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At least 91 records · Page 5

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NASA NTRS1990

Phase compatibilities of YBa2Cu3O(9-delta) type structure in quintenary systems Y-Ba-Cu-O-X (impurity)

Electrical transport properties of the oxidic high T(sub c) superconductors are significantly affected by the presence of minor amounts of various elements adventing as impurities, e.g., from the chemical environment during manufacturing. YBa2Cu3O(9-delta) is prone to an extinction of the superconductivity on (partial) substitution of all four elemental components. E.g., Pr (for Y), La (for Ba), Zn (for Cu) or peroxygroup (for O) substituents will alter some of the superconductivity preconditions, like mixed valence state in Cu3O7/O(9-delta) network or structural distortion of the network. Although various pseudoternary chemical equilibrium phase diagrams of the Y(O)-Ba(O)-Cu(O) system now are available, no consensus is generally shown, however, this is partly due to lack of compatible definitions of the equilibrium conditions. Less information is available about the phase compatibilities in the appropriate quaternary phase diagram (including oxygen) and virtually no information exists about any pentenary phase diagrams (including one impurity). Unfortunately, complexity of such systems, stemming both from number of quaternary or pentenary compounds and from visualizing the five-component phase system, limits this presentation to more or less close surroundings of the YBa2Cu3O(9-delta) type phase in appropriate pseudoquaternary or pseudopseudoternary diagrams, involving Y-Ba-Cu and O, O-CO2, alkaline metals, Mg and alkaline earths, and Sc and most of the 3-d and 4-f elements. The systems were investigated by means of x ray diffraction, neutron diffraction and chemical analytical methods on samples prepared by sol-gel technique from citrates. The superconductivity was characterized by measuring the diamagnetic susceptibility by SQUID.

Karen, P.↗

NASA NTRS1991

Theoretical study of the bonding of Sc, Y, and La singly charged and dipositive ions to C2H2, C2H4, and C3H6

The interaction of the Sc and Y singly-charged and dipositive ions with C2H2, C2H4, and C3H6 is studied using electronic structure calculations that include high levels of electron correlation. These results are compared with comparable calculations performed previously for La(+) and La(2+). For C2H2 and C2H4, all three metal ions insert into the C-C pi bond, making a three-membered ring. The optimal structures for the MC3H6(+) ions all involve rearrangement to make a four-membered ring. The strength of the metal-ligand bond for the singly charged ions follows the order La greater than Sc equal to about Y. In contrast, the bonds involving the dipositive ions are electrostatic, so that the binding energy increases as the size of the ion decreases, leading to the trend Sc greater than Y greater than La.

Bauschlicher, Charles W., Jr.↗

NASA NTRS1991

Y-MP floating point and Cholesky factorization

The floating point arithmetics implemented in the Cray 2 and Cray Y-MP computer systems are nearly identical, but large scale computations performed on the two systems have exhibited significant differences in accuracy. The difference in accuracy is analyzed for Cholesky factorization algorithm, and it is found that the source of the difference is the subtract magnitude operation of the Cray Y-MP. The results from numerical experiments for a range of problem sizes are presented, and an efficient method for improving the accuracy of the factorization obtained on the Y-MP is presented.

Carter, Russell↗

NASA NTRS1992

Airborne measurements of total reactive odd nitrogen (NO(y))

Airborne total reactive odd nitrogen measurements were made during August and September 1986 over the continental United States and off the west coast over the Pacific Ocean during NASA's Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 2 program. Measurements were made in the marine and continental boundary layer and the free troposphere up to 6.1 km altitude. NO(y) mixing ratios between 24 pptv and more than 1 ppbv were found, with median values of 101 pptv in the marine boundary layer, 298 pptv in the marine free troposphere, and 288 pptv in the continental free troposphere, respectively. The marine troposphere exhibited layered structure which was also seen in the simultaneously measured ozone mixing ratio and dew point temperature. The averaged vertical NO(y) profile over the ocean does not show a distinct gradient. The NO(y) mixing ratio over the continent decreases with increasing altitude. The latter is consistent with our understanding that the continents are the major source region for these gases.

Huebler, G.↗

NASA NTRS1993

High-resolution Y-axis readout for delay-line microchannel anodes

We have devised a method to obtain a high-resolution Y-axis event position determination from microchannel plate delay-line detectors. The method is based on the double-delay-line wedge-wedge charge partition principle of Lampton et al. (1990), where the X axis is read out by a pair of side-by-side delay lines and the Y-axis coordinate is determined by comparing the charges on the two parallel delay lines. However, our new method abandons the common field return path of the two delay lines, and splits them into two adjacent but independent transformer coupled timing circuits having no common ground connection except via the charge-measurement amplifiers. Thus we eliminate the large electrostatic capacitance of the delay lines from the input of the charge-measuring system. Over its limited working field, the spatial resolution in the Y coordinate can easily equal the resolution provided in the X coordinate by the delay line. Applications to extreme and FUV photon-counting spectroscopy are envisioned.

Raffanti, R.↗

NASA NTRS1932

Comparison of Full-Scale Propellers Having R.A.F.-6 and Clark Y Airfoil Sections

In this report the efficiencies of two series of propellers having two types of blade sections are compared. Six full-scale propellers were used, three having R. A. F.-6 and three Clark Y airfoil sections with thickness/chord ratios of 0.06, 0.08, and 0.10. The propellers were tested at five pitch setting, which covered the range ordinarily used in practice. The propellers having the Clark Y sections gave the highest peak efficiency at the low pitch settings. At the high pitch settings, the propellers with R. A. F.-6 sections gave about the same maximum efficiency as the Clark Y propellers and were more efficient for the conditions of climb and take-off.

Freeman, Hugh B↗

NASA NTRS1933

Wind-tunnel tests of a Clark Y wing with a narrow auxiliary airfoil in different positions

Aerodynamic force tests were made on a combination of a Clark Y wing and a narrow auxiliary airfoil to find the best location of the auxiliary airfoil with respect to the main wing. The auxiliary was a highly cambered airfoil of medium thickness having a chord 14.5 per cent that of the main wing. It was tested in 141 different positions ahead of, above, and behind the nose portion of the main wing, the range of the test points being extended until the best aerodynamic conditions were covered. A range of positions was found in which the combination of main wing and auxiliary gave substantially greater aerodynamic efficiency and higher maximum lift coefficients (based on total area) than the main Clark Y wing alone. In the optimum position tested, considering both the maximum lift and the speed-range ratio, the combination of main wing and auxiliary gave an increase in the maximum lift coefficient of 32 per cent together with an increase in the ratio of 21 per cent of the respective values for the main Clark Y wing alone.

Weick, Fred E↗

NASA NTRS1994

Cross-tail current, field-aligned current, and B(y)

Orbits of individual charged particles were traced in a one-dimensional magnetic field model that included a uniform cross-tail component B(sub yo). The effects of B(sub yo) on the cross-tail current distribution j(sub y)(z), the average cross-tail drift velocity(nu(sub y)z), and the average pitch angle change(delta alpha) experienced during current sheet encounters were calculated. The addition of a B(sub yo) that exceeded several tenths of one nanotesla completely eliminated all resonance effects for odd-N orbits. An odd-N resonance involves ions that enter and exit the current sheet on the same side. Pitch angles of nearly all such ions changed substantially during a typical current sheet interaction, and there was no region of large cross-tail drift velocity in the presence of a modest B(sub yo). the addition of a very large B(sub yo) guide field in the direction that enhances the natural drift produces a large j(y) and small (Delta alpha) for ions with all energies. The addition of a modest B(sub yo) had less effect near even-N resonances. In this case, ions in a small energy range were found to undergo so little change in pitch angle that particles which originated in the ionosphere would pass through the current sheet and return to the conjugate ionosphere. Finally, the cross-tail drift of ions from regions dominated by stochastic orbits to regions dominated by either resonant or guiding center orbits was considered. The ion drift speed changed substantially during such transitions. The accompanying electrons obey the guiding center equations, so electron drift is more uniform. Any difference between gradients in the fluxes associated with electron and ion drifts requires the presence of a Birkeland current in order to maintain charge neutrality. This plasma sheet region therefore serves as a current generator. The analysis predicts that the resulting Birkeland current connects to the lowest altitude equatorial regions in which ions drift to or from a point at which stochastic orbits predominate. The proposed mechanism appears only in analyses that include non-guiding-center effects.

Kaufmann, Richard L.↗

NASA NTRS1994

Summertime distribution and relations of reactive odd nitrogen species and NO(y) in the troposphere over Canada

We report here large-scale features of the distribution of NO(x), HNO3, PAN, particle NO3, and NO(y) in the troposphere from 0.15 to 6 km altitude over central Canada. These measurements were conducted in July - August 1990 from the NASA Wallops Electra aircraft as part of the joint United States-Canadian Arctic Boundary Layer Expedition (ABLE) 3B-Northern Wetlands Study. Our findings show that this region is generally NO(x) limited, with NO(x) mixing ratios typically 20-30 parts per trillion by volume (pptv). We found little direct evidence for anthropogenic enhancement of mixing ratios of reactive odd nitrogen species and NO(y) above those in 'background' air. Instead, it appears that enhancements in the mixing ratios of these species were primarily due to emissions from several day old or CO-rich-NO(x)-poor smoldering local biomass-burning fires. NO(x) mixing ratios in biomass-burning impacted air masses were usually less than 50 pptv, but those of HNO3 and PAN were typically 100-300 pptv representing a twofold-threefold enhancement over 'background' air. During our study period, inputs of what appeared to be aged tropical air were a major factor influencing the distribution of reactive odd nitrogen in the midtroposphere over northeastern North America. These air masses were quite depleted in NO(y) (generally less than 150 pptv), and a frequent summertime occurrence of such air masses over this region would imply a significant influence on the reactive odd nitrogen budget. Our findings show that the chemical composition of aged air masses over subarctic Canada and those documented in the Arctic during ABLE 3A have strikingly similar chemistries, suggesting large-scale connection between the air masses influencing these regions.

Talbot, R. W.↗

NASA NTRS1995

Penetration of the interplanetary magnetic field B(sub y) magnetosheath plasma into the magnetosphere: Implications for the predominant magnetopause merging site

Magnetosheath plasma peertated into the magnetospere creating the particle cusp, and similarly the interplanetary magnetic field (IMF) B(sub y) component penetrates the magnetopause. We reexamine the phenomenology of such penetration to investigate implications for the magnetopause merging site. Three models are popular: (1) the 'antiparallel' model, in which merging occurs where the local magnetic shear is largest (usually high magnetic latitude); (2) a tilted merging line passing through the subsolar point but extending to very high latitudes; or (3) a tilted merging line passing through the subsolar point in which most merging occurs within a few Earth radii of the equatorial plane and local noon (subsolar merging). It is difficult to distinguish between the first two models, but the third implies some very different predictions. We show that properties of the particle cusp imply that plasma injection into the magnetosphere occurs most often at high magnetic latitudes. In particular, we note the following: (1) The altitude of the merging site inferred from midaltitude cusp ion pitch angle dispersion is typically 8-12 R(sub E). (2) The highest ion energy observable when moving poleward through the cusp drops long before the bulk of the cusp plasma is reached, implying that ions are swimming upstream against the sheath flow shortly after merging. (3) Low-energy ions are less able to enter the winter cusp than the summer cusp. (4) The local time behavior of the cusp as a function of B(sub y) and B(sub z) corroborates predictions of the high-latitude merging models. We also reconsider the penetration of the IMF B(sub y) component onto closed dayside field lines. Our approach, in which closed field lines ove to fill in flux voids created by asymmetric magnetopause flux erosion, shows that strich subsolar merging cannot account for the observations.

Newell, Patrick T.↗

NASA NTRS1995

Effect of Nb2O5 and V2O5 addition on the superconducting properties of YBa2Cu3O(y) thin films

The effect of Nb2O5 and V2O5 addition on the superconducting properties and microstructure of YBa2Cu3O(y) has been studied in thin films. Polycrystalline targets for laser ablation were prepared by mixing high purity V2O5 or Nb2O5 powders with a well characterized YBa2Cu3O(y) powder in the range 0 to 4 wt percent by solid state reaction method. Thin films (approximately 1500 A thickness) of the above targets were grown on (100) SrTiO3 (STO) and (100) LaAlO3 (LAO) substrates at 700 C temperature by pulsed laser deposition (PLD) technique. In the case of Nb2O5 addition we have noticed an increase in J(sub c) up to 0.5 wt percent and higher additive concentration (greater than 0.5 wt percent) have degraded the superconducting properties. However, in the case of V2O3 addition, there is an improvement in current density and microstructural properties up to 1 wt percent and the superconducting properties degrade for concentrations greater than 1 wt percent. The best J(sub c) for 0.5 wt percent of Nb2O5 added YBCO thin film is 1.6 x 10(exp 6) A/sq cm and for that of V2O5 added sample is 3.4 x 10(exp 6) A/sq cm at 77 K as compared to the pure YBa2Cu3O(y) (YBCO) film J(sub c) (1.2 x 10(exp 6) A/sq cm) observed on STO substrates. The reason for improvement in J(sub c) and microstructural properties in the case of V2O5 addition could be due to the low melting of V2O5 (690 C) which can act as a very good surfactant during deposition. Over all, we have realized that Nb2O5 addition or V2O5 addition to YBCO have shown significant improvement over the undoped YBa2Cu3O(7-x) films grown under identical conditions.

Srinivas, S.↗

NASA NTRS1995

Chemical stabilization and high pressure synthesis of Ba-free Hg-based superconductors, (Hg,M)Sr2Ca(n-1)Cu(n)O(y)(n=1 to approximately 3)

A homologous series of new Hg-based HTSC compounds, (Hg,M)Sr2Ca(n - 1)Cu(n)P(y) with n = 1 to 3, have been synthesized. The stabilization of the pure phases have been accomplished by chemical doping of third elements such as M = Cr, Mo and Re. While the Hgl2O1(n = 1) phase was readily obtained in this way, it was necessary to simultaneously dope Y into the Ca site to stabilize the Hg1212(n = 2) phase. On the other hand, single-phase Y-free Hg1212(n = 2) and Hg1223 (n = 3) samples were synthesized only under a high pressure of 6 GPa. In sharp contrast to the Ba containing compounds, all the samples prepared in the present study have been quite stable during the synthesis and no deterioration in air has been observed after the preparation.

Kishio, K.↗

NASA NTRS1995

Airborne measurements of NO, NO2, and NO(sub y) as related to NASA's TRACE-A field program

The Georgia Tech group's effort on NASA's GTE program and TRACE-A field mission primarily involved analysis and interpretation of the measurement data base obtained during the TRACE-A field campaign. These investigations focused on the distribution of ozone and ozone precursors over the south Atlantic and nearby continental regions of Africa and South Africa. The Transport and Atmospheric Chemistry near the Equator-Atlantic (TRACE-A) Mission was designed with the goal of investigating tropospheric trace gas distributions, sources, and photochemical state over the southern Atlantic. Major scientific issues related to N(x)O(y) tropospheric chemistry addressed in this program included: (1) what controls the tropospheric ozone budget over the southern Atlantic? (2) What are the spatial distributions of CO, CO2, NO, NO2, NO(sub y), O3, NMHC, H2O3, etc. over the southern Atlantic? (3) How does long range transport of long-lived NO(y) compounds affect the more reactive NO(x) budget in southern Atlantic troposphere?

Bradshaw, John↗

NASA NTRS2001

Formation of Y(x)Nd(1-x) Ba2Cu3O(7-delta) (0 = or < x < or = 0.7) Superconductors from an Undercooled Melt Via Aero-Acoustic Levitation

Melt processing of RE123 superconductors has gained importance in recent years. While the first high temperature superconductors (HTSCs) were made using traditional ceramic press and sinter technology, recent fabrication efforts have employed alternate processing techniques including laser ablation and ion beam assisted deposition for thin film fabrication of tapes and wires and melt growth for bulk materials. To optimize these techniques and identify other potential processing strategies, phase relation studies on HTSCs have been conducted on a wide variety of superconducting compounds using numerous processing strategies. This data has enhanced the understanding of these complex systems and allowed more accurate modeling of phase interactions. All of this research has proved useful in identifying processing capabilities for HTSCs but has failed to achieve a breakthrough for wide spread application of these materials. This study examines the role of full to partial substitution of Nd in the Y123 structure under rapid solidification conditions. Aero-acoustic levitation (AAL) was used to levitate and undercool RE123 in pure oxygen binary alloys with RE = Nd an Y along a range of compositions corresponding to Y(x)Nd(1-x) Ba2Cu3O(7-delta) (0 = or < x < or = 0.7) which were melted by a CO2 laser. Higher Y content spheres could not be melted in the AAL and were excluded from this report. Solidification structures were examined using scanning electron microscopy, electron dispersive spectroscopy, and powder x-ray diffraction to characterize microstructures and identify phases.

Gustafson, D. E.↗

NASA NTRS2001

Delayed Alumina Scale Spallation on Rene'n5+y: Moisture Effects and Acoustic Emission

The single crystal superalloy Rene'N5 (with or without Y-doping and hydrogen annealing) was cyclically oxidized at 1150 C for 1000 hours. After considerable scale growth (>= 500 hours), even the adherent alumina scales formed on Y-doped samples exhibited delayed interfacial spallation during subsequent water immersion tests, performed up to one year after oxidation. Spallation was characterized by weight loss, the amount of spalled area, and acoustic emission response. Hydrogen annealing (prior to oxidation) reduced spallation both before and after immersion, but without measurably reducing the bulk sulfur content of the Y-doped alloys. The duration and frequency of sequential, co-located acoustic emission events implied an interfacial crack growth rate at least 10(exp -3) m/s, but possibly higher than 10(exp 2) m/s. This is much greater than classic moisture-assisted slow crack growth rates in bulk alumina (10(exp -6) to 10(exp -3) m/s), which may still have occurred undetected by acoustic emission. An alternative failure sequence is proposed: an incubation process for preferential moisture ingress leads to a local decrease in interfacial toughness, thus allowing fast fracture driven by stored strain energy.

Smialek, James L.↗

NASA NTRS2004

Sintering of BaCe(sub 0.85)Y(sub 0.15)O(sub 3-delta) with/without SrTiO3 Dopant

The perovskite composition, BaCe(sub 0.85)Y(sub 0.15)O(sub 3-delta), displays excellent protonic conduction at high temperatures making it a desirable candidate for hydrogen separation membranes. This paper reports on the sintering behavior of BaCe(sub 0.85)Y(sub 0.15)O(sub 3-delta) powders doped with SrTiO3. Two methods were used to synthesize BaCe(sub 0.85)Y(sub 0.15)O(sub 3-delta) powders: (1) solid state reaction and (2) wet chemical co-precipitation. Co-precipitated powder crystallized into the perovskite phase at 1000 C for 4 hrs. Complete reaction and crystallization of the perovskite phase by solid state was achieved by calcining at 1200 C for 24 hrs. Solid state synthesis produced a coarser powder with an average particle size of 1.3 microns and surface area of 0.74 sq m/g. Co-precipitation produced a finer powder with a average particle size of 65 nm and surface area of 14.9 sq m/g. Powders were doped with 1, 2, 5, and 10 mole % SrTiO3. Samples were sintered at 1450 C, 1550 C and 1650 C. SrTiO3 enhances sintering, optimal dopant level is different for powders synthesized by solid state and co-precipitation. Both powders exhibit similar grain growth behavior. Dopant levels of 5 and 10 mole % SrTiO3 significantly enhances the grain size.

Dynys, F.↗

NASA NTRS2006

Pulsed Laser Deposition of BaCe(sub 0.85)Y(sub 0.15)0(sub 3) FILMS

Pulsed laser deposition has been used to grow nanostructured BaCe(sub 0.85)Y(sub 0.15)0(sub 3) films. The objective is to enhance protonic conduction by reduction of membrane thickness. Sintered samples and laser targets were prepared by sintering BaCe(sub 0.85)Y(sub 0.15)O(sub 3) powders derived by solid state synthesis. Films 2 to 6 m thick were deposited by KrF excimer laser on Si and porous Al2O3 substrates. Nanocrystalline films were fabricated at deposition temperatures of 600-800 C deg at O2 pressure of 30 mTorr and laser fluence of 1.2 J/cm square. Films were characterized by x-ray diffraction, scanning electron microscopy and electrical impedance spectroscopy. Dense single phase BaCe(sub 0.85)Y((sub 0.15) 0(sub 3) films with a columnar growth morphology is observed, preferred crystal growth was found to be dependent upon deposition temperature and substrate type. Electrical conductivity of bulk samples produced by solid state sintering and thin film samples were measured over a temperature range of 100 C deg to 900 C deg in moist argon. Electrical conduction of the fabricated films was 1 to 4 orders of magnitude lower than the sintered bulk samples. With respect to the film growth direction, activation energy for electrical conduction is 3 times higher in the perpendicular direction than the parallel direction.

Dynys, F. W.↗

NASA NTRS2006

Non-spherical Lobate Chondrules in CO3.0 Y-81020: General Implications for the Formation of Low-FeO Porphyritic Chondrules in CO Chondrites

Non-spherical chondrules (arbitrarily defined as having aspect ratios greater than or equal to 1.20) in CO3.0 chondrites comprise multi-lobate, distended, and highly irregular objects with rounded margins; they constitute approx. 70% of the type-I (low-FeO) porphyritic chondrules in Y-81020, approx. 75% of such chondrules in ALHA77307, and approx. 60% of those in Colony. Although the proportion of non-spherical type-I chondrules in LL3.0 Semarkona is comparable (approx. 60%), multi-lobate OC porphyritic chondrules (with lobe heights equivalent to a significant fraction of the mean chondrule diameter) are rare. If the non-spherical type-I chondrules in CO chondrites had formed from totally molten droplets, calculations indicate that they would have collapsed into spheres within approx. 10(exp -3) s, too little time for their 20-micrometer-size olivine phenocrysts to have grown from the melt. These olivine grains must therefore be relicts from an earlier chondrule generation; the final heating episode experienced by the non-spherical chondrules involved only minor amounts of melting and crystallization. The immediate precursors of the individual non-spherical chondrules may have been irregularly shaped chondrule fragments whose fracture surfaces were rounded during melting. Because non-spherical chondrules and circular chondrules form a continuum in shape and have similar grain sizes, mineral and mesostasis compositions, and modal abundances of non-opaque phases, they must have formed by related processes. We conclude that a large majority of low-FeO chondrules in CO3 chondrites experienced a late, low-degree melting event. Previous studies have shown that essentially all type-II (high-FeO) porphyritic chondrules in Y-81020 formed by repeated episodes of low-degree melting. It thus appears that the type-I and type-II porphyritic chondrules in Y-81020 (and, presumably, all CO3 chondrites) experienced analogous formation histories. Because these two types constitute approx. 95% of all CO chondrules, it is clear that chondrule recycling was the rule in the CO chondrule-formation region and that most melting events produced only low degrees of melting. The rarity of significantly non-spherical, multi-lobate chondrules in Semarkona may reflect more-intense heating of chondrule precursors in the ordinary-chondrite region of the solar nebula.

Rubin, Alan E.↗