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  • 1
    facet.materialart.
    Unknown
    In:  CASI
    Publication Date: 2006-07-02
    Description: The interaction of heat, mass, and momentum transport in the floating zone method for growing single crystals from the melt is examined. Methods for detailed numerical simulation of the transport phenomena in a floating zone are developed. Results of the calculations are combined with experiments to determine the effects of solidification induced, surface tension driven, and buoyancy driven convection in establishing dopant redistribution in the melt and the roles of heat transfer in crystal and melt and melt/solid interface shape in determining crystal quality. State of the art finite element techniques were developed for calculating the influence of natural convection in the melt on the shape of a melt/crystal interface and dopant segregation in the crystal. These techniques are demonstrated for solidification by the Bridgman technique. Numerical techniques are developed that calculate the shapes of both the melt/solid and melt/gas interfaces simultaneously with the thermal fields in melt and solid. Models for the fluid flows due to the rotation of the feed and crystal rods are completed and the effects of these flows on dopant segregation are studied, especially in the case of zones longer than can be achieved on Earth.
    Keywords: ASTRONAUTICS (GENERAL)
    Type: NASA. Marshall Space Flight Center Float Zone Workshop; p 175-199
    Format: text
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  • 2
    Publication Date: 2019-06-28
    Description: This research program is composed of theoretical and experimental studies of solutal convection and its effects on crystal growth and segregation in binary and pseudo-binary systems with large liquidus-solidus separation. The theoretical program has resulted in the development of a transient simulation of the growth of concentrated alloys in microgravity and has been used to study the dynamics of GeSi, PbSnTe, HgCdTe growth. Finite element analysis has also been developed for studying the interactions of buoyancy-driven convection in the melt caused by temperature and concentration variations with melt-solid interface morphology and alloy segregation in the grown crystal. These calculations are being compared directly to experimental measurements of segregation and interface shape for GeSi growth and GaGe growth. The results for SiGe indicate that careful design of the thermal system can result in almost diffusion-controlled growth on Earth because of the stablizing influence of the concentration gradient ahead of the solidification front.
    Keywords: ASTRONAUTICS (GENERAL)
    Type: NASA, Washington Microgravity Sci. and Appl. Program Tasks; p 11
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  • 3
    Publication Date: 2019-06-28
    Description: Research is directed at development of a detailed model of mass and heat transfer and chemical reaction in the pyrolysis of silane for the growth of thin amorphous silicon substrates incorporating laser heating of the gas phase above the film. The model will be the basis for evaluation of the relative importances of the decomposition of SiH4 in the vapor phase, mass transfer of the intermediate species, e.g., SiH2, and the evolution of hydrogen gas. Plans are also underway for developing a model for homogeneous nucleation of Si in the vapor phase to model the rate limitations observed at high gas-phase temperatures and high partial pressures of silane. Work was concentrated on an almost one-dimensional model for the coupling of the CO2 laser beam for heat transfer of the vapor phase with simple kinetic models for SiH4 decomposition and subsequent absorption of Si vapor on the substrate. Mass transfer in the vapor phase is assumed to be solely by diffusion. The role of convection in the vapor phase caused by the large changes in density in and around the center of the laser beam will be analyzed to evaluate the potential of microgravity experiments for increasing the uniformity of the film and the deposition rate.
    Keywords: ASTRONAUTICS (GENERAL)
    Type: NASA, Washington Microgravity Sci. and Appl. Program Tasks; p 94
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  • 4
    Publication Date: 2019-06-28
    Description: Research is aimed at the fundamental understanding of the interactions of heat and mass transport and fluid mechanics in crystal growth from the melt, especially in how it pertains to experiments supported by the Microgravity Science and Applications program. Emphasis has been on studies of the vertical Bridgman system, on small-scale floating zone systems proposed for space flight, and on quantitative prediction of nonlinear transitions in melt/solid interface morphologies leading to the formation of dendrites in directional solidification. The analysis of vertical Bridgman growth has lead to the complete numerical analysis of transport phenomena in directional solidification and to a detailed comparison of calculations to the GaGe growth experiments of Wang and Witt. Calculations show the central role of the ampoule material in determining the radial temperature gradients in the melt and thus, the intensity of convection. Results indicate that diffusion-controlled growth can not be achieved on Earth with conventional growth systems, but are feasible in microgravity
    Keywords: ASTRONAUTICS (GENERAL)
    Type: NASA, Washington Microgravity Sci. and Appl. Program Tasks; p 8-10
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  • 5
    Publication Date: 2019-07-13
    Description: The processing of materials in a low-gravity (low-g) or microgravity environment is investigated by NASA for scientific and commercial utilization, and process and product development. The elimination of gravity is shown to create unique materials, and improve ground-based processes, such as convection, sedimentation, buoyancy, and containerless processing. Commercial applications are discussed, including the manufacturing of silicon ribbon, turbine blades, and various pharmaceuticals in space. Commercial incentives and needs are also discussed, including the technical exchange agreement, in which NASA and a company agree to cooperate in the conduct and analysis of research programs. In addition to establishing and demonstrating scientific and technological precepts for analyzing and using low-g environments, NASA is establishing legal and management mechanisms to share cost and risk of early commercial ventures.
    Keywords: ASTRONAUTICS (GENERAL)
    Type: National Symposium and Exhibition Material and process applications - Land, sea, air, space; Apr 28, 1981 - Apr 30, 1981; Los Angeles, CA
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  • 6
    facet.materialart.
    Unknown
    In:  Other Sources
    Publication Date: 2019-07-13
    Description: The paper examines the scientific and commercial aspects of Materials Processing in the Space program. The elimination of gravity driven convection in molten materials can preclude undesirable stirring and mixing during crystal growth, and improve the casting of alloys and composites, chemical reactions, and the separation of biological materials. The elimination of hydrostatic pressure will allow alloy heat-treatment without distortion and growth of heavy crystals, such as thorium oxide, and containerless processing of liquids and molten materials. On the other hand, more sophisticated process control and diagnostic methods in sample preparation and temperature control must be developed, concluding that space made products of commercial interest are likely to be low volume, high value items.
    Keywords: ASTRONAUTICS (GENERAL)
    Type: AAS PAPER 79-239 , Annual Meeting; Oct 29, 1979 - Nov 01, 1979; Los Angeles, CA
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  • 7
    Publication Date: 2019-08-27
    Description: Goals for planetary exploration during the next decade include: (1) determine how our solar system formed, and understand whether planetary systems are a common phenomenon through out the cosmos; (2) explore the diverse changes that planets have undergone throughout their history and that take place at present, including those that distinguish Earth as a planet; (3) understand how life might have formed on Earth, whether life began anywhere else in the solar system, and whether life (including intelligent beings) might be a common cosmic phenomenon; (4) discover and investigate natural phenomena that occur under conditions not realizable in laboratories; (5) discover and inventory resources in the solar system that could be used by human civilizations in the future; and (6) make the solar system a part of the human experience in the same way that Earth is, and hence lay the groundwork for human expansion into the solar system in the coming century. The plan for solar system exploration is motivated by these goals as well as the following principle: The solar system exploration program will conduct flight programs and supporting data analysis and scientific research commensurate with United States leadership in space exploration. These programs and research must be of the highest scientific merit, they must be responsive to public excitement regarding planetary exploration, and they must contribute to larger national goals in technology and education. The result will be new information, which is accessible to the public, creates new knowledge, and stimulates programs of education to increase the base of scientific knowledge in the general public.
    Keywords: ASTRONAUTICS (GENERAL)
    Type: NASA-TM-111082 , NAS 1.15:111082
    Format: application/pdf
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