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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Trace element compositions of spinel-rich refractory inclusions from the Murchison meteorite

Three spinel-rich, perovskite-bearing, pyroxene-rimmed, hibonite-free inclusions from the Murchison C2 chondrite have volatility controlled, modified group II REE patterns. Two have high Eu/Yb ratios, two have small positive Ce anomalies and one has high refractory siderophiles for a group II inclusion. Two additional spinel-rich inclusions have subchondritic Ce/La ratios and negative Eu and Yb anomalies, the Eu anomalies being larger than the Yb ones. One of these inclusions is enriched in La relative to all HREE and the other is enriched in HREE relative to all LREE except La, possibly suggesting the presence of an ultrarefractory component in the latter. One hibonite-rich inclusion has a group III REE pattern with a negative Ce anomaly. A calcium dialuminate-bearing inclusion known to have crystallized from a melt at 2100-1800K has a modified group II REE pattern, rather than a pattern which would be predicted from the evaporation expected from such a droplet, e.g., a group III or ultrarefractory pattern. Volatile element contents of these inclusions are among the lowest found in any refractory inclusions.

Yoneda, S.↗
NASA NTRS1960

The Blunt Plate In Hypersonic Flow

The sonic-wedge characteristics method has been used to obtain the shock shapes and surface pressure distributions on several blunt two-dimensional shapes in a hypersonic stream for several values of the ratio of specific heats. These shapes include the blunt slab at angle of attack and power profiles of the form yb = a)P, where 0 les than m less than 1, Yb and x are coordinates of the body surface, and a is a constant. These numerical results have been compared with the results of blast-wave theory, and methods of predicting the pressure distributions and shock shapes are proposed in each case. The effects of a free-stream conical-flow gradient on the pressure distribution on a blunt slab in hypersonic flow were investigated by the sonic-wedge characteristics method and were found to be sizable in many cases. Procedures which are satisfactory for reducing pressure data obtained in conical flows with small gradients are presented.

Baradell, Donald L.↗
NASA NTRS2009

Further Analysis of Thermal Control Coatings on MISSE for Aerospace Applications

Many different passive thermal control materials were flown as part of the Materials on International Space Station Experiment. Engineers and scientists at the Marshall Space Flight Center have analyzed a number of these materials, including Z93P zinc oxide/potassium silicate coating, YB-71P zinc orthotitanate/potassium silicate coating, NZOT, which is a low-cost alternative to YB-71P, several electrically conductive/static dissipative thermal control coatings, as well as black coatings for part marking and automated rendezvous and capture. These were exposed to the low Earth orbital environment of atomic oxygen, ultraviolet radiation, thermal cycling, and hard vacuum, though atomic oxygen exposure was very limited for some samples. Results from the one-year exposure of MISSE-3 and MISSE-4 are compared to the four-year exposure of MISSE-1 and MISSE-2. Solar absorptance, infrared emittance, and mass measurements indicate the durability of these materials to withstand the space environment. The effect of contamination from an active space station on the performance of white thermal control coatings is discussed.

Kenny, Mike↗
NASA NTRS2016

Magnesium and Titanium Isotopic Compositions of an Unusual Hibonite-Perovskite Refractory Inclusion from Allende: It Is Fun

Introduction: Hibonite-rich refractory inclusions are among the first solids that formed in the solar nebula, and thus provide constraints on the earliest environment in the Solar System. An unusual hibonite-perovskite inclusion from Allende, SHAL, consists of a large (approximately 500 by 200 microns) single hibonite crystal and coexisting blocky perovskite (approximately 200 microns in size). The hibonite is characterized by chemical and oxygen isotopic compositions similar to those in the FUN (Fractionated and Unknown Nuclear anomalies) inclusion HAL. However, the rare earth element (REE) patterns measured at different spots of SHAL hibonite are highly variable, ranging from Group II-like (light REEs enriched relative to heavy REEs) to Group III-like (relatively flat with slight Eu depletions), but overall contrast largely with that of HAL, especially in the Ce and Yb abundances. This implies that SHAL hibonite formed and underwent distillation processes under more reducing conditions. Interestingly, the accompanying perovskite has uniform, unfractionated oxygen isotopic compositions (averaging delta (sup 17) O equals delta (sup 18) O equals −7 per mille) and REE abundances that are completely different from those of SHAL hibonite. This has been interpreted that perovskite and hibonite may not be co-genetic. Here we performed Al-Mg and Ti isotopic measurements of SHAL hibonite and perovskite to determine if the FUN characteristics are observed in these two isotope systems, and to further constrain the origin and evolution of SHAL. Results: Isotopic measurements of Al-Mg and Ti in SHAL were performed on the UCLA CAMECA ims-1290 ion microprobe by following the analytical protocols described in [1]. The Al-Mg and Ti data obtained in both terrestrial standards and SHAL hibonite and perovskite are shown below. Both SHAL hibonite and perovskite, despite very high (sup 27) Al to (sup 24) Mg ratios, are devoid of (sup 26) Mg excesses that can be attributed to the decay of (sup 26) Al. Delta (sup 25) Mg (mass-dependent fractionation) in hibonite is approximately −5 per mille per atomic mass unit relative to Madagascar hibonite, but is not well constrained for perovskite due to very large uncertainties owing to extremely low Mg contents. Similar to Mg isotopes, SHAL hibonite and perovskite show essentially the same Ti isotopic compositions, with anomalies in (sup 50) Ti of approximately 14 per mille, but the former shows greater Ti isotope fractionation than the latter (2.5 per mille per atomic mass unit versus 0 per mille). Discussion and Conclusions: The Al-Mg and Ti isotopic compositions of SHAL hibonite are consistent with those of HAL, suggesting that SHAL hibonite is a FUN inclusion and likely formed prior to homogenization of (sup 26) Al and Ti isotope variations in the solar nebula. However, the formation mechanisms for SHAL and HAL differ, given the differences in the REE patterns and degrees of oxygen mass-dependent fractionation. The Group-II to Group-III like REE patterns, the Yb depletions, and negative delta (sup 25) Mg observed in SHAL hibonite are all consistent with condensation of the hibonite precursor in a reducing environment.. The lack of large Ce depletions in SHAL hibonite implies that distillation processes that fractionated hibonite's oxygen isotopes must have taken place under a reducing condition, but the extent to which SHAL hibonite was distilled appears to be less than HAL because of the smaller degree of oxygen mass-dependent fractionation. The perovskite shares essentially the same Ti and Mg isotopic compositions as hibonite and probably formed in the same reservoir.. The ultrarefractory REE pattern seen in perovskite likely resulted from gas-solid fractionation which depleted HREEs in this reservoir. This process also explains why SHAL hibonite is generally depleted in HREEs relative to LREEs.

Liu, M.-C.↗
NASA NTRS2017

Thermochemistry of Calcium-Magnesium-Aluminum-Silicate (CMAS) and Components of Advanced Thermal and Environmental Barrier Coating Systems

There is increasing interest in the degradation mechanism studies of thermal and environmental barrier coatings (TEBCs) of gas turbines by molten CaO-MgO-Al(exp. 2)O(exp. 3)-SiO(exp. 2) CMAS). CMAS minerals are usually referred as silicon-containing sand dust and volcano ash materials that are carried by the intake air into gas turbines, e.g. in aircraft engines, and their deposits often react at high temperatures (greater than 1200 degrees C) with the engine turbine coating systems and components. The high temperature reactions causes degradation and accelerated failure of the static and rotating components of the turbine engines. We discuss some results of the reactions between the CMAS and Rare-Earth (RE = Y, Yb, Dy, Gd, Nd and Sm) - oxide stabilized ZrO(exp. 2) or HfO(exp. 2) systems, and the stability of the resulting oxides and silicates. Plasma sprayed hollow tube samples (outside diameter = 4.7 mm, wall thickness = 0.76 mm and = 26 mm height) were half filled with CMAS powder, wrapped and sealed with platinum foil, and heat treated at 1310 degrees C for 5h. Samples were characterized by differential scanning calorimetry (DSC), X-ray diffraction, and cross-section electron microscopy analysis and energy dispersive X-ray spectroscopy. It was found that CMAS penetrated the samples at the grain boundaries and dissolved the TEBC materials to form silicate phases containing the rare-earth elements. Furthermore, it was found that apatite crystalline phases were formed in the samples with total rare-earth content higher than 12 mol% in the reaction zone for the ZrO(exp. 2) system. In general, samples with the nominal compositions (30YSZ), HfO(exp. 2)-7Dy(exp. 2)O(exp. 2) and ZrO(exp. 2)-9.5Y(exp. 2)O(exp. 3)-2.25Gd(exp. 2)O(exp. 3)-2.25Yb(exp. 2)O(exp. 3) exhibited lower reactivity or more resistance to CMAS than the other coating compositions of this work.

degradation↗
NASA NTRS2020

Calorimetric Measurements of the Thermodynamic Properties of RE-Silicate Coating Materials

Thermodynamic quantities of coatings materials and siliceous debris-induced corrosion products are crucial to understand in order to develop mitigation strategies necessary to improve the durability of gas-turbine engines. Siliceous induced corrosion can occur when debris consisting mainly of CaO-MgO-Al2O3-SiO2 (CMAS) is ingested by aircraft engines during and after take-off, which sticks to hot surfaces and forms calcium rare-earth silicate oxyapatites. In this work, high-temperature oxide melt drop solution calorimetry (HT drop solution calorimetry) was used to obtain the enthalpies of formation for RE silicate (RE2Si2O7, RE2SiO5 where RE = Yb, Er, Y, Dy, Nd, Lu and Gd) environmental barrier coatings (EBCs) and the calcium RE silicate oxyapatite Ca2RE8(SiO4)6O2 (RE = Yb, Er, Y, Dy, Nd, Gd and Sm) corrosion products. Trends in the enthalpy of formation as a function of the ionic potential of the rare-earth cations in their related crystallographic sites are discussed.

Costa, Gustavo↗
NASA NTRS2022

The reliability of the small-core Lanthanide effective core potentials

The reliability of the small-core Lanthanide effective core potentials (ECP) is tested using MF and MF(3), for M=Eu, Gd, Tb, and Yb and the atomic excitation energies for Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. In some case the ECP and all-electron (AE) results are in good agreement, while in others there are significant difference. The difference are much larger when the segmented basis set is used in conjunction with the ECP than when the atomic natural orbital (ANO) basis set is used. The study of the atoms suggests that problems for lanthanide containing molecules are associated with poor atomic excitation energies in the ECP treatment and even using the ANO basis set does not completely solve the problem. We note that the problem appears to be more severe for density functional approaches than for traditional correlation methods. We suggest that additional studies and new effective core potentials may be required for the Lanthanide atoms.

effective core potentials, atomic excitation, atom↗
NASA NTRS1973

Studies of beta Coronae Borealis. II - Identification of the lanthanides

A table presented contains the identification of Ce III (Sugar 1965), Pr III (Sugar 1961), Nd III (Crosswhite 1972), Sm III (Crosswhite 1969), Ho II, and Yb II (Corliss and Tech 1972) lines. The tolerance for identifications was 0.04 A. Each of these ionic identifications is based primarily on only a few of the lines listed while the remaining lines which are normally blended add support.

Adelman, S. J.↗
NASA NTRS1974

Minor and trace element distribution in melilite and pyroxene from the Allende meteorite

Melilite and pyroxene were separated from a coarsely crystalline chondrule in the Allende meteorite and analyzed by microprobe and spark source mass spectrometer techniques. Elemental abundances in the bulk chondrule are consistent with a mixture of equal amounts of the two minerals, as observed microscopically. The lanthanide distributions are markedly different in the two minerals; relative to chondrite abundances, melilite shows progressive depletion of the lanthanides La-Sm, a positive Eu anomaly, and relatively constant abundances of the heavier lanthanides (Gd-Yb, and Y) whereas pyroxene shows progressive enrichment towards the heavier lanthanides, on which is superimposed a negative Eu anomaly. Both minerals, and the bulk chondrule, show unusual concentrations of the platinum metals, but the crystallographic site or sites of these metals remains to be determined.

Mason, B.↗
NASA NTRS1975

Fractionation in the solar nebula - Condensation of yttrium and the rare earth elements

The condensation of Y and the rare earth elements (REE) from the solar nebula may be controlled by thermodynamic equilibrium between gas and condensed solids. Highly fractionated REE patterns may result if condensates are removed from the gas before condensation is complete. It is found that the fractionation is not a smooth function of REE ionic radius but varies in an extremely irregular pattern. Both Yb and Eu are predicted to be extremely depleted in the early condensate without the requirement of condensation in the divalent state. The model is discussed with respect to a highly fractionated pattern observed by Tanaka and Masuda (1973), in a pink Ca-Al-rich inclusion from the Allende meteorite and can account for the abundances of each REE determined. According to the model this inclusion represents a condensate from a previously fractionated gas rather than from a gas of solar composition. Before the condensation of this inclusion, an earlier condensate was formed and was removed from equilibrium with the gas.

Boynton, W. V.↗
NASA NTRS1974

The contamination of lunar highland rocks by KREEP - Interpretation by mixing models

A mixing model method was used to determine the component abundance in the Apollo 16 and 17 soils. This method uses the chemical composition of the soils for up to 30 elements and a weighted least-squares mixing model technique. Elements included in the calculations are: Si, Ti, Al, Ca, Fe, Mg, P, Cr, Mn, Na, K, Rb, Ba, U, Th, La, Ce, Sm, Eu, Sr, Yb, Y, Sc, V, Zr, Nb, Co, Ni, Li, Au, and Ir. The method was used to examine the possibility that some of the highland rocks such as VHA and low-K Fra Mauro basalt are mixtures. The results of the mixing model calculations show that it is possible that these rocks are mixtures of KREEP, troctolite, 'anorthosites', and a meteoritic component. The Rb-Sr systematics are consistent with such a model for the genesis of the highland rocks. KREEP has high relative concentrations of Rb, U, and radiogenic Sr and Pb and its model age of about 4.4 AE dominates the model age of all the 'contaminated' highland rocks.

Schonfeld, E.↗
NASA NTRS1976

Drive tube 60009 - A chemical study of magnetic separates of size fractions from five strata

Each bulk soil and both the magnetic and nonmagnetic components of the 90-150 micron and below 20 micron fractions of five soils from drive tube 60009 were analyzed. Samples were analyzed for FeO, Na2O, Sc, Cr, Co, Ni, Hf, Ta, Th, La, Ce, Sm, Eu, Tb, Yb, and Lu by neutron activation analysis. Several samples were fused and analyzed for major elements by electron microprobe analysis. Compositional variations are not systematically related to depth. The compositions of the five soils studied are well explained by a two-component mixing model whose end members are a submature Apollo 16-type soil and an extremely immature anorthositic material similar to 60025. There is evidence that the anorthositic component had received a small amount of exposure before these soils were mixed. After mixing, the soils received little exposure suggesting mixing and deposition on a rapid time scale.

Blanchard, D. P.↗
NASA NTRS1979

Trace elements in the Allende meteorite. IV - Amoeboid olivine aggregates

INAA data for Ca, Sc, Hf, La, Ce, Sm, Eu, Tb, Yb, Lu, Os, Ir, Ru, Na, Cl, Br, Fe, Mn, Cr, Co, Au, As, and Sb are presented for ten amoeboid aggregates from the Allende meteorite. Only one lacks olivine. Seven of the remainder, as a group, have cosmic proportions of refractory lithophile and siderophile elements and appear to have formed when coarse-grained Allende inclusion material underwent partial reaction with a low-temperature nebular gas and mixture with FeO-rich olivine. The other two have highly fractionated abundances of refractory elements relative to one another compared to Cl chondrites, including Group II REE patterns, and probably formed by the mixing of fine-grained Allende inclusion material with FeO-rich olivine. Non-refractory siderophile components are also different in composition in each type of amoeboid olivine aggregate.

Grossman, L.↗
NASA NTRS1980

Experimental mineral/liquid partition coefficients of the rare earth elements /REE/, Sc and Sr for perovskite, spinel and melilite

Experimental determinations of the mineral/liquid partition coefficients of REE (La, Sm, Eu, Gd, Tb, Yb and Lu), Sc and Sr are reported for the minerals perovskite, spinel and melilite in synthetic systems. Perovskite concentrates light REE with respect to the residual liquid but shows no preference for heavy REE. Spinel greatly discriminates against the incorporation of REE, especially light REE, into its crystal structure. The partition of REE into melilite from a silicate liquid is quite dependent upon both the bulk melt and melilite solid-solution (gehlenite and akermanite components) compositions. As such, melilite can be enriched in REE or will reject REE with corresponding strong negative or strong positive Eu anomalies, respectively.

Nagasawa, H.↗
NASA NTRS1982

Trace element concentrations in the isotopically unique Allende inclusion, EK 1-4-1

The bulk sample and mineral separates of the isotopically unique Allende inclusion, EK 1-4-1, are analyzed by instrumental neutron activation for the concentrations of REE, Sc, Fe, Co, Cr, Na, and Ir, and REE concentrations were also determined by mass spectrometric isotope dilution for two density separates. Results show that the inclusion is enriched in light REE compared to heavy REE with a positive Yb anomaly, and has a high Cr concentration, both of which indicate group II affinity, although EK 1-4-1 is a coarse-grained group I inclusion. On the other hand, high Ir, and Sc, and low FeO and Co concentrations were found in the bulk inclusion, and low Na concentrations were found in mineral separates, which indicate an affinity to group I. In addition, for melilite and pyroxene fractions REE, Sc, Co, and Fe were determined to have distributions that were substantially different from those observed for normal Allende type B inclusions.

Nagasawa, H.↗
NASA NTRS1984

Thermal barrier coating system

A high temperature oxidation resistant, thermal barrier coating system is disclosed for a nickel cobalt, or iron base alloy substrate. An inner metal bond coating contacts the substrate, and a thermal barrier coating covers the bond coating. NiCrAlR, FeCrAlR, and CoCrAlR alloys are satisfactory as bond coating compositions where R=Y or Yb. These alloys contain, by weight, 24.9-36.7% chromium, 5.4-18.5% aluminum, and 0.05 to 1.55% yttrium or 0.05 to 0.53% ytterbium. The coatings containing ytterbium are preferred over those containing yttrium. An outer thermal barrier coating of partial stabilized zirconium oxide (zirconia) which is between 6% and 8%, by weight, of yttrium oxide (yttria) covers the bond coating. Partial stabilization provides a material with superior durability. Partially stabilized zirconia consists of mixtures of cubic, tetragonal, and monoclinic phases.

Stecura, S.↗
NASA NTRS1984

Chemical compositions of refractory inclusions in the Murchison C2 chondrite

Neutron activation has been used to analyze samples from ten refractory inclusions in Murchison for 36 elements. Two inclusions, BB-5 and MUCH-1, have large negative Yb anomalies unaccompanied by correspondingly large and negative Eu anomalies; six inclusions have group II or III patterns, or variants of such patterns. The frequency of highly fractionated REE patterns, and the absence of group I patterns, suggest that refractory inclusions in Murchison stopped equilibrating with the nebular gas at higher temperatures than most Allende coarse grained inclusions. Murchison inclusions tend to be lower in nonrefractory elements than Allende coarse grained inclusions, further suggesting that, on average, the former underwent less severe secondary alteration than the latter.

Ekambaram, V.↗