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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
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

Analysis of X-Ray Microradiographs of Al-Au Interface Quench Profile using Modeling of Solidification Including Double-Diffusion and Convection in the Melt

Experimental data on Al-0.8Au horizontal solidification of a 1 mm thick specimen in a BN crucible shows the effect of growth rate on the solidification interface shape. For translation rates below 0.5 micron/s the interface maintains a plain and flat shape. When the translation rate is 3 to 5 micron/s or more, the interface appearance changes to two planar zones, with the zone closer to the bottom having higher inclination. The interface shapes were measured by first quenching in place during growth. X-ray microscopy shows the interface shape within the quenched sample by viewing through the side of the specimen. In order to provide theoretical explanation of the phenomena, numerical modeling was undertaken using finite element code FIDAP. Double diffusion convection in Al-0.8Au melt and crystal-melt interface curvature during directional solidification was analyzed numerically. Actual thermophysical properties of Al-0.8Au including the binary Al-Au phase diagram were used. Although convection in the sample is weak, for the slower translation rate convection and diffusion is sufficient for the redistribution of initial compositional stratification caused by gravity. When translation rate is raised, neither convection nor diffusion can provide proper mixing so that solidification temperatures differ significantly near the bottom within the bulk of the sample. As a result, the solid-liquid interface appears to have two planar zones with different inclination.

Bune, Andris V.↗
NASA NTRS1999

Melt Growth of a Nonlinear Optical Crystal Triethylphosphine Sulfide using Modified Bridgman-Stockbarger Technique

Bulk single crystals of triethylphosphine sulfide (C2H5)3P(S), a potential nonlinear optical organic material has been grown from melt using Bridgman- Stockbarger method. Commercially available material triethylphosphine sulfide procured from Johnson Matthey was purified by physical vapor transport using low pressure sublimaton (about 30 mTorr). Modified Bridgman-Stockbarger technique is characterized by the smallest possible quantity of product in the melt for a short period of time. The temperature gradient was chosen to be about 10 C/cm and the ampoule was lowering rate was chosen to be 0.2-0.3 mm/h. For various growth conditions, several single crystal of triethylphosphine sulfide have been grown with sizes 10X10X15mm(exp 3). The second harmonic generation (SHG) efficiency was measured and has been found to be comparable to phase matched potassium dihydrogen phosphate crystals. Further characterization of the grown single crystals is in progress and the results will be presented at the symposium.

Curry, K.↗
NASA NTRS1999

Reducing and Inducing Convection in Ge-Si Melts with Static Magnetic Field

Results of a study of the effectiveness of using static magnetic fields to reduce convection in Ge-Si melts will be presented. Lenz's law causes a retardation of convection when a static magnetic field is applied to an electrically conducting liquid. However, during the solidification of a solid-solution system such as Ge-Si, the interface is neither isothermal nor isoconcentrational. The variation of temperature and chemical composition along the interface causes thermoelectric currents to be generated within the solidifying material (and the container if it is electrically conductive). These currents, in the presence of a magnetic field, can cause movement (stirring, convection) in the melt which can exceed convection induced by normal thermosolutal mechanisms. Crystals have been grown by both the Bridgman and floating-zone methods. Clear evidence for the existence of this thermoelectromagnetic convection, especially in the case of Si floating-zone growth, will be presented.

Szofran, Frank R.↗
NASA NTRS1999

Determination of the Wetting Angle of Germanium and Germanium-Silicon Melts on Different Substrate Materials

During Bridgman growth of semiconductors detachment of the crystal and the melt meniscus has occasionally been observed, mainly under microgravity (microg) conditions. An important factor for detached growth is the wetting angle of the melt with the crucible material. High contact angles are more likely to result in detachment of the growing crystal from the ampoule wall. In order to achieve detached growth of germanium (Ge) and germanium-silicon (GeSi) crystals under 1g and microg conditions, sessile drop measurements were performed to determine the most suitable ampoule material as well as temperature dependence of the surface tension for GeSi. Sapphire, fused quartz, glassy carbon, graphite, SiC, pyrolytic Boron Nitride (pBN), AIN, and diamond were used as substrates. Furthermore, different cleaning procedures and surface treatments (etching, sandblasting, etc.) of the same substrate material and their effect on the wetting behavior were studied during these experiments. pBN and AIN substrates exhibited the highest contact angles with values around 170 deg.

Kaiser, Natalie↗
NASA NTRS1999

Use of Traveling Magnetic Fields to Control Melt Convection

An axially traveling magnetic wave induces a meridional basic flow in a molten cylindrical zone. This flow can be beneficial for crystal growth applications. In particular, it can be used to effectively stir the melt in long cylindrical columns. Also, it can beneficially modify the thermal and species concentration fields in the melt and the interface shape of the growing crystal. The basic theory of such an application is developed and preliminary data from a mercury column experiment are also presented.

Ramachandran, N.↗
NASA NTRS1999

Melting Efficiency During Plasma Arc Welding

A series of partial penetration Variable Polarity Plasma Arc welds were made at equal power but various combinations of current and voltage on 2219 aluminum. Arc Efficiency was measured calorimetrically and ranged between 48% and 66%. Melting efficiency depends on the weld pool shape. Increased current increases the melting efficiency as it increases the depth to width ratio of the weld pool. Higher currents are thought to raise arc pressure and depress the liquid at the bottom of the weld pool causing a more nearly two dimensional heat flow condition.

McClure, J.C.↗
NASA NTRS1998

Osmium Solubility in Silicate Melts: New Efforts and New Results

In a recent paper, Borisov and Palme reported the first experimental results on the partitioning of Os between metal (Ni-rich OsNi alloys) and silicate melt of anorthite-diopside eutectic composition at 1400 C and 1 atm total pressure and and at function of O2 from 10(exp -8) to 10(exp -12) atm. Experiments were done by equilibrating OsNi metal loops with silicate melt. Metal and glass were analyzed separately by INAA. D(sup 0s) ranged from 10(exp 6) to 10(exp 7), which is inconsistent with core/ mantle equilibrium for HSEs and favors the late veneer hypothesis. Unfortunately, there was practically no function of O2 dependence of Os partitioning, and the scatter of experimental results was quite serious, so the formation of Os nuggets was suspected. This new set of experiments was specifically designed to avoid of at least minimize the nugget problem

Borisov, A.↗
NASA NTRS1999

Reactive Infiltration of Silicon Melt Through Microporous Amorphous Carbon Preforms

The kinetics of unidirectional capillary infiltration of silicon melt into microporous carbon preforms have been investigated as a function of the pore morphology and melt temperature. The infiltrated specimens showed alternating bands of dark and bright regions, which corresponded to the unreacted free carbon and free silicon regions, respectively. The decrease in the infiltration front velocity for increasing infiltration distances, is in qualitative agreement with the closed-form solution of capillarity driven fluid flow through constant cross section cylindrical pores. However, drastic changes in the thermal response and infiltration front morphologies were observed for minute differences in the preforms microstructure. This suggests the need for a dynamic percolation model that would account for the exothermic nature of the silicon-carbon chemical reaction and the associated pore closing phenomenon.

Sangsuwan, P.↗
NASA NTRS1999

Use of Traveling Magnetic Fields to Control Melt Convection

An axially travelling magnetic wave induces a meridional base flow in an electrically conducting molten cylindrical zone. This flow can be beneficial for crystal growth applications. In particular, it can be effectively used to stir the melt in long cylindrical columns. It can also be tailored to modify the thermal and species concentration fields in the melt and to control the interface shape of the growing crystal. The basic theory of such an application is developed and data from a preliminary mercury column experiment are presented.

Ramachandran, Narayanan↗
NASA NTRS2000

Processing and Properties of SiC/MoSi2-SiC Composites Fabricated by Melt Infiltration

Hi-Nicalon SiC fiber reinforced MoSi2-SiC matrix composites (SiC/MoSi2-SiC) have been fabricated by the melt infiltration approach. The composite consists of approximately 60 vol%, 2-D woven BN/SiC coated Hi-Nicalon SiC fibers and approximately 40 vol% MoSi2-SiC matrix. The room temperature tensile properties and thermal conductivity of the SiC/MoSi2-SiC composites were measured and compared with those of the melt infiltrated SiC/SiC composites. The influence oi fiber architecture on tensile properties was also evaluated. Results indicate that the primary modulus, stress corresponding to deviation from linearity, and transverse thermal conductivity values for the SiC/MoSi2-SiC composites are significantly lower than those for the SiC/SiC composites. Microcracking of the matrix due to the large difference in thermal expansion between MoSi2 and SiC appears to be the reason for the lower matrix dominated properties of SiC/MoSi2-SiC composites.

Bhatt, Ramakrishna T.↗
NASA NTRS1999

The Considere Condition and Rapid Stretching of Linear and Branched Polymer Melts

We analyze the onset of "necking" and subsequent filament failure during the transient uniaxial elongation of viscoelastic fluid samples in extensional rheometers. In the limit of rapid elongation (such that no molecular relaxation occurs), the external work applied is all stored elastically and the Considere criterion originally developed in solid mechanics can be used to quantitatively predict the critical Hencky strain to failure. By comparing the predictions of the Doi-Edwards model for linear homopolymer melts with those of the "Pom-Pom" model for prototypical branched melts we show that the critical strain to failure in rapid elongation of a rubbery material is intimately linked to the molecular topology of the chain, especially the degree of chain branching. The onset of necking instability is monotonically shifted to larger Hencky strains as the number of branches is increased. Numerical computations at finite Deborah numbers also show that there is an optimal range of deformation rates over which homogeneous extensions can be maintained to large strain. We also consider other rapid homogeneous stretching deformations, such as biaxial and planar stretching, and show that the degree of stabilization afforded by inclusion of material with long-chain branching is a sensitive function of the imposed mode of deformation.

McKinley, Gareth H.↗
NASA NTRS2000

Mineralogy, Petrology, Chemistry, and Ar-39 - Ar-40 and Ages of the Caddo County IAB Iron: Evidence for Early Partial Melt Segregation of a Gabbro Area Rich in Plagioclase-Diopside

We found coarse-grained gabbroic material rich in plagioclase and diopside in the Caddo County IAB iron meteorite. The polished thin sections studied were made from areas rich in Al and Ca detected by a micro-focus X-ray fluorescence (XRF) mapping technique. The gabbro is not a clast within a breccia, but rather this area is located mainly at silicate-metal boundaries only a few cm away from an area with fine-grained, ultramafic silicate similar to winonaites. Medium-grained orthopyroxene and olivine are found in transitional areas showing no disturbance of their crystalline textures. A vein-like region, starting at the area rich in fine-grained mafic silicate, extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside. This texture and our accumulated knowledge of the formation mechanism of IAB/winonaltes meteorites, suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts of chondritic source materials. Compositional data on two mineralogically distinct samples of the gabbro-rich portion of the inclusion were obtained by INAA. Compared to an average of LAB silicate inclusions or winonaites, the Caddo County gabbro is enriched in the incompatible lithophile elements Na, Ca, Sc, REE and Hf, which is consistent with a melt origin for the gabbro. The cosmogenic space exposure age of Caddo County (511 Ma) is significantly younger than exposure ages of some other IAB meteorites, An 39Ar-40Ar age determination of the gabbroic material indicates a series of upward steps in age from 4.516 Ga to 4.523 Ga, with a few high temperature ages up to 4.54 Ga. The older age could approximate the primary recrystallization age of silicates. The stepped Ar age spectrum may indicate differences in Ar closure temperatures during slow cooling of -2-20'C/Myr in the parent body. Alternatively, the younger Ar-Ar ages may date a shock event which occurred while Caddo County was hot and which also created textures such as a linear metal vein with rounded zigzag walls in a diopside crystal and slightly miss-oriented rounded plagioclase domain.

Takeda, Hiroshi↗
NASA NTRS2001

Method of Preparing Polymers with Low Melt Viscosity

This invention is an improvement in standard polymerizations procedures, i.e., addition-type and step-growth type polymerizations, wherein monomers are reacted to form a growing polymer chain. The improvement includes employing an effective amount of a trifunctional monomer (such as a trifunctional amine anhydride, or phenol) in the polymerization procedure to form a mixture of polymeric materials consisting of branced polymers, star-shaped polymers, and linear polymers. This mixture of polymeric materials has a lower melt temperature and a lower melt viscosity than corresponding linear polymeric materials of equivalent molecular weight.

Jensen, Brian J.↗
NASA NTRS1999

Trapped Melt in IIIAB Irons: Solid/Liquid Elemental Partitioning During the Fractionation of the IIIAB Magma

Group IIIAB, the largest iron-meteorite group, shows compositional trends (including a three-order-of-magnitude It concentration range) indicating that it formed by fractional crystallization of a metallic magma. Because about 200 irons are available, and all degrees of crystallization are well represented, IIIAB offers an excellent set of samples for the study of crystallization at all depths of the asteroidal core. On log-log Ir-Au, and Ir-As diagrams IIIAB forms a broad band; the breadth represents real meteorite-to-meteorite variations, far outside experimental or sampling uncertainties. A successful model must explain the width of this band; I suggest that it mainly resulted from the trapping of parental magma within the crystallizing solid. Because S is essentially insoluble in metal, the abundance of FeS is a measure of the fraction of trapped liquid. The trapped-melt model is supported by the observation that irons having higher S contents plot closer to the inferred composition of the magmatic parental liquid. The lowest S values are found in the irons occupying the left envelope of the IIIAB Ir-Au or Ir-As compositional fields, thus it is this set of irons that should be interpreted as the solid products of a fractionating magma. This simplifies the modeling of the crystallization process and allows inferences regarding the distribution ratios for other elements in the evolved IIIAB system. The large (multiton) Cape York irons show wide variations in their trapped-melt fractions; their compositions seem best understood in terms of a low initial S content of the IIIAB magma, about 20 mg/g. The inferred initial IIIAB distribution coefficient for Ir, 4.6, is much higher than published values based on laboratory studies of low-S systems; I suggest that low-S (and low-P) partition-ratio measurements tend to err in the direction of unity. In IIIAB distribution coefficients for Au, As, and Ni were still < 1 when the most evolved IIIAB irons formed, another indication of a low initial S content.

Wasson, John T.↗
NASA NTRS2000

Use of Traveling Magnetic Fields to Control Melt Convection

An axially traveling magnetic wave induces a meridional base flow in an electrically conducting molten cylindrical zone. This flow can be beneficial for crystal growth applications. In particular, it can be effectively used to stir the melt in long cylindrical columns. It can also be tailored to modify the thermal and species concentration fields in the melt and to control the interface shape of the growing crystal. The basic theory of such an application is developed and data from a preliminary mercury column experiment are presented.

Ramachandran, Narayanan↗
NASA NTRS1997

Comparison of Chain Conformation of Poly(vinyl alcohol) in Solutions and Melts from Quantum Chemistry Based Molecular Dynamics Simulations

Confirmations of 2,4-dihydroxypentane (DHP), a model molecule for poly(vinyl alcohol), have been studied by quantum chemistry (QC) calculations and molecular dynamics (MD) simulations. QC calculations at the 6-311G MP2 level show the meso tt conformer to be lowest in energy followed by the racemic tg, due to intramolecular hydrogen bond between the hydroxy groups. The Dreiding force field has been modified to reproduce the QC conformer energies for DHP. MD simulations using this force field have been carried out for DHP molecules in the gas phase, melt, and CHCl3 and water solutions. Extensive intramolecular hydrogen bonding is observed for the gas phase and CHCl3 solution, but not for the melt or aqueous solution, Such a condensed phase effect due to intermolecular interactions results in a drastic change in chain conformations, in agreement with experiments.

Jaffe, Richard↗