Centrifugal Materials Processing

Summary

This is a brief review of the influence of centrifugation on materials processing. The emphasis in this summary is on papers presented at the Third International Workshop on Materials Processing at High Gravity. This highly successful meeting was held June 2–8, 1996 on the campus of Clarkson University in Potsdam, New York, under the sponsorship of Corning Corporation and the International Center for Gravity Materials Science and Applications. The present volume constitutes the proceedings of this workshop.

The workshop began with our discussion of the influence of centrifugation on transport phenomena. We pointed out that centrifugation not only increases the acceleration, but also introduces the Coriolis force and a spatial dependence for the acceleration vector. Increasing the acceleration to a few g_e greatly increases sedimentation of second-phase particles, but should have little effect on sedimentation of the components in ordinary solutions or on the pressure in a liquid. Much of this material is in the present paper.

The primary stimulus for increased activity in centrifugal materials processing was the 1988 report by Liya Regel and Huguette Rodot of uniform Ag doping in PbTe grown by a gradient freeze technique. This observation indicated diffusion-controlled segregation at a particular acceleration -- an unexpected and surprising result. At the Second International Workshop in 1993, numerical modeling indicated that a sharp minimum in convection should occur at the rotation rate where the acceleration vector is nearly perpendicular to the solid-liquid interface. At the present Workshop, Friedrich and Müller confirmed the Regel-Rodot results, both experimentally and theoretically. They obtained a maximum in the effective distribution coefficient for Ga in Ge at a particular rotation rate. They used a scaling analysis, in which they equated the axial and radial buoyancy terms to their corresponding Coriolis terms, in order to predict a minimum in convection versus rotation rate for a concave interface shape. Their three-dimensional numerical model predicted that this convection should be steady, with circumferential rotational features. These features were, in turn, confirmed by the flow visualization experiments of Skudarnov, Regel and Wilcox.

At the 1996 Workshop, Arnold and Regel showed numerical results for rotation of a cylinder about its own axis. They predicted a sharp minimum in buoyancy-driven convection for for example, analysis of impurity concentration to the ends of the crystals and over entire cross sections, microstructure, dislocation content, inclusions, and electrical properties.

Similarly, additional research should be performed on solution crystal growth, vapor transport, chemical vapor deposition, polymerization, and welding. Other opportunities exist in Bridgman-Stockbarger solidification, electrodeposition, fabrication and joining of composite materials, and fine particle processing.

Flow visualization and temperature measurements should be performed and compared with theoretical predictions.