Modeling Directional Solidification

The objective of the research is to develop tools of use in explaining the results of directional solidification experiments in space. These tools consist of mathematical and experimental models. Experiments are also being flown at low-g to validate the ground-based results. Many metals and semiconductors directionally solidified in space had a diameter less than that of the containing ampoule. In fact this occurred whenever the ampoule contained a gas space. This phenomenon has been attributed in a vague way to lack of wetting of the ampoule wall by the melt. The actual liquid behavior by use of low temperature analogs is being observed aboard the KC-135. In the first flight, water was placed in plastic ampoules (not wet) with ceramic end plugs which were wet by the water to simulate a solid-liquid interface. Unfortunately during storage prior to the flight the water began to wet the ampoule wall. Discrete bubbles formed down the axis of the ampoules. New ampoule materials are being used now, with testing for stability prior to the next flight. A theoretical model has also been developed for this phenomenon.