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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Stiffness and damping characteristics of aluminum in creep

Tensile creep tests conducted at 200 C were performed on annealed commercially pure aluminum specimens in order to measure the dominant elevated temperature dislocation processes. Testing consisted of applying small lateral loads to measure flexural stiffness, and vibrating the specimens laterally in order to measure dynamic modulus and internal damping. It was concluded that (1) the strain hardening increased static stiffness and decreased internal damping during early creep, and (2) the dynamic modulus remained essentially constant at the elastic value during creep. These results imply that primary creep may constitute a mechanism of recovery of dislocatory disorder induced by yielding the material during loading, and that the inelastic modulus utilized as a mathematical concept in several creep buckling theories is not a directly measurable material property.

Berkovits, A.↗
NASA NTRS1986

Theoretical Study of the Transverse Dielectric Constant of Superlattices and Their Alloys

The optical properties of III to V binary and ternary compounds and GaAs-Al(x)Ga(1-x)As superlattices are determined by calculating the real and imaginary parts of the transverse dielectric constant. Emphasis is given to determining the influence of different material and superlattice parameters on the values of the index of refraction and absorption coefficient. In order to calculate the optical properties of a material, it is necessary to compute its electronic band structure. This was accomplished by introducing a partition band structure approach based on a combination of the vector k x vector p and nonlocal pseudopotential techniques. The advantages of this approach are that it is accurate, computationally fast, analytical, and flexible. These last two properties enable incorporation of additional effects into the model, such as disorder scattering, which occurs for alloy materials and excitons. Furthermore, the model is easily extended to more complex structures, for example multiple quantum wells and superlattices. The results for the transverse dielectric constant and absorption coefficient of bulk III to V compounds compare well with other one-electron band structure models and the calculations show that for small frequencies, the index of refraction is determined mainly by the contibution of the outer regions of the Brillouin zone.

Kahen, K. B.↗
NASA NTRS1987

Wide-angle X-ray scattering study of heat-treated PEEK and PEEK composite

Samples of poly(etheretherketone) (PEEK) neat resin and APC-2 carbon fiber composite were subjected to various heat treatments, and the effect of quenching and annealing treatments was studied by wide-angle X-ray scattering. It is found that high-temperature treatments may introduce disorder into neat resin and composite PEEK when followed by rapid cooling. The disorder is metastable and can revert to ordered state when the material is heated above its glass transition temperature and then cooled slowly. The disorder may result from residual thermal stresses.

Cebe, Peggy↗
NASA NTRS1993

Effect of disorder on the optical properties of short period superlattices

The optical properties of disordered short period superlattices are studied using a one-dimensional tight-binding model. A difference vector and disorder structure factor are proposed to characterize the disordered superlattice. The density of states, participation number, and optical absorption coefficients for both ordered and disordered superlattices are calculated as a function of energy. The results show that introduction of disorder into an indirect band gap material enhances the optical transition near the indirect band edge.

Strozier, J. A.↗
NASA NTRS2019

Investigation of Stacking Fault and Antiphase Boundary Segregation in Ni-Based Superalloys Using Density Functional Theory

Ni-base superalloys have a long history of use in jet turbine engines, and efforts to improve their performance in that application are ongoing. It is known that the precipitation of the Ni3Al γ' phase within the disordered FCCγ phase strengthens the overall material. However, in the high-temperature environment found inside a turbine engine during operation, creep can cause the γ' phase to transform to different, weaker phases along stacking faults, leading to a deterioration of performance. In the γ' phase one mode of creep deformation is the formation of stacking fault ribbons, which consist of intrinsic stacking faults further shearing into antiphase boundaries (APBs). It is also known that certain alloying additions exhibit segregation to stacking faults. If segregating elements could be identified which segregate to the intrinsic stacking fault, but not to the APB, the inclusion of such elements could lead to improved creep strength in these alloys. To investigate this possibility, a density functional investigation of the segregation of W, Mo and Cr to both a superlattice intrinsic stacking fault (SISF) and an APB was performed. It was found that W, Mo and Cr all exhibit segregation to the SISF. In contrast, for the APB, Cr was either energy-neutral, or segregates, depending on the presence of additional nearby Cr, or on the specific lattice site upon which it was placed, while Mo and W did not segregate. Because W and Mo segregate to the SISF but not to the APB, the inclusion of these elements could provide a degree of protection against creep-related deterioration.

Good, Brian S.↗
NASA NTRS1982

Prolonged triboluminescence in clays and other minerals

Samples of various clays and minerals were ground or fractured and monitored with a liquid scintillation spectrometer in order to obtain triboluminescent decay curves. Kaolinite samples displayed several million counts/min after grinding, with a surface area emission estimated at tens of billions of photons/sq cm of surface. The photon production rates varied with the origin of the sample, and kaolinite continually yielded higher production rates than bentonite. The addition of water to the samples slightly increased the count rate of emitted light, while the addition of the fluorescent molecule substance tryptofan significantly enhanced the count rate. Freezing smears of kaolinite and montmorillonite in liquid nitrogen and in a salt ice mixture also induced triboluminescence in the montmorillonite. A possible connection between powdery triboluminescent materials formed in mining industries and respiratory disorders among miners is suggested.

Lahav, N.↗
NASA NTRS1979

The 16-39 micron spectroscopy of oxygen-rich stars

Airborne observations of the 16-39 microns spectra of ten oxygen-rich stars with excess emission in the infrared was obtained. The stars show excess emission attributed to circumstellar dust grains in the 16-39 microns region in the form of a broad hump peaking near 18 microns and falling smoothly to longer wavelengths. The emission is similar in character to the emission from the Trapezium region of the Orion nebula indicating the grain materials are quite similar in these objects. The existence of a feature in the 20 microns region is consistent with the 0-Si-0 bending resonance expected for silicate material. The lack of any sharp structure in the spectra indicates the silicate is in an amorphous, disordered form. A simple model of small grains of carbonaceous chondrite silicate material in a diffuse circumstellar envelope is shown to give a good qualitative fit to the observed 8-39 microns circumstellar spectra. Comparison of the observed spectra with the model spectra indicates the grain emissivity falls as 1/lambda squared from 20 microns to 40 microns.

Forrest, W. J.↗
NASA NTRS1979

16-39 micron spectroscopy of oxygen-rich stars

Airborne observations of the 16-39-micron spectra of 10 oxygen-rich stars with excess emission in the infrared have been obtained. The stars show excess emission attributed to circumstellar dust grains in the 16-39-micron region in the form of a broad hump peaking near 18 microns and falling smoothly to longer wavelengths. The emission is similar in character to the emission from the Trapezium region of the Orion Nebula, indicating the grain materials are quite similar in these objects. The existence of a feature in the 20-micron region is consistent with the O-Si-O bending resonance expected for silicate material. The lack of any sharp structure in the spectra indicates the silicate is in an amorphous, disordered form. A simple model of small grains of carbonaceous chondrite silicate material in a diffuse circumstellar envelope is shown to give a good qualitative fit to the observed 8-39-micron circumstellar spectra. Comparison of the observed spectra with the model spectra indicates the grain emissivity falls as the inverse square of wavelength from 20 to 40 microns.

Forrest, W. J.↗
NASA NTRS1980

Structures and textures of the Murchison and Mighei carbonaceous chondrite matrices

High-resolution transmission electron microscopy has confirmed earlier observations that the character of the Murchison and Mighei fine-grained matrices is complex in mineralogy and texture. Layer structure minerals occur as planar laths, rounded grains or subhedral grains, and range in size from less than 100 A to about 1 micrometer. Serpentine-type and brucite-type structures predominate in the CM matrices. The occurrence of Povlen chrysolite and a vein of disordered mixed-layer and brucite-type material cutting a large lizardite-type grain suggests that at least some of the matrix materials were formed by alteration of preexisting material.

Mackinnon, I. D. R.↗
NASA NTRS1972

Influence of silicon on friction and wear of iron-cobalt alloys

Sliding friction and wear experiments were conducted with ternary ordered alloys of iron and cobalt containing various amounts of silicon to 5 weight percent. The friction and wear of these alloys were compared to those for binary iron-cobalt alloys in the ordered and disordered states and to those for the conventionally used bearing material, 440-C. Environments in which experiments were conducted included air, argon, and 0.25percent stearic acid in hexadecane. Results indicate that a ternary iron - cobalt - 5-percent-silicon alloy exhibits lower friction and wear than the simple binary iron-cobalt alloy. It exhibits lower wear than 440-C in all three environments. Friction was lower for the alloy in argon than in air. Auger analysis of the surface of the ternary alloy indicated segregation of silicon at the surface as a result of sliding.

Buckley, D. H.↗
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 NTRS1985

Raman structural studies of the nickel electrode

Raman spectroscopy is sensitive to empirically controlled nickel electrode structural variations, and has unique potential for structural characterization of these materials. How the structure relates to electrochemical properties is examined so that the latter can be more completely understood, controlled, and optimized. Electrodes were impregnated and cycled, and cyclic voltammetry is being used for electrochemical characterization. Structural variation was observed which has escaped detection using other methods. Structural changes are induced by: (1) cobalt doping, (2) the state of change or discharge, (3) the preparation conditions and type of buffer used, and (4) the formation process. Charged active mass has an NiOOH-type structure, agreeing with X-ray diffraction results. Discharged active mass, however, is not isostructural with beta-Ni(OH)2. Chemically prepared alpha phases are not isostructural either. A disordered structural model, containing point defects, is proposed for the cycled materials. This model explains K(+) incorporation. Band assignments were made and spectra interpreted for beta-Ni(OH)2, electrochemical NiOOH and chemically precipitated NiOOH.

Cornilsen, B. C.↗
NASA NTRS1988

Modeling of mechanical properties of II-VI materials

This paper reviews some new developments in the theory of alloy correlations, order-disorder transitions, and solidus phase-transition curves. It is argued that semiconductor alloys are never truly random, and the various phenomena that drive deviations from random arrangements are introduced. Likely consequences of correlations on the ability to fine-tune the lattice match of epitaxial layers to substrates, on vacancy formation, on diffusion, and on vapor-phase crystal growth are discussed. Examples are chosen for the alloys Hg(1-x)Cd(x)Te, Hg(1-x)Zn(x)Te, Cd(1-y)Zn(y)Te, and CdSe(1-y)Te(y).

Sher, A.↗
NASA NTRS1978

X-ray diffractometer studies of shocked materials

This study explores the utility of X-ray diffractometer scans for determination of shock pressure histories of geological materials from meteorite impact sites. The technique is based on quantification of increased crystal lattice disorder with increasing shock pressure as expressed by decreasing diffraction peak amplitude and pronounced line-broadening caused by decreasing mosaic domain size and increasing strain. The ratio of peak height (PH) to half width (HW) decreases systematically with increasing shock pressure. Data are given for experimentally shocked quartz, feldspars, pyroxene and olivine along with data for granitic materials from the Piledriver nuclear event and from the Ries Crater, Germany. Although the technique in principle may be capable of yielding relatively accurate pressure determinations, its application to naturally shocked materials may be severely limited.

Hanss, R. E.↗
NASA NTRS1975

Experimental shock metamorphism of maximum microcline

A series of recovery experiments are conducted to study the behavior of single-crystal perthitic maximum microcline shock-loaded to a peak pressure of 417 kbar. Microcline is found to deform in a manner similar to quartz and other alkali feldspars. It is observed that shock-induced cleavages occur initially at or slightly below the Hugoniot elastic limit (60-85 kbar), that shock-induced rather than thermal disordering begins above the Hugoniot elastic limit, and that all types of planar elements form parallel to crystallographic planes of low Miller indices. When increasing pressure, it is found that bulk density, refractive indices, and birefringence of the recovered material decrease and approach diaplectic glass values, whereas disappearance and weakening of reflections in Debye-Sherrer patterns are due to disordering of the feldspar lattice.

Robertson, P. B.↗
NASA NTRS2002

Interaction of high intensity focused ultrasound with biological materials

This work is motivated by the possible medical application of focused ultrasound in minimally invasive treatment of a variety of disorders including those associated with soft tissue or disk element disruption in the vicinity of the spine causing impingement on the spinal cord.

focused ultrasound HIFU PZFlex↗
NASA NTRS2015

Experimentally Shocked and Altered Basalt: Laboratory Analogs for Calibration of Mars Remote Sensing and In Situ Data

Calciumphosphate (likely chloroapatite) is formed in the alteration experiments and is more abundant in the altered and shocked sample probably due to increased surface area exposed to alteration fluids resulting from shock damage in the form of both brittle and structural deformation to the starting material (Figs 1 & 3). Apatite forms in basic conditions so the closed system alteration experiment must be buffered by the basalt starting material to create a fluid chemistry environment evolving from neutral at the start to alkaline after 21 days at 160 C. Plagioclase feldspar in the unshocked sample (Fig. 2) has undergone a solid-state transformation to maskelynite, a disordered phase that is not manifest in the XRD pattern of the shocked sample (Fig.4). Olivine and ulvospinel that are present in the starting material can be detected by XRD in the shocked and altered sample (Fig. 4). Tungsten from the sample holder used in the shock experiments dominates the XRD pattern of the shocked and altered sample (Fig. 4). Samples were weighed after the alteration experiments to determine mass loss and predict the amount of material available for the planned analyses from the shock experiments. Within the constraints of these experiments, mass loss is negligible. The samples will next be characterized by Moessbauer and Vis-Near IR spectroscopy, the results of which will be compared to the Mars Exploration Rovers and Mars Reconnaissance Orbiter data sets respectively.

Bell, M. S.↗