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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Absorption and fluorescence of alexandrite and of titanium in sapphire and glass

The fluorescence and absorption data for titanium in crystalline sapphire and titanium doped into two silicate and one phosphate glass structures are analyzed. It is observed that the Ti-doped silicate glass sample exhibits no absorption related to the Ti(III) ion, the Ti-doped phosphate glass is deep blue, the absorption line width of the glass samples are a factor of two larger than that of sapphire, and the absorption peak for the Ti in the glass shifted about 100 nm to the red from the Ti:sapphire absorption peak. This shift reveals that the Ti(III) ion is sensitive to the crystalline environment and not to the glass environment. The photoluminescence spectra for Ti-doped sapphire and alexandrite are compared. It is detected that the Ti:sapphire exhibits a broader spectrum than that for alexandrite with a peak at 750 nm. The three zero phonon transitions of Ti:Al2O3 at liquid nitrogen temperatures are studied.

Byvik, C. E.↗
NASA NTRS1987

Containerless high temperature property measurements by atomic fluorescence

Containerless high temperature processing and material property measurements are discussed. Researchers developed methods for non-contact suspension, heating, and property measurement for materials at temperatures up to 3,680K, the melting point of tungsten. New, scientifically interesting results were obtained in Earth-based research. These results and the demonstration of new methods and techniques form a basis for further advances under the low gravity environment of space where containerless conditions are more easily achieved. Containerless high temperature material property investigations that have been completed in this and our earlier projects include measurements of fluorine LaB sub 6 reaction kinetics at 1,000 to 1,500K; optical property measurements on sapphire (Al2O3) at temperatures up to the melting point (2,327K); and vapor pressure measurements for LaB sub 6 at 2,000 to 2,500K, for molybdenum up to 2,890K and for tungsten up to 3,680K. Gas jet levitation which is applicable to any solid material, and electromagnetic levitation of electrical conductors were used to suspend the materials of interest. Non-contact heating and property measurements were achieved by optical techniques, i.e., laser heating, laser induced fluorescence measurements of vapor concentrations, and optical pyrometry for specimen temperatures.

Nordine, Paul C.↗
NASA NTRS1986

A high-temperature heat sensitive element

This invention concerns the high-temperature heat sensitive element which is stable at high temperatures. A solid solution of the main component MgO-Al2O3-Cr2O3-Fe2O3 which contains spinel crystal structure is mixed with the secondary component ZrO2 at the mol ratio of 100 : 0.1 to 5.0 and sintered to prepare a high-temperature heat sensitive element.

Oguro, M.↗
NASA NTRS1987

Alumina-supported Pd-Ag catalysts for low-temperature CO and methanol oxidation

Pd-Ag bimetallic catalysts, supported on gamma-Al2O3, have been evaluated as exhaust catalysts for methanol-fueled vehicles. Laboratory studies have shown that a 0.01% Pd-5% Ag catalyst has greater CO and CH3OH oxidation activity than either 0.01% Pd or 5% Ag catalysts alone. Moreover, Pd and Ag interact synergistically in the bimetallic catalyst to produce greater CO and CH3OH oxidation rates and lower yields of methanol partial oxidation products than expected from a mixture of the single-component catalysts. The Pd-Ag synergism results from Pd promoting the rate of O2 adsorption and reaction with CO and CH3OH on Ag. Rate enhancement by the bimetallic catalyst is greatest at short reactor residence times where the oxygen adsorption rate limits the overall reaction rate.

Mccabe, R. W.↗
NASA NTRS1986

Degradation mechanisms in thermal-barrier coatings

The degradation mechanism in a thermal barrier-coating system subjected to prolonged heating in air as well as to thermal cycling was studied. Bond-coat oxidation was found to be the most important reason for degradation. The oxidation produced NiO as well as Al2O3 in one set of samples, but the variation in initial coating structure made it difficult to resolve systematic differences between isothermally heated and thermally cycled samples. However, the contribution to degradation from changes in substrate composition seemed less in the cycled sample.

Shinde, S. L.↗
NASA NTRS1986

Technology for active laser remote sensing from space

The requirements for the experiments to be performed on the laser atmospheric sounder and altimeter facility of the polar orbiting platform are considered. The proposed instruments for the Lidar In-Space Technology Experiment designed to calibrate and verify the radiative transfer equation for laser energies at wavelengths of 1060, 532, and 355 nm, and to measure the vertical distribution of aerosols, the optical thickness of high altitude clouds, and the top of clouds are described. Research in the area of tunable laser materials and Lidar transmitters, in particular Ti:Al2O3 materials and Lidar transmitters, aimed at increasing wavelength tunability, efficiency, and the lifetime of solid-state lasers for utilization on Space Station polar orbiting platforms is discussed.

Sokoloski, M. M.↗
NASA NTRS1986

Thin-film temperature sensors for gas turbine engines Problems and prospects

The erosion and corrosion of thermocouples used to measure the temperature in turbine engines are studied. Structural and metallurgical interactions and instabilities at thermocouple interfaces are analyzed. Consideration is given to the adhesion, dielectric quality, surface topography, and hardness of the thermal oxides; it is observed that the structural and thermoelectric stability of thin-film thermocouple elements depends on adhesion, surface topography, and dielectric strength. The electrical conductivity and impurity content of the oxide scale are evaluated. Methods for improving the adhesion of thermocouples on the alumina surfaces are described. Compositional inhomogeneities in the sensors and contamination of the thermocouple elements are examined. The fabrication of the thermocouples is discussed. It is noted that Al2O3 and Si3N4 are useful for developing stable thermocouple elements on the surface of the blades and vanes.

Budhani, R. C.↗
NASA NTRS1987

A vaporization model for iron/silicate fractionation in the Mercury protoplanet

A study has been carried out on the vaporization of a totally molten silicate magma of chondritic composition heated into the range 2500-3500 K. The motivation for this was to determine the changes in the composition of the mantle that would occur in the Mercury protoplanet should that body have been subjected to the high-temperature phase in the evolution of the primitive solar nebula, but the results are of more general interest. An empirical model based on ideal mixing of complex components was used to describe the nonideal magma. It is found that vaporization of about 70-80 percent of the original amount of silicate from a chondritic planet is required to produce an iron-rich body with a mean uncompressed density equal to that deduced for Mercury. At this point the silicate is depleted in the alkalis, FeO, and SiO2, and enriched in CaO, MgO, Al2O3, and TiO2 relative to chondritic material.

Fegley, Bruce, Jr.↗
NASA NTRS1986

Space debris - Electrodynamic effects

It is shown that electrodynamic forces play a crucial role in the orbital dynamics of the small Al2O3 particles that are injected into the terrestrial magnetosphere during solid rocket propellant burns. Due to the simplified model that is used to describe the plasma distribution in the magnetosphere, the present results are necessarily preliminary. Even so, it appears that the smallest particles (a = 0.1 micron) will be rapidly eliminated from the magnetosphere. Evaluation of the fate of the somewhat larger (a = 1 micron) particles awaits the construction of a more complete and realistic model of the magnetospheric plasma.

Horanyi, M.↗
NASA NTRS1988

Process Makes High-Grade Silicon

Reactants and electrolyte recycled to yield relatively pure product. Process based on reaction of silicon dioxide with aluminum. Aluminum for process taken from operating aluminum-electrolysis cell, and Al2O3 product returned to same cell. Aluminum continually recycled and purified, and purity of silicon high as well.

Keller, Rudolf↗
NASA NTRS1987

The effect of sulfur and zirconium Co-doping on the oxidation of NiCrAl

The adhesion behavior of Al2O3 scales formed on NiCrAl+Zr alloys was examined as a function of both sulfur and zirconium doping levels. In general, very high levels of zirconium were required to counteract the detrimental effects of sulfur. A sulfur-zirconium adherence map was constructed, as determined from the oxidation and spalling behavior in 1100 C cyclic tests. For low sulfur alloys, the amount of zirconium required for adherence at any given sulfur level can be described by Zr greater than 600 S sup 0.2 (in ppma). These results underscore the importance of sulfur to adhesion mechanisms and suggests that sulfur gettering is a first order effect of reactive element additions to MCrAl alloys.

Smialek, James L.↗
NASA NTRS1987

Structure and grain coarsening during the processing of engineering ceramics

Studies have been made of three ceramic systems (Al2O3, Y2O3/MgO, and SiC/C/B), both to explore a surface area/density diagram approach to examining the coarsening processes during sintering and to explore an alternative coarsening parameter, i.e., the grain boundary surface area (raising it at a given value of the density) and not the pore surface area; therefore, pinning of the grain boundaries by solid-solution drag is the only function evidenced by these results. The importance of such pinning even at densities as low as 75% of theoretical is linked to the existence of microstructural inhomogeneities. The early stages of sintering of Y2O3 powder have been examined using two techniques, BET surface area analysis and transmission electron microscopy. Each has given some insight into the process occurring and, used together, have provided some indication of the effect of MgO on coarsening during sintering. Attempts to further elucidate effects of MgO on the coarsening behavior of Y2O3 by the surface area/density diagram approach were unsuccessful due to masking effects of contaminating reactions during sintering and/or thermal etching. The behavior of the undoped SiC which only coarsens can be clearly distinguished by the surface area/density diagram from that of SiC/C/B which also concurrently densifies. Little additional information was obtainable by this method due to unfavorable sample etching characteristics. The advantages, disadvantages, and difficulties of application of these techniques to the study of coarsening during sintering are discussed.

Shaw, Nancy J.↗
NASA NTRS1987

What SNC meteorites tell us about Martian magmatism

The SNC meteorites record relatively recent (less than b.y.) basaltic volcanism on a hydrous planet with a core. This much was known about Mars even before the Viking mission. The SNCs tell that the basalts were unusually low in Al2O3 and, despite overall similarities in major elements, apparently sampled two isotopically distinct regions of Mars: one is relatively depleted in incompatible elements (Nahklite source) with a U-Pb model age of 4.3 b.y. and is probably a portion of the upper Martian mantle, whereas the other is relatively enriched in incompatible elements (Shergottite source) with a 4.5 b.y. U-Pb age and is probably Martian crust assimulated into the mantle-derived Shergottite magma. The SNCs tell that the overall abundances of moderately volatile elements, such as alkalies, are higher on Mars than on earth. Given extensive volcanism and a depleted mantle, the present day absence of a thick atmosphere is more likely due to catastrophic removal of a primordial atmosphere than lack of degassing.

Longhi, John↗
NASA NTRS1987

Phase relations of a high-Mg basalt from the Aleutian Island arc - Implications for primary island arc basalts and high-Al basalts

An experimental investigation of a primitive high-Mg basalt, MK-15, collected from lava flows of the Unalaska Island in the Aleutian Island arc has been conducted in order to study primary and parental island arc basalts and the development of island arc magmas. The results suggest a model in which high-Al basalts are generated by moderate amounts of crystal fractionation from more primitive (high Mg/Mg + Fe, lower Al2O3) basaltic magmas near the arc crust-mantle boundary. Somewhere between 20-30 depth, significant amounts of clinopyroxene and olivine, with lesser amounts of spinel and possibly amphibole, fractionate, forming layer of olivine-clinopyroxenite at the base of the arc crust.

Gust, D. A.↗
NASA NTRS1988

Tunable solid-state lasers - An emerging technology for remote sensing of planetary atmospheres

The present development status and prospective (1990s) performance-improvement evaluation of tunable solid-state laser technology notes recent trends toward spectrum coverage over the 0.20-14.0 microns range, in addition to dramatic increases in efficiency, service life, and reliability. It is judged that the Ti:Al2O3 laser and the AgGaSe2 optical parametric oscillator pumped by a Ho:YAG laser could cover the near-IR and mid-IR regions of the spectrum. Laser diodes operating at 0.78 microns should provide an excellent pump for a Ho:YAG laser.

Barnes, Norman P.↗
NASA NTRS1988

Enthalpies of formation of CaAl4O7 and CaAl12O19 (hibonite) by high temperature, alkali borate solution calorimetry

Enthalpies of formation were determined for two calcium aluminate phases, CaAl4O7 and CaAl12O19, using high-temperature alkali borate solution calorimetry. The aluminates were synthesized by multiple-cycle heating and grinding stoichiometric mixtures of CaCO3 and Al2O3, and the products were characteized by X-ray diffraction and SEM microbeam analysis. The data on impurities (CaAl4O7 was found to be about 89.00 percent pure by weight and the CaAl12O19 samples about 91.48 percent pure) were used to correct the heat of solution values of the synthetic products. The enthalpies of formation, at 1063 K, from oxides, were found to be equal to -(25.6 + or - 4.7) kJ/g.f.w. for CaAl4O7 and -(33.0 + or - 9.7) kJ/g.f.w. for CaAl12O19; the respective standard enthalpies of formation from elements, at 298 K, were estimated to be -4007 + or - 5.2 kJ/g.f.w. and -10,722 + or - 12 kJ/g.f.w.

Geiger, C. A.↗
NASA NTRS1988

Light-weight, high-performance metal matrix composites

Recent years have seen the development of a wide range of light-weight, high-performance aluminum powder metallurgy composites that combine both standard (6061 and 2124) and specialty matrix compositions (Al-Mg-Cu-Li and Al-Fe-Ce) with a wide variety of discontinuous reinforcements (Al2O3, B4C, and SiC). This paper reviews the fabrication and damage tolerance performance of these light-weight, high-performance composites. Particular attention is given to developing a general framework for understanding the interrelationship existing between microstructure, thermomechanical processing and ductility/fracture toughness behavior in these composite systems.

Rack, H. J.↗
NASA NTRS1988

Containerless laser-induced flourescence study of vaporization and optical properties for sapphire and alumina

Evaporation of aluminum oxide was studied from 1800 to 2327 K by laser-induced flourescence (LIF) detection of Al atom vapor over sapphire and alumina spheres that were levitated in an argon gas jet and heated with a continuous wave CO2 laser. Optical properties were determined from apparent specimen temperatures measured with an optical pyrometer and true temperatures deduced from the LIF intensity versus temperature measurements using the known temperature dependence of the Al atom vapor concentration in equilibrium with Al2O3. The effects of impurities and dissolved oxygen on the high-temperature optical properties of aluminum oxide were discussed.

Nordine, Paul C.↗