426 research outputs found
Computational Materials Techniques for Thermal Protection Solutions: Materials and Process Design
Integrated computational materials techniques that span the atomistic and continuum scales have the potential to aid the design and manufacturing of thermal protection materials. Two cases demonstrating the practical application of these methods are discussed. Case one examines the selection of a high temperature coating for carbon/carbon, with the target application being a solar thermal propulsion heat exchanger. The performance of various refractory metal and metal-carbide coatings is characterized considering extreme thermal (3500 K) and chemical (hydrogen flows) conditions. The recession rate, hydrogen leakage, and likelihood of mechanical failure are characterized and provide directions for further experimental investigation. Case two examines the process optimization of a heat shield material composed of a woven silica fiber preform and cyanate ester resin. Frequently, internal voids were found to be present in this composite after the resin infusion and curing stages of manufacturing. Using the manufacturing conditions, computations indicate that both water adsorption and resin cure shrinkage are contributing factors to void formation. The results suggest an alternative process configuration for curing that would mitigate voids
Molecular Dynamics Simulations of Liquid and Polymer Electrolytes for Energy Storage Devices
Advancing beyond current lithium-ion technology is necessary in order to enable energy storage devices for electric airplanes. Electrolyte stability is a key limiting factor, yet the design of improved electrolytes remains a formidable challenge. Molecular dynamics (MD) simulations are a powerful tool for studying electrolytes, since they can be used to evaluate structural, thermodynamic, and transport properties, and can provide molecular-level detail often inaccessible to experimental techniques. Our computational materials groups at the NASA Ames Research Center has developed models and methods to accurately simulate both liquid and polymer electrolytes.We report the results from atomistic MD simulations of several electrolyte materials, with lithium salts dissolved in ionic liquids, dimethoxyethane (DME), and polyethylene oxide (PEO). For improved accuracy, we employ polarizable models, where each atom is given an environment-dependent atomic dipole. The simulations accurately predict bulk transport properties, including viscosity, diffusion, and ionic conductivity, in quantitative agreement with available experimental data. Moreover, the simulations provide important insights into the solvation structure of the lithium ions.We also report the results from coarse-grained MD simulations of polyanion electrolytes. In order to more efficiently capture the longer length- and time-scales of these systems, we employ a generic bead-spring model. These simulations provide important insight into how the polymer chain architecture and ionic interaction strengths affect the ionic aggregation behavior and cation dynamics. Despite the simplicity of the model, the simulations yield qualitative agreement with experimental data for similar systems
Magnons, their Solitonic Avatars and the Pohlmeyer Reduction
We study the solitons of the symmetric space sine-Gordon theories that arise
once the Pohlmeyer reduction has been imposed on a sigma model with the
symmetric space as target. Under this map the solitons arise as giant magnons
that are relevant to string theory in the context of the AdS/CFT
correspondence. In particular, we consider the cases S^n, CP^n and SU(n) in
some detail. We clarify the construction of the charges carried by the solitons
and also address the possible Lagrangian formulations of the symmetric space
sine-Gordon theories. We show that the dressing, or Backlund, transformation
naturally produces solitons directly in both the sigma model and the symmetric
space sine-Gordon equations without the need to explicitly map from one to the
other. In particular, we obtain a new magnon solution in CP^3. We show that the
dressing method does not produce the more general "dyonic" solutions which
involve non-trivial motion of the collective coordinates carried by the
solitons.Comment: 52 page
The Gauged Vector Model in Four-Dimensions: Resolution of an Old Problem?
A calculation of the renormalization group improved effective potential for
the gauged U(N) vector model, coupled to fermions in the fundamental
representation, computed to leading order in 1/N, all orders in the scalar
self-coupling , and lowest order in gauge coupling , with
of order , is presented. It is shown that the theory has two phases, one of
which is asymptotically free, and the other not, where the asymptotically free
phase occurs if , and
. In the asymptotically free phase, the effective
potential behaves qualitatively like the tree-level potential. In the other
phase, the theory exhibits all the difficulties of the ungauged
vector model. Therefore the theory appears to be consistent (only) in the
asymptotically free phase.Comment: Latex, 18 pages plus 3 figures using epsf. Substantially revised to
correct a factor of 2 error in the previous version of equation (2.5b). This
has significant effects on the results. The model has also been revised to
include fermion
The effective action and quantum gauge transformations
The local symmetry transformations of the quantum effective action for
general gauge theory are found. Additional symmetries arise under consideration
of background gauges. Together with "trivial" gauge transformations, vanishing
on mass shell, they can be used for construction simple gauge generators. For
example, for the Yang-Mills theory the classically invariant effective action
is obtained, reproducing DeWitt's result. For rank one theories a natural
generalization is proposed.Comment: Revtex, 11 pages; added reference
Gauge dependence of effective gravitational field
The problem of gauge independent definition of effective gravitational field
is considered from the point of view of the process of measurement. Under
assumption that dynamics of the measuring apparatus can be described by the
ordinary classical action, effective Slavnov identities for the generating
functionals of Green functions corresponding to a system of arbitrary
gravitational field measured by means of scalar particles are obtained. With
the help of these identities, the total gauge dependence of the non-local part
of the one-loop effective apparatus action, describing the long-range quantum
corrections, is calculated. The value of effective gravitational field inferred
from the effective apparatus action is found to be gauge-dependent. A probable
explanation of this result, referring to a peculiarity of the gravitational
interaction, is given.Comment: Revised version as publishe
Chiral transition and monopole percolation in lattice scalar QED with quenched fermions
We study the interplay between topological observables and chiral and Higgs
transitions in lattice scalar QED with quenched fermions. Emphasis is put on
the chiral transition line and magnetic monopole percolation at strong gauge
coupling. We confirm that at infinite gauge coupling the chiral transition is
described by mean field exponents. We find a rich and complicated behaviour at
the endpoint of the Higgs transition line which hampers a satisfactory analysis
of the chiral transition. We study in detail an intermediate coupling, where
the data are consistent both with a trivial chiral transition clearly separated
from monopole percolation and with a chiral transition coincident with monopole
percolation, and characterized by the same critical exponent .
We discuss the relevance (or lack thereof) of these quenched results to our
understanding of the \chupiv\ model. We comment on the interplay of magnetic
monopoles and fermion dynamics in more general contexts.Comment: 29 pages, 13 figures included, LaTeX2e (elsart
Analytic and Numerical Study of Preheating Dynamics
We analyze the phenomenon of preheating,i.e. explosive particle production
due to parametric amplification of quantum fluctuations in the unbroken case,
or spinodal instabilities in the broken phase, using the Minkowski space
vector model in the large limit to study the non-perturbative issues
involved. We give analytic results for weak couplings and times short compared
to the time at which the fluctuations become of the same order as the tree
level,as well as numerical results including the full backreaction.In the case
where the symmetry is unbroken, the analytic results agree spectacularly well
with the numerical ones in their common domain of validity. In the broken
symmetry case, slow roll initial conditions from the unstable minimum at the
origin, give rise to a new and unexpected phenomenon: the dynamical relaxation
of the vacuum energy.That is, particles are abundantly produced at the expense
of the quantum vacuum energy while the zero mode comes back to almost its
initial value.In both cases we obtain analytically and numerically the equation
of state which turns to be written in terms of an effective polytropic index
that interpolates between vacuum and radiation-like domination. We find that
simplified analysis based on harmonic behavior of the zero mode, giving rise to
a Mathieu equation forthe non-zero modes miss important physics. Furthermore,
analysis that do not include the full backreaction do not conserve energy,
resulting in unbound particle production. Our results do not support the recent
claim of symmetry restoration by non-equilibrium fluctuations.Finally estimates
of the reheating temperature are given,as well as a discussion of the
inconsistency of a kinetic approach to thermalization when a non-perturbatively
large number of particles is created.Comment: Latex file, 52 pages and 24 figures in .ps files. Minor changes. To
appear in Physical Review D, 15 December 199
Holography of AdS vacuum bubbles
We consider the fate of AdS vacua connected by tunneling events. A precise
holographic dual of thin-walled Coleman--de Luccia bounces is proposed in terms
of Fubini instantons in an unstable CFT. This proposal is backed by several
qualitative and quantitative checks, including the precise calculation of the
instanton action appearing in evaluating the decay rate. Big crunches manifest
themselves as time dependent processes which reach the boundary of field space
in a finite time. The infinite energy difference involved is identified on the
boundary and highlights the ill-defined nature of the bulk setup. We propose a
qualitative scenario in which the crunch is resolved by stabilizing the CFT, so
that all attempts at crunching always end up shielded from the boundary by the
formation of black hole horizons. In all these well defined bulk processes the
configurations have the same asymptotics and are finite energy excitations.Comment: version submitted to journal. Note added referring to previous work
on holographic instantons
Can Asymptotic Series Resolve the Problems of Inflation?
We discuss a cosmological scenario in which inflation is driven by a
potential which is motivated by an effective Lagrangian approach to gravity. We
exploit the recent arguments \cite{ARZ} that an effective Lagrangian
which, by definition, contains operators of arbitrary dimensionality is in
general not a convergent but rather an asymptotic series with factorially
growing coefficients. This behavior of the effective Lagrangian might be
responsible for the resolution of the cosmological constant problem. We argue
that the same behavior of the potential gives a natural realization of the
inflationary scenario.Comment: 12 pages, uses Late
- …