Dresden 2006 – wissenschaftliches Programm

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SYSS: Structure Formation and Self-Organization in non-equilibrium Systems

SYSS 3: Structure Formation and Self-Organization in non-equilibrium Systems - Poster

SYSS 3.5: Poster

Donnerstag, 30. März 2006, 16:00–18:00, P1

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

In our experiments we study instabilities of both the spherical and the flat solid-liquid interfaces and apply it in a controlled manner to obtain symmetric crystals. Xenon dendrites growing in steady-state from pure melt show a characteristic spacing between the side branches, depending on undercooling. The interface instability of the Mullins-Sekerka type leads to side branches. The growth velocity and the temporal development of a branch can significantly differ from a branch on the opposite side of the dendrite. The interaction between adjacent branches on all sides cancels out a specific number of them but it is not predictable which will be retained. In this sense such a steady state dendrite is ’statistical symmetric’. By an external perturbation we can influence the branching of the growing tip. i) A single short-time heating pulse of the melt initiates synchronously side branches in all growth directions. Statistical processes govern their further development as the system relaxes to the steady-state behavior some seconds after the pulse. ii) Stabilizing the temperature of the melt above the melting temperature for some minutes leads to melting and to a reduced curvature of the solid-liquid interface. A sharp temperature drop restarts growth and initiates new side branches growing symmetrically on four sides with contours remaining in good correlation for the time it takes the crystal to grow some ten tip radii. They even seem to interact over a macroscopic distance of more than 50 tip radii (about 1 mm).