio in the high-velocity gas. We show that
the suggestion that the high velocity gas is optically-thin presents many
problems. We show that the relative strengths of the high velocity wings arise
naturally in an optically-thick BLR component. An optically-thick model
successfully explains the equivalent widths of the lines, the Ly-alpha/H-alpha
ratios and flatter Balmer decrements in the line wings, the strengths of CIII]
and the lambda 1400 blend, and the strong variability of high-velocity,
high-ionization lines (especially HeII and HeI).Comment: 34 pages in AASTeX, including 10 pages of figures. Submitted to
Astrophysical Journa
The UV Properties of the Narrow Line Quasar I Zwicky 1
I Zw 1 is the prototype narrow line quasar. We report here the results of our
study of the UV emission of I Zw 1 using a high S/N (50-120) spectrum obtained
with the HST FOS. The following main new results are obtained: 1. The Mg II and
Al III doublets are partially/fully resolved. The measured doublet ratios
verify theoretical predictions that the lines are thermalized in the BLR. 2. A
weak associated UV absorption system is detected in N~V, and possibly also in C
IV and Lya, suggesting an outflow with a velocity of 1870 km/s and velocity
dispersion <300 km/s. 3. Lines from ions of increasing ionization level show
increasing excess blue wing flux, and an increasing line peak velocity shift,
reaching a maximum blueshift of about 2000 km/s for He II 1640. This may
indicate an out-flowing component in the BLR, where the ionization level
increases with velocity, and which is visible only in the approaching
direction. The highest velocity part of this outflow may produce the associated
UV absorption system. 4. The small C III] 1909 EW, and the small C III]
1909/Lya and C III] 1909/Si III] 1892 flux ratios indicate a typical BLR
density of 10^11, i.e. about an order of magnitude larger than implied by C
III] 1909 in most quasars. A BLR component of a higher density is implied by
the EW and doublet ratio of the Al III 1857 doublet. 5. Prominent Fe II UV 191
emission is seen, together with weaker line emission at 1294 and 1871 A. These
three features have been proposed as evidence for significant Lya pumping of
the 8-10 eV levels of Fe II. 6. Significant Fe III emission is present. The Fe
III UV 34 and UV 48 multiplets are clearly resolved, and Fe III UV 1, UV 47, UV
50, and UV 68 may also be present. (Shortened version)Comment: 28 pages, 1 table and 7 figures included. Uses aas2pp4.sty. Scheduled
for the Astrophysical Journal November 10, 1997 issue, Vol. 48
An Atlas of Computed Equivalent Widths of Quasar Broad Emission Lines
We present graphically the results of several thousand photoionization
calculations of broad emission line clouds in quasars, spanning seven orders of
magnitude in hydrogen ionizing flux and particle density. The equivalent widths
of 42 quasar emission lines are presented as contours in the particle density -
ionizing flux plane for a typical incident continuum shape, solar chemical
abundances, and cloud column density of . Results are
similarly given for a small subset of emission lines for two other column
densities ( and ), five other incident
continuum shapes, and a gas metallicity of 5 \Zsun. These graphs should prove
useful in the analysis of quasar emission line data and in the detailed
modeling of quasar broad emission line regions. The digital results of these
emission line grids and many more are available over the Internet.Comment: 16 pages, LaTeX (AASTeX aaspp4.sty); to appear in the 1997 ApJS: full
contents of the 9 photoionization grids presented in this paper may be found
at http://www.pa.uky.edu/~korista/grids/grids.htm
X-ray Insights Into Interpreting CIV Blueshifts and Optical/UV Continua
We present 0.5-8.0 keV Chandra observations of six bright quasars that
represent extrema in quasar emission-line properties -- three quasars each with
small and large blueshifts of the CIV emission line with respect to the
systemic redshift of the quasars. Supplemented with seven archival Chandra
observations of quasars that met our selection criteria, we investigate the
origin of this emission-line phenomenon in the general context of the structure
of quasars. We find that the quasars with the largest CIV blueshifts show
evidence, from joint-spectral fitting, for intrinsic X-ray absorption (N_H ~
10^22 cm^-2). Given the lack of accompanying CIV absorption, this gas is likely
to be highly ionized, and may be identified with the shielding gas in the
disk-wind paradigm. Furthermore, we find evidence for a correlation of
alpha_uv, the ultraviolet spectral index, with the hardness of the X-ray
continuum; an analysis of independent Bright Quasar Survey data from the
literature supports this conclusion. This result points to intrinsically red
quasars having systematically flatter hard X-ray continua without evidence for
X-ray absorption. We speculate on the origins of these correlations of X-ray
properties with both CIV blueshift and alpha_uv and discuss the implications
for models of quasar structure.Comment: 9 figs, 25 pages, AASTeX; accepted for publication in A
Chandra Multiwavelength Project: Normal Galaxies at Intermediate Redshift
(abridged) We have investigated 136 Chandra extragalactic sources without
broad optical emission lines, including 93 galaxies with narrow emission lines
(NELG) and 43 with only absorption lines (ALG). Based on fx/fo, Lx, X-ray
spectral hardness and optical emission line diagnostics, we have conservatively
classified 36 normal galaxies (20 spirals and 16 ellipticals) and 71 AGNs. We
found no statistically significant evolution in Lx/LB, within the limited z
range. We have built log(N)-log(S), after correcting for completeness based on
a series of simulations. The best-fit slope is -1.5 for both S and B energy
bands, which is considerably steeper than that of the AGN-dominated cosmic
background sources, but slightly flatter than the previous estimate, indicating
normal galaxies will not exceed the AGN population until fx ~ 2 x 10-18 erg s-1
cm-2 (a factor of ~5 lower than the previous estimate). A group of NELGs appear
to be heavily obscured in X-rays, i.e., a typical type 2 AGN. After correcting
for intrinsic absorption, their X-ray luminosities could be Lx > 10^44 erg s-1,
making them type 2 quasar candidates. While most X-ray luminous ALGs (XBONG -
X-ray bright, optically normal galaxy candidates) do not appear to be
significantly absorbed, we found two heavily obscured objects, which could be
as luminous as an unobscured broad-line quasar. Among 43 ALGs, we found two E+A
galaxy candidates with strong Balmer absorption lines, but no [OII] line. The
X-ray spectra of both galaxies are soft and one of them has a nearby close
companion galaxy, supporting the merger/interaction scenario rather than the
dusty starburst hypothesis.Comment: 31 pages, 9 figures, accepted for publication in ApJ (20 June 2006,
v644), replaced with minor correction
Combined ground and aerial measurements resolve vent-specific gas fluxes from a multi-vent volcano
Volcanoes with multiple summit vents present a methodological challenge for determining vent-specific gas emissions. Here, using a novel approach combining multiple ultraviolet cameras with synchronous aerial measurements, we calculate vent-specific gas compositions and fluxes for Stromboli volcano. Emissions from vent areas are spatially heterogeneous in composition and emission rate, with the central vent area dominating passive emissions, despite exhibiting the least explosive behaviour. Vents exhibiting Strombolian explosions emit low to negligible passive fluxes and are CO2-dominated, even during passive degassing. We propose a model for the conduit system based on contrasting rheological properties between vent areas. Our methodology has advantages for resolving contrasting outgassing dynamics given that measured bulk plume compositions are often intermediate between those of the distinct vent areas. We therefore emphasise the need for a vent-specific approach at multi-vent volcanoes and suggest that our approach could provide a transformative advance in volcano monitoring applications
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