16 research outputs found
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. E53, 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 1+delta(|sin(theta)|+|cos(theta)|). The phase-field
results are consistent with the scaling law "Lambda inversely proportional to
the square root of V" 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.Comment: 1O pages, 2 tables, 17 figure
Surface effects in nucleation and growth of smectic B crystals in thin samples
We present an experimental study of the surface effects (interactions with
the container walls) during the nucleation and growth of smectic B crystals
from the nematic in free growth and directional solidification of a mesogenic
molecule () called CCH4 in thin (of thickness in the 10
m range) samples. We follow the dynamics of the system in real time with a
polarizing microscope. The inner surfaces of the glass-plate samples are coated
with polymeric films, either rubbed polyimid (PI) films or monooriented
poly(tetrafluoroethylene) (PTFE) films deposited by friction at high
temperature. The orientation of the nematic and the smectic B is planar. In
PI-coated samples, the orientation effect of SmB crystals is mediated by the
nematic, whereas, in PTFE-coated samples, it results from a homoepitaxy
phenomenon occurring for two degenerate orientations. A recrystallization
phenomenon partly destroys the initial distribution of crystal orientations. In
directional solidification of polycrystals in PTFE-coated samples, a particular
dynamics of faceted grain boundary grooves is at the origin of a dynamical
mechanism of grain selection. Surface effects also are responsible for the
nucleation of misoriented terraces on facets and the generation of lattice
defects in the solid.Comment: 15 pages, 24 figures, submitted to PR
Quantitative determination of the solidus line in the dilute limit of succinonitrile–camphor alloys
International audienceDifferent phase diagram measurements for succinonitrile–camphor alloys to date have yielded different values of the solute partition coefficient and the freezing range of the alloy. These parameters are critical to model solidification microstructure evolution. New measurements are made to precisely characterize the dilute limit of the succinonitrile–camphor phase diagram using thin-sample directional solidification experiments where convection is negligible, so that solute transport in the melt is purely diffusive, and the temperature gradient is constant in time. These results are confirmed through complementary measurements by differential scanning calorimetry and isothermal annealing. Possible measurement uncertainties in previously measured solidus lines are discussed. Experimental results were further confirmed using a boundary layer model of transient planar interface dynamics