Dendrite engineering on xenon crystals.
The experimental work presented focuses on transient growth, morphological transitions, and control of xenon dendrites. Dendritic free growth is perturbed by two different mechanisms: Shaking and heating up to the melting temperature. Spontaneous and metastable multitip configurations are stabilized, coarsening is reduced, leading to a denser sidebranch growth, and a periodic tip splitting is found during perturbation by shaking. On the other hand, heating leads to controlled sidebranching and characteristic transitions of the tip shape. A deterministic behavior is found besides the random-noise-driven growth. The existence of a limit cycle is supported by the findings. Together the two perturbation mechanisms allow a "dendrite engineering"--i.e., a reproducible controlling of the crystal shape during its growth. The tip splitting for dendritic free growth is found not to be a splitting of the tip in two; rather, the respective growth velocities of the main tip and the fins change. The latter then surpass the main tip and develop into new tips. The occurrence of three- and four-tip configurations is explained with this mechanism. Finite-element calculations of the heat flow and the convective flow in the growth vessel show that the idea of a single axisymmetric toroidal convection roll across the whole growth vessel has to be dropped. The main effect of convection under Earth's gravity is the compression of the diffusive temperature field around the downward-growing tip. A model to explain the symmetry of dendritic crystals--e.g., snow crystals--is developed, based on the interaction of crystal shape and heat flow in the crystal.
Laboratorium für Festkörperphysik, ETH Zürich, 8093 Zürich, Switzerland. mfell@phys.ethz.ch
Marco
M
Fell
Jörg
J
Bilgram