67 research outputs found
A dissolution rate study of beryllium in hydrofluoric acid
Beryllium was shown to react stoichiometrically with HF, HCl, and H2SO4 acids in accordance with a plus two valence. These reactions were investigated by the hydrogen evolution method.
Dissolution rates were determined in five concentrations, 0.01, 0.05, 0.10, 0.50, and 0,75 N, of HF acid at 10, 20, 30, 40 and 50°C, This was accomplished by the hydrogen evolution method and the results were based on the tested stoichiometry. Beryllium has a high dissolution rate in HF acid, which approximates the following under similar conditions: 10 times Al, 4 times Ti, and 1.5 times Zr. A kinetic evaluation of the dissolution results revealed a reaction order of 0.88 and an activation energy of 6.8 Kcal/mole, Unfortunately, due to the existence of complex surface conditions, these values proved to be inadequate to deduce a conclusive mechanism for the reaction. However, a tentative hypothesis is offered which assumes a diffusionally controlled process that is hindered by a BeO film. Future studies involving potential and difference effect measurements should prove effectual in developing an explanation of this reaction.
A preliminary investigation of the HCl dissolution reaction revealed a very different mode of attack which appeared to be primarily electrochemical. A most interesting feature of this reaction was its capacity to exhibit the phenomenon known as the chunk effect”. This resulted in fragments of the beryllium being dislodged into the solution during attack by the dilute acid. Limited dissolution rate data for this reaction indicated it to be more complex than for the HF acid reaction.
A review of the literature revealed no evidence of previous quantitative corrosion studies on beryllium in the above three acids --Abstract, pages ii-iii
Connectionist model-based stereo vision for telerobotics
Autonomous stereo vision for range measurement could greatly enhance the performance of telerobotic systems. Stereo vision could be a key component for autonomous object recognition and localization, thus enabling the system to perform low-level tasks, and allowing a human operator to perform a supervisory role. The central difficulty in stereo vision is the ambiguity in matching corresponding points in the left and right images. However, if one has a priori knowledge of the characteristics of the objects in the scene, as is often the case in telerobotics, a model-based approach can be taken. Researchers describe how matching ambiguities can be resolved by ensuring that the resulting three-dimensional points are consistent with surface models of the expected objects. A four-layer neural network hierarchy is used in which surface models of increasing complexity are represented in successive layers. These models are represented using a connectionist scheme called parameter networks, in which a parametrized object (for example, a planar patch p=f(h,m sub x, m sub y) is represented by a collection of processing units, each of which corresponds to a distinct combination of parameter values. The activity level of each unit in the parameter network can be thought of as representing the confidence with which the hypothesis represented by that unit is believed. Weights in the network are set so as to implement gradient descent in an energy function
Asteroseismic measurement of slow, nearly uniform surface-to-core rotation in the main-sequence F star KIC 9244992
We have found a rotationally split series of core g-mode triplets and surface p-mode multiplets in a main-sequence F star, KIC 9244992. Comparison with models shows that the star has a mass of about 1.45 M�, and is at an advanced stage of main-sequence evolution in which the central hydrogen abundance mass fraction is reduced to about 0.1. This is the second case, following KIC 11145123, of an asteroseismic determination of the rotation of the deep core and surface of an A-F main-sequence star. We have found, essentially model independently, that the rotation near the surface, obtained from p-mode splittings, is 66 d, slightly slower than the rotation of 64 d in the core, measured by g-mode splittings. KIC 9244992 is similar to KIC 11145123 in that both are near the end of main-sequence stage with very slow and nearly uniform rotation. This indicates the angular momentum transport in the interior of an A-F star during the main-sequence stage is much stronger than that expected from standard theoretical formulations
Polycyclic Aromatic Hydrocarbon and Emission Line Ratios in Active Galactic Nuclei and Starburst Galaxies
We study the Polycyclic Aromatic Hydrocarbons (PAH) bands, ionic emission
lines, and Mid-infrared continuum properties, in a sample of 171 emission line
galaxies taken from literature plus 15 new active galactic nuclei (AGN) Spitzer
spectra. The continuum shape steeply rises for longer wavelengths and can be
fitted with a warm blackbody distribution of T=150-300K. The brightest PAH
spectral bands (6.2, 7.7, 8.6, 11.3, and 12.7m) and the forbidden emission
lines of [Si II] 34.8m, [Ar II] 6.9, [S III] 18.7 and 33.4 were detected
in all the Starbursts and in ~80% of the Seyfert~2. Taking under consideration
only the PAH bands at 7.7m, 11.3m, and 12.7m we find they are
present in ~80% of the Seyfert 1, while only half of this type of activity show
the 6.2m and 8.6 PAH bands. The observed intensities ratios for neutral
and ionized PAHs (6.2/7.7 x 11.3/7.7) were compared to theoretical intensity
ratios, showing that AGNs have higher ionization fraction and larger PAH (> 180
carbon atoms) than SB galaxies. The ratio between the ionized (7.7) and the
neutral PAH bands (8.6 and 11.3) are distributed over different ranges for AGNs
and SB galaxies, suggesting that these ratios could depend on the ionization
fraction, as well as on the hardness of the radiation field. The ratio between
the 7.7 and 11.3 bands is nearly constant with the increase of [Ne III]15.5/[Ne
II], indicating that the fraction of ionized to neutral PAH bands does not
depend on the hardness of the radiation field. The equivalent width of both PAH
features show the same dependence with [Ne III]/[Ne II], suggesting that the
PAH, emitting either ionized (7.7) or neutral (11.3) bands, may be destroyed
with the increase of the hardness of the radiation field.Comment: Accepted by Ap
High-global warming potential F-gas emissions in California: comparison of ambient-based versus inventory-based emission estimates, and implications of refined estimates.
To provide information for greenhouse gas reduction policies, the California Air Resources Board (CARB) inventories annual emissions of high-global-warming potential (GWP) fluorinated gases, the fastest growing sector of greenhouse gas (GHG) emissions globally. Baseline 2008 F-gas emissions estimates for selected chlorofluorocarbons (CFC-12), hydrochlorofluorocarbons (HCFC-22), and hydrofluorocarbons (HFC-134a) made with an inventory-based methodology were compared to emissions estimates made by ambient-based measurements. Significant discrepancies were found, with the inventory-based emissions methodology resulting in a systematic 42% under-estimation of CFC-12 emissions from older refrigeration equipment and older vehicles, and a systematic 114% overestimation of emissions for HFC-134a, a refrigerant substitute for phased-out CFCs. Initial, inventory-based estimates for all F-gas emissions had assumed that equipment is no longer in service once it reaches its average lifetime of use. Revised emission estimates using improved models for equipment age at end-of-life, inventories, and leak rates specific to California resulted in F-gas emissions estimates in closer agreement to ambient-based measurements. The discrepancies between inventory-based estimates and ambient-based measurements were reduced from -42% to -6% for CFC-12, and from +114% to +9% for HFC-134a
Recombination Line vs. Forbidden Line Abundances in Planetary Nebulae
Recombination lines (RLs) of C II, N II, and O II in planetary nebulae (PNs)
have been found to give abundances that are much larger in some cases than
abundances from collisionally-excited forbidden lines (CELs). The origins of
this abundance discrepancy are highly debated. We present new spectroscopic
observations of O II and C II recombination lines for six planetary nebulae.
With these data we compare the abundances derived from the optical
recombination lines with those determined from collisionally-excited lines.
Combining our new data with published results on RLs in other PNs, we examine
the discrepancy in abundances derived from RLs and CELs. We find that there is
a wide range in the measured abundance discrepancy Delta(O+2) = log O+2(RL) -
log O+2(CEL), ranging from approximately 0.1 dex up to 1.4 dex. Most RLs yield
similar abundances, with the notable exception of O II multiplet V15, known to
arise primarily from dielectronic recombination, which gives abundances
averaging 0.6 dex higher than other O II RLs. We compare Delta(O+2) against a
variety of physical properties of the PNs to look for clues as to the mechanism
responsible for the abundance discrepancy. The strongest correlations are found
with the nebula diameter and the Balmer surface brightness. An inverse
correlation of Delta(O+2) with nebular density is also seen. Similar results
are found for carbon in comparing C II RL abundances with ultraviolet
measurements of C III].Comment: 48 pages, 14 figures, accepted for publication in the Astrophysical
Journal Supplemen
ISO SWS Observations of H II Regions in NGC 6822 and I ZW 36: Sulfur Abundances and Temperature Fluctuations
We report ISO SWS infrared spectroscopy of the H II region Hubble V in NGC
6822 and the blue compact dwarf galaxy I Zw 36. Observations of Br alpha, [S
III] at 18.7 and 33.5 microns, and [S IV] at 10.5 microns are used to determine
ionic sulfur abundances in these H II regions. There is relatively good
agreement between our observations and predictions of S^+3 abundances based on
photoionization calculations, although there is an offset in the sense that the
models overpredict the S^+3 abundances. We emphasize a need for more
observations of this type in order to place nebular sulfur abundance
determinations on firmer ground. The S/O ratios derived using the ISO
observations in combination with optical data are consistent with values of
S/O, derived from optical measurements of other metal-poor galaxies.
We present a new formalism for the simultaneous determination of the
temperature, temperature fluctuations, and abundances in a nebula, given a mix
of optical and infrared observed line ratios. The uncertainties in our ISO
measurements and the lack of observations of [S III] lambda 9532 or lambda 9069
do not allow an accurate determination of the amplitude of temperature
fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we
illustrate the diagnostic power and limitations of our new method.Comment: 32 Pages total, including 6 encapsulated postscript figures (one with
two parts). Accepted for Publication in the 20 Dec 2002 Ap
Exploring the dust content of SDSS DR7 damped Lyman alpha systems at 2.155.2
We have studied a sample of 1084 intervening absorption systems with 2.155.2, having log(N) 20.0 in the spectra of QSOs in
Sloan Digital Sky Survey (SDSS) data release 7 (DR7), with the aim of
understanding the nature and abundance of the dust and the chemical abundances
in the DLA absorbers. Composite spectra were constructed for the full sample
and several subsamples, chosen on the basis of absorber and QSO properties.
Average extinction curves were obtained for the samples by comparing their
geometric mean composite spectra with those of two samples of QSOs, matching in
z and i magnitude with the DLA sample, one sample without any absorbers
along their lines of sight and the other without any DLAs along their lines of
sight irrespective of the presence of other absorption systems. While the
average reddening in the DLA sample is small, we find definite evidence for the
presence of dust in subsamples based on absorber properties, in particular the
strength of metal absorption lines. DLAs along lines of sight to QSOs which are
not colour selected are found to be more dusty compared to those along the
lines of sight to the more numerous colour selected QSOs. From these studies
and from the strengths of absorption lines in the composite spectra, we
conclude that 10% of the DLAs in SDSS DR7 cause significant reddening,
have stronger absorption lines and have higher abundances as compared to the
rest of the sample. The rest of the sample shows little reddening. Due to the
dominant color selection method used to target QSOs in the SDSS DR7, this
fraction of 10% likely represents a lower limit for the global fraction of
dusty DLAs at high-z.Comment: 12 pages, 9 figures. To appear in MNRA
A novel poly-ε-lysine based implant, Proliferate®, for promotion of CNS repair following spinal cord injury
The limited regenerative capacity of the CNS poses formidable challenges to the repair of spinal cord injury (SCI). Two key barriers to repair are (i) the physical gap left by the injury, and (ii) the inhibitory milieu surrounding the injury, the glial scar. Biomaterial implantation into the injury site can fill the cavity, provide a substrate for cell migration, and potentially attenuate the glial scar. We investigated the biological viability of a biocompatible and biodegradable poly-ε-lysine based biomaterial, Proliferate®, in low and high cross-linked forms and when coated with IKVAV peptide, for SCI implantation. We demonstrate altered astrocyte morphology and nestin expression on Proliferate® compared to conventional glass cell coverslips suggesting a less reactive phenotype. Moreover Proliferate® supported myelination in vitro, with myelination observed sooner on IKVAV-coated constructs compared with uncoated Proliferate®, and delayed overall compared with maintenance on glass coverslips. For in vivo implantation, parallel-aligned channels were fabricated into Proliferate® to provide cell guidance cues. Extensive vascularisation and cellular infiltration were observed in constructs implanted in vivo, along with an astrocyte border and microglial response. Axonal ingrowth was observed at the construct border and inside implants in intact channels. We conclude that Proliferate® is a promising biomaterial for implantation following SCI
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