Faceted growth of a Smectic B crystal in thin sample directional solidification

摘要

A crystal growing from the liquid exhibits a faceted or a nonfaceted shape according to the strength of the anisotropy of the properties of the solid-liquid interface (surface tension and interfacial kinetics). When the anisotropy is small, the interface is rough on the atomic scale and the solidification dynamics is limited by heat or impurity diffusion (diffusion limited growth) which has been extensively studied in the last two decades. On the other hand faceted crystals are observed when the interfacial anisotropy is large. A facet corresponds to a particular crystal lattice plane and is smooth on the atomic scale. The growth kinetics of a facet depends on the actual structure of the crystal on the atomic level. For a perfect crystal the growth of the facet takes place by nucleation of new terraces and propagation of atomic steps. If the nucleation barrier is high the facet can be blocked at relatively large undercoolings (ΔT of a few K). On the other hand, the propagation of the interface of a crystal with imperfections (e.g. dislocations) that act as sources of new atomic steps on the interface becomes effective even for small undercoolings. Such a non local, highly non linear interfacial kinetics, which cannot be easily studied on the macroscopic scale with the traditional methods (optical microscopy, continuum models) used in the case of diffusion limited growth, induces a very " irregular'' large-scale dynamics.