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MM: Metall- und Materialphysik

MM 25: Poster Session

MM 25.15: Poster

Mittwoch, 29. März 2006, 15:30–17:30, P4

Controlling of structure formation in crystal growth — •Marco Fell and Jörg Bilgram — Laboratorium für Festkörperphysik, ETH, 8093 Zürich

Instabilities of the solid-liquid interface of xenon dendrites growing from pure melt give rise to the formation of side branches at arbitrary locations in the first some tip radii behind the tip. Side branches show a characteristic spacing, depending on undercooling, but the development on opposite sides of the dendrite is uncorrelated which, together with the coarsening effect, leads to a ’statistical symmetry’ of dendritic crystals. We control the branching of a single crystal in our experiments by two kinds of external perturbation: i) A sole short-time heating pulse initiates synchronously side branches in all growth directions. Statistical processes govern their further development as the system relaxes to the steady-state growth some seconds after the pulse. ii) Melting the crystal by stabilizing the temperature of the melt slightly above melting temperature for some minutes leads to a reduction of interface curvature. Then a sharp temperature drop restarts the growth and new side branches are initiated symmetrically in all growth directions. They seem to interact over a macroscopic distance of more than 50 tip radii ( 1 mm) across the dendrite as they perform higher order branchings simultaneously.

A dendrite tip reacts upon the temperature drop by showing a hysteresis behavior in its radius. The branching as the result of unstable interfaces can be controlled by specific heating. The instability of spherical as well as flat solid-liquid interfaces are used to obtain a highly symmetric crystal in a controlled manner.