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At least 919 records · Page 51

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NASA NTRS1984

A method for interface shape control during Bridgman type crystal growth of HgCdTe alloys

A method is described for interface shape control of Hg(1-x)Cd(x)Te alloys during Bridgman-type crystal growth. The authors have used this method for several years to obtain the flat or convex interface shapes required for simultaneously reducing radial compositional variations and crystal defects. The method turns to advantage the thick-walled, fused-silica ampules required for growing these alloys. A proper combination of furnace geometry (two isothermal zones separated by a thin barrier), upper- and lower-zone temperatures, and growth rate is required. Examples of results are reported.

Szofran, F. R.↗

NASA NTRS1986

Making a Lightweight Battery Plaque

Plaque formed in porous plastic by electroless plating. Lightweight plaque prepared by electroless plating of porous plastic contains embedded wire or expanded metal grid. Plastic may or may not be filled with soluble pore former. If it contains soluble pore former, treated to remove soluble pore former and increase porosity. Porous plastic then clamped into rig that allows plating solutions to flow through plastic. Lightweight nickel plaque used as electrode substrate for alkaline batteries, chiefly Ni and Cd electrodes, and for use as electrolyte-reservoir plates for fuel cells.

Reid, M. A.↗

NASA NTRS1985

Growth of Solid Solution Crystals

The major objective of this program is to determine the conditions under which single crystals of solid solutions can be grown from the melt in a Bridgman configuration with a high degree of chemical homogeneity. The central aim is to assess the role of gravity in the growth process and to explore the possible advantages for growth in the absence of gravity. The alloy system being investigated is the solid solution semiconductor with x-values appropriate for infrared detector applications in Hg sub (1-x) Cd sub x Te the 8 to 14 micro m wavelength region. Both melt and Te-solvent growth are being considered. The study consists of an extensive ground-based experimental and theoretical research effort followed by flight experimentation where appropriate. Experimental facilities have been established for the purification, casting, and crystal growth of the alloy system. Facilities have been also established for the metallurgical, compositional, electric and optical characterization of the alloys. Crystals are being grown by the Bridgman-Stockbarger method and are analyzed by various experimental techniques to evaluate the effects of growth conditions on the longitudinal and radial compositional variations and defect densities in the crystals.

Lehoczky, S. L.↗

NASA NTRS1985

Fluid Dynamics and Thermodynamics of Vapor Phase Crystal Growth

The ground-based research effort under this program is concerned with systematic studies of the effects of variations: (1) of the relative importance of buoyancy-driven convection, and (2) of diffusion and viscosity conditions on crystal properties. These experimental studies are supported by thermodynamic characterizations of the systems, based on which fluid dynamic parameters can be determined. The specific materials under investigation include: the GeSe-GeI4, Ge-GeI4, HgTe-HgI2, and Hg sub (1-x)Cd sub (x) Te-HgI2 systems. Mass transport rate studies of the GeSe-GeI system as a function of orientation of the density gradient relative to the gravity vector demonstrated the validity of flux anomalies observed in earlier space experiments. The investigation of the effects of inert gases on mass flux yielded the first experimental evidence for the existence of a boundary layer in closed ampoules. Combined with a thorough thermodynamic analysis, a transport model for diffusive flow including chemical vapor transport, sublimation, and Stefan flow was developed.

Wiedemeier, H.↗

NASA NTRS1985

Vapor Growth of Alloy-Type Semiconductor Crystals

The present effort is part of a continuing research program directed towards the investigation of basic vapor transport phenomena and of crystal growth properties of electronic materials. The primary purpose of ground-based studies is the development and definition of optimum experimental parameters for flight experiments. The ground-based effort includes the investigation of gravity-driven convection effects on mass transport rates and on crystal morphology for different orientations of the density gradient with respect to the gravity vector, and as a function of pressure and of temperature. In addition to the experimental tasks, theoretical efforts involve the quantitative thermodynamic analysis of the systems under investigation, the computation of fluid dynamic parameters, and the consideration of other possible effects on fluid flow under vertical, stabilizing and microgravity conditions. The specific experiments to be performed in a microgravity environment include the investigation of vapor transport and crystal growth phenomena of the GeSe-Xenon system and of the mass flux and growth of bulk and layer-type crystals of Hg sub (1-x) Cd sub x Te using HgI as a transport agent.

Wiedemeier, H.↗

NASA NTRS1985

Dynamics of Bridgman-Stockbarger growth of non-dilute binary alloys

The coupling between heat and mass transfer in the Bridgman-Stockbarger growth of nondilute crystals induces growth rate transients that are longer than those caused by heat transfer alone. The dynamics of the diffusion-controlled growth for thermophysical properties representative of Pb(1-x)Sn(x)Te and Hg(1-x)Cd(x)Te are computed from a one-dimensional solidification model which includes melt-crystal phase behavior. Results show that the growth rate response to a step change in pull rate for both systems differs from the exponential decay observed for dilute alloys and that growth rates cannot be inferred from axial segregation data in nondilute systems due to the dependence of the segregation coefficient on composition.

Bourret, E. D.↗

NASA NTRS1985

Chemical compositional study of 35 iron meteorites and its application in taxonomy

Structural and compositional data are reported as a guide to the classification of 35 iron meteorites. The Xinjiang iron meteorite, previously classified as III AB, is reclassified as III E on the basis of its lower Ga/Ni and Ge/Ni ratios, its wider, swollen kamacite bands, and the ubiquitous presence of haxonite, (Fe,Ni)22C. The Dongling (III CD) appears not to be a new meteorite, but to be paired with the Nantan. Four Antarctic iron meteorites, IAB Allan Hills A77250, A77263, A77289, and A77290, are classified as a paired meteorite because of their similarities in structure and in concentrations of various elements. It is shown that Cu shares certain properties with Ga and Ge, which makes them excellent taxonomic parameters.

Wang, D.↗

NASA NTRS1986

Kinematic evidence of satellite galaxy populations in the potential wells of first-ranked cluster galaxies

The velocities of 38 centrally positioned galaxies (r much less than 100 kpc) were measured relative to the velocity of the first-ranked galaxy in 14 rich clusters. Analysis of the velocity distribution function of this sample and of previous data shows that the population cannot be fit by a single Gaussian. An adequate fit is obtained if 60 percent of the objects lie in a Gaussian with sigma = 250 km/s and the remainder in a population with sigma = 1400 km/s. All previous data sets are individually consistent with this conclusion. This suggests that there is a bound population of galaxies in the potential well of the central galaxy in addition to the normal population of the cluster core. This is taken as supporting evidence for the galactic cannibalism model of cD galaxy formation.

Cowie, L. L.↗

NASA NTRS1986

Chemical systematics of the Shergotty meteorite and the composition of its parent body (Mars)

Sixty elements in two bulk samples of Shergotty meteorite and 30 elements in various mineral separates of Shergotty were identified, using mainly INAA and RNAA techniques. In addition, elements leached out from powdered samples of Shergotty and EETA 79001 meteorites by 0.1 N HCl, as well as the elements of their residues, were analyzed. The results have indicated that Shergotty meteorite is homogeneous in its major element composition, but heterogeneous with respect to large-ion lithophile elements, such as K, Ba, Sr, Zr, Hf, Ta, Th, and rare-earth elements (REEs). It is even more heterogeneous with respect to volatile elements, such as Cd, Te, Tl, and Bi, and the siderophiles Au and Ag. The REE patterns of the Shergotty and EETA 79001 residues are identical, indicating that the parent magmas of both meteorites are compositionally similar. However, their leachate (phosphate) patterns are different, suggesting two components for the Shergotty, one of which is similar to the EETA 79001 leachate.

Laul, J. C.↗

NASA NTRS1985

Preliminary results of a quantitative comparison of the spectral signatures of Landsat Thematic Mapper (TM) and Modular Optoelectronic Multispectral Scanner (MOMS).

Operationally acquired Thematic Mapper and experimental MOMS-01 data are evaluated quantitatively concerning the systems spectral response and performance for geoscientific applications. Results show the two instruments to be similar in the spectral bands compared. Although the MOMS scanner has a smaller IFOV, it has a lower modulation transfer function performance for small, low contrast features as compared to Thematic Mapper. This deficiency does not only occur when MOMS was switched to the low gain mode. It is due to the CD arrays used (ITEK CCPD 1728).

Bodechtel, J.↗

NASA NTRS1986

Observational constraints on bending the wide-angle tailed radio galaxy 1919+479

A wide range of new observations of the wide-angle tailed radio galaxy 1919+479 and its environs are presented. Multifrequency and multiconfiguration VLA observations are described, ranging in resolution from 0.7 arcsec to 40 arcsec. Distributions of source structures at different scale sizes, spectral indices, and polarizations are presented. Wide-field photographic and CCD imaging data are presented along with a review of the measured galaxy redshifts in the field. A newly reprocessed Einstein IPC X-ray image of the cluster is shown, and the X-ray emission's relationship to the radio morphology is described. The radio, optical, and X-ray data are used to constrain the cD galaxy dynamics and models for bending the radio structure. In particular, the possible role of jet collisions with clouds in the intracluster medium is considered. The applicability of cloud collision models to wide-angle tails in general is considered.

Burns, J. O.↗

NASA NTRS1987

Chemical studies of H chondrites. I - Mobile trace elements and gas retention ages

Trends for 16 trace elements (Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, K, Rb, Sb, Se, Te, Tl, and Zn), chosen to span a broad geochemical and thermal response range, in 44 H4-6 chondrites, differ widely from those in L4-6 chondrites. In particular, H chondrites classified as heavily shocked petrologically do not necessarily exhibit Ar-40 loss and vice versa. The clear-cut causal relationship between siderophile and mobile element loss with increasing late shock seen in L chondrites is not generally evident in the H group. H chondrite parent material experienced an early high temperature genetic episode that mobilized a substantial proportion of these trace elements so that later thermal episodes resulted in more subtle, collateral fractionations. Mildly shocked L chondrites escaped this early high temperature event, indicating that the two most numerous meteorite groups differ fundamentally in genetic history.

Lingner, David W.↗

NASA NTRS1987

Growth rate of CdS by vapor transport in a closed ampoule

Vapor transport of CdS in a closed ampoule due to a temperature gradient was investigated. Comparisons between the theoretical and experimental growth rate for the following conditions were made: (1) with and without the presence of Ar gas, (2) convective contribution to the growth rate in the vertical and horizontal configurations, and (3) the effect of starting material composition on the growth rate. It was found that Cd reacts with fused silica ampoules and causes the contamination of CdS samples.

Su, Ching-Hua↗

NASA NTRS1987

Computational vortex dynamics in two and three dimensions

A discussion of some solutions and outstanding problems of dissipationless and nearly dissipationless flows in two and three dimensions is presented. In particular, two-dimensional steady-state solutions, linear stability and nonlinear evolutions including axisymmetrization of isolated vortex regions and merger or condensation of like-signed vorticity regions, are reviewed. These results are obtained from pseudo-spectral codes, contour dynamical (CD) codes and a new Hamiltonian moment-model of the Euler equations

Melander, Mogens V.↗

NASA NTRS1987

On the thermal instability of galactic and cluster halos

The paper presents a detailed study of thermal instabilities in cooling flows associated with galaxies and clusters of galaxies. In the case of purely radiation-driven accretion onto a central object such as the cD galaxy M87, it is found that the gas is largely subject to overstability, rather than to monotonic instability. If thermal conductivity is taken into account, the flow is stabilized on scales of several kiloparsecs, even if the conductivity is appreciably reduced (e.g., about 1 percent) with respect to the Spitzer value. In no case are the globular perturbations (i.e., perturbations with comparable radial and azimuthal dimensions) found to be monotonically unstable. The paper presents numerical solutions of the local dispersion relation for the cooling flow in M87 and discusses the possible consequences of the results for a correct understanding of cooling flows.

Malagoli, A.↗

NASA NTRS1987

The status of the international Halley watch

More than 1000 professional astronomers worldwide actually observed Halley's comet from the ground. Preliminary logs from the observers indicate that 20-40 Gbytes of data were acquired in eight professional disciplines and as much as 5 Gbytes in the amateur network. The latter will be used to fill in gaps in the Archive and to provide a visual light curve. In addition roughly 400 Mbytes of data were taken on Comet Giacobini-Zinner. Data will be accepted for archiving until early 1989. The permanent archive will consist of a set of CD-ROMs and a set of books, publication of both to be completed by mid-1990. Data from the space missions will be included but only on the CDs. From every indication, the ground based effort and the space missions complimented each other beautifully, both directly in the solution of spacecraft navigation problems and indirectly in the solution of scientific problems. The major remaining concern is that scientists submit their data to the Archive before the 1989 deadline.

Newburn, Ray L., Jr.↗

NASA NTRS1987

Grace DAKASEP alkaline battery separator

The Grace DAKASEP separator was originally developed as a wicking layer for nickel-zinc alkaline batteries. The DAKASEP is a filled non-woven separator which is flexible and heat sealable. Through modification of formulation and processing variables, products with a variety of properties can be produced. Variations of DAKASEP were tested in Ni-H2, Ni-Zn, Ni-Cd, and primary alkaline batteries with good results. The properties of DAKASEP which are optimized for Hg-Zn primary batteries are shown in tabular form. This separator has high tensile strength, 12 micron average pore size, relatively low porosity at 46-48 percent, and consequently moderately high resistivity. Versions were produced with greater than 70 percent porosity and resistivities in 33 wt percent KOH as low as 3 ohm cm. Performance data for Hg-Zn E-1 size cells containing DAKASEP with the properties shown in tabular form, are more reproducible than data obtained with a competitive polypropylene non-woven separator. In addition, utilization of active material is in general considerably improved.

Giovannoni, R. T.↗

NASA NTRS1987

Growth of solid solution single crystals

Based on the thermophysical properties of Hg sub 1-x Cd sub x Te alloys, the reasons are discussed for the failure of conventional Bridgman-Stockbarger growth methods to produce high quality homogeneous crystals in the presence of Earth's gravity. The deleterious effects are considered which arise from the dependence of the thermophysical properties on temperature and composition and from the large amount of heat carried by the fused silica ampules. An improved growth method, developed to optimize heat flow conditions, is described and experimental results are presented. The problems associated with growth in a gravitational environment are discussed. The anticipated advantages of growth in microgravity are given and the implications of the requirements for spaceflight experiments are summarized.

Lehoczky, S. L.↗