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At least 1,063 records · Page 59

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NASA NTRS1977

Optical properties of ion beam textured metals

Copper, silicon, aluminum, titanium and 316 stainless steel were textured by 1000 eV xenon ions from an 8 cm diameter electron bombardment ion source. Simultaneously sputter-deposited tantalum was used to facilitate the development of the surface microstructure. Scanning electron microscopy of the ion textured surfaces revealed two types of microstructure. Copper, silicon, and aluminum developed a cone structure with an average peak-to-peak distance ranging from 1 micron for silicon to 6 microns for aluminum. Titanium and 316 stainless steel developed a serpentine ridge structure. The average peak-to-peak distance for both of these materials was 0.5 micron. Spectral reflectance was measured using an integrating sphere and a holraum reflectometer. Total reflectance for air mass 0 and 2, solar absorptance and total emittance normalized for a 425 K black body were calculated from the reflectance measurements.

Hudson, W. R.↗

NASA NTRS1979

Experimental laboratory measurement of thermophysical properties of selected coal types

A number of bituminous coals of moderate to high plasticity were examined, along with portions of their extrudates from the JPL 1.5-inch 850 F screw extruder. Portions of the condensed pyrolysis liquids released during extrusion, and of the gaseous products formed during extrusion were also analyzed. In addition to the traditional determinations, the coals and extrudates were examined in terms of microstructure (especially extractable fractions), thermal analysis (especially that associated with the plastic state), and reactivity towards thermal and catalyzed hydroliquefaction. The process of extrusion increases the fixed carbon content of coals by about 5% and tends to increase the surface area. Coals contaning 25% or more DMF-extractable material show an increase in extractables as a result of extrusion; those initially containing less than 20% extractables show a decrease as a result of extrusion. Both the raw and extruded samples of Kentucky #9 coal are highly reactive towards hydroliquefaction, undergoing conversions of 75 to 80% in 15 min and 85-94% in 60 min in a stirred clave.

Lloyd, W. G.↗

NASA NTRS1990

Theory and experimental technique for nondestructive evaluation of ceramic composites

The important ultrasonic scattering mechanisms for SiC and Si3N4 ceramic composites were identified by examining the interaction of ultrasound with individual fibers, pores, and grains. The dominant scattering mechanisms were identified as asymmetric refractive scattering due to porosity gradients in the matrix material, and symmetric diffractive scattering at the fiber-to-matrix interface and at individual pores. The effect of the ultrasonic reflection coefficient and surface roughness in the ultrasonic evaluation was highlighted. A new nonintrusive ultrasonic evaluation technique, angular power spectrum scanning (APSS), was presented that is sensitive to microstructural variations in composites. Preliminary results indicate that APSS will yield information on the composite microstructure that is not available by any other nondestructive technique.

Generazio, Edward R.↗

NASA NTRS1992

Review of the physical and mechanical properties and potential applications of the B2 compound NiAl: Unabridged version of a paper published in International materials review

Considerable work has been performed on NiAl over the last three decades, with an extremely rapid growth in research on this intermetallic occurring in the last few years due to recent interest in this material for electronic and high temperature structural applications. However, many physical properties and the controlling fracture and deformation mechanisms over certain temperature regimes are still in question. Part of this problem lies in the incomplete characterization of many of the alloys previously investigated. Fragmentary data on processing conditions, chemistry, microstructure and the apparent difficulty in accurately measuring composition has made direct comparison between individual studies sometimes tenuous. Therefore, the purpose of this review is to summarize all available mechanical and pertinent physical properties on NiAl, stressing the most recent investigations, in an attempt to understand the behavior of NiAl and its alloys over a broad temperature range.

Noebe, Ronald D.↗

NASA NTRS1993

Physical and mechanical properties of the B2 compound NiAl

Noebe, R. D.↗

NASA NTRS1997

Effect of a growth hormone treatment on bone orthotropic elasticity in dwarf rats

A refinement of the current ultrasonic elasticity technique was used to measure the orthotropic elastic properties of rat cortical bone as well as to quantify changes in elastic properties, density, and porosity of the dwarf rat cortex after a treatment with recombinant human growth hormone (rhGH). The ultrasonic elasticity technique was refined via optimized signal management of high-frequency wave propagation through cubic cortical specimens. Twenty dwarf rats (37 days old) were randomly assigned to two groups (10 rats each). The dwarf rat model (5-10% of normal GH) was given subcutaneous injections of either rhGH or saline over a 14-day treatment period. Density was measured using Archimedes technique. Porosity and other microstructural characteristics were also explored via scanning electron microscopy and image analysis. Statistical tests verified significant decreases in cortical orthotropic Young's (-26.7%) and shear (-16.7%) moduli and density (-2.42%) concomitant with an increase in porosity (+125%) after rhGH treatments to the dwarf model (p < 0.05). A change in material symmetry from orthotropy toward planar isotropy within the radial-circumferential plane after GH treatments was also noted. These results demonstrate some alteration in bone properties at this time interval. Structural implications of these changes throughout physiological loading regimens should be explored.

NASA Discipline Musculoskeletal↗

NASA NTRS2005

Fracture Toughness in Advanced Monolithic Ceramics - SEPB Versus SEVENB Methods

Fracture toughness of a total of 13 advanced monolithic ceramics including silicon nitrides, silicon carbide, aluminas, and glass ceramic was determined at ambient temperature by using both single edge precracked beam (SEPB) and single edge v-notched beam (SEVNB) methods. Relatively good agreement in fracture toughness between the two methods was observed for advanced ceramics with flat R-curves; whereas, poor agreement in fracture toughness was seen for materials with rising R-curves. The discrepancy in fracture toughness between the two methods was due to stable crack growth with crack closure forces acting in the wake region of cracks even in SEVNB test specimens. The effect of discrepancy in fracture toughness was analyzed in terms of microstructural feature (grain size and shape), toughening exponent, and stable crack growth determined using back-face strain gaging.

Choi, S. R.↗

NASA NTRS2006

Assessments of Fracture Toughness of Monolithic Ceramics-SEPB Versus SEVNB Methods

Fracture toughness of a total of 13 advanced monolithic ceramics including silicon nitrides, silicon carbide, aluminas, and glass ceramic was determined at ambient temperature by using both single edge precracked beam (SEPB) and single edge v-notched beam (SEVNB) methods. Relatively good agreement in fracture toughness between the two methods was observed for advanced ceramics with flat R-curves; whereas, poor agreement in fracture toughness was seen for materials with rising R-curves. The discrepancy in fracture toughness between the two methods was due to stable crack growth with crack closure forces acting in the wake region of cracks even in SEVNB test specimens. The effect of discrepancy in fracture toughness was analyzed in terms of microstructural feature (grain size and shape), toughening exponent in R-curve, and stable crack growth determined using back-face strain gaging.

Choi, Sung R.↗

NASA NTRS2009

Joining and Integration of Silicon Nitride Ceramics for Aerospace and Energy Systems

Light-weight, creep-resistant silicon nitride ceramics possess excellent high-temperature strength and are projected to significantly raise engine efficiency and performance when used as turbine components in the next-generation turbo-shaft engines without the extensive cooling that is needed for metallic parts. One key aspect of Si3N4 utilization in such applications is its joining response to diverse materials. In an ongoing research program, the joining and integration of Si3N4 ceramics with metallic, ceramic, and composite materials using braze interlayers with the liquidus temperature in the range 750-1240C is being explored. In this paper, the self-joining behavior of Kyocera Si3N4 and St. Gobain Si3N4 using a ductile Cu-based active braze (Cu-ABA) containing Ti will be presented. Joint microstructure, composition, hardness, and strength as revealed by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Knoop microhardness test, and offset compression shear test will be presented. Additionally, microstructure, composition, and joint strength of Si3N4/Inconel 625 joints made using Cu-ABA, will be presented. The results will be discussed with reference to the role of chemical reactions, wetting behavior, and residual stresses in joints.

Singh, M.↗

NASA NTRSDate not supplied

Thermochemical and microstructural contributions of high temperature particle erosion durability in CMAS exposed EBCs

Particulate/debris damage caused by ingestion of calcium magnesium aluminosilicates (CMAS) hinders the use of environmental barrier coatings (EBCs) to protect SiC-based ceramic matrix composite components in next generation gas turbine engines. Similarly, ingestion of any debris in the engine can lead to mechanical damage and recession of coatings due to particulate erosion. Investigating particulate interactions at relevant engine conditions is crucial in determining limiting mechanisms in the operating lifetime of EBCs. This study assesses the effects of extrinsic phase formation and microstructural changes due to CMAS interactions on the erosion durability of Yb 2 Si 2 O 7 -based EBCs in NASA Glenn’s Erosion Burner Rig Facility. CMAS exposures and erosion testing were carried out at 1316°C. Using 60 µm Al 2 O 3 particles as the erodent material, the effects of CMAS loading on erosion durability at various impingement angles were evaluated.

Jamesa L. Stokes↗

NASA NTRS2013

Transmission Electron Microscopy of Itokawa Regolith Grains

Introduction: In a remarkable engineering achievement, the JAXA space agency successfully recovered the Hayabusa space-craft in June 2010, following a non-optimal encounter and sur-face sampling mission to asteroid 25143 Itokawa. These are the first direct samples ever obtained and returned from the surface of an asteroid. The Hayabusa samples thus present a special op-portunity to directly investigate the evolution of asteroidal sur-faces, from the development of the regolith to the study of the effects of space weathering. Here we report on our preliminary TEM measurements on two Itokawa samples. Methods: We were allocated particles RA-QD02-0125 and RA-QD02-0211. Both particles were embedded in low viscosity epoxy and thin sections were prepared using ultramicrotomy. High resolution images and electron diffraction data were ob-tained using a JEOL 2500SE 200 kV field-emission scanning-transmission electron microscope. Quantitative maps and anal-yses were obtained using a Thermo thin-window energy-dispersive x-ray (EDX) spectrometer. Results: Both particles are olivine-rich (Fo70) with μm-sized inclusions of FeS and have microstructurally complex rims. Par-ticle RA-QD02-0125 is rounded and has numerous sub-μm grains attached to its surface including FeS, albite, olivine, and rare melt droplets. Solar flare tracks have not been observed, but the particle is surrounded by a continuous ~50 nm thick, stuctur-ally disordered rim that is compositionally similar to the core of the grain. One of the surface adhering grains is pyrrhotite show-ing a S-depleted rim (~8-10 nm thick) with nanophase Fe metal grains (<5 nm) decorating the outermost surface. The pyrrhotite displays a complex superstructure in its core that is absent in the S-depleted rim. Particle RA-QD02-0211 contains solar flare particle tracks (~2x109 cm-2) and shows a structurally disordered rim ~100 nm thick. The track density corresponds to a surface exposure of ~103-104 years based on the track production rate of [1]. The dis-ordered rim is nanocrystalline with minor amorphous material between crystalline domains. Quantitative element maps show the outermost ~10 nm of the disordered rim is Si-rich. Discussion and Conclusions: Both particles record the ef-fects of space weathering processes on Itokawa. Noguchi et al. [2] proposed that the disordered rims they observed on Itokawa particles largely result from solar wind radiation damage and we arrive at a similar conclusion for the two particles we analyzed. The microstructure of the S-depleted layer on the pyrrhotite grain in RA-QD02-0125 is similar to that observed in troilite irradiated with ~1018 4 kV He+ [3, 4]. Prolonged irradiation has also been shown to disorder pyrrhotite such that the superstructure reflec-tions are lost [5].

Keller, Lindsay P.↗

NASA NTRS1990

Effect of hydrogen on the strength and microstructure of selected ceramics

Ceramics in monolithic form and as composite constituents in the form of fibers, matrices, and coatings are currently being considered for a variety of high-temperature applications in aeronautics and space. Many of these applications involve exposure to a hydrogen-containing environment. The compatibility of selected ceramics in gaseous high-temperature hydrogen is assessed. Environmental stability regimes for the long term use of ceramic materials are defined by the parameters of temperature, pressure, and moisture content. Thermodynamically predicted reactions between hydrogen and several monolithic ceramics are compared with actual performance in a controlled environment. Morphology of hydrogen attack and the corresponding strength degradation is reported for silicon carbide, silicon nitride, alumina, magnesia, and mullite.

Herbell, Thomas P.↗

NASA NTRS1991

Effect of hydrogen on the strength and microstructure of selected ceramics

Herbell, Thomas P.↗

NASA NTRS1987

Effect of hydrogen on deformation structure and properties of CMSX-2 nickel-base single-crystal superalloy

Material used in this study was a heat of the alloy CMSX-2. This nickel-based superalloy was provided in the form of oriented single crystals, solutionized for 3 hrs at 1315 C. It was then usually heat treated as follows: 1050 C/16h/air cool + 850 C/48h/air cool. The resulting microstructure is dominated by cuboidal, ordered gamma precipitates with a volume fraction of about 75% and an average size of 0.5 microns. In brief, the most compelling hydrogen induced-changes in deformation structure are: (1) enhanced dislocation accumulation in the gamma matrix; and (2) more extensive cross-slip of superdislocations in the gamma precipitates. The enhanced dislocation density in gamma acts to decrease the mean free path of a superdislocation, while easier cross slip hinders superdislocation movement by providing pinning points in the form of sessile jobs. Both processes contribute to the increase of flow stress and the notable work hardening that occurs prior to fracture.

Dollar, M.↗

NASA NTRS2004

Atmospheric Tar Balls: Particles from Biomass and Biofuel Burning

Tar balls are amorphous, carbonaceous spherules that occur in the tropospheric aerosol as a result of biomass and biofuel burning. They form a distinct group of particles with diameters typically between 30 and 500 nm and readily identifiable with electron microscopy. Their lack of a turbostratic microstructure distinguishes them from soot, and their morphology and composition (approximately 90 mol% carbon) renders them distinct from other carbonaceous particles. Tar balls are particularly abundant in slightly aged (minutes to hours old) biomass smoke, indicating that they likely form by gas-to-particle conversion within smoke plumes. The material of tar balls is initially hygroscopic; however, the particles become largely insoluble as a result of free radical polymerization of their organic molecules. Consequently, tar balls are primarily externally mixed with other particle types, and they do not appreciably increase in size during aging. When tar balls coagulate with water-bearing particles, their material may partly dissolve and no longer be recognizable as distinct particles. Tar balls may contain organic compounds that absorb sunlight. They are an important, previously unrecognized type of carbonaceous (organic) atmospheric particle.

Posfai, Mihaly↗

NASA NTRS2013

Micromechanical Characterization and Testing of Carbon Based Woven Thermal Protection Materials

Woven thermal protection system (TPS) materials are one of the enabling technologies for mechanically deployable hypersonic decelerator systems. These materials can be simultaneously used for thermal protection and as structural load bearing members during the entry, descent and landing operations. In order to ensure successful thermal and structural performance during the atmospheric entry, it is important to characterize the properties of these materials, once they have been subjected to entry like conditions. The present paper focuses on mechanical characteristics of pre-and post arc-jet tested woven TPS samples at different scales. It also presents the observations from scanning electron microscope and computed tomography images, and explains the changes in microstructure after being subjected to combined thermal-mechanical loading environments.

Agrawal, Parul↗

NASA NTRS2013

Nano-Ceramic Coated Plastics

Plastic products, due to their durability, safety, and low manufacturing cost, are now rapidly replacing cookware items traditionally made of glass and ceramics. Despite this trend, some still prefer relatively expensive and more fragile ceramic/glassware because plastics can deteriorate over time after exposure to foods, which can generate odors, bad appearance, and/or color change. Nano-ceramic coatings can eliminate these drawbacks while still retaining the advantages of the plastic, since the coating only alters the surface of the plastic. The surface coating adds functionality to the plastics such as self-cleaning and disinfectant capabilities that result from a photocatalytic effect of certain ceramic systems. These ceramic coatings can also provide non-stick surfaces and higher temperature capabilities for the base plastics without resorting to ceramic or glass materials. Titanium dioxide (TiO2) and zinc oxide (ZnO) are the candidates for a nano-ceramic coating to deposit on the plastics or plastic films used in cookware and kitchenware. Both are wide-bandgap semiconductors (3.0 to 3.2 eV for TiO2 and 3.2 to 3.3 eV for ZnO), so they exhibit a photocatalytic property under ultraviolet (UV) light. This will lead to decomposition of organic compounds. Decomposed products can be easily washed off by water, so the use of detergents will be minimal. High-crystalline film with large surface area for the reaction is essential to guarantee good photocatalytic performance of these oxides. Low-temperature processing (<100 C) is also a key to generating these ceramic coatings on the plastics. One possible way of processing nanoceramic coatings at low temperatures (< 90 C) is to take advantage of in-situ precipitated nanoparticles and nanostructures grown from aqueous solution. These nanostructures can be tailored to ceramic film formation and the subsequent microstructure development. In addition, the process provides environment- friendly processing because of the aqueous solution. Low-temperature processing has also shown versatility to generate various nanostructures. The growth of low-dimensional nanostructures (0-D, 1-D) provides a means of enhancing the crystallinity of the solution-prepared films that is of importance for photocatalytic performance. This technology can generate durable, fully functional nano-ceramic coatings (TiO2, ZnO) on plastic materials (silicone, Teflon, PET, etc.) that can possess both photocatalytic oxide properties and flexible plastic properties. Processing cost is low and it does not require any expensive equipment investment. Processing can be scalable to current manufacturing infrastructure.

Cho, Junghyun↗

NASA NTRS1979

Structure of deformed silicon and implications for low-cost solar cells

The paper reports on an investigation of the microstructure and minority carrier lifetime of silicon in uniaxially compressed silicon samples, the objective of which was to determine if it is feasible to produce silicon solar cells from sheet formed by high temperature deformation. It is reported that recrystallization was found to be incomplete in both fine and large grained materials, and that the major mode of recrystallization appears to be migration of existing boundaries into the deformed regions. Also, minority carrier diffusion length was found to be drastically reduced after deformation, perhaps due to contamination or cooling rate, and recovered only slightly with annealing. It is concluded that these results suggest that high temperature deformation of silicon for direct production of sheet for high efficiency solar cells is not practical. It is noted that potential may exist for its use as a coarse grained substrate.

Mardesich, N.↗