Interface stability and defect formation during crystal growth

摘要

Unidirectional solidification experiments have been carried out in organic crystals with the aim of improving our knowledge on the effects of constraints on the interface morphology and to increase our understanding of the growth of anisotropic materials. The experimental information shows that lateral constraints such as a sharp change in the cross-sectional area in the solid liquid interface path, can produce important changes in the microstructure if the interface morphology is planar, cellular or dendritic;The study of anisotropic materials cover several topics. It is first shown that slight anisotropy does not influence the dendrite tip selection criterion. This conclusion is obtained from the analysis of the relationship between tip radius and velocity for dendrites growing under the steady state condition for two different materials, CBr[subscript]4 and C[subscript]2Cl[subscript]6, which have different surface energy anisotropy values. The values of the dendrite operating parameters [sigma][superscript]* are compared with the predictions of the solvability theory and the morphological stability theory. The experiments show better agreement with the latter theory;Critical experiments have been designed and carried out to find the response functions which determine the composition and temperature of the interface as a function of velocity in faceted materials. The experiments, carried out in Napthalene-Camphor system, indicate a strong temperature dependence of the planar interface growth which can be correlated with the step growth mechanism. Experiments on the interface instability show an important dependence on the crystallographic orientation;Unidirectional solidification experiments in zone refined Napthalene confined in very thin cells (gap size ≤ 50[mu]m) have proven to be a good method to study the defect production at the solid liquid interface. Specifically, the photoelastic properties of Napthalene provides a visual examination of the stress configuration. The photoelastic fringes correlate with the stress produced at the interface during the solidification. A finite element model is proposed that assumes the stress to be due to an inhomogeneous concentration distribution in the solid near the interface;Unidirectional solidification experiments have been carried out with Tert Butyl Alcohol which indicate that the instability is similar to that for nonfaceted materials. A morphological transition which is similar to the cellular-dendritic transition in nonfaceted materials is identified;The analysis of the transient times in unidirectional solidification experiments, carried out in the Succinonitrile-Acetone, system, indicates that the presence of thermomigration and convection can produce important departures from the theoretical models which consider solute diffusion as the only mechanism present.