47,338 research outputs found
Interface characteristics in an {\alpha}+{\beta} titanium alloy
The alpha/beta interface in Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) is investigated via
centre of symmetry analysis, both as-grown and after 10% cold work.
Semi-coherent interface steps are observed at a spacing of 4.5 +/-1.13 atoms in
the as-grown condition, in good agreement with theory prediction (4.37 atoms).
Lattice accommodation is observed, with elongation along [-1 2 -1 0]alpha and
contraction along [1 0 -1 0]alpha . Deformed alpha exhibited larger, less
coherent steps with slip bands lying in {110}beta. This indicates dislocation
pile-up at the grain boundary, a precursor to globularisation, offering insight
into the effect of deformation processing on the interface, which is important
for titanium alloy processing route design.Comment: Revised after revie
Transport between two fluids across their mutual flow interface: the streakline approach
Mixing between two different miscible fluids with a mutual interface must be
initiated by fluid transporting across this fluid interface, caused for example
by applying an unsteady velocity agitation. In general, there is no necessity
for this physical flow barrier between the fluids to be associated with
extremal or exponential attraction as might be revealed by applying Lagrangian
coherent structures, finite-time Lyapunov exponents or other methods on the
fluid velocity. It is shown that streaklines are key to understanding the
breaking of the interface under velocity agitations, and a theory for locating
the relevant streaklines is presented. Simulations of streaklines in a
cross-channel mixer and a perturbed Kirchhoff's elliptic vortex are
quantitatively compared to the theoretical results. A methodology for
quantifying the unsteady advective transport between the two fluids using
streaklines is presented
Stable unidimensional arrays of coherent strained islands
We investigate the equilibrium properties of arrays of coherent strained
islands in heteroepitaxial thin films of bidimensional materials. The model we
use takes into account only three essential ingredients : surface energies,
elastic energies of the film and of the substrate and interaction energies
between islands via the substrate. Using numerical simulations for a simple
Lennard-Jones solid, we can assess the validity of the analytical expressions
used to describe each of these contributions. A simple analytical expression is
obtained for the total energy of the system. Minimizing this energy, we show
that arrays of coherent islands can exist as stable configurations. Even in
this simple approach, the quantitative results turn out to be very sensitive to
some details of the surface energy.Comment: 24 pages, 7 figures. to be published in Surface Scienc
Resilience and Controllability of Dynamic Collective Behaviors
The network paradigm is used to gain insight into the structural root causes
of the resilience of consensus in dynamic collective behaviors, and to analyze
the controllability of the swarm dynamics. Here we devise the dynamic signaling
network which is the information transfer channel underpinning the swarm
dynamics of the directed interagent connectivity based on a topological
neighborhood of interactions. The study of the connectedness of the swarm
signaling network reveals the profound relationship between group size and
number of interacting neighbors, which is found to be in good agreement with
field observations on flock of starlings [Ballerini et al. (2008) Proc. Natl.
Acad. Sci. USA, 105: 1232]. Using a dynamical model, we generate dynamic
collective behaviors enabling us to uncover that the swarm signaling network is
a homogeneous clustered small-world network, thus facilitating emergent
outcomes if connectedness is maintained. Resilience of the emergent consensus
is tested by introducing exogenous environmental noise, which ultimately
stresses how deeply intertwined are the swarm dynamics in the physical and
network spaces. The availability of the signaling network allows us to
analytically establish for the first time the number of driver agents necessary
to fully control the swarm dynamics
Two-dimensional hole precession in an all-semiconductor spin field effect transistor
We present a theoretical study of a spin field-effect transistor realized in
a quantum well formed in a p--doped ferromagnetic-semiconductor-
nonmagnetic-semiconductor-ferromagnetic-semiconductor hybrid structure. Based
on an envelope-function approach for the hole bands in the various regions of
the transistor, we derive the complete theory of coherent transport through the
device, which includes both heavy- and light-hole subbands, proper modeling of
the mode matching at interfaces, integration over injection angles, Rashba spin
precession, interference effects due to multiple reflections, and gate-voltage
dependences. Numerical results for the device current as a function of
externally tunable parameters are in excellent agreement with approximate
analytical formulae.Comment: 9 pages, 11 figure
Phase Transformation Dynamics in Porous Battery Electrodes
Porous electrodes composed of multiphase active materials are widely used in
Li-ion batteries, but their dynamics are poorly understood. Two-phase models
are largely empirical, and no models exist for three or more phases. Using a
modified porous electrode theory based on non-equilibrium thermodynamics, we
show that experimental phase behavior can be accurately predicted from free
energy models, without artificially placing phase boundaries or fitting the
open circuit voltage. First, we simulate lithium intercalation in porous iron
phosphate, a popular two-phase cathode, and show that the zero-current voltage
gap, sloping voltage plateau and under-estimated exchange currents all result
from size-dependent nucleation and mosaic instability. Next, we simulate porous
graphite, the standard anode with three stable phases, and reproduce
experimentally observed fronts of color-changing phase transformations. These
results provide a framework for physics-based design and control for
electrochemical systems with complex thermodynamics
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