118 research outputs found
Ceramic/metal nanocomposites by lyophilization: Processing and HRTEM study
5 páginas, 8 figuras, 1 tabla.-- El pdf del artículo es el manuscrito de autor.This work describes a wet-processing route based on spray-freezing and subsequent lyophilization
designed to obtain nanostructured ceramic/metal powders. Starting from the ceramic powder and the
corresponding metal salt, a water-based suspension is sprayed on liquid nitrogen. The frozen powders are
subsequently freeze-dried, calcined and reduced. The material was analyzed using X-ray diffraction
analysis at all stages. High resolution transmission electron microscopy studies showed a uniform
distribution of metal nanoparticles on the ceramic grain surfaces, good interfaces and high crystallinity,
with an average metal particle size in the nanometric range.This work was supported by the Spanish Ministry of Science and
Innovation (MICINN) under the project MAT2009-14542-C02 and
the Government of the Principality of Asturias through PCTI and
ERDF (European Regional Development Fund) 2007-2013 under
project PC10-65. C.F. Gutierrez-
Gonzalez acknowledges CSIC and ESF for the concession of a JAE-Doc
2009 grant. S. Agouram thanks the Spanish Ministry of Science and
European Social Fund for financial support.Peer reviewe
Strongly nonlinear dynamics of electrolytes in large ac voltages
We study the response of a model micro-electrochemical cell to a large ac
voltage of frequency comparable to the inverse cell relaxation time. To bring
out the basic physics, we consider the simplest possible model of a symmetric
binary electrolyte confined between parallel-plate blocking electrodes,
ignoring any transverse instability or fluid flow. We analyze the resulting
one-dimensional problem by matched asymptotic expansions in the limit of thin
double layers and extend previous work into the strongly nonlinear regime,
which is characterized by two novel features - significant salt depletion in
the electrolyte near the electrodes and, at very large voltage, the breakdown
of the quasi-equilibrium structure of the double layers. The former leads to
the prediction of "ac capacitive desalination", since there is a time-averaged
transfer of salt from the bulk to the double layers, via oscillating diffusion
layers. The latter is associated with transient diffusion limitation, which
drives the formation and collapse of space-charge layers, even in the absence
of any net Faradaic current through the cell. We also predict that steric
effects of finite ion sizes (going beyond dilute solution theory) act to
suppress the strongly nonlinear regime in the limit of concentrated
electrolytes, ionic liquids and molten salts. Beyond the model problem, our
reduced equations for thin double layers, based on uniformly valid matched
asymptotic expansions, provide a useful mathematical framework to describe
additional nonlinear responses to large ac voltages, such as Faradaic
reactions, electro-osmotic instabilities, and induced-charge electrokinetic
phenomena.Comment: 30 pages, 17 eps-figures, RevTe
Nonlinear electrochemical relaxation around conductors
We analyze the simplest problem of electrochemical relaxation in more than
one dimension - the response of an uncharged, ideally polarizable metallic
sphere (or cylinder) in a symmetric, binary electrolyte to a uniform electric
field. In order to go beyond the circuit approximation for thin double layers,
our analysis is based on the Poisson-Nernst-Planck (PNP) equations of dilute
solution theory. Unlike most previous studies, however, we focus on the
nonlinear regime, where the applied voltage across the conductor is larger than
the thermal voltage. In such strong electric fields, the classical model
predicts that the double layer adsorbs enough ions to produce bulk
concentration gradients and surface conduction. Our analysis begins with a
general derivation of surface conservation laws in the thin double-layer limit,
which provide effective boundary conditions on the quasi-neutral bulk. We solve
the resulting nonlinear partial differential equations numerically for strong
fields and also perform a time-dependent asymptotic analysis for weaker fields,
where bulk diffusion and surface conduction arise as first-order corrections.
We also derive various dimensionless parameters comparing surface to bulk
transport processes, which generalize the Bikerman-Dukhin number. Our results
have basic relevance for double-layer charging dynamics and nonlinear
electrokinetics in the ubiquitous PNP approximation.Comment: 25 pages, 17 figures, 4 table
Реалізація цифрової системи автоматичного керування безперервним об'єктом, на основі фізичної моделі теплового об'єкта з використанням scada системи ZENON
У статті наведена методика поетапної реалізації системи цифрового керування безперервним об'єктом, реалізованої на персональному комп'ютері, що дозволила максимально наблизити модель до реальної системи керування й виконати ефективне тестування її функціонування в невиробничих умовах
Diffuse-Charge Dynamics in Electrochemical Systems
The response of a model micro-electrochemical system to a time-dependent
applied voltage is analyzed. The article begins with a fresh historical review
including electrochemistry, colloidal science, and microfluidics. The model
problem consists of a symmetric binary electrolyte between parallel-plate,
blocking electrodes which suddenly apply a voltage. Compact Stern layers on the
electrodes are also taken into account. The Nernst-Planck-Poisson equations are
first linearized and solved by Laplace transforms for small voltages, and
numerical solutions are obtained for large voltages. The ``weakly nonlinear''
limit of thin double layers is then analyzed by matched asymptotic expansions
in the small parameter , where is the
screening length and the electrode separation. At leading order, the system
initially behaves like an RC circuit with a response time of
(not ), where is the ionic diffusivity, but nonlinearity
violates this common picture and introduce multiple time scales. The charging
process slows down, and neutral-salt adsorption by the diffuse part of the
double layer couples to bulk diffusion at the time scale, . In the
``strongly nonlinear'' regime (controlled by a dimensionless parameter
resembling the Dukhin number), this effect produces bulk concentration
gradients, and, at very large voltages, transient space charge. The article
concludes with an overview of more general situations involving surface
conduction, multi-component electrolytes, and Faradaic processes.Comment: 10 figs, 26 pages (double-column), 141 reference
The study of the mechanism of an electrode reaction by first- and second-order techniques A theoretical approach
Abstract
The theoretical expressions for the faradaic admittance and the faradaic demodulation voltage are rewritten, introducing the thermodynamic restrictions proposed by Reinmuth in 1972 and without any specification of the mechanism of the electrode reaction.
The result is applied to general first-order kinetics of the type CECEC…etc. A treatment is also given in the case that one or more of the steps are of a higher order. Although the overall rate equation becomes extremely complex in such a case, it is shown that the expressions describing the faradaic admittance and the faradaic demodulation voltage are relative simple.
From some numerical examples it is made clear that, in general, by these methods discrimination between possible mechanisms can be realized on the basis of the potential dependences of the overall rate constant, operational transfer coefficient, irreversibility quotient and demodulation voltage. This work is believed to provide a framework for future quantitative studies of electrode reactions. Some ideas about its possible further elaboration are given
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