25,747 research outputs found
Adjoint-based predictor-corrector sequential convex programming for parametric nonlinear optimization
This paper proposes an algorithmic framework for solving parametric
optimization problems which we call adjoint-based predictor-corrector
sequential convex programming. After presenting the algorithm, we prove a
contraction estimate that guarantees the tracking performance of the algorithm.
Two variants of this algorithm are investigated. The first one can be used to
solve nonlinear programming problems while the second variant is aimed to treat
online parametric nonlinear programming problems. The local convergence of
these variants is proved. An application to a large-scale benchmark problem
that originates from nonlinear model predictive control of a hydro power plant
is implemented to examine the performance of the algorithms.Comment: This manuscript consists of 25 pages and 7 figure
Thermal reaction of Al/Ti bilayers with contaminated interface
We have studied some new aspects of thermal reactions in Al/Ti bilayers in which the interface is purposely contaminated with oxygen. After annealing at a temperature of 460â°C, an Al_3Ti compound forms at the interface, moreover some Al diffuses through the Ti to form a compound at the free surface. The amount of aluminum at the free surface can be as large as at the interface. Nucleation and lateral growth of Al_3Ti at the interface are locally unfavorable. This results in a competition between the lateral growth of Al_3Ti at the Al/Ti interface and the diffusion of Al to the free surface. Once full coverage by Al_3Ti is obtained at the Al/Ti interface, the diffusion of Al to the surface becomes negligible
Thermodynamic dislocation theory of high-temperature deformation in aluminum and steel
The statistical-thermodynamic dislocation theory developed in previous papers
is used here in an analysis of high-temperature deformation of aluminum and
steel. Using physics-based parameters that we expect theoretically to be
independent of strain rate and temperature, we are able to fit experimental
stress-strain curves for three different strain rates and three different
temperatures for each of these two materials. Our theoretical curves include
yielding transitions at zero strain in agreement with experiment. We find that
thermal softening effects are important even at the lowest temperatures and
smallest strain rates.Comment: 7 pages, 8 figure
Closed-form Absorption Probability of Certain D=5 and D=4 Black Holes and Leading-Order Cross-Section of Generic Extremal p-branes
We obtain the closed-form absorption probabilities for minimally-coupled
massless scalars propagating in the background of D=5 single-charge and D=4
two-charge black holes. These are the only two examples of extremal black holes
with non-vanishing absorption probabilities that can be solved in closed form
for arbitrary incident frequencies. In both cases, the absorption probability
vanishes when the frequency is below a certain threshold, and we discuss the
connection between this phenomenon and the behaviour of geodesics in these
black hole backgrounds. We also obtain leading-order absorption cross-sections
for generic extremal p-branes, and show that the expression for the
cross-section as a function of frequency coincides with the leading-order
dependence of the entropy on the temperature in the corresponding near-extremal
p-branes.Comment: Latex (3 times), 20 page
Submicrometric Films of Surface-Attached Polymer Network with Temperature-Responsive Properties
Temperature-responsive properties of surface-attached
poly(N-isopropylacrylamide) (PNIPAM) network films with well-controlled
chemistry are investigated. The synthesis consists of cross-linking and
grafting preformed ene-reactive polymer chains through thiol--ene click
chemistry. The formation of surface-attached and cross-linked polymer films has
the advantage of being wellcontrolled without any caution of no-oxygen
atmosphere or addition of initiators. PNIPAM hydrogel films with same
cross-link density are synthesized on a wide range of thickness, from
nanometers to micrometers. The swelling-collapse transition with temperature is
studied by using ellipsometry, neutron reflectivity, and atomic force
microscopy as complementary surface-probing techniques. Sharp and high
amplitude temperature-induced phase transition is observed for all
submicrometric PNIPAM hydrogel films. For temperature above LCST,
surface-attached PNIPAM hydrogels collapse similarly but without complete
expulsion of water. For temperature below LCST, the swelling of PNIPAM
hydrogels depends on the film thickness. It is shown that the swelling is
strongly affected by the surface attachment for ultrathin films below 150
nm. For thicker films above 150 nm (to micrometers), surface-attached polymer
networks with the same cross-link density swell equally. The density profile of
the hydrogel films in the direction normal to the substrate is confronted with
in-plane topography of the free surface. It results that the free interface
width is much larger than the roughness of the hydrogel film, suggesting
pendant chains at the free surface.Comment: in Langmuir, American Chemical Society, 2015, LANGMUIR, 31 (42),
pp.11516-1152
Enhancement of the Spin Accumulation at the Interface Between a Spin-Polarized Tunnel Junction and a Semiconductor
We report on spin injection experiments at a Co/AlO/GaAs interface
with electrical detection. The application of a transverse magnetic field
induces a large voltage drop at the interface as high as 1.2mV for a
current density of 0.34 nA.. This represents a dramatic increase of
the spin accumulation signal, well above the theoretical predictions for spin
injection through a ferromagnet/semiconductor interface. Such an enhancement is
consistent with a sequential tunneling process via localized states located in
the vicinity of the AlO/GaAs interface. For spin-polarized carriers
these states act as an accumulation layer where the spin lifetime is large. A
model taking into account the spin lifetime and the escape tunneling time for
carriers travelling back into the ferromagnetic contact reproduces accurately
the experimental results
Large-scale bottleneck effect in two-dimensional turbulence
The bottleneck phenomenon in three-dimensional turbulence is generally
associated with the dissipation range of the energy spectrum. In the present
work, it is shown by using a two-point closure theory, that in two-dimensional
turbulence it is possible to observe a bottleneck at the large scales, due to
the effect of friction on the inverse energy cascade. This large-scale
bottleneck is directly related to the process of energy condensation, the
pile-up of energy at wavenumbers corresponding to the domain size. The link
between the use of friction and the creation of space-filling structures is
discussed and it is concluded that the careless use of hypofriction might
reduce the inertial range of the energy spectrum
Indium oxide diffusion barriers for Al/Si metallizations
Indium oxide (In2O3) films were prepared by reactive rf sputtering of an In target in O2/Ar plasma. We have investigated the application of these films as diffusion barriers in Si/In2O3/Al and Si/TiSi2.3/In2O3/Al metallizations. Scanning transmission electron microscopy together with energy dispersive analysis of x ray of cross-sectional Si/In2O3/Al specimens, and electrical measurements on shallow n + -p junction diodes were used to evaluate the diffusion barrier capability of In2O3 films. We find that 100-nm-thick In2O3 layers prevent the intermixing between Al and Si in Si/In2O3/Al contacts up to 650°C for 30 min, which makes this material one of the best thin-film diffusion barriers on record between Al and Si. (The Si-Al eutectic temperature is 577°C, Al melts at 660°C.) When a contacting layer of titanium silicide is incorporated to form a Si/TiSi2.3/In2O3/Al metallization structure, the thermal stability of the contact drops to 600°C for 30 min heat treatment
U-duality as General Coordinate Transformations, and Spacetime Geometry
We show that the full global symmetry groups of all the D-dimensional maximal
supergravities can be described in terms of the closure of the internal general
coordinate transformations of the toroidal compactifications of D=11
supergravity and of type IIB supergravity, with type IIA/IIB T-duality
providing an intertwining between the two pictures. At the quantum level, the
part of the U-duality group that corresponds to the surviving discretised
internal general coordinate transformations in a given picture leaves the
internal torus invariant, while the part that is not described by internal
general coordinate transformations can have the effect of altering the size or
shape of the internal torus. For example, M-theory compactified on a large
torus T^n can be related by duality to a compactification on a small torus, if
and only if n\ge 3. We also discuss related issues in the toroidal
compactification of the self-dual string to D=4. An appendix includes the
complete results for the toroidal reduction of the bosonic sector of type IIB
supergravity to arbitrary dimensions D\ge3.Comment: Latex, 28 page
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