The preparation of solid materials involves the control and manipulation of fluids in ways that are sensitive to gravitational influences. Although these effects are pervasive, surprisingly little is understood about phenomena such as natural convection and containerless processes under boundary conditions of interest to materials processing. Recent emphasis has focused on process fundamentals involving areas such as fluid dynamics, heat flow, and thermophysical property measurement as can be seen in this Symposium. Such a knowledge base is essential to any sensible evolution of the space environment as a capability for studying materials processing and preparing limited quantities of materials under quiescent or containerless conditions for subsequent assessment on earth. A brief overview of current work will be presented, together with possible future directions.

A few months ago, european activities in the field of microgravity research appeared as being an addition of single programmatic elements without enough logics, coordination and scientific background. Under the active pressure and guidance of many european scientists who became more and more aware of the basic interest of materials research in the microgravity environment, and due to the efforts of the ESA executive, the situation could evolve in a significant way, succeeding now in getting the official recognition of the necessity and content of a coherent european microgravity programme by both the ESA Council and the community of classical Space scientists. This presentation will deal with the scientific motivations of this new-born Programme through both the initial status and the most rational ambitions for future. The poirts of emphasis of the necessary multidisciplinary cooperative exertion together with the possible involvement of these scientists who could not derive benefit from any of the existing national programmes will then be stressed. At last the resulting priority objectives will be briefly reviewed before discussing the needed coordination with other european and non-european microgravity activities.

In the near future, a large segment of the scientific community will have an opportunity to perform materials processing experiments on the Space Shuttle in the reduced gravity environment of space. Many of these experiments will require containerless processing techniques that provide manipulation and control of weightless (molten) materials without physical contact with container walls or other holding devices. A variety of containerless processing technologies are now being developed for space and ground-based materials processing facilities. The utilization of air jets or high intensity acoustic, electromagnetic or electrostatic fields can produce forces that support and manipulate materials. Most of the present containerless research is directed toward the development of high temperature systems capable of melting and resolidifying materials. This paper will review the materials processing capabilities and level of developmental progress of each technique. An introduction to available NASA test facilities will also be given.

Ultraviolet laser induced atomic fluorescence has been used to characterize supersonic jet aerodynamic levitation experiments. The levitated specimen was a 0.4 cm sapphire sphere that was separately heated at temperatures up to 2327K by an infrared laser. The supersonic jet expansion and thermal gradients in the specimen wake were studied by measuring spatial variations in the concentration of atomic Hg added to the levitating argon gas stream. Further applications of atomic fluorescence in containerless experiments, such as ideal gas fluorescence thermometry and containerless process control are discussed.

A technique is described for the dynamic measurements of selected thermophysical properties of electrically conducting substances in the range 1500 K to the melting temperature of the specimen. The technique is based on rapid resistive self-heating of the specimen from room temperature to any desired high temperature in less than one second by the passage of an electrical current pulse through it and on measuring the pertinent quantities, such as current, voltage and temperature, with millisecond resolution. The technique was applied to the measurements of heat capacity, electrical resistivity, hemispherical total emittance, normal spectral emittance, thermal expansion, melting temperature, heat of fusion, and temperature and energy of solid-solid phase transformations. Upper temperature of the measurements has been limited by the melting temperature of the specimen, at which point, the specimen collapses due to the gravitational force. In order to be able to extend the measurements to the liquid phase, performance of the dynamic experiments in a near-zero gravity environment is suggested.

Several processes in various fields require uniformly thick coatings and layers on small particles. The particles may be used as carriers of catalytic materials (platinum or other coatings), as laser fusion targets (various polymer or metallic coatings), or for biological or other tracer or interactive processes. We have devisea both molecular beam and electro-dynamic techniques for levitation of the particles during coating and electrodynamic methods of controlling and transporting the particles between coating steps and to final use locations. Both molecular beam and electrodynamic techniques are describea and several advantages and limitations of each will be discussed. A short movie of an operating electrodynamic levitation and transport apparatus will be shown.

Inertial confinement fusion (ICF) glass microballoons (GMBs) levitated in a focusing radiator acoustic device can be coated with liquid materials by deploying the liquid into the levitation field with a stepped-horn atomizer. The GMB can be forced to the center of the coating liquid with a strong acoustically generated centering force. Water solutions of organic polymers, uv-curable liquid organic monomers, and paraffin waxes have been used to prepare solid coatings on the surface of GMBs using this technique.

The basic theory of electromagnetic levitation of conducting specimens by alternating fields is reviewed. Emphasis is placed upon simple approximation formulae useful in planning and interpreting laboratory measurements. Recent work involving computer solutions for forces and heating by complex field coil configurations, including those with polyphase excitation, is described. Applications and limitations of the technique both terrestrially and in a microgravity environment are discussed.

We report here a study of the crystal phase nucleation in undercooled droplets of Pd-Si alloys with composition near the Pd84.5Si15.5 eutectic composition. Molten droplets are released at the top of a drop tube and solidify (to either a crystalline or glassy state) during descent. This provides a containerless (and nearly gravity free) environment so that nucleation due to container walls or vibrations is eliminated. We found that crystallization, due to homogeneous nucleation, is bypassed in droplets of 1 mm diameter when cooled at 760 K/sec. From this an upper limit of the homogeneous nucleation rate is estimated. Results are compared with our previously published study of nucleation in 0.06mm to 0.33 mm diameter droplets, which indicated that nucleation results from heterogeneous surface nucleation and that the number of these nuclei is dependent on the atmosphere in the drop tube.

A 32 meter evacuated drop tube has been used to investigate the solidification of Nb-Ge alloys after deep undercooling. Samples have been supercooled as much as 500°K below the liquidus by using free-fall conditions to eliminate crucible induced nucleation. Final microstructures are dependent on the quenching rates at the bottom of the drop tube with a striking extension of the β phase solubility limit at the higher quenching rates.

During the past few years we have been studying several of the physical processes relevant to the production of spherical shells for inertial confinement fusion targets, both in a microgravity environment and in a containerless environment. The work has led to the development of several experimental facilities. Those which are most unique are described here, and fall into three categories as follows: 1. Ones which provide an induced low- or microgravity containerless environment, such as a vertical drag-free wind tunnel, two differing low-pressure and/or high-temperature drop towers for processing metallic or metallic-glass specimens, and a neutral buoyancy tank, 2. Ones providing containerless processing capability, such as a focusing radiator and an electrostatic levitator and 3. Ones providing extended microgravity and containerless capabilities, such as the KC-135 aircraft and the Space Processing Application Rockets. The physical processes which we have been studying include, but are not limited to, those which establish the shell sphericity, concentricity, surface topology, material properties, coatings, heating and cooling requirements and the effects of gravity on fusion pellet fabrication processes.

High quality metallic and metallic-glass microballoons (MMB and MGMB) are of considerable interest for fusion target applications on account of the intrinsic properties of these materials such as high density, high strength and high atomic number. We report the first successfully formed submillimeter and millimeter spherical shells of tin and of a gold-lead-antimony alloy by means of the hollow-jet instability technique developed by one of us (JMK). Examination of tin specimens by means of SEM has revealed that surface quality varied from poor to excellent. Whereas this metal has been employed only as a convenient and inexpensive material, the gold alloy is important because it is hard, has high atomic number, and may be solidified into the amorphous state through the provision of a modest cooling rate. We have produced AuPbSb spherules up to 1.5 mm in diameter using LN2 or chilled methanol as a coolant, and have found that these amorphous samples possess a superb surface smoothness compatible with fusion target requirements. Hollow spheres currently made of this alloy have an average O.D. of 2000 μm.

We report preliminary results of the design and operation of electrostatic levitators we have developed at JPL. Discussion is focussed on the single-axis levitators for groundbased operation and for operation in the reduced gravity environment. Design principles and the performance of final products are described.

A collimated hole structure air jet levitation system has been developed which can be used to levitate hollow glass microspheres used in inertial confinement fusion studies. An ellipsoidal furnace has been added to the system to provide a heating source. A video camera and a 16 mm movie camera connected to a microsphere system provide real time observation as well as permanent documentation of the experiments. Microspheres have been levitated at temperatures over 1400°C for over 10 minutes at a time.

A constricted-tube gas flow levitator first developed by Berge, Oran, and Theiss shows promise both as a spacepositioning device and as a levitator for ground-based work. We present results of laboratory studies which were designed to predict the behavior of the device in a low-g environment.

The techniques of acoustic positioning without the use of resonant cavities have been explored and developed over a number of years and the currently available capabilities are reviewed. The performance and characteristics of these nonresonant acoustic systems are described in regard to containerless processing with emphasis on the low-g environment. This includes manipulation and mixing of liquid drops, super-cooling phenomena, heating to temperatures of 1600°C or higher, rapid cooling solidification, and surface shape control. Some possible applications are measurements of physical properties of substances at high temperatures or of highly reactive specimens, and the formation of unique glasses and alloys of new compositions.

Fluid states are central to melt and vapor processing. Fluids flow. As they do they convect heat, solutes, and inclusions. Liquid/gas interfaces have tension. Their importance rises when scale is small or effective gravity low.

The forces that move fluids and mold fluid interfaces are recounted here. Fundamentals of viscous flow, convective transport, and capillary phenomena are summarized with special attention to the roles of gravity in liquid processing. Research needs are highlighted.

Velocity and temperature measurements are taken of heat induced surface-tension driven flows in both silicone oil and Fluorinert FC-43. Each fluid is contained in an open rectangular box and heated from above by a single strand of electrically heated nichrome wire suspended slightly above the fluid's surface. Velocity measurements are obtained for both liquids with a laser anemometer system. For silicone oil the general flow pattern is recorded using time-lapse photography. The surface temperature measurements are taken by a radiation thermometer and the bulk temperature distributions are measured by thermocouples. The velocity and temperature measurements are compared with numerical solutions obtained for the present configuration.

Thermocapillarity can drive steady flows in float-zone configurations. These flows can become unstable and exhibit time-periodic oscillations. In order to obtain insight into such phenomena, studies are made of a simplified system consisting of liquid in a two-dimensional slot. Steady flows and their instabilities are examined.