8,976 research outputs found
The Fundamental Aspects of Iron Ore Reduction
Iron ore reduction is the conversion of iron oxide minerals to metallic iron. The chemical reactions inv-olved take place in the blast furnace during the prod-uction of hot metal or in the several proposed solid-
state processes that produce sponge iron. Because there are three oxides of iron, hematite magnetite and wustite and because both carbon and hydrogen are used as reducing agents, the fundamental aspects of the reduction process are rather complex. It is therefore important that these fundamentals be throughly understood in the development
of a new process if sucess is to be achieved
Development and screening of selective catalysts for the synthesis of clean liquid fuels
This article is a compilation of the research carried out under EEC contract EN3V-0400-D at the Institut für Energieverfahrenstechnik in Jülich and at the Faculty of Chemical Technology and Materials Science, Delft, concerning the development and screening of copper/cobalt-based catalysts for the synthesis of alcohol mixtures from syngas. Analogous work, based on copper/zinc oxide/alumina catalysts, has been performed at the Faculty of Chemical Technology in Twente University at Enschede. This work is described in a companion paper. Comparative tests of several catalysts in a pressure micropulse reactor and in a plug flow tubular reactor, carried out at the Institut für Technische Chemie, TU Braunschweig, are presented in a second companion paper. \ud
In the discussion section of the present article the results obtained by the joint groups are compared with the initial objectives of the programme
Entanglement Sharing and Decoherence in the Spin-Bath
The monogamous nature of entanglement has been illustrated by the derivation
of entanglement sharing inequalities - bounds on the amount of entanglement
that can be shared amongst the various parts of a multipartite system.
Motivated by recent studies of decoherence, we demonstrate an interesting
manifestation of this phenomena that arises in system-environment models where
there exists interactions between the modes or subsystems of the environment.
We investigate this phenomena in the spin-bath environment, constructing an
entanglement sharing inequality bounding the entanglement between a central
spin and the environment in terms of the pairwise entanglement between
individual bath spins. The relation of this result to decoherence will be
illustrated using simplified system-bath models of decoherence.Comment: 5 pages, 1 figure v2: 6 pages 2 figures, additional example and
reference
Holographic Superconductors with Lifshitz Scaling
Black holes in asymptotically Lifshitz spacetime provide a window onto finite
temperature effects in strongly coupled Lifshitz models. We add a Maxwell gauge
field and charged matter to a recently proposed gravity dual of 2+1 dimensional
Lifshitz theory. This gives rise to charged black holes with scalar hair, which
correspond to the superconducting phase of holographic superconductors with z >
1 Lifshitz scaling. Along the way we analyze the global geometry of static,
asymptotically Lifshitz black holes at arbitrary critical exponent z > 1. In
all known exact solutions there is a null curvature singularity in the black
hole region, and, by a general argument, the same applies to generic Lifshitz
black holes.Comment: 23 pages, 4 figures; v2: added references; v3: matches published
versio
Domain wall mobility in nanowires: transverse versus vortex walls
The motion of domain walls in ferromagnetic, cylindrical nanowires is
investigated numerically by solving the Landau-Lifshitz-Gilbert equation for a
classical spin model in which energy contributions from exchange, crystalline
anisotropy, dipole-dipole interaction, and a driving magnetic field are
considered. Depending on the diameter, either transverse domain walls or vortex
walls are found. The transverse domain wall is observed for diameters smaller
than the exchange length of the given material. Here, the system behaves
effectively one-dimensional and the domain wall mobility agrees with a result
derived for a one-dimensional wall by Slonczewski. For low damping the domain
wall mobility decreases with decreasing damping constant. With increasing
diameter, a crossover to a vortex wall sets in which enhances the domain wall
mobility drastically. For a vortex wall the domain wall mobility is described
by the Walker-formula, with a domain wall width depending on the diameter of
the wire. The main difference is the dependence on damping: for a vortex wall
the domain wall mobility can be drastically increased for small values of the
damping constant up to a factor of .Comment: 5 pages, 6 figure
A Component Based Heuristic Search Method with Evolutionary Eliminations
Nurse rostering is a complex scheduling problem that affects hospital
personnel on a daily basis all over the world. This paper presents a new
component-based approach with evolutionary eliminations, for a nurse scheduling
problem arising at a major UK hospital. The main idea behind this technique is
to decompose a schedule into its components (i.e. the allocated shift pattern
of each nurse), and then to implement two evolutionary elimination strategies
mimicking natural selection and natural mutation process on these components
respectively to iteratively deliver better schedules. The worthiness of all
components in the schedule has to be continuously demonstrated in order for
them to remain there. This demonstration employs an evaluation function which
evaluates how well each component contributes towards the final objective. Two
elimination steps are then applied: the first elimination eliminates a number
of components that are deemed not worthy to stay in the current schedule; the
second elimination may also throw out, with a low level of probability, some
worthy components. The eliminated components are replenished with new ones
using a set of constructive heuristics using local optimality criteria.
Computational results using 52 data instances demonstrate the applicability of
the proposed approach in solving real-world problems.Comment: 27 pages, 4 figure
Modeling Excitable Systems: Reentrant Tachycardia
Excitable membranes are an important type of nonlinear dynamical system and
their study can be used to provide a connection between physical and biological
circuits. We discuss two models of excitable membranes important in cardiac and
neural tissues. One model is based on the Fitzhugh-Nagumo equations and the
other is based on a three-transistor excitable circuit. We construct a circuit
that simulates reentrant tachycardia and its treatment by surgical ablation.
This project is appropriate for advanced undergraduates as a laboratory
capstone project, or as a senior thesis or honors project, and can also be a
collaborative project, with one student responsible for the computational
predictions and another for the circuit construction and measurements.Comment: 9 pages, twocolumn, revised and published in American Journal of
Physic
Quantum Transition State Theory for proton transfer reactions in enzymes
We consider the role of quantum effects in the transfer of hyrogen-like
species in enzyme-catalysed reactions. This study is stimulated by claims that
the observed magnitude and temperature dependence of kinetic isotope effects
imply that quantum tunneling below the energy barrier associated with the
transition state significantly enhances the reaction rate in many enzymes. We
use a path integral approach which provides a general framework to understand
tunneling in a quantum system which interacts with an environment at non-zero
temperature. Here the quantum system is the active site of the enzyme and the
environment is the surrounding protein and water. Tunneling well below the
barrier only occurs for temperatures less than a temperature which is
determined by the curvature of potential energy surface near the top of the
barrier. We argue that for most enzymes this temperature is less than room
temperature. For physically reasonable parameters quantum transition state
theory gives a quantitative description of the temperature dependence and
magnitude of kinetic isotope effects for two classes of enzymes which have been
claimed to exhibit signatures of quantum tunneling. The only quantum effects
are those associated with the transition state, both reflection at the barrier
top and tunneling just below the barrier. We establish that the friction due to
the environment is weak and only slightly modifies the reaction rate.
Furthermore, at room temperature and for typical energy barriers environmental
degrees of freedom with frequencies much less than 1000 cm do not have a
significant effect on quantum corrections to the reaction rate.Comment: Aspects of the article are discussed at
condensedconcepts.blogspot.co
A Note On R-Parity Violation and Fermion Masses
We consider a class of supersymmetric SU(3)\times SU(2)\times U(1) multihiggs
models in which R-parity is violated through bilinear Higgs-lepton
interactions. The required, due to R-parity violation, higgs-lepton rotations
introduce an alternative way to generate the phenomenologically desirable
fermion mass matrix structures independently of the equality of Yukawas,
possibly imposed by superstring or other unification.Comment: 8 pages, uses LaTeX2
The Heisenberg antiferromagnet on an anisotropic triangular lattice: linear spin-wave theory
We consider the effect of quantum spin fluctuations on the ground state
properties of the Heisenberg antiferromagnet on an anisotropic triangular
lattice using linear spin-wave theory. This model should describe the magnetic
properties of the insulating phase of the kappa-(BEDT-TTF)_2 X family of
superconducting molecular crystals. The ground state energy, the staggered
magnetization, magnon excitation spectra and spin-wave velocities are computed
as a function of the ratio between the second and first neighbours, J2/J1. We
find that near J2/J1 = 0.5, i.e., in the region where the classical spin
configuration changes from a Neel ordered phase to a spiral phase, the
staggered magnetization vanishes, suggesting the possibility of a quantum
disordered state. In this region, the quantum correction to the magnetization
is large but finite. This is in contrast to the frustrated Heisenberg model on
a square lattice, for which the quantum correction diverges logarithmically at
the transition from the Neel to the collinear phase. For large J2/J1, the model
becomes a set of chains with frustrated interchain coupling. For J2 > 4 J1, the
quantum correction to the magnetization, within LSW, becomes comparable to the
classical magnetization, suggesting the possibility of a quantum disordered
state. We show that, in this regime, quantum fluctuations are much larger than
for a set of weakly coupled chains with non-frustated interchain coupling.Comment: 10 pages, RevTeX + epsf, 5 figures Replaced with published version.
Comparison to series expansions energies include
- …