37 research outputs found
Expanded Strings in the Background of NS5-branes via a M2-brane, a D2-brane and D0-branes
Classical configurations of a M2-brane, a D2-brane and D0-branes are
investigated in the background of an infinite array of M5-branes or NS5-branes.
On the M2-brane, we discuss three kinds of configurations, such as a sphere, a
cylinder and a torus-like one. These are stabilized by virtue of the background
fluxes of M5-branes. The torus-like M2-brane configuration has winding and
momentum numbers of 11th direction, and in terms of the type IIA superstring
theory, this corresponds to a torus-like D2-brane with electric and magnetic
fluxes on it. We also reproduce the same configuration from a non-abelian
Born-Infeld action for D0-branes. It will be a construction of closed strings
from D0-branes. An electric flux quantization condition on the D2-brane is also
discussed in terms of D0-branes.Comment: 33 pages, 6 figures, references and footnote added, confusing
expressions and introduction are improved, version to appear in JHE
Indirect Exchange Interaction between two Quantum Dots in an Aharonov-Bohm Ring
We investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between
two spins located at two quantum dots embedded in an Aharonov-Bohm (AB) ring.
In such a system the RKKY interaction, which oscillates as a function of the
distance between two local spins, is affected by the flux. For the case of the
ferromagnetic RKKY interaction, we find that the amplitude of AB oscillations
is enhanced by the Kondo correlations and an additional maximum appears at half
flux, where the interaction is switched off. For the case of the
antiferromagnetic RKKY interaction, we find that the phase of AB oscillations
is shifted by pi, which is attributed to the formation of a singlet state
between two spins for the flux value close to integer value of flux.Comment: 10 pages, 5 figure
Meson Spectroscopy in AdS/CFT with Flavour
We compute the meson spectrum of an N=2 super Yang-Mills theory with
fundamental matter from its dual string theory on AdS_5 x S_5 with a D7-brane
probe. For scalar and vector mesons with arbitrary R-charge the spectrum is
computed in closed form by solving the equations for D7-brane fluctuations; for
matter with non-zero mass m_q it is discrete, exhibits a mass gap of order m_q
/ sqrt(g_s N) and furnishes representations of SO(5) even though the manifest
global symmetry of the theory is only SO(4). The spectrum of mesons with large
spin J is obtained from semiclassical, rotating open strings attached to the
D7-brane. It displays Regge-like behaviour for J << sqrt(g_s N), whereas for J
>> sqrt(g_s N) it corresponds to that of two non-relativistic quarks bound by a
Coulomb potential. Meson interactions, baryons and `giant gauge bosons' are
briefly discussed.Comment: LaTeX, 39 pages, 4 figures, uses epsf. v2: typos corrected.
references adde
Catalysts based in cerium oxide for wet oxidation of acrylic acid in the prevention of environmental risks
Acrylic acid is a refractory compound for the non-catalytic wet oxidation (WO) process and can seriously damage the environment when released in industrial effluents. Oxidation of acrylic acid by catalytic wet oxidation (CWO) was studied in slurry conditions in a high-pressure batch reactor at 200 °C and 15 bar of oxygen partial pressure. Several solid cerium-based catalysts prepared in our laboratory were used (Ag/Ce, Co/Ce, Mn/Ce, CeO, MnO) and evaluated in terms of activity, selectivity and stability. Mn/Ce shows the higher activity in 2 h with 97.7% reduction of total organic carbon (TOC) followed by: MnO(95.5%)>Ag/Ce(85.0%)>Co/Ce(65.1%)>CeO(61.2%). Attempts were also carried out to analyze the influence of different Mn/Ce molar ratios. High percentages of Mn lead to practically total organic carbon concentration (TOC) abatements while low ratios lead to the formation of non-oxidizable compounds. Acrylic acid was readily degraded by all the catalysts pointing out the high importance of using a catalytic process. pH was an indicator of the reaction pathway and acetic acid was found as the major reaction intermediate compound; however it is completely oxidized after 2 h with exception for Co/Ce, CeO and MnO. Carbon adsorption and leaching of metals were poorly found for Mn/Ce indicating high stability. The catalyst microstructure after the reaction was analyzed and formation of whiskers of [beta]-MnO2 (or less probably MnOOH) were observed at the catalyst surface. Therefore, Mn/Ce revealed to be a promising catalyst for the treatment of effluents containing acrylic acid; nevertheless, its commercialization depends on further research.http://www.sciencedirect.com/science/article/B6TF6-4BC2MG4-1/1/af59fe86614137b4456d53b240ee61c
Catalytic studies in wet oxidation of effluents from formaldehyde industry
Development and design of active catalysts for the oxidation of formaldehyde present in wastewaters is of great importance. In this context, catalytic performance studies for oxidation of high formaldehyde containing solutions (1500 ppm) were carried out in a semibatch high-pressure reactor at 190-220°C and 15-35 bar of oxygen partial pressure. The removal efficiency of total organic carbon (TOC) was evaluated experimentally under different solid catalysts, using several heterogeneous composite oxides prepared in our laboratory (Mn/Ce, Co/Ce and Ag/Ce), as well as a commercial catalyst (CuO-ZnO/Al2O3). The Mn/Ce catalyst was the more active leading to high TOC conversions (99.4%). The TOC reductions were lower using Co/Ce, Ag/Ce and CuO-ZnO/Al2O3, respectively, 71.3%, 54.2% and 78.7%. Attempts were made to identify the influence of different Mn/Ce ratios. A high molar fraction of Mn leads to high TOC abatements. During the preparation procedure of catalysts the drying temperature does not affect significantly the catalytic area while increasing calcination temperature leads to lower surface areas. Carbon capacity adsorption was not observed for the Mn/Ce and CuO-ZnO/Al2O3 catalyst and leaching of the correspondent metals, Mn, Cu and Zn, was not significant. The catalytic wet oxidation (CWO) process was studied with an industrial high formaldehyde concentration effluent (800 ppm). Using an Mn/Ce catalyst the formaldehyde concentration decreased from 800 to 0.1 ppm and ammoniacal nitrogen from 420 to 155 ppm while 91.7% in TOC reduction was observed. These conversions were higher than those obtained with the commercial CuO-ZnO/Al2O3 catalyst. Therefore, Mn/Ce catalysts seem to be interesting options for the treatment of effluents from formaldehyde industry by CWO process.http://www.sciencedirect.com/science/article/B6TFK-47YGF5W-6/1/e0a79ca1a3c6dc28fdc3c33a6ce69ac