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  • 1
    Publication Date: 2011-08-19
    Description: A Petrov-Galerkin/finite-element method is used to analyze the effect of furnace configuration and ampoule design on the temperature field, the convection in the melt, the shape of the melt-solid interface, and the segregation of solute in the crystal, in the directional solidification of several dilute and nondilute binary semiconductor alloys. The vertical Bridgman-Stockbarger system leads to a two-cell flow structure, and the constant gradient furnace has only a single cell near the interface. Radial temperature gradients interact with the solute field to cause the previously predicted sideways diffusive instability, and addition of a solute that is less dense than the bulk melt and that is incorporated upon solidification decreases the intensity of the flow near the interface by increasing the melt density there. The present results have application to understanding of the effects on solute segregation of microgavity solidification and of applied magnetic fields.
    Keywords: MATERIALS PROCESSING
    Type: Journal of Crystal Growth (ISSN 0022-0248); 80; 155-190
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  • 2
    Publication Date: 2011-08-19
    Description: A previous finite-element analysis of vertical Bridgman growth for dilute and nondilute alloys is extended to include the effect of a vertically-aligned magnetic field in the limit of zero magnetic Reynolds number. Calculations are presented for growth of a dilute gallium-germanium alloy in a vertically stabilized Bridgman-Stockbarger system and in a furnace with a uniform temperature gradient imposed along the ampoule. Steady cellular convection driven by radial temperature gradients causes good axial and radial mixing in both systems without a magnetic field. A weak magnetic field decreases the intensity of convection and the effectiveness of solute mixing. The radial nonuniformity is greatest for an intermediate field strength. Stronger fields suppress flow recirculation completely, and lead to uniform solute segregation across the crystal and to diffusion-controlled axial segregation.
    Keywords: MATERIALS PROCESSING
    Type: Journal of Crystal Growth (ISSN 0022-0248); 89; 2-3,; 339-356
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  • 3
    Publication Date: 2011-08-19
    Description: Transport processes in the vertical Bridgman growth of Hg(1-x)Cd(x)Te were investigated by numerical analysis of an idealized quasi-steady-state model of this system. The analysis was specifically directed at an explanation for the radial and axial segregation data obtained by Szofran and Lehoczky (1984) and Szofran et al. (1984) on this system, who reported, respectively, on the observations of diffusion-controlled axial segregation and on the presence of large compositional nonuniformities of CdTe across the radius of the crystal. It is shown that the data sets of these groups can be explained by the results of a detailed analysis of heat transfer, convection, and species transport which account for thermosolutal convection in the melt and for heat transfer mechanisms that dominate the HgCdTe alloy. Predictions from this analysis is applicable to the growth of other materials, because they suggest conditions for the growth of radially uniform crystals with nearly constant axial composition profiles.
    Keywords: MATERIALS PROCESSING
    Type: Journal of Crystal Growth (ISSN 0022-0248); 96; 3, Ju; 609-627
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  • 4
    Publication Date: 2019-07-13
    Description: Herein we report on a preliminary study to assess the use of the Hubble Space Telescope (HST) for the direct detection and spectroscopic characterization of exoplanets and debris disks - an application for which HST was not originally designed. Coronagraphic advances may enable the design of a science instrument that could achieve limiting contrasts approx.10deg beyond 275 milli-arcseconds (4 lambda/D at 800 nm) inner working angle, thereby enabling detection and characterization of several known jovian planets and imaging of debris disks. Advantages of using HST are that it already exists in orbit, it's primary mirror is thermally stable and it is the most characterized space telescope yet flown. However there is drift of the HST telescope, likely due to thermal effects crossing the terminator. The drift, however, is well characterized and consists of a larger deterministic components and a smaller stochastic component. It is the effect of this drift versus the sensing and control bandwidth of the instrument that would likely limit HST coronagraphic performance. Herein we discuss the science case, quantifY the limiting factors and assess the feasibility of using HST for exoplanet discovery using a hypothetical new instrument. Keywords: Hubble Space Telescope, coronagraphy, exoplanets, telescopes
    Keywords: Astronomy
    Type: GSFC.CPR.4894.2011 , SPIE 7731, Astronomical Telescopes and Instrumentation, San Diego CA, June 2010; Jun 01, 2010; San Diego, CA; United States
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  • 5
    Publication Date: 2019-07-18
    Description: The planned testing of the Advanced Mirror System Demonstrator program being performed by Ball Aerospace are presented.
    Keywords: Instrumentation and Photography
    Type: 2nd Annual Technology Days; May 22, 2002 - May 24, 2002; Huntsville, AL; United States
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  • 6
    Publication Date: 2019-07-18
    Description: The Sub-scale Beryllium Mirror Demonstrator that was successfully tested to demonstrate cryogenic figuring of a bare mirror has been coated with a protected gold reflective surface and retested at cryogenic temperatures. Results showing less than 9 nm rms surface distortion attributable to the added coating are presented.
    Keywords: Instrumentation and Photography
    Type: 2nd Annual Technology Days; May 22, 2002 - May 24, 2002; Huntsville, AL; United States
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  • 7
    Publication Date: 2019-07-18
    Description: The current status of the Advanced Mirror System Demonstrator program being performed by Ball Aerospace is presented. The hexagonal low-areal density Beryllium mirror blank has been fabricated and undergoing polishing at the time of this presentation.
    Keywords: Instrumentation and Photography
    Type: 2nd Annual Technology Days; May 22, 2002 - May 24, 2002; Huntsville, AL; United States
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  • 8
    Publication Date: 2019-07-19
    Description: Recent near-infrared spectroscopy of Kuiper Belt objects and Centaurs indicates considerable spectral diversity among them. Some have entirely bland spectra with no discernible spectral features (e.g., Chiron), while 5145 Pholus has a very active spectrum with absorption bands of H2O, CH3OH, and probably the mineral olivine present. In addition, the strong red color of Pholus indicates the presence of organic solids. Among the KBOs, 1993 SC has an active spectrum with the probably presence of hydrocarbons and possibly the ices of H2O and N2. The diversity among these spectra and the implications that such diversity has for models of the formation of the formation of the planets will be discussed.
    Keywords: Astronomy
    Type: Protostars and Planets IV Conference; Jul 06, 1998 - Jul 10, 1998; Santa Barbara, CA; United States
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  • 9
    Publication Date: 2019-08-14
    Description: The analysis of Kim and Brown (1989), which described the pseudo-steady state of the HgCdTe melt/crystal system grown by the vertical Bridgman method, is extended to the study of the transients during the crystal growth and, in particular, at the beginning of the growth. The time-dependent numerical simulations developed show the development of a diffusion layer in the melt and the direct relationship between this layer and the intensity and the pattern of convection in the melt. It is shown that the spread of the solute layer occurs on a diffusive time scale, with the initial transient to the steady-state growth rate lasting several hours for the slow growth rates that are typical of the HgCdTe growth. The simulations demonstrate all the features observed experimentally by Szofran et al. (1984).
    Keywords: MATERIALS PROCESSING
    Type: Journal of Crystal Growth (ISSN 0022-0248); 114; 3, No; 411-434
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  • 10
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    In:  Other Sources
    Publication Date: 2019-07-13
    Description: Overall program completeness is optimized by selecting the ordered set of observations and targets that maximizes the efficiency for the ensemble of stars. We describe the optimization approach and report on completeness sensitivity to instrument throughput, inner working angle, instrument sensitivity, observational overhead, exo-zodiacal brightness, and target revisit constraints.
    Keywords: Instrumentation and Photography
    Type: Terrestrial Planet Finder - Coronagraph STDT #5; Nov 30, 2005; Pasadena, CA; United States
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