Velocity and Morphology Transition in Dendritic Growth
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VELOCITY AND MORPHOLOGY TRANSITION IN DENDRITIC GROWTH E. RAZ and A. CHAIT NASA Lewis Research Center, Cleveland, OH. 44135 ABSTRACT The growth velocity and morphology of crystals growing in a supercooled aqueous solution of ammonium chloride (28 wt%/o) with small amounts of copper sulfate (0.05 to-1 wt%) have been examined. For the [100] growth direction at low supercooling and for the [110) direction at greater supercooling the crystals start to grow slowly with a faceted interface. As the supercooling is continuously increased, the growth direction becomes [111]. Dendrites with pointed tips develop but continue to grow slowly. With further increase insupercooling, the velocity increases 20- to 40fold and the tip shape changes from a cusp to a thin and almost paraboloid interface while the growth direction remains [111]. The velocity transition is gradual when the concentration is below 0.1 wt 0/%but is sharper at hig her concentrations. The transition temperature decreases with increasing copper sulfate concentration. We postulate that the observed transition indicates a dynamic roughening phenomenon, where the supercooling is sufficiently high to create many nucleation islands on the faceted interface. In this case the interface behaves as an atomically rough surface and dendrite growth occurs at an interface temperature that is below the equilibrium temperature. One of the open questions in dendritic growth is the interplay between the microscopic interfacial dynamics (e.g. surface tension, anisotropy and attachment of atoms to the interface) and the macroscopic dynamics (solutal and thermal fields) and how they mutually govern the morphology and kinetics of crystal growth [1]. Morphology transition in dendritic growth [1, 2] is one aspect of this mutuality. It is known that adding a small amount of impurity to a melt or to a solution can significantly affect the crystallization process. From a macroscopic viewpoint, impurity segregation is intricately coupled with the interface morphology via the governing equations and may greatly affect the growth morphology. Theoretical works [3,4] concerning impurity as an additional component of the diffusion equations predict an increase in growth velocity for small amounts of impurity and then a decrease in velocity ifthe concentration of the impurity exceeds a certain value. They also predict a decrease of the radius of the advancing tip. These predictions correspond to experimental works done with the binary system of succinonitrile and acetone. In addition, impurity atoms on the interface and their inclusion in the solid lattice can significantly change the interfacial energy and dramatically affect the morphology and the growth kinetics. This case is too complicated to handle theoretically, nevertheless, studying this effect can open windows for better understanding of the growth process. In this work we report the effect small amounts of copper sulfate have on the morphology and growth kinetics of ammonium chloride dendrites growing from a supersaturated aqueous s
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