948 Matching Results

Search Results

Advanced search parameters have been applied.

Electrical Resistivity of Lanthanum, Praseodymium, Neodymium, and Samarium

Description: The electrical resistivities of polycrystalline samples of La, Pr, Nd, and Sm are reported in the temperature range 1.3 to 300 deg K. La exhibits a superconducting transition at 5.8 deg K. The curve for Pr has slope changes at 61 and 95 deg K. The Nd curve shows small jumps at 5 and 20 deg K. Sm shows slope changes at 14 and 106 deg K. (auth)
Date: March 15, 1961
Creator: Alstad, J.K.; Colvin, R.V.; Legvold, S. & Spedding, F.H.
Partner: UNT Libraries Government Documents Department

Fast neutron radiography research at ANL-W

Description: Thirty-seven different elements were tested for their suitability as converter screens for direct and indirect fast neutron radiography. The use of commercial X-ray scintillator screens containing YTaO{sub 4}, LaOBr:Tm, YTaO{sub 4}:Nb, YTaO{sub 4}:Tm, CaWO{sub 4}, BaSO{sub 4}:Sr, and GdO{sub 2}S:Tb was also explored for direct fast neutron radiography. For the indirect radiographic process, only one element, holmium, was found to be better than copper. Iron was also found to work as well as copper. All other elements that were tested were inferior to copper for indirect fast neutron radiography. For direct fast neutron radiography, the results were markedly different. Copper was found to be a poor material to sue, as thirty-two of the elements performed better than the copper. Tantalum was found to be the best material to use. Several other materials that also performed remarkably well include, in order of decreasing utility, gold, lutetium, germanium, dysprosium, and thulium. Several interesting results were obtained for the commercial X-ray scintillator screens. Most notably, useful radiographs were produced with all of the various scintillation screens. However, the screens containing YTaO{sub 4}:Nb offered the greatest film densities for the shortest exposure times. Screens using GdSO{sub 4}:Tb provided the best resolution and clearest images at the sacrifice of exposure time. Also, as previous researchers found, scintillator screens offered significantly shorter exposure times than activation foils.
Date: June 1, 1996
Creator: Klann, R.T. & Natale, M.D.
Partner: UNT Libraries Government Documents Department

Evaluation of Oxide Dispersion Strengthened (ODS) molybdenum alloys

Description: A series of fourteen (14) novel high-strength molybdenum alloy compositions containing a dispersion of very fine (< 1 {mu}m diameter) oxide particles were consolidated using two proprietary powder metallurgy techniques. The developmental compositions were evaluated to determine the microstructural stability and mechanical properties from cryogenic (-148{degrees}F) to elevated temperatures (4000{degrees}F) for material in the as-swaged (>98% cold work) condition and for as-swaged material in the heat treated condition. Extremely fine oxide particle sizes (<1000 {Angstrom}) were observed by Transmission Electron Microscopy (TEM) for a number of the experimental compositions in the as-swaged condition. A one hour recrystallization temperature as high as 3990{degrees}F was measured and a ductile-to-brittle transition temperature as low as {approximately}58{degrees}F for material in the recrystallized condition was determined. The preliminary results support the alloy design concept feasibility.
Date: December 31, 1995
Creator: Bianco, R. & Buckman, R.W. Jr.
Partner: UNT Libraries Government Documents Department

Sn-substituted LaNi{sub 5} alloys for metal hydride electrodes

Description: This research examines the efficacy of tin additions to LaNi{sub 5} in improving the hydrogen storage capacity of the material during charging/discharging. Alloys were prepared using high energy ball milling (mechanical alloying), a technique superior to arc casting for alloying elements with a wide disparity in melting points. Characterization by X-ray diffraction and Rietveld analysis shows that tin preferentially occupies the Ni(3g) sites in the LaNi{sub 5} structure, and the unit cell volume increases linearly with tin content to the maximum tin solubility of 7.33 atomic percent (LaNi{sub 4.56}Sn{sub 0.44}). The authors found that powders prepared by mechanical alloying and not exposed to air require no activation to induce hydrogen absorption. The hydrogen storage capacity in the gas and electrochemical phase was measured as a function of tin content. They found that with increasing tin, the plateau pressure decreases logarithmically, whereas the hydrogen storage capacity decreases linearly.
Date: May 1, 1995
Creator: Wasz, M.L.; Schwarz, R.B.; Srinivasan, S. & Sridhar Kumar, M.P.
Partner: UNT Libraries Government Documents Department

Hollow cathode for positive ion sources

Description: Development to incorporate hollow cathodes into high power ion sources for neutral beam injection systems is being pursued. Hollow tube LaB/sub 6/-type cathodes, similar to a UCLA design, have been constructed and tested in several ORNL ion source configurations. Results of testing include arc discharge parameters of >1000 and 500 amps for 0.5 and 10 second pulse lengths, respectively. Details of cathode construction and additional performance results are discussed.
Date: January 1, 1979
Creator: Schechter, D.E.; Kim, J. & Tsai, C.C.
Partner: UNT Libraries Government Documents Department

Automated apparatus for Sieverts' studies of metal hydrides. [U deuteride; La/sub 5. 25/ Ni hydride]

Description: An automated system, controlled by a small desktop computer, that carries out Sieverts' experiments on metal hydride systems is described. Data (equilibrium pressures versus temperatures at constant hydrogen-to-metal ratios) are recorded for later analysis and plotting. The computer also controls valve positions during the experiments. Results for two metal hydride systems, the uranium deuteride system in the plateau region and the La/sub 5/ /sub 25/Ni hydride system at low concentrations, are presented as examples. Data included on the latter system have not been reported previously. The techniques used in Sieverts' experiments and the sources of errors are discussed.
Date: December 1, 1980
Creator: Carstens, D.H.W. & David, W.R.
Partner: UNT Libraries Government Documents Department

On the validity of the band model for high T/sub c/ superconductors

Description: The discovery of high T/sub c/ superconductivity in cupric oxides has led, amongst other things, to numerous electronic bandstructure studies of these compounds. The foundation of these calculations is Density Functional Theory (DFT) and they are implemented with the Local Density Approximation (LDA) or Local Spin Density (LSD) approximation. In this article we will review what we have learned from these calculations regarding the electronic structure of the family of high T/sub c/ materials. To answer this we have to find out how relevant the LSD/LDA is for the electronic properties of cupric oxides. It is well known that the application of LDA DFT for transition metal oxides is fraught with difficulty and controversy. Although the LDA describes the variation of the equilibrium volume through the 3d transition metal oxides, including the volume expansion associated with Mott insulators, the theory fails to describe FeO and CoO as antiferromagnetic insulators, and in the case of NiO and MnO where the LDA does yield an insulating ground state, the band gap is grossly underestimated. Evidently the high-T/sub c/ materials are systems that are on the verge of undergoing a Mott transition; consequently, it is necessary to be circumspect regarding the applicability of LDA DFT both in the metallic (superconducting) and magnetic insulating states. 23 refs., 12 figs.
Date: January 1, 1988
Creator: Temmerman, W.M.; Guo, G.Y.; Szotek, Z.; Durham, P.J. & Stocks, G.M.
Partner: UNT Libraries Government Documents Department

NQR study of local structures and cooling rate dependent superconductivity in La{sub 2}CuO{sub 4+{delta}}

Description: Structural properties of oxygen-annealed polycrystals of La{sub 2}CuO{sub 4 + {delta}} ({delta}{approximately}0.03) have been studied using {sup 139}La NQR spectroscopy. Superconducting critical temperatures were found to depend on the rate of cooling through a narrow temperature range at about 195K. Preliminary analysis of the {sup 139}La NQR spectra suggest that the oxygen-rich phase-separated region is composed of two structurally distinct phases, both of which are metallic and super-conducting. One phase has a structure closely related to the stoichiometric oxygen-poor compound. The second shows a considerable amount of apical oxygen disorder, a large shift in NQR frequency {nu}{sub Q}, and a volume fraction which increases with cooling rate. The formation of the second phase below {minus}200K is indicative of the freezing Of CuO{sub 6} octahedral tilting. Abrupt shifts in {nu}{sub Q} above {Tc} were also observed for both phases, suggestive of a local structural anomaly or charge transfer to the Cu-O plane.
Date: October 1, 1992
Creator: Reyes, A. P.; Ahrens, E. T.; Hammel, P. C.; Heffner, R. H.; Thompson, J. D.; Canfield, P. C. et al.
Partner: UNT Libraries Government Documents Department

Operation of a Dudnikov type Penning source with LaB/sub 6/ cathodes

Description: The Dudnikov type Penning source has been operated successfully with LaB/sub 6/ cathodes in a cesium-free discharge. It is found that the extracted H/sup -/ current density is comparable to that of the cesium-mode operation and H/sup -/ current density of 350 mA/cm/sup 2/ have been obtained for an arc current of 55 A. The H/sup -/ yield is closely related to the source geometry and the applied magnetic field. Experimental results demonstrate that the majority of the H/sup -/ ions extracted are formed by volume processes in this type of source operation.
Date: October 1, 1986
Creator: Leung, K. N.; DeVries, G. J.; Ehlers, K. W.; Jackson, L. T.; Stearns, J. W.; Williams, M. D. et al.
Partner: UNT Libraries Government Documents Department

In situ observation of radiation induced amorphization of crystals with apatite structure

Description: Temperature dependence of amorphization doses for 1.5 MeV Kr ion-irradiated Ca{sub 10}(PO{sub 4}){sub 6}F{sub 2} and Ca{sub 2}La{sub 8}(SiO{sub 4}){sub 6}O{sub 2} were studied with in situ transmission electron microscopy. At low temperatures, Ca{sub 10}(PO{sub 4}){sub 6}F{sub 2} became amorphized at lower dose than Ca{sub 2}La{sub 8}(SiO{sub 4}){sub 6}O{sub 2}. However, the critical amorphization dose increased much more rapidly for the former above 350 K than the latter. At 475 K, Ca{sub 10}(PO{sub 4}){sub 6}F{sub 2} required nearly five times of higher dose to become amorphous than Ca{sub 10}La{sub 8}(SiO{sub 4}){sub 6}O{sub 2}. Based on the dose-temperature curves, activation energies for the crystal structure recovery process were determined to be 0.07 eV for Ca{sub 10}(PO{sub 4}){sub 6}F{sub 2} and 0.13 eV for Ca{sub 2}La{sub 8}(SiO{sub 4}){sub 6}O{sub 2} respectively. The lower critical amorphization dose and lower activation energy for recovery of Ca{sub 10}(PO{sub 4}){sub 6}F{sub 2} are attributed to the weaker P-O bond and the higher fluorine mobility in the structure.
Date: June 1, 1993
Creator: Wang, L. M.; Ewing, R. C.; Cameron, M.; Weber, W. J. & Crowley, K. D.
Partner: UNT Libraries Government Documents Department

Scintillation materials for medical applications. Annual progress report, January 1, 1991--December 31, 1992

Description: Scintillators are beginning to attract renewed attention because modern High Energy Physics accelerators are placing unprecedented demands of quantity and quality of detector materials and Positron Emission Tomography (PET), used by the medical field. Both applications required materials for scintillator detectors with properties beyond those delivered by traditional scintillators. Thallium doped halides are very efficient, but slow and chemically unstable. Two modern developments, namely the very fast BaF{sub 2}, which owed its success to the newly discovered crossover transitions, and CeF{sub 3}, which carried a promise of fast components, more practical wavelengths and attractive efficiency. Since traditional scintillators (Tl doped halides) are very efficient, and could be even more efficient at larger concentrations of Tl, if it were not for concentration quenching. However Tl transitions are spin forbidden and slow. Both ills could be remedied by replacing Tl with Ce, whose transitions are allowed and which is known to form fully concentrated compounds of high photoluminescent efficiency and no quenching. These materials, plus new Ce-doped materials, exhibiting highly promising properties for medical applications, became the target of our studies.
Date: December 31, 1992
Creator: Lempicki, A. & Wojtowicz, A. J.
Partner: UNT Libraries Government Documents Department

Nanosecond length electron pulses from a laser-excited photocathode

Description: A photocathode made from polycrystalline lanthanum hexaboride (LaB{sub 6}) has produced nanosecond length electron pulses when excited by an excimer laser at 308nm. Peak currents in excess of 1A have been observed, with quantum yields of 4 {times} 10{sup {minus}5} being measured. A method for extracting the electrons from an emission-limited cathode, plasma extraction, has been demonstrated. This technique uses a low power continuous discharge to provide the electric field needed to extract the photoelectrons. This technique may be useful in producing high repetition rate short pulse ion sources. 10 refs., 4 figs.
Date: June 1, 1989
Creator: Young, A.T.; D'Etat, B.; Stutzin, G.C.; Leung, K.N. & Kunkel, W.B.
Partner: UNT Libraries Government Documents Department

NMR studies of oxygen-doped La{sub 2}CuO{sub 4+{delta}}

Description: The observation that the structure of this material is sensitive to levels of doping sufficient to produce superconductivity ({Tc} {approximately}40 K) suggests an important role for structure in determining its electronic properties. Here we discuss unusual features of phase separation and studies of cooling rate dependence of the superconducting {Tc} relevant to this proposition.
Date: August 11, 1993
Creator: Hammel, P. C.; Reyes, A. P.; Ahrens, E. T.; MacLaughlin, D. E.; Thompson, J. D.; Fisk, Z. et al.
Partner: UNT Libraries Government Documents Department

Charge delocalization and structural response in layered La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}: Enhanced Jahn-Teller distortion in the metallic regime

Description: Temperature dependent neutron diffraction studies on melt-grown crystals of the layered compound La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} reveal a dramatic structural response to the onset of ferromagnetism and the coincident insulator-metal transition. Unlike the related manganite perovskites, whose Jahn-Teller distorted octahedra become more regular at temperatures below the metal-insulator transition, the MnO{sub 6} octahedra of this layered material are more severely distorted when the charge is itinerant than when it is localized.
Date: November 1996
Creator: Mitchell, J. F.; Argyriou, D. N. & Jorgensen, J. D.
Partner: UNT Libraries Government Documents Department

Anomalous Hall effect in Gd-doped La{sub 2/3}Ca{sub 1/3}MnO{sub 3}

Description: Ceramic samples of (La{sub 1-x}Gd{sub x}){sub 2/3}Ca{sub 1/3}MnO{sub 3} were prepared and used as targets to grow films onto LaAlO{sub 3} substrates by pulsed laser deposition. Electrical resistance and thermopower, measured vs temperature and applied magnetic fields indicate transport dominated by positive small polarons in the high temperature paramagnetic state. The Hall effect was measured in 0.5 {mu}m thick films of composition x=0 and x=0.25. No evidence for extraordinary hall effect was found in the paramagnetic regime. Instead, the magnitude of the Hall coefficient decreases exponentially with temperature. This behavior and its anomalous negative sign are interpreted to result from face-diagonal hopping of small polarons in the Mn sublattice.
Date: October 1, 1996
Creator: Jaime, M.; Hardner, H.; Salamon, M.B.; Rubinstein, M.; Dorsey, P. & Emin, D.
Partner: UNT Libraries Government Documents Department

The local Jahn-Teller effect in (La/Sr){sub n+1}Mn{sub n}O{sub 3n+1}

Description: Lattice polarons form an important constituent in the mechanism of magnetism and transport of the (La/Sr){sub n+1}Mn{sub n}O{sub 3n+1} (for n = 2 and {infinity}) colossal magnetoresistance manganites. The coupling of the polaronic distortions to the local Jahn-Teller (JT) effect has been characterized by the pulsed neutron pair density function (PDF) analysis. Distortions of equal magnitude to the ones found in perovskites arising from a JT effect are found in the 2-dimensional layer crystals. The coupling of the distortions to the lattice is similar in both systems suggesting that lattice polarons are formed independently to the crystal dimensionality.
Date: December 31, 1998
Creator: Louca, D.; Kwei, G.H. & Egami, T.
Partner: UNT Libraries Government Documents Department

Methyl chloride via oxyhydrochlorination of methane. Quarterly technical progress report No. 16, July 1995--September 1995

Description: Continuous progress is being made toward the contract objectives in our attempt to produce {open_quotes}Methyl Chloride via Oxyhydrochlorination of Methane.{close_quotes} Specifically, gains have been achieved in Task 1.0 Catalyst Development and Separation Unit Operation Development and Task 4.0 PDU Startup. Catalyst development continued forward to examine the copper, lanthanum, and lithium loadings as well as support surface area in four factor, two level, fully crossed design experiments carried out at five temperatures between 290{degrees}C and 310{degrees}C. Eight of the sixteen runs have been completed. Evaluation of these runs as a three factor, two level, fully crossed experiment has yielded some useful information. The choice of the Cu/La/Li metals system has been confirmed, and the high Cu, low La, low Li system hasn`t proven itself to be the most active at this point. Chemical feeds were brought on-line to the PDU. Small amounts of methyl chloride were made before the system was shut down due to equipment failure. Glass-lined equipment was found to be failing due to chemical etching. Replacement equipment with other materials of construction have been ordered. In addition, the FTIR windows clouded soon after chemical feeds were started. This materials problem is also being investigated.
Date: March 1, 1996
Partner: UNT Libraries Government Documents Department

Advanced materials for solid oxide fuel cells

Description: Purpose of the research is to improve the properties of current state- of-the-art materials used for SOFCs. The project includes interconnect development, high-performance cathode, electrochemical testing, and accelerated testing. This document reports results of mechanical tests (bend strength, elastic modulus, fracture strength) of acceptor-substituted lanthanum chromite (interconnect material).
Date: December 31, 1996
Creator: Armstrong, T.R.; Stevenson, J. & Paulik, S.
Partner: UNT Libraries Government Documents Department

Processing of LaCrO{sub 3} for solid oxide fuel cell applications. April 1994--April 1995

Description: A 5-yr program has the objectives of developing LaCrO{sub 3}-based interconnect powders which densify when in contact with anode and cathode materials for solid oxide fuel cells and developing high- performance cathodes, anodes, and interfaces for planar SOFCs. This report is divided into LaCrO{sub 3} sintering studies and SOFC performance studies. Major achievements during the past year included: Developing processing skills for fabricating single cells, incorporating a Pt reference electrode into the electrolyte for separating electrode effects, developing processing-microstructure- property relations for a number of anodes, and developing experimental techniques for measuring cell performance.
Date: July 1, 1995
Creator: Huebner, W. & Anderson, H.U.
Partner: UNT Libraries Government Documents Department

Magnetism of cuprate oxides

Description: A review is given of current neutron scattering experiments on cuprate oxides. We first discuss the extensive neutron measurements on high-Tc oxides: La{sub 2-x}Sr{sub x}CuO{sub 4} and related (La{sub 1.6-x}Nd{sub 0.4})Sr{sub x}CuO{sub 4}. The second topic is the spin- Peierls system Cu{sub 1-x}Zn{sub x}GeO{sub 3}, where a new type of antiferromagnetic phase has been discovered. 17 refs, 8 figs.
Date: November 1, 1996
Creator: Shirane, G.
Partner: UNT Libraries Government Documents Department

Synthesis, structure, and superconductivity of La{sub 2-x}K{sub x}CuO{sub 4}

Description: Structural and superconducting properties of La{sub 2-x}K{sub x}CuO{sub 4} were studied by neutron and x-ray powder diffraction, ac magnetic susceptibility, and dc electrical resistivity. Superconductivity is observed in both orthorhombic and tetragonal phases with x{ge}0.06 and is absent for x{le}0.04. At room temperature, the system undergoes an orthorhombic-to-tetragonal phase transition at x{similar_to}0.07. Diffraction studies show no evidence of a low temperature tetragonal phase at 50 K. Comparison of these structural and superconducting properties with earlier data indicate that K contents have previously been highly overestimated.
Date: May 1, 1994
Creator: Keane, P.M.; Hinks, D.G. & Jorgensen, J.D.
Partner: UNT Libraries Government Documents Department