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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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Fracture theory for amorphous high polymers below glass transition temperature, considering viscoelastic effects on structural defect stress concentration
Research projects on defect structure, mechanical behavior, electrical and optical properties, and thermodynamics of solid materials at university
Mechanical properties of lithium fluoride single crystals, taking into account equilibrium lattice defect structures
Doped silver carbonate defect structure and doping effect on reaction kinetics by X rays and electron paramagnetic resonance, finding new crystal structure
Surface impurity and structural defect analysis on thermally grown silicon oxide integrated circuit
The behavior and properties of the refractory carbides, nitrides, and borides are being investigated by NASA as part of its research aimed at developing superior heat resistant materials for aerospace applications. Fundamental studies on the electronic and defect structures of the carbides indicate that there is promise for improving the strength and ductility of these materials. Studies of the zirconium-carbon-oxygen system show that zirconium oxycarbides of different compositions and lattice parameters can be formed between 1500 and 1900 C and are stable below 1500 C. More applied studies show that hot working generally improves the microstructure and therefore the strength of TiC and NbC. Sintering studies on UN indicate that very high densities can be achieved. Hot pressing of cermets of HfN and HfC produces good mechanical properties for high temperature bearing applications.
The elastic and plastic deformation behavior of high-purity aluminum and of dilute aluminum alloys is reviewed. Reliable property data, including elastic moduli, elastic coefficients, tensile, creep, fatigue, hardness, and impact are presented. Single crystal tensile results are discussed. Rather comprehensive reference lists, containing publications of the past 20 years, are included for each of the above categories. Defect structures and mechanisms responsible for mechanical behavior are presented. Strengthening techniques (alloys, cold work, irradiation, quenching, composites) and recovery are briefly reviewed.
Certain anions OH(-), F(-) and Gl(-) are shown to enhance grain growth in MgO. The magnitude of their effect decreases in the order in which the anions are listed and depends on their location (solid-solution, second phase) in the MgO lattice. As most anions exhibit relatively high vapor pressures at sintering temperatures, they retard densification and invariably promote residual porosity. The role of anions on grain growth rates was studied in relation to their effect on pore mobility and pore removal; the atomic process controlling the actual rates was determined from observed kinetics in conjunction with the microstructural features. With respect to controlling mechanisms, the effects of all anions are not the same. OH(-) and F(-) control behavior through creation of a defect structure and a grain boundary liquid phase while Cl(-) promotes matter transport within pores by evaporation-condensation. Studies on an additional anion, S to the minus 2nd power gave results which were no different from undoped MgO, possibly because of evaporative losses during hot pressing. Hence, the effect of sulphur is negligible or undetermined.
Germanium and GaAs crystals were investigated for studies on photovoltaic effects, chemical etching and epitaxial growth according to the overall objective to assess the defect structure of single crystalline materials. A brief survey of basic theory and topographical techniques is provided; examples of topographs are presented.
Progress in a program to produce high speed, thin, wide silicon sheets for fabricating 10% efficient solar cells is reported. An EFG ribbon growth system was used to perform growth rate and ribbon thickness experiments. A new, wide ribbon growth system was developed. A theoretical study of stresses in ribbons was also conducted. The EFG ribbons were observed to exhibit a characteristic defect structure which is orientation dependent in the early stages of growth.
A general overview of materials experiments done on several Apollo missions and Skylab as well as the ground research that led to these experiments is given. Demonstrations of composite solidification, electrophoresis, and fluid flow were developed for Apollo. Fourteen different experiments in metal processing and single crystal growth were done in the space processing facility on Skylab. Metals were melted and solidified in a variety of configurations using different methods of heating to produce improved alloys and composite materials. Single crystals were grown by directional solidification and vapor transport to achieve uniform crystalline perfection, decreased structural defects, and homogeneity of the dopant material.
Metal fiber reinforced superalloys are being considered for construction of critical components in turbine engines that operate at high temperature. The problems involved in fabricating refractory metal alloys into wire form in such a manner as to maximize their strength properties without developing excessive structural defects are described. The fundamental principles underlying the development of such alloy fibers are also briefly discussed. The progress made to date in developing tungsten, tantalum and columbium base alloys for fiber reinforcement is reported and future prospects for alloy fiber development considered.
The paper discusses structural defects in low-cost silicon sheets and their effect on the electronic properties related to solar cell performance. Experimental data are presented on the influence of grain boundaries on minority carrier diffusion length, impurity defect interaction, and variable surface recombination velocity. An analytical model of the effect of grain boundaries on solar cell performance is constructed based on these results.
Two areas of investigation which are required for maximum utilization of the flight experiment results from the solution growth of crystals of triglycine sulfate (TGS) are addressed. The use of an ultramicroscope to characterize structural defects in the grown TGS crystals is discussed. Mie theory is applied to the light scattering expected from idealized crystal defects in order to predict the sensitivity of ultramicroscopy in the detection of small defects. Secondly, the measurement of optical properties of TGS solutions in water is discussed. This data is needed in order to interpret interferometric studies of mass transport through the TGS solution during the growth phase of the flight experiment. In particular, refractive index data is obtained for a wide range of temperatures and concentrations of TGS solutions. Absorbance measurements in the ultraviolet have also been made for the same samples.
Two large cast ingots were evaluated. Solar cell performance versus substrate position within the ingots was obtained and the results are presented. Dendritic web samples were analyzed in terms of structural defects, and efforts were made to correlate the data with the performance of solar cells made from the webs.
The silicon-on-ceramic (SOC) process produces thin layers of silicon on inexpensive ceramic substrates. The layers are produced by unidirectional solidification of molten silicon, and are polycrystalline. Solar cells made from SOC material have performed with a conversion efficiency of 10.5%. In connection with the objective to improve the efficiency of SOC solar cells, an investigation was conducted to assess the relative importance of grain boundaries and impurities. In order to separate the two effects, light-beam-induced current (LBIC) measurements of the short-circuit current were made using a micron-sized light beam from a monochromator. The LBIC measurements are closely related to solar cell performance since the technique identifies the various spatial and spectral contributions to solar cell current. On the basis of the obtained results, it appears that structural defects are more important than chemical defects in limiting the performance of present SOC solar cells.
Light-beam-induced currents yield information about solar cell material. Apparatus measures short-circuit current generated when spot of concentrated light is scanned across grains and grain boundaries in material under test. Technique used to evaluate SOC samples for diffusion and recombination effects of cell processing and chemical and structural defects.
Semiconductor crystal growth and segregation problems are examined in the context of their relationship to material properties, and some of the problems are illustrated with specific experimental results. The compositional and structural defects encountered in semiconductors are largely associated with gravity-induced convective currents in the melt; additional problems are introduced by variations in stoichiometry. It is demonstrated that in near-zero gravity environment, crystal growth and segregation takes place under ideal steady-state conditions with minimum convective interference. A discussion of the advantages of zero-gravity crystal growth is followed by a summary of problems arising from the absence of gravitational forces.