1,721,002 research outputs found
Erratum: Step energy and step interactions on the reconstructed GaAs(001) surface (Physical Review B - Condensed Matter and Materials Physics (2014) 90 (115314) DOI: 10.1103/PhysRevB.90.115314)
No full textWe report here the corrected values for the step energies and the step interactions calculated in our paper for a large set of different step configurations. We have found that one parameter, i.e., the smearing parameter governing the band occupation around the Fermi level, was not converged enough to predict the step properties with sufficient accuracy. Since the surfaces are metallic a smearing function had to be used for the state occupation around the Fermi level. We used the smearing function proposed by Marzari and Vanderbilt. We found that the parameter entering this expression is a particularly sensitive one. The previous paper used a commonly chosen value of 0.02 Ry. However, we found that a much smaller value for this parameter was necessary to obtain well converged values for the surface energies. The convergence of the smearing parameter has been carefully checked. Figure presented
Si and Ge based metallic core/shell nanowires for nano-electronic device applications
Get PDFOne dimensional heterostructure nanowires (NWs) have attracted a large attention due to the possibility of easily tuning their energy gap, a useful property for application to next generation electronic devices. In this work, we propose new core/shell NW systems where Ge and Si shells are built around very thin As and Sb cores. The modification in the electronic properties arises due to the induced compressive strain experienced by the metal core region which is attributed to the lattice-mismatch with the shell region. As/Ge and As/Si nanowires undergo a semiconducting-to-metal transition on increasing the diameter of the shell. The current-voltage (I-V) characteristics of the nanowires show a negative differential conductance (NDC) effect for small diameters that could lead to their application in atomic scale device(s) for fast switching. In addition, an ohmic behavior and upto 300% increment of the current value is achieved on just doubling the shell region. The resistivity of nanowires decreases with the increase in diameter. These characteristics make these NWs suitable candidates for application as electron connectors in nanoelectronic devices
Concentration Gradient Driven Natural Convection in Soluble Lead Redox Flow Batteries
No full textLow cost and long cycle-life energy storage systems are needed to harness renewable
sources of energy at large scale. Among the options available, redox
ow batteries
(RFB) offer the maximum potential. The vanadium based RFB offers long cycle
life but requires high initial investment and running cost. The membrane-less
soluble lead redox
ow battery (SLRFB) offers a low cost alternative. Since it
uses a common electrolyte for both the electrodes without a proton exchange
membrane in-between, it is likely to be easy to operate and maintain. Soluble
lead redox
ow battery (SLRFB) is currently under development. Our group
has earlier established the dominant role concentration gradient driven natural
convection
ow plays in this electrochemical system.
The present work focuses on developing new designs that harness natural convection
flow for effcient charge-discharge operation of a single SLRFB cell. We
take fi rst step in this direction by establishing the ability of a model developed in
our group to match experimental measurements. The validated model is then used
to test new designs for cell and electrodes. The model considers
fluid
flow, potential
eld, electro-deposition and electro-dissolution reactions on electrode surfaces,
buildup of deposits, and transport of ionic species under convection, diffusion, and
electric fi eld induced migration. The highly coupled physics makes simulations
computation intensive, hence 2-d approximation is used.
A batch cell with wall mounted electrodes (standard cell) and additional electrolyte
above (top) and below (bottom) them is studied for model validation. The
simulation results are obtained without re tting any parameters, and are shown
to be independent of grid size. The results con rm natural convection induced
electrolyte circulation. The strong circulation predicted on anode compared to
cathode is attributed to electric field driven migration of Pb2+ being opposed to
diffusional
flux on anode and in the same direction on cathode. The cell potential
during charge remains constant until the depletion of active material Pb2+ becomes
signi cant. During discharge, the deposit pro le on electrode controls the performance.
The measurements validate the model predictions quantitatively. The 2-d
approximation used in the model, without which a single simulation would take
weeks to complete, is tested by varying cell depth. The measurements validate the
2-d approximation made in the model.
The model predicted velocity eld is tested against measurements obtained
using particle image velocimetry (PIV) technique for the standard wall mounted
electrode cell. Reliable measurements are obtained in regions away from electrode
walls. The resolution and accuracy of measurements near electrode walls is poor
due to blurring of images. The measurements validate the model predicted features
reasonably well.
A new cell con guration with electrodes mounted o cell walls (lift cell) is
studied using simulations and measurements. The new design is inspired by lift
and bubble column reactors. The model predicts large scale circulation in lift
cell. The velocity fi eld induced between the electrodes is more intense for o -
wall compared to on-wall electrodes. The electrolyte circulating through space
between wall and electrodes at an order of magnitude smaller velocities impacts
cell performance signi cantly. It extends constant potential charge and discharge
phases by mixing electrolyte everywhere in the cell. Simulations predict that
providing a small gap, as little as 2 mm, above and below the electrodes, it enough
to bring about complete mixing in lift cell. A number of measurements made on
lift cells validate model predictions quite well.
Splitting electrodes and staggering them in lift cell con figuration so that boundary
layer starts to grow afresh from lower edge of each segment is seen in simulations
to i) decrease charging potential, and ii) increase charging time for the same
total current density. The simulations show that addition of an external
ow loop
allows light electrolyte formed during charge to enter
ow loop from the top and
dense electrolyte to enter the cell from the bottom to bring about mixing. The
opposite happens during discharge. A comparison among the two strategies shows
that split electrodes provide better mixing and lowest charging potential for same
electrolyte volume and fixed current density.
Batch mode of operation is eventually impacted by depletion of active material
even for the case of perfect mixing. Continuous feed of active species is necessary
for charge and discharge over long periods. The effect of
ow rate (from low to
moderate) is studied in detail for standard cell using simulations and measurements.
The simulations show constant cell potential during charge and discharge
for average
ow velocity in range of 2 107 to 2 103 m/s. The measurements
could be carried in
ow velocity range of 6:9 105 to 2 103 m/s. The measurements
validate model predictions well. The simulations carried out for forced
convection alone show that cell potential during charge and discharge keeps changing
with time, similar to diffusion limited response for no
ow cases, for average
ow velocity smaller than 4:5 104 m/s.
There is an asymmetric response of anode and cathode for the same extent of
deposition and dissolution of active material on the two electrodes during charge
and discharge in vertical orientation. The measurements carried out for horizontal
orientation of electrodes (standard cell) with anode at the bottom in one case and
cathode at the bottom in another show signi ficant differences. These differences
over short charging phase are unexplained. The measurements point to decreased
depletion at long time for anode at the bottom confi guration. The latter is consistent
with intense natural convection
ow on anode in vertical con figuration.
The investigations carried out in the present work establish that the model
is able to predict cell performance for a number of new designs introduced here,
without any re tting of the model parameters. Some of the predictions for the
new designs are also validated experimentally. The model can thus be used for
evaluating new designs. Among the tested designs, of-wall electrodes, a small
space above and below electrodes to let circulatory
ow establish, splitting of
electrodes for intense circulation, and unusually weak external
flows at low cost
for long charge and discharge phase hold promise for improved performance
Studies On Phase Inversion
No full textAgitated dispersions of one liquid in another immiscible liquid are widely used in chemical industry in operations such as liquid-liquid extraction, suspension polymerisation, and blending of polymers. When holdup of the dispersed phase is increased, in an effort to increase the productivity, at a critical holdup, the dispersed phase catastrophically becomes the continuous phase and vice versa. This phenomenon is known as phase inversion.
Although the inversion phenomenon has been studied off and on over the past few decades, the mechanism of phase inversion (PI) has yet not become clear. These studies have however brought out many interesting aspects of PI, besides unravelling the effect of physical and operational variables on PL Experiments show that oil-in-water (o/w) and water-in-oil (w/o) dispersions behave very differently, e.g water drops in w/o dispersions contain oil droplets in them, but oil drops in o/w dispersions contain none, dispersed phase hold up at which inversion occurs increases with agitation speed for w/o dispersions but decreases for o/w dispersions.
A common feature of both types of dispersions however is that as agitation speed is increased to high values, inversion holdups reach a constant value. A further increase in agitation speed does not change inversion hold up. Although this finding was first reported a long time ago, the implications it may have not received any attentions. In fact, the work reported in the literature since then does not even mention it. The present work shows that this finding has profound implications. Starting with the finding that at high agitation speed inversion hold up does not change with agitation speed, the present work shows that inversion hold up also does not change with agitator diameter, type of agitator and vessel diameter. In these experiments, carried out in agitated vessel, energy was introduced as a point source. The experiments carried out with turbulent flow in annular region of two coaxial cylinders, inner one rotating, in which energy is introduced nearly uniformly throughout the system, show that the inversion holdup remains unchanged. These results indicate that constant values of inversion holdups for a given liquid-liquid systems (o/w and w/o) are properties of the liquid-liquid systems alone, independent of geometrical and operational parameters. A new hypothesis is proposed to explain the new findings.
Phase inversion is considered to occur as a result of imbalance between breakup and coalescence of drops. Electrolytes, which affect only coalescence of drops, were therefore added to the system to investigate the effect of altering coalescence of drops on phase inversion. The experiments performed in the presence of electrolyte KI at various concentrations indicate that addition of electrolyte increases the inversion holdup for both o/w and w/o dispersions for three types of systems: non polar-water, polar-water and immiscible organic-organic. Higher the concentration of electrolyte used, higher was the holdup required for phase inversion. These findings indicate that while the addition of electrolyte increases coalescence of drops in lean dispersions, it has exactly opposite effect on imbalance of breakage and coalescence of drops at high holdups near phase inversion point. The opposite effect of electrolytes in lean and concentrated dispersions could be explained qualitatively, but only in part in the light of a new theory, involving multi-particle interactions.
The phase inversion phenomenon is quantified in a simple manner by testing the
breakage and coalescence rate expressions available in literature. It has been found
that, equilibrium drop size (where breakage and coalescence events are in dynamic
equilibrium) approaches infinity near phase inversion holdup which is not an ex
perimentally observed fact. To capture the catastrophic nature of phase inversion,
two steady state approach is proposed. The two steady states namely the stable
steady state and unstable steady state, are achieved by modifying the expression
for coalescence frequency on the basis of (i) shear coalescence mechanism and, (ii)
recognising the fact that at high dispersed phase holdup the droplets are already in
contact with each other at all times and hence rendering the second order coales
cence process to a first order one. Using two steady states approach, catastrophic
phase inversion is shown to occur at finite drop size
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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