1,043 research outputs found
Investigating the void structure of the polyamide active layers of thin-film composite membranes
The potential presence of voids in the fully-aromatic polyamide active layers of thin-film composite (TFC) membranes for water purification was studied in a selection of commercial membranes with a broad range of performance levels. The membranes were characterized for their potential void fractions using three independent methods: (i) analysis of transmission electron microscopy (TEM) images of membrane cross-sections, (ii) water uptake measurements by quartz crystal microbalance (QCM), and (iii) estimates of the effective refractive indices of active layers by spectroscopic ellipsometry. Results revealed that voids having tens of nanometers in diameter exist in the fully-aromatic polyamide active layers of TFC membranes, the voids fill up with water when immersed in it, and the voids account for a significant volume fraction of the active layers (i.e., 15-32% for the membranes studied). It was concluded that the voids in polyamide active layers do not form passageways connecting the feed and permeate sides, but rather are cavities disconnected from the feed side. In addition, it was also concluded that the globular features observable in TEM images of membrane cross sections that had been previously identified as voids or nodules are indeed voids, and not nodules. The finding that a significant volume fraction of fully-aromatic polyamide active layers corresponds to water-filled voids has deep implications on various aspects of TFC membrane science and technology. For example, we illustrate how the presence of voids can potentially increase the effective water permeability of the active layer by as much as a factor of â5 compared with the case of an equivalent active layer without any voids. The methods developed in this study to measure void volume fraction represent useful tools for future membrane characterization studies, and the void fractions measured can be used as input or calibration parameters in future modeling studies of active layer formation or water and solute transport
Answer Set Programming for Non-Stationary Markov Decision Processes
Non-stationary domains, where unforeseen changes happen, present a challenge
for agents to find an optimal policy for a sequential decision making problem.
This work investigates a solution to this problem that combines Markov Decision
Processes (MDP) and Reinforcement Learning (RL) with Answer Set Programming
(ASP) in a method we call ASP(RL). In this method, Answer Set Programming is
used to find the possible trajectories of an MDP, from where Reinforcement
Learning is applied to learn the optimal policy of the problem. Results show
that ASP(RL) is capable of efficiently finding the optimal solution of an MDP
representing non-stationary domains
Oxygen abundances in unevolved metal-poor stars from near-UV OH lines
We have performed a detailed oxygen abundance analysis of 23 metal-poor
(-3.0<[Fe/H]<-0.3) unevolved halo stars and one giant through the OH bands in
the near UV, using high-resolution echelle spectra. Oxygen is found to be
overabundant with respect to iron in these stars, with the [O/Fe] ratio
increasing from 0.6 to 1 between [Fe/H]=-1.5 and -3.0. The behavior of the
oxygen overabundance with respect to [Fe/H] is similar to that seen in previous
works based on OI IR triplet data (Abia and Rebolo 1989; Tomkin et al. 1992;
Cavallo, Pilachowski, and Rebolo 1997). Contrary to the previously accepted
picture, our oxygen abundances, derived from low-excitation OH lines, agree
well with those derived from high-excitation lines of the triplet. For nine
stars in common with Tomkin et al. we obtain a mean difference of 0.00+/-0.11
dex with respect to the abundances determined from the triplet using the same
stellar parameters and model photospheres. For four stars in our sample we have
found measurements of the [OI] 6300 A line in the literature, from which we
derive oxygen abundances consistent (average difference 0.09 dex) with those
based on OH lines, showing that the long standing controversy between oxygen
abundances from forbidden and permitted lines in metal-poor unevolved stars can
be resolved. Our new oxygen abundances show a smooth extension of the
Edvardsson et al.'s (1993) [O/Fe] versus metallicity curve to much lower
abundances, with a slope -0.31+/- 0.11 (taking into account the error bars in
both oxygen abundances and metallicities) in the range -3<[Fe/H]<-1.Comment: 17 pages, 8 figures, 3 table
On the Mechanism of the Copper-Catalyzed Enantioselective 1,4-Addition of Grignard Reagents to α,ÎČ-Unsaturated Carbonyl Compounds
The mechanism of the enantioselective 1,4-addition of Grignard reagents to α,ÎČ-unsaturated carbonyl compounds promoted by copper complexes of chiral ferrocenyl diphosphines is explored through kinetic, spectroscopic, and electrochemical analysis. On the basis of these studies, a structure of the active catalyst is proposed. The roles of the solvent, copper halide, and the Grignard reagent have been examined. Kinetic studies support a reductive elimination as the rate-limiting step in which the chiral catalyst, the substrate, and the Grignard reagent are involved. The thermodynamic activation parameters were determined from the temperature dependence of the reaction rate. The putative active species and the catalytic cycle of the reaction are discussed.
Administration-time dependent effects of aspirin on blood pressure in untreated hypertensive patients
A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: Variation of the Metallicity Distribution Function Along the Sagittarius Stream
We present reliable measurements of the metallicity distribution function
(MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarf
spheroidal (dSph) galaxy, based on high resolution, echelle spectroscopy of
candidate M giant members of the Sgr system. The Sgr MDF is found to evolve
significantly from a median [Fe/H] ~-0.4 in the core to ~-1.1 dex over a Sgr
leading arm length representing ~2.5-3.0 Gyr of dynamical (i.e. tidal
stripping) age. This is direct evidence that there can be significant chemical
differences between current dSph satellites and the bulk of the stars they have
contributed to the halo. Our results suggest that Sgr experienced a significant
change in binding energy over the past several Gyr, which has substantially
decreased its tidal boundary across a radial range over which there must have
been a significant metallicity gradient in the progenitor galaxy. By accounting
for MDF variation along the debris arms, we approximate the MDF Sgr would have
had several Gyr ago. We also analyze the MDF of a moving group of M giants we
previously discovered towards the North Galactic Cap having opposite radial
velocities to the infalling Sgr leading arm stars there and propose that most
of these represent Sgr trailing arm stars overlapping the Sgr leading arm in
this part of the sky. If so, these trailing arm stars further demonstrate the
strong MDF evolution within the Sgr stream.Comment: The new version is a much-expanded version moved from ApJL to ApJ and
will appear in Dec. 1 editio
A Consistency Test of Spectroscopic Gravities for Late-Type Stars
Chemical analyses of late-type stars are usually carried out following the
classical recipe: LTE line formation and homogeneous, plane-parallel,
flux-constant, and LTE model atmospheres. We review different results in the
literature that have suggested significant inconsistencies in the spectroscopic
analyses, pointing out the difficulties in deriving independent estimates of
the stellar fundamental parameters and hence,detecting systematic errors.
The trigonometric parallaxes measured by the HIPPARCOS mission provide
accurate appraisals of the stellar surface gravity for nearby stars, which are
used here to check the gravities obtained from the photospheric iron ionization
balance. We find an approximate agreement for stars in the metallicity range -1
<= [Fe/H] <= 0, but the comparison shows that the differences between the
spectroscopic and trigonometric gravities decrease towards lower metallicities
for more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadow
upon the abundance analyses for extreme metal-poor stars that make use of the
ionization equilibrium to constrain the gravity. The comparison with the
strong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a)
confirms that this method provides systematically larger gravities than the
ionization balance. The strong-line gravities get closer to the physical ones
for the stars analyzed by Fuhrmann, but they are even further away than the
iron ionization gravities for the stars of lower gravities in Edvardsson's
sample. The confrontation of the deviations of the iron ionization gravities in
metal-poor stars reported here with departures from the excitation balance
found in the literature, show that they are likely to be induced by the same
physical mechanism(s).Comment: AAS LaTeX v4.0, 35 pages, 10 PostScript files; to appear in The
Astrophysical Journa
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