334 research outputs found
Assessing the Impact of Adlayer Description Fidelity on Theoretical Predictions of Coking on Ni(111) at Steam Reforming Conditions
Methane steam reforming is an important industrial
process for hydrogen production, employing Ni as a low-cost, highly
active catalyst, which, however, suffers from coking due to methane
cracking. Coking is the accumulation of a stable poison over time,
occurring at high temperatures; thus, to a first approximation, it can be
treated as a thermodynamic problem. In this work, we developed an Ab
initio kinetic Monte Carlo (KMC) model for methane cracking on
Ni(111) at steam reforming conditions. The model captures C−H
activation kinetics in detail, while graphene sheet formation is described
at the level of thermodynamics, to obtain insights into the “terminal
(poisoned) state” of graphene/coke within reasonable computational
times. We used cluster expansions (CEs) of progressively higher fidelity to systematically assess the influence of effective cluster
interactions between adsorbed or covalently bonded C and CH species on the “terminal state” morphology. Moreover, we compared
the predictions of KMC models incorporating these CEs into mean-field microkinetic models in a consistent manner. The models
show that the “terminal state” changes significantly with the level of fidelity of the CEs. Furthermore, high-fidelity simulations predict
C−CH island/rings that are largely disconnected at low temperatures but completely encapsulate the Ni(111) surface at high
temperatures
Interoperable Credentials Management for Wholesale Banking
A gap exists between wholesale-banking business practices and security best practices: wholesale banks operate within the boundaries of contract law, while security best practices often relies upon a benevolent trusted party outside the scope of straightforward contracts. While some business domains may be able to bridge this gap, the ultra-high-value transactions used in business-to-business banking substantially increase the size of the gap. The gap becomes most apparent when regarded from the perspective of interoperability. If a single user applies the same credential to sign high-value transactions at multiple banks, then the trusted-party model becomes overly cumbersome and conflicts with an acceptable concept of liability. This paper outlines the business complexities of wholesale banking and proposes a solution called Partner Key Management (PKM). PKM technology manages the credentials required to authenticate users and sign transactions. This paper presents PKM technology by describing an interoperable protocol, requisite data structures, and an interoperable XML definition. The paper uses formal methods to demonstrate a security equivalence between revocation options within PKM against the security offered by the traditional Public Key Infrastructure (PKI), a technology that features the benevolent trusted party
Time-resolved spectroscopy of multi-excitonic decay in an InAs quantum dot
The multi-excitonic decay process in a single InAs quantum dot is studied
through high-resolution time-resolved spectroscopy. A cascaded emission
sequence involving three spectral lines is seen that is described well over a
wide range of pump powers by a simple model. The measured biexcitonic decay
rate is about 1.5 times the single-exciton decay rate. This ratio suggests the
presence of selection rules, as well as a significant effect of the Coulomb
interaction on the biexcitonic wavefunction.Comment: one typo fixe
Polarization-Correlated Photon Pairs from a Single Quantum Dot
Polarization correlation in a linear basis, but not entanglement, is observed
between the biexciton and single-exciton photons emitted by a single InAs
quantum dot in a two-photon cascade. The results are well described
quantitatively by a probabilistic model that includes two decay paths for a
biexciton through a non-degenerate pair of one-exciton states, with the
polarization of the emitted photons depending on the decay path. The results
show that spin non-degeneracy due to quantum-dot asymmetry is a significant
obstacle to the realization of an entangled-photon generation device.Comment: 4 pages, 4 figures, revised discussio
When and why direct transmission models can be used for environmentally persistent pathogens
Variants of the susceptible-infected-removed (SIR) model of Kermack & McKendrick (1927) enjoy wide application in epidemiology, offering simple yet powerful inferential and predictive tools in the study of diverse infectious diseases across human, animal and plant populations. Direct transmission models (DTM) are a subset of these that treat the processes of disease transmission as comprising a series of discrete instantaneous events. Infections transmitted indirectly by persistent environmental pathogens, however, are examples where a DTM description might fail and are perhaps better described by models that comprise explicit environmental transmission routes, so-called environmental transmission models (ETM). In this paper we discuss the stochastic susceptible-exposed-infected-removed (SEIR) DTM and susceptible-exposed-infected-removed-pathogen (SEIR-P) ETM and we show that the former is the timescale separation limit of the latter, with ETM host-disease dynamics increasingly resembling those of a DTM when the pathogen’s characteristic timescale is shortened, relative to that of the host population. Using graphical posterior predictive checks (GPPC), we investigate the validity of the SEIR model when fitted to simulated SEIR-P host infection and removal times. Such analyses demonstrate how, in many cases, the SEIR model is robust to departure from direct transmission. Finally, we present a case study of white spot disease (WSD) in penaeid shrimp with rates of environmental transmission and pathogen decay (SEIR-P model parameters) estimated using published results of experiments. Using SEIR and SEIR-P simulations of a hypothetical WSD outbreak management scenario, we demonstrate how relative shortening of the pathogen timescale comes about in practice. With atttempts to remove diseased shrimp from the population every 24h, we see SEIR and SEIR-P model outputs closely conincide. However, when removals are 6-hourly, the two models’ mean outputs diverge, with distinct predictions of outbreak size and duration
A revised speech spectrum for STI calculations
The ability of the Speech Transmission Index (STI) to predict speech intelligibility under noisy conditions is
highly dependent on the assumed spectrum of the speech signal. Examination of the literature showed that the
long-term average speech spectrum of male talkers differs substantially from the speech spectrum recommended
for STI calculations (IEC 60268-16). To explore these issues, the long-term average speech spectrum of forty
male British English people was first measured, compared with the available literature and proposed for STI
calculations. Then, using several voice alarm systems, the influence of the measured spectrum on STI calculations
was assessed and comparisons made with the standard speech spectrum. The results showed significant STI
differences under noisy conditions and considerable reductions in the required electrical power with the use of
the new proposed male spectrum. This indicated that the current STI method could benefit from a revised speech
spectrum
Impact of a pyridazine derivative on tripartite synapse ultrastructure in hippocampus: a three-dimensional analysis
IntroductionWe previously discovered a pyridazine derivative compound series that can improve cognitive functions in mouse models of Alzheimer’s disease. One of the advanced compounds from this series, LDN/OSU-0215111-M3, was selected as the preclinical development candidate. This compound activates local protein translation at the perisynaptic astrocytic process (PAP) and enhances synaptic plasticity sequentially. While biochemical evidence supports the hypothesis that the compound enhances the structural plasticity of the tripartite synapse, its direct structural impact has not been investigated.MethodsVolume electron microscopy was used to study the hippocampal tripartite synapse three-dimensional structure in 3-month-old wild-type FVB/NJ mice after LDN/OSU-0215111-M3 treatment.ResultsLDN/OSU-0215111-M3 increased the size of tertiary apical dendrites, the volume of mushroom spines, the proportion of mushroom spines containing spine apparatus, and alterations in the spine distribution across the surface area of tertiary dendrites. Compound also increased the number of the PAP interacting with the mushroom spines as well as the size of the PAP in contact with the spines. Furthermore, proteomic analysis of the isolated synaptic terminals indicated an increase in dendritic and synaptic proteins as well as suggested a possible involvement of the phospholipase D signaling pathway. To further validate that LDN/OSU-0215111-M3 altered synaptic function, electrophysiological studies showed increased long-term potentiation following compound treatment.DiscussionThis study provides direct evidence that pyridazine derivatives enhance the structural and functional plasticity of the tripartite synapse
Studies of Millimeter-Wave Atmospheric Noise Above Mauna Kea
We report measurements of the fluctuations in atmospheric emission
(atmospheric noise) above Mauna Kea recorded with Bolocam at 143 and 268 GHz
from the Caltech Submillimeter Observatory (CSO). The 143 GHz data were
collected during a 40 night observing run in late 2003, and the 268 GHz
observations were made in early 2004 and early 2005 over a total of 60 nights.
Below 0.5 Hz, the data time-streams are dominated by atmospheric noise in all
observing conditions. The atmospheric noise data are consistent with a
Kolmogorov-Taylor (K-T) turbulence model for a thin wind-driven screen, and the
median amplitude of the fluctuations is 280 mK^2 rad^(-5/3) at 143 GHz and 4000
mK^2 rad^(-5/3) at 268 GHz. Comparing our results with previous ACBAR data, we
find that the normalization of the power spectrum of the atmospheric noise
fluctuations is a factor of 80 larger above Mauna Kea than above the South Pole
at millimeter wavelengths. Most of this difference is due to the fact that the
atmosphere above the South Pole is much drier than the atmosphere above Mauna
Kea. However, the atmosphere above the South Pole is slightly more stable as
well: the fractional fluctuations in the column depth of precipitable water
vapor are a factor of sqrt(2) smaller at the South Pole compared to Mauna Kea.
Based on our atmospheric modeling, we developed several algorithms to remove
the atmospheric noise, and the best results were achieved when we described the
fluctuations using a low-order polynomial in detector position over the 8
arcmin field of view (FOV). However, even with these algorithms, we were not
able to reach photon-background-limited instrument photometer (BLIP)
performance at frequencies below 0.5 Hz in any observing conditions.Comment: 48 pages, 16 figures, accepted for publication in Ap
Nonlinear Evolution of Gravitational Fragmentation Regulated by Magnetic Fields and Ambipolar Diffusion
We present results from an extensive set of simulations of gravitational
fragmentation in the presence of magnetic fields and ambipolar diffusion. The
average fragmentation spacing in the nonlinear phase of evolution is in
excellent agreement with the prediction of linear perturbation theory. The time
scale for nonlinear growth and runaway of the first core is times
the calculated growth time \taugm of the eigenmode with minimum growth time,
when starting from a uniform background state with small-amplitude white-noise
perturbations. Subcritical and transcritical models typically evolve on a
significantly longer time scale than the supercritical models. Infall motions
in the nonlinear fully-developed contracting cores are subsonic on the core
scale in subcritical and transcritical clouds, but are somewhat supersonic in
supercritical clouds. Core mass distributions are sharply peaked with a steep
decline to large masses, consistent with the existence of a preferred mass
scale for each unique set of dimensionless free parameters. However, a sum
total of results for various initial mass-to-flux ratios yields a broad
distribution reminiscent of observed core mass distributions. Based on our
results, we conclude that fragmentation spacings, magnitude of infall motions,
core shapes, and, especially, the curvature of magnetic field morphology, may
serve as indirect observational means of determining a cloud's ambient
mass-to-flux ratio.Comment: v2, 13 figures, New Astronomy, animations can be obtained at
http://www.astro.uwo.ca/~basu/pb.ht
Seasonal Cholera from Multiple Small Outbreaks, Rural Bangladesh
Clinical and environmental Vibrio cholerae organisms collected from February 2004 through April 2005 were systematically isolated from 2 rural Bangladeshi locales. Their genetic relatedness was evaluated at 5 loci that contained a variable number of tandem repeats (VNTR). The observed minimal overlap in VNTR patterns between the 2 communities was consistent with sequential, small outbreaks from local sources
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