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Microgravity Research Opportunities for the 1990s: Chapter 6 Microgravity Research Opportunities for the 1990s PART II—SCIENTIFIC ISSUES 6 Materials Science and Processing This chapter discusses the areas of microgravity research generally identified as materials science. As organized in this report, they include metals and alloys, organic materials and polymers, the growth of inorganic single crystals, the growth of epitaxial layers on single-crystal substrates, and ceramics and glasses. METALS AND ALLOYS REPORT MENU NOTICE BACKGROUND AND INTRODUCTION MEMBERSHIP PREFACE One of the major desires of modern materials scientists and physical EXECUTIVE SUMMARY metallurgists is to understand the formation, structure, and properties of materials PART I from the atomic and molecular levels (0.1 to 1 nm) up through the mesoscopic CHAPTER 1 CHAPTER 2 level (0.1 to 100 m). Many physical properties of materials are influenced PART II directly by this microstructure (i.e., the morphology, size scale, spatial CHAPTER 3 distribution, interface characteristics, and composition of different phases or CHAPTER 4 structures that are present within a material). The relationship between the CHAPTER 5 microstructure of a material and its physical properties and the manipulation of CHAPTER 6 various processing parameters to obtain a given microstructure constitute key CHAPTER 7 elements in the modern study of materials. PART III 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) [6/18/2004 11:17:28 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 transport phenomena occurring in the melt and the kinetics of the solidification interface. Most metals and alloys are solidified from a fluid state. Since it is often desirable to mix two or more components to obtain alloys or composites with improved physical properties, the solidification process becomes complicated by the fact that different atoms do not generally enter the solid in the same proportion as they occur in the melt. This chemical segregation produces local variations in composition in the melt that, in turn, change the temperature at which various fractions of the solid will form. The situation described here is complicated further by the presence of convective and advective liquid flows that also modify the local composition and temperature of the melt. The microgravity environment, by reducing these gravity-driven phenomena, clearly offers new opportunities to metallurgists to develop and enhance control of materials processing. 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. Body force effects such as sedimentation, hydrostatic pressure, and deformation are similarly reduced. Weak noncontacting forces derived from acoustic, electromagnetic, or electrostatic fields can be used to position specimens while they are being processed, thus avoiding contamination of reactive melts with their containers. To accomplish the objectives discussed above, it is necessary to conduct a series of carefully chosen, well-conceived experiments that clearly delineate the advantages and limitations of microgravity research. These experiments should be designed to produce new results that could not have been obtained by terrestrial experiments. FUNDAMENTAL RESEARCH AREAS-METALS AND ALLOYS Nucleation and Metastable States In order for a material to transform to a more ordered phase (vapor to solid or liquid to solid), it is first necessary to form an aggregate or cluster of molecules above a critical size to initiate the process. Such an aggregate may (by heterogeneous nucleation) form on a foreign surface, such as a container wall or a freely floating mote, or it may form spontaneously (by homogeneous nucleation) from random internal thermodynamic fluctuations. Homogeneous nucleation can occur only if the melt is cooled far below its normal freezing temperature without solidifying. Heterogeneous nucleation almost always occurs first, provided there are impurities that can act as nucleation sites. Understanding and controlling phase nucleation are extremely important for control of the overall solidification process. file:///C|/SSB_old_web/mgoppch6.htm (2 of 38) [6/18/2004 11:17:28 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 For example, if a fine-grained casting is desired, one would try to produce a very large number of crystal nuclei and, ideally, distribute them randomly throughout the melt. Gravity-driven convection plays an important role in this process, as was demonstrated in a series of early experiments at reduced gravity using Space Processing Applications Rockets (SPARs) 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. It is often desirable to suppress, or even avoid nucleation entirely, in order to be able to supercool a melt to a temperature well below its normal thermodynamic freezing point. Solidification of deeply undercooled metallic melts is usually initiated by a single nucleation site, and freezing tends to be rapid. These melts generally produce a refined dendritic microstructure, often exhibiting enhanced mechanical properties. If the solidification is rapid enough, the solute atoms will not have sufficient time to arrive at positions of lowest energy (i.e., near their equilibrium configurations); metastable crystalline or even amorphous phases can thus be produced. A metastable phase can have a crystalline structure different from that of the equilibrium phase, which can greatly alter some of the material's physical properties in technologically significant ways. Perhaps the best-known metallurgical example of a metastable crystalline phase is steel, which is hardened by formation of the metastable phase martensite. Martensite is a nonequilibrium tetragonal phase containing carbon and iron that forms upon rapidly quenching austenite, which is a cubic, equilibrium phase consisting of carbon dissolved in the face-centered cubic form of iron. An amorphous phase may be thought of as a liquid-like structure, frozen in place, that lacks long-range crystalline order. Ordinary soda-lime silica glass is a classic example of an amorphous engineering solid. Amorphous materials tend to be highly resistant to chemical attack because there are no crystalline grain boundaries on which most chemical reactions occur. Similarly, glassy amorphous structures can be achieved in some metallic alloys by rapid solidification. One example is Metglas®—a product developed by Allied Signal Corporation that consists mainly of iron, silicon, and boron. The absence of grain structure makes it extremely easy to magnetize and demagnetize Metglas® with little magnetic hysteresis loss, thus making this material useful in transformer cores, where it provides greater efficiency than the conventional, crystalline iron-silicon material. By contrast, a new rapidly solidified iron-boron-neodymium alloy, developed recently by General Motors Corporation, is an extremely good permanent magnet, with high coercivity, because its fine grain structure tends to "pin" the magnetic domains and prevent demagnetization. These examples demonstrate how crucially the physical properties and performance of a material depend on its microstructure. The ability to melt, supercool, and subsequently solidify alloys without a supporting container or crucible removes a major source of impurities and heterogeneous nucleation sites, thus permitting very large undercoolings to be achieved. On Earth this can be accomplished for small metallic samples by file:///C|/SSB_old_web/mgoppch6.htm (3 of 38) [6/18/2004 11:17:28 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 levitating them on a gas-film or in an electromagnetic field and then releasing the molten droplets into a long, vertical, evacuated tube and allowing them to solidify during free-fall. These techniques have been used extensively over many years but have severe limitations. It is not possible, for example, to control the heating and levitating force independently in an electromagnetic levitator. Use of a quench gas to cool the sample often promotes surface nucleation. There is also some evidence that flows in the melt driven by the induced alternating currents may stimulate nucleation and limit the amount of undercooling. These difficulties can be overcome in a drop tube, but it is not yet possible to observe the falling droplet closely enough as it cools in order to make accurate temperature measurements. Also, the sample volume and the amount of undercooling are limited by the time available for the fall (typically a few seconds). In reduced gravity, the sample may be positioned by a much lower induced current or may even be allowed to float freely. This significantly decouples the heating field from the levitation field and greatly reduces the amount of stirring in the melt. The temperature may then be measured optically during cooling and the solidification process, and thermophysical properties such as specific heat, heat of fusion, and thermal diffusivity can be determined calorimetrically. 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. It may even be possible to perform critical supercooling experiments to evaluate various theories of nucleation that have never been verified experimentally. Convenient methods for obtaining extremely low partial pressures of reactive gases, such as oxygen, must also be developed since oxides and other reaction products will form rapidly on molten samples and, in some systems, act as effective nucleation sites. Prediction and Control of Microstructure Microstructure can be manipulated to a significant degree by controlling the rate at which solidification takes place and the temperature distribution in the region of the crystal-melt interface. This is often accomplished by the process of directional solidification, in which heat is extracted unidirectionally either by placing the bottom of the crucible containing the molten specimen on a chill-block (gradient freeze method) or by slowly lowering the crucible through a temperature gradient (Bridgman method).* If the temperature everywhere in the melt is above the local freezing (liquidus) temperature, a smooth, almost featureless, solidification front will result. This is desirable for growing single crystals or for file:///C|/SSB_old_web/mgoppch6.htm (4 of 38) [6/18/2004 11:17:28 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 producing aligned composites. However, in virtually all alloys, the atoms are incorporated into the solid phase in proportions different from the melt. This leads to a region ahead of the freezing interface in which the solute concentration is altered from its mean composition. Within this thin region or boundary layer, the equilibrium freezing point in the melt can be lower than the freezing point of the original alloy. If the temperature gradient in the melt is not steep enough, which is generally the case in a slowly frozen casting, a situation known as "constitutional supercooling" will arise, in which the melt ahead of the interface is below its equilibrium freezing temperature. At the onset of constitutional supercooling, the initially smooth interface becomes unstable and forms a series of protuberances or cells. At higher degrees of constitutional supercooling, the interface breaks down completely as these cells and bumps elongate and develop side branches to form tree-like structures called dendrites. When this happens, partial solidification proceeds over a substantial distance ahead of the totally frozen material. The region between the advancing solid and dendrite tip is called the "mushy" zone because it is composed of a fine, micrometer-length scale mixture of liquid and solid. The heat and mass transfer processes that take place in this two-phase "mushy" zone determine the microstructure of the solid. Although these processes are understood qualitatively, the detailed quantitative descriptions required to control the microstructure remain poorly understood. A great deal of theoretical work has been devoted to obtaining accurate descriptions of dendrite growth rate, shape, and primary and secondary arm spacing-all as functions of the solidification and materials parameters.1 Generally such fundamental theories ignore the important effects of convection in order to remain tractable. Convective flows are usually unavoidable in Earth gravity because of the lateral density gradients that must exist as a result of the solute rejection described above. In fact, short-duration, low-gravity experiments carried out on sounding rockets and in aircraft flying parabolic (low-gravity) 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), was carried out recently aboard the United States Microgravity (USMP-2) mission launched March 3, 1994. Preliminary results obtained from the IDGE show that the terrestrial dendritic growth data used previously to test diffusion-based theories were, in fact, heavily corrupted by buoyancy-induced convection. As a result, these fundamental theories, each one purportedly providing the basis of dynamical pattern selection, now remain in question.3 The discussion, thus far, applies only to alloy systems that have one solid phase, (i.e., to solid solutions in which the atoms in the solid are completely miscible or to dilute alloys in which the minor constituent does not exceed the limit of solid solubility). Most alloy systems of practical interest, however, display only partial miscibility in the solid state, a situation that produces a more complicated behavior involving polyphase reactions. An important class of materials exhibiting this behavior is the eutectics in which a melt consisting of a file:///C|/SSB_old_web/mgoppch6.htm (5 of 38) [6/18/2004 11:17:28 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 homogeneous solution of the two components solidifies to form a solid with segregated regions of quite different compositions. A good example of this type of behavior occurs in cast iron, whose melt consists of a solution of iron and carbon. On freezing, flakes or nodules of almost pure graphite form in a crystalline matrix of nearly pure iron. The morphology or form of these graphite particles largely determines the mechanical properties of the material. Despite many years of intensive research, all the kinetic and thermodynamic factors that determine the graphite morphology are still not understood. By carefully controlling the direction in which heat is extracted (directional solidification), interesting, controlled, two-phase microstructures can be produced in a variety of eutectic systems. In some systems, it is possible to grow thin rods of one composition and crystal structure that are embedded in a matrix of yet another composition and crystal structure, with both aligned along the direction of heat flow. In other systems the resulting structure comprises a series of alternating thin slabs, or lamellae, of the two compositions. These structures are termed "in situ composites" and often exhibit highly anisotropic properties because of the aligned microstructure. For example, some turbine blades manufactured by directional solidification for high-performance jet engines exhibit this aligned structure and show improved strength and creep resistance. In another example, the theoretical magnetic coercivity in a manganese-bismuth eutectic alloy was approached by using directional solidification to match the eutectic rod diameter to the size of a single elongated magnetic domain. The classical theory of eutectic spacing selection assumes only diffusive species transport, because convective effects were thought to be unimportant on the length scales involved. However, in a series of microgravity experiments for the manganese-bismuth eutectic, it was found that the rod diameter and spacing were considerably smaller than predicted from this theory. 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. Obviously, there is still much to be learned about the science of eutectic solidification. Another effect that is important in the evolution of the microstructure of an alloy is the phenomenon of ripening or coarsening. It can be shown from thermodynamics that the melting point of a solid particle is reduced slightly by the curvature of its surface (Gibbs-Thomson effect). This effect comes about because of the interfacial energy change associated with the motion of the curved surface during melting or freezing. A planar surface (zero curvature) would not experience a change in energy on melting or freezing. Given a distribution of solid particles with varying curvatures in a melt, the larger particles will grow at the expense of smaller particles as the system tends to lower its free energy by reducing its total surface area. This effect, known as Ostwald ripening file:///C|/SSB_old_web/mgoppch6.htm (6 of 38) [6/18/2004 11:17:28 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 or phase coarsening, is important in a large class of dispersion-hardened alloys in which fine particles are either added to or caused to precipitate from the melt during solidification. Since the strengthening effect of the dispersed particles diminishes as the particle size increases, the process of coarsening must be understood and controlled. Similarly, in castings and weldments, dendrite arms coarsen by the same process, which again affects the final microstructure and properties. The classical theory of Ostwald ripening was developed in 1961. An approximate solution was obtained that is valid in the limit of zero volume fraction of solid particles. In metallurgical applications, however, the dispersed phase makes up a significant fraction of the system. Also, it appears that neighboring particles interact strongly with one another, an effect that is not accounted for in the approximation used in the classical theory. A number of attempts have been made recently to modify the classical theory, but so far none of these theories can predict experimentally observed phenomena. This might be due to the fact that none of the theories considers convective transport of the continuous liquid phase (because the fluid channel sizes are small); yet it might be an important factor, as well as an additional complication. Well-defined microgravity experiments are needed to confirm the quantitative aspects of the underlying physics in the theory of Ostwald ripening under the conditions of pure diffusive transport. Such experiments need to be performed before the effects of convection and fluid motions can be incorporated into the theory. 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) a range of compositions is always encountered for which the melt will separate into two immiscible liquids. Since the two liquids are of different composition, they will invariably have different mass densities. In Earth gravity the two liquid phases will stratify before they can be frozen, resulting in a highly chemically segregated solid. It was reasoned that this buoyancy-driven sedimentation could be avoided by solidifying such systems in microgravity. When actually performed, such experiments have been only partially successful in producing the expected fine dispersions uniformly distributed throughout the final solid.5 These experiments indicate that effects other than gravity-induced buoyancy become important in the solidification of monotectic systems. An extensive series of ground-based experiments, using systems that have similarly configured (monotectic) phase diagrams, and which in some instances can be made neutrally buoyant, has uncovered a rich variety of interfacial energy-driven effects (such as critical wetting). These monotectic alloys exhibit a variety of interesting microstructures, which have been scientifically classified through the help of microgravity research.6 file:///C|/SSB_old_web/mgoppch6.htm (7 of 38) [6/18/2004 11:17:28 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 A clear example of the dominating influence of interfacial energy effects occurs in monotectic alloy systems below the critical temperature. Over a temperature range below the critical temperature, one of the liquids always exhibits perfect (or critical) wetting behavior, so that one fluid phase essentially encapsulates the other and, even more curiously, the wetting phase will perfectly coat any container or surface that it contacts. If final freezing occurs within the temperature range of critical wetting, then massive phase separation will be produced in the microstructure. Alternatively, if the temperature for the monotectic reaction, which produces a solid- and liquid-phase pair from a different liquid composition, falls below the critical range, then useful (dispersed) microstructure control is possible. A continuous, well-aligned two-phase structure with uniform spacing between phases can often be achieved in a directional growth process.7,8 In the latter case, melt flows driven by the temperature and composition dependence of surface energy can become dominant. The minority liquid phase can form droplets that will migrate in a thermal gradient (thermophoresis) as well as in a composition gradient. Larger droplets move faster than smaller ones, so that the smaller ones will be overtaken and agglomerated with the bigger ones. Again, massive phase separation will result. In fact, depending on the alloy system, thermal gradients as small as 1 K/cm in low gravity can have the same motive effect as Earth gravity in causing massive segregation. A second case of dominating interfacial factors occurs in containerless processing, where the extent of wetting between the solid(s) forming from a melt controls the degree of undercooling and the resulting microstructures. Brazing, soldering, and welding operations represent additional technologically important processes that are influenced greatly by interfacial phenomena. Here both wetting and surface tension-driven flows can be primary influences in achieving or not achieving success. It is clear that interfacial phenomena are common to numerous metals processing strategies. Although limited studies of interfacial effects are always possible on Earth using density-matched immiscible systems, density matching per se under the influence of terrestrial gravity can be accomplished only at a single temperature. 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. The rejected component sets up a compositional gradient in the melt at the solidification interface. With gravity file:///C|/SSB_old_web/mgoppch6.htm (8 of 38) [6/18/2004 11:17:28 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 present, any lateral density gradient resulting from composition or thermal gradients will produce prompt convective stirring. Convective mixing of the enriched solute rejected from a growing crystal-melt interface with the bulk melt causes a spatially varying composition (i.e., segregation) to occur throughout the solidified specimen. This phenomenon is called macrosegregation. When the mixing rate varies in time, the temporal variation results in a solid with a profile that might exhibit bands of composition that vary along its length. Macrosegregation can be controlled to some extent by extracting heat unidirectionally from the bottom of the specimen to minimize lateral thermal and solutal gradients. However, if the rejected component of the melt at the interface is less dense than the bulk melt, a phenomenon known as a double-diffusive convection can develop. Channels or plumes of the lighter melt will rise from the interface, resulting in solidified regions with a grossly different composition (known as "freckles"). Macrosegregation also plays a significant role in the formation of in situ composites with aligned microstructure. 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. Strong magnetic fields can slow these convective flows but cannot eliminate them. Thus, magnetic fields are generally of limited effectiveness for controlling macrosegregation. Microgravity offers the opportunity to produce unique microstructures in a variety of systems (e.g., off-eutectic compositions, monotectics, and peritectics) that tend to phase-separate in normal gravity, as well as in solid solutions that are subject to double-diffusive convection. Liquid- phase sintering of dense tungsten particles with lighter molten transition alloys (typically Fe-Ni) is a good example of an important terrestrial process that would be assisted if performed in a microgravity environment. Experiments in microgravity are being flown by NASA to test theories of solidification, seeking a more basic understanding of the way in which microstructures are formed. Since the solidification process involves the transport of heat and mass and a moving phase boundary, one way to test the theories is to encode them as a mathematical model and then compare the predicted thermal and solute fields with those observed experimentally. This allows evaluation of competing theories using the same experimental data. Most of the mathematical models that have been developed thus far consider only steady accelerations. Additional modeling development efforts are clearly needed to account for transient, random, and oscillatory accelerations (collectively termed "g-jitter") and their effects on heat and mass transport. The ability to design and interpret scientific experiments in the microgravity program is often predicated on the available knowledge of the relevant solute and temperature fields. Mathematical and numerical models are an indispensable tool to augment experimental measurements for this purpose. There are two distinct ways to use the mathematical models that describe the physical transport processes of interest: (1) to design and then theoretically interpret scientific file:///C|/SSB_old_web/mgoppch6.htm (9 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 experiments performed in microgravity, and (2) to obtain values for certain thermophysical properties, which are difficult to measure terrestrially, by using microgravity experiments, and then employ these data to improve model-based predictions of terrestrial processes used in industry. In order to improve the chances of a successful flight experiment and take maximum advantage of the relatively few flight opportunities that will be available over the next decade, candidate experiments must be carefully modeled mathematically for both terrestrial and microgravity conditions. Even for a six-order-of-magnitude reduction in gravity, significant flows can still result if the residual gravity vector is perpendicular to a large density gradient. 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. This is a problem not only for scientists modeling microgravity experiments, but also for many industrial researchers who are using the computational methods made possible by supercomputers to model the solidification of complex castings. The lack of high-temperature thermophysical data is due in part to the extreme difficulty of making accurate measurements on melts under terrestrial gravity. Recent European experiments on Spacelab-1 and D-1 have shown that diffusion coefficients for a variety of molten alloys measured in space differ considerably from the accepted values obtained on Earth. This situation is believed to occur either because of wall effects (such measurements are often made in capillary tubes) or because of uncompensated convective transport, which is virtually impossible to avoid in a terrestrial setting. Also, the effect of thermo-diffusion (Soret-Dufour effect) was found in space experiments to be as much as an order of magnitude larger than previously estimated. No accurate measurements had been made of this effect for solidification processes conducted on Earth. This effect, which causes a mixture of atoms to become separated in a thermal gradient according to their atomic mass, now appears to play a more important role in mass transport in many terrestrial processes than had been realized heretofore. The following are some thermophysical properties,9 only a few of which can be advantageously measured in microgravity, that are of interest in the development of reliable models for metals processing: Emissivity, electrical conductivity, and optical properties; Calorimetry including specific heats and heats of mixing, formation, and transformations; file:///C|/SSB_old_web/mgoppch6.htm (10 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 Transport coefficients including thermal conductivity, viscosity, and diffusion constants; Density data; Thermodynamic moduli, including thermal expansion coefficients and compressibility; Vapor pressures and activity coefficients; Surface tension and interfacial energies; and Equations of state. FUTURE SCIENTIFIC DIRECTIONS—METALS AND ALLOYS As indicated in the previous section, a number of important scientific problems in metallurgy are being addressed by experiments currently sponsored by NASA's microgravity science and applications program. However, other important areas can be identified that are just beginning to be developed. These areas include powder metal processing, electrolytic processes, joining, ultrapurification, ultrahigh-vacuum processing, nanomaterials, and cuprate superconductors. Powder metallurgy represents an important technology for processing many dense refractory metals or materials that tend to phase-separate when melted in unit gravity. Indeed, some of the gravity-related problems discussed previously can be avoided by standard powder processing: for example, by compacting a mixture of the components in the form of a fine powder and heating it just below the melting point for some time (sintering). Theoretical density can often be achieved by applying high pressure (hot isostatic pressing) or by raising the temperature so that one of the components melts (liquid-phase sintering). Since the powder metal process is designed ab initio to minimize the effects of gravity, one would not expect to see any large differences between Earth- and space-processed samples. 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. Little has been reported in the United States concerning electrodeposition, electropolishing, or corrosion in reduced gravity. Convection file:///C|/SSB_old_web/mgoppch6.htm (11 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 increasing the fracture toughness, either through elimination of flaws or by making the ceramic less sensitive to the flaw population (increasing the critical flaw size or increasing the fracture energy). Raw materials for ceramic and glass manufacture traditionally are earthy, oxide materials that are mined in high volume at low cost and are subjected to relatively little processing. The products made from them are commodity items such as bricks, tile, and glass windows. Modern technical or engineering ceramics are higher-value-added materials that have superior properties by virtue of their more sophisticated processing and tighter control over raw materials. 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. Specific recommendations for increased emphasis include: Interactive research on new materials synthesis that is linked with characterization and analysis of the product; Basic research on synthetic solid-state inorganic chemistry to produce new compounds; Synthesis of ultrapure materials, for example, fibers with low oxygen or carbon impurity levels; Research on techniques for synthesis to net-shape, that is, learning how to do synthesis, processing, and forming in a single step; Research on methods for processing ceramic materials far from equilibrium; and Research on processing of artificially structured or, as they are sometimes called, functionally gradient materials. Whether the availability of a free-fall environment is relevant to these research areas is an unanswered question. MICROGRAVITY AS A VARIABLE IN CERAMICS SYNTHESIS AND PROCESSING file:///C|/SSB_old_web/mgoppch6.htm (27 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 Evaluating the applicability of the microgravity environment to opportunities and problems in ceramics R&D requires a good definition of that environment. Two aspects of this environment seem relevant to ceramics: (1) the possibility of levitating a specimen for high-temperature containerless processing and (2) the suppression of buoyancy-driven convection. In both cases, the applicability is to gas- or liquid-phase phenomena. Thus, for applications of microgravity to ceramics, one should look to where liquid or gas phases are important. Some areas of liquid- and gas-phase processing of ceramics are discussed below. Melt Synthesis Ceramics are very seldom made directly by melt processing, mainly because their melting temperatures are very high (for those that do not decompose first), the melts react severely with available crucible materials, and undesirable microstructures are obtained on cooling. Thus, ceramics typically are made by sintering of powders. Those ceramic powders are produced by a variety of techniques, including upgrading of mined deposits, solid-state reactions, solid- liquid reactions, and gas-phase reactions. For some powder compositions, such as high-purity Al2O3 or SiC, sintering and densification are entirely by solid-state processes. For other ceramics, a second-phase liquid may be present in sintering (up to 15% by volume). The unanswered question is what difference microgravity might make for ceramic melts. 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. Ceramic superconductors, such as YBa2Cu3Ox, have been proposed for levitation melting to minimize crucible contamination.54 However, here the problem is not reaction with the container but that YBa2Cu3Ox melts incongruently and on cooling does not give the stoi- chiometric compound-a problem not addressed by levitation. Many of the benefits of microgravity containerless melting are available on Earth using skull or cold- wall melting. The relative merits of space-based containerless melting and terrestrial cold-wall melting have not been assessed for ceramic synthesis. Vaporization is frequently a problem in ceramic and glass synthesis and processing. For example, Si3N4 vaporizes incongruently at temperatures below those required for significant solid-state diffusion. Thus, Si3N4 ceramics are made with additives that lower the sintering temperature (to 1700-1800ºC) by producing small amounts of liquid phases. Most commercial glasses are made from high-temperature melts. Reaction with crucible materials is always a concern for glass melting, but it is usually dealt with case by case. Glasses crystallize by a nucleation and growth file:///C|/SSB_old_web/mgoppch6.htm (28 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 mechanism. In some cases the crystallization is desired, as with glass-ceramics. For glasses, however, crystallization is undesirable and nucleation must be suppressed. Heterogeneous nucleation occurs at phase interfaces, as in the melt- crucible wall or on impurities or heterogeneities in the melt. Containerless melting could be advantageous for some glasses. Glass melts frequently experience preferential loss of the more volatile components, changing the composition of the resulting glass. Processing in a convection-free environment could lead to higher levels of saturation in the atmosphere at the solid or liquid surface that would suppress evaporation. Lower rates of evaporation would expand the available processing windows for many materials and allow shorter processing times. Optical fibers are the application of glasses that has the most stringent requirements for purity and homogeneity. Optical fibers are pulled from preforms made by a CVD process and are not melt synthesized. 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. Solution Synthesis A recent trend in ceramic synthesis is use of near-room-temperature solution techniques to synthesize ceramic precursors. Increasingly, advanced ceramics are synthesized from highly processed, chemically prepared powders. Some of the more promising routes to advanced ceramic powders are sol-gel processing, precipitation from solution, gas-phase synthesis, and powder-surface modification. The sol-gel method is a familiar example. In that technique, organometallic reagents in solution are hydrolyzed and condensed to form an inorganic polymeric gel that, when dried and fired, gives the desired ceramic composition. These chemical methods can generate controlled size distributions, extremely reactive precursors, unusually shaped particles, and gels. Solution methods permit intimate mixing of components, easy dispersion of second phases, and surface modification of the precursor particles. Liquid precursor solutions also can be used to make thin films by dipping or spinning, and because of the high reactivity of the precursor particles, film consolidation occurs at moderate temperatures. This advantage is being exploited in research on the synthesis of electronic ceramic films such as PZT (lead zirconate-lead titanate) and YBa2Cu3O7-x from solution. One potential application might be in epitaxial growth of films from solution. That is one example of biomimetic synthesis of ceramics, a proposed approach to ceramic synthesis that mimics biological processes through the use of self-assembling monolayers. Whether reduction of convective transport would be useful to biomimetic synthesis should be determined experimentally. file:///C|/SSB_old_web/mgoppch6.htm (29 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 Chemical preparation of ceramics is a broad subject. For example, synthesis of ceramic precursors by polymerization of metal alkoxides using hydrolysis and condensation reactions is a rapidly expanding area of ceramics. Ultrafine ceramic particles with enhanced surface reactivity (such as SiO2) can be synthesized through nucleation or condensation reactions in gas-phase aerosols. Ceramic powders can be prepared under hydrothermal conditions in which the solubilities of many oxides are greatly enhanced, permitting direct precipitation of oxides from solution that is not possible under ambient temperature and pressure. Hydrothermal growth is well established for growth of quartz crystals, but the full potential of the technique has not been exploited. Glass, which is also considered a ceramic, has a lack of crystallinity as its most distinguishing feature. Glass precursors can be synthesized in the same ways as ceramics, for example, the process for making the silica boules from which optical fiber is pulled: SiCl4 + O2 (C2H2 flame) SiO2 + CO2 + H2O + HCl. Glasses can also be made by using the alkoxide solution route described above for sol-gel ceramics. Powder Synthesis in Microgravity One of the main objectives in ceramic powder synthesis is to make particles that are fine, homogeneous, and unagglomerated. Preparation by precipitation from solution requires rapid introduction and mixing of reagents. 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. Microgravity would seem most obviously of benefit in situations where convection and particle settling must be minimized. In traditional powder synthesis, the amount of liquid is a small fraction of the total mass, and capillary effects should be much larger than those due to convection. Ordinary gas-phase synthesis techniques involve forced flow of the gases, and it is not clear how large an effect is introduced by convection. It is possible to design experiments with gas flow rates low enough so that convection is relatively important. The significant practical problem in gas-phase synthesis is avoiding agglomeration of very fine particles. Understanding nucleation and growth of those particles is an important basic science issue. In summary, gravity may be a significant variable in solution synthesis of ceramics, but that has yet to be demonstrated. When ceramic powder is the goal, reactions normally are run very quickly in a flow reactor or with stirring to obtain a file:///C|/SSB_old_web/mgoppch6.htm (30 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 very fine, homogeneous precipitate. Forced mixing is the rule, and any convection is a very minor effect. In sol-gel synthesis of bulk objects such as aerogels, convection may play a more important role, and some gelation experiments in microgravity may be fruitful. CERAMIC AND GLASS PROCESSING IN MICROGRAVITY Processing refers to the steps involved in going from the precursor powder to the final object. There can be a great deal of overlap between synthesis and processing because some schemes are more or less continuous from raw material to final product. As already discussed, much traditional ceramic processing is by sintering of powders. Melt processing of ceramics is rare, being used mostly for glasses and single-crystal growth (e.g., ruby laser crystals). There are several practical reasons for this, among them the fact that the melting points of many important ceramics are very high (greater than 2000ºC for Al2O3) and ceramic melts react with many crucible materials. Other ceramics, such as SiC and Si3N4, cannot be melt processed because they melt incongruently. Where reaction with crucible materials is the limiting factor, containerless processing in microgravity may have significant advantages. There might be opportunities for microgravity research in solution processing of ceramics. One example is making sol-gel films by dip coating. Use of ceramics as films is a rapidly expanding area with applications as diverse as sensors, microelectronic memory elements, and antireflection coatings. It is important to determine the physical and chemical factors that control film formation during dip and spin coating and to develop strategies to tailor film porosity and microstructure for sensor, membrane, protective, and photonic applications. A major issue is the balance between gravitational draining, solvent evaporation, solute particle interactions, and surface tension effects. A key to the latter is the hydrostatic capillary pressure, which has rarely been measured in a film or gel.56 Solution deposition is of interest for preparing other kinds of ceramic thin films because of the low capital investment costs, the ability to closely control composition, and the relative ease of process integration with other technologies. One problem with the use of chemical deposition methods to prepare thin films is lack of a fundamental understanding of the effects of solution chemistry variations and structural evolution on film properties. For example, the process variables used in each step of the deposition of PZT ferroelectric thin films (e.g., solution preparation conditions, heat treatment temperatures, times, and ramp rates) determine the ferroelectric properties of the film. 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. file:///C|/SSB_old_web/mgoppch6.htm (31 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 BASIC SCIENCE There are a few fundamental issues in ceramics, in addition to those already mentioned, for which the microgravity environment might prove useful. Perhaps the most obvious is the study of crystal nucleation and growth in glass melts. This question is important technically because of its relation to glass- ceramics (where one wants controlled crystallization) and to optical glasses (where crystallization is to be avoided). This line of research is being pursued in the NASA containerless melt program.57 There is a large body of work in the scientific literature from the 1960s and early 1970s on glass crystallization.58 The challenge to microgravity research is to show how microgravity can be used for critical experiments that will provide new advances in our understanding of crystal nucleation and growth. A second fundamental research opportunity is mass transport or diffusion studies in glass and ceramic melts. For many systems of interest, the data are inaccurate or unknown. Diffusion studies in microgravity would not be troubled by convective mixing, and better data could be obtained in much less time. One area of application is using diffusion data to test theories that classify glasses as either strong or fragile, a concept that is related to the degree of polymerization in the melt. RECOMMENDATIONS AND CONCLUSIONS The above discussion identifies a number of areas in which a microgravity environment might be of benefit to ceramics research and development. The following is a prioritized listing of those points: 1. The development of a general method for contamination-free synthesis using containerless melting would provide a capability not available on Earth. Examples are containerless melting of glasses to suppress heterogeneous nucleation and containerless processing to produce research specimens of new glasses for optoelectronic applications. 2. A fundamental study of crystal nucleation and growth in glass melts in microgravity could provide information difficult to obtain on Earth. 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. 4. The suppression of free evaporation from melt surfaces could allow synthesis at higher temperatures than can be done on Earth. file:///C|/SSB_old_web/mgoppch6.htm (32 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 5. The epitaxial growth of films from solution, including biomimetic synthesis (self-assembling monolayers) of ceramics, should be studied. REFERENCES 1. Glicksman, M.E., and S.P. Marsh. 1993. The dendrite. Pp. 1107-1112 in Handbook of Crystal Growth, Vol. 1b, D.J.T. Hurle, ed. Elsevier, Amsterdam. 2. Johnston, M.H., and R.A. Parr. 1982. Met. Trans., 13B:85. 3. Glicksman, M.E., and M. Koss. 1994. Dendritic growth velocities in microgravity. Phys. Rev. Lett., 73(4):573-576. 4. Pirich, Ron G., and D.J. Larson. 1982. Materials processing in the reduced gravity environment of space. Materials Research Society Symposium Proceedings, 9:523-531. 5. Lacy, L.L., and G.H. Otto. 1975. AIAA Journal, 13:219. 6. Grugel, R.N., T.A. Lograsso, and A. Hellawell. 1982. Directional solidification of alloys in systems containing a liquid miscibility gap. Pp. 553-561 in Materials Processing in the Reduced Gravity Environment of Space. Elsevier Science Publishing Co., Inc. 7. Andrews, J.B., A.L. Schmale, and A.C. Sandlin. 1992. J. Cryst. Growth, 119:152-159. 8 Hayes, J.L., and J.B. Andrews. 1994. Experimental methods for microgravity materials science. R.A. Schiffman and J.B. Andrews, eds. TMS, The Minerals, Metals and Materials Society Proceedings, 6:167-174. 9. Margrave, John L. 1982. Heat capacities of liquid metals above 1500 K. Pp. 39-42 in Materials Processing in the Reduced Gravity Environment of Space. Elsevier Science Publishing Co., Inc. 10. National Aeronautics and Space Administration. 1989. NASA Laser Light Scattering Advanced Technology Development Workshop-1988. NASA Conference Publication 10033. 11. Vanderhoff, J.W., et al. 1983. Preparation of large particle size monodisperse latexes in microgravity. P. 17 in Materials Sciences Under Microgravity, Abstracts of the 4th European Symposium, Madrid, Spain. file:///C|/SSB_old_web/mgoppch6.htm (33 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 12. Micale F.J., J.W. Vanderhoff, and R.S. Snyder. 1977. Analysis of electrophoresis on Apollo 16. Separation and Purification Methods, 6:1-59. 13. Owen, R.B. 1980. Optical measurements and tests performed in a low- gravity environment. Society of Photo-Optical Instrumentation Engineers Journal, 255:74-81. 14. Riley, C., D. Coble, and G. Maybee. 1987. Electrodeposition of metals and metal/cermet composites in low gravity. AIAA Paper 87-0510. American Institute for Aeronautics and Astronautics, Washington, D.C. 15. Dorsinville, R., et al. 1989. Optical Letters, 14(23):1321. 16. Kajzar, F., and J. Messier. 1986. Resonance enhancement in cubic susceptibility of Langmuir-Blodgett multi-layers of polydiacetylene. Thin Solid Films, 132:11. 17. Ho, Z.Z., C.Y. Ju, and W.M. Hetherington III. 1987. 3rd harmonic- generation in phthalocyanines. J. Appl. Phys, 62:716. 18. Debe, M.K., R.J. Poirier, D. Erickson, T.N. Tommet, D.R. Field, and K.M. White. 1990. Effect of gravity on copper phthalocyanine thin films. Thin- Solid-Films-Switzerland 186:257-288. 19. Debe, M.K., and K.K. Kam. 1990. Effect of gravity on copper phthalocyanine thin films. Thin-Solid-Films-Switzerland, 186:289-325. 20. Debe, M.K., and R.J. Poirier. 1990. Effect of gravity on copper phthalocyanine thin films. Thin-Solid-Films-Switzerland, 186:327-347. 21. Debe, M.K., E.L. Cook, R.J. Poirier, L.R. Miller, G.J. Follett, and S.M. Spiering. 1987. Polymer Preprints, 28(2):453. 22. Debe, M.K. 1986. Industrial materials processing experiments on board the space shuttle orbiter. J. Vac. Sci. Technol., A4(3):273. 23. Debe, M.K., E.L. Cook, R.J. Poirier, L.R. Miller, G.J. Follett, and S.M. Spiering. 1987. Polymer Preprints, 28(2):453. 24. Debe, M.K. 1986. Industrial materials processing experiments on board the space shuttle orbiter. J. Vac. Sci. Technol., A4(3):273. 25. National Aeronautics and Space Administration. n.d. Research and Technology Operating Plan (RTOP): Study of Organic Nonlinear Optical Thin Films and Bulk Crystals. Chemistry and Polymeric Materials Branch, Marshall Space Flight Center, Ala. file:///C|/SSB_old_web/mgoppch6.htm (34 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 26. Heeger, A.J., and D.R. Ulrich, eds. 1988. Nonlinear optical properties of polymers. Symposium Proceedings Ser., Vol. 109. North Holland Press, New York. 27. Witt, A.F., H.C. Gatos, M. Lichtensteiger, and C.J. Herman. 1978. Crystal-growth and segregation under zero gravity-Ge. J. Electrochem. Soc., 125:1832. 28. Witt, A.F., H.C. Gatos, M. Lichtensteiger, M.C. Lavine, and C.J. Herman. 1978. Crystal growth and segregation under zero gravity: germanium. J. Electrochem. Soc. 125:276. 29. European Space Agency. 1991. Summary Review of Sounding Rocket Experiments in Fluid Science and Materials Science. O. Minster. ESA- 1132. February. 30. Matthiesen, D.H. 1991. Growth of electronic materials in microgravity. P. 111 in Proceedings of the Microgravity Science Symposium, Moscow, May 13- 19. American Institute of Aeronautics and Astronautics, Washington, D.C. 31. Zemskov, V.S., M.R. Raukhman, E.A. Kozitsina, I.V. Barmin, and A.S. Senchenkov. Experiments on directional crystallization of indium antimonide on 'FOTON' automatic satellites. P. 124 in Proceedings of the Microgravity Science Symposium, Moscow, May 13-19. American Institute of Aeronautics and Astronautics, Washington, D.C. 32. Croell, A., W. Muller, and R. Nitsche. 1986. Floating-zone growth of surface-coated silicon under microgravity. J. Cryst. Growth, 79:65. 33. Eyer, A., and H. Leiste. 1985. Striation-free silicon crystals by float- zoning with surface-coated melt. J. Cryst. Growth, 71:249. 34. Walter, H.U. 1974. Seeded, containerless solidification of indium antimonide. P. 257 in Proceedings of the 3rd Space Processing Symposium. NASA TMX-70252. 35. Larson, D.J. 1993. Orbital processing of ZnCdTe. Presented at the 14th International Astronautical Congress, Graz, Austria. 36. Iwan, J., D. Alexander, J. Ouazzani, and F. Rosenberger. 1989. J. Cryst. Growth, 97:285. 37. Alexander, J.I.D., and C.A. Lundquist. 1988. AIAA Journal, 26:193. 38. USSR Scientific Industrial Association, Electronic Industry Ministry. 1991. AIAA Microgravity Science Symposium, Moscow. file:///C|/SSB_old_web/mgoppch6.htm (35 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 39. Private communication, F. Lemkey. 40. Kawase, T., A. Kawasaki, and K. Tada. 1986. Gallium arsenide and related compounds. Inst. Phys. Conf. Ser., 82:27. 41. Price, M.W., R.N. Andrews, C.H. Su, S.L. Lehoczky, and F.R. Sofran. 1994. The effect of a transverse magnetic field on the microstructure of directionally solidified CdTe. J. Cryst. Growth, 137:201-207. 42. Su, Ching-Hua, S.L. Lehoczky, and F.R. Sofran. 1991. Directional solidification of HgCdTe and HgZnTe in a transverse magnetic field. J. Cryst. Growth, 109:392-400. 43. Hurle, D.T.J. 1992. Gravity related phenomena in materials science. Presented at the European International Space Year Conference, Munich. 44. Nambu, K., R. Nakai, M. Yokogawa, K. Matsumoto, K. Koe, and K. Tada. 1987. Gallium arsenide and related compounds. Inst. Phys. Conf. Ser., 91:141. 45. Ostrogorsky, A.G., F. Mosel, and M.T. Schmidt. 1991. Diffusion controlled distribution of solute in SN-1-percent-BI specimens solidified by the submerged heater method. J. Cryst. Growth, 110:950. 46. Bonner, W., B.J. Skromme, E. Berry, H.L. Gilchrist, and R.E. Nahory. 1988. Bulk single-crystal Ga1-xInxAs-LEC growth and characterization. Inst. Phys. Conf. Ser., 96:337-342, and private communication. 47. Larson, D.J., Jr., A. Levy, D. Gilles, J.I.D. Alexander, and F.M. Carlson. 1991. Proceedings of the SPIT International Symposium on Optical Science and Engineering, Vol. 1557. 48. Lagowski, J., H.C. Gatos, and F.P. Dabkowski. 1985. Partially confined configuration for the growth of semiconductor crystals from the melt in zero-gravity environment. J. Cryst. Growth, 72:595. 49. Jensen, K., D.I. Fotiadis, and T.J. Mountziaris. 1991. Detailed models of the MOCVD process. J. Cryst. Growth, 107:1. 50. Pfeiffer, L., K.W. West, H.L. Stormer, and K.W. Baldwin. 1989. Electron mobilities exceeding 107 cm2/s in modulation-doped GaAs. Appl. Phys. Lett. 55:1888. 51. National Research Council. 1989. Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of Materials. National file:///C|/SSB_old_web/mgoppch6.htm (36 of 38) [6/18/2004 11:17:29 AM]

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Microgravity Research Opportunities for the 1990s: Chapter 6 Academy Press, Washington, D.C. 52. Evans, J.W., and L.C. de Jonge, eds. 1991. The Production of Inorganic Materials. Macmillan Publishing Company, New York. 53. ASM. 1991. Ceramics and Glasses, Vol. 4, Engineered Materials Handbook. ASM International, Metals Park, Ohio. 54. Hofmeister, W.H., et al. 1992. P. 47 in NASA Technical Memorandum 4349. 55. MacChesney, J.B., and D.J. DiGiovanni. 1990. Materials development of optical fiber. J. Am. Ceram. Soc., 73(12):3537-3559. 56. Brinker, C.J., A.J. Hurd, G.C. Frye, and C.S. Ashley. 1991. Fundamentals of sol-gel dip coating. Thin Solid Films, 201:97-98. 57. The references to Michael Weinberg's work in NASA Technical Memorandum 4349 (February 1992) summarize much of what has been done. 58. Simmons, J.H., D.R. Uhlmann, and G.H. Beall, eds. 1982. Nucleation and crystallization in glasses. Advances in Ceramics, Vol. 4. American Ceramic Society, Columbus, Ohio. *For a discussion of both methods, see section below, Growth of Inorganic Single Crystals. file:///C|/SSB_old_web/mgoppch6.htm (37 of 38) [6/18/2004 11:17:29 AM]