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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

An advanced microcomputer design for processing of semiconductor materials

In the Get Away Special 330 payload two germanium samples doped with gallium will be processed. The aim of the experiments is to create a planar solid/liquid interface, and to study the breakdown of this interface as the crystal growth rate increases. For the experiments a gradient furnace was designed which is heated by resistive heaters. Cooling is provided by circulating gas from the atmosphere in the cannister through cooling channels in the furnace. The temperature along the sample are measured by platinum/rhodium thermocouples. The furnace is controlled by a microcomputer system, based upon the processor 80C88. A data acquisition system is integrated into the system. In order to synchronize the different actions in time, a multitask manager is used.

Bjoern, L.↗
NASA NTRS1989

Efficient estimation of diffusion during dendritic solidification

A very efficient finite difference method has been developed to estimate the solute redistribution during solidification with diffusion in the solid. This method is validated by comparing the computed results with the results of an analytical solution derived by Kobayashi (1988) for the assumptions of a constant diffusion coefficient, a constant equilibrium partition ratio, and a parabolic rate of the advancement of the solid/liquid interface. The flexibility of the method is demonstrated by applying it to the dendritic solidification of a Pb-15 wt pct Sn alloy, for which the equilibrium partition ratio and diffusion coefficient vary substantially during solidification. The fraction eutectic at the end of solidification is also obtained by estimating the fraction solid, in greater resolution, where the concentration of solute in the interdendritic liquid reaches the eutectic composition of the alloy.

Yeum, K. S.↗
NASA NTRS1990

Hydrodynamic interactions in directional solidification

A binary liquid undergoes unidirectional solidification. The one-dimensional steady state is susceptible to morphological instability that causes the solid/liquid interface to change from a planar state to a cellular pattern. This paper examines the effects on this transition of volume-change convection, buoyancy-driven convection or forced flows. It emphasizes how flows alter stability limits, create scale and pattern changes in morphology, and create, through coupling, new instabilities. Emphasis is placed on the physical mechanisms of the interactions.

Davis, Stephen H.↗
NASA NTRS1990

Influence of convection on microstructure

In eutectic growth, as the solid phases grow they reject atoms to the liquid. This results in a variation of melt composition along the solid/liquid interface. In the past, mass transfer in eutectic solidification, in the absence of convection, was considered to be governed only by the diffusion induced by compositional gradients. However, mass transfer can also be generated by a temperature gradient. This is called thermotransport, thermomigration, thermal diffusion or the Soret effect. A theoretical model of the influence of the Soret effect on the growth of eutectic alloys is presented. A differential equation describing the compositional field near the interface during unidirectional solidification of a binary eutectic alloy was formulated by including the contributions of both compositional and thermal gradients in the liquid. A steady-state solution of the differential equation was obtained by applying appropriate boundary conditions and accounting for heat flow in the melt. Following that, the average interfacial composition was converted to a variation of undercooling at the interface, and consequently to microstructural parameters. The results obtained show that thermotransport can, under certain circumstances, be a parameter of paramount importance.

Wilcox, William R.↗
NASA NTRS1990

Influence of convection on rod spacing of eutectics

This paper describes a three-dimensional numerical model to study the influence of convection on the rod-like microstructure of an eutectic system. This model is based on a central finite difference approach. By applying it, the average concentration near the solid/liquid interface of a growing rod-like eutectic was determined for eutectic compositions C(e) of 0.03, 0.05, and 0.10. Following Jackson and Hunt (1966), the average interfacial composition was converted to a change of undercooling at the interface and, finally, to spacing between the rods. The change in rod spacing with increasing intensity of convection was calculated assuming the eutectic grows at minimum interfacial undercooling. It was confirmed that an increase in convection should coarsen the microstructure (i.e., the rod spacing increases with increasing intensity of stirring).

Caram, R.↗
NASA NTRS1990

Long-wave interactions in morphological and convective instabilities

A binary liquid that undergoes directional solidification is susceptible to morphological and solutal-convective instabilities that cause the solid/liquid interface to change from a planar to a cellular state. This paper gives derivations for those long-wave evolution equations that describe the weak couplings between convection and interface morphology and gives some analytical results obtainable from these.

Riley, D. S.↗
NASA NTRS1993

Thermal effects in rapid directional solidification - Linear theory

We study the morphological instability of the planar solid/liquid interface for a unidirectionally-solidified dilute binary mixture. We use a model developed by Boettinger et al. (1985, 1986), Aziz (1982), and Jackson et al. (1980), which allows for nonequilibrium effects on the interface through velocity-dependent segregation and attachment kinetics. Two types of instabilities are found in the linear stability analysis: (1) a cellular instability, and (2) an oscillatory instability driven by disequilibrium effects. Merchant and Davis (1990) characterized these instabilities subject to the frozen-temperature approximation (FTA). The present work relaxes the FTA by including the effects of latent heat and the full temperature distribution. Thermal effects slightly postpone the onset of the cellular instability but dramatically postpone the onset of the oscillatory instability; however, the absolute-stability conditions, at which at high speed the cellular and oscillatory instabilities are suppressed, remain unchanged from the FTA.

Huntley, D. A.↗
NASA NTRS1995

IJEMS: Iowa Joint Experiment in Microgravity Solidification

The Iowa Joint Experiment in Microgravity Solidification (IJEMS) is a cooperative effort between Iowa State University and the University of Iowa to study the formation of metal-matrix composites in a microgravity environment. Of particular interest is the interaction between the solid/liquid interface and the particles in suspension. The experiment is scheduled to fly on STS-69, Space Shuttle Endeavor on August 3, 1995. This project is unique in its heavy student participation and cooperation between the universities involved.

Bendle, John R.↗
NASA NTRS1996

Shear Stabilization of a Solidifying Front

The manufacturing of multi-component single crystals with uniform material properties is frequently hampered by the presence of morphological instabilities during the solidification. We discuss the influence of shear flows on the linear and nonlinear stability of the solid/liquid interface during the directional solidification of binary alloys. The flows are generated by nonplanar harmonic translations of the crystal parallel to the mean interface position. Oscillations with physically realizable amplitudes and frequencies are found useful for stabilization purposes.

Davis, Stephen H.↗
NASA NTRS1997

STS-87 Day 12 Highlights

On this twelfth day of the STS-87 mission, the flight crew, Cmdr. Kevin R. Kregel, Pilot Steven W. Lindsey, Mission Specialists Winston E. Scott, Kalpana Chawla, and Takao Doi, and Payload Specialist Leonid K. Kadenyuk continue to look at how plant growth and composite materials are affected by microgravity. The astronauts use the globebox facility to process samples for the Particle Engulfment and Pushing by a Solid/Liquid Interface experiment.

Source record↗
NASA NTRS1997

STS-87 Day 11 Highlights

On this eleventh day of the STS-87 mission, the flight crew, Cmdr. Kevin R. Kregel, Pilot Steven W. Lindsey, Mission Specialists Winston E. Scott, Kalpana Chawla, and Takao Doi, and Payload Specialist Leonid K. Kadenyuk continue to look at how plant growth and composite materials are affected by microgravity. The astronauts will use the Middeck Globebox Facility to process samples for the Particle Engulfment and Pushing by a Solid/Liquid Interface experiment. PEP is studying the formation of composite materials, attempting to accurately map the roles of gravity-induced convection and sedimentation in the process by removing the gravity from the equation.

Source record↗
NASA NTRS1998

Melt Convection Effects in the Bridgman Crystal Growth of an Alloy Under Microgravity Conditions

The solidification of a dilute bismuth-tin alloy under Bridgman crystal growth conditions is investigated in support of NASA's MEPHISTO space shuttle flight experiment. Computations are performed in two-dimensions with a uniform grid. The simulation includes the species-concentration, temperature and flow fields, as well as conduction in the ampoule. Fully transient simulations have been performed; no simplifying steady state approximations are used. Results are obtained under microgravity conditions for pure bismuth, and Bismuth-0.1 at.% Sn and Bi-1.0 at.% Sn alloys. The concentration dependence of the melting temperature is neglected; the solid/liquid interface temperature is assumed to be the melting temperature of pure bismuth for all cases studied. For the Bi-1.0 at.% Sn case the results indicate that a secondary convective cell, driven by solutal gradients, forms near the interface. The magnitude of the velocities in this cell increases with time; this causes increasing solute segregation at the liquid/solid interface.

Simpson James E.↗
NASA NTRS1999

An Experimental and Computational Study of Directional Solidification in Transparent Materials

An experimental and numerical study of the horizontal Bridgman growth of pure succinonitrile (SCN) and of a succinonitrile- 1.0 mol.% acetone alloy (SCN- 1.0 mol.% ACE) has been performed. Experiments at growth rates of 0, 2 and 40 micron/s were investigated. The solid/liquid interface was stable (non-dendritic and non-cellular); however, it was not flat. Rather, it was significantly distorted by the influence of convection in the melt and, for the growth cases, by the moving temperature boundary conditions along the ampoule. For the alloy, the interface was.determined to be unstable at growth rates greater than 2.8 micron/s, but stable for the no-growth and 2 micron/s growth cases. When compared to the pure SCN interface, the alloy interface forms closer to the cold zone, indicating that the melting temperature has been suppressed by the addition of the alloying element. Two-dimensional computer simulations were performed for the no-growth case for both the pure and alloy materials. These simulations indicate that a primary longitudinal convective cell is formed in the melt. The maximum magnitude of velocity was calculated to be 1.515 mm/s for pure SCN and 1.724 mm/s for the alloy. The interface shape predicted by the computer simulation agrees well with the experimentally determined shape for the pure SCN case. In ongoing work, numerical simulations of the process during growth conditions are being performed.

Simpson, James E.↗
NASA NTRS2001

Cellular Spacing Selection During the Directional Solidification of Binary Alloys. A Numerical Approach

The evolution of cellular solid/liquid interfaces from an initially unstable planar front was studied by means of a two-dimensional computer simulation. The developed numerical model makes use of an interface tracking procedure and has the capability to describe the dynamics of the interface morphology based on local changes of the thermodynamic conditions. The fundamental physics of this formulation was validated against experimental microgravity results and the predictions of the analytical linear stability theory. The performed simulations revealed that in certain conditions, based on a competitive growth mechanism, an interface could become unstable to random perturbations of infinitesimal amplitude even at wavelengths smaller than the neutral wavelength, lambda(sub c), predicted by the linear stability theory. Furthermore, two main stages of spacing selection have been identified. In the first stage, at low perturbations amplitude, the selection mechanism is driven by the maximum growth rate of instabilities while in the second stage the selection is influenced by nonlinear phenomena caused by the interactions between the neighboring cells. Comparison of these predictions with other existing theories of pattern formation and experimental results will be discussed.

Catalina, Adrian V.↗
NASA NTRS2001

A New Analytical Approach to Predict Spacing Selection in Lamellar and Rod Eutectic Systems

In this paper, we reexamine the Jackson and Hunt (JH) theory and relax the assumption of isothermal solid/liquid interface(SLI) used in their treatment. A modification of the term B. in the expression of the solute concentration profile is also proposed. Based on the predictions of this modified theory the traditional definitions of regular and irregular eutectics are discussed. For regular eutectics the new model identifies a range of spacing within the limits defined by the minimum undercooling of the alpha and beta phase. For the irregular Al-Si eutectic system in particular we identified two different spacing selection mechanisnis: a) for a particular growth rate, a nearly isothermal interface can be achieved at a unique minimum spacing lambda(sub I); b) the average spacing in the microstructure (lambda(sub av) > lambda(sub I)) is essentially dictated by the undercooling of the faceted phase. Based on the modified theoretical model a semi-empirical expression has been developed to account for the influence of the temperature gradient. The theoretical calculations have been found to be in good agreement with the published experimental measurements.

Catalina, Adrian V.↗
NASA NTRS2001

Shuttle Flight Experiment on USMP-4: In Situ Monitoring of Crystal Growth Using MEPHISTO

This reports on the MEPHISTO-4 experiment on the Space Shuttle Columbia, STS-87, November 19-December 5, 1997. Involved were NASA; the University of Florida at Gainesville; groups from France that developed and built the furnace; the National Institute of Standards and Technology; The University of New South Wales, Australia; and Purdue University. This was a solidification study in which three long rods of Bismuth- 1 at.% Tin were directionally solidified. The goals were to solidify in an environment free of natural convection; to determine the relationship among solidification growth velocity, growth mode, and temperature; and determine the diffusivity of Sn in Bi. The flight samples grew with a planar solid/liquid interface at velocities less than 3.4 gm/s, and cellular growth was present at velocities greater than 6.7 um/s; grain orientation influenced the planar to cellular transition. The temperature gradient in the liquid was 204 K/cm. The s/l interface was flat with slight concavity. Diffusion-dominated conditions were present during MEPHISTO-4. The Seebeck technique was used to determine the s/I interface temperature during growth, however, to date, analysis of the Seebeck results has not yielded a reliable measurement of the interface temperature. The partition coefficient for Bi alloyed with Sn was measured, k = 0.029.

Abbaschian, Reza↗
NASA NTRS2002

A New Analytical Approach to Predict Spacing Selection in Lamellar and Rod Eutectic Systems

The Jackson and Hunt (JH) theory has been modified to relax the assumption of isothermal solid/liquid interface(SLI) used in their treatment. Based on the predictions of this modified theory the traditional definitions of regular and irregular eutectics are revised. For regular eutectics the new model identifies a range of spacing within the limits defined by the minimum undercooling of the alpha and beta phase. For the irregular Al-Si eutectic system two different spacing selection mechanisms were identified: a) for a particular growth rate, a nearly isothermal interface can be achieved at a unique minimum spacing lambda(sub I); b) the average spacing (lambda(sub av) greater than lambda(sub I)) is essentially dictated by the undercooling of the faceted phase. Based on the modified theoretical model a semiempirical expression has been developed to account for the influence of the temperature gradient, which is dominant in the irregular Al-Si system. The behavior of the Fe-Fe3C eutectic is also discussed The theoretical calculations have been found to be in good agreement with the published experimental measurements.

Catalina, Adrian V.↗
NASA NTRS2004

Direct Observation of Controlled Melting and Resolidification of Succinonitrile Mixtures in a Microgravity Environment

In support of the Pore Formation and Mobility Investigation (PFMI) direct observation of experiments on the controlled melting and subsequent resolidification of succinonitrile were conducted in the glovebox facility (GBX) of the International Space Station (ISS). Samples were prepared on ground by filling glass tubes, 1 cm ID and approximately 30 cm in length, with pure succinonitrile (SCN) and SCN-Water mixtures under 450 millibar of nitrogen. Experimental processing parameters of temperature gradient and translation speed, as well as camera settings, were remotely monitored and manipulated from the ground Telescience Center (TSC) at the Marshall Space Flight Center. Sample temperatures are monitored by six in situ thermocouples. Real time visualization during melt back revealed bubbles of different sizes initiating at the solid/liquid interface, their release, interactions, and movement into the temperature field ahead of them. Subsequent re-solidification examined planar interface breakdown and the transition to steady-state dendritic growth. A preliminary analysis of the observed phenomena and its implication to future microgravity experiments is presented and discussed.

Grugel, R. N.↗