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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Thermal-capillary analysis of small-scale floating zones Steady-state calculations

Galerkin finite element analysis of a thermal-capillary model of the floating zone crystal growth process is used to predict the dependence of molten zone shape on operating conditions for the growth of small silicon boules. The model accounts for conduction-dominated heat transport in the melt, feed rod and growing crystal and for radiation between these phases, the ambient and a heater. Surface tension acting on the shape of the melt/gas meniscus counteracts gravity to set the shape of the molten zone. The maximum diameter of the growing crystal is set by the dewetting of the melt from the feed rod when the crystal radius is large. Calculations with small Bond number show the increased zone lengths possible for growth in a microgravity environment. The sensitivity of the method to the shape and intensity of the applied heating distribution is demonstrated. The calculations are compared with experimental observations.

Duranceau, J. L.↗
NASA NTRS1987

Floating frame grounding system

The development of a floating frame grounding system (FFGS) for the 40- by 80-foot low speed wind tunnel facility at the NASA Ames Research Center National Full Scale Aerodynamics Complex is addresssed. When electrical faults are detected, the FFGS ensures a ground path for the fault current. In addition, the FFGS alerts the tunnel operator when a mechanical foul occurs.

Forsyth, T. J.↗
NASA NTRS1988

Figured grazing incidence mirrors from reheated float glass

High throughput grazing incidence mirrors have been fabricated from float glass using a new inexpensive technique. An array of twelve such mirrors including grazing angles from 3 to 15 deg has been fabricated and tested. Optical measurement of this array shows a line spread function of 6.3-min of arc FWHM with 90 percent of the energy enclosed within 13 min of arc. Three-mirror arrays are used to provide one-dimensional focusing for a diffuse extreme ultraviolet spectrometer. This paper presents the fabrication techniques and testing procedures used, as well as the mirror performance results.

Labov, Simon E.↗
NASA NTRS1987

Laminar mixing in a small floating zone

The relationship between the flow and solute fields during steady mass transfer of a dilute component is analyzed for multi-cellular rotating flows in the floating zone process of semiconductor growth. When the recirculating flows are weak in relation to the rate of crystal growth, a closed-form solution clearly shows the link between the convection pattern in the melt and the solute distribution across the surface of the growing solid. In the limit of strong convection, finite element calculations demonstrate the tendency of the composition to become uniform over the majority of the melt. The solute segregation in the product crystal is greatest when the recirculating motion is comparable to the rate of crystal growth, and points to the danger in attempting to grow compositionally uniform materials from a nearly convectionless melt.

Harriott, George M.↗
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 NTRS1989

Steady-state thermal-solutal diffusion in a float zone

A model for a float zone in a thin vertical sheet is used to study the formation of the zone configuration for both pure and binary systems of Si and GeGa. Equations describing the steady two-dimensional diffusion of solute in the melt and diffusion of heat in the feed material and the product crystal are presented. The material properties, asymptotic solutions for the temperature and concentration profiles, and melting, solidifying, and melt/gas interfacial shapes are determined in the small aspect ratio limit.

Young, G. W.↗
NASA NTRS1989

Amazon capims (floating grassmats) - A source of C-13 enriched methane to the troposphere

The C-13 isotopic composition of methane emitted to the troposphere from Amazon capims (floating grassmats) ranged from -36.9 to -48.0, per mil averaging -44.4 + or - 4.2 per mil. All pools of methane associated with the grassmats were enriched; methane withdrawn from plant stems ranged from -39 to -49 per mil while bubbles stirred from the root mat averaged -41.4 per mil. As the CH4 flux from these habitats makes up some 40 percent of the total flux from the Amazon floodplain, methane emissions from the region as a whole must be enriched in.

Chanton, Jeffrey↗
NASA NTRS1990

Porous-Floating-Gate Field-Effect Transistor

Porous-floating-gate, "vertical" field-effect transistor proposed as programmable analog memory device especially suitable for use in electronic neural networks. Analog value of electrical conductance of device represents synaptic weight (strength of synaptic connection) repeatedly modified by application of suitable writing or erasing voltage. Suited for hardware implementations of massively parallel neural-network architectures for two important reasons: vertical transistor structure requires only two external electrodes, and use of tailored amorphous semiconductors provides choice of very wide range of low conductivity values, dictated by overall power dissipation requirements in massively parallel neural-network circuits.

Thakoor, Anilkumar P.↗
NASA NTRS1990

Fresh look at floating shock fitting

A fast implicit upwind procedure for the two-dimensional Euler equations is described that allows accurate computations of shocked flows on nonadapted meshes. Away from shocks, the second-order accurate upwinding is based on the split-coefficient-matrix (SCM) method. In the presence of shocks, the difference stencils are modified using a floating shock fitting technique. Rapid convergence to steady-state solutions is attained with a diagonalized approximate factorization (AF) algorithm. Results are presented for Riemann's problem, for a regular shock reflection at an inviscid wall, for supersonic flow past a cylinder, and for a transonic airfoil. All computed shocks are ideally sharp and in excellent agreement with other numerical results or 'exact' solutions. Most importantly, this has been accomplished on unusually crude meshes without any attempt to align grid lines with shock fronts or to cluster grid lines around shocks.

Hartwich, PETER-M.↗
NASA NTRS1990

Thermocapillary convection in a model float-zone

A finite-element method has been used to study thermocapillary convection in a model of relevance to the float-zone, crystal-growth process. The geometry consists of a pair of horizontal isothermal surfaces with a Boussinesq liquid suspended between them. Because of the variation of surface tension with temperature, the temperature gradient along the free surface drives thermocapillary convection. The free surface is allowed to deform and its location is calculated along with the velocity and temperature fields. Cases in which thermocapillarity dominates and others in which it interacts with an unstable axial buoyancy gradient are treated. In order to obtain information of possible interest to potential microgravity applications, the influence of the Grashof number is investigated.

Neitzel, G. P.↗
NASA NTRS1991

Toward a formal verification of a floating-point coprocessor and its composition with a central processing unit

Discussed here is work to formally specify and verify a floating point coprocessor based on the MC68881. The HOL verification system developed at Cambridge University was used. The coprocessor consists of two independent units: the bus interface unit used to communicate with the cpu and the arithmetic processing unit used to perform the actual calculation. Reasoning about the interaction and synchronization among processes using higher order logic is demonstrated.

Pan, Jing↗
NASA NTRS1990

Thermocapillary flow and melt/solid interfaces in floating-zone crystal growth under microgravity

Computer simulation of steady-state axisymmetrical heat transfer and fluid flow was conducted to study thermocapillary flow and melt/solid interfaces in floating-zone crystal growth under microgravity. The effects of key variables on the extent of thermocapillary flow in the melt zone, the shapes of melt/solid interfaces and the length of the melt zone were discussed. These variables are: (1) the temperature coefficient of surface tension (or the Marangoni number), (2) the pulling speed (or the Peclet number), (3) the feed rod radius, (4) the ambient temperature distribution, (5) the heat transfer coefficient (or the Biot number), and (6) the thermal diffusivity of the material (or the Prandtl number).

Lan, C. W.↗
NASA NTRS1991

A hardware-oriented algorithm for floating-point function generation

An algorithm is presented for performing accurate, high-speed, floating-point function generation for univariate functions defined at arbitrary breakpoints. Rapid identification of the breakpoint interval, which includes the input argument, is shown to be the key operation in the algorithm. A hardware implementation which makes extensive use of read/write memories is used to illustrate the algorithm.

O'Grady, E. Pearse↗
NASA NTRS1992

Floating Ring-Groove Lapper

Tool fits out-of-round seal groove and laps it to fine finish without binding. Includes floating lapping pieces riding freely in groove, and are curved to match nominal diameter of groove. One lapping piece tightened so it does not move relative to disk.

Williams, Robert L., Sr.↗
NASA NTRS1991

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

The energy-stability theory has been used to study the stability properties of thermocapillary convection in a half-zone model of the float-zone crystal-growth process. It is concluded that the energy theory provides a sufficient condition for stability to disturbances of arbitrary amplitude. Results of the computation of energy-stability limits are found to be in good agreement with experimentally measured bounds for oscillatory thermocapillary convection.

Neitzel, G. P.↗
NASA NTRS1991

Energy stability of thermocapillary convection in a model of the float-zone crystal-growth process. II - Nonaxisymmetric disturbances

Energy-stability theory has been applied to investigate the stability properties of thermocapillary convection in a half-zone model of the float-zone crystal-growth process. An earlier axisymmetric model has been extended to permit nonaxisymmetric disturbances, thus determining sufficient conditions for stability to disturbances of arbitrary amplitude. The results for nonaxisymmetric disturbances are compared with earlier axisymmetric results, with linear-stability results for a geometry with an infinitely long aspect ratio and with stability boundaries from recent laboratory experiments.

Neitzel, G. P.↗
NASA NTRS1991

Y-MP floating point and Cholesky factorization

The floating point arithmetics implemented in the Cray 2 and Cray Y-MP computer systems are nearly identical, but large scale computations performed on the two systems have exhibited significant differences in accuracy. The difference in accuracy is analyzed for Cholesky factorization algorithm, and it is found that the source of the difference is the subtract magnitude operation of the Cray Y-MP. The results from numerical experiments for a range of problem sizes are presented, and an efficient method for improving the accuracy of the factorization obtained on the Y-MP is presented.

Carter, Russell↗
NASA NTRS1992

Heat transfer and fluid flow in floating-zone crystal growth with a mostly covered melt surface

Computer simulation was conducted to study a modified floating-zone crystal growth process, in which the melt surface is mostly covered with a heated ring. The growth of 6-mm diameter single crystals of NaNO3 was considered, and the effects of the following parameters were studied: (1) temperature of the ring, (2) growth rate, (3) surface tension-temperature coefficient of the melt, (4) thermal expansion coefficient of the melt, and (5) gravity. It was demonstrated that thermocapillary convection in the melt zone is reduced significantly in this modified process. The model was checked against the measured lengths of the meniscus near the growth front and axial temperature distribution in a growing crystal, and the agreement was good.

Lan, C. W.↗