364 research outputs found
Palatini approach to Born-Infeld-Einstein theory and a geometric description of electrodynamics
The field equations associated with the Born-Infeld-Einstein action are
derived using the Palatini variational technique. In this approach the metric
and connection are varied independently and the Ricci tensor is generally not
symmetric. For sufficiently small curvatures the resulting field equations can
be divided into two sets. One set, involving the antisymmetric part of the
Ricci tensor , consists of the field equation for
a massive vector field. The other set consists of the Einstein field equations
with an energy momentum tensor for the vector field plus additional
corrections. In a vacuum with the field
equations are shown to be the usual Einstein vacuum equations. This extends the
universality of the vacuum Einstein equations, discussed by Ferraris et al.
\cite{Fe1,Fe2}, to the Born-Infeld-Einstein action. In the simplest version of
the theory there is a single coupling constant and by requiring that the
Einstein field equations hold to a good approximation in neutron stars it is
shown that mass of the vector field exceeds the lower bound on the mass of the
photon. Thus, in this case the vector field cannot represent the
electromagnetic field and would describe a new geometrical field. In a more
general version in which the symmetric and antisymmetric parts of the Ricci
tensor have different coupling constants it is possible to satisfy all of the
observational constraints if the antisymmetric coupling is much larger than the
symmetric coupling. In this case the antisymmetric part of the Ricci tensor can
describe the electromagnetic field, although gauge invariance will be broken.Comment: 12 page
Gauge Formalism for General Relativity and Fermionic Matter
A new formalism for spinors on curved spaces is developed in the framework of
variational calculus on fibre bundles. The theory has the same structure of a
gauge theory and describes the interaction between the gravitational field and
spinors. An appropriate gauge structure is also given to General Relativity,
replacing the metric field with spin frames. Finally, conserved quantities and
superpotentials are calculated under a general covariant form.Comment: 18 pages, Plain TEX, revision, explicit expression for superpotential
has been adde
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Tunable composite membranes for gas separations. Progress report, August 1, 1995--October 31, 1995
The fourth quarter of continued to see progress on five fronts: (a) instrument development, (2) polymer synthesis, (3) membrane fabrication and microscopy, (4) composite membrane fabrication and (5) permeability measurements. We have examined zeolite-polymer composite membranes with different zeolite loadings (10%, 25% and 50% by weight) using light and scanning electron microscopies in order to determine particle size distribution and homogeneity of dispersion. The distribution of zeolite (NaX) particles was fairly uniform across the length and width and excellent across the thickness of the film. The NaX crystallites displayed a reasonably uniform size distribution ranging between 2--5 {mu}m and this was carried over into the composite films made with the lower loading levels indicating that aggregation during the casting process was not occurring. At the highest loadings some clustering of the zeolites was observed
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Tunable Composite Membranes for Gas Separations.
Solution cast membranes of poly(3-dodecylthiophene) (PDDT) were studied for the room temperature separation of N{sub 2}, 0{sub 2}, and C0{sub 2} procedure for fabricating reproducible, smooth, uniformly thick (-35-pm), defect-free membranes was established. Permeability values were measured for as-cast PDDT membranes (PO{sub 2} = 9.4, PN{sub 2} = 20.2, PCO{sub 2} = 88. 2 Barrers) and selectivity values were calculated (XO{sub 2}/N{sub 2} = 2.2, XC0{sub 2}/N{sub 2} = 9.4). Chemically induced doping (-23%) with SbCI5 resulte in a decrease in permeability (PN{sub 2} = 3.5, P0{sub 2} =10.5, PCO{sub 2} = 48.5 Barrers) and a corresponding increase in permselectivity (X 0{sub 2}/N{sub 2} = 0, (xCO{sub 2}/N{sub 2} =14.0)). Membrane undoping with hydrazine partially reversed these trends (PN{sub 2} = 5.4, P0{sub 2} = 15.1, PCO{sub 2} = 62.9 Barrers), (XO{sub 2}/N{sub 2} = 2.8), (XCO{sub 2}/N{sub 2} =I 1. 6). The chemical composition cast, doped, and undoped PDDT membranes were determined using elemental analysis and energy dispersive x-ray spectrometry. Membrane microstructure was investigated by optical microscopy, TappingModeTM atomic force microscopy and scanning electron microscopy. The composition and microscopy results were correlated with changes in gas-transport properties. Two papers were presented at the Meeting of the North American Membranes Society, (June 2-4,1997, Baltimore, MD)
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Mixed-Matric Membranes for CO2 and H2 Gas Separations Using Metal-Organic Framework and Mesoporus Hybrid Silicas
In this work, we have investigated the separation performance of polymer-based mixed-matrix membranes containing metal-organic frameworks and mesoporous hybrid silicas. The MOF/Matrimid{reg_sign} and MOP-18/Matrimid{reg_sign} membranes exhibited improved dispersion and mechanical strength that allowed high additive loadings with reduced aggregation, as is the case of the 80 wt% MOP-18/Matrimid{reg_sign} and the 80% (w/w) Cu-MOF/Matrimid{reg_sign} membranes. Membranes with up to 60% (w/w) ZIF-8 content exhibited similar mechanical strength and improved dispersion. The H{sub 2}/CO{sub 2} separation properties of MOF/Matrimid{reg_sign} mixed-matrix membranes was improved by either keeping the selectivity constant and increasing the permeability (MOF-5, Cu-MOF) or by improving both selectivity and permeability (ZIF-8). In the case of MOF-5/Matrimid{reg_sign} mixed-matrix membranes, the H{sub 2}/CO{sub 2} selectivity was kept at 2.6 and the H{sub 2} permeability increased from 24.4 to 53.8 Barrers. For the Cu-MOF/Matrimid{reg_sign} mixed-matrix membranes, the H{sub 2}/CO{sub 2} selectivity was kept at 2.05 and the H{sub 2} permeability increased from 17.1 to 158 Barrers. These two materials introduced porosity and uniform paths that enhanced the gas transport in the membranes. When ZIF-8/Matrimid{reg_sign} mixed-matrix membranes were studied, the H{sub 2}/CO{sub 2} selectivity increased from 2.9 to 4.4 and the permeability of H{sub 2} increased from 26.5 to 35.8 Barrers. The increased H{sub 2}/CO{sub 2} selectivity in ZIF-8/Matrimid{reg_sign} membranes was explained by the sieving effect introduced by the ZIF-8 crystals (pore window 0.34 nm) that restricted the transport of molecules larger than H{sub 2}. Materials with microporous and/or mesoporous cavities like carbon aerogel composites with zeolite A and zeolite Y, and membranes containing mesoporous ZSM-5 showed sieving effects for small molecules (e.g. H{sub 2} and CO{sub 2}), however, the membranes were most selective for CO{sub 2} due to the strong interaction of the zeolites with CO{sub 2}. For example, at 30 wt% ZSM-5 loading, the CO{sub 2}/CH{sub 4} selectivity increased from 34.7 (Matrimid{reg_sign}) to 56.4. The large increase in selectivity was the result of the increase in CO{sub 2} permeability from 7.3 (Matrimid{reg_sign}) to 14.6 Barrers. At 30 wt% ZSM-5 loading, the H{sub 2}/CH{sub 4} separation was also improved from 83.3 (Matrimid{reg_sign}) to 136.7 with an increase in H{sub 2} permeability from 17.5 (Matrimid{reg_sign}) to 35.3 Barrers. The 10% carbon aerogel-zeolite A and -zeolite Y composite/Matrimid{reg_sign} membranes exhibited an increase in the CO{sub 2}/CH{sub 4} separation from 34.7 to 71.5 (zeolite A composite) and to 57.4 (zeolite Y composite); in addition, the membrane exhibited an increase in the CO{sub 2}/N{sub 2} separation from 33.1 to 50 (zeolite A composite) and to 49.4 (zeolite Y composite), indicating that these type of materials have affinity for CO{sub 2}. The inclusion of mesoporosity enhanced the dispersion of the additive allowing loadings of up to 30% (w/w) without the formation of non-selective voids
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Tunable Composite Membranes for Gas Separations.
Poly(3-dodecylthiophene) films were solution cast and subsequently subjected to chemical oxidation (doping), followed by chemical undoping. The microstructure of each form of the membrane was determined by optical microscopy (OM), scanning electron microscopy (SEM) and TappingMode Atomic Force Microscopy (TMAFM). Energy dispersive x-ray spectrometry (EDS) was used to elucidate the chemical composition of the membranes. Changes in microstructure after exposure to or protection from the laboratory atmosphere, and after permeability measurements, were assessed by these same techniques to estimate the environmental stability of the membranes. Although dramatic changes in topology occur for films exposed to the laboratory atmosphere, these are greatly reduced when the films are stored in containers that limit the access of moisture. Films exposed to dry gases in the permeameter exhibit essentially no change to their original microstructures
Description of a new species of Tetranematichthys (Siluriformes: Auchenipteridae) from the lower Amazon basin, Brazil
Miniature catfishes of the genus Gelanoglanis (Siluriformes: Auchenipteridae): monophyly and the description of a new species from the upper rio Tapajós basin, Brazil
Effect of thermal treatments on sputtered silver nanocluster/ silica composite coatings on soda-lime glasses: ionic exchange and antibacterial activity
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