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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Study of ceramic products and processing techniques in space

An analysis of the solidification kinetics of beta alumina in a zero-gravity environment was carried out, using computer-simulation techniques, in order to assess the feasibility of producing high-quality single crystals of this material in space. The two coupled transport processes included were movement of the solid-liquid interface and diffusion of sodium atoms in the melt. Results of the simulation indicate that appreciable crystal-growth rates can be attained in space. Considerations were also made of the advantages offered by high-quality single crystals of beta alumina for use as a solid electrolyte; these clearly indicate that space-grown materials are superior in many respects to analogous terrestrially-grown crystals. Likewise, economic considerations, based on the rapidly expanding technological applications for beta alumina and related fast ionic conductors, reveal that the many superior qualities of space-grown material justify the added expense and experimental detail associated with space processing.

Markworth, A. J.↗
NASA NTRS1975

Analysis of liquid metal embrittlement from a bond energy viewpoint

Absorption induced embrittlement of solid metals by certain liquid metals is analyzed through an Engel-Brewer calculation of the solid-liquid interaction energy, and of the effect of the latter in reducing fracture surface energy. The reduction in fracture surface energy is estimated by comparison of the electronic contribution to the solid-liquid interaction energy with solid-solid bond energy for some 40 liquid-solid couples. Regular solution theory is used to estimate mutual solubility as the relative difference in parameter values. Embrittlement can be predicted by using reduction in fracture surface energy and solubility parameter difference as critical variables. The effect of solute additions to the liquid on the degree of embrittlement is interpreted via the same two variables; the principal effect of solutes is to modify solubility relationships at the solid-liquid interface.

Kelley, M. J.↗
NASA NTRS1976

RF shaping of silicon ribbon

Electromagnetic force generated by radiofrequency coil is used to shape molten silicon. Shaping coil surrounds melt near solid-liquid interface and induces current in surface region of melt nearly equal to but opposite coil current.

Pelhank, D. A.↗
NASA NTRS1976

Analysis of surface tension driven flow in floating zone melting

Surface tension driven flow in a cylindrical melt suspended between two rods was investigated by numerical solution of the steady state differential equations for heat and momentum transfer. Radiation heating and electron beam heating were considered approximately. For small values of the driving force, one rotating ring was formed in the top half of the zone, and its mirror image in the bottom half. At larger driving forces, secondary cells form which probably would undergo oscillatory motion. The influence of Prandtl number, zone movement, and buoyancy on the convection was also studied. The primary resistance to mass transfer in the laminar regime was in the center of the zone rather than at the solid-liquid interfaces.

Chang, C. E.↗
NASA NTRS1982

Investigation of compositional segregation during unidirectional solidification of solid solution semiconducting alloys

Compositional segregation of solid solution semiconducting alloys in the radial direction during unidirectional solidification was investigated by calculating the effect of a curved solid liquid interface on solute concentration at the interface on the solid. The formulation is similar to that given by Coriell, Boisvert, Rehm, and Sekerka except that a more realistic cylindrical coordinate system which is moving with the interface is used. Analytical results were obtained for very small and very large values of beta with beta = VR/D, where V is the velocity of solidification, R the radius of the specimen, and D the diffusivity of solute in the liquid. For both very small and very large beta, the solute concentration at the interface in the solid C(si) approaches C(o) (original solute concentration) i.e., the deviation is minimal. The maximum deviation of C(si) from C(o) occurs for some intermediate value of beta.

Wang, J. C.↗
NASA NTRS1983

Studies of convection in a solidifying binary mixture at reduced gravity

A great deal of interest was generated recently in the possibility of producing new materials in the reduced gravity environment provided during the forthcoming missions of Spacelab. The range of possibilities extend from producing large crystals of uniform properties to manufacturing materials with unique properties. Most of these processes involve the solidification of materials from the liquid state. Convective motions within the liquid during solidification can influence the local material composite and the shape of the solid-liquid interface which may result in solids with non-uniform properties and crystal defects. The microgravity environment of Spacelab is being viewed as one in which the buoyancy forces are eliminated so that convection driven by thermal gradients does occur, resulting in an improved solidification process. However, convection may occur for other reasons and whether convection is negligible or not during solidification constitutes processing in low-gravity environment. Little information exists presently on convection during solidification under such circumstances. A continuation of an analytical investigation into the nature of convective motion in a binary liquid layer due to surface tension forces during its solidification is reported. The onset of convection will be determined through a stability analysis which is described.

Antar, B. N.↗
NASA NTRS1984

Finite-difference fluid dynamics computer mathematical models for the design and interpretation of experiments for space flight

Numerical methods are used to design a spherical baroclinic flow model experiment of the large scale atmosphere flow for Spacelab. The dielectric simulation of radial gravity is only dominant in a low gravity environment. Computer codes are developed to study the processes at work in crystal growing systems which are also candidates for space flight. Crystalline materials rarely achieve their potential properties because of imperfections and component concentration variations. Thermosolutal convection in the liquid melt can be the cause of these imperfections. Such convection is suppressed in a low gravity environment. Two and three dimensional finite difference codes are being used for this work. Nonuniform meshes and implicit iterative methods are used. The iterative method for steady solutions is based on time stepping but has the options of different time steps for velocity and temperature and of a time step varying smoothly with position according to specified powers of the mesh spacings. This allows for more rapid convergence. The code being developed for the crystal growth studies allows for growth of the crystal as the solid-liquid interface. The moving interface is followed using finite differences; shape variations are permitted. For convenience in applying finite differences in the solid and liquid, a time dependent coordinate transformation is used to make this interface a coordinate surface.

Roberts, G. O.↗
NASA NTRS1984

The Effect of Solidification Rate on Morphological Stability

At low solidification rates, the criterion for the onset of morphological instability parallels closely the criterion of constitutional supercooling. At somewhat larger rates of solidification, however, the results of the perturbation theory of morphological instability differ significantly from the predictions of constitutional supercooling. This arises because the critical wave length for instability decreases as solidification rate increases and thus the effects of capillarity (solid-liquid surface tension) play a strong stabilizing role. This gives rise to the concept of absolute stability, according to which the system will always be stable for a sufficiently large rate of solidification. This enhanced stabilization by capillarity is present only so long as local equilibrium is maintained at the solid-liquid interface. If the interfacial temperature drops below its equilibrium value by an amount dependent on growth rate, oscillatory morphological instabilities can occur. The differences among these various stability criteria are illustrated by means of some simple two-dimensional diagrams that should supplant the conventional plots of (temperature gradient)/(growth rate) vs. alloy concentration.

Sekerka, R. F.↗
NASA NTRS1984

Low Angle Silicon Sheet Growth: a Review of Progress, Problems and Promise

High growth rates which are the key to a successful low cost silicon ribbon technology is discussed. The heat of fusion is conducted along the grown ribbon for some distance before it can be dissipated. The conduction path is limited to the cross sectional area of the ribbon, since the solid-liquid interface is perpendicular to the pull direction. Horizontal ribbon growth is achieved by providing an extended solidification interface, parallel within a few degrees, to the pull direction. The heat of fusion need only be conducted through the thickness of the ribbon and is dissipated from the upper surface which comprises an area equal to the interface. Growth of up to 15cm wide ribbon and continuous growth of ribbons as long as 120 meters is demonstrated. Areal productivity as high as 2.7 sq/m/hr is shown for a single 15 cm ribbon, while typical values of approximately 1 sq/m/hr are obtained in the growth of 5 cm ribbon.

H. E. Bates↗
NASA NTRS1984

Hot stage and sample cell design for the solidification of transparent materials with and without forced convection

A temperature gradient stage design for optical microscopy is described. Exceptional thermal stability is the major feature. The stage is used to study crystal growth phenomena occurring at the solid-liquid interface. The apparatus is designed to use transparent organic solutions as models for the study of metal-like solidification. The stage provides a controlled thermal environment for unidirectional solidification of low melting temperature materials. Freezing rate is regulated by mechanically sliding in the stage a thin glass cell containing the materials being studied. Two cell assemblies are described. One type is used for convection-free and the other for controlled forced convection studies of the solidification interface.

Kaukler, W. F.↗
NASA NTRS1984

Electrochemical etching of the semiconductor lead tin telluride

Convective currents during the Bridgman growth of a compound semiconductor lead to temperature fluctuations at the solid-liquid interface. These temperature fluctuations in turn lead to microscopic compositional variations in the solid. Electrochemical principles have been applied to develop three etches which delineate the variations in the compound semiconductor, lead tin telluride, and allow optical studies of the growth kinetics of this material. Use of these etches has shown periodic lines during the initial growth, with indications of oscillatory instabilities developing in later stages of growth, and, finally, complete breakdown of the interface.

Barber, P. G.↗
NASA NTRS1985

The Influence of Low Frequency Mechanical Vibrations on the Growth of Single Crystals

The optimum conditions for crystal growth are usually achieved either by suppressing convective fluid flows (e.g., by the use of a low-gravity environment) or by over-riding thermal and solutal convection by the use of a strong stirring action. A novel stirring technique has been developed which involves subjecting a vertical crucible to a circle in a horizontal plane (without rotation). Use of an amplitude of 3 mm at a frequency of approx 6 Hz produced complete mixing of a non-uniform aqueous liquid in a few seconds. The mixing action involved the downward flow of liquid in the outer annulus of the liquid, driven by surface waves. When the downward flowing liquid reaches the bottom of the crucible, it is reflected in a central, upward flowing spiral. This flow pattern should be beneficial for crystal growth by the Bridgman method since it will sweep impurities away from the walls and produce a more convex solid-liquid interface. Initial attempts to apply the new stirring technique to CdTe crystal growth did not show significant improvement in the number of crystals nucleated, but the interface shape appeared to be close to that predicted.

Feigelson, R. S.↗
NASA NTRS1985

Mephisto: Interfacial Destabilization in Metal Alloys

The destabilizing mechanisms at a solidification interface were studied to obtain information on the kinetics and morphologies in the transient and steady state, and to separate the influences of liquid phase instabilities from interfacial instabilities. A differential seebeck voltage measurements technique was developed to provide a continuous record of the solid-liquid interface temperature as the solidification rate is varied to determine the kinetic coefficients. Signal processing and noise suppression techniques allow nonovolt precision which corresponds to mK accuracy for the interfacial temperature.

Favier, J. J.↗
NASA NTRS1984

The effect of anisotropic crystal-melt surface tension on grain boundary groove morphology

The shape of a stationary solid-liquid interface in a temperature gradient near a grain boundary in a pure material is calculated for anisotropic crystal-melt surface tension and equal thermal conductivities of crystal and melt. Results are compared with those for the well-known problem of the two-dimensional equilibrium shape of a crystal. For small anisotropy, the resulting interface shapes have continuously turning tangents but differ in detail from the grain boundary groove shapes that have been calculated for isotropic surface tension. For larger anisotropy, the interface shapes have discontinuities in slope as a result of missing orientations.

Voorhees, P. W.↗
NASA NTRS1984

Solidification behavior of low and high thermal conductivity materials in a Bridgman-Stockbarger furnace

The solid-liquid interface position and the temperature gradients in both the solid and liquid at the interface have been studied in a modified Bridgman-Stockbarger crystal growth furnace. These crystal growth factors have been studied as a function of ampoule translation rate, materials properties, and the size and temperature of a small auxiliary heater placed at the edge of the furnace hot zone. It has been found that the interface position with respect to a furnace reference point is essentially constant during a run for a low thermal conductivity material whereas the interface position changes continuously during a run with high thermal conductivity material. However, the ampoule translation rate and auxiliary heater conditions produce interface position changes in both high and low thermal conductivity materials.

Ejim, T. I.↗
NASA NTRS1984

Thermal convection during Bridgman crystal growth

Numerical experiments are used to study thermally driven flows which occur during vertical Bridgman crystal growth of a single component fluid. The solid-liquid interface was specified as parabolic and flow patterns were calculated for various insulation thicknesses, Grashof, Prandtl, and Biot numbers. When the melt is on top and the gravity vector is axially downward it is shown that flow persists as long as a radial temperature gradient is present. If the interface is convex, as viewed from the liquid, a single cell is observed. A concave interface exhibits multiple counterrotating cells. The insulation thickness and Grashof, Prandtl, and Biot numbers influence the flow in a quantitative manner.

Carlson, Frederick M.↗
NASA NTRS1974

Enzyme activity in terrestrial soil in relation to exploration of the Martian surface

Sensitive tests for the detection of extracellular enzyme activity in Martian soil was investigated using simulated Martian soil. Enzyme action at solid-liquid water interfaces and at low humidity were studied, and a kinetic scheme was devised and tested based on the growth of microorganisms and the oxidation of ammonium nitrite.

Mclaren, A. D.↗
NASA NTRS1972

Critical levitation loci for spheres on cryogenic fluids.

The conditions which allow a fluid to support a sphere having a higher specific gravity than its own are investigated. Three basic parameters which together define the maximum floating conditions are considered, including the Bond number, the wetting angle of the interface, and the ratio of solid-liquid specific gravities. The Bond number represents the ratio of buoyancy to surface tension forces. An analysis of the forces at the sphere-liquid interface is conducted to determine the optimum levitation loci, that is, the conditions permitting flotation of a maximum density sphere. Data for glycerine, carbon tetrachloride, and water spheres floating on a liquid nitrogen surface appear to be in good agreement with the analysis. Data for Teflon spheres on water also appear to be in agreement with the analytical results.

Hendricks, R. C.↗