Containerless Processing Technology

Summary

The virtual absence of hydrostatic pressure in a microgravity environment allows liquids to be confined solely by their surface tension and offers the possibility of melting and solidifying materials without physical contact. This “containerless processing” eliminates contaminants that may be introduced by the container or crucible and may be used to obtain ultrapure materials. This is especially important for systems that are highly corrosive in the melt. Elimination of container-induced nucleation also permits melts to be deeply undercooled before solidification. This offers the possibility of studying homogeneous nucleation and solidification in undercooled melts. Such solidification can be extremely rapid and may produce unique microstructures including metastable or amorphous phases in metals. The range of glass formation may be extended in borderline glass forming systems, which may result in new glasses with unusual properties. Since such glasses are formed from melts confined only by surface tension, they will have perfectly spherical pristine surfaces, undamaged by grinding or other shaping processes. It may also be possible to manipulate the melt to form aspherical shapes, or to draw optical fibers from a containerless melt.

The dynamics of drops and bubbles may also be studied using containerless techniques. Detailed photographs of droplets merging were obtained in a simple demonstration on Skylab. The drop dynamics module on Spacelab 3 is dedicated to a more elaborate set of experiments to study the vibrational and rotational behavior of liquid droplets. The centering forces on a bubble suspended in a droplet have also been investigated in a variety of low-gravity experiments to the point where the role of droplet oscillations are understood well enough to permit the routine formation of thin-walled spherical shells up to a few millimeters in diameter that have a high degree of concentricity. Such shells have been used for fuel containment in inertially confined fusion experiments [1] and are also finding a variety of other uses. Finally, the ability to suspend a high temperature melt without physical contact is useful for measurement of high temperature thermophysical properties. Techniques hate been developed for independent measurement of emissivity as a function lof temperature which permits the determination of heat capacity, enthalpy, heat of fusion, etc. for metals such as W, Mo, Nb, and other refractory materials. [2] Surface tension and viscosity may also be determined without physical contact in low to moderate viscosity samples by inducing oscillations in the melt and observing their period and decay, or in high viscosity material by rotating of the sample and measuring the distortion that results. [3]

There are a variety of techniques for containerless processing that can be used to some degree in Earth’s gravity. These techniques consist of suspending or levitating a specimen using non-contacting forces such as gas pressure from an acoustic field or air stream, or electric forces from an electrostatic or electromagnetic field. Such techniques are limited to small samples in Earth’s gravity because the hydrostatic pressure must be confined by surface tension, and distortions from sphericity are unavoidable. Also, the heating and cooling of the sample is generally not independent of the levitation process. Free-fall facilities such as Marshall Space Flight Center’s (MSFC) drop tube and Jet Propulsion Laboratory’s (JPL) aerodynamic drop facility avoid this difficulty, but the limited free-fall distance restricts the sample to a size that can be solidified in the time available. Nevertheless, these techniques have produced much valuable information on containerless processing technology and serve as an indispensible adjunct to microgravity experiments for testing new concepts, screening prospective sample materials, and developing experimental protocol.

The use of the microgravity environment of an orbiting vehicle extends the free-fall time more or less indefinitely and allows much larger samples to be processed as well as close observation and manipulation of the sample during processing. However, the small residual accelerations in an orbiting vehicle require the use of some form of non-contacting positioning force, similar to that used to levitate samples in Earth’s gravity, to hold the sample in the confines of its processing chamber for extended periods of time.

The techniques and facilities for containerless processing, both in space and on Earth, will be described in the remainder of this chapter.