28,274 research outputs found
Three thousand hour electrical test of an alumina trilayer at 1325 K
Prolonged electrical test of bonded alumina trilayer at 1325 K in vacuum environmen
Circumplanetary disks around young giant planets: a comparison between core-accretion and disk instability
Circumplanetary disks can be found around forming giant planets, regardless
of whether core accretion or gravitational instability built the planet. We
carried out state-of-the-art hydrodynamical simulations of the circumplanetary
disks for both formation scenarios, using as similar initial conditions as
possible to unveil possible intrinsic differences in the circumplanetary disk
mass and temperature between the two formation mechanisms. We found that the
circumplanetary disks mass linearly scales with the circumstellar disk mass.
Therefore, in an equally massive protoplanetary disk, the circumplanetary disks
formed in the disk instability model can be only a factor of eight more massive
than their core-accretion counterparts. On the other hand, the bulk
circumplanetary disk temperature differs by more than an order of magnitude
between the two cases. The subdisks around planets formed by gravitational
instability have a characteristic temperature below 100 K, while the core
accretion circumplanetary disks are hot, with temperatures even greater than
1000 K when embedded in massive, optically thick protoplanetary disks. We
explain how this difference can be understood as the natural result of the
different formation mechanisms. We argue that the different temperatures should
persist up to the point when a full-fledged gas giant forms via disk
instability, hence our result provides a convenient criteria for observations
to distinguish between the two main formation scenarios by measuring the bulk
temperature in the planet vicinity.Comment: 12 pages, 9 figures, 1 table, accepted for publication at MNRA
Electrical heating tests of uranium dioxide external fuel configuration at emitter temperature of 1900 K
Testing of two fuel clad specimens for thermionic reactor application is described. The annular UO2 fuel was clad on both sides with tungsten; heat rejection was radially inward. The tests were intended to study inner clad stability, fuel redistribution, and fuel melting problems. The specimens were tested in a vacuum chamber using electron bombardment heating. Fuel structural changes were studied using periodic gammagraphs and posttest metallography. The first specimen test was terminated at 50 hours because of a braze failure. The second specimen was tested for 240 hours when an outer clad leak developed due to a tungsten-water reaction. The fuel developed numerous cracks on cooldown but the inner clad remained dimensionally stable. The fuel cover gas did not impede the rate of fuel redistribution. Posttest examination showed the fuel had not melted during operation
Nuclei embedded in an electron gas
The properties of nuclei embedded in an electron gas are studied within the
relativistic mean-field approach. These studies are relevant for nuclear
properties in astrophysical environments such as neutron-star crusts and
supernova explosions. The electron gas is treated as a constant background in
the Wigner-Seitz cell approximation. We investigate the stability of nuclei
with respect to alpha and beta decay. Furthermore, the influence of the
electronic background on spontaneous fission of heavy and superheavy nuclei is
analyzed. We find that the presence of the electrons leads to stabilizing
effects for both decay and spontaneous fission for high electron
densities. Furthermore, the screening effect shifts the proton dripline to more
proton-rich nuclei, and the stability line with respect to beta decay is
shifted to more neutron-rich nuclei. Implications for the creation and survival
of very heavy nuclear systems are discussed.Comment: 35 pages, latex+ep
Stabilizing the surface morphology of Si1–x–yGexCy/Si heterostructures grown by molecular beam epitaxy through the use of a silicon-carbide source
Si1–x–yGexCy/Si superlattices were grown by solid-source molecular beam epitaxy using silicon carbide as a source of C. Samples consisting of alternating layers of nominally 25 nm Si1–x–yGexCy and 35 nm Si for 10 periods were characterized by high-resolution x-ray diffraction, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry to determine strain, thickness, and composition. C resonance backscattering and secondary ion mass spectrometries were used to measure the total C concentration in the Si1–x–yGexCy layers, allowing for an accurate determination of the substitutional C fraction to be made as a function of growth rate for fixed Ge and substitutional C compositions. For C concentrations close to 1%, high-quality layers were obtained without the use of Sb-surfactant mediation. These samples were found to be structurally perfect to a level consistent with cross-sectional TEM (< 10^7 defects/cm^2) and showed considerably improved homogeneity as compared with similar structures grown using graphite as the source for C. For higher Ge and C concentrations, Sb-surfactant mediation was found to be required to stabilize the surface morphology. The maximum value of substitutional C concentration, above which excessive generation of stacking fault defects caused polycrystalline and/or amorphous growth, was found to be approximately 2.4% in samples containing between 25 and 30% Ge. The fraction of substitutional C was found to decrease from roughly 60% by a factor of 0.86 as the Si1–x–yGexCy growth rate increased from 0.1 to 1.0 nm/s
Growing Massive Black Hole Pairs in Minor Mergers of Disk Galaxies
We perform a suite of high-resolution smoothed particle hydrodynamics
simulations to investigate the orbital decay and mass evolution of massive
black hole (MBH) pairs down to scales of ~30 pc during minor mergers of disk
galaxies. Our simulation set includes star formation and accretion onto the
MBHs, as well as feedback from both processes. We consider 1:10 merger events
starting at z~3, with MBH masses in the sensitivity window of the Laser
Interferometer Space Antenna, and we follow the coupling between the merger
dynamics and the evolution of the MBH mass ratio until the satellite galaxy is
tidally disrupted. While the more massive MBH accretes in most cases as if the
galaxy were in isolation, the satellite MBH may undergo distinct episodes of
enhanced accretion, owing to strong tidal torques acting on its host galaxy and
to orbital circularization inside the disk of the primary galaxy. As a
consequence, the initial 1:10 mass ratio of the MBHs changes by the time the
satellite is disrupted. Depending on the initial fraction of cold gas in the
galactic disks and the geometry of the encounter, the mass ratios of the MBH
pairs at the time of satellite disruption can stay unchanged or become as large
as 1:2. Remarkably, the efficiency of MBH orbital decay correlates with the
final mass ratio of the pair itself: MBH pairs that increase significantly
their mass ratio are also expected to inspiral more promptly down to
nuclear-scale separations. These findings indicate that the mass ratios of MBH
pairs in galactic nuclei do not necessarily trace the mass ratios of their
merging host galaxies, but are determined by the complex interplay between gas
accretion and merger dynamics.Comment: 5 pages, 4 figures, replaced to match accepted version on Ap
Effects of zinc on microalgal biofilms in intertidal and subtidal habitats
Microalgal biofilms are sensitive to environmental conditions. Impacts of contaminants on assemblages of marine biofilm are often investigated in laboratories or in mesocosms. Such experiments are rarely representative of the effects of contaminants on biofilms under natural conditions. Studies in field situations, with enough power to detect impacts, are necessary to develop a better understanding of the effects of contaminants on ecological processes. Metals are a common contaminant of marine systems and can cause disturbances to assemblages. Using a new technique to experimentally deliver contaminants to microalgal assemblages, hypotheses were tested regarding the effects of zinc on microalgal biofilms growing on settlement panels in subtidal and intertidal habitats. PAM fluorometry was used to assess the amount and physiological state of biofilms on panels. Control panels deployed for 1 month in each habitat had significantly greater amounts of biofilm than those exposed to zinc. After deployment for 3 months, the results varied with location. The observed effects on the biofilm did not, however, cause significant changes in the macro-invertebrate assemblages that developed on the panels
Electronic properties of Si/Si1–x–yGexCy heterojunctions
We have used admittance spectroscopy and deep-level transient spectroscopy to characterize electronic properties of Si/Si1–x–yGexCy heterostructures. Band offsets measured by admittance spectroscopy for compressively strained Si/Si1–x–yGexCy heterojunctions indicate that incorporation of C into Si1–x–yGexCy lowers both the valence- and conduction-band edges compared to those in Si1–xGex by an average of 107 ± 6 meV/% C and 75 ± 6 meV/% C, respectively. Combining these measurements indicates that the band alignment is type I for the compositions we have studied, and that these results are consistent with previously reported results on the energy band gap of Si1–x–yGexCy and with measurements of conduction band offsets in Si/Si1–yCy heterojunctions. Several electron traps were observed using deep-level transient spectroscopy on two n-type heterostructures. Despite the presence of a significant amount of nonsubstitutional C (0.29–1.6 at. %), none of the peaks appear attributable to previously reported interstitial C levels. Possible sources for these levels are discussed
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