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At least 253 records · Page 14

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NASA NTRS1984

Predictions of nucleation theory applied to Ehrenfest thermodynamic transitions

A modified nucleation theory is used to determine a critical nucleus size and a critical activation-energy barrier for second-order Ehrenfest thermodynamic transitions as functions of the degree of undercooling, the interfacial energy, the heat-capacity difference, the specific volume of the transformed phase, and the equilibrium transition temperature. The customary approximations of nucleation theory are avoided by expanding the Gibbs free energy in a Maclaurin series and applying analytical thermodynamic expressions to evaluate the expansion coefficients. Nonlinear correction terms for first-order-transition calculations are derived, and numerical results are presented graphically for water and polystyrene as examples of first-order and quasi-second-order transitions, respectively.

Barker, R. E., Jr.↗

NASA NTRS1984

Impurity effects in dendritic solidification

A quantitative calculation of growth rates and tip radii is presented for dendrites growing in undercooled dilute solutions. Included in the calculations are the capillary corrections to the steady-state Ivantsov needle-crystal solutions. The results show good agreement with available experimental data and support the validity of the marginal-stability theory of dendritic growth.

Karma, A.↗

NASA NTRS1984

Effect of applied magnetic fields during directional solidification of eutectic Bi-Mn

Samples of rod eutectics Bi/MnBi were directionally solidified in a growth-up Bridgman-Stockbarger configuration in the presence of a transverse magnetic field up to 3 kg to determine whether gravity-driven convective effects could be reduced or eliminated. The experiments were carried out over a range of furnace velocities, V, of 0.2 to 50 cm per hour with a thermal gradient at the liquid-solid interface of 100 C/cm and 150 C/cm. Morphological, thermal and magnetic analyses were carried out on samples grown with and without an applied magnetic field. For samples grown at V greater than 3 cm per hour in a transverse magnetic field, reduced mean rod diameter and interrod spacing occurred as well as undercooling and increased coercive strength. The data agreed with that obtained for low-g growth at 50 cm per hour and 30 cm per hour.

Decarlo, J. L.↗

NASA NTRS1985

Pattern selection in a boundary-layer model of dendritic growth in the presence of impurities

Presently analyzed, in the context of a boundary-layer model, is the problem of pattern selection in dendritic growth in a situation where impurities are present in the undercooled liquid. It is found that the tip-velocity selection criterion that has been proposed recently for the geometrical model and the boundary-layer model of a pure substance can be extended, in a nontrivial way, to this more complex situation where two coupled diffusion fields (temperature and solute) determine the interface dynamics. This model predicts a sharp enhancement of tip velocity in good qualitative agreement with experiment. This agreement is consistent with the conjecture that a solvability condition can be used to determine the operating point of the dendrite in the full nonlocal problem.

Karma, A.↗

NASA NTRS1986

Fundamentals of Alloy Solidification

Potential benefits of microgravity processing discussed. Symposium held at Lewis Research Center in September of 1984 on subject of microgravity and some basic metallurgical factors involved in production of metals. General metallurgical areas of interest were metal solidification and processing. Five specific areas covered included undercooling of liquids, porosity, microstructure, solidification, and segregation. Theme of symposium: Possible benefits of microgravity processing and beneficial effects on industry processing. Information readily lends itself to inclusion in educational programs at college level.

Harf, F.↗

NASA NTRS1985

Solidification Fundamentals

The fundamental aspects of a variety of solidification phenomena were studied, taking advantage of the unique opportunities offered by the microgravity environment of space. Containerless processing and an understanding of the role of fluids induced by gravitational effects are the two major objectives justifying an experimental program in reduced gravity. The solidification phenomena which are likely to benefit are: macro and microsegregation, columnar-equiaxed transition, pore formation, and undercooling.

Laxmanan, V.↗

NASA NTRS1985

Modification of the Electromagnetic Levitator (EML) Hardware

The goals of this project are: (1) to study the upgrade requirements and approaches needed for incorporation of an Electromagnetic Levitator (EML) into the shuttle orbiter, (2) to work with members of the Electromagnetic Containerless Processing science working group (SWG) to define future experiments for the EML, and (3) to assist these investigators in further development of ground-based experiment techniques to the limits possible in the terrestrial gravitational environment. Present work is directed toward: (1) upgrading the EML flight apparatus to meet requirements of safety and integration interfaces with the MSL orbiter carrier, (2) development of new experiment components required to carry out approved experiments in undercooled solidification and associated fluid flow studies directed by MIT, and (3) construction, test, qualification and integration assistance for the EML MSL flight package.

Frost, R. T.↗

NASA NTRS1985

Acoustic Levitation Containerless Processing

This research program consists of the development of acoustic containerless processing systems with applications in the areas of research in material sciences, as well as the production of new materials, solid forms with novel and unusual microstructures, fusion target spheres, and improved optical fibers. Efforts have been focused on the containerless processing at high temperatures for producing new kinds of glasses. Also, some development has occurred in the areas of containerlessly supporting liquids at room temperature, with applications in studies of fluid dynamics, potential undercooling of liquids, etc. The high temperature area holds the greatest promise for producing new kinds of glasses and ceramics, new alloys, and possibly unusual structural shapes, such as very uniform hollow glass shells for fusion target applications. High temperature acoustic levitation required for containerless processing has been demonstrated in low-g environments as well as in ground-based experiments. Future activities include continued development of the signals axis acoustic levitator.

Whymark, R. R.↗

NASA NTRS1985

A study of the applicability of nucleation theory to quasi-thermodynamic transformations of second and higher Ehrenfest-order

Transient and steady-state phenomena in temperature, stress, and electric, field intensity in ferroelectric polymers were investigated. The application and extension of the theory in the primary stage to the polarization domain nucleation and growth in ferroelectric polymers were developed. The kinetics of this growth were investigated. Expressions describing nucleation under the influence of an electric field were found through the expansion of the Gibbs' free energy in a Maclaurin series. The series was expanded in the electric field strength rather than the degree of undercooling. The resulting expressions were manipulated and applied to the case of nucleation of polarized domains in ferroelectric polymers. The kinetics of the nucleation and growth of polarized domains are also investigated. This was accomplished through the modification of the Johnson-Mehl-Avrami treatment of crystallization kinetics to be applicable to the growth of polarization domains in ferroelectric materials.

Barker, R. E., Jr.↗

NASA NTRS1984

Study of eutectic formation

A theory was developed for the influence of convection on the microstructure of lamellar eutectics. Convection is predicted to produce a coarser microstructure, especially at low freezing rates and large volume fractions of the minority phase. Similary convection is predicted to lower the interfacial undercooling, especially at low freezing rates. Experiments using spin-up/spin-down were performed on the Mn-Bi eutectic. This stirring had a dramatic effect on the microstructure, not only making it coarser but at low freezing rates also changing the morphology of the MnBi. The coarsering persisted to moderately high freezing rates. At the lowest freezing rate, vigorous stirring caused the MnBi to be concentrated at the periphery of the ingot and absent along the center. Progress was made on developing a technique for revealing the three-dimensional microstructure of the MnBi eutectic by time-lapse videotaping while etching.

Wilcox, W. R.↗

NASA NTRS1985

Compact acoustic levitation device for studies in fluid dynamics and material science in the laboratory and microgravity

An ultrasonic levitation device operable in both ordinary ground-based as well as in potential space-borne laboratories is described together with its various applications in the fields of fluid dynamics, material science, and light scattering. Some of the phenomena which can be studied by this instrument include surface waves on freely suspended liquids, the variations of the surface tension with temperature and contamination, the deep undercooling of materials with the temperature variations of their density and viscosity, and finally some of the optical diffraction properties of transparent substances.

Trinh, E. H.↗

NASA NTRS1986

Melt segregation in plagioclase-poikilitic mesosiderites

The Budalan and Mincy mesosiderites contain a poikilitic-plagioclase matrix with orthopyroxene chadacrysts and interstitial-subophitic inverted pigeonite. Orthopyroxene chadacrysts in both mesosiderites are uniformly more aluminous than orthopyroxene clasts, suggesting that they were not derived from clasts by metamorphism. Interstitial inverted pigeonite is more ferroan than adjacent orthopyroxene in the matrix, consistent with the crystallization of a melt with the sequence orthopyroxene followed by pigeonite. The magnesium chadcrysts in Mincy could not have formed from a melt in equilibrium with the clasts but could have crystallized from impact melt. The most Mg chadacrysts are enclosed in large reversely zoned plagioclase crystals as a result of the undercooling in melt-lacking plagioclase clasts and associated nuclei. Mincy contains both plagioclase-poor and plagioclase-rich regions, explained by a separation of silicate melt into pools. Reckling Peak A80258, a plagioclase-poikilitic mesosiderite with a very high chadacryst/plagioclase ratio, resembles Mincy material from which melt has been extracted. It is suggested that the origin of the plagioclase-poikilitic mesosiderites is impact melting of a metal-silicate mixture.

Hewins, R. H.↗

NASA NTRS1986

Cellular solidification of transparent monotectics

Understanding how liquid phase particles are engulfed or pushed during freezing of a monotectic is addressed. The additional complication is that the solid-liquid interface is nonplanar due to constitutional undercooling. Some evidence of particle pushing where the particles are the liquid phase of the montectic was already observed. Cellular freezing of the succinonitrile-glycerol system also occurred. Only a few compositions were tested at that time. The starting materials were not especially pure so that cellular interface observed was likely due to the presence of unkown impurities, the major portion of which was water. Topics addressed include: the effort of modeling the particle pushing process using the computer, establishing an apparatus for the determination of phase diagrams, and the measurement of the temperature gradients with a specimen which will solidify on the temperature gradient microscope stage.

Kaulker, W. F.↗

NASA NTRS1987

Apparatus ad method for quiescent containerless processing of high temperature metals and alloys in low gravity

The electron bombardment furnace consists of two confinement grid sections which may be moved and separated from each other. Inside the bombardment furnace, a tungsten element is enclosed. The material specimen is located within the tungsten element and grounded by means of grounded support wires connected to the respective sections of the furnace. The material specimen is supported on the ground wires and heated by electron bombardment until melt occurs. The furnace sections are separated in opposite directions causing the ground wires to pull from the surfaces of the specimen, leaving the specimen freely suspended in the process chamber without the action of external forces. The specimen remains in its melt condition in the processing chamber where it can be undercooled without external forces acting on the specimen, which would cause dynamic nucleation.

Robinson, Michael B.↗

NASA NTRS1986

Cellular and dendritic growth in a binary melt - A marginal stability approach

A simple model for the constrained growth of an array of cells or dendrites in a binary alloy in the presence of an imposed positive temperature gradient in the liquid is proposed, with the dendritic or cell tip radius calculated using the marginal stability criterion of Langer and Muller-Krumbhaar (1977). This approach, an approach adopting the ad hoc assumption of minimum undercooling at the cell or dendrite tip, and an approach based on the stability criterion of Trivedi (1980) all predict tip radii to within 30 percent of each other, and yield a simple relationship between the tip radius and the growth conditions. Good agreement is found between predictions and data obtained in a succinonitrile-acetone system, and under the present experimental conditions, the dendritic tip stability parameter value is found to be twice that obtained previously, possibly due to a transition in morphology from a cellular structure with just a few side branches, to a more fully developed dendritic structure.

Laxmanan, V.↗

NASA NTRS1986

Solvability conditions for dendritic growth in the boundary-layer model with capillary anisotropy

This paper is concerned primarily with the development of an analytic approach to the theory of steady-state velocity selection in the boundary-layer model of dendritic solidification. The two-dimensional version of this model with a fourfold crystalline anisotropy alpha in the surface tension is considered. By extending a WKB method introduced in an earlier paper, the alpha dependence of the selected growth rate is determined in the limit of small alpha; and this rate is studied for large alphas in the limit in which the dimensionless undercooling approaches unity. Portions of the paper are devoted to a reinterpretation of the mathematical structure of the solvability condition in problems of this kind.

Langer, J. S.↗

NASA NTRS1987

Processing materials in space - The history and the future

The development of materials processing in space, and some of the Soyuz, Apollo, Skylab, and Shuttle orbital materials experiments are reviewed. Consideration is given to protein crystal growth, electrophoresis, low-gravity isoelectric focusing, phase partitioning, a monodisperse latex reactor, semiconductor crystal growth, solution crystal growth, the triglycine sulfate experiment, vapor crystal growth experiments, the mercuric iodide experiment, electronic and electrooptical materials, organic thin films and crystalline solids, deep undercooling of metals and alloys, magnetic materials, immiscible materials, metal solidification research, reluctant glass-forming materials, and containerless glass formation. The space processing apparatuses and ground facilities, for materials processing are described. Future facilities for commercial research, development, and manufacturing in space are proposed.

Chassay, Roger↗

NASA NTRS1986

Glass formation and crystallization of high lead content PbO-B2O3 compositions

The glass-forming and undercooling ability of PbO-B2O3 melts in the 2PbO.B2O3 to 4PbO.B2O3 composition range were studied. The glass formation propensities were investigated as a function of cooling rate and sample mass. A qualitative investigation of the crystallization process(es) preventing glass formation was made, and it was concluded that under normal circumstances heterogeneous nucleation was the common occurrence. Hence, it was concluded that such compositions are prime candidates for containerless experiments aboard the Space Shuttle.

Weinberg, Michael C.↗