601 research outputs found
An efficient composite growing N-doped TiO2 on multi-walled carbon nanotubes through sol-gel process
High-activity, visible-light-driven photocatalysts were prepared by forming N-doped TiO2 on multi-walled carbon nanotubes (MWCNTs). The use of MWCNTs as the support in a N-doped TiO2 system favored electron trapping, because the recombination process could be retarded, thus promoting photocatalytic activity. The prepared photocatalysts were systematically characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunaure-Emmett-Teller (BET) spectroscopy, and UV-Vis diffuse reflectance spectroscopy (UV-Vis/DRS). The results indicated that the N-doped TiO2 coated on MWCNTs improved the surface area and slightly modified the optical properties of the composite. The activities of the photocatalysts were probed by photodegradation of methanol in the presence of visible light irradiation. The experimental results revealed that the strong interphase linkage between the MWCNTs and the N-doped TiO2 played a significant role in improving photocatalytic activity. However, the mechanical process for MWCNT-TiO2-x N (x) mixtures showed lower activity than just pure N-doped TiO2. In this study, N-doped TiO2 precursors coated with pretreated MWCNTs during a sol-gel process could effectively form a MWCNT-TiO2-x N (x) composite. The composite showed excellent activity and effectively enhanced the efficiency of N-doped TiO2 under the visible light region
Improving the Activity of Rh/Al2O3 Catalyst for NO Reduction by Na Addition in the Presences of H2O and O-2
The effect of Na addition on the performance of Rh/Al2O3 catalyst for NO reduction with CO in the presence of H2O and O-2 was investigated. The reacted catalysts were analyzed by the FTIR technique to identify the products for further investigation on the possible catalytic reaction mechanisms and the reasons behind the H2O poisoning. Experimental results show that the removal efficiency of NO by Rh/Al2O3 catalyst was 63% at 250 degrees C but that decreased as the H2O content increased. Adding Na to modify the Rh/ Al2O3 catalyst significantly enhanced the conversion of NO to 99% at 250-300 degrees C even as the H2O content was 1.6 vol%. The FTIR analyses results reveal that the abundant H2O in the flue gas can compete with NO to adsorb on the surfaces of Rh/Al2O3 and Rh-Na/Al2O3 catalysts and further enhance the formation of NO3 that reacts with H. The effects of H2O on Rh/Al2O3 and Rh-Na/Al2O3 catalysts can be eliminated by increasing the reaction temperature to higher than 300 degrees C. Rh-Na/Al2O3 is a feasible catalyst for NO reduction at such condition with relative high H2O and O-2 contents
Energy Spectrum of Bloch Electrons Under Checkerboard Field Modulations
Two-dimensional Bloch electrons in a uniform magnetic field exhibit complex
energy spectrum. When static electric and magnetic modulations with a
checkerboard pattern are superimposed on the uniform magnetic field, more
structures and symmetries of the spectra are found, due to the additional
adjustable parameters from the modulations. We give a comprehensive report on
these new symmetries. We have also found an electric-modulation induced energy
gap, whose magnitude is independent of the strength of either the uniform or
the modulated magnetic field. This study is applicable to experimentally
accessible systems and is related to the investigations on frustrated
antiferromagnetism.Comment: 8 pages, 6 figures (reduced in sizes), submitted to Phys. Rev.
Radiative Corrections to Fixed Target Moller Scattering Including Hard Bremsstrahlung Effects
We present a calculation of the complete electroweak radiative
corrections to the Moller scattering process e^-e^- -> e^-e^-, including hard
bremsstrahlung contributions. We study the effects of these corrections on both
the total cross section and polarization asymmetry measured in low energy fixed
target experiments. Numerical results are presented for the experimental cuts
relevant for E-158, a fixed target e^-e^- experiment being performed at SLAC;
the effect of hard bremsstrahlung is to shift the measured polarization
asymmetry by approximately +4%. We briefly discuss the remaining theoretical
uncertainty in the prediction for the low energy Moller scattering polarization
asymmetry.Comment: 22 pgs; minor clarifications added and typos fixe
Assertion-based proof checking of Chang-Roberts leader election in PVS
We report a case study in automated incremental assertion-based proof checking with PVS. Given an annotated distributed algorithm, our tool ProPar generates the proof obligations for partial correctness, plus a proof script per obligation. ProPar then lets PVS attempt to discharge all obligations by running the proof scripts. The Chang-Roberts algorithm elects a leader on a unidirectional ring with unique identities. With ProPar, we check its correctness with a very high degree of automation: over 90% of the proof obligations is discharged automatically. This case study underlines the feasibility of the approach and is, to the best of our knowledge, the first verification of the Chang-Roberts algorithm for arbitrary ring size in a proof checker
Compatibility of DAMA/LIBRA dark matter detection with other searches
The DAMA/NaI and DAMA/LIBRA annual modulation data, which may be interpreted
as a signal for the existence of weakly interacting dark matter (WIMPs) in our
galactic halo, are examined in light of null results from other experiments. We
use the energy spectrum of the combined DAMA modulation data given in 36 bins,
and include the effect of channeling. Several statistical tools are implemented
in our study: likelihood ratio with a global fit and with raster scans in the
WIMP mass and goodness-of-fit (g.o.f.). These approaches allow us to
differentiate between the preferred (global best fit) and allowed (g.o.f.)
parameter regions. It is hard to find WIMP masses and couplings consistent with
all existing data sets. For spin-independent (SI) interactions, the best fit
DAMA regions are ruled out to the 3 C.L., even with channeling taken
into account. However, for WIMP masses of ~8 GeV some parameters outside these
regions still yield a moderately reasonable fit to the DAMA data and are
compatible with all 90% C.L. upper limits from negative searches, when
channeling is included. For spin-dependent (SD) interactions with proton-only
couplings, a range of masses below 10 GeV is compatible with DAMA and other
experiments, with and without channeling, when SuperK indirect detection
constraints are included; without the SuperK constraints, masses as high as ~20
GeV are compatible. For SD neutron-only couplings we find no parameters
compatible with all the experiments. Mixed SD couplings are examined: e.g. ~8
GeV mass WIMPs with a_n = +/- a_p are found to be consistent with all
experiments. In short, there are surviving regions at low mass for both SI and
SD interactions; if indirect detection limits are relaxed, some SD proton-only
couplings at high masses also survive.Comment: 45 pages, 23 figures. v2: Added references and minor revisions. v3:
improvements to some null experiment analyses and DAMA g.o.f. statistical
constraints; added DAMA total event rate constrain
Chern-Simons Theory for Magnetization Plateaus of Frustrated - Heisenberg model
The magnetization curve of the two-dimensional spin-1/2 -
Heisenberg model is investigated by using the Chern-Simons theory under a
uniform mean-field approximation. We find that the magnetization curve is
monotonically increasing for , where the system under zero
external field is in the antiferromagnetic N\'eel phase. For larger ratios of
, various plateaus will appear in the magnetization curve. In
particular, in the disordered phase, our result supports the existence of the
plateau and predicts a new plateau at .
By identifying the onset ratio for the appearance of the 1/2-plateau
with the boundary between the N\'eel and the spin-disordered phases in zero
field, we can determine this phase boundary accurately by this mean-field
calculation. Verification of these interesting results would indicate a strong
connection between the frustrated antiferromagnetic system and the quantum Hall
system.Comment: RevTeX 4, 4 pages, 3 EPS figure
Observation of Two New N* Peaks in J/psi -> and Decays
The system in decays of is limited to be
isospin 1/2 by isospin conservation. This provides a big advantage in studying
compared with and experiments which mix
isospin 1/2 and 3/2 for the system. Using 58 million decays
collected with the Beijing Electron Positron Collider, more than 100 thousand
events are obtained. Besides two well known
peaks at 1500 MeV and 1670 MeV, there are two new, clear peaks in
the invariant mass spectrum around 1360 MeV and 2030 MeV. They are the
first direct observation of the peak and a long-sought "missing"
peak above 2 GeV in the invariant mass spectrum. A simple
Breit-Wigner fit gives the mass and width for the peak as MeV and MeV, and for the new peak above 2 GeV
as MeV and MeV, respectively
Constraints on dark matter particles charged under a hidden gauge group from primordial black holes
In order to accommodate increasingly tighter observational constraints on
dark matter, several models have been proposed recently in which dark matter
particles are charged under some hidden gauge group. Hidden gauge charges are
invisible for the standard model particles, hence such scenarios are very
difficult to constrain directly. However black holes are sensitive to all gauge
charges, whether they belong to the standard model or not. Here, we examine the
constraints on the possible values of the dark matter particle mass and hidden
gauge charge from the evolution of primordial black holes. We find that the
existence of the primordial black holes with reasonable mass is incompatible
with dark matter particles whose charge to mass ratio is of the order of one.
For dark matter particles whose charge to mass ratio is much less than one, we
are able to exclude only heavy dark matter in the mass range of 10^(11) GeV -
10^(16) GeV. Finally, for dark matter particles whose charge to mass ratio is
much greater than one, there are no useful limits coming from primordial black
holes.Comment: accepted for publication in JCA
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