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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 NTRS2002

Ceramic Inclusions In Powder Metallurgy Disk Alloys: Characterization and Modeling

Powder metallurgy alloys are increasingly used in gas turbine engines, especially as the material chosen for turbine disks. Although powder metallurgy materials have many advantages over conventionally cast and wrought alloys (higher strength, higher temperature capability, etc.), they suffer from the rare occurrence of ceramic defects (inclusions) that arise from the powder atomization process. These inclusions can have potentially large detrimental effect on the durability of individual components. An inclusion in a high stress location can act as a site for premature crack initiation and thereby considerably reduce the fatigue life. Because these inclusions are exceedingly rare, they usually don't reveal themselves in the process of characterizing the material for a particular application (the cumulative volume of the test bars in a fatigue life characterization is typically on the order of a single actual component). Ceramic inclusions have, however, been found to be the root cause of a number of catastrophic engine failures. To investigate the effect of these inclusions in detail, we have undertaken a study where a known population of ceramic particles, whose composition and morphology are designed to mimic the 'natural' inclusions, are added to the precursor powder. Surface connected inclusions have been found to have a particularly large detrimental effect on fatigue life, therefore the volume of ceramic 'seeds' added is calculated to ensure that a minimum number will occur on the surface of the fatigue test bars. Because the ceramic inclusions are irregularly shaped and have a tendency to break up in the process of extrusion and forging, a method of calculating the probability of occurrence and expected intercepted surface and embedded cross-sectional areas were needed. We have developed a Monte Carlo simulation to determine the distributions of these parameters and have verified the simulated results with observations of ceramic inclusions found in macro slices from extrusions and forgings. The ultimate goal of this study will be to use probabilistic methods to determine the reliability detriment that can be attributed to these ceramic inclusions.

Bonacuse, Pete↗
NASA NTRS1979

Structure and electrical activity of planar defects in EFG ribbons

Optical, electron beam induced current (EBIC), and transmission electron microscopy were used to study the structure and electrical activity of planar defects in EFG silicon. What appears to be twin boundaries by both optical microscopy plus etching, and by EBIC are in reality systems of microtwins, some of which are only a few atomic lattice planes thick. The electrical activity of planar defects appears to be correlated with emission of dislocations especially at termination points. Impurity effects may also play a role. Twin boundaries per se appear not to be electrically active.

Ast, D. G.↗
NASA NTRS1986

Studies of implanted iron in silicon by channeling and Rutherford backscattering

Different amounts of 100-keV iron ions have been implanted into high-resistivity p-type FZ-silicon samples. The implantation damage, recovery of damage during various annealing periods and temperatures, movement of iron atoms under annealing and oxidation, and the kinds of defects created after implantation, annealing, or oxidation are all investigated by channeling and backscattering measurements. It is found that the critical fluence of 100-keV iron implanted into silicon at room temperature is about 2.5 x 10 to the 14th Fe/sq cm, and that iron atoms are gettered by silicon oxidation. In this supersaturated region, iron atoms diffuse slightly towards bulk silicon during high-temperature annealing (greater than or equal to 1100 C) but not at all during low-temperature annealing (less than or equal to 1000 C) in dry nitrogen ambient.

Wang, P. W.↗
NASA NTRS2000

Method For Growth of Crystal Surfaces and Growth of Heteroepitaxial Single Crystal Films Thereon

A method of growing atomically-flat surfaces and high quality low-defect crystal films of semiconductor materials and fabricating improved devices thereon is discussed. The method is also suitable for growing films heteroepitaxially on substrates that are different than the film. The method is particularly suited for growth of elemental semiconductors (such as Si), compounds of Groups III and V elements of the Periodic Table (such as GaN), and compounds and alloys of Group IV elements of the Periodic Table (such as SiC).

Powell, J. Anthony↗
NASA NTRS1992

Lack of deleterious effect of slow-release sodium fluoride treatment on cortical bone histology and quality in osteoporotic patients

We evaluated the effects of intermittent slow-release sodium fluoride (SRNaF) and continuous calcium citrate therapy on cortical bone histology, reflection ultrasound velocity (material strength) and back-scattered electron image analysis (BEI) in 26 osteoporotic patients before and following therapy. All measurements were made on transiliac crest bone biopsies obtained before and following 2 years of therapy in each patient. For all 26 patients there were no significant changes in cortical bone histomorphometric parameters. In 15 patients in whom bone material quality was assessed by reflection ultrasound, there was no change in velocity (4000 +/- 227 SD to 4013 +/- 240 m/s). BEI disclosed no mineralization defects or the presence of woven bone. Mean atomic number (density) of bone increased slightly, but significantly (9.261 +/- 0.311 to 9.457 +/- 0.223, P = 0.031). While these changes are less marked than those observed for cancellous bone, they indicate that this form of therapy does not adversely affect cortical bone remodelling.

NASA Discipline Regulatory Physiology↗
NASA NTRS2001

Ceramic Inclusions in Powder Metallurgy Disk Alloys: Characterization and Modeling

Powder metallurgy alloys are increasingly used in gas turbine engines, especially in turbine disk applications. Although powder metallurgy materials have many advantages over conventionally cast and wrought alloys (higher strength, higher temperature capability, etc.), they suffer from the rare occurrence of ceramic defects (inclusions) that are inherent to the powder atomization process. These inclusions can have a potentially large detrimental effect on the durability of individual components. An inclusion in a high stress location can act as a site for premature crack initiation and thereby considerably reduce the fatigue life. Because these inclusions are exceedingly rare, they typically do not reveal themselves in the process of characterizing the material for a particular application (the cumulative volume of the test bars in a fatigue life characterization is typically on the order of a single actual component). Ceramic inclusions have, however, been found to be the root cause of a number of catastrophic engine failures. To investigate the effect of these inclusions in detail, we have undertaken a study where known populations of ceramic particles, whose composition and morphology are designed to mimic the "natural" inclusions, are added to the precursor powder. Surface-connected inclusions have been found to have a particularly large detrimental effect on fatigue life; therefore, the quantity of ceramic "seeds" added is calculated to ensure that a minimum number will intersect the surface of the fatigue test bars. Because the ceramic inclusions are irregularly shaped and have a tendency to break up in the process of extrusion and forging, a method of calculating the probability of occurrence and expected intercepted surface area was needed. We have developed a Monte Carlo simulation to determine the distributions of these parameters and have verified the simulated results with observations of ceramic inclusions found in macroscopic slices from extrusions and forgings. Fatigue specimens have been machined from Udimet 720 (a powder metallurgy superalloy) forgings, to determine the effects of the inclusions on fatigue life. The ultimate goal of this study will be to use probabilistic methods to determine the reliability detriment that can be attributed to these ceramic inclusions. This work has been supported by the Ultra Safe and Ultra- Efficient Engine Technologies programs.

Bonacuse, Peter J.↗
NASA NTRSDate not supplied

H20 Phase Characterization and Icing Mitigation Through Quantum Sensing in Silicon Carbide Non-Metal Defects

The primary focus of aircraft icing research has been on ice accretion on wings for its serious adverse consequences. When icing occurs on a wing, the change in airfoil shape results in a decrease in lift and an increase in drag, leading to potentially fatal accidents. Although the issue was recognized in the 1920s, the icing problem is still an area of ongoing research due to the complexity of the icing phenomena. A much-improved understanding of how certain weather conditions produce different icing characteristics will lead to new mitigation methods and a better quantification of current methods. This can be done by better understanding of the molecular bonding energy produced by ice polytype combinations when interacting with different surface materials in various environments. This work is currently developing both models that more accurately describe the quantum signatures of H2O states (i.e. supercooled, ice, liquid, glass phase...) and chip-based detectors to evaluate these signatures. These sensors utilize ion defects in Silicon Carbide (SiC) as extremely sensitive atomic magnetic detectors. This effort leverages both the decades-long SiC development expertise and infrastructure at NASA Glenn and its growing capabilities in quantum metrology.

Daniel R. Hart↗
NASA NTRS1987

The role of electronic mechanisms in surface erosion and glow phenomena

Experimental studies of desorption induced by electronic transitions (DIET) are described. Such studies are producing an increasingly complete picture of the dynamical pathways through which incident electronic energy is absorbed and rechanneled to produce macroscopic erosion and glow. These mechanistic studies can determine rate constants for erosion and glow processes in model materials and provide valuable guidance in materials selection and development. Extensive experiments with electron, photon, and heavy particle irradiation of alkali halides and other simple model materials have produced evidence showing that: (1) surface erosion, consisting primarily in the ejection or desorption of ground-state neutral atoms, occurs with large efficiencies for all irradiated species; (2) surface glow, resulting from the radiative decay of desorbed atoms, likewise occurs for all irradiating species; (3) the typical mechanism for ground-state neutral desorption is exciton formation, followed by relaxation to a permanent, mobile electronic defect which is the precursor to bond-breaking in the surface or near-surface bulk of the material; and (4) the mechanisms for excited atom formation may include curve crossing in atomic collisions, interactions with surface defect or impurity states, or defect diffusion.

Haglund, Richard F., Jr.↗
NASA NTRS2002

Issues and Consequences of Atomic Oxygen Undercutting of Protected Polymers in Low Earth Orbit

Hydrocarbon based polymers that are exposed to atomic oxygen in low Earth orbit are slowly oxidized which results in recession of their surface. Atomic oxygen protective coatings have been developed which are both durable to atomic oxygen and effective in protecting underlying polymers. However, scratches, pin window defects, polymer surface roughness and protective coating layer configuration can result in erosion and potential failure of protected thin polymer films even though the coatings are themselves atomic oxygen durable. This paper will present issues that cause protective coatings to become ineffective in some cases yet effective in others due to the details of their specific application. Observed in-space examples of failed and successfully protected materials using identical protective thin films will be discussed and analyzed. Proposed approaches to prevent the failures that have been observed will also be presented.

Banks, Bruce A.↗
NASA NTRS1996

Atomic Oxygen Durability Testing of an International Space Station Solar Array Validation Coupon

An International Space Station solar array validation coupon was exposed in a directed atomic oxygen beam for space environment durability testing at the NASA Lewis Research Center. Exposure to atomic oxygen and intermittent tensioning of the solar array were conducted to verify the solar array#s durability to low Earth orbital atomic oxygen and to the docking threat of plume loading both of which are anticipated over its expected mission life of fifteen years. The validation coupon was mounted on a specially designed rotisserie. The rotisserie mounting enabled the solar and anti-solar facing side of the array to be exposed to directed atomic oxygen in a sweeping arrival process replicating space exposure. The rotisserie mounting also enabled tensioning, in order to examine the durability of the array and its hinge to simulated plume loads. Flash testing to verify electrical performance of the solar array was performed with a solar simulator before and after the exposure to atomic oxygen and tensile loading. Results of the flash testing indicated little or no degradation in the solar array#s performance. Photographs were also taken of the array before and after the durability testing and are included along with comparisons and discussions in this report. The amount of atomic oxygen damage appeared minor with the exception of a very few isolated defects. There were also no indications that the simulated plume loadings had weakened or damaged the array, even though there was some erosion of Kapton due to atomic oxygen attack. Based on the results of this testing, it is apparent that the International Space Station#s solar arrays should survive the low Earth orbital atomic oxygen environment and docking threats which are anticipated over its expected mission life.

Forkapa, Mark J.↗
NASA NTRS1983

Diffusion barriers

The choice of the metallic film for the contact to a semiconductor device is discussed. One way to try to stabilize a contact is by interposing a thin film of a material that has low diffusivity for the atoms in question. This thin film application is known as a diffusion barrier. Three types of barriers can be distinguished. The stuffed barrier derives its low atomic diffusivity to impurities that concentrate along the extended defects of a polycrystalline layer. Sacrificial barriers exploit the fact that some (elemental) thin films react in a laterally uniform and reproducible fashion. Sacrificial barriers have the advantage that the point of their failure is predictable. Passive barriers are those most closely approximating an ideal barrier. The most-studied case is that of sputtered TiN films. Stuffed barriers may be viewed as passive barriers whose low diffusivity material extends along the defects of the polycrystalline host.

Nicolet, M. A.↗
NASA NTRS2018

Light Microscopy Module Biophysics - 4 (LMM-B4)

Compare incorporation of protein aggregates into growing protein crystals on ISS (International Space Station) and on Earth. Measure growth rates in 1g (1 gravity) versus microgravity (micro-g) for different size aggregates of proteins. Compare the defect density and crystal quality via fluorescent-based atomic force microscopy and X-ray diffraction quality of crystals grown at different rates in a 1g environment.

Penn, Benjamin G.↗
NASA NTRS2002

Fast Three-Dimensional Method of Modeling Atomic Oxygen Undercutting of Protected Polymers

A method is presented to model atomic oxygen erosion of protected polymers in low Earth orbit (LEO). Undercutting of protected polymers by atomic oxygen occurs in LEO due to the presence of scratch, crack or pin-window defects in the protective coatings. As a means of providing a better understanding of undercutting processes, a fast method of modeling atomic-oxygen undercutting of protected polymers has been developed. Current simulation methods often rely on computationally expensive ray-tracing procedures to track the surface-to-surface movement of individual "atoms." The method introduced in this paper replaces slow individual particle approaches by substituting a model that utilizes both a geometric configuration-factor technique, which governs the diffuse transport of atoms between surfaces, and an efficient telescoping series algorithm, which rapidly integrates the cumulative effects stemming from the numerous atomic oxygen events occurring at the surfaces of an undercut cavity. This new method facilitates the systematic study of three-dimensional undercutting by allowing rapid simulations to be made over a wide range of erosion parameters.

Snyder, Aaron↗
NASA NTRS1991

Atomic oxygen undercutting of LDEF aluminized Kapton multilayer insulation

Atomic oxygen undercutting is a potential threat to vulnerable spacecraft materials which have been shielded with an atomic oxygen protective coating. This is due to atomic oxygen attack of oxidizable materials at the point of microscopic defects in the protective coatings which occur during fabrication and handling, or from micrometeoroid and debris bombardment in space. An aluminized Kapton multilayer insulation sample which was flown on the leading edge of the Long Duration Exposure Facility (LDEF) was used to study low Earth orbit (LEO) directed ram oxygen undercutting. Cracks in the aluminized coatings around the vent holes provided excellent locations for evaluation of atomic oxygen undercutting. The undercutting profiles were compared to Monte Carlo models which predict LEO ram atomic oxygen attack. The shape of the undercurrent profile was found to vary with crack width, which is proportional to the number of atomic oxygen atoms entering the crack. The resulting atomic oxygen undercut profiles which occurred on LDEF indicated wide undercut cavities in spite of the fixed ram orientation. Potential causes of the observed undercutting profiles will be presented. Implications of the undercutting profiles relevant to Space Station Freedom will also be discussed.

Degroh, Kim K.↗
NASA NTRS2002

Durability Issues for the Protection of Materials from Atomic Oxygen Attack in Low Earth Orbit

Low Earth orbital atomic oxygen is capable of eroding most polymeric materials typically used on spacecraft. Solar array blankets, thermal control polymers, and carbon fiber matrix composites are readily oxidized to become thinner and less capable of supporting the loads imposed upon them. Protective coatings have been developed that are durable to atomic oxygen to prevent oxidative erosion of the underlying polymers. However, the details of the surface roughness, coating defect density, and coating configuration can play a significant role as to whether or not the coating provides long duration atomic oxygen protection. Identical coatings on different surface roughness surfaces can have drastically different durability results. Examples and analysis of the causes of resultant differences in atomic oxygen protection are presented. Implications based on in-space experiences, ground laboratory testing, and computational modeling indicate that thin film vacuum-deposited aluminum protective coatings offer much less atomic oxygen protection than sputter-deposited silicon dioxide coatings.

Banks, Bruce↗
NASA NTRS1994

Simulation studies for surfaces and materials strength

Computer simulation studies were conducted to investigate materials properties. All the calculations were carried out using atomic level simulation techniques which are based on semiempirical or model functions. During this project, functions with varying degrees of complexity were derived and employed in simulations. Investigations include covalently bonded materials as well as systems involving metal atoms. For small clusters calculated results provided information on various energy- and structure-related properties along with vibrational characteristics. Also, energy barriers for configurational transitions were calculated for selected cases. Simulation calculations for surfaces produced new results in areas related to surface energies, reconstructions and relaxations, surface defects, surface stresses as well as adsorption and nucleation processes. Simulations were also performed on bulk materials. Calculated results provided an atomic level understanding on energetics and structures of point defects, crystal stability, elastic properties, and materials strength for various systems. Calculations involving polymeric materials include studies of polar polymer melts and polymer/solid interfaces. Simulations employing specially developed codes provided significant information about energetics and conformational characteristics of different polymeric chains.

Halicioglu, Timur↗