Microgravity Research Opportunities for the 1990s (1995) / Chapter Skim
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Microgravity Research Opportunities for the 1990s: Chapter 6
Microgravity Research Opportunities for the 1990s: Chapter 6
Pages 83-119
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From page 83... ...
CHAPTER 8 APPENDIX A Metallurgy has evolved over the millennia from a purely empirical art to a APPENDIX B modern branch of materials science, with considerable progress being made in understanding some of the simpler metallurgical systems and processes from first principles. A key to this progress has been the improved ability to control the solidification process, an important scientific feat that has come about through an improved understanding of the interrelationships between heat and mass file:///C|/SSB_old_web/mgoppch6.htm (1 of 38)
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From page 84... ...
The microgravity environment provided by an orbiting spacecraft offers new opportunities in control of the solidification process. Reduction of convective velocities permits, in some cases, more precise control of the temperature and composition of the melt.
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From page 85... ...
and at increased gravity using a centrifuge. This research is an example of how microgravity experiments may be used to elucidate the essential features of a solidification process and to suggest better control strategies for use on Earth-in this case, by enhancing convection or mechanically stirring the melt to distribute the crystal nuclei.
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From page 86... ...
Initial microgravity experiments on metals and alloys should focus on identifying factors that limit the degree of undercooling, such as the amount of superheat required to dissolve or destroy potential nucleation sites in a melt, the use of so-called fluxing agents to remove impurities and prevent oxidation, and the effects of stirring and melt shear on promoting heterogeneous nucleation. Once techniques for obtaining large undercoolings have been determined for an alloy, the emphasis of subsequent experiments might be on forming metastable and amorphous phases from deeply undercooled melts and determining their properties.
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From page 87... ...
trajectories have shown evidence that both the primary and secondary arm spacings change as the force of gravity is altered.2 Therefore, it was considered scientifically important to test the first-order fundamental theories that ignore convection in a microgravity environment before adding modifications to correct for convective effects that are encountered in terrestrial processes. Such a critical test, known as the Isothermal Dendritic Growth Experiment (IDGE)
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From page 88... ...
Interestingly enough, ground control experiments, which had convective flows, yielded results that agreed well with the classical theory.4 Growth in strong magnetic fields, on the other hand, which suppress convective flows, produced results similar to those from the spaceflight experiments, clearly indicating that convective effects are in fact important in the process. European experimenters on Spacelab-1 and D-1 have found similar results with other eutectic systems, agreement with the classical theory in other systems, and even larger spacings than predicted by classical theory in yet other systems.
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From page 89... ...
Phase Separating Systems and Interfacial Phenomena Another class of polyphase materials that is of interest to microgravity research involves monotectic systems, which are characterized by a compositional region of liquid-phase immiscibility. At any given temperature (below the critical temperature)
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From page 90... ...
Thus, the microgravity environment provides a unique opportunity to study and quantify a range of interfacial phenomena in order to suggest better materials processing strategies on Earth and under microgravity conditions. Heat and Mass Transport The freezing temperature of any alloy is dependent on composition, and a freezing solid tends to accept one component of a melt more readily than the other components; thus, the first-to-freeze solid will always have a composition different from that of the remaining solid.
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Even with a well-controlled directional solidification process such as the Bridgman method, lateral thermal gradients occur that will produce some convective stirring. Thermosolutal convection occurs in Earth gravity if the rejected component is less dense than the bulk melt.
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From page 92... ...
This can easily occur in a directional solidification experiment unless care is taken to ensure that the residual acceleration environment is controlled and that the experiment is configured to minimize its effects, for example, by application of a strong magnetic field. Thermophysical Properties Another serious deficiency in our ability to model solidification processes is a paucity of accurate thermophysical property data for many alloys in the molten state.
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From page 93... ...
Rapid particle growth during liquid-phase sintering is an anomalous processing effect that might arise from sedimentation induced convection or from some unidentified interfacial phenomenon that promotes coalescence. Well-designed microgravity experiments should provide some valuable insight as to the sensitivity of such powder processes to these effects.
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From page 94... ...
Nucleation kinetics and the achievement of metastable phase states, such as metallic glasses and nanostructures, are areas of scientific interest that would benefit from achieving deep supercooling in the microgravity environment by elimination of container surfaces and from reduction of melt flows due to buoyancy-driven convection.
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From page 95... ...
Thus, more careful consideration reveals a rationale for studying organic systems in the low-gravity environment that is not dissimilar to the reasons supporting scientific uses of microgravity research for other materials (e.g., inorganic crystal and protein crystal growth)
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From page 96... ...
(2) During copolymerizations, convection flows reduce the thickness of the boundary layer of preferentially rejected monomers from near the chain ends, so copolymers reacted in reduced gravity tend to produce materials that have greater uniformity in their distributions of average molecular weights and chain lengths.10 Finally, in recent years there has been growing interest in the field of nonlinear optical (NLO)
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From page 97... ...
Shadowgraphs showed the absence of solutal "plumes" at the electrode surface in low gravity. The feasibility of using electrochemical techniques to assist and stimulate polymerization to prepare NLO polymer thin films for use in devices was demonstrated15 with prepared films of polythiophene and a homologous series of thiophene-based polymers that had NLO conversion efficiencies among the largest ever observed for polymers and comparable to the optical performance obtained for the polydiacetylenes.16 Recent research17-20 indicates that better-quality thin films for use in NLO devices might be obtained by closed-cell physical vapor transport (PVT)
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From page 98... ...
Given some of the apparent benefits of microgravity in the growth of some protein single crystals, selected examples of important organic materials should also be considered candidates for microgravity studies. In contrast to inorganic materials, organics and polymers are nearly infinite in variety and number.
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Reasonable goals for studying the growth of inorganic single crystals under microgravity conditions are to contribute to an understanding of the fundamental processes that take place during crystal growth, to provide benchmark crystals of higher quality than can be obtained terrestrially, and/or to provide useful experimental data that cannot be obtained (or obtained as accurately) in terrestrial studies.
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From page 100... ...
, which may provide advances in Department of Defense and NASA detector applications. SUMMARY OF TERRESTRIAL GROWTH OF INORGANIC SINGLE CRYSTALS The commercial growth of single-crystal boules (particularly of semiconductors, and frequently of laser crystals)
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From page 101... ...
For this reason the molten zone is usually a thin slice whose maximum thickness depends on the diameter of the boule. Floating zone growth under microgravity conditions is not subject to gravity-induced flow, and so large stable molten regions will be limited only by surface tension forces.
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The effects of particulates may be expected to be reduced for crystals grown under microgravity conditions if the circulation is reduced by eliminating or reducing convective flow. The elimination of convection also can eliminate impurity striations.
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Such studies provide an existence proof of the possibility of generating improved materials and suggest the possibility of generating fundamental information on bulk inorganic crystal growth. These and other studies have demonstrated that the microgravity environment is indeed unique and that the effects of convection may, in some cases, be removed.
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From page 104... ...
The terrestrial production of high-quality semiconductor crystals on a commercial scale is now done by the pulling, float zone, or Bridgman method with apparatus that permits precise temperature control over very large volumes. One area in which considerations of microgravity may have impinged on commercial practice is in the use of magnetic fields to reduce convection.
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From page 105... ...
The design and execution of microgravity experiments that lead to a better fundamental understanding of crystal growth have proved elusive, and the committee is recommending against the growth of large-diameter inorganic crystals under low gravity. The best approach to understanding the details of file:///C|/SSB_old_web/mgoppch6.htm (23 of 38)
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From page 106... ...
If some of the versatility of terrestrial experimentation can be achieved in the microgravity environment, there are opportunities for microgravity research that will impact terrestrial bulk crystal growth. Priority should be given to transport studies including studies of solute and self-diffusion, heat diffusion, and Soret diffusion.
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From page 107... ...
Since the manufacture of heterostructures by MOCVD will be accomplished terrestrially, the impact of research in microgravity is limited. Further, given the complexity of the calculations for that situation, it is not clear that there is relevant information to be obtained through microgravity research.
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From page 108... ...
The committee concludes from studies of ultrapure high-mobility GaAs grown by MBE50 that, with sufficient contamination reduction and increased pumping speed, any required great gains in purity can be made with terrestrial studies. The committee discards the idea that epitaxial layers will be manufactured in space and notes that, at a reasonable cost, much improvement in the vacuum environment can be achieved terrestrially.
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From page 109... ...
It is to this latter class of materials that the microgravity environment has the greatest relevance. CURRENT ISSUES IN CERAMICS RESEARCH As discussed in several publications,51-53 the greatest needs and opportunities in ceramics lie in the areas of synthesis and processing.
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From page 110... ...
Availability of a general method for contamination-free synthesis using containerless melting would be of value for producing research specimens of new high-temperature materials. Lack of promising new techniques is probably one of the main reasons that melt synthesis of ceramics or inorganic compounds is no longer a very active research area.
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From page 111... ...
Their manufacture is already highly developed and fibers with optical losses less than 1 dB/km in lengths of 30 km are routinely drawn in a single pull.55 New materials for optical fibers and signal amplifiers are under development. It is conceivable that containerless processing could be useful for producing research specimens of some materials.
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From page 112... ...
One typical method, used for electronic ceramics such as ZnO varistors and YBa2Cu3Ox, is to pump the solutions into an ultrasonic mixing cell where reaction occurs on the order of micro- to milliseconds and the products are carried away to be removed by filtration. The microgravity environment does not naturally provide any of those conditions.
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Reduction of convective flows in the microgravity environment may be of benefit in the preparation of high-quality ceramic films, particularly where epitaxy is desired. This would be especially useful if it led to improved processing on Earth.
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3. Mass transport and diffusion studies of glass and ceramic melts under microgravity conditions should generate more precise data than the data available from terrestrial measurements.
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1982. Materials processing in the reduced gravity environment of space.
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: Study of Organic Nonlinear Optical Thin Films and Bulk Crystals. Chemistry and Polymeric Materials Branch, Marshall Space Flight Center, Ala.
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1991. AIAA Microgravity Science Symposium, Moscow.
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1991. Directional solidification of HgCdTe and HgZnTe in a transverse magnetic field.
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* For a discussion of both methods, see section below, Growth of Inorganic Single Crystals.
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