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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Stability and instability of thermocapillary convection in models of the float-zone crystal-growth process

This project was concerned with the determination of conditions of guaranteed stability and instability for thermocapillary convection in a model of the float-zone crystal-growth process. This model, referred to as the half-zone, was studied extensively, both experimentally and theoretically. Our own earlier research determined, using energy-stability theory, sufficient conditions for stability to axisymmetric disturbances. Nearly all results computed were for the case of a liquid with Prandtl Number Pr = 1. Attempts to compute cases for higher Prandtl numbers to allow comparison with the experimental results of other researchers were unsuccessful, but indicated that the condition guaranteeing stability against axisymmetric disturbances would be a value of the Marangoni number (Ma), significantly higher than that at which oscillatory convection was observed experimentally. Thus, additional results were needed to round out the stability picture for this model problem. The research performed under this grant consisted of the following: (1) computation of energy-stability limits for non-axisymmetric disturbances; (2) computation of linear-stability limits for axisymmetric and non-axisymmetric disturbances; (3) numerical simulation of the basic state for half- and full-zones with a deformable free surface; and (4) incorporation of radiation heat transfer into a model energy-stability problem. Each of these is summarized briefly below.

Neitzel, G. P.↗
NASA NTRS2000

Float-Zone Growth of Gel-xSix (x ? 10 at .%) Single Crystals: Influence of Thermocapillary and Solutocapillary Convection

Gel-xSix (x ? 10 at.%) single crystals have been grown with the float-zone technique applied within a monoellipsoid mirror furnace. The feed crystal consisted of synthesized GeO.95siO.05 polycrystalline material. Two Boron-doped (much much more than 1-2 (raised dot) 1017 at/cm3) crystals were grown under identical conditions using <100> Ge seeds. One Ge:Ga-doped (much much more than 1018 at/cm3) reference crystal was also grown in the same furnace. Compositional profiles were measured with an energy dispersive analysis by X-ray (EDAX). The maximum silicon concentration attained was 10 at.%. Micrographs of the etched crystals show sharp changes in interface curvature at the crystal edges. These are considered to be caused by solutal Marangoni convection, whose strength is assumed to be a direct function of silicon concentration, Theoretical considerations show that the flow direction, velocity, and strength varies significantly from a solutal Marangoni convection regime directly in front of the solid-liquid interface to a thermal Marangoni convection regime within the bulk melt.

Campbell, Tom A.↗
NASA NTRS1974

Float zone processing in a weightless environment

Results are given for investigations into: (1) the physical limits which set the maximum practical diameters of Si crystals that can be processed by the float-zone method in a near weightless environment, and (2) the economic impact of large, space-produced Si crystals on the electronics industry. The stability of the melt is evaluated. Heat transfer and fluid flow within the melt as dependent on the crystal size and the degree and type of rotation imparted to the melt are studied. Methods of utilizing the weightless environment for the production of large, stress-free Si crystals of uniform composition are proposed. The economic effect of large size Si crystals, their potential applications, likely utilization and cost advantages in LSI, integrated circuits, and power devices are also evaluated. Foreseeable advantages of larger diameter wafers of good characteristics and the possibilities seen for greater perfection resulting from stress-free growth are discussed.

Fowle, A. A.↗
NASA NTRS1986

Fluid mechanics and mass transfer in melt crystal growth: Analysis of the floating zone and vertical Bridgman processes

This research program focuses on analysis of the transport mechanisms in solidification processes, especially one of interest to the Microgravity Sciences and Applications Program of NASA. Research during the last year has focused on analysis of the dynamics of the floating zone process for growth of small-scale crystals, on studies of the effect of applied magnetic fields on convection and solute segregation in directional solidification, and on the dynamics of microscopic cell formation in two-dimensional solidification of binary alloys. Significant findings are given.

Brown, R. A.↗
NASA NTRS1985

Fluid Flow in Crystal Growth: Analysis of the Verticle Bridgman and Floating Zone Process

Research is aimed at the fundamental understanding of the interactions of heat and mass transport and fluid mechanics in crystal growth from the melt, especially in how it pertains to experiments supported by the Microgravity Science and Applications program. Emphasis has been on studies of the vertical Bridgman system, on small-scale floating zone systems proposed for space flight, and on quantitative prediction of nonlinear transitions in melt/solid interface morphologies leading to the formation of dendrites in directional solidification. The analysis of vertical Bridgman growth has lead to the complete numerical analysis of transport phenomena in directional solidification and to a detailed comparison of calculations to the GaGe growth experiments of Wang and Witt. Calculations show the central role of the ampoule material in determining the radial temperature gradients in the melt and thus, the intensity of convection. Results indicate that diffusion-controlled growth can not be achieved on Earth with conventional growth systems, but are feasible in microgravity

Brown, R. A.↗
NASA NTRS2012

Characterization of 4H <000-1> Silicon Carbide Films Grown by Solvent-Laser Heated Floating Zone

Commercially available bulk silicon carbide (SiC) has a high number (>2000/sq cm) of screw dislocations (SD) that have been linked to degradation of high-field power device electrical performance properties. Researchers at the NASA Glenn Research Center have proposed a method to mass-produce significantly higher quality bulk SiC. In order for this bulk growth method to become reality, growth of long single crystal SiC fibers must first be achieved. Therefore, a new growth method, Solvent-Laser Heated Floating Zone (Solvent-LHFZ), has been implemented. While some of the initial Solvent-LHFZ results have recently been reported, this paper focuses on further characterization of grown crystals and their growth fronts. To this end, secondary ion mass spectroscopy (SIMS) depth profiles, cross section analysis by focused ion beam (FIB) milling and mechanical polishing, and orientation and structural characterization by x-ray transmission Laue diffraction patterns and x-ray topography were used. Results paint a picture of a chaotic growth front, with Fe incorporation dependant on C concentration.

Woodworth, Andrew, A↗
NASA NTRS1985

Potential productivity benefits of float-zone versus Czochralski crystal growth

Efficient mass production of single-crystal silicon is necessary for the efficient silicon solar arrays needed in the coming decade. However, it is anticipated that there will be difficulty growing such volumes of crystals using conventional Czochralski (Cz) methods. While the productivity of single crystals might increase with a crystal diameter increase, there are two obstacles to the mass production of large diameter Czochralski crystals, the long production cycle due to slow growth rate and the high heat requirements of the furnaces. Also counterproductive would be the large resistivity gradient along the growth direction of the crystals due to impurity concentration. Comparison between Float zone (FZ) and Cz crystal growth on the basis of a crystal 150 mm in diameter is on an order of two to four times in favor of the FZ method. This advantage results from high growth rates and steady-state growth while maintaining a dislocation-free condition and impurity segregation.

Abe, T.↗
NASA NTRS2014

Behavior of Particle Depots in Molten Silicon During Float-Zone Growth in Strong Magnetic Fields

Solar cells made from directionally solidified silicon cover 57% of the photovoltaic industry's market [1]. One major issue during directional solidification of silicon is the precipitation of foreign phase particles. These particles, mainly SiC and Si3N4, are precipitated from the dissolved crucible coating, which is made of silicon nitride, and the dissolution of carbon monoxide from the furnace atmosphere. Due to their hardness and size of several hundred micrometers, those particles can lead to severe problems during the wire sawing process for wafering the ingots. Additionally, SiC particles can act as a shunt, short circuiting the solar cell. Even if the particles are too small to disturb the wafering process, they can lead to a grit structure of silicon micro grains and serve as sources for dislocations. All of this lowers the yield of solar cells and reduces the performance of cells and modules. We studied the behaviour of SiC particle depots during float-zone growth under an oxide skin, and strong static magnetic fields. For high field strengths of 3T and above and an oxide layer on the sample surface, convection is sufficiently suppressed to create a diffusive like regime, with strongly dampened convection [2, 3]. To investigate the difference between atomically rough phase boundaries and facetted growth, samples with [100] and [111] orientation were processed.

Jauss, T.↗
NASA NTRS2014

Behavior of Particle Depots in Molten Silicon During Float-Zone Growth in Strong Static Magnetic Fields

In the photovoltaics industry, the largest market share is represented by solar cells made from multicrystalline silicon, which is grown by directional solidification. During the growth process, the silicon melt is in contact with the silicon nitride coated crucible walls and the furnace atmosphere which contains carbon monoxide. The dissolution of the crucible coating, the carbon bearing gas, and the carbon already present in the feedstock, lead to the precipitation of silicon carbide, and silicon nitride, at later stages of the growth process. The precipitation of Si3N4 and SiC particles of up to several hundred micrometers in diameter leads to severe problems during the wire sawing process for wafering the ingots. Furthermore the growth of the silicon grains can be negatively influenced by the presence of particles, which act as nucleation sources and lead to a grit structure of small grains and are sources for dislocations. If doped with Nitrogen from the dissolved crucible coating, SiC is a semi conductive material, and can act as a shunt, short circuiting parts of the solar cell. For these reasons, the incorporation of such particles needs to be avoided. In this contribution we performed model experiments in which the transport of intentionally added SiC particles and their interaction with the solid-liquid interface during float zone growth of silicon in strong steady magnetic fields was investigated. SiC particles of 7μm and 60μm size are placed in single crystal silicon [100] and [111] rods of 8mm diameter. This is achieved by drilling a hole of 2mm diameter, filling in the particles and closing the hole by melting the surface of the rod until a film of silicon covers the hole. The samples are processed under a vacuum of 1x10(exp -5) mbar or better, to prevent gas inclusions. An oxide layer to suppress Marangoni convection is applied by wet oxidation. Experiments without and with static magnetic field are carried out to investigate the influence of melt convection on the distribution of particles and their incorporation into the crystal. The field strengths applied by a superconducting magnet are 1T, 3T, 4.5T, and 5T. The increase in field strength dampens the melt flow, and so this study provides comparative data to the crystal growth experiment to be carried out onboard the sounding rocket mission TEXUS 51, where purely diffusive growth condition will be achieved under microgravity conditions.

Jauss, T.↗
NASA NTRS1975

Studies of rotating liquid floating zones on Skylab IV

Liquid zones of water, soap solution and soap foam were deployed between two aligned circular disks which were free to rotate about the zone axis in the microgravity environment of Skylab IV. Such a configuration is of interest in the containerless handling of melts for possible future space processing crystal growth experiments. Three basic types of zone surface deformation and instability were observed for these rotational conditions; axisymmetric shape changes under single disk rotation, nonaxisymmetric, whirling, C-modes for long zones with equal rotation of both disks, and capillary wave phenomena for short zones with equal rotation of both disks. The sources of these instabilities and the conditions promoting them are analyzed in detail from video tape recordings of the Skylab experiments.

Carruthers, J. R.↗
NASA NTRS1976

Space processing float zone thermal analysis

Thermal analysis (BETA) computer program adaptations were prepared to analyze phase change histories in crystal specimens. The first program (BETA-CYL) treats right circular cylinder configurations and the second, more general, program (BETA-BOR) treats a generalized body-of-revolution configuration. A series of computer runs were made for silicon material to determine boundary conditions which produce flat solidification interfaces while, at the same time, minimizing peak temperatures in the molten zone. Flat solidification interfaces are a goal believed by some investigators to be required to produce high quality semiconductor materials. The thermal effects of convection in a molten zone were examined and found to be negligible in comparison to the conduction heat transfer of the melt.

Pogson, J. T.↗
NASA NTRS1976

Analysis of surface tension driven flow in floating zone melting

Surface tension driven flow in a cylindrical melt suspended between two rods was investigated by numerical solution of the steady state differential equations for heat and momentum transfer. Radiation heating and electron beam heating were considered approximately. For small values of the driving force, one rotating ring was formed in the top half of the zone, and its mirror image in the bottom half. At larger driving forces, secondary cells form which probably would undergo oscillatory motion. The influence of Prandtl number, zone movement, and buoyancy on the convection was also studied. The primary resistance to mass transfer in the laminar regime was in the center of the zone rather than at the solid-liquid interfaces.

Chang, C. E.↗
NASA NTRS1980

Float zone silicon for IR detectors

An overview of the state of the art of processing silicon for use as infrared detectors is presented. Improvements in the process that could be obtained in a low-g environment were examined. The various requirements for the different types of Si detector materials are summarized.

Kern, E. L.↗
NASA NTRS1980

Float zone refining: Space problems and solutions. A speculative proposal

A facility for the automatic growth of 60 cm long boules of Si up to 10 cm diameter is described. The facility is 21 ft tall, weighs 17,000 lb (without the sand ballast), and consumes 60 kW power. As a point of reference, the facility will cost approximately $500K without automation which will be developed. The total price is expected to be approximately $1.2 million.

Gill, G.↗