527 research outputs found
AC electrokinetic phenomena over semiconductive surfaces: effective electric boundary conditions and their applications
Electrokinetic boundary conditions are derived for AC electrokinetic (ACEK)
phenomena over leaky dielectric (i.e., semiconducting) surfaces. Such boundary
conditions correlate the electric potentials across the
semiconductor-electrolyte interface (consisting of the electric double layer
(EDL) inside the electrolyte solutions and the space charge layer (SCL) inside
the semiconductors) under AC electric fields with arbitrary wave forms. The
present electrokinetic boundary conditions allow for evaluation of induced zeta
potential contributed by both bond charges (due to electric polarization) and
free charges (due to electric conduction) from the leaky dielectric materials.
Subsequently, we demonstrate the applications of these boundary conditions in
analyzing the ACEK phenomena around a semiconducting cylinder. It is concluded
that the flow circulations exist around the semiconducting cylinder and are
shown to be stronger under an AC field with lower frequency and around a
cylinder with higher conductivity.Comment: 29 pages, 4 figure
Measurements of the Casimir-Lifshitz force in fluids: the effect of electrostatic forces and Debye screening
In this work, we present detailed measurements of the Casimir-Lifshitz force
between two gold surfaces (a sphere and a plate) immersed in ethanol and study
the effect of residual electrostatic forces, which are dominated by static
fields within the apparatus and can be reduced with proper shielding.
Electrostatic forces are further reduced by Debye screening through the
addition of salt ions to the liquid. Additionally, the salt leads to a
reduction of the Casimir-Lifshitz force by screening the zero-frequency
contribution to the force; however, the effect is small between gold surfaces
at the measured separations and within experimental error. An improved
calibration procedure is described and compared to previous methods. Finally,
the experimental results are compared to Lifshitz's theory and found to be
consistent for the materials used in the experiment.Comment: 11 figures. PRA in pres
Porous silicon formation and electropolishing
Electrochemical etching of silicon in hydrofluoride containing electrolytes
leads to pore formation for low and to electropolishing for high applied
current. The transition between pore formation and polishing is accompanied by
a change of the valence of the electrochemical dissolution reaction. The local
etching rate at the interface between the semiconductor and the electrolyte is
determined by the local current density. We model the transport of reactants
and reaction products and thus the current density in both, the semiconductor
and the electrolyte. Basic features of the chemical reaction at the interface
are summarized in law of mass action type boundary conditions for the transport
equations at the interface. We investigate the linear stability of a planar and
flat interface. Upon increasing the current density the stability flips either
through a change of the valence of the dissolution reaction or by a nonlinear
boundary conditions at the interface.Comment: 18 pages, 8 figure
Non-mean-field theory of anomalously large double-layer capacitance
Mean-field theories claim that the capacitance of the double-layer formed at
a metal/ionic conductor interface cannot be larger than that of the Helmholtz
capacitor, whose width is equal to the radius of an ion. However, in some
experiments the apparent width of the double-layer capacitor is substantially
smaller. We propose an alternate, non-mean-field theory of the ionic
double-layer to explain such large capacitance values. Our theory allows for
the binding of discrete ions to their image charges in the metal, which results
in the formation of interface dipoles. We focus primarily on the case where
only small cations are mobile and other ions form an oppositely-charged
background. In this case, at small temperature and zero applied voltage dipoles
form a correlated liquid on both contacts. We show that at small voltages the
capacitance of the double-layer is determined by the transfer of dipoles from
one electrode to the other and is therefore limited only by the weak
dipole-dipole repulsion between bound ions, so that the capacitance is very
large. At large voltages the depletion of bound ions from one of the capacitor
electrodes triggers a collapse of the capacitance to the much smaller
mean-field value, as seen in experimental data. We test our analytical
predictions with a Monte Carlo simulation and find good agreement. We further
argue that our ``one-component plasma" model should work well for strongly
asymmetric ion liquids. We believe that this work also suggests an improved
theory of pseudo-capacitance.Comment: 19 pages, 14 figures; some Monte Carlo results and a section about
aqueous solutions adde
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
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
Size-dependent spinodal and miscibility gaps for intercalation in nano-particles
Using a recently-proposed mathematical model for intercalation dynamics in
phase-separating materials [Singh, Ceder, Bazant, Electrochimica Acta 53, 7599
(2008)], we show that the spinodal and miscibility gaps generally shrink as the
host particle size decreases to the nano-scale. Our work is motivated by recent
experiments on the high-rate Li-ion battery material LiFePO4; this serves as
the basis for our examples, but our analysis and conclusions apply to any
intercalation material. We describe two general mechanisms for the suppression
of phase separation in nano-particles: (i) a classical bulk effect, predicted
by the Cahn-Hilliard equation, in which the diffuse phase boundary becomes
confined by the particle geometry; and (ii) a novel surface effect, predicted
by chemical-potential-dependent reaction kinetics, in which
insertion/extraction reactions stabilize composition gradients near surfaces in
equilibrium with the local environment. Composition-dependent surface energy
and (especially) elastic strain can contribute to these effects but are not
required to predict decreased spinodal and miscibility gaps at the nano-scale
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
Pseudo-single crystal electrochemistry on polycrystalline electrodes : visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction
The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing âpseudo-single-crystalâ electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe2+/3+ couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe2+ oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale
Critical Review of Theoretical Models for Anomalous Effects (Cold Fusion) in Deuterated Metals
We briefly summarize the reported anomalous effects in deuterated metals at
ambient temperature, commonly known as "Cold Fusion" (CF), with an emphasis on
important experiments as well as the theoretical basis for the opposition to
interpreting them as cold fusion. Then we critically examine more than 25
theoretical models for CF, including unusual nuclear and exotic chemical
hypotheses. We conclude that they do not explain the data.Comment: 51 pages, 4 Figure
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