256 research outputs found
2-Sulfoethylammonium trifluoromethanesulfonate as an Ionic Liquid for High Temperature PEM Fuel Cells
2-Sulfoethylammonium trifluoromethanesulfonate ([2-Sea+][TfO−]) represents a novel class of proton-conducting ionic liquids (PILs) based on aminoalkylsulfonic acids. The fundamental suitability of [2-Sea+][TfO−] for application as a protic electrolyte in high temperature PEM fuel cells (HT-PEFCs) was investigated up to a temperature of 130°C. A comparison was made against a state-of-the-art electrolyte, phosphoric acid. [2-Sea+][TfO−] is electrochemically and thermally stable up to 140°C. The specific conductivity of 95 wt% [2-Sea+][TfO−] aqueous solution at 130°C is ≈20 times lower compared to 95 wt% H3PO4. The strong coupling of ion transport and viscous flow suggests a vehicular ion (proton) transport in [2-Sea+][TfO−]. 95 wt% [2-Sea+][TfO−] shows superior kinetics in terms of oxygen reduction reaction (ORR) on polycrystalline Pt compared to 95 wt% H3PO4 at temperatures greater than 90°C in a fuel cell-applicable potential range. Double layer capacitances suggest a complex double layer structure, including adsorbed [2-Sea+][TfO−] and water, as well as intermediates of oxygen reduction and Pt oxidation. Potential and temperature-dependent ORR kinetics in the presence of 95 wt% [2-Sea+][TfO−] yield different Tafel slopes (b = 82–139 mV) and symmetry factors (β = 0.46–0.96), indicating changes in surface coverages of the adsorbed species and possibly also a change in the reaction mechanism
Автоматический регулятор частоты и мощности тепловых станций
Процесс производства и передачи энергии на электростанции динамичен и постоянно подвержен случайным возмущающим воздействиям, что приводит к изменению частоты, поэтому без автоматического управления его функционирование невозможно. Удерживать частоту и контролировать распределение мощности можно только с помощью автоматического управления. Настройка параметров, изучение и исследование переходных процессов, протекающие в автоматических регуляторах синхронных машин, является актуальной задачей для электроэнергетики.The process of production and transmission of energy at a power plant is dynamic and constantly subject to accidental disturbances, which leads to a change in frequency, so without automatic control its operation is impossible. Automatic control can regulate the frequency and control the power distribution. The adjustment of parameters, study and investigation of transient processes occurring in automatic regulators of synchronous machines is an urgent task for the electrical power system
Minimal Model for Sand Dunes
We propose a minimal model for aeolian sand dunes. It combines an analytical
description of the turbulent wind velocity field above the dune with a
continuum saltation model that allows for saturation transients in the sand
flux. The model provides a qualitative understanding of important features of
real dunes, such as their longitudinal shape and aspect ratio, the formation of
a slip face, the breaking of scale invariance, and the existence of a minimum
dune size.Comment: 4 pages, 4 figures, replaced with publishd versio
Minimal model for aeolian sand dunes
We present a minimal model for the formation and migration of aeolian sand
dunes. It combines a perturbative description of the turbulent wind velocity
field above the dune with a continuum saltation model that allows for
saturation transients in the sand flux. The latter are shown to provide the
characteristic length scale. The model can explain the origin of important
features of dunes, such as the formation of a slip face, the broken scale
invariance, and the existence of a minimum dune size. It also predicts the
longitudinal shape and aspect ratio of dunes and heaps, their migration
velocity and shape relaxation dynamics. Although the minimal model employs
non-local expressions for the wind shear stress as well as for the sand flux,
it is simple enough to serve as a very efficient tool for analytical and
numerical investigations and to open up the way to simulations of large scale
desert topographies.Comment: 19 pages, 22 figure
Corridors of barchan dunes: stability and size selection
Barchans are crescentic dunes propagating on a solid ground. They form dune
fields in the shape of elongated corridors in which the size and spacing
between dunes are rather well selected. We show that even very realistic models
for solitary dunes do not reproduce these corridors. Instead, two instabilities
take place. First, barchans receive a sand flux at their back proportional to
their width while the sand escapes only from their horns. Large dunes
proportionally capture more than they loose sand, while the situation is
reversed for small ones: therefore, solitary dunes cannot remain in a steady
state. Second, the propagation speed of dunes decreases with the size of the
dune: this leads -- through the collision process -- to a coarsening of barchan
fields. We show that these phenomena are not specific to the model, but result
from general and robust mechanisms. The length scales needed for these
instabilities to develop are derived and discussed. They turn out to be much
smaller than the dune field length. As a conclusion, there should exist further
- yet unknown - mechanisms regulating and selecting the size of dunes.Comment: 13 pages, 13 figures. New version resubmitted to Phys. Rev. E.
Pictures of better quality available on reques
A Continuum Saltation Model for Sand Dunes
We derive a phenomenological continuum saltation model for aeolian sand
transport that can serve as an efficient tool for geomorphological
applications. The coupled differential equations for the average density and
velocity of sand in the saltation layer reproduce both known equilibrium
relations for the sand flux and the time evolution of the sand flux as
predicted by microscopic saltation models. The three phenomenological
parameters of the model are a reference height for the grain-air interaction,
an effective restitution coefficient for the grain-bed interaction, and a
multiplication factor characterizing the chain reaction caused by the impacts
leading to a typical time or length scale of the saturation transients. We
determine the values of these parameters by comparing our model with wind
tunnel measurements. Our main interest are out of equilibrium situations where
saturation transients are important, for instance at phase boundaries
(ground/sand) or under unsteady wind conditions. We point out that saturation
transients are indispensable for a proper description of sand flux over
structured terrain, by applying the model to the windward side of an isolated
dune, thereby resolving recently reported discrepancies between field
measurements and theoretical predictions.Comment: 11 pages, 7 figure
Collision dynamics of two barchan dunes simulated by a simple model
The collision processes of two crescentic dunes called barchans are
systematically studied using a simple computer simulation model. The simulated
processes, coalescence, ejection and reorganization, qualitatively correspond
to those observed in a water tank experiment. Moreover we found the realized
types of collision depend both on the mass ratio and on the lateral distance
between barchans under initial conditions. A simple set of differential
equations to describe the collision of one-dimensional (1D) dunes is
introduced.Comment: 4 pages, 5 figures : To be published in Journal of the Physical
Society of Japa
Synthesis of Indium Nanowires by Galvanic Displacement and Their Optical Properties
<p>Abstract</p> <p>Single crystalline indium nanowires were prepared on Zn substrate which had been treated in concentrated sulphuric acid by galvanic displacement in the 0.002 mol L<sup>−1</sup>In<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>-0.002 mol L<sup>−1</sup>SeO<sub>2</sub>-0.02 mol L<sup>−1</sup>SDS-0.01 mol L<sup>−1</sup>citric acid aqueous solution. The typical diameter of indium nanowires is 30 nm and most of the nanowires are over 30 μm in length. XRD, HRTEM, SAED and structural simulation clearly demonstrate that indium nanowires are single-crystalline with the tetragonal structure, the growth direction of the nanowires is along [100] facet. The UV-Vis absorption spectra showed that indium nanowires display typical transverse resonance of SPR properties. The surfactant (SDS) and the pretreatment of Zn substrate play an important role in the growth process. The mechanism of indium nanowires growth is the synergic effect of treated Zn substrate (hard template) and SDS (soft template).</p
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