A study of enhancing critical current densities (J(sub c)) and critical temperature (T(sub c)) of high-temperature superconductors

The development of pure phase 123 and Bi-based 2223 superconductors has been optimized. The pre-heat processing appears to be a very important parameter in achieving optimal physical properties. The synthesis of pure phases in the Bi-based system involves effects due to oxygen partial pressure, time, and temperature. Orientation/melt-sintering effects include the extreme c-axis orientation of Yttrium 123 and Bismuth 2223, 2212, and 2201 phases. This orientation is conductive to increasing critical currents. A procedure was established to substitute Sr for Ba in Y-123 single crystals

The Crystal and Molecular Structure of an Asymmetric Diacetylene Monomer, 6-(2-methyl-4-nitroanilino)-2,4-hexadiyne-1-ol

The crystal and molecular structure of an asymmetric diacetylene monomer has been determined from x-ray diffraction data. The crystals, obtained from an acetone/pentane solution, are orthorhombic, Fdd2 with Z = 16 in a unit cell having dimensions of a = 42.815(6) A, b = 22.224(5) A, c = 4.996(l) A. The structure was solved by direct methods and refined by least- squares techniques to an R(sub F) of 6.4% for 988 reflections and 171 variables. The diacetylene chains are disposed in the unit cell in a complex manner in order to satisfy the hydrogen- bonding, crystal packing, and symmetry requirements of the system. The solid state polymerization mechanism is discussed with respect to the geometric disposition of the diacetylene chains. These chains are far apart and incorrectly oriented with respect to each other to permit polymerization in the crystal by means of 1,4-addition, consistent with the Baughman mechanistic model

Liquid encapsulated float zone process and apparatus

The process and apparatus for growing crystals using float zone techniques are described. A rod of crystalline materials is disposed in a cylindrical container, leaving a space between the rod and container walls. This space is filled with an encapsulant, selected to have a slightly lower melting point than the crystalline material. The rod is secured to a container end cap at one end and to a shaft at its other end. A piston slides over the rod and provides pressure to prevent loss of volatile components upon melting of the rod. Prior to melting the rod the container is first heated to melt the encapsulant, with any off-gas from this step being vented to a cavity behind the piston. The piston moves slightly forward owing to volume change upon melting of the encapsulant, and the vent passageway is closed. The container is then moved longitudinally through a heated zone to progressively melt sections of the rod as in conventional float zone processes. The float zone technique may be used in the microgravity environment of space

Microgravity Processing of Oxide Superconductors

Considerable effort has been concentrated on the synthesis and characterization of high T(sub c) oxide superconducting materials. The YBaCuO system has received the most intense study, as this material has shown promise for the application of both thin film and bulk materials. There are many problems with the application of bulk materials- weak links, poor connectivity, small coherence length, oxygen content and control, environmental reactivity, phase stability, incongruent melting behavior, grain boundary contamination, brittle mechanical behavior, and flux creep. The extent to which these problems are intrinsic or associated with processing is the subject of controversy. This study seeks to understand solidification processing of these materials, and to use this knowledge for alternative processing strategies, which, at the very least, will improve the understanding of bulk material properties and deficiencies. In general, the phase diagram studies of the YBaCuO system have concentrated on solid state reactions and on the Y2BaCuO(x) + liquid yields YBa2Cu3O(7-delta) peritectic reaction. Little information is available on the complete melting relations, undercooling, and solidification behavior of these materials. In addition, rare earth substitutions such as Nd and Gd affect the liquidus and phase relations. These materials have promising applications, but lack of information on the high temperature phase relations has hampered research. In general, the understanding of undercooling and solidification of high temperature oxide systems lags behind the science of these phenomena in metallic systems. Therefore, this research investigates the fundamental melting relations, undercooling, and solidification behavior of oxide superconductors with an emphasis on improving ground based synthesis of these materials

Dielectric study of dynamics of organic glasses

The dynamics of organic compounds 2-cyclo-octylamino-5-nitropyridine (COANP), (S)-2-N--(methylbenzylamino)-5-nitropyridine (MBANP), 2-(N-prolinol)-5-nitropyridine (PNP), and N-(4-nitrophenyl)-(L)-prolinol (NPP) were studied by dielectric relaxation spectroscopy in the frequency range of 10 Hz-2 MHz and differential scanning calorimetry (DSC). The dielectric and DSC studies showed that COANP, MBANP and PNP underwent glass transition. However, NPP crystallized so rapidly upon cooling that the glass state could not be observed. It was found that the crystalline process of COANP did not slow the structure relaxation of COANP glass. The relaxation times fitted well to the empirical Vogel-Fulcher equation = expEa/kb(T-TVF). The activation energies Ea and the Vogel-Fulcher temperature TVF were 54.5 meV and 239 K for COANP, 86.2 meV and 249 K for MBANP and 84.9 meV and 245 K for PNP, respectively. The crystalline temperatures of COANP and MBANP were given as 300 K and 330 K, respectively. An anomalous behaviour of the dielectric permittivity of PNP glass was observed

Sur quelques phases du système thallium-vanadium-souffre

International audienceDans le système Tl-V-S, mise en évidence des phases..

Étude structurale de la phase Tl3VS4

International audienceThe crystal structure of Tl3VS4 was determined by x-ray and refined to R = 0.049. The compound is cubic, space group I4̅3m, with a 7.504(5) Å; d.(exptl.) = 6.17(6) and d.(calculated) = 6.23 for Z = 2. The structure consists of VS4 tetrahedra held together by Tl ions. The structure proposed by C. Crevecoeur (1964) is basically correct

Étude structurale de la phase Tl3VS4

International audienceThe crystal structure of Tl3VS4 was determined by x-ray and refined to R = 0.049. The compound is cubic, space group I4̅3m, with a 7.504(5) Å; d.(exptl.) = 6.17(6) and d.(calculated) = 6.23 for Z = 2. The structure consists of VS4 tetrahedra held together by Tl ions. The structure proposed by C. Crevecoeur (1964) is basically correct

La structure cristalline de l'oxyfluorotantalate de potassium K2Ta2O3F6

The structure of potassium tantalum oxyfluoride K₂Ta₂O₃F₆ has been determined from a Patterson synthesis and refined by least squares (R = 0.065). The orthorhombic unit cell has the space group Pnma (a = 10.212 (5) Å ; b = 5.581 (3) Å ; c = 14.499 (7) Å) and contains four motifs. The structure is made up of double infinite zigzag chains along the [010] direction. These chains are formed by the (TaO₃F₃) ⁴⁻ octahedra sharing three oxygen corners of a common face. The K⁺ ions occupy the vacant sites between the files and ensure the cohesion of the lattice.La structure de l'oxyfluorotantalate de potassium K₂Ta₂O₃F₆ a été déterminée à partir d'une synthèse de Patterson et affinée par la méthode des moindres carrés (R = 0,065). La maille élémentaire de symétrie orthorhombique comporte le groupe spatial Pnma (a = 10,212 (5) Å ; b = 5,581 (3) Å ; c = 14,499 (7) Å) et contient quatre motifs formulaires. Le réseau est formé de chaînes doubles se développant en zigzag dans la direction de l'axe b, constituées d'octaèdres (TaO₃F₃) ⁴⁻ mettant en commun trois sommets oxygénés d'une même face. Des ions K⁺ occupent les sites vacants entre les files assurant la cohésion du réseau.Vlasse Marcus, Chaminade Jean-Pierre, Pouchard Michel. La structure cristalline de l'oxyfluorotantalate de potassium K2Ta2O3F6. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 99, 1, 1976. pp. 3-7

Deux nouveaux oxyfluoroniobates de potassium

International audienceK6Nb6.5O14.5F9.5 (I) and K12Nb15.5O35.5F18.5 (II) were prepared by reaction in the solid phase of stoichiometric mixtures of K2NbF7, Nb2O5, and KNbO3 in a dry atm. The compounds are isotypic with the analogous Ta phases. The compounds are hexagonal, space group P6̅, with a 13.126 (8) and c 3.883 (5) Å for (I), and a 19.26 (1) and c 3.873 (7) Å for (II)