199 research outputs found
Kinetic cross coupling between non-conserved and conserved fields in phase field models
We present a phase field model for isothermal transformations of two
component alloys that includes Onsager kinetic cross coupling between the
non-conserved phase field and the conserved concentration field. We also
provide the reduction of the phase field model to the corresponding macroscopic
description of the free boundary problem. The reduction is given in a general
form. Additionally we use an explicit example of a phase field model and check
that the reduced macroscopic description, in the range of its applicability, is
in excellent agreement with direct phase field simulations. The relevance of
the newly introduced terms to solute trapping is also discussed
Universal Dynamics of Phase-Field Models for Dendritic Growth
We compare time-dependent solutions of different phase-field models for
dendritic solidification in two dimensions, including a thermodynamically
consistent model and several ad hoc models. The results are identical when the
phase-field equations are operating in their appropriate sharp interface limit.
The long time steady state results are all in agreement with solvability
theory. No computational advantage accrues from using a thermodynamically
consistent phase-field model.Comment: 4 pages, 3 postscript figures, in latex, (revtex
Phase-Field Formulation for Quantitative Modeling of Alloy Solidification
A phase-field formulation is introduced to simulate quantitatively
microstructural pattern formation in alloys. The thin-interface limit of this
formulation yields a much less stringent restriction on the choice of interface
thickness than previous formulations and permits to eliminate non-equilibrium
effects at the interface. Dendrite growth simulations with vanishing solid
diffusivity show that both the interface evolution and the solute profile in
the solid are well resolved
Crossover Scaling in Dendritic Evolution at Low Undercooling
We examine scaling in two-dimensional simulations of dendritic growth at low
undercooling, as well as in three-dimensional pivalic acid dendrites grown on
NASA's USMP-4 Isothermal Dendritic Growth Experiment. We report new results on
self-similar evolution in both the experiments and simulations. We find that
the time dependent scaling of our low undercooling simulations displays a
cross-over scaling from a regime different than that characterizing Laplacian
growth to steady-state growth
Dynamics of driven interfaces near isotropic percolation transition
We consider the dynamics and kinetic roughening of interfaces embedded in
uniformly random media near percolation treshold. In particular, we study
simple discrete ``forest fire'' lattice models through Monte Carlo simulations
in two and three spatial dimensions. An interface generated in the models is
found to display complex behavior. Away from the percolation transition, the
interface is self-affine with asymptotic dynamics consistent with the
Kardar-Parisi-Zhang universality class. However, in the vicinity of the
percolation transition, there is a different behavior at earlier times. By
scaling arguments we show that the global scaling exponents associated with the
kinetic roughening of the interface can be obtained from the properties of the
underlying percolation cluster. Our numerical results are in good agreement
with theory. However, we demonstrate that at the depinning transition, the
interface as defined in the models is no longer self-affine. Finally, we
compare these results to those obtained from a more realistic
reaction-diffusion model of slow combustion.Comment: 7 pages, 9 figures, to appear in Phys. Rev. E (1998
Polychromatic angle resolved IBIC analysis of silicon power diodes
This paper describes both an experimental methodology based on the Ion Beam
Induced Charge (IBIC) technique and the relevant interpretative model, which
were adopted to characterize the electronic features of power diodes. IBIC
spectra were acquired using different proton energies (from 1.2 to 2.0 MeV),
angles of incidence, and applied bias voltages. The modulation of the ion probe
range, combined with the modulation of the extensions of the depletion layer,
allowed the charge collection efficiency scale to be accurately calibrated, the
dead layer beneath the thick (6 micrometer) Al electrode and the minority
carrier lifetime to be measured. The analysis was performed by using a
simplified model extracted from the basic IBIC theory, which proved to be
suitable to interpret the behaviour of the IBIC spectra as a function of all
the experimental conditions and to characterize the devices, both for what
concerns the electrostatics and the recombination processes.Comment: 24 pagese,10 figure
Controlling crystal symmetries in phase-field crystal models
We investigate the possibility to control the symmetry of ordered states in
phase-field crystal models by tuning nonlinear resonances. In two dimensions,
we find that a state of square symmetry as well as coexistence between squares
and hexagons can be easily obtained. In contrast, it is delicate to obtain
coexistence of squares and liquid. We develop a general method for constructing
free energy functionals that exhibit solid-liquid coexistence with desired
crystal symmetries. As an example, we develop a free energy functional for
square-liquid coexistence in two dimensions. A systematic analysis for
determining the parameters of the necessary nonlinear terms is provided. The
implications of our findings for simulations of materials with simple cubic
symmetry are discussed.Comment: 19 pages, 6 figure
Flame propagation in random media
We introduce a phase-field model to describe the dynamics of a
self-sustaining propagating combustion front within a medium of randomly
distributed reactants. Numerical simulations of this model show that a flame
front exists for reactant concentration , while its vanishing at
is consistent with mean-field percolation theory. For , we find
that the interface associated with the diffuse combustion zone exhibits kinetic
roughening characteristic of the Kardar-Parisi-Zhang equation.Comment: 4, LR541
Efficient Computation of Dendritic Microstructures using Adaptive Mesh Refinement
We study dendritic microstructure evolution using an adaptive grid, finite
element method applied to a phase-field model. The computational complexity of
our algorithm, per unit time, scales linearly with system size, rather than the
quadratic variation given by standard uniform mesh schemes. Time-dependent
calculations in two dimensions are in good agreement with the predictions of
solvability theory, and can be extended to three dimensions and small
undercoolingsComment: typo in a parameter of Fig. 1; 4 pages, 4 postscript figures, in
LateX, (revtex
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