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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Czochralski growth of crystals - Simple models for growth rate and interface shape

A simple model for the crystal growth by the Czochralski (CZ) process has been proposed based on semiquantitative arguments. The model provides empirical relationships for the dependence of the pulling rate and the interface shape on the important process variables such as crystal radius, crucible temperature, height of the melt level, and the height of the exposed portion of the crucible wall. The parameters of the model can be evaluated by matching the results obtained from a detailed mathematical model of the CZ process or from extensive experimental data. The model has, therefore, the potential application for determining the best process conditions and for on-line control and optimization of the crystal puller to grow crystals with constant diameter and nearly planar interface.

Srivastava, R. K.↗
NASA NTRS1987

Determining Optical Axes of Uniaxial Crystals

Polarizing-microscope concept adapted for thick samples. Optical axis of crystal usually found by examining sample thinner than 1 mm between crossed polarizing plates. Frequently impractical to cut off small sample of crystal for testing, technique modified to accommodate large crystals. Ability to circumvent effect of birefringence has applications where laser beams must be transmitted through uniaxial crystals, as in laser diagnostics of contained flows in systems requiring windows for optical access.

Schock, H. J.↗
NASA NTRS1987

Crystal Growth in Liquid-Encapsulated Float Zone

Suitably chosen liquid encapsulant placed around melt zone in float-zone crystal-growth system performs four important functions enhancing purity and reducing strains and dislocations in final crystal. In new technique, grow dislocation-free crystals with precisely controlled composition even from materials not amenable to conventional float-zone crystal-growth method. Support provided by encapsulant make it practical to process materials of low surface tension.

Naumann, Robert J.↗
NASA NTRS1986

The crystallization of tough thermoplastic resins in the presence of carbon fibers

The crystallization kinetics of the thermoplastic resins poly(phenylene sulfide) (PPS) and poly(aryl-ether-ether-ketone) (PEEK) in the presence and in the abscence of carbon fibers was studied. How carbon fiber surfaces in composites affect the crystallization of tough thermoplastic polymers that may serve as matrix resins were determined. The crystallization kinetics of such substances can provide useful information about the crystallization mechanisms and, thus, indicate if the presence of carbon fibers cause any changes in such mechanisms.

Theil, M. H.↗
NASA NTRS1986

Quantitative Schlieren analysis applied to holograms of crystals grown on Spacelab 3

In order to extract additional information about crystals grown in the microgravity environment of Spacelab, a quantitative schlieren analysis technique was developed for use in a Holography Ground System of the Fluid Experiment System. Utilizing the Unidex position controller, it was possible to measure deviation angles produced by refractive index gradients of 0.5 milliradians. Additionally, refractive index gradient maps for any recorded time during the crystal growth were drawn and used to create solute concentration maps for the environment around the crystal. The technique was applied to flight holograms of Cell 204 of the Fluid Experiment System that were recorded during the Spacelab 3 mission on STS 51B. A triglycine sulfate crystal was grown under isothermal conditions in the cell and the data gathered with the quantitative schlieren analysis technique is consistent with a diffusion limited growth process.

Brooks, Howard L.↗
NASA NTRS1986

Microgravity crystallization of macromolecules: An interim report and proposal for continued research

An initial investigation exploring the effects of gravity on the crystallization of macromolecular systems has been completed. Monodisperse poly(ethylene), molecular weight 48,000 was melted and recrystallized under gravitational conditions: 0, 1, and 2 g. No correlations to gravitational environment were noted for the 20 C/min melt, as monitored with a photodensitometer system. However, post-crystallization testing of the recrystallized samples revealed thicker samples with more regions of large, well defined spherulites for the zero gravity crystallization environment. The results of the post-crystallization analysis have been reviewed and the results related to nucleation concerns. Finally, birefringence data, consistent with, but not explained by, the nucleation scenarios is detailed, and further investigations are proposed.

Goldberg, Benjamin E.↗
NASA NTRS1987

Growth of Triglycine Sulfate (TGS) crystals aboard Spacelab-3

An experiment to study the growth of single crystals of triglycine sulfate (NH2CH2COOH)3 H2SO4 (TGS) was successfully carried out on the Spacelab-3 mission during April 29 to May 6, 1985. Two crystals of TGS were grown during the flight, using a specially developed cooled sting technique of solution crystal growth. For the first time in any flight experiment the growth was monitored on-board as well as on ground by video-schlieren technique. Hundreds of holograms were taken for the solution/crystal interaction during the growth process. Preliminary results indicate that the optical system worked very well and the quality of reconstructed holograms is satisfactory. The cooled sting technique was successfully demonstrated. Holographic interferograms indicate convection free, diffusion limited growth. Some of the preliminary results of crystal quality are also presented.

Lal, R. B.↗
NASA NTRS1987

Laser schlieren crystal monitor

A system and method for monitoring the state of a crystal which is suspended in a solution is described which includes providing a light source for emitting a beam of light along an optical axis. A collimating lens is arranged along the optical axis for collimating the emitted beam to provide a first collimated light beam consisting of parallel light rays. By passing the first collimated light beam through a transparent container, a number of the parallel light rays are deflected off the surfaces of said crystal being monitored according to the refractive index gradient to provide a deflected beam of deflected light rays. A focusing lens is arranged along optical axis for focusing the deflected rays towards a desired focal point. A knife edge is arranged in a predetermined orientation at the focal point; and a screen is provided. A portion of the deflected beam is blocked with the knife edge to project only a portion of the deflected beam. A band is created at one edge of the image of the crystal which indicates the state of change of the surface of the crystal being monitored.

Owen, Robert B.↗
NASA NTRS1986

Ground-based experiments on the minimization of convection during the growth of crystals from solution

Crystals were grown from solution in the earth's gravitational field with the substantial absence of convection by (1) downward unidirectional growth in an isothermally cooled solution and (2) upward unidirectional growth in a positive vertical thermal gradient. Growth rates were 1.3 to 1.8 times the predictions for convectionless growth, and an order of magnitude less than for growth with vigorous convection. Crystals grown in the isothermal configuration were of comparable or superior quality to those grown with substantial convection. The imposition of a 0.5 to 0.8 C/mm gradient greatly stabilized the growth interface so that flat (001) surfaces could be grown on KDP. Crystals grown in a thermal gradient, however, were filled with far more fluid inclusions than crystals grown isothermally.

Nerad, B. A.↗
NASA NTRS1986

Growth kinetics of tetragonal lysozyme crystals

A method for immobilizing protein crystals in small volumes to determine growth rates on various faces is applied to study the growth kinetics of the (100) face of tetragonal hen-egg white lysozyme crystals at different degrees of bulk saturation. In normal gravity, transport is found to be dominated by convection for crystal sizes larger than a few microns, while in a microgravity environment, transport is diffusion-limited for sizes up to a few mm. It is found that convection can be significant even in microgravity for crystals approaching cm sizes, and that lysozyme growth is limited by surface kinetics in normal gravity.

Pusey, M.↗
NASA NTRS1986

Inorganic and protein crystal growth - Similarities and differences

Transport and interface kinetic concepts for the design and control of inorganic crystal growth experiments are reviewed, and their applications and limitations in protein crystal growth are considered. It is suggested that the interfacial concentration gradients are steeper for faster crystallization, and that the interfacial concentration distributions for the protein and the precipitant can differ significantly. Results show that uniformity in crystal composition and steady-state conditions in growth kinetics are favored by larger sample size, since surface-tension gradients drive strong in microgravity experiments and in small samples on earth.

Rosenberger, F.↗
NASA NTRS1987

Protein crystal growth in space

The advantages of protein crystallization in space, and the applications of protein crystallography to drug design, protein engineering, and the design of synthetic vaccines are examined. The steps involved in using protein crystallography to determine the three-dimensional structure of a protein are discussed. The growth chamber design and the hand-held apparatus developed for protein crystal growth by vapor diffusion techniques (hanging-drop method) are described; the experimental data from the four Shuttle missions are utilized to develop hardware for protein crystal growth in space and to evaluate the effects of gravity on protein crystal growth.

Bugg, C. E.↗
NASA NTRS1986

An apparatus for protein crystal growth studies

An apparatus is described for observing protein crystals growing under varying growth conditions, and for changing or manipulating crystal orientation for viewing selected faces or growth modes. This flexibility permits a wide variety of protein crystal growth experiments to be performed using the same crystal(s).

Pusey, Marc L.↗
NASA NTRS1984

Coupled convective instabilities at crystal-melt interfaces

The stability of the parallel flow between a vertical crystal-melt interface and a vertical wall held at a temperature above the melting point of the crystal is analyzed for Prandtl numbers, P, ranging from 0.01 to 100. Three modes of instability occur: (1) a buoyant mode, (2) a shear mode, and (3) a coupled crystal-melt mode. The buoyant and shear modes are similar to those that occur for flow between two vertical rigid walls held at different temperatures. For Prandtl numbers greater than approximately two, the coupled crystal-melt mode occurs at a lower Grashof number than the other two modes. Specific results are given for succinonitrile (P = 22.8) and lead (P = 0.0225). These calculations and similar calculations for a cylindrical geometry were motivated by and are in general agreement with recent experiments on succinonitrile.

Coriell, S. R.↗
NASA NTRS1988

Electrochemical growth of linear conducting crystals in microgravity

Much attention has been given to the synthesis of linear conducting materials. These inorganic, organic, and polymeric materials have some very interesting electrical and optical properties, including low temperature superconductivity. Because of the anisotropic nature of these compounds, impurities and defects strongly influences the unique physical properties of such crystals. Investigations have demonstrated that electrochemical growth has provided the most reproducible and purest crystals. Space, specifically microgravity, eliminates phenomena such as buoyancy driven convection, and could permit formation of crystals many times purer than the ones grown to date. Several different linear conductors were flown on Get Away Special G-007 on board the Space Shuttle Columbia, STS 61-C, the first of a series of Project Explorer payloads. These compounds were grown by electrochemical methods, and the growth was monitored by photographs taken throughout the mission. Due to some thermal problems, no crystals of appreciable size were grown. The experimental results will be incorporated into improvements for the next 2 missions of Project Explorer. The results and conclusions of the first mission are discussed.

Cronise, Raymond J., IV↗
NASA NTRS1988

Unified constitutive model for single crystal deformation behavior with applications

Single crystal materials are being used in gas turbine airfoils and are candidates for other hot section components because of their increased temperature capabilities and resistance to thermal fatigue. Development of a constitutive model which assesses the inelastic behavior of these materials has been studied in 2 NASA programs: Life Prediction and Constitutive Models for Engine Hot Section Anisotropic Materials and Biaxial Constitutive Equation Development for Single Crystals. The model has been fit to a large body of constitutive data for single crystal PWA 1480 material. The model uses a unified approach for computing total inelastic strains (creep plus plasticity) on crystallographic slip systems reproducing observed directional and strain rate effects as a natural consequence of the summed slip system quantities. The model includes several of the effects that have been reported to influence deformation in single crystal materials, such as shear stress, latent hardening, and cross slip. The model is operational in a commercial Finite Element code and is being installed in a Boundary Element Method code.

Walker, K. P.↗
NASA NTRS1988

Anisotropic constitutive modeling for nickel-base single crystal superalloys

An anisotropic constitutive model was developed based on crystallographic slip theory for nickel base single crystal superalloys. The constitutive equations developed utilizes drag stress and back stress state variables to model the local inelastic flow. Specially designed experiments were conducted to evaluate the existence of back stress in single crystal superalloy Rene N4 at 982 C. The results suggest that: (1) the back stress is orientation dependent; and (2) the back stress state variable is required for the current model to predict material anelastic recovery behavior. The model was evaluated for its predictive capability on single crystal material behavior including orientation dependent stress-strain response, tension/compression asymmetry, strain rate sensitivity, anelastic recovery behavior, cyclic hardening and softening, stress relaxation, creep and associated crystal lattice rotation. Limitation and future development needs are discussed.

Sheh, Michael Y.↗
NASA NTRS1987

Devitrification and delayed crazing of SiO2 on single-crystal silicon and chemically vapor-deposited silicon nitride

The linear growth rate of cristobalite was measured in thin SiO2 films on silicon and chemically vapor-deposited silicon nitride. The presence of trace impurities from alumina furnace tubes greatly increased the crystal growth rate. Under clean conditions, the growth rate was still 1 order-of-magnitude greater than that for internally nucleated crystals in bulk silica. Crystallized films cracked and lifted from the surface after exposure to atmospheric water vapor. The crystallization and subsequent crazing and lifting of protective SiO2 films on silicon nitride should be considered in long-term applications.

Choi, Doo Jin↗