Investigation of Faceted Solid-Liquid Interface Behavior During the Directional Solidification of Salol

During solidification of materials, a microstructure forms at the solid-liquid interface that depends on both the physical parameters of the material (in particular the surface energy and kinetic attachment anisotropies) and the processing parameters such as thermal gradient G, growth velocity v, and composition of the sample. The microstructure plays a critical role in determining the final properties of the material. One can distinguish two different macroscopic types of interface: rough where the kinetic attachment of atoms is fast and heat and solute diffusion in the liquid is dominant; and faceted where the kinetic attachment of atoms is anisotropic and slow and considered as the dominant effect. In this last, the crystals grow with smooth surfaces at the atomic scale, and facets at macroscopic scale. Despite numerous researches, particularly for semi-conductors such as silicon, the understanding of the faceted growth is still a significant challenge.<br/>In this work, directional solidification experiments of organic transparent materials are used to evidence and analyze mechanisms involved during the growth of faceted solid-liquid interfaces. Such transparent model materials are chosen to have similar solid-liquid interface properties as technological materials studied, and thus to form similar microstructures, with the advantages of being transparent to visible light, so that common optical techniques can be used to follow their growth. We use salol, which is orthorhombic in structure and grows from its melt with a distinct macroscopic faceted interface because of its very high degree of anisotropy. Directional solidifications are performed in the ECODIS device a Bridgman furnace designed for thin samples geometries, equipped with an optical microscope to observe solid-liquid interface dynamics <i>in situ</i> and real time.<br/>A highly faceted solid-liquid interface appears and keeps evolving till the end of the experiment. The facet angles are characterized and it shows that they correspond mostly to {1 1 1} and {1 0 0} crystallographic planes. Velocities of the different facets are measured and they vary with facet orientation relatively to the thermal gradient. These measurements allow to establish kinetic growth laws which depend on the crystallographic nature of the facet. Facets are also studied in terms of size evolution and distribution, which are affected by facet competition and interaction with defects.