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Electron irradiation effects in YBa/sub 2/Cu/sub 3/O/sub 7-delta/ single crystals

Description: Defect structures in YBa/sub 2/Cu/sub 3/O/sub 7/minus/delta/ produced by electron irradiation at 300/degree/K, were investigated by transmission electron microscopy. Threshold energies for the production of visible defects were determined to b 152 keV and 131 keV (+- 7 keV) in directions near the a- and b-axes, respectively (b > a, both perpendicular to c, the long axis in the orthorhombic structure). During above-threshold irradiations in an electron flux of 3 x 10 /sup 18/ cm/sup /minus/2/s/sup /minus/1/, extended defects were observed to form and grow to sizes of 10--50 nm over 15 minutes, in material thicknesses varying between 20 and 200 nm. Upon irradiation between the a- and b-thresholds, movement of twin plane boundaries and shrinkage of twinned volume were observed. All these findings suggest oxygen atom displacements in the basal plane with recoil energies near 20 eV. Above-threshold irradiations also show the collapse of c-axis long-range order into a planar faulted defect structure with short range order peaks at 1.2 c and 1.07 c, depending on the irradiation direction. 9 refs., 4 figs.
Date: April 1, 1988
Creator: Kirk, M.A.; Baker, M.C.; Liu, J.Z.; Lam, D.J. & Weber, H.W.
Partner: UNT Libraries Government Documents Department

Electron microscopy of ceramic superconductors

Description: The critical current Jc is at least as important as Tc (transition temperature) for applications in superconducting materials. Jc is strongly dependent on microstructure and, in consequence, electron microscopy will continue to be important in the development of practical ceramic superconductors. We will review the progress that has been made over the past year or so in studying the superconductors by electron microscopy techniques of all kinds--conventional, high resolution, analytical, etc. A thorough review is impossible but a bibliography is available, as well as two special issues of Journals. 25 refs., 9 figs.
Date: January 1, 1988
Creator: Mitchell, T.E. & Roy, T.
Partner: UNT Libraries Government Documents Department

Preparation of oxide superconductors

Description: Preparation of bulk superconductors by cold pressing and sintering of metal oxides has been optimized in this study. A knowledge of phase behavior was used in developing a flux method for growing superconducting single crystals of yttrium and gadolinium based superconductors. Means of achieving single-phase, high-density samples of (Rare Earth)-Ba/sub 2/Cu/sub 3/O/sub 7/ with optimal electronic properties are reviewed. 2 refs., 2 figs.
Date: January 1, 1988
Creator: Peterson, D.E.
Partner: UNT Libraries Government Documents Department

Nonlinear materials for frequency conversion

Description: Two figures of merit, the threshold power (P/sub th/) and the limiting volume (V/sub min/) can be used to compare the relative efficiency and economy of new harmonic generating crystals. The properties of barium metaborate and L-Arginine phosphate are used to illustrate the effect of nonlinearity, birefringence, and damage threshold on these figures of merit.
Date: March 22, 1988
Creator: Velsko, S.P. & Eimerl, D.
Partner: UNT Libraries Government Documents Department

Preparation and characterization of isotopic oxygen-enriched yttrium barium copper oxide

Description: The preparation of labeled superconducting yttrium barium copper oxides from the labeled metal nitrates is described. The materials were characterized by a variety of physical techniques, and their superconducting properties were measured. Trends are seen in the structural parameters of the materials obtained by the nitrate route and also in isotopically enriched materials prepared via gas-phase exchange. These structural changes are accompanied by changes in superconducting properties. The /sup 18/O-enriched yttrium barium copper oxide prepared via the nitrate route has a T/sub c/depressed by 33/degree/K to 59/degree/K, whereas a /sup 17/O sample has a T/sub c/ between the /sup 18/O material and similarly prepared /sup 16/O sample. 15 refs., 1 fig., 3 tabs.
Date: January 1, 1988
Creator: Ott, K.C. & Smith, J.L.
Partner: UNT Libraries Government Documents Department

Ordered structures in YBa/sub 2/Cu/sub 3/O/sub 7-y/, La/sub 2-x/Sr/sub x/CuO/sub 4-par. delta/ and related perovskites

Description: Electron microscopy and electron diffraction studies have been performed on the superconducting oxides YBa/sub 2/Cu/sub 3/O/sub 7-y/, GdBa/sub 2/Cu/sub 3/O/sub 7-y/ and La/sub 2-x/Sr/sub x/CuO/sub 4-par. delta/ as well as the related perovskites La/sub 2/CuO/sub 4/, Eu/sub 2/CuO/sub 4/ and Gd/sub 2/CuO/sub 4/. Extra reflections are commonly observed in all the cases. For example, in the 123 compounds, in situ heating leads to transformations from orthorhombic to tetragonal with a loss of twin structure; on cooling the oxygen vacancies re-order in the basal plane to give 1/3(100)* or 1/4(110)* diffraction spots. In tetragonal Eu/sub 2/CuO/sub 4/ and Gd/sub 2/CuO/sub 4/, the (001) diffraction patterns often have extra spots at 1/2(110)* or at 1/4(110)* positions. Extra spots are also observed in La/sub 2/CuO/sub 4/ (orthorhombic) but this is probably due to the space group being primitive rather than centered. These observations are discussed in terms of the ordering of oxygen vacancies in the Cu-O planes of the various perovskite structures. 6 figs.
Date: January 1, 1988
Creator: Mitchell, T.E.; Roy, T.; Fisk, Z. & Smith, J.L.
Partner: UNT Libraries Government Documents Department

Fermi liquid theory for high temperature superconductors

Description: In this article the Fermi liquid theory of metals is discussed starting from Luttinger's theorem. The content of Luttinger's Theorem and its implications for microscopic theories of high temperature superconductors are discussed. A simple quasi-2d Fermi liquid theory is introduced and some of its properties are calculated. It is argued that a number of experiments on YBa/sub 2/Cu/sub 3/O/sub 6+x/, x > 0.5, strongly suggest the existence of a Fermi surface and thereby a Fermi liquid normal state. 25 refs., 1 fig.
Date: January 1, 1988
Creator: Bedell, K.S.
Partner: UNT Libraries Government Documents Department

Ion beam analysis and modification of thin film high temperature superconductors

Description: The purpose of the present paper is to show that in addition to modifying materials, ion beams can be used in more gentle but very powerful way to explore what happens to a ceramic thin film as a function processing. The discussions which will follow will be concerned exclusively with the new and exciting class of ceramic, the high temperature superconductor (HTS). We will discuss the application of various ion beam backscattering techniques, as well as examine the use of ion implantation in the processing of these materials. 34 refs., 25 figs.
Date: January 1, 1988
Creator: Nastasi, M.
Partner: UNT Libraries Government Documents Department

Electronic structure of the quenched superconductivity materials Y/sub 1-x/Pr/sub x/Ba/sub 2/Cu/sub 3/O/sub 7-delta/

Description: The superconductivity of the Y/sub 1-x/Pr/sub x/Ba/sub 2/Cu/sub 3/O/sub 7-delta/ system is quenched as x increases. It has been speculated that Pr has valence 4+, resulting in extra charge in the Cu-O planes, and causing T/sub c/-quenching. To study the Pr electronic state, we have measured valence band resonant photoemission of the Y/sub 1-x/Pr/sub x/Ba/sub 2/Cu/sub 3/O/sub 7- delta/ system for x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0. We find that the Pr valence is close to 3+ for all x and that the extracted Pr 4f spectral weight has a complex lineshape, implying extensive Pr 4f/O 2p hybridization which probably causes the T/sub c/-quenching by disrupting the electronic or magnetic structure of the x = 0 material, perhaps via a Pr-Cu superexchange interaction. 36 refs., 4 figs.
Date: January 1, 1988
Creator: Kang, J.S.; Allen, J.W.; Shen, Z.X.; Ellis, W.P.; Yeh, J.J.; Lee, B.W. et al.
Partner: UNT Libraries Government Documents Department

Antiferromagnetism in NdBa/sub 2/Cu/sub 3/O/sub 6. 1/

Description: Neutron scattering experiments performed on single crystals of NdBa/sub 2/Cu/sub 3/O/sub 6.1/ reveal antiferromagnetic order for T < T/sub N/ = 385 +- 2K in which the structure is characterized by the magnetic wave vector (1/2 1/2 1/2). The magnetic intensities are accounted for the Cu/sup + +/ spins coupled antiferromagnetically in the CuO/sub 2/ planes. The oxygen deficient layers exhibit a small staggered magnetization epsilonS which induces the spin ordering /minus/S, /minus/epsilonS, /minus/S, +S, +epsilonS, +S, along the tetragonal axis. The average staggered magnetization evaluated at room temperature is about 0.40 ..mu..BETA in the CuO/sub 2/ planes and about 0.04 ..mu..BETA in the oxygen deficient layers. 8 refs., 2 figs., 1 tab.
Date: January 1, 1988
Creator: Moudden, A.H.; Shirane, G.; Tranquada, J.M.; Birgeneau, R.J.; Endoh, Y.; Yamada, K. et al.
Partner: UNT Libraries Government Documents Department

Preparation and some properties of H/sub x/REBa/sub 2/Cu/sub 3/O/sub 7/ (RE = Er, Nd, Dy, Ho, Gd) superconductors

Description: Hydrogen gas has been found to react directly with the subject oxides. Most exhibit a small region of H solid solubility which remains superconducting with a slight increase in critical temperature (1 to 2K). At higher H contents a hydrogen rich hydride phase precipitates which is amorphous. The H rich phases are not superconducting at 4K or above. Neutron diffraction studies of D/sub 1.55/YBa/sub 2/Cu/sub 3/O/sub 7/ confirm that the D rich phase is amorphous but that its precipitation causes increased microtwinning in the D saturated oxide. 12 refs., 3 figs.
Date: January 1, 1988
Creator: Johnson, J.; Suenaga, M.; Thompson, P. & Reilly, J.
Partner: UNT Libraries Government Documents Department

A novel excitonic mechanism for high temperature superconductivity

Description: We propose a novel mechanism for superconductivity, based on intra and interband Cu/longleftrightarrow/O charge transfer excitations in oxide superconductors. The dynamic polarizability of the environment surrounding CuO/sup 2/ planes plays an important role in enhancing T/sub c/. The ''sandwich'' structure in which CuO/sub 2/ planes are separated by a highly polarizable medium is ideally suited for this mechanism. Our proposal is consistent with a variety of available data, and suggests several new experimental directions. 9 refs., 2 figs.
Date: January 1, 1988
Creator: Tesanovic, Z.; Bishop, A.R. & Martin, R.L.
Partner: UNT Libraries Government Documents Department

Magnetic field penetration into the high temperature superconductor YBa/sub 2/Cu/sub 3/O/sub 7/minus/x/

Description: The penetration depth of a magnetic field into a superconducting YBa/sub 2/Cu/sub 3/O/sub 7-x/ film was measured by polarized neutron reflection. The sample comprised an epitaxial film with the c-axis of its orthorhombic structure perpendicular to the film's surface. Measurements at 14 K showed that a magnetic field (parallel to the surface) penetrates into the surface over a depth of 1400 /angstrom/. 9 refs., 3 figs.
Date: July 1, 1988
Creator: Mansour, A.; Hilleke, R.O.; Felcher, G.P.; Laibowitz, R.B.; Chaudhari, P. & Parkin, S.S.P.
Partner: UNT Libraries Government Documents Department

Excitonic Superconductivity in Copper Oxides

Description: We discuss the possibility of excitonic superconductivity in high T/sub c/ copper oxides. The Hamiltonians describing CuO/sub 2/ planes supports both antiferromagnetism and low-lying Cu /longleftrightarrow/ O intra- and interband charge fluctuations. One crosses from one regime to another as the number of holes per unit cell increases. The high T/sub c/ superconductivity takes place at hole concentrations most favorable for intraband charge transfer excitations. The dynamic polarizability of the environment surrounding CuO/sub 2/ planes plays an important role in enhancing T/sub c/. 15 refs., 4 figs.
Date: 1988~
Creator: Tesanovic, Z.; Bishop, A. R.; Martin, R. L. & Harris, C.
Partner: UNT Libraries Government Documents Department

Superconducting wires

Description: The requirement of high critical current density has prompted extensive research on ceramic processing of high-T/sub c/ superconductors. An overview of wire fabrication techniques and the limitations they impose on component design will be presented. The effects of processing on microstructure and critical current density will also be discussed. Particle alignment has been observed in extruded samples which is attributed to high shear stresses during plastic forming. Composites of superconductor and silver in several configurations have been made with little deleterious effect on the superconducting properties. 35 refs., 2 figs., 1 tab.
Date: June 1, 1988
Creator: Lanagan, M.T.; Poeppel, R.B.; Singh, J.P.; Dos Santos, D.I.; Lumpp, J.K.; Dusek, J.T. et al.
Partner: UNT Libraries Government Documents Department

Immobilization of IFR salt wastes in mortar

Description: Portland cement-base mortars are being considered for immobilizing chloride salt wastes produced by the fuel cycles of Integral Fast Reactors (IFR). The IFR is a sodium-cooled fast reactor with metal alloy fuels. It has a close-coupled fuel cycle in which fission products are separated from the actinides in an electrochemical cell operating at 500/degree/C. This cell has a liquid cadmium anode in which the fuels are dissolved and a liquid salt electrolyte. The salt will be a mixture of either lithium, potassium, and sodium chlorides or lithium, calcium, barium, and sodium chlorides. One method being considered for immobilizing the treated nontransuranic salt waste is to disperse the salt in a portland cement-base mortar that will be sealed in corrosion-resistant containers. For this application, the grout must be sufficiently fluid that it can be pumped into canister-molds where it will solidify into a strong, leach-resistant material. The set times must be longer than a few hours to allow sufficient time for processing, and the mortar must reach a reasonable compressive strength (/approximately/7 MPa) within three days to permit handling. Because fission product heating will be high, about 0.6 W/kg for a mortar containing 10% waste salt, the effects of elevated temperatures during curing and storage on mortar properties must be considered.
Date: January 1, 1988
Creator: Fischer, D.F. & Johnson, T.R.
Partner: UNT Libraries Government Documents Department

Photoemission from single-crystal EuBa/sub 2/Cu/sub 3/O/sub 6+x/ cleaved below 20K: Metallic-to-insulating surface transformation

Description: Valence band ultraviolet photoemission spectra (UPS) of single-crystal EuBa/sub 2/Cu/sub 3/O/sub 6+x/ (x > 0.6) samples cleaved in vacuum at 20 K demonstrate that the metallic superconducting phase undergoes an irreversible transformation via near-surface oxygen loss to an insulating state upon annealing above 50 K. Freshly cleaved surfaces at 20 K exhibit a density of states at the Fermi level comparable to that of copper, and have both O(2p) and Cu(3d) character at E/sub F/ based on the photon energy dependence of the intensity. Reasonably good agreement between band structure calculations and the present data would suggest theoretical models using the band state as a starting point. 18 refs., 2 figs.
Date: January 1, 1988
Creator: List, R.S.; Arko, A.J.; Fisk, Z.; Cheong, S.W.; Conradson, S.D.; Thompson, J.D. et al.
Partner: UNT Libraries Government Documents Department

Effects of Fe substitutions in YBa/sub 2/Cu/sub 3/O/sub 7/

Description: Both near edge features and EXAFS results unequivocally demonstrate that Fe preferentially substitutes for Cu(1) atoms at the linear chain site for YBa/sub 2/(Cu/sub 1-x/Fe/sub x/)/sub 3/O/sub 7+delta/, where x = 0.02 to 0.15. Electronic and atomic structural changes associated with Fe content are discussed. 5 refs., 2 figs.
Date: January 1, 1988
Creator: Yang, C.Y.; Heald, S.M.; Tranquada, J.M.; Xu, Y.; Wang, Y.L.; Moodenbaugh, A.R. et al.
Partner: UNT Libraries Government Documents Department

Shock consolidation of YBa/sub 2/Cu/sub 3/O/sub 7-x/ powders

Description: Yttrium-Barium-Copper Oxide ceramics, commonly known as Y-123, have been shock compacted by numerous investigators. Unfortunately most of them, initially, had not succeeded in obtaining crack/melt-free monoliths. What is now known from these and similar efforts is that the shock pressure must be low and that the temperature excursion due to the shock and related strains be minimized and kept below the decomposition temperature of the Y-123 compound. This paper describes the efforts and considerations in some of the shock work performed at Los Alamos National Laboratory. 12 refs., 8 figs., 1 tab.
Date: January 1, 1988
Creator: Staudhammer, K.P.
Partner: UNT Libraries Government Documents Department

The upper critical field of high temperature superconductors

Description: Resistive and magnetic measurements of the superconducting transition in good quality single crystals of YBa/sub 2/Cu/sub 3/O/sub 7/minus/delta/ and La/sub 2/minus/x/Sr/sub x/CuO/sub 4/ and in good polycrystalline samples of Ba/sub 1/minus/x/K/sub x/BiO/sub 3/ are used to derive the temperature dependence and anisotropy of the upper critical field. Comparison of the results shows several common features, most notably the existence of upward curvature in the temperature dependence of the upper critical field. The definition of T/sub c/ from resistance curves and the origin of the upward curvature in the upper critical field are discussed.
Date: January 1, 1988
Creator: Crabtree, G.W.; Kwok, W.K.; Welp, U.; Burriel, R.; Claus, H.; Vandervoort, K.G. et al.
Partner: UNT Libraries Government Documents Department

Material selection for electrooptic deflectors

Description: The selection of a material for a practical device is generally guided by a number of criteria, including cost, size, difficulty of fabrication, durability, driver requirements, and system constraints. A quantitative analysis can usually be made for comparison, or a figure of merit can be computed. In the case of materials for electrooptical (EO) devices the choice is often made based on the availability of materials meeting some minimum system requirement. For fast EO deflectors, where a large number of resolvable spots is required, the choice of materials is quite limited. A model of just such a device is proposed; it is based on the resolution of 400 spots and reasonable boundary conditions. The model predicts that to be successful, an EO material must be chosen that has a linear EO coefficient (r/sub 33/) of at least 336 pm/V. A survey was conducted of the EO materials which are generally available. Based on the model and the survey, Czochralski crystal growth of strontium barium niobate (SBN:60) is recommended. Although SBN:60 does not have the largest EO coefficient, it may be the easiest to grow in the required size and optical quality, thus satisfying the availability criterion. It should be borne in mind that many materials may be grown by this technique and there are many new and potential applications for EO materials. 92 refs., 18 figs., 14 tabs.
Date: September 1, 1988
Partner: UNT Libraries Government Documents Department

Atomic imaging and microanalysis of ceramics

Description: This paper is a short review of electron microscopy techniques, as applied to modern ceramics. Examples: representative of the significance of modern electron microscopy, methods of atomic resolution imaging, diffraction and spectroscopy in the task of characterising, and understanding typical ceramic materials are given. (JL)
Date: October 1, 1988
Creator: Thomas, G. & Ramesh, R.
Partner: UNT Libraries Government Documents Department

Radiation induced amorphization in YBa2Cu3O7 and GdBa2Cu3O7 superconductors

Description: The response of YBa2Cu3O7 and GdBa2Cu3O7 high temperature superconductors to particle irradiation is examined. Both ion and electron irradiations have been shown to first produce an orthorhombic-to-tetragonal transformation at doses roughly a factor of 10 higher. Analysis of the displacement stoichiometry that results from 120, 300, and 1000 keV electron irradiations, 400 and 500 keV O irradiations, and 300 keV helium irradiations indicate that the orthorhombic-to-tetragonal transformation is driven by O atoms displacements either alone or in the presence of metal atom displacements and that the transformation to the amorphous phase is driven by displacements on the Y, Gd, or other rare earth atom site. 10 refs., 3 figs., 1 tab.
Date: January 1, 1988
Creator: Parkin, Don M. & Nastasi, Michael
Partner: UNT Libraries Government Documents Department

Radiation damage in barium fluoride detector materials

Description: To develop radiation hard detectors, particularly for high energy physics studies, radiation damage is being studied in BaF/sub 2/, both undoped and doped with La, Ce, Nd, Eu, Gd and Tm. Some dopants reduce radiation damage. In La doped BaF/sub 2/ they reduce the unwanted long lifetime luminescence which interferes with the short-lived fluorescence used to detect particles. Radiation induced coloring is being studied with facilities for making optical measurements before, during and after irradiation with /sup 60/C0 gamma rays. Doses of 10/sup 6/ rad, or less, create only ionization induced charge transfer effects since lattice atom displacement damage is negligible at these doses. All crystals studied exhibit color center formation, between approximately 200 and 800 nm, during irradiation and color center decay after irradiation. Thus only measurements made during irradiation show the total absorption present in a radiation field. Both undoped and La doped BaF/sub 2/ develop damage at minimum detectable levels in the UV---which is important for particle detectors. For particle detector applications these studies must be extended to high dose irradiations with particles energetic enough to cause lattice atom displacement damage. In principle, the reduction in damage provided by dopants could apply to other applications requiring radiation damage resistant materials.
Date: January 1, 1988
Creator: Levey, P.W.; Kierstead, J.A. & Woody, C.L.
Partner: UNT Libraries Government Documents Department