Jean-Marc Debierre

Phase-field approach for faceted solidification.

We extend the phase-field approach to model the solidification of faceted materials. Our approach consists of using an approximate gamma plot with rounded cusps that can approach arbitrarily closely the true gamma plot with sharp cusps that correspond to faceted orientations. The phase-field equations are solved in the thin-interface limit with local equilibrium at the solid-liquid interface [A. Karma and W.-J. Rappel, Phys. Rev. E 53, R3017 (1996)]. The convergence of our approach is first demonstrated for equilibrium shapes. The growth of faceted needle crystals in an undercooled melt is then studied as a function of undercooling and the cusp amplitude delta for a gamma plot of the form gamma=gamma0[1+delta(/sin theta/+/cos theta/)]. The phase-field results are consistent with the scaling law Lambda approximately V(-1/2) observed experimentally, where Lambda is the facet length and V is the growth rate. In addition, the variation of V and Lambda with delta is found to be reasonably well predicted by an approximate sharp-interface analytical theory that includes capillary effects and assumes circular and parabolic forms for the front and trailing rough parts of the needle crystal, respectively.

Measuring kinetic coefficients by molecular dynamics simulation of zone melting.

Molecular dynamics simulations are performed to measure the kinetic coefficient at the solid-liquid interface in pure gold. Results are obtained for the (111), (100), and (110) orientations. Both Au(100) and Au(110) are in reasonable agreement with the law proposed for collision-limited growth. For Au(111), stacking fault domains form, as first reported by Burke, Broughton, and Gilmer [J. Chem. Phys. 89, 1030 (1988)]. The consequence on the kinetics of this interface is dramatic: the measured kinetic coefficient is three times smaller than that predicted by collision-limited growth. Finally, crystallization and melting are found to be always asymmetrical and here again the effect is much more pronounced for the (111) orientation.

Enhanced conductivity in ionic conductor-insulator composites: Experiments and numerical model

We study ionic conductivity of the model composites, copper(I) bromide-titanium dioxyde, in a large domain of composition. The experimentally observed enhancement of conductivity is interpreted by a numerical model taking into account grain size effects and interactions between ionic conductor and insulator grains.

Magnetic cluster formation in Li x Ni 1 − x O compounds: Experiments and numerical simulations

The magnetic properties of

Nonequilibrium molecular dynamics simulation of rapid directional solidification

{\mathrm{Li}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}

Role of step-flow advection during electromigration-induced step bunching

compounds with x ranging between 0.4 and 0.49 are investigated. Magnetization and ac susceptibility measured at temperatures between 2 K and 300 K reveal a high sensitivity to x, the excess lithium concentration. We introduce a percolation model describing the formation of Ni clusters and use an Ising model to simulate their magnetic properties. Numerical results, obtained by a Monte Carlo technique, are compared to the experimental data. We show the existence of a critical concentration,

Modelling the two-dimensional polymerization of 1,4-benzene diboronic acid on a Ag surface

{x}_{c}=0.432,

Electromigration-induced step meandering on vicinal surfaces : Nonlinear evolution equation

locating the Ni percolation threshold. The system is superparamagnetic for

Low temperature resistivity of Au-YBa2Cu3O7-delta sintered materials: Influence of porosity and secondary phases

xg{x}_{c},

Static and dynamic scaling for chain-chain aggregation in two dimensions

while it is ferrimagnetic for