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Toward a Microgravity Research Strategy (Appendix E) Toward a Microgravity Research Strategy E Glasses and Ceramics STATUS Several reports have been published that address the overall field of microgravity research and its various subdisciplines, including glasses and ceramics. In addition to reviewing material prepared by the Glasses and Ceramics Discipline Working Group, the committee examined the most recent pertinent reports.1-4 A large part of ceramics and glass research is concerned with controlling the minute flaws that determine the manner in which materials (e.g., pottery, bricks, concrete, glass, and others) fail. Ceramics, held together by strong covalent and ionic bonds, cannot absorb impacts by means of plastic deformation, as do metals. Instead, they crack, often catastrophically. Most of the current research in structural ceramics focuses on toughening mechanisms to REPORT MENU help inhibit failure. the essence of ceramic properties is as much in the internal NOTICE structure of their crystals as in their chemical composition. A ruby and a fire brick MEMBERSHIP may be made from the same substance—aluminum oxide—yet they look and SUMMARY behave quite differently. the key difference lies in how the crystals within the CHAPTER 1 structure are arranged. When ceramic and glass structures are ordered or CHAPTER 2 aligned, they can be a great deal stronger than when crystal boundaries or CHAPTER 3 disorder are present. Because many of the defects leading to the failure of CHAPTER 4 ceramic and glass materials emanate from the surface, processing conditions are APPENDIX A important in the quest to improve mechanical performance. APPENDIX B APPENDIX C APPENDIX D Most advanced ceramics for automobile and aircraft engines, bearings, APPENDIX E bio-implants, and microchip wafers involve powder processing and consolidation APPENDIX F techniques. Little benefit is foreseen from the study of traditional solid-state processing/consolidation techniques under microgravity conditions. Avoidance of a container, however, may be an important factor it processing reactive ceramic materials. Harmful effects associated with containers include chemical and structural (e.g., crystal nuclei) contamination of the melt and limitations of the temperature to which the melt can be superheated. Low gravity offers the possibility to perform containerless processing, thus avoiding the contamination and nucleation effects produces by the container. In fact, containerless file:///C|/SSB_old_web/cmgr92appende.htm (1 of 5) [6/18/2004 11:10:05 AM]
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Toward a Microgravity Research Strategy (Appendix E) processing has become somewhat synonymous with ceramic and glass research in space. Containerless processing is an excellent example of how to use low gravity to overcome a problem that has nothing to do with gravity. Containers interfere with what one is trying to accomplish in crystal growth. Chemical reactions at the walls can degrade the quality of a crystal being produced or constrain it' shape. Removing the container is the issue, while eliminating gravity is the path to the solution. Unfortunately, it seems that interest in developing containerless processing techniques has been the driving force for research on ceramics ant glasses, rather than the converse. Containerless science, however, include experimental and theoretical studies of the behavior of melts when removes from the physical contact of confining walls. Such studies include nucleation, glass formation in undercooled states, thermophysical measurements kinetics of purification, and buoyancy and convection in levitated samples While it is envisioned that glasses and ceramics might be scrutinized to produce selected benchmark materials that can serve as standards to which future terrestrial technology could aspire, it is highly unrealistic to expect that a technology for either glass or ceramic processing could be deployed economically in space in the foreseeable future. MAJOR RESEARCH ACCOMPLISHMENTS Two experiments on the containerless processing of several different glasses have been flown on the Space Shuttle. These studies examines glass nucleation and crystallization processes and used containerless processing with acoustic positioning to avoid contamination. Both experiences a number of difficulties but did accomplish the melting; several glass crystallization paths, along with bubble behavior and resulting homogenization were analyzed during cooling. The containerless suspension system did not always keep the sample in place during melting, and fluoride dissociation from a zirconium fluoride melt hindered the predicted ease of optical fiber formation. Studies of the dynamics of bubbles and droplets, which may lead to the removal of gas-filled bubbles during containerless melting, have captures the interest of experts in fluid physics and dynamics. Containerless processing has demonstrated the advantage of allowing for significantly lower critical cooling rates, thus extending the range of materials that can be obtained in glassy form. Early drop tube experiments showed the possibility of glass formation in tantalum oxide systems in which glass formation is difficult. Theories of acoustic behavior in a furnace of particular geometry and theories of bubble behavior have been explored extensively. The fabrication of microballoons and porous foams through the understanding of bubble migration is an example of the common research objectives of initial studies. file:///C|/SSB_old_web/cmgr92appende.htm (2 of 5) [6/18/2004 11:10:05 AM]
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Toward a Microgravity Research Strategy (Appendix E) RESEARCH PROSPECTS AND OPPORTUNITIES Containerless processing of materials from the melt certainly offers the possibility of minimizing contamination from crucible materials, particularly for higher-melting-point glasses and the new oxide superconductors, for example, yttrium barium cuprate. However, similar results can be obtained by variations on the skull melting process, in which the material acts as its own container or is levitated on a flowing gaseous stream. Therefore, containerless melting in the low- gravity environment is not absolutely necessary. New glasses may or may not be possible by containerless melting. This depends on whether heterogeneous nucleation occurs at the free surface and on whether bulk impurities or compositional fluctuations will promote heterogeneous nucleation. A low-gravity environment is advantageous for the study of immiscible liquids. Containerless melting becomes important when one of the phases preferentially wets the container, a situation that seems virtually unavoidable. The preparation of ultrapure glass fibers for optical communications is a relevant subject for the microgravity program. Most glass fibers today are fabricated from silica-based systems and are prepared by chemical vapor deposition. The chemicals used here are ultrapure so that optical losses due to contaminants can be minimized. However, the silica-based systems are limited in certain parts of the infrared spectrum by fundamental absorption effects. For these wavelengths, other glassy materials (such as halide systems), which do not have chemical sources suitable for vapor deposition, have been proposed. Thus, optical glass fibers must be prepared by pulling them directly from the molten glass precursor. Unfortunately, the need to contain these melts in a normal- gravity environment results in serious contamination. Consequently, containerless processing methods appear to be very attractive for this application. Much preliminary work to screen materials and to explore the relative effects of container contamination and nucleation effects on bulk glasses in novel materials systems can be performed in a reduced-gravity environment. The current microgravity program has been developing containerless processing techniques for many years; in fact, the U.S. program is leading European and former Soviet efforts. Therefore, the exploitation of ultrapure glass fibers from new materials systems is worth studying in microgravity. The preparation of powders by gas-phase reactions in fluidized bed reactors makes study of the kinetics of powder nucleation and growth extremely difficult because of convective currents and particle settling caused by gravity. With only gas diffusion as the major transport process, the nucleation and growth of these particles might be studied more carefully in microgravity by laser light scattering techniques, for example. If the kinetics of flow reactor processes can be understood, then powders of controlled size and size distribution can be designed. In addition, the agglomeration processes of these particles in the gas phase due to particle collisions can be understood. Finally, experiments similar to those described above for gas-phase reactions can be performed in aqueous systems. In this case, the attractive van der Waals forces can be overcome by charged particles, by applying the principles of electrical double-layer theory, or by the principles of steric hindrance for organic dispersants. Not only could file:///C|/SSB_old_web/cmgr92appende.htm (3 of 5) [6/18/2004 11:10:05 AM]
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Toward a Microgravity Research Strategy (Appendix E) optimum dispersant concentrations be determined, but fundamental studies of agglomeration could also be carried out over a range of particle sizes far greater than could be achieved in a one-gravity environment. A low-gravity environment is advantageous for the elimination of thermal and compositional currents that lead to compositional inhomogeneity in crystals and may also permit the fabrication of new composites of constituent materials with differences in density. Further, the effects of studies on Earth-based processes in the low-gravity environment can be greater than any actual use of the low-gravity environment for the processing of glass and ceramic materials. The virtual absence of both desirable and undesirable convective effects in low- gravity processing permits the study of complex processes when convective complications are eliminated. REFERENCES 1. Panel on Microgravity Research, Solid State Sciences Committee, Board on Physics and Astronomy. 1986. Microgravity Science and Applications: Report on a Workshop. National Academy Press, Washington, D.C. 2. Review Committee, 1. Robert Schrieffer, chairman. 1987. Review of Microgravity Science and Applications Flight Programs—January-March 1987. Universities Space Research Association, Washington, D.C. 3. Space Applications Board. 1988. Industrial Applications of the Microgravity Environment. National Academy Press. Washington, D.C. 4. Aeronautics and Space Engineering Board. 1989. Report of the Committee on a Commercially Developed Space Facility. National Academy Press, Washington, D.C. file:///C|/SSB_old_web/cmgr92appende.htm (4 of 5) [6/18/2004 11:10:05 AM]