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Space Processing is a fledgling and somewhat controversial subject. The scientific community as well as NASA have not yet quite made up their minds as to what to do with Space Processing. The scientific society views Space Processing with some suspicion, and rightfully so. They're not sure whether this is something real or just a Class “A” con job. NASA has made a lot of promises to this program and has delivered very little. Space Processing has a good chance of becoming a self-fulfilled prophecy. Therefore, responsibility is laid on the shoulders of those of us who believe in this program to convince the scientific community that there is something real there and lobby NASA that they should either “put up or shut up.” This is one of the reasons why I took up the challenge to fly in the Space Shuttle and to live and train with these “Space jocks” for a total of about three years.
Chemical Vapor Deposition (CVD) processes are generally carried out under large temperature gradients. These gradients, temperature dependent fluid properties, and the earth's gravitational field give rise to buoyancy-driven free convective fluid flow which augments heat and mass transport in the CVD reactor. Under certain conditions, this free convective flow may alter the gas phase chemistry associated with the deposition process. In order to understand these free convective effects and their implications for the deposition process, a computational model describing the combined effects of fluid mechanics and chemistry has been developed. This model uses a coupled chemical equilibrium/mass transport code in conjunction with a 2-D elliptic fluid dynamics code to describe gas phase species profiles and deposition rates. This paper briefly describes the development of the model, its use, and the results of typical calculations.
Containerless drop tube processing allows for significant levels of liquid undercooling through control of parameters such as sample size, surface coating and cooling rate. A laboratory scale (3m) drop tube has been developed which allows the under-cooling and solidification behavior of powder samples to be eval-uated under low gravity free fall conditions. The level of undercooling obtained in an InSb-Sb eutectic alloy has been eval-uated by comparing the eutectic spacing in drop tube samples with a spacing/undercooling relationship established using thermal analysis techniques. Undercoolings of 0.17 and 0.23 Te were produced by processing under vacuum and He gas conditions respec-tively. Alternatively, the formation of an amorphous phase in a Ni-Nb eutectic alloy indicates that undercooling levels of approximately 500°C were obtained by drop tube processing. The influence of droplet size and gas environment on undercooling behavior in the Ni-Nb eutectic was evaluated through their effect on the amorphous/crystalline phase ratio. To supplement the structural analysis, heat flow modeling has been developed to describe the undercooling history during drop tube processing and the model has been tested experimentally.
Rockwell International has long endeavored to stimulate industrial utilization of space for materials processing. A successful introductory briefing program to acquaint nonaerospace industry with the space environment, microgravity process phenomena, experiment hardware, and the programs available to conduct research in space has encouraged several companies to initiate space processing research projects.
To help satisfy industry's microgravity experiment hardware requirements, Rockwell has developed a multipurpose materials processing laboratory for use on the Space Shuttle. The Fluids Experiment Apparatus (FEA) has been flown to perform floating zone crystal growth and purification research and is currently being used to support further crystal growth research with advanced materials for Rockwell. Other companies are preparing experiments that are expected to be conducted in the FEA on future Space Shuttle missions.
Rockwell is developing, with NASA, a program that will allow industry to plan and fly microgravity materials processing experiments within a few months–much faster than the current one to two year lead time. This low-cost program, patterned after the NASA Joint Endeavor Program, provides Space Shuttle flight services and use ot the FEA to conduct scientific investigations. Rockwell plans to offer experiment integration and support services to industry as needed.
The growth of dendrites in pure melts and alloys is controlled by diffusion-limited transport of heat and/or solute. The presence of temperature or concentration gradients within a molten phase subject to gravitational forces generally promotes convection, which in turn, modifies the diffusion processes. The vigor of melt convection is controlled by several parameters often expressed as a lumped dimensionless group, the Grashof number Gr = gβΔTℓ3/ν2, where g is the acceleration due to gravity; is the volumetric expansion coefficient; ΔT is the undercooling; ν is the kinematic viscosity; and ℓ is the relevant length scale, e.g., the characteristic diffusion distance. Dendritic growth, by its nature, does not permit independent manipulation of the controlling length scale, ℓ, which is determined by materials properties (e.g. diffusion coefficient or thermal diffusivity) and the undercooling or supersaturation. The reduction of g through orbital free fall is often the only practical way to lower Gr sufficiently to permit careful observation of the morphological and kinetic characteristics of isothermal dendritic growth. Previously conducted ground-based studies and the current approach to performing these studies in low earth orbit will be described.
The Space Shuttle Columbia carried an Alloy Undercooling Experiment on its STS 61-C mission in January, 1986. The experiment was performed in the electromagnetic levitator (EML) designed and produced by the General Electric Company. A sample of Ni-32.5wt% Sn eutectic was melted and solidified under microgravity conditions in the Space Shuttle. The specimen achieved only a fairly small undercooling, probably less than 30 K.
The specimen was examined by optical and scanning electron microscopy. The surface and cross-sectional microstructures were primarily composed of normal lamellar eutectic, but showed several interesting features, including an apparent surface nucleation site, curved dendrites with non-orthogonal secondary arms, dendrite fragments with extremely fine arm spacing, submicron precipitates, and faceted crystals. The results of the space experiment are presented and compared with ground-based results obtained with the same alloy.
Ground-based and short-duration low gravity experiments have been carried out with the use of ultrasonic levitators to study the dynamics of freely suspended liquid drops under the influence of predominantly capillary and acoustic radiation forces. Some of the effects of the levitating field on the shape as well as the fluid flow fields within the drop have been determined. The development and refinement of measurement techniques using levitated drops with size on the order of 2mm in diameter have yielded methods having direct application to experiments in microgravity. In addition, containerless melting, undercooling, and freezing of organic materials as well as low melting metals have provided experimental data and observation on the application of acoustic positioning techniques to materials studies.
We have carried out extensive experimentation on the physical vapor transport growth of mercurous chloride, which is an important material for opto-electronic devices. Because of the extraordinary combination of properties found in Hg2Cl2, including transmittance from 0.36 to 20 μm, anomalously slow soung velocity, high birefringence, and large acousto-optic diffraction efficiency, we are exploring the conditions for growth of large crystals of device quality. Our experiments have led so far to measured Hg2C12 growth rates that were orders of magnitude smaller than those expected by theories based on laminar, unidirectional flow between source and sink. Slight disturbances of the solid-vapor interface give rise to instabilities which lead to interfacial convection. This kind of convection should enhance the growth rate due to the acceleration of the materials exchange by hydrodynamic coupling, which is contrary to our findings. Interfacial convection is often connected with buoyancy-induced instabilities which dominate other instabilities under 1-g conditions. Despite attempts to reduce the Rayleigh numbers in our system, buoyancy-induced instabilities might be the cause of discrepancy between the measured and calculated growth rates. Observations of these phenomena under a microgravity environment might permit a better assessment of the transport mechanisms in physical vapor phase crystal growth of Hg2C12.
Immiscible GaBi alloys were solidified during free fall in the Marshall Space Flight Cente4 drop tower which provides about 4.5 seconds of low gravity (about 10−4 g, g = 980 cm/s2 ). We have measured the electrical resistivity and magnetic susceptibility as a function of pressure (up to 18 kbar) and temperature (300K to 4.2K) of drop tower (DT) sample and ground control (GC) sample prepared under identical conditions except for gravity. At ambient pressure the electrical resistance of the DT sample exhibits a broad maximum at 100K, while that of GC sample decreases rapidly as temperature decreases. Both DT and GC samples becomesuperconducting at 7.7K (Tc2). However, a minor second superconducting phase with a transition temperature at 8.3K (Tc1) is observed only in the DT samples.
The aim of these experiments in the Gradient Furnace with Quenching device (GFQ) during the German D-1 Mission was to study the influence of thermal convection on
– the diffusion in the melt ahead of the solidification front in an AlCu-alloy
– the stability of a smooth solidification front in an Al-Cualloy and
– the morphology of the solidification front in an AlSi-alloy
during the directional solidification of the binary alloys AlCu with 0.3 wght.-% Cu and AlSi with 7.0 wght.-% Si. Altogether five samples had been successfully processed during the D1-Mission. After getting the complete values from the process data (above all the velocity of the solidification front and the temperature gradient) reference experiments were made in the same GFQ under lg-conditions.
Additional experiments under lg with a transparent fluid were done to obtain information about the thermal convection in a cylindrical cell while measuring temperature distribution at the same time.
A first result having the same crystallization condition is the more than twice thicker diffusion boundary layer in AlCu 0,3-specimen than in the lg-reference specimen. The diffusion coefficient and the dendritic morphology are to be determined. The evaluation of all specimens (flight- and lg-samples) is not finished in the moment.
An Electrodynamic Thermogravimetric Analysis (EDTGA) Instrument has been developed for the study of kinetics of gas-solid reactions and phase transformations of levitated particles at high temperature. The levitated particles may be heated with a focused laser beam and their temperature and weight monitored during the process. The particles that can be studied are, approximately, 10 to 150 microns in diameter and it is aimed to produce temperature pulses comparable to those encountered in plasma torches and reactors. The instrumentation implemented in the system permits the measurement and or control of relative temperature, weight change, and energy flux to the particle with a time resolution of less than one millisecond. Selected results obtained from pulse heating aluminum oxide and silicon dioxide are presented.
Charged drop levitation is described for two different kinds of levitators. It is demonstrated that the feedback-controlled electrostatic levitator is capable of levitating a several milimeter size large drop in 1 g, and it can be used in various experiments such as crystal growth, supercooling and solidification, and drop dynamics. Charged droplet levitation in a vertical, linear quadrupole levitator is described, and its advantages are demonstrated taking the charged drop instability experiment as an example. The cause of the observed premature burstings are speculated to be not by the Rayleigh limit but, rather, by an electron avalanch in the surrounding gaseous medium. No evidence was found that the burtings accompaied by mass loss, in direct contrast to the most of the previous reports by Doyle et.al.and Abbas and Latham.
Finite reservoir effects on capillary spreading at small reservoir dimensions are explored numerically and related to wave propagation in combustion synthesis of TiC from elemental Ti and C. In addition a convective instability which may be activated by gravitational, surface tension or contact forces may cause void regions to coalesce, thereby altering the void distribution in combustion synthesis products.
The Consortium for Materials Development in Space amalgamates private industries, the federal government, and universities for a common goal, commercial developments in space. The Consortium embraces research and development projects that benefit from unique attributes of space and that also rely on innovative applications of physical chemistry, material transport and their interactions. Three projects employ vapor transport of material from a solid source at one end of a sealed container to a growing crystal at the other end. The fourth and fifth projects have desired surface properties of materials as their objectives. In one of these, surfaces are electrodeposited, often with inert solid particles codeposited in the surface layer. The other project investigating surface properties makes use of the atomic oxygen environment outside a spacecraft. A surface exposed toward the direction of orbital motion is impacted by a beam predominantly of 5 electron volt oxygen atoms. This can produce unusual chemical reactions and surface morphology. The sixth project uses demixing of immiscible polymers in low-g to better understand the role of phase segregation in the properties of polymer blends and to accomplish the purification of materials by partitioning between two liquid phases.
We have studied the spatial profile of the thermal transients that occur during and following the current pulsing associated with Peltier Interface Demarcation during directional solidification. Results for pure Bi are presented in detail and compared with corresponding results for the Bi/MnBi eutectic. Significant thermal transients occur throughout the sample that can be accounted for by the Peltier effect, the Thomson effect, and Joule heating. We have separated these effects and studied their behavior as a function of time, current density, and position with respect to the solid/liquid interface.
An experiment in floating-zone processing of indium was performed on Space Transportation System (STS) Flight 41-D on September 4, 1984. An indium crystal 1 cm in diameter and 7 cm long was grown. The polycrystal-line indium used as the feedstock had been doped with approximately 100 ppm thallium to investigate the distribution coefficient of thallium in indium; the result was K =CS/CL< 0.03. Indium was chosen for study because its low melting point and the low vapor pressure of the melt at that temperature would simplify the apparatus and reduce the electrical energy and cooling requirements. Another advantage is that the properties of liquid indium probably preclude time-dependent thermocapillary flows. The experiment also served as the first flight test of the Fluids Experiment Apparatus (FEA), a new multiple-purpose apparatus we have developed for low-cost, entry-level materials processing, physics and chemistry experiments in Earth-orbit. The experiment confirmed the flight-worthiness of the FEA and the suitability of indium for exploratory floating-zone experiments in Earth-orbit.
The 100 meter Drop Tube at NASA Marshall Space Flight Center provides an excellent opportunity to study the effects of containerless, microgravity processing in metals and alloys. In a series of experiments high melting temperature pure metals were melted in an electron beam furnace and dropped in vacuum. Sample sizes ranged from 0.175-1.2 grams. Large undercoolings on the order of 18% of the melting temperature were observed in Ti, Zr, Nb, Mo, Rh, Ta, and Pt. Undercoolings of 5-18% Tm were observed in Ru and Ir. These undercooling results are consistent, repeatable, and occur in a high percentage of experiments. The experimental technique will be presented as well as the resultant microstructures of undercooled drops. The data will be discussed with respect to nucleation theory.
The free fall of a liquid-metal drop and heat transfer from the drop to its environment are described for both a gaseous atmosphere and vacuum. A simple model, in which the drop is assumed to fall rectilinearly with behavior like that of a rigid particle, is developed first, then possible causes of deviation from this behavior are discussed. The model is applied to describe solidification of drops in a drop tube. Possible future develop-ments of the model are suggested.
Product morphology in metal casting and semiconductor crystal growth is strongly influenced by the nature of the solid-liquid interface during solidification. Since in situ observation of solification in these technologically important systems is not possible, investigators have resorted to the use of physical models, such as water and organic liquids, which while they are transparent to visible light, suffer from the fact that their Prandtl numbers are too high. Molten alkali halides are better physical models in this respect, and are transparent to visible light. The purpose of the present study is to examine solidification in the LiCl-KCl system to determine if phenomena such as solute rejection can be observed by laser schlieren imaging.
Materials processing in a microgravity environment is aimed at developing commercial materials as well as investigating basic phenomena to improve earth-based processing. Materials research in space has dealt with glasses and ceramics, crystal growth, electronic materials, metals and alloys, polymers, composites, and biological materials. Battelle has been conducting research in this area since the early-1970s. Several important results have been obtained in: immiscible alloys, containerless under-cooling of clustering alloys, sol-gel glasses, and collagen fibers.
More recently, Battelle's Advanced Materials Center for the Commercial Development of Space (CCDS) has been established to utilize the microgravity environment in the commercial development of composite and mixed-phase materials with substantially improved properties. Currently, the Center is conducting research in catalysts (variant-phase chlorides, zeolites, and mixed oxides), polymer systems, electronic materials (float-zone crystal growth on Type II-VI semiconductor crystals, particularly CdTe), and con-trolled- porosity glass. The present program focuses on a proof of principle for each research thrust, utilizing ground-based and suborbital facilities, together with modeling to demonstrate the potential for producing commercially important materials.
Each of these research programs is outlined. In addition, the more important developments in each of the major categories of microgravity materials research is reviewed.