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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Peculiar spherules from Antarctica and their origin

Two calcium titanium oxide spherules (CTS) and an iron chromium nickel oxide one (FCN) were obtained from a segment of ice core collected at Mizuho Station, East Antarctica. An anomalous REE abundance pattern enriched in Sm by a factor of about 10 relative to the typical pattern of terrestrial perovskites was found. Subsequently, more than forty spherules from ten other depth ranges of the ice core were analyzed: (1) both CTS and FCN occur in every depth range; (2) CTS, FCN, and other are in the ratio of about 2:21 among all the analyzed spherules; (3) all of CTS show the same chemical and mineralogical characteristics as those previously obtained; (4) CTS contains some amounts of Cr and Fe, and/or FCN contains Ca and Ti; and (5) two composite particles exist among all the spherules. It is implied that CTS and FCN had originated in a common natural material and from a common natural process.

Tazawa, Y.↗
NASA NTRS1987

High-surface-area, dual-function oxygen electrocatalysts for space power applications

The processes of hydration/dehydration and carbonation/decarbonation are investigated as an approach to provide higher surface area mixed metal oxides that are more active electrochemically. These materials are candidates for use as electrocatalysts and electrocatalyst supports for alkaline electrolyzers and fuel cells. For the case of the perovskite, LaCoO3 , higher surface areas were achieved with no change in structure and a more active oxygen electrocatalyst.

Ham, David O.↗
NASA NTRS1986

Refractory minerals in interplanetary dust

A newly studied interplanetary dust particle contains a unique set of minerals that closely resembles assemblages in the refractory, calcium- and aluminum-rich inclusions in carbonaceous chondrite meteorites. The set of minerals includes diopside, magnesium-aluminum spinel, anorthite, perovskite, and fassaite. Only fassaite has previously been identified in interplanetary dust particles. Diopside and spinel occur in complex symplectic intergrowths that may have formed by a reaction between condensed melilite and the solar nebula gas. The particle represents a new link between interplanetary dust particles and carbonaceous chondrites; however, the compositions of its two most abundant refractory phases, diopside and spinel, differ in detail from corresponding minerals in calcium- and aluminum-rich inclusions.

Christoffersen, Roy↗
NASA NTRS1987

High-pressure study of the new Y-Ba-Cu-O superconducting compound system

Hydrostatic effects on the superconducting transition temperature of the Y-Ba-Cu-O compound system, resistively, up to 19 kbar are investigated. It is found that pressure has little effect on the superconducting state of Y-Ba-Cu-O, in marked contrast to the behavior of the K2NiF4-phase La-Ba-Cu-O and La-Sr-Cu-O systems. It is suggested that this effect may be due to chemical pressure associated with the smaller Y atoms already present in Y-Ba-Cu-O. X-ray powder-diffraction studies show that the high-temperature superconductivity in Y-Ba-Cu-O can only be attributed to one or more phases with structures different from the cubic perovskite or tetragonal layered ones.

Hor, P. H.↗
NASA NTRS1987

Refractory interplanetary dust particles

Criteria are described by which refractory interplanetary dust particles (IDPs) can be differentiated from the products of spacecraft debris. These criteria have been used to discover and characterize IDPs that are composed predominantly of refractory phases. Two of these particles contain hibonite, perovskite, spinel, refractory glass, and a melilite; only hibonite was identified within a third. The grain size for all particles ranges from 0.05 to 1 micrometer, so that they are much finer grained than the refractory calcium- and aluminum-rich inclusions in meteorites. The glass-containing refractory IDPs may be primitive nebular condensates that never completely crystallized and thus have been preserved extant.

Zolensky, Michael E.↗
NASA NTRS1988

Chemical durability of high-temperature superconductor YBa2Cu3O(7-x) in aqueous environments

The stability of YBa2Cu3O(7-x) in water and 100-percent humidity has been investigated at three temperatures, using pH measurements, X-ray diffraction, and scanning electron microscopy. The oxide-ceramic superconductor is highly unstable; it reacts rapidly with water and degrades in moisture. Dissolution of the oxide perovskite in water is highly incongruent. The corrosion products are found to be BaCO3, CuO, O2, etc. Barium hydroxide is first formed and further reacts with atmospheric CO2 to form needle-shaped crystals of BaCO3. For any practical applications, devices made from these materials would have to be protected with an impermeable coating to prevent deterioration from atmosphere.

Bansal, Narottam P.↗
NASA NTRS1988

Effect of fluoride doping on the transition temperature of YBa2Cu3O(6.5+delta)

The effects of F content on the Tc of superconducting YBa2Cu3F(x)O(6.5+delta-0.5x) (x = 0.0165-1.65) prepared by the solid-state reaction method (Bandsahl and Sandkuhl, 1988) are investigated experimentally. The results are presented in tables and graphs and characterized. Tc is found to increase steadily from 90.8 to 93.4 K as x is increased from 0.0165 to 0.066, but to decrease as x is further increased (Tc = 90.3 K at x = 1.65). The structure of the samples with x = 0.066 or lower is found to be single-phase perovskite.

Bansal, Narottam P.↗
NASA NTRS1988

Antiferromagnetism and superconductivity in oxygen-deficient YBa2Cu3O(x)

Positive-muon spin-rotation and -relaxation measurements of the oxygen-deficient perovskite YBa2Cu3O(x) have revealed local antiferromagnetic order for x = 6.0-6.4 with a Neel temperature TN that decreases rapidly with increasing oxygen content x. For slowly annealed samples with x = 6.35-6.5 the superconducting transition temperature Tc increases smoothly with x from 25 K at x = 6.348 to 60 K at x = 6.507. Two such samples with x = 6.348 and x = 6.400 appear to 'switch' from superconductivity to antiferromagnetic order at lower temperatures.

Brewer, J. H.↗
NASA NTRS1988

Superconductivity in the high-Tc Bi-Ca-Sr-Cu-O system - Phase identification

Four phases are observed in superconducting Bi-Ca-Sr-Cu-O samples. The superconducting phase, with onset temperature near 120 K, is a 15.4-A-layered compound with composition near Bi2Ca1Sr2Cu2O9 and an A-centered orthorhombic unit subcell 5.41 x 5.44 x 30.78 A. X-ray diffraction and electron microscopy data are consistent with a structure of alternating perovskite and Bi2O2 layers. High-resolution transmission electron microscopy images reveal a b-axis superstructure of 27.2 A, numerous (001) stacking faults, and other defects.

Hazen, R. M.↗
NASA NTRS1988

The origin of type C inclusions from carbonaceous chondrites

The origin of type C inclusions found in carbonaceous chondrites and the relationship of the type C inclusions to other Ca- and Al-rich chondrules are discussed on the basis of major-element composition of these objects. The results on bulk composition analysis of primary phase assemblages in eight Allende-type inclusions and an Allende glass-spinel inclusion were found to be consistent with the following scenario: (1) the condensation of melilite + spinel + perovskite +/- hibonite +/- CaAl4O7; (2) the reaction of melilite + spinel with SiO2(g); (3) the removal of inclusions from further interaction with the reacting gas; (4) melting with little or no volatilization; (5) crystallization; (6) alteration; and (7) incorporation of inclusions into host meteorite. Type C inclusions are considered to be precursors to Al-rich chondrules in ordinary and enstatite chondrites, i.e., they are considered to be a spinel-rich class of type A that reacted with silica from a coexisting gas to produce anorthite plus diopside.

Beckett, John R.↗
NASA NTRS1988

Structural chemistry of Au(III)-substituted Ba2YCu3O(7-delta)

A series of gold-substituted perovskite superconductors Ba2Y(Cu/1-x/Aux)3O(7-delta)(x = 0-0.1) was synthesized. For x = 0.1, there was no change in the a and b lattice parameters (a = 3.826 A and b = 3.889 A), but a 0.06 A c-axis expansion to 11.75 A was observed. Substituted gold was found to be trivalent by X-ray photoelectron spectroscopy. Replacing Cu(1) in the copper oxide chain with a slight reordering of oxygen is consistent with c-axis expansion. The formal charge of the site remains trivalent, while remaining Cu in the chains is reduced to Cu(I), resulting in an oxygen stoichiometry of less than 7. Finally, no large effect on Tc is observed (Tc = 89 K for x = 0.10), in contrast to the effect of a number of other metal ion dopants. These results are discussed relative to the chemistry of Au(III) and to the use of the metal in structures containing gold and ceramic superconductors.

Hepp, A. F.↗
NASA NTRS1988

Interaction of Au, Ag, and Bi ions with Ba2YCu3O(7-y) - Implications for superconductor applications

Results are presented on the reactions of Au, Ag, and Bi ions with Ba2YCu3O(7-y) oxides and on the properties of the resultant materials. The results indicate that Au(3+) structural chemistry makes gold an excellent candidate for multiphase structures of the Ba2Y(Cu/1-x/Au/x/)3O(7-y)-type substituted superconductors. Silver is structurally and chemically compatible with the perovskite structure, but when it forms a second phase, it does so without the destruction of the superconducting phase, making silver a useful metal for metal/ceramic applications. On the other hand, bismuth was shown to degrade Tc phase or to form other phases, indicating that it may not be useful in applications with rare-earth-based superconductors.

Hepp, A. F.↗
NASA NTRS1990

Ternary and quaternary oxides of Bi, Sr, and Cu

Before the discovery of superconductivity in an oxide of Bi, Sr, and Cu, the system Bi-Sr-Cu-O had not been studied, although several solid phases had been identified in the two-component regions of the ternary system Bi2O3-SrO-CuO. The oxides Sr2CuO3, SrCu2O2, SrCuO2, and Bi2CuO4 were then well known and characterized, and the phase diagram of the binary system Bi2O3 -SrO had been established in the temperature range 620 to 1000 C. Besides nine solutions of compositions Bi(2-2x) Sr(x) O(3-2x) and different symmetries, this diagram includes three definite compounds of stoichiometries Bi(2)SrO4, Bi2Sr2O5, and Bi2Sr3O6 (x = 0.50, 0.67 and 0.75 respectively), only the second of which with known unit-cell of orthorhombic symmetry, dimensions (A) a = 14.293(2), b = 7.651(2), c = 6.172(1), and z = 4. The first superconducting oxide in the system Bi-Sr-Cu-O was initially formulated as Bi2Sr2Cu2O(7+x), with an orthorhombic unit-cell of parameters (A) a = 5.32, b = 26.6, c = 48.8. In a preliminary study the same oxide was formulated with half the copper content, Bi(2)Sr(2)CuO(6+x), and indexed its reflections assuming an orthorhombic unit-cell of dimensions (A) a = 5.390(2), b = 26.973(8), c = 24.69(4). Subsequent studies by diffraction techniques have confirmed the composition 2:2:1. A new family of oxygen-deficient perovskites, was characterized, after identifying by x ray diffraction the phases present in the products of thermal treatments of about 150 mixtures of analytical grade Bi2O3, Sr(OH)2-8H2O and CuO at different molar ratios. X ray diffraction data are presented for some other oxides of Bi and Sr, as well as for various quaternary oxides, among them an oxide of Bi, Sr, and Cu.

Casais, M. T.↗
NASA NTRS1990

Electrochemical incineration of wastes

The novel technology of waste removal in space vehicles by electrochemical methods is presented to convert wastes into chemicals that can be eventually recycled. The important consideration for waste oxidation is to select a right kind of electrode (anode) material that should be stable under anodic conditions and also a poor electrocatalyst for oxygen and chlorine evolution. On the basis of long term electrolysis experiments on seven different electrodes and on the basis of total organic carbon reduced, two best electrodes were identified. The effect of redox ions on the electrolyte was studied. Though most of the experiments were done in mixtures of urine and waste, the experiments with redox couples involved 2.5 M sulfuric acid in order to avoid the precipitation of redox ions by urea. Two methods for long term electrolysis of waste were investigated: (1) the oxidation on Pt and lead dioxide electrodes using the galvanostatic methods; and (2) potentiostatic method on other electrodes. The advantage of the first method is the faster rate of oxidation. The chlorine evolution in the second method is ten times less then in the first. The accomplished research has shown that urine/feces mixtures can be oxidized to carbon dioxide and water, but current densities are low and must be improved. The perovskite and Ti4O7 coated with RuO2 are the best electrode materials found. Recent experiment with the redox agent improves the current density, however, sulphuric acid is required to keep the redox agent in solution to enhance oxidation effectively. It is desirable to reduce the use of acid and/or find substitutes.

Bhardwaj, R. C.↗
NASA NTRS1989

Properties of Pb(1-x)Bi(x)/Ba2YCu3O(x) composites - Reaction of Ba2YCu3O(x) with Pb and Bi

This paper describes experiments conducted with a purpose of fabricating superconducting composites from Pb, Bi, and Ba2YCu3O(x), together the results of SEM examinations and energy dispersive spectroscopy performed on products. Results showed a limited utility of Pb and Pb(1-x)Bi(x) as matrices for ceramic superconductors. It was found that cold pressing followed by sintering at 200 C resulted in a composite which excluded flux below 90 K but did not show zero electrical resistance until the metal superconducting transition. Processing at 400 or 950 C resulted in oxygen depleted perovskite and/or metal oxides; these materials displayed greatly degraded superconducting properties. Finally, processing at 800 C resulted in high Tc only for composites containing more than 90 wt pct ceramic.

Hepp, A. F.↗
NASA NTRS1990

Presolar isotopic and chemical signatures in hibonite-bearing refractory inclusions from the Murchison carbonaceous chondrite

The mineralogy and the major-element composition of phases within individual hibonite-bearing inclusions were determined, using SEM with EDAX analysis, for 37 hibonite-bearing inclusions which comprised 26 spinel-hibonite-perovskite inclusions, ten platy hibonite crystal fragments, and one blue hibonite aggregate. Results from all inclusions studied indicated the presence of correlations in hibonite isotopic systematics and chemistry. Namely, no inclusion had both a large Ti-50 anomaly and radiogenic Mg-26, and inclusions that were depleted in the ultrarefractory elements had radiogenic Mg-26 with the exception of those which a had a large Ti-50 anomaly. These features establish a relationship between chemical and isotopic characteristics of the inclusions which must predate their formation. It is suggested that depletions in ultrarefractory elements observed in these inclusions may be a memory of stellar condensation and are not necessarily related to a condensation event in the solar nebula.

Ireland, Trevor R.↗
NASA NTRS1991

Hibonite-bearing microspherules - A new type of refractory inclusions with large isotopic anomalies

This paper presents petrographic description as well as results on the major- and trace-element chemistry and on Mg, Ca, and Ti isotopic compositions of three refractory inclusions, including 3413-1/31 inclusion from Lance and Murchison 7-228 and 7-753, the mineralogy of which is dominated by the oxide minerals spinel, hibonite, and perovskite. The microspherules examined seem to constitute a separate class of refractory inclusions, characterized by a distinct morphology and mineralogy; large excesses of Ca-48 and Ti-50, and Mg-26 depletions. It is suggested that this type of inclusions must have formed early, prior to the dilution of isotopic anomalies by mixing processes, and in an area characterized by excesses of Ca-48 and Ti-50, depletions of Mg-26, and a lack of Al-26.

Ireland, T. R.↗
NASA NTRS1992

Catalysts for ultrahigh current density oxygen cathodes for space fuel cell applications

The objective was to identify promising electrocatalyst/support systems for oxygen cathodes capable of operating at ultrahigh current densities in alkaline fuel cells. Such cells will require operation at relatively high temperatures and O2 pressures. A number of materials were prepared, including Pb-Ru and Pb-Ir pyrochlores, RuO2 and Pt-doped RuO2, lithiated NiO and La-Ni perovskites. Several of these materials were prepared using techniques that had not been previously used to prepare them. Particularly interesting was the use of the alkaline solution technique to prepare Pt-doped and Pb-Ru pyrochlores in high area form. Also interesting was the use of the fusion (melt) method for preparing the Pb-Ru pyrochlore. Several of the materials were also deposited with platinum. Well-crystallized Pb2Ru2O(7-y) was used to fabricate very high performance O2 cathodes with good stability in room temperature KOH. This material was also found to be stable over a useful potential range at approx. 140 C in concentrated KOH. For some of the samples, fabrication of the gas-fed electrodes could not be fully optimized during this project period. Future work may be directed at this problem. Pyrochlores that were not well-crystallized were found to be unstable in alkaline solution. Very good O2 reduction performance and stability were observed with Pb2RuO(7-y) in a carbon-based gas-fed electrode with an anion-conducting membrane placed on the electrolyte side of the electrode. The performance came within a factor of about two of that observed without carbon. High area platinum and gold supported on several conductive metal oxide supports were examined. Only small improvements in O2 reduction performance at room temperature were observed for Pb2Ru2O(7-y) as a support because of the high intrinsic activity of the pyrochlore. In contrast, a large improvement was observed for Li-doped NiO as a support for Pt. Very poor performance was observed for Au deposited on Li-NiO at approx. 150 C. Nearly reversible behavior was observed for the O2/OH(-) couple for Li-doped NiO at approx. 200 C. The temperature dependence for the O2 reduction was examined.

Tryk, Donald A.↗