Porosity effects in the ionization of cesium on tungsten.
Cesium vapor ionization on porous tungsten substrate creates gradients in surface atom concentrations diminishing ionization efficiency
IONIZATION POTENTIAL↗
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Toward Understanding Pore Formation and Mobility During Controlled Directional Solidification in a Microgravity Environment Investigation (PFMI)
Porosity in the form of "bubbles and pipes" can occur during controlled directional solidification processing of metal alloys. It is detrimental to material properties and precludes obtaining meaningful scientific results. On Earth, density differences allow an initiated bubble can rise through the liquid and "pop" at the surface resulting in a sound casting. This is not likely to occur in a microgravity environment and, unfortunately, a number of experiments conducted in microgravity have suffered from porosity effects. The current investigation is a systematic effort towards understanding porosity formation and mobility during controlled directional solidification in a microgravity environment. This will be investigated by utilizing a transparent material, succinonitrile (SCN), in conjunction with a translating temperature gradient stage so that direct observation and recording of pore generation and mobility can be made. The talk will cover the porosity problem, the details of the proposed experiments and the experimental hardware, and the expectations from the microgravity experiments.
Grugel, R. N.↗
Pore Formation and Mobility (PFMI): An International Space Station Glovebox Investigation
Porosity in the form of "bubbles and pipes" can occur during controlled directional solidification processing of metal alloys. It is detrimental to material properties and precludes obtaining meaningful scientific results. On Earth, density differences allow an initiated bubble can rise through the liquid and "pop" at the surface resulting in a sound casting. This is not likely to occur in a microgravity environment and, unfortunately, a number of experiments conducted in microgravity have suffered from porosity effects. The current investigation is a systematic effort towards understanding porosity formation and mobility during controlled directional solidification in a microgravity environment. This will be investigated by utilizing a transparent material, succinonitrile (SCN), in conjunction with a translating temperature gradient stage so that direct observation and recording of pore generation and mobility can be made. The talk will cover the porosity problem, the details of the proposed experiments and the experimental hardware, and the expectations from the microgravity experiments.
Grugel, R. N.↗
The influence of high thermal gradient casting, hot isostatic pressing and alternate heat treatment on the structure and properties of a single crystal nickel base superalloy
A development program has been conducted to improve the cyclic properties of the PWA 1480 single-crystal superalloy by reducing or entirely eliminating casting porosity at fatigue-initiation sites, through the use of improved casting process parameters and HIPing; potential mechanical property improvements in a high-pressure hydrogen environment were also sought in alternatives to the standard coating and heat-treatment cycle. High thermal gradient casting was found to yield a reduction in overall casting porosity density and pore sizes. The most dramatic mechanical property improvement resulted from HIPing.
Fritzemeier, L. G.↗
Controls on Martian Hydrothermal Systems: Application to Valley Network and Magnetic Anomaly Formation
Models of hydrothermal groundwater circulation can quantify limits to the role of hydrothermal activity in Martian crustal processes. We present here the results of numerical simulations of convection in a porous medium due to the presence of a hot intruded magma chamber. The parameter space includes magma chamber depth, volume, aspect ratio, and host rock permeability and porosity. A primary goal of the models is the computation of surface discharge. Discharge increases approximately linearly with chamber volume, decreases weakly with depth (at low geothermal gradients), and is maximized for equant-shaped chambers. Discharge increases linearly with permeability until limited by the energy available from the intrusion. Changes in the average porosity are balanced by changes in flow velocity and therefore have little effect. Water/rock ratios of approximately 0.1, obtained by other workers from models based on the mineralogy of the Shergotty meteorite, imply minimum permeabilities of 10(exp -16) sq m2 during hydrothermal alteration. If substantial vapor volumes are required for soil alteration, the permeability must exceed 10(exp -15) sq m. The principal application of our model is to test the viability of hydrothermal circulation as the primary process responsible for the broad spatial correlation of Martian valley networks with magnetic anomalies. For host rock permeabilities as low as 10(exp -17) sq m and intrusion volumes as low as 50 cu km, the total discharge due to intrusions building that part of the southern highlands crust associated with magnetic anomalies spans a comparable range as the inferred discharge from the overlying valley networks.
Harrison, Keith P.↗
Bidirectional Brush Seals: Post-Test Analysis
A post-test analysis of a set of inside-diameter/outside-diameter (ID/OD) bidirectional brush seals used in three-port wave rotor tests was undertaken to determine brush bristle and configuration wear, pullout, and rotor coating wear. The results suggest that sharp changes in the pressure profiles were not well reflected in bristle tip configuration patterns or wear. Also, positive-to-negative changes in axial pressure gradients appeared to have little effect on the backing plates. Although the brushes had similar porosities, they had very different unpacked arrays. This difference could explain the departure of experimental data from computational fluid dynamics flow predictions for well-packed arrays at higher pressure drops. The rotor wear led to "car-track" scars (upper and lower wear bands) with a whipped surface between the bands. Those bands may have resulted from bristle stiffening at the fence and gap plates during alternate portions of the rotor cycle. Within the bristle response range the wear surface reflected the pressure distribution effect on bristle motion. No sacrificial metallurgical data were taken. The bristles did wear, with correspondingly more wear on the ID brush configurations than on the OD configurations; the complexity in constructing the ID brush was a factor.
Hendricks, Robert C.↗
Direct Observation of Pore Formation and Bubble Mobility during Controlled Melting and Resolidification in Microgravity
Detailed studies on the controlled melting and subsequent re-solidification of succinonitrile were conducted in the microgravity environment aboard the International Space Station (ISS) using the PFMI apparatus (Pore Formation and Mobility Investigation) located in the ISS glovebox facility (GBX). Samples were initially prepared on ground by filling glass tubes, 1 cm ID and approximately 30 cm in length, with pure succinonitrile (SCN) under 450 millibar of nitrogen. During Space processing, experimental parameters like temperature gradient and translation speed, for melting and solidification, were remotely monitored and controlled from the ground Telescience Center (TSC) at the Marshall Space Flight Center. Real time visualization during controlled melting revealed bubbles of different sizes initiating at the solid/liquid interface, and traveling up the temperature gradient ahead of them. Subsequent controlled re-solidification of the SCN revealed the details of porosity formation and evolution. A preliminary analysis of the melt back and re- solidification and its implications to future microgravity materials processing is presented and discussed.
Grugel, Richard N.↗
Dynamic, High-Temperature, Flexible Seal
New seal consists of multiple plies of braided ceramic sleeves filled with small ceramic balls. Innermost braided sleeve supported by high-temperature-wire-mesh sleeve that provides both springback and preload capabilities. Ceramic balls reduce effect of relatively high porosity of braided ceramic sleeves by acting as labyrinth flow path for gases and thereby greatly increasing pressure gradient seal can sustain. Dynamic, high-temperature, flexible seal employed in hypersonic engines, two-dimensional convergent/divergent and vectorized-thrust exhaust nozzles, reentry vehicle airframes, rocket-motor casings, high-temperature furnaces, and any application requiring non-asbestos high-temperature gaskets.
Steinetz, Bruce M.↗
Laser Ablation Surface Treatment of Structural Aerospace Composites and Metal Alloys for Adhesive Bonding
NASA Langley Research Center (LaRC) has been involved over the past decade in the development of laser ablation as a method to surface treat aerospace structural fiber reinforced composites (CFRPs) and metal alloys. [1- 18] Laser surface treatment has been shown to be a rapid, high precision, and reproducible method for surface treatment of these materials, and is amenable to automation. The laser ablation process removes surface contamination such as silicones, creates surface roughness, and chemically activates a surface for bonding. The degree of ablation is controllable by adjusting laser parameters. In the case of CFRPs, ablation can be performed without damage to any underlying carbon fibers. In the case of metal alloys, ablation is possible without surface embrittlement, heat damage or creation of undesirable grain structures or morphologies. The optimum laser treatment conditions are substrate dependent and generally are influenced by laser wavelength and pulse duration (typically fixed for a laser system), and laser power. In the case of metal alloys such as Ti-6Al-4V, the laser ablation process eliminates the need for several steps of the conventional processes, including hydrofluoric (HF) acid and caustic treatment. These steps are often conducted in dip tanks and require costly facilities and environmentally unfriendly liquids. In addition, the laser can create nano-porosity on the Ti-6Al-4V alloy surface allowing for sol-gel penetration and thereby creating a gradient interface with improved temperature and mechanical performance. NASA LaRC has also been involved with the development of in-line characterization methods compatible with the laser treatment process to rapidly determine the presence of key contaminates such as silicones that, even at very low concentrations, are a threat to adhesive bonding. A spectroscopic technique called micro-laser induced breakdown spectroscopy (u-LIBS) has been developed to rapidly detect silicones at concentrations below those known to cause bonding problems. This technique requires no sample preparation, vacuum or inert atmosphere, has no practical size limitations, and provides nearly instantaneous feedback to the operator. This presentation will give an overview and update of these developments with specific examples highlighted.
Connell, John W.↗
Computational Analysis of the Effect of Porosity on Shock Cell Strength at Cruise
A computational flow field analysis is presented of the effect of core cowl porosity on shock cell strength for a modern separate flow nozzle at cruise conditions. The goal of this study was to identify the primary physical mechanisms by which the application of porosity can reduce shock cell strength and hence the broadband shock associated noise. The flow is simulated by solving the asymptotically steady, compressible, Reynoldsaveraged Navier-Stokes equations on a structured grid using an implicit, up-wind, flux-difference splitting finite volume scheme. The standard two-equation k - epsilon turbulence model with a linear stress representation is used with the addition of a eddy viscosity dependence on total temperature gradient normalized by local turbulence length scale. Specific issues addressed in this study were the optimal area required to weaken a shock impinging on the core cowl surface and the optimal level of porosity and placement of porous areas for reduction of the overall shock cell strength downstream. Two configurations of porosity were found to reduce downstream shock strength by approximately 50%.
Massey, Steven J.↗
Evolution and Transport of Water in the Upper Regolith of Mars
Long standing theoretical predictions [1-3], as well as recent spacecraft observations [4] indicate that large quantities of ice is present in the high latitudes upper decimeters to meters of the Martian regolith. At shallower depths and warmer locations small amounts of H2O, either adsorbed or free, may be present transiently. An understanding of the evolution of water based on theoretical and experimental considerations of the processes operating at the Martian environment is required. In particular, the porosity, diffusivity, and permeability of soils and their effect on water vapor transport under Mars-like conditions have been estimated, but experimental validation of such models is lacking. Goal: Three related mechanisms may affect water transport in the upper Martian regolith. 1) diffusion along a concentration gradient under isobaric conditions, 2) diffusion along a thermal gradient, which may give rise to a concentration gradient as ice sublimes or molecules desorb from the regolith, and 3) hydraulic flow, or mass motion in response to a pressure gradient. Our combined theoretical and experimental investigation seeks to disentangle these mechanisms and determine which process(es) are dominant in the upper regolith over various timescales. A detailed one-dimensional model of the upper regolith is being created which incorporates water adsorption/ desorption, condensation, porosity, diffusivity, and permeability effects. Certain factors such as diffusivity are difficult to determine theoretically due to the wide range of intrinsic grain properties such as particle sizes, shapes, packing densities, and emergent properties such as tortuosity. An experiment is being designed which will allow us to more accurately determine diffusivity, permeability, and water desorption isotherms for regolith simulants.
Hudson, T. L.↗
Advanced SiC/SiC Ceramic Composites For Gas-Turbine Engine Components
NASA Glenn Research Center (GRC) is developing a variety of advanced SiC/SiC ceramic composite (ASC) systems that allow these materials to operate for hundreds of hours under stress in air at temperatures approaching 2700 F. These SiC/SiC composite systems are lightweight (approximately 30% metal density) and, in comparison to monolithic ceramics and carbon fiber-reinforced ceramic composites, are able to reliably retain their structural properties for long times under aggressive gas-turbine engine environments. The key for the ASC systems is related first to the NASA development of the Sylramic-iBN Sic fiber, which displays higher thermal stability than any other SiC- based ceramic fibers and possesses an in-situ grown BN surface layer for higher environmental durability. This fiber is simply derived from Sylramic Sic fiber type that is currently produced at ATK COI Ceramics (COIC). Further capability is then derived by using chemical vapor infiltration (CVI) and/or polymer infiltration and pyrolysis (PIP) to form a Sic-based matrix with high creep and rupture resistance as well as high thermal conductivity. The objectives of this study were (1) to optimize the constituents and processing parameters for a Sylramic-iBN fiber reinforced ceramic composite system in which the Sic-based matrix is formed at COIC almost entirely by PIP (full PIP approach), (2) to evaluate the properties of this system in comparison to other 2700 F Sylramic-iBN systems in which the matrix is formed by full CVI and CVI + PIP, and (3) to examine the pros and cons of the full PIP approach for fabricating hot-section engine components. A key goal is the development of a composite system with low porosity, thereby providing high modulus, high matrix cracking strength, high interlaminar strength, and high thermal conductivity, a major property requirement for engine components that will experience high thermal gradients during service. Other key composite property goals are demonstration at high temperatures of high environmental resistance and high creep resistance, which in turn will result in long component life. Data are presented from a variety of laboratory tests on simple two-dimensional panels that examine these properties and compare the performance of the optimized full PIP system with those of the full CVI and CVI + PIP hybrid systems. Underlying mechanisms for performance differences in the various systems are discussed. Remaining issues for further property enhancement and for application of the full PIP approach for engine components are also discussed, as well as on-going approaches at NASA to solve these issues.
H. M. Yun↗
Measurement of Heat Transfer in Unbonded Silica Fibrous Insulation and Comparison with Theory
Effective thermal conductivity of a high porosity unbonded silica fibrous insulation specimen was measured over a pressure range of 0.001 to 750 torr (0.1 to 101.3 x 10(exp 3) Pa), and with large temperature gradients maintained across the sample thickness: hot side temperature range of 360 to 1360 K, with the cold side at room temperature. The measurements were compared with the theoretical solution of combined radiation/conduction heat transfer. The previously developed radiation heat transfer model used in this study is based on a modified diffusion approximation, and uses deterministic parameters that define the composition and morphology of the medium: distributions of fiber size and orientation, fiber volume fractions, and the spectral complex refractive index of the fibers. The close agreement between experimental and theoretical data further verifies the theoretical model over a wide range of temperatures and pressures.
Daryabeigi, Kamran↗
Pore Formation and Mobility Investigation (PFMI): Concept, Hardware Development, and Initial Analysis of Experiments Conducted Aboard the International Space Station
Porosity in the form of "bubbles and pipes" can occur during controlled directional solidification processing of metal alloys. This is a consequence that 1) precludes obtaining any meaningful scientific results and 2) is detrimental to desired material properties. Unfortunately, several Microgravity experiments have been compromised by porosity. The intent of the PFMl investigation is to conduct a systematic effort directed towards understanding porosity formation and mobility during controlled directional solidification (DS) in a microgravity environment. PFMl uses a pure transparent material, succinonitrile (SCN), as well as SCN "alloyed" with water, in conjunction with a translating temperature gradient stage so that direct observation and recording of pore generation and mobility can be made. PFMl is investigating the role of thermocapillary forces and temperature gradients in affecting bubble dynamics as well as other solidification processes in a microgravity environment. This presentation will cover the concept, hardware development, operations, and the initial results from experiments conducted aboard the International Space Station.
Grugel, Richard N.↗
Pore Formation and Mobility Investigation (PFMI): Concept, Hardware Development and Initial Analysis of Experiments
Porosity in the form of "bubbles and pipes" can occur during controlled directional solidification processing of metal alloys. This is a consequence that 1) precludes obtaining any meaningful scientific results and 2) is detrimental to desired material properties. Unfortunately, several Microgravity experiments have been compromised by porosity. The intent of the PFMI investigation is to conduct a systematic effort directed towards understanding porosity formation and mobility during controlled directional solidification (DS) in a microgravity environment. PFMI uses a pure transparent material, succinonitrile (SCN), as well as SCN "alloyed" with water, in conjunction with a translating temperature gradient stage so that direct observation and recording of pore generation and mobility can be made. PFMI is investigating the role of thermocapillary forces and temperature gradients in affecting bubble dynamics as well as other solidification processes in a microgravity Environment. This presentation will cover the concept, hardware development, operations, and the initial results from experiments conducted aboard the International Space Station. .
Grugel, Richard N.↗
Impacts of free-floating objects: Unique space station experiments
The transfer of momentum and kinetic energy between planetary bodies forms the basis for wide ranging problems in planetary science ranging from the collective long term effects of minor perturbations to the catastrophic singular effect of a major collision. Although the collisional transfer of momentum and energy was discussed over the last two decades, major issues remain that largely reflect current limitations in Earth based experimental conditions and 3-D numerical codes. Two examples with potential applications in a Space Station laboratory, are presented: asteroid spin rates and orientations, and planetary disruption/spin rates. Asteroid spin rate and orientation experiments are needed wherein free floating nonspining and spining objects of varying strength, porosity, and volatility are impacted at varying velocities and angles. A space station platform also could provide an opportunity to test important facets of planetary disruption/spin rate models by allowing freely suspended spherical targets of varying viscosities, internal density gradients, and spin rates.
Schultz, P. H.↗
Interaction of Porosity with a Planar Solid/Liquid Interface
In this paper, an investigation of the interaction between gas porosity and a planar solid/liquid (SL) interface is reported. A two-dimensional numerical model able to accurately track sharp SL interfaces during solidification of pure metals and alloys is proposed. The finite difference method and a rectangular undeformed grid are used for computation. The SL interface is described through the points of intersection with the grid lines. Its motion is determined by the thermal and solute gradients at each particular point. Changes of the interface temperature because of capillarity or solute redistribution as well as any perturbation of the thermal and solute field produced by the presence of non-metallic inclusions can be computed. To validate the model, the dynamics of the interaction between a gas pore and a solidification front in metal alloys was observed using a state of the art X-ray Transmission Microscope. The experiments included observation of the distortion of the SL interface near a pore, real-time measurements of the growth rate and the change in shape of the porosity during interaction with an advancing SL interface in pure Al and Al-0.25 wt% Au alloy. In addition, porosity induced solute segregation patterns surrounding a pore were also quantified.
Catalina, Adrian V.↗
Interaction of Porosity with a Planar Solid/Liquid Interface
In this article, an investigation of the interaction between gas porosity and a planar solid/liquid (SL) interface is reported. A two-dimensional numerical model able to accurately track sharp SL interfaces during solidification of pure metals and alloys is proposed. The finite-difference method and a rectangular undeformed grid are used for computation. The SL interface is described through the points of intersection with the grid lines. Its motion is determined by the thermal and solute gradients at each particular point. Changes of the interface temperature because of capillarity or solute redistribution as well as any perturbation of the thermal and solute field produced by the presence of non-metallic inclusions can be computed. To validate the model, the dynamics of the interaction between a gas pore and a solidification front in metal alloys was observed using a state of the art X-ray transmission microscope (XTM). The experiments included observation of the distortion of the SL interface near a pore, real-time measurements of the growth rate, and the change in shape of the porosity during interaction with the SL interface in pure Al and Al-0.25 wt pct Au alloy. In addition, porosity-induced solute segregation patterns surrounding a pore were also quantified.
Catalina, Adrian V.↗