CRYSTAL-GROWTH FROM LIQUID COLUMNS

The microgravity environment within the Space Station Mir, within the Spacelab, and within the forthcoming Space Station Freedom offers the possibility of growing large crystals from liquid zones with free fluid surfaces, which are supported by surface tension only. The idea is to reduce convection in the fluid phase and, at the same time, to exclude heterogeneous nucleation at the container wall. On the other hand, the free fluid surface poses several problems. There are stability criteria for liquid columns which depend on height and volume, Bond number, frequency of rotation, etc. There exist resonance frequencies, which have to be avoided, and there is Marangoni convection caused by the fact that the temperature assumes a maximum between the melting front and the solidafication front. Respective stability diagrams are presented; the resonance frequencies and their impact on the tolerable residual vibrations are discussed. Problems of Marangoni convection are addressed. It may aid material transport, if it stays laminar, but may become fatal if it becomes oscillatory or even turbulent. Results of relevant microgravity experiments are presented. © 1990.