1,779 research outputs found
Nickel catalysts for internal reforming in molten carbonate fuel cells
Natural gas may be used instead of hydrogen as fuel for the molten carbonate fuel cell (MCFC) by steam reforming the natural gas inside the MCFC, using a nickel catalyst (internal reforming). The severe conditions inside the MCFC, however, require that the catalyst has a very high stability. In order to find suitable types of nickel catalysts and to obtain more knowledge about the deactivation mechanism(s) occurring during internal reforming, a series of nickel catalysts was prepared and subjected to stability tests at 973 K in an atmosphere containing steam and lithium and potassium hydroxide vapours. All the catalysts prepared showed a significant growth of the nickel crystallites during the test, especially one based on ¿-Al2O3 and a coprecipitated Ni/Al2O3 sample having a very high nickel content. However, this growth of nickel crystallites only partially explained the very strong deactivation observed in most cases. Only a coprecipitated nickel/alumina catalyst with high alumina content and a deposition-precipitation catalyst showed satisfactory residual activities. Addition of magnesium or lanthanum oxide to a coprecipitated nickel/alumina catalyst decreased the stability.\ud
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Adsorption and retention of the alkali was the most important factor determining the stability of a catalyst in an atmosphere containing alkali hydroxides. This is because the catalyst bed may remain active if a small part of the catalyst bed retains all the alkali
Dynamics of the Solar Chromosphere. II. Ca II H2V and K2V Grains versus Internetwork Fields
We use the Advanced Stokes Polarimeter at the NSO/Sacramento Peak Vacuum Tower Telescope to search for spatio-
temporal correlations between enhanced magnetic fields in the quiet solar internetwork photosphere and the occurrence
of Ca II H2v grains in the overlying chromosphere.We address the question of whether the shocks that produce
the latter are caused by magnetism-related processes,or whether they are of purely hydrodynamic nature. The
observations presented here are the first in which sensitive Stokes polarimetry is combined synchronously with high-
resolution Ca II H spectrometry. We pay particular attention to the nature and significance of weak polarization
signals from the internetwork domain,obtaining a robust estimate of our magnetographic noise level at an apparent
flux density of only 3 Mxcm^-2 . For the quiet Sun internetwork area analyzed here,we find no direct correlation
between the presence of magnetic features with apparent flux density above this limit and the occurrence of H2v
brightenings.This result contradicts the one-to-one correspondence claimed by Sivaraman &Livingston (1982).We
also find no correspondence between H2v grains and the horizontal-?eld internetwork features discovered by Lites
et al.(1996)
Temperature dependence of the superconducting gap anisotropy in BiSrCaCuO
We present the first detailed data of the momentum-resolved, temperature
dependence of the superconducting gap of ,
complemented by similar data on the intensity of the photoemission
superconducting condensate spectral area. The gap anisotropy between the
and directions increases markedly with increasing
temperature, contrary to what happens for conventional anisotropic-gap
superconductors such as lead. Specifically, the size of the superconducting gap
along the direction decreases to values indistinguishable from zero
at temperatures for which the gap retains virtually full value along the
direction.Comment: APS_REVTEX. 19 pages, including 8 figures, available upon request.
UW-Madison preprin
Crystal structure and stability of gyrase–fluoroquinolone cleaved complexes from Mycobacterium tuberculosis
Mycobacterium tuberculosis (Mtb) infects one-third of the world’s population and in 2013 accounted for 1.5 million deaths. Fluoroquinolone antibacterials, which target DNA gyrase, are critical agents used to halt the progression from multidrug-resistant tuberculosis to extensively resistant disease; however, fluoroquinolone resistance is emerging and new ways to bypass resistance are required. To better explain known differences in fluoroquinolone action, the crystal structures of the WT Mtb DNA gyrase cleavage core and a fluoroquinolone-sensitized mutant were determined in complex with DNA and five fluoroquinolones. The structures, ranging from 2.4- to 2.6-Å resolution, show that the intrinsically low susceptibility of Mtb to fluoroquinolones correlates with a reduction in contacts to the water shell of an associated magnesium ion, which bridges fluoroquinolone–gyrase interactions. Surprisingly, the structural data revealed few differences in fluoroquinolone–enzyme contacts from drugs that have very different activities against Mtb. By contrast, a stability assay using purified components showed a clear relationship between ternary complex reversibility and inhibitory activities reported with cultured cells. Collectively, our data indicate that the stability of fluoroquinolone/DNA interactions is a major determinant of fluoroquinolone activity and that moieties that have been appended to the C7 position of different quinolone scaffolds do not take advantage of specific contacts that might be made with the enzyme. These concepts point to new approaches for developing quinolone-class compounds that have increased potency against Mtb and the ability to overcome resistance
Dynamics of the Quiet Solar Chromosphere
The solar chromosphere has never been static although it
was often modeled so.Even the quiet-sun internetwork chromosphere
has become thoroughly dynamic with the acoustic shock interpretation
of the Ca II K 2V grains. We concentrate on the latter in this brief review.
Recent analysis of ASP data confirms that their excitation is more likely
set acoustically than magnetically.TRACE imagery permits seeing-free
studies of their occurrence patterns
Ballistic versus diffusive magnetoresistance of a magnetic point contact
The quasiclassical theory of a nanosize point contacts (PC) between two
ferromagnets is developed. The maximum available magnetoresistance values in PC
are calculated for ballistic versus diffusive transport through the area of a
contact. In the ballistic regime the magnetoresistance in excess of few
hundreds percents is obtained for the iron-group ferromagnets. The necessary
conditions for realization of so large magnetoresistance in PC, and the
experimental results by Garcia et al are discussedComment: 4 pages, TEX, 1 Figur
Surface spin-transfer torque and spin-injection effective field in ferromagnetic junctions: Unified theory
We consider theoretically a current flowing perpendicular to interfaces of a
spin-valve type ferromagnetic metallic junction. For the first time an
effective approach is investigated to calculate a simultaneous action of the
two current effects, namely, the nonequilibrium longitudinal spin injection and
the transversal spin-transfer surface torque. Dispersion relation for
fluctuations is derived and solved. Nonlinear problem is solved about steady
state arising due to instability for a thick enough free layer.Comment: 9 page
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