Velocity and Morphology Transition in Dendritic Growth

ABSTRACTThe growth velocity and morphology of crystals growing in a supercooled aqueous solution of ammonium chloride (28 wt%) 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 in supercooling, the velocity increases 20- to 40-fold 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% but is sharper at higher 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.