888 research outputs found
Space-charge mechanism of aging in ferroelectrics: an exactly solvable two-dimensional model
A mechanism of point defect migration triggered by local depolarization
fields is shown to explain some still inexplicable features of aging in
acceptor doped ferroelectrics. A drift-diffusion model of the coupled charged
defect transport and electrostatic field relaxation within a two-dimensional
domain configuration is treated numerically and analytically. Numerical results
are given for the emerging internal bias field of about 1 kV/mm which levels
off at dopant concentrations well below 1 mol%; the fact, long ago known
experimentally but still not explained. For higher defect concentrations a
closed solution of the model equations in the drift approximation as well as an
explicit formula for the internal bias field is derived revealing the plausible
time, temperature and concentration dependencies of aging. The results are
compared to those due to the mechanism of orientational reordering of defect
dipoles.Comment: 8 pages, 4 figures. accepted to Physical Review
Incremental expansions for Hubbard-Peierls systems
The ground state energies of infinite half-filled Hubbard-Peierls chains are
investigated combining incremental expansion with exact diagonalization of
finite chain segments. The ground state energy of equidistant infinite Hubbard
(Heisenberg) chains is calculated with a relative error of less than for all values of using diagonalizations of 12-site (20-site)
chain segm ents. For dimerized chains the dimerization order parameter as a
function of the onsite repulsion interaction has a maximum at nonzero
values of , if the electron-phonon coupling is lower than a critical
value . The critical value is found with high accuracy to be
. For smaller values of the position of the maximum of is
approximately , and rapidly tends to zero as approaches from
below. We show how our method can be applied to calculate breathers for the
problem of phonon dynamics in Hubbard-Peierls systems.Comment: 4 Pages, 3 Figures, REVTE
An Artificial SEI Layer Based on an Inorganic Coordination Polymer with Self-Healing Ability for Long-Lived Rechargeable Lithium-Metal Batteries
Upon immersion of a lithium (Li) anode into a diluted 0.05 to 0.20 M dimethoxyethane solution of the phosphoric-acid derivative (CFCHO)P(O)OH (HBFEP), an artificial solid-electrolyte interphase (SEI) is generated on the Li-metal surface. Hence, HBFEP reacts on the surface to the corresponding Li salt (LiBFEP), which is a Li-ion conducting inorganic coordination polymer. This film exhibits – due to the reversibly breaking ionic bonds – self-healing ability upon cycling-induced volume expansion of Li. The presence of LiBFEP as the major component in the artificial SEI is proven by ATR-IR and XPS measurements. SEM characterization of HBFEP-treated Li samples reveals porous layers on top of the Li surface with at least 3 μm thickness. Li−Li symmetrical cells with HBFEP-modified Li electrodes show a three- to almost fourfold cycle-lifetime increase at 0.1 mA cm in a demanding model electrolyte that facilitates fast battery failure (1 M LiOTf in TEGDME). Hence, the LiBFEP-enriched layer apparently acts as a Li-ion conducting protection barrier between Li and the electrolyte, enhancing the rechargeability of Li electrodes
Design of human-like posture prediction for inverse kinematic posture control of a humanoid robot
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references (p. 53-54).A method and system has been developed to solve the kinematic redundancy for a humanoid redundant manipulator based on forward kinematic equation and the optimization of human-like constraints. The Multiple Objective Optimization (MOO) is preformed using a Genetic Algorithms (GA) and implemented using the Genetic and Evolutionary Algorithm Matlab Toolbox. The designed system is illustrated on a simple redundant 3 degree of freedom (dof) manipulator and is set up for a more complicated redundant 7 dof manipulator. The 7 dof manipulator is modeled from the Stan Winston studio's Leonardo, an 61 dof expressive humanoid robot. It has been found that the inverse kinematic solution to a 3dof model arm converged within 1% error of the solution within .05 mins processor time using the discomfort human-like constraint in 2d space. Similarly, the inverse kinematic solution to a 7 dof model arm consisting of Leonardo's right arm geometry was found to converge within 1% error within .20 mins processor time using the discomfort human-like constraint in 3D space. The full kinematic model of Leonardo is developed and future efficiency optimizations are posed to move towards the real-time motion control of a redundant humanoid robot by way of human-like posture prediction.by Derik Thomann.S.B
Small unilamellar liposomes as a membrane model for cell inactivation by cold atmospheric plasma treatment
International audienceCold atmospheric plasma is thought to be a promising tool for numerous biomedical applications due to its ability to generate a large diversity of reactive species in a controlled way. In some cases, it can also generate pulsed electric fields at the zone of treatment, which can induce processes such as electroporation in cell membranes. However, the interaction of these reactive species and the pulse electric field with cells in a physiological medium is very complex and still need a better understanding in order to be useful for future applications. A way to reach this goal is to work with model cell membranes such as liposomes, with the simplest physiological liquid and in a controlled atmosphere in order to limit the number of parallel reactions and processes. In this work, where this approach has been chosen, 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC) small unilamellar vesicles (SUV) has been synthesized in phosphate buffered aqueous solution and this solution has been treated by a nanosecond pulsed plasma jet under a pure nitrogen atmosphere. Only the composition of the plasma gas has been changed in order to generate different cocktails of reactive species. After the quantification of the main plasma reactive species in the PBS solution, structural, surface charge state, and chemical modifications generated on the plasma treated liposomes, due to the interaction with the plasma reactive species, has been carefully characterized. These results allow going further in the understanding of the effect of plasma reactive species on model cell membranes in physiological liquids. Permeation through the liposomal membrane and reaction of plasma reactive species with molecules encapsulated inside the liposomes has also been evaluated. New processes of degradation are finally presented and discussed, which come from the specific conditions of plasma treatment under pure nitrogen atmosphere
Quantum instability in a dc-SQUID with strongly asymmetric dynamical parameters
A classical system cannot escape out of a metastable state at zero
temperature. However, a composite system made from both classical and quantum
degrees of freedom may drag itself out of the metastable state by a sequential
process. The sequence starts with the tunneling of the quantum component which
then triggers a distortion of the trapping potential holding the classical
part. Provided this distortion is large enough to turn the metastable state
into an unstable one, the classical component can escape. This process reminds
of the famous baron Muenchhausen who told the story of rescuing himself from
sinking in a swamp by pulling himself up by his own hair--we thus term this
decay the `Muenchhausen effect'. We show that such a composite system can be
conveniently studied and implemented in a dc-SQUID featuring asymmetric
dynamical parameters. We determine the dynamical phase diagram of this system
for various choices of junction parameters and system preparations.Comment: 12 pages, 12 figure
Aging of poled ferroelectric ceramics due to relaxation of random depolarization fields by space-charge accumulation near grain boundaries
Migration of charged point defects triggered by the local random
depolarization field is shown to plausibly explain aging of poled ferroelectric
ceramics providing reasonable time and acceptor concentration dependences of
the emerging internal bias field. The theory is based on the evaluation of the
energy of the local depolarization field caused by mismatch of the
polarizations of neighbor grains. The kinetics of charge migration assumes
presence of mobile oxygen vacancies in the material due to the intentional or
unintentional acceptor doping. Satisfactory agreement of the theory with
experiment on the Fe-doped lead zirconate titanate is demonstrated.Comment: theory and experiment, 22 pages, 3 figure
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