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At least 127 records · Page 7

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NASA NTRS1982

The influence of acceleration forces on dendritic growth and grain structure

The results of experiments on the tin-15 wt pct lead system are presented, showing the effects on microstructure of solidification in the presence of acceleration forces from 0.0001 to 5 g for three cooling rates. An increase in the acceleration level is shown to drive fluid flow and cause dendrite remelting, fragmentation, and macrosegregation. The cooling rate impacts the final structure through its control of dendrite arm spacings and permeability to fluid flow. At the low (0.0001 g) acceleration, dendrite arm spacings deviated from the predicted relationship to cooling rate. An explanation for this anomaly is given which considers the temperature and concentration gradients in the low-gravity environment.

Johnston, M. H.↗

NASA NTRS1983

Solidifying Bi/MnBi at Low Gravity

Directional solidification of Bi/MnBi magnetic alloy under low gravity enhances magnetic properties. Magnetic properties of low-gravity Bi/MnBi alloy make attractive material for use in motors and other small electrical and electronic components.

Drauch, J.↗

NASA NTRS1982

Mercuric iodide (HgI2) growth for nuclear detectors

The purpose of this investigation is to grow more-perfect mercuric iodide crystals in a low-gravity environment by taking advantage of diffusion-controlled growth conditions and by avoiding the problem of strain dislocations produced by the crystal's weight. This crystal has considerable practical importance as a sensitive gamma-ray detector and energy spectrometer that can operate at ambient temperature, as compared to presently available detectors that must be cooled to near liquid nitrogen temperatures. However, the performance of mercuric iodide crystals only rarely approaches the expected performance, presumably because some of the free electrical charges produced within the crystal are not collected at the electrodes, but instead remain trapped or immobilized at crystal defects. An efficient high atomic number semiconductor detector capable of operating at room temperature utilizing single HgI2 crystals offers a greater potential than existing detector technology.

Schnepple, W.↗

NASA NTRS1983

Preliminary science report on the directional solidification of hypereutectic cast iron during KC-135 low-G maneuvers

An ADSS-P directional solidification furnace was reconfigured for operation on the KC-135 low-g aircraft. The system offers many advantages over quench ingot methods for study of the effects of sedimentation and convection on alloy formation. The directional sodification furnace system was first flown during the September 1982 series of flights. The microstructure of the hypereutectic cast iron sample solidified on one of these flights suggests a low-g effect on graphite morphology. Further experiments are needed to ascertain that this effect is due to low-gravity and to deduce which of the possible mechanisms is responsible for it.

Curreri, P. A.↗

NASA NTRS1983

Method and apparatus for supercooling and solidifying substances

An enclosure provides a containerless environment in which a sample specimen is positioned. The specimen is heated in the containerless environment, and the specimen melt is dropped through the tube in which it cools by radiation. The tube is alternatively backfilled with an inert gas whereby the specimen melt cools by both radiation and convection during its free fall. During the free fall, the sample is in a containerless, low-gravity environment which enhances supercooling in the sample and prevents sedimentation and thermal convection influences. The sample continues to supercool until nucleation occurs which is detected by silicon photovoltaic detectors. The sample solidifies after nucleation and becomes completely solid before entering the detachable catcher. The amount of supercooling of the specimen can be measured by knowing the cooling ratio and determining the time for nucleation to occur.

Lacy, L. L.↗

NASA NTRS1983

Mass loss, levitation, accretion, and the sharp-lined features in hot white dwarfs

A study has been conducted of eight white dwarfs, including seven DA and one He-rich types. The study is based on high-resolution observations conducted with the aid of the International Ultraviolet Explorer. Four of the dwarfs show features related to heavy elements which are not interstellar in origin. It is tentatively suggested that, at least in the hottest low-gravity DA white dwarfs, the observed narrow-lined features are formed in expanding halos or winds associated with the white dwarfs. Theoretically, stable white dwarf halos should actually be coronae with temperatures in excess of 1,000,000 K. However, the observed narrow-lined features do not suggest such high temperatures. The observed radial velocities suggest weak stellar winds in two hot white dwarfs, namely, G191-B2B and 2111+49. It is tentatively proposed that radiative levitation can explain the appearance of the observed metallic lines in the hot DA white dwarfs.

Bruhweiler, F. C.↗

NASA NTRS1982

Influence of gravity driven convection on the directional solidification of Bi/MnBi eutectic composites

The role of gravity on Bridgman-Stockharger directional solidification of eutectic Bi/MnBi has been studied in reduced gravity aboard NASA sounding rocket SPAR flight experiments and contrasted with normal gravity investigations. The directional solidification of eutectic Bi/MnBi results in a low volume fraction, faceted/nonfaceted aligned rod eutectic whose MnBi rod size, interrod spacing, thermal and magnetic properties are sensitive functions of solidification processing conditions. The morphology of the low-gravity samples showed striking differences compared with identically processed, normal gravity samples grown in the same apparatus. The MnBi rod diameter and interrod spacing distributions were significantly smaller, approximately 50 percent, for the low gravity samples compared with identically processed one gravity samples. Accompanying the smaller MnBi rod diameters observed in the flight samples, was an increase in permanent magnet properties which reached greater than 97 percent of the theoretical maximum. Gravitationally induced thermal instabilities in one-gravity which result in irregular interface movement and associated difficulty of the faceted MnBi phase to branch are suggested to explain the morphological differences between one and low gravity solidification.

Pirich, R. G.↗

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

Separation processes during binary monotectic alloy production

Observation of microgravity solidification processes indicates that outside of sedimentation, at least two other important effects can separate the phases: critical-point wetting and spreading; and thermal migration of second-phase droplets due to interfacial tension gradients. It is difficult to study these surface tension effects while in a unit gravity field. In order to investigate the processes occurring over a temperature range, i.e., between a consolute point and the monotectic temperature, it is necessary to use a low-gravity environment. The MSFC drop tube (and tower), the ballistic trajectory KC-135 airplane, and the Space Shuttle are ideal facilities to aid formation and testing of hypotheses. Much of the early work in this area focuses on transparent materials so that process dynamics may be studied by optical techniques such as photography for viewing macro-processes; holography for studying diffusional growth; spinodal decomposition and coalescence; ellipsometry for surface wetting and spreading effects; and interferometry and spectroscopy for small-scale spatial resolution of concentration profiles.

Frazier, D. O.↗

NASA NTRS1984

Computational modeling of jet induced mixing of cryogenic propellants in low-G

The SOLA-ECLIPSE Code is being developed to enable computational prediction of jet induced mixing in cryogenic propellant tanks in a low-gravity environment. Velocity fields, predicted for scale model tanks, are presented which compare favorably with the available experimental data. A full scale liquid hydrogen tank for a typical Orbit Transfer Vehicle is analyzed with the conclusion that coupling an axial mixing jet with a thermodynamic vent system appears to be a viable concept for the control of tank pressure.

Hochstein, J. I.↗

NASA NTRS1984

Space processing applications rocket project. SPAR 8

The Space Processing Applications Rocket Project (SPAR) VIII Final Report contains the engineering report prepared at the Marshall Space Flight Center (MSFC) as well as the three reports from the principal investigators. These reports also describe pertinent portions of ground-based research leading to the ultimate selection of the flight sample composition, including design, fabrication, and testing, all of which are expected to contribute immeasurably to an improved comprehension of materials processing in space. This technical memorandum is directed entirely to the payload manifest flown in the eighth of a series of SPAR flights conducted at the White Sands Missile Range (WSMR) and includes the experiments entitled Glass Formation Experiment SPAR 74-42/1R, Glass Fining Experiment in Low-Gravity SPAR 77-13/1, and Dynamics of Liquid Bubbles SPAR Experiment 77-18/2.

Chassay, R. P.↗

NASA NTRS1984

Optical observations of unidirectional solidification and related fluid parameters in microgravity

This paper updates a previous paper in this journal on shadowgraph and schlieren studies of gravity related fluid flow during solidification. The research has now been extended to include interferometric data by the use of a Mach-Zehnder interferometer on a NASA KC-135 aircraft flying a parabolic trajectory. It has thus been possible to obtain quantitative temperature and concentration profiles in an ammonium chloride metal-model material being solidified in microgravity. The results indicate that the growth interface is more stable under low-gravity conditions than in a terrestrial environment.

Owen, R. B.↗

NASA NTRS1983

Materials processing in space; Proceedings of the Special Conference, Cincinnati, OH, May 4, 5, 1982

An international review of materials processing in space is provided, taking into account the NASA materials processing in space program, European activities in the field of microgravity research, and Japanese material processing tests in space. The subjects considered are related to materials processing in a microgravity environment, industrial and university applications and involvement, and challenges and prospectives of microgravity research in space. Attention is given to a review of the containerless processing technologies and facilities, glass research in space, bubble behavior in molten glass in a temperature gradient, semiconductor crystal growth and segregation problems on earth and space, seeded crystal growth of pseudobinary systems with large liquidus-solidus separation, avenues and incentives for commercial use of a low-gravity environment, and the Universities Space Research Association and its role in the materials processing in space program.

Dunbar, B. J.↗

NASA NTRS1983

Scientific results from the materials processing in space program

Scientific experiments conducted in a low-gravity environment make it possible to study effects, such as surface tension-driven flows, which are often masked by buoyancy-driven flows. A summary is provided of several early experiments which demonstrate that unwanted convective flows can be effectively suppressed in experiments conducted in orbiting spacecraft. Possible applications of low-G processing are discussed, taking into account Bridgman growth of solid solution alloy-type semiconductors, Bridgman growth of eutectic systems, and float-zone crystal growth. The control of macrosegregation and phase separation in castings is considered, giving attention also to containerless melting and solidification. A description of experiment facilities and opportunities is provided.

Naumann, R.↗

NASA NTRS1983

Optimization studies for the growth of Pb(1-x)Sn(x)Te in space

A study of the effects of convection on the quality of crystals grown by the Bridgman technique has been initiated. This study is to provide a basis for the utilization of the low-gravity environment furnished by the Space Shuttle in the development of crystals with a better quality than obtainable under terrestrial conditions. A series of ground-based studies has been started with the objective to optimize the results of Shuttle experiments. A description is presented of preliminary results of some of these studies. Attention is given to a thermo-solutal convection analysis, the development of a technique for interface demarcation, the growth of Pb(1-x)Sn(x)Te crystals, thermophysical property measurements, and solutal diffusion coefficients.

Crouch, R. K.↗

NASA NTRS1984

Computational modeling of jet induced mixing of cryogenic propellants in low-G

The SOLA-ECLIPSE Code is being developed to enable computational prediction of jet induced mixing in cryogenic propellant tanks in a low-gravity environment. Velocity fields, predicted for scale model tanks, are presented which compare favorably with the available experimental data. A full scale liquid hydrogen tank for a typical Orbit Transfer Vehicle is analyzed with the conclusion that coupling an axial mixing jet with a thermodynamic vent system appears tobe a viable concept for the control of tank pressure. Previously announced in STAR as N84-25000

Hochstein, J. I.↗

NASA NTRS1984

SPAR X Technical Report for Experiment 76-22 Directional Solidification of Magnetic Composites

The effects of gravity on Bridgman-Stockbarger directional solidification of off-eutectic Bi/MnBi were studied in reduced gravity aboard the SPAR X flight and compared to normal-gravity investigations and previous eutectic Bi/MnBi SPAR flight experiments. The directional solidification of off-eutectic Bi/MnBi results in either a dendritic structure connected with local cooperative growth or a coupled low volume fraction faceted/non faceted aligned rod eutectic whose Mn macrosegregation, MnBi rod size, interrod spacing, and thermal and magnetic properties are sensitive functions of the solidification processing conditions. Two hypoeutectic and two hypereutectic samples were solidified during 605 sec of furnace travel, with an initial 265 sec low-gravity interval. Comparison Earth-gravity samples were solidified in the same furance assembly under identical processing conditions. Macrosegregation in the low-g samples was consistent with a metastable increase in Mn solubility in the Bi matrix, in partial agreement with previous Bi/MnBi SPAR findings of MnBi volume reduction.

Bethin, J.↗

NASA NTRS1984

NASA OAST perspective

An advanced OTV is one of a number of advanced STS vehicles that the NASA OAST Space Systems Division Transportation Systems Office identified as candidates for future vehicle development. Vehicle requirements as well as technology needs and need dates were established and technology programs initiated to support those potential developments in a timely manner. It is assumed that advanced OTV will be space based and fully reusable for low cost operations, use aeroassist for return to low-Earth-orbit, and evolve to a man-rated system. The propulsion system will need to maintain high performance over a wide thrust range for mission flexibility, ranging from the transfer of large, acceleration limited structures from LEO to GEO, to demanding high reliability round-trip manned missions. Technology advances are needed in propulsion, aerobraking, low-gravity cryogenic fluid management, and in environmentally compatible, low-loss cryogenic tankage. In addition, diagnostic instrumentation for monitoring the health of on-board components and systems, and automated check-out capability will enhance low-cost space based OTV operations.

Stephenson, F.↗