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  • 1
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    Morphological instabilities in directional solidification of a binary alloy - End effects (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: This paper considers the unidirectional solidification of a binary alloy for which the liquid and solid phases are bounded by endwalls. In order to account for the transport of latent heat and solute, thermal and solutal boundary layers must be placed at the solidifying interface. Further, under the assumption of fixed temperature gradients, the presence of the endwalls leads to a velocity of solidification that decreases with time, and hence to an unsteady basic state having a planar interface. From a stability analysis of this state, a nonlinear long-wave evolution equation of Sivashinsky type is derived, with modified coefficients, that shows how the onset of cellular structure is delayed by the presence of endwalls.
    Keywords: SOLID-STATE PHYSICS
    Type: SIAM Journal on Applied Mathematics (ISSN 0036-1399); 49; 152-164
    Format: text
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  • 2
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    Directional solidification in an imperfect furnace (1987)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The upward directional solidification of a binary alloy in which the rejected solute is more dense than the solvent is considered. The system is stably stratified both thermally and solutally. If heat is lost through the sidewalls, horizontal thermal gradients drive a steady double-diffusive convection. Solidification cells with small aspect ratios (width/length) A are considered, and asymptotic solutions to the steady double-diffusive system are obtained as A tends to 0 under the assumption heat losses are also small. When the thermal Grashof number is large, buoyancy boundary layers are at the sidewalls. The shape of the crystal-melt interface is determined by the coupled effects of local thermodynamic equilibrium, diffusion and convection.
    Keywords: SOLID-STATE PHYSICS
    Type: PCH/PhysicoChemical Hydrodynamics (ISSN 0191-9059); 9; 1-2,; 9-14
    Format: text
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  • 3
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    Shallow cells in directional solidification (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The existing theory on two-dimensional transitions (appropriate to thin parallel-plate geometries) is presented in such a way that it is possible to identify easily conditions for the onset of shallow cells. Conditions are given under which succinonitrile-acetone mixtures should undergo supercritical bifurcation in experimentally accessible ranges. These results suggest a means for the quantitative test of the Mullins and Sekerka (1964) model and its weakly nonlinear extensions.
    Keywords: SOLID-STATE PHYSICS
    Type: Physical Review Letters (ISSN 0031-9007); 63; 573-575
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  • 4
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    Directional solidification with heat losses (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The upward directional solidification of a dilute binary alloy in a tall, thin, two-dimensional slot is considered. Heat losses from the sidewalls produce curved isotherms, interface deformations, and melt convection. An asymptotic approach is used which relates the small heat loss to the thinness of the solidification cell, and solutions are obtained for the resulting melt convection, interface deflection, and the distribution of heat and solute. The model is compared to the numerical results of Chang and Brown (1983) which exclude the effects of solute buoyancy and thermodynamic equilibrium at the interface. The radial segregation at the interface (consistent with both interface shapes and temperature fields) is computed, and it is found that the radial segregation is independent of interfacial shapes; the radial segregation cannot be calculated from only a knowledge of the interface shape.
    Keywords: SOLID-STATE PHYSICS
    Type: Journal of Crystal Growth (ISSN 0022-0248); 91; 4, Se; 538-556
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  • 5
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    Similarity solutions for phase-change problems (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A modification of Ivantsov's (1947) similarity solutions is proposed which can describe phase-change processes which are limited by diffusion. The method has application to systems that have n-components and possess cross-diffusion and Soret and Dufour effects, along with convection driven by density discontinuities at the two-phase interface. Local thermal equilibrium is assumed at the interface. It is shown that analytic solutions are possible when the material properties are constant.
    Keywords: SOLID-STATE PHYSICS
    Type: Metallurgical Transactions A - Physical Metallurgy and Materials Science (ISSN 0360-2133); 20A; 225-235
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  • 6
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    Studies of superconducting materials with muon spin rotation (1989)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The muon spin rotation/relaxation technique was found to be an exceptionally effective means of measuring the magnetic properties of superconductors, including the new high temperature superconductor materials, at the microscopic level. The technique directly measures the magnetic penetration depth (type II superconductors (SC's)) and detects the presence of magnetic ordering (antiferromagnetism or spin-glass ordering were observed in some high temperature superconductor (HTSC's) and in many closely related compounds). Extensive studies of HTSC materials were conducted by the Virginia State University - College of William and Mary - Columbia University collaboration at Brookhaven National Laboratory and TRIUMF (Vancouver). A survey of LaSrCuO and YBaCaCuO systems shows an essentially linear relationship between the transition temperature T(sub c) and the relaxation rate. This appears to be a manifestation of the proportionality between T(sub c) and the Fermi energy, which suggests a high energy scale for the SC coupling, and which is not consistent with the weak coupling of phonon-mediated SC. Studies of LaCuO and YBaCuO parent compounds show clear evidence of antiferromagnetism. YBa2Cu(3-x)CO(x)O7 shows the simultaneous presence of spin-glass magnetic ordering and superconductivity. Three-dimensional SC, (Ba, K) BiO3, unlike the layered CuO-based compounds, shows no suggestion of magnetic ordering. Experimental techniques and theoretical implications are discussed.
    Keywords: SOLID-STATE PHYSICS
    Type: Alabama A & M Univ., NASA-HBCU Space Science and Engineering Research Forum Proceedings; p 211-21
    Format: application/pdf
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  • 7
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    Crystal growing from the melt (1987)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The mechanical and electrical properties of crystals produced by a unidirectional process depend strongly on the temperature and flow fields since these control the concentration of solute at the melt-crystal interface. The solute gradient there drives morphological instabilities that lead to cellular or dendritic interfaces. In the presentation several features of flow-solidification interactions will be discussed. These will include the effects of convection driven by density changes and buoyancy and the imposition of forced flow.
    Keywords: SOLID-STATE PHYSICS
    Type: Interdisciplinary issues in materials processing and manufacturing; Dec 13, 1987 - Dec 18, 1987; Boston, MA; United States
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  • 8
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    Electron beam flash evaporation for YBaCuO and BiCaSrCuO thin films (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: An electron beam flash evaporation technique is described which reproduces the stoichiometry of superconducting YBa(2)Cu(3)O(x) (123), Bi(2)CaSr(2)Cu(2)O(y) (2122), and Bi(2)Ca(2)Sr(2)Cu(3)O(z) (2223) source powders. Films of each material deposited with this technique exhibit zero resistance temperatures of 78 K or above when furnace annealed; 75 K zero temperatures are obtained for 123 films in situ annealed with atomic oxygen at 775 C. Transport critical current densities are 10,000 A/sq cm in 25-micron stripes patterned from 123 films, and 60,000 A/sq cm in 1-mm stripes of 2122 and 2223 films using a 1-microV/mm measurement criteria. Typical normal state resistivities are 700 micro-ohm cm for 2122 and 2223 films and 5 micro-ohm cm for 123 films. The 2122 films are highly textured with the c axis normal to the substrate. All results are for films deposited on MgO.
    Keywords: SOLID-STATE PHYSICS
    Type: Journal of Applied Physics (ISSN 0021-8979); 66; 4903-490
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  • 9
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    The effect of oxygen pressure on volatility and morphology of LaB6 single crystal cathodes (1986)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The effect of oxygen pressure on the volatility and morphology of single crystal LaB6 cathodes, heated to different temperatures, was investigated. At a temperature of 1600 K, an increase of oxygen pressure from 1 x 10 to the -8th torr to 1 x 10 to the -6th torr has led to a 100-fold enhancement in cathode volatility. The enhancement effect of oxygen pressure diminished with increasing temperature: at a cathode operating temperature of 1900 K, the volatility enhancement due to the same oxygen pressure was negligible. It was shown that the faceting frequently observed during evaporation of conically shaped emitters is due to a crystallograpic anisotropy of the oxidation rate of LaB6. No facet formation occurs during evaporation at oxygen pressures below -110 to the -8th torr.
    Keywords: SOLID-STATE PHYSICS
    Type: AD-A181173 , AFOSR-TR-87-0634 , Journal of Vacuum Science and Technology B (ISSN 0734-211X); 4; 112-115
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