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NASA activities in the framework of the 11-agency federal Small Business Innovation Research program are outlined in tables and graphs and briefly characterized. Statistics on the program are given; the technical topics covered are listed; and the procedures involved in evaluating applications for support are discussed. A number of typical defects in proposals are indicated, and recommendations for avoiding them are provided.
Preferential oriented multiwalled carbon nanotubes were prepared by the injection chemical vapor deposition (CVD) method using either cyclopentadienyliron dicarbonyl dimer or cycloctatetraene iron tricarbonyl as the iron catalyst source. The catalyst precursors were dissolved in toluene as the carrier solvent for the injections. The concentration of the catalyst was found to influence both the growth of the nanotubes as well as the amount of iron in the deposited material. As deposited the multiwalled carbon nanotubes contained as little as 2.8% iron by weight. The material was deposited onto tantalum foil and fused silica substrates. The nanotubes were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and thermogravimetric analysis. This synthetic route provides a simple and scalable method to deposit MWNTs with a low defect density, low metal content and preferred orientation.
Preferential oriented multiwalled carbon nanotubes were prepared by the injection chemical vapor deposition (CVD) method using either cyclopentadienyliron dicarbonyl dimer or cyclooctatetraene iron tricarbonyl as the iron catalyst source. The catalyst precursors were dissolved in toluene as the carrier solvent for the injections. The concentration of the catalyst was found to influence both the growth (i.e., MWNT orientation) of the nanotubes, as well as the amount of iron in the deposited material. As deposited, the multiwalled carbon nanotubes contained as little as 2.8% iron by weight. The material was deposited onto tantalum foil and fused silica substrates. The nanotubes were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and thermogravimetric analysis. This synthetic route provides a simple and scalable method to deposit MWNTs with a low defect density, low metal content and a preferred orientation. Subsequently, a small start-up was founded to commercialize the deposition equipment. The contrast between the research and entrepreneurial environments will be discussed.
The performance of commercially available silicon carbide (SiC) power devices is limited due to inherently high density of screw dislocations (SD), which are necessary for maintaining polytype during boule growth and commercially viable growth rates. The NASA Glenn Research Center (GRC) has recently proposed a new bulk growth process based on axial fiber growth (parallel to the c-axis) followed by lateral expansion (perpendicular to the c-axis) for producing multi-faceted m-plane SiC boules that can potentially produce wafers with as few as one SD per wafer. In order to implement this novel growth technique, the lateral homoepitaxial growth expansion of a SiC fiber without introducing a significant number of additional defects is critical. Lateral expansion is being investigated by hot wall chemical vapor deposition (HWCVD) growth of 6H-SiC am-plane seed crystals (0.8mm x 0.5mm x 15mm) designed to replicate axially grown SiC single crystal fibers. The post-growth crystals exhibit hexagonal morphology with approximately 1500 m (1.5 mm) of total lateral expansion. Preliminary analysis by synchrotron white beam x-ray topography (SWBXT) confirms that the growth was homoepitaxial, matching the polytype of the respective underlying region of the seed crystal. Axial and transverse sections from the as grown crystal samples were characterized in detail by a combination of SWBXT, transmission electron microscopy (TEM) and Raman spectroscopy to map defect types and distribution. X-ray diffraction analysis indicates the seed crystal contained stacking disorders and this appears to have been reproduced in the lateral growth sections. Analysis of the relative intensity for folded transverse acoustic (FTA) and optical (FTO) modes on the Raman spectra indicate the existence of stacking faults. Further, the density of stacking faults is higher in the seed than in the grown crystal. Bundles of dislocations are observed propagating from the seed in m-axis lateral directions. Contrast extinction analysis of these dislocation lines reveals they are edge type basal plane dislocations that track the growth direction. Polytype phase transition and stacking faults were observed by high-resolution TEM (HRTEM), in agreement with SWBXT and Raman scattering.
An accidental fire involving the Lithium-Ion (Li-ion) battery in a laptop computer is one of the most likely fire scenarios on-board a spacecraft. These fires can occur from a defect in the battery that worsens with time, over-charging the battery and leading to failure or accidental damage caused by thermal runaway. While this is a relatively likely fire scenario, very little is known about the how a laptop computer fire would impact a sealed spacecraft. The heat release would likely cause a pressure rise, possibly exceeding the pressure limit of the vehicle and causing a relief valve to open. The combustion products from the fire could pose a short-term and long-term health hazard to the crew and the fire itself could cause injury to the crew and damage to the spacecraft. Despite the hazard posed by a laptop fire, there is little quantitative data on the fire size, heat release and toxic product formation. This paper presents the results of initial attempts to quantify the fire resulting from a failed laptop fire tested at the NASA White Sands Test Facility (WSTF). The data from the testing is useful to attempt to determine the fire size and characteristics such as maximum heat release rate, total heat release, maximum temperatures and fire duration are determined. Using existing models and correlations for fires, the measured fire characteristics are extrapolated to laptop fires on a vehicle the approximate size of the Orion spacecraft.
Developments in fracture mechanics and elastic wave theory enhance the understanding of many physical phenomena in a mathematical context. Available literature in the material, and fracture characterization by NDT, and the related mathematical methods in mechanics that provide fundamental underlying principles for its interpretation and evaluation are reviewed. Information on the energy release mechanism of defects and the interaction of microstructures within the material is basic in the formulation of the mechanics problems that supply guidance for nondestructive evaluation (NDE).
Solar dynamic (SD) space power systems require durable, high emittance surfaces on a number of critical components, such as heat receiver interior surfaces and parasitic load radiator (PLR) elements. To enhance surface characteristics, an alumina-titania coating has been applied to 500 heat receiver thermal energy containment canisters and the PLR of NASA Lewis Research Center's (LeRC) 2 kW SD ground test demonstrator (GTD). The alumina-titania coating was chosen because it had been found to maintain its high emittance under vacuum (less than or equal to 10(exp -6) torr) at high temperatures (1457 F (827 C)) for an extended period (approximately 2,700 hours). However, preflight verification of SD systems components, such as the PLR require operation at ambient pressure and high temperatures. Therefore, the purpose of this research was to evaluate the durability of the alumina-titania coating at high temperature in air. Fifteen of sixteen alumina-titania coated Incoloy samples were exposed to high temperatures (600 F (316 C) to l500 F (816 C)) for various durations (2 to 32 hours). Samples, were characterized prior to and after heat treatment for reflectance, solar absorptance, room temperature emittance and emittance at 1,200 F (649 C). Samples were also examined to detect physical defects and to determine surface chemistry using optical microscopy, scanning electron microscopy operated with an energy dispersive spectroscopy (EDS) system, and x ray photoelectron spectroscopy (XPS). Visual examination of the heat-treated samples showed a whitening of samples exposed to temperatures of 1,000 F (538 C) and above. Correspondingly, the optical properties of these samples had degraded. A sample exposed to 1,500 F (816 C) for 24 hours had whitened and the thermal emittance at 1,200 F (649 C) had decreased from the non-heat treated value of 0.94 to 0.62. The coating on this sample had become embrittled with spalling off the substrate noticeable at several locations. Based on this research it is recommended that preflight testing of SD components with alumina-titania coatings be restricted to temperatures no greater than 600 F (316 C) in air to avoid optical degradation. Moreover, components with the alumina-titania coating are likely to experience optical property degradation with direct atomic oxygen exposure in space.
ZnSe, a II-VI semiconductor with a large direct band gap of 2.7 eV at room temperature and 2.82 eV at 10 K, is considered a promising material for optoelectric applications in the blue-green region of the spectrum. Photoemitting devices and diode laser action has been demonstrated as a result of decades of research. A key issue in the development of II-VI semiconductors is the control of the concentration of the various impurities. The II-VI semiconductors seem to defy the effort of high level doping due to the well known self compensation of the donors and the acceptors. A good understanding of roles of the impurities and the behavior of the various intrinsic defects such as vacancies, interstitials and their complexes with impurities is necessary in the development and application of these materials. Persistent impurities such as Li and Cu have long played a central role in the photoelectronic properties of many II-VI compounds, particularly ZnSe. The shallow centers which may promote useful electrical conductivity are of particular interest. They contribute the richly structured near gap edge luminescence, containing weak to moderate phonon coupling and therefore very accessible information about the energy states of the different centers. Significance of those residual impurities which may contribute such centers in II-VI semiconductors must be fully appreciated before improved control of their electrical properties may be possible. Low temperature photoluminescence spectroscopy is an important source of information and a useful tool of characterization of II-VI semiconductors such as ZnSe. The low temperature photoluminescence spectrum of a ZnSe single crystal typically consists of a broad band emission peaking at 2.34 eV, known as the Cu-green band, and some very sharp lines near the band gap. These bands and lines are used to identify the impurity ingredients and the defects. The assessment of the quality of the crystal based on the photoluminescence analysis is then possible. In this report we present the characterization of a ZnSe single crystal as grown by the physical vapor transport method, with special intention paid to the possible effects of the gravitational field to the growth of the crystal.
Acousto-ultrasonics utilizes simulated stress waves to detect and quantify defect states, damage conditions, and variations of mechanical properties in fiber reinforced composites. The term acousto-ultrasonics denotes a combination of aspects of acoustic emission methodology with ultrasonic materials characterization. The acousto-ultrasonic approach was developed to deal primarily with evaluation of the integrated effect of minor flaws and diffuse flaw populations of subcritical flaws in composite and bonded structures. These factors singly and collectively also influence acousto-ultrasonic measurements that, in turn, correlate with dynamic response and mechanical property variations. Since it was first introduced, the acousto-ultrasonic approach was successfully applied to a variety of materials, including polymeric, metallic, and ceramic matrix composites; adhesively bonded materials; paper and wood products; cable and rope; and also human bone. Examples of applications and limitations of the approach are reviewed. Basic methods and guidelines are discussed. The underlying hypothesis and theory development needs are indicated.
Progress in the processing and fabrication of metal matrix composites (MMC's) requires appropriate mechanical and nondestructive testing methods. These methods are needed to characterize properties, assess integrity, and predict the life of engine components such as compressor rotors, blades, and vanes. Capabilities and limitations of several state-of-the-art nondestructive evaluation (NDE) technologies are investigated for characterizing titanium MMC rings for gas turbine engines. The use of NDE technologies such as x-ray computed tomography, radiography, and ultrasonics in identifying fabrication-related problems that caused defects in components is examined. Acousto-ultrasonics was explored to assess degradation of material mechanical properties by using stress wave factor and ultrasonic velocity measurements before and after the burst testing of the rings.
The objective of the present study was to define the optimum conditions for using replication-defective adenovirus (Ad) to transfer the gene for the green fluorescent protein (GFP) to the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei and cells of the neurohypophysis (NH). As indicated by characterizing cell survival over 15 days in culture and in electrophysiological whole cell patch-clamp studies, viral concentrations up to 2 x 10(7) pfu/coverslip did not affect viability of transfected PVN and NH cultured cells from preweanling rats. At 2 x 10(7) pfu, GFP gene expression was higher (40% of GFP-positive cells) and more sustained (up to 15 days). Using a stereotaxic approach in adult rats, we were able to directly transduce the PVN, SON, and NH and visualize gene expression in coronal brain slices and in the pituitary 4 days after injection of Ad. In animals receiving NH injections of Ad, the virus was retrogradely transported to PVN and SON neurons as indicated by the appearance of GFP-positive neurons in cultures of dissociated cells from those brain nuclei and by polymerase chain reaction and Western blot analyses of PVN and SON tissues. Adenoviral concentrations of up to 8 x 10(6) pfu injected into the NH did not affect cell viability and did not cause inflammatory responses. Adenoviral injection into the pituitary enabled the selective delivery of genes to the soma of magnocellular neurons. The experimental approaches described here provide potentially useful strategies for the treatment of disordered expression of the hormones vasopressin or oxytocin. Copyright 2000 Academic Press.
Objectives of this research are: (1) to measure the hysteresis loop, Kerr rotation angle, anisotropy energy profile, Hall voltage, and magnetoresistance of thin-film magneto-optic media using our loop-tracer; (2) measure the wavelength-dependence of the Kerr rotation angle, Theta(sub k), and ellipticity, epsilon(sub k), for thin-film media using our magneto-optic Kerr spectrometer (MOKS); (3) measure the dielectric tensor of thin-film and multilayer samples using our variable-angle magneto-optic ellipsometer (VAMOE); (4) measure the hysteresis loop, coercivity, remanent magnetization, saturation magnetization, and anisotropy energy constant for thin film magnetic media using vibrating sample magnetometry; (5) observe small magnetic domains and investigate their interaction with defects using magnetic force microscopy; (6) perform static read/write/erase experiments on thin-film magneto-optic media using our static test station; (7) integrate the existing models of magnetization, magneto-optic effects, coercivity, and anisotropy in an interactive and user-friendly environment, and analyze the characterization data obtained in the various experiments, using this modeling package; (8) measure focusing- and tracking-error signals on a static testbed, determine the 'feedthrough' for various focusing schemes, investigate the effects of polarization and birefringence, and compare the results with diffraction-based calculations; and (9) measure the birefringence of optical disk substrates using two variable angle ellipsometers.
Optical and acoustooptical techniques for bonded surface inspection and interface wave detection are discussed. Interface wave propagation models based on viscous, elastic, and viscoelastic mechanical properties are reviewed. Gross boundary anomalies such as large defects and localized disbond regions were measured using conventional high frequency pulse echo and pitch catch ultratechniques. Bond durabilty is estimated by comparing detected wave attenuation versus frequency as a function of prebond surface preparation with the experimental limit obtained for perfect welded contact. Sensitivity of interface acoustic field periodicity to both residual and dc applied stress at the boundary surface is observed as well as hysteresis due to viscoelastic polymeric behavior as the applied stress is cycled.
Water ice nanoclusters are useful analogs for studying a variety of processes that occur within icy grains in the extraterrestrial environment. The surface of ice nanoclusters prepared in the laboratory is similar to the surface of interstellar ice grains. In cold molecular clouds, the silicate cores of interstellar grains are typically approx. 100 nm in diameter and have a coating of impure amorphous water ice. Depositional, thermal and radiolytic processes leave the surface and subsurface molecules in a disordered state. In this state, structural defects become mobile and reactions of trapped gases and small molecules can occur. The large surface area of nanocluster deposits relative to their bulk allows for routine observation of such surface-mediated processes. Furthermore, the disordered surface and subsurface layers in nanocluster deposits mimic the structure of amorphous ice rinds found on interstellar dust grains. Transmission Electron Microscopy (TEM has been used tn characterize the crystallinity, growth mechanism, and size distribution of nanoclusters formed from a mixture of water vapor with an inert carrier gas that has been rapidly cooled to 77K. E M imaging reveals a Gaussian size distribution around a modal diameter that increases from approx. 15 to 30 nm as the percentage of water vapor within the mixture increases from 0.5 to 2.007, respectively . TEM bright and dark field imaging also reveals the crystalline nature of the clusters. h4any of the clusters show a mosaic structure in which crystalline domains originate at the center Other images show mirror planes that are separated by approx. 10 nm. Electron diffraction patterns of these clusters show that the clusters are composed of cubic ice with only a small hexagonal component. Further, the crystalline domain size is approximately the same as the modal diameter suggesting that the clusters are single crystals.
Due to the difficulty of consolidation, the risk of fractures, and the high cost of therapy, the orthopedic field views the restoration of bony defects or post-traumatic diseases as a significant issue. In recent years, efforts have been made toward guided bone regeneration as a viable solution. In this study, natural rubber latex (NRL) and latex–collagen (NRL-COL) membranes were developed and characterized, and their effects on bone regeneration in rat tibial lesions were assessed. According to the physicochemical findings, the hydrolyzed COL was effectively incorporated into the NRL membrane to produce a more robust membrane (348% increase in stress) and showed good biocompatibility for human dermal fibroblast cells and in hemolytic activity assay. According to bone analysis using computed microtomography, a substantial difference in bone volume (BV) was found among NRL, control (CONT), and NRL-COL. NRL-COL exhibited the greatest value for the bone volume/total volume ratio (BV/TV). The presence of NRL and COL was significant for the analysis of BV and BV/TV, respectively. All the experimental groups demonstrated a higher bone quality and fracture resistance, as shown by the mechanical properties of the bone. In the COL group, both neoformation of bone and total collagen levels were greater, indicating that treated defects had been closed and mature bone cells were present. Due to its angiogenic properties and the low cost of NRL membrane production, the NRL-COL membrane could be a good substitute for upcoming bone regeneration applications.
Crystals of Cd(1-x)Zn(x)Te grown by Physical Vapor Transport (PVT) using self-seeding 'contactless' techniques were characterized using synchrotron radiation (reflection, transmission, and Laue back-reflection X-ray topography). Crystals of low (x = 0.04) and high (up to x approx. = 0.4) ZnTe content were investigated. Twins and defects such as dislocations, precipitates, and slip bands were identified. Extensive inhomogeneous strains present in some samples were found to be generated by interaction (sticking) with the pedestal and by composition gradients in the crystals. Large (up to about 5 mm) oval strain fields were observed around some Te precipitates. Low angle grain boundaries were found only in higher ZnTe content (x greater than or equal to 0.2) samples.
Tethered particle microscopy is a powerful tool to study the dynamics of DNA molecules and DNA-protein complexes in single-molecule experiments. We demonstrate that stroboscopic total internal reflection microscopy can be used to characterize the three-dimensional spatiotemporal motion of DNA-tethered particles. By calculating characteristic measures such as symmetry and time constants of the motion, well-formed tethers can be distinguished from defective ones for which the motion is dominated by aberrant surface effects. This improves the reliability of measurements on tether dynamics. For instance, in observations of protein-mediated DNA looping, loop formation is distinguished from adsorption and other nonspecific events.
Microscopic aspects of interfacial reactions are discussed, such as interdiffusion, formation of intermetallic phases, generation and annihilation of lattice defects, effect of temperature, grain size, etc., which normally occur in diffusion bonding processes. Relationships between properties and microstructure in thin film couples are examined utilizing a unique combination of contact resistance measurements and characterization by transmission electron microscopy.