Search NASA⌕ Search

SEARCH · Search NASA

Results for “melt growth”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Large area sheet task: Advanced Dendritic Web Growth Development

A melt level control system was implemented to provide stepless silicon feed rates from zero to rates exactly matching the silicon consumed during web growth. Bench tests of the unit were successfully completed and the system mounted in a web furnace for operational verification. Tests of long term temperature drift correction techniques were made; web width monitoring seems most appropriate for feedback purposes. A system to program the initiation of the web growth cycle was successfully tested. A low cost temperature controller was tested which functions as well as units four times as expensive.

Duncan, C. S.↗
NASA NTRS1995

Thermal Characterization of the Universal Multizone Crystallizator

The Universal Multizone Crystallizator (UMC) is a special apparatus for crystal growth under terrestrial and microgravity conditions. The use of twenty-five zones allows the, UMC to be used for several normal freezing growth techniques. The thermal profile is electronically translated along the stationary sample by systematically reducing the power to the control zones. Elimination of mechanical translation devices increases the systems reliability while simultaneously reducing the size and weight. This paper addresses the UMC furnace design, sample cartridge, typical thermal profiles and corresponding power requirements necessary for two normal freezing techniques: dynamic gradient freeze and zone melting crystal growth.

Watring, D. A.↗
NASA NTRS1993

Convection effects on radial segregation and crystal melt interface in vertical Bridgman growth

We analytically study the influence of convection caused by horizontal heat transfer through the sides of a vertical Bridgman apparatus. We consider the case when the heat transfer across the side walls is small so that the resulting interfacial deformation and fluid velocities are also small. This allows us to linearize the Navier-Stokes equations and express the interfacial conditions about a planar interface through a Taylor expansion. Using a no tangential stress conditions on the side walls, asymptotic expressions for both the interfacial slope, and radial segregation at the crystal-melt interface are obtained in closed form in the limit of large thermal Rayleigh number. It is suggested that these can be reduced by appropriately controlling a specific heat transfer property at the edge of the insulation zone in the solid side.

Tanveer, S.↗
NASA NTRS1994

Century/millennium internal climate oscillations in an ocean-atmosphere-continental ice sheet model

We demonstrate in a simple climate model that there exist nonlinear feedbacks between the atmosphere, ocean, and ice sheets capable of producing century/millennium timescale internal oscillations resembling those seen in the paleoclimate record. Feedbacks involve meridional heat and salt transports in the North Atlantic, surface ocean freshwater fluxes associated with melting and growing continental ice sheets in the northen hemisphere and with Atlantic to Pacific water vapor transport. The positive feedback between the production of North Atlantic Deep Water (NADW) and the meridional salt transport by the Atlantic thermohaline circulation tends to destabilize the climate system, while the negative feedback between the freshwater flux, either to or from the continental ice sheets, and meridional heat flux to the high-latitude North Atlantic, accomplished by the thermohaline circulation, stabilizes the system. The thermohaline circulation plays a central role in both positive and negative feedbacks because of its transport of both heat and salt. Because of asymmetries between the growth and melt phases the oscillations are, in general, accompanied by a growing or decreasing ice volume over each cycle, which in the model is reflected by increasing or decreasing mean salinity.

Birchfield, Edward G.↗
NASA NTRS2017

The Effect of Firn-Aquifer Drainage on the Greenland Subglacial System or Subglacial Efficiency and Storage Modified by the Temporal Pattern of High-Elevation Meltwater Input

Ice flow in marginal region of the Greenland Ice Sheet dynamically responds to summer melting as surface meltwater is routed through the supraglacial hydrologic system to the bed of the ice sheet via crevasses and moulins. Given the expected increases in surface melt production and extent, and the potential for high elevation surface-to-bed connections, it is imperative to understand how meltwater delivered to the bed from different high-elevation supraglacial storage features affects the evolution of the subglacial hydrologic system and associated ice dynamics. Here, we use the two-dimensional subglacial hydrologic model, GLaDS, which includes distributed and channelized water flow, to test how the subglacial system of an idealized outlet glacier responds to cases of high-elevation firn-aquifer-type and supraglacial-lake-type englacial drainage over the course of 5 years. Model outputs driven by these high elevation drainage types are compared to steady-state model results, where the subglacial system only receives the 1980- 2016 mean MERRA-2 runoff via low-elevation moulins. Across all experiments, the subglacial hydrologic system displays inter-annual memory, resulting in multiyear declines in subglacial pressure during the onset of seasonal melting and growth of subglacial channels. The gradual addition of water in firn-aquifer-type drainage scenarios resulted in small increases in subglacial water storage but limited changes in subglacial efficiency and channelization. Rapid, supraglacial- lake-type drainage resulted in short-term local increases in subglacial water pressure and storage, which gave way to spatially extensive decreases in subglacial pressure and downstream channelization. These preliminary results suggest that the character of high-elevation englacial drainage can have a strong, and possibly outsized, control on subglacial efficiency throughout the ablation zone. Therefore, understanding both how high elevation meltwater is stored supraglacially and the probability of crevassing at high elevations will play an important role in how the subglacial system, proglacial discharge and ice motion will respond to future increases in surface melt production and runoff.

Andrews, Lauren C.↗
NASA NTRS2000

Detached and Floating-Zone Growth of Semiconductor Crystals on the ISS

Understanding the mechanism of detached Bridgman growth and establishing the growth of large scale germanium-silicon crystals by the float-zone technique are the key points of the project "RDGS - Reduction of Defects in Germanium-Silicon". The contact angle of the melt and the growth angle of the crystal are essential parameters which allow a controlled use of detached growth. The contact angle was determined for a variety of different substrates and melt compositions; pBN showed the highest value for pure germanium as well as for germanium-rich GeSi melts. The growth angle of Ge(sub 0.95) Si(sub 0.05) was measured to be 8.5-10.5 degrees which concurs with the values of pure germanium and silicon, respectively. The temperature dependence and the concentration dependence of the surface tension were determined for concentrations up to 10at% silicon (partial derivative (gamma)/partial derivative T=-0.08 (raised dot) 10(exp -3)N/m (raised dot) K, partial derivative (gamma)/partial derivative (C)=2.2 (raised dot) 10(exp -3)N/m (raised dot) at%). Using these values, the critical Marangoni number indicating the transition to time-dependent thermocapillary flow will be exceeded for the growth of large scale float-zone crystals onboard the ISS. Therefore, suitable tools for flow control are required.

Dold, P.↗
NASA NTRS1970

Preliminary terrestrial based experiments on gravity-affected crystal growth

Tin was melted in a heating assembly secured to the arm of a centrifuge. The furnace was allowed to pivot and reach its equilibrium angle of swing for the gravity force being experienced. The crucible was cooled during rotation to allow the growth of single crystals. The crystals were etched for the purpose of observing the growth striations. Slices were removed from some of the crystals to permit observation of the striations in the interior. Visual analyses were made with a scanning electron microscope. Preliminary conclusions relating the appearance of the striations to gravity forces and the affected growth mechanisms are presented. Further experiments that will verify these conclusions and determine other gravity effects are proposed.

Johnston, M. H.↗
NASA NTRS1980

Ice conditions on the Chesapeake Bay as observed from LANDSAT during the winters of 1977, 1978 and 1979

The LANDSAT observations during the winters of 1977, 1978 and 1979, which were unusually cold in the northeastern U.S. and in the Chesapeake Bay area, were evaluated. Abnormal atmospheric circulation patterns displaced cold polar air to the south, and as a result, the Chesapeake Bay experienced much greater than normal icing conditions during these 3 years. The LANDSAT observations of the Chesapeake Bay area during these winters demonstrate the satellite's capabilities to monitor ice growth and melt, to detect ice motions, and to measure ice extent.

Foster, J. L.↗
NASA NTRS1981

Float Zone Workshop

Presentations made on space based float zone crystal growth and melt behavior are summarized and handouts illustrating the presentations are presented.

E. L. Kern↗
NASA NTRS1983

Materials processing in the reduced-gravity environment of space

Studies of the effects of gravitation on the behavior of fluids and the shapes of materials for the years 1977-1982 are reviewed, with a focus on ground-based research which complements and gives direction to experimentation in the reduced-gravity environment of space. Fluid-behavior topics discussed include crystal growth from melts and from vapor, dendritic solidification and casting, multiphase alloy solidification, electrophoresis, convection behavior, and special heat-flow considerations characteristic of the space environment. With regard to solids, such subjects as containerless positioning technology and its applications (in glass preparation, liquid drop and shell studies, nucleation experiments, and measurements of fundamental properties of molten reactants), liquid floating zones, solid self-deformation, and sedimentation and buoyancy effects are considered and illustrated.

Carruthers, J. R.↗
NASA NTRS1985

Sensitivity of a thermodynamic sea ice model with leads to time step size

The characteristics of sea ice models, developed to study the physics of the growth and melt of ice at the ocean surface and the variations in ice extent, depend on the size of the time step. Thus, to study longer-term variations within a reasonable computer budget, a model with a scheme allowing longer time steps has been constructed. However, the results produced by the model can definitely depend on the length of the time step. The sensitivity of a model to time-step size can be reduced by appropriate approaches. The present investigation is concerned with experiments which use a formulation of a lead parameterization that can be considered as a first step toward the development of a lead parameterization suitable for a use in long-term climate studies.

Ledley, T. S.↗
NASA NTRS1986

The effect of fluid flow due to the crystal-melt density change on the growth of a parabolic isothermal dendrite

The Ivantsov (1947) analysis of an isolated isothermal dendrite (with zero surface tension) growing into a supercooled liquid is extended to include the effects of the fluid flow due to volume contraction or expansion upon solidification. For an axisymmetric paraboloidal dendrite, an analytic solution to the Navier-Stokes equations is obtained. The magnitude of the flow is proportional to the relative density change epsilon, and the flow becomes negligible far from the surface of the dendrite. The temperature field consistent with this flow can also be found explicitly. The well-known expression that relates the dimensionless supercooling to the Peclet number in the absence of fluid flow is modified for nonzero epsilon, but the effect is of order epsilon and hence is seen to be minor for most values of epsilon and dimensionless supercooling that occur in practice.

Mcfadden, G. B.↗
NASA NTRS1986

The materials processing research base of the Materials Processing Center

An annual report of the research activities of the Materials Processing Center of the Massachusetts Institute of Technology is given. Research on dielectrophoresis in the microgravity environment, phase separation kinetics in immiscible liquids, transport properties of droplet clusters in gravity-free fields, probes and monitors for the study of solidification of molten semiconductors, fluid mechanics and mass transfer in melt crystal growth, and heat flow control and segregation in directional solidification are discussed.

Latanision, R. M.↗
NASA NTRS1993

Imaging radar studies of polar ice

A vugraph format presentation is given. The following topics are discussed: scientific overview, radar data opportunities, sea ice investigations, and ice sheet investigations. The Sea Ice Scientific Objectives are as follows: (1) to estimate globally the surface brine generation, heat flux, and fresh water advection (as ice); (2) to monitor phasing of seasonal melt and freeze events and accurately estimate melt and growth rates; and (3) to develop improved treatment of momentum transfer and ice mechanics in coupled air-sea-ice models.

Carsey, Frank↗
NASA NTRS1992

A toy model of sea ice growth

My purpose here is to present a simplified treatment of the growth of sea ice. By ignoring many details, it is possible to obtain several results that help to clarify the ways in which the sea ice cover will respond to climate change. Three models are discussed. The first deals with the growth of sea ice during the cold season. The second describes the cycle of growth and melting for perennial ice. The third model extends the second to account for the possibility that the ice melts away entirely in the summer. In each case, the objective is to understand what physical processes are most important, what ice properties determine the ice behavior, and to which climate variables the system is most sensitive.

Thorndike, Alan S.↗
NASA NTRS1994

Arctic sea ice concentrations from special sensor microwave imager and advanced very high resolution radiometer satellite data

Nearly coincident data from the special sensor microwave imager (SSM/I) and the advanced very high resolution radiometer (AVHRR) are used to compute and compare Arctic sea ice concentrations for different regions and times of the year. To help determine overall accuracies and to highlight sources of differences between passive microwave, optical wavelength, and thermal wavelength data, ice concentrations are estimated using two operational SSM/I ice concentration algorithms and with visible- and thermal-infrared wavelength AVHRR data. All algorithms capture the seasonal patterns of ice growth and melt. The ranges of differences fall within the general levels of uncertainty expected for each method and are similar to previous accuracy estimates. The estimated ice concentrations are all highly correlated, with uniform biases, although differences between individual pairs of observations can be large. On average, the NASA Team algorithm yielded 5% higher ice concentrations than the Bootstrap algorithm, while during nonmelt periods the two SSM/I algorithms agree to within 0.5%. These seasonal differences are consistent with the ways that the 19-GHz and 37-GHz microwave channels are used in the algorithms. When compared to the AVHRR-derived ice concentrations, the Team-algorithm results are more similar on average in terms of correlation and mean differences. However, the Team algorithm underestimates concentrations relative to the AVHRR output by 6% during cold months and overestimates by 3% during summer. Little seasonal difference exists between the Bootstrap and AVHRR results, with a mean difference of about 5%. Although the mean differences are less between the SSM/I-derived concentrations and concentrations estimated using AVHRR channel 1, the correlations appear substantially better between the SSM/I data and concentrations derived from AVHRR channel 4, particularly for the Team algorithm output.

Emery, W. J.↗
NASA NTRS2013

Improving Surface Mass Balance Over Ice Sheets and Snow Depth on Sea Ice

Surface mass balance (SMB) over ice sheets and snow on sea ice (SOSI) are important components of the cryosphere. Large knowledge gaps remain in scientists' abilities to monitor SMB and SOSI, including insufficient measurements and difficulties with satellite retrievals. On ice sheets, snow accumulation is the sole mass gain to SMB, and meltwater runoff can be the dominant single loss factor in extremely warm years such as 2012. SOSI affects the growth and melt cycle of the Earth's polar sea ice cover. The summer of 2012 saw the largest satellite-recorded melt area over the Greenland ice sheet and the smallest satellite-recorded Arctic sea ice extent, making this meeting both timely and relevant.

Snow↗
NASA NTRS2021

Arctic Cloud Response to a Perturbation in Sea Ice Concentration: The North Water Polynya

Surface and atmosphere energy exchanges play an important role in the Arctic climate system by influencing the lower atmospheric stability and humidity, sea ice melt and growth, and surface temperature. Sea ice significantly alters the character of these energy exchanges relative to ice-free ocean. The observed decline in Arctic sea ice since 1979 motivates questions related to the evolving role of surface-atmosphere coupling and potential feedbacks on the Arctic system. Due to the strong wintertime cloud warming effect, a critical question concerns the potential response of low clouds to Arctic sea ice decline. Previous approaches relied on interannual variability to investigate the cloud response to sea ice decline. However, the covariation between atmospheric conditions and sea ice makes it difficult to define an observational control when using interannual variability. To circumvent this difficulty, we exploit the recurring North Water polynya, an episodic opening in the northern Baffin Bay sea ice, as a natural laboratory to isolate the cloud response to a rapid, near-step perturbation in sea ice. Our results show that during the event, (a) low-cloud cover is 10%–33% larger over the polynya than nearby sea ice, (b) cloud liquid water content is up to 400% larger over the polynya than nearby sea ice, and (c) the surface cloud radiative effect is 18 W/sq. m larger over the polynya than nearby sea ice. Our results provide evidence that the low-cloud response during a polynya is a positive feedback lengthening the event.

Emily E. Monroe↗