Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex-situ tapes

Ex-situ Powder-In-Tube MgB2 tapes prepared with ball-milled, undoped powders showed a strong enhancement of the irreversibility field H*, the upper critical field Hc2 and the critical current density Jc(H) together with the suppression of the anisotropy of all of these quantities. Jc reached 104 A/cm2 at 4.2 K and 10 T, with an irreversibility field of about 14 T at 4.2 K, and Hc2 of 9 T at 25 K, high values for not-doped MgB2. The enhanced Jc and H* values are associated with significant grain refinement produced by milling of the MgB2 powder, which enhances grain boundary pinning, although at the same time also reducing the connectivity from about 12% to 8%. Although enhanced pinning and diminished connectivity are in opposition, the overall influence of ball milling on Jc is positive because the increased density of grains with a size comparable with the mean free path produces strong electron scattering that substantially increases Hc2, especially Hc2 perpendicular to the Mg and B planes.Comment: 26 pages, 9 figures, submitted to J. Appl. Phy

The Grenoble High Magnetic Field Laboratory

The Grenoble High Magnetic Field Laboratory (GHMFL) was built to meet the requirements of scientists for experiments implying the use of high magnetic fields. It is established in Grenoble, France. It is a French-German laboratory, jointly operated by the Max Planck Institut Air Festkorperforschung, in Stuttgart (MPI-FKF) and the Centre National de la Recherche Scientifique (CNRS). Scientists have access to the magnetic field facility provided they have written a proposal for experiments in high magnetic fields and have obtained the approval of the Program Committee to carry out their research. Since the foundation of the laboratory, the research effort has been mainly devoted to solid state physics (i.e. research on metals, superconductors, semiconductors, magnetism, etc.) which can be performed in a volume as small as a few cml. Five years ago the laboratory has undertaken the development of 20 MW magnets. Two of them are now in operation and deliver fields up to 30 T in a 50 mm room temperature bore. A 20 MW magnet is composed of two concentric 10 MW magnets. The outer 10 MW part alone achieves a field up to 10 T in a 376 min diameter bore. It is obvious that this large diameter is convenient for MHD flow studies under high magnetic fields. (C) 2001 Elsevier Science Ltd. All rights reserved

Characterization of transport properties variations with magnetic field and temperature of ITER-candidate NbTi strands

While the International Thermonuclear Experimental Reactor (ITER) conceptual design retained the Nb3Sn for toroidal field (TF) and central solenoid (CS) coils, the low working field (around 6 T) promoted the choice of NbTi for the poloidal field (PF) coils. EU has carried out the experimental study of industrial NbTi strands and cables with different internal structures in order to choose the one which generate the lowest losses when used in the PF operating conditions (i.e. pulsed field). CEA has contributed to this project through the experimental study of the transport properties variations with respect to magnetic field and temperature of two candidate strands. One of them contains an internal CuNi barrier and the other is Ni-plated. A homemade cryostat is used to control the temperature of the sample which is wound on a VAMAS-like mandrel. J(c) measurements are presented here and subsequently the parameters deduced from scaling laws and their variation with temperature between 4.2 and 7 K and with field up to I 1 T. A comparison between the two strands characteristics and ITER PF coils criteria is also discussed. The results are in good agreement with literature and lie inside an acceptable range in spite of some discrepancy with the ITER PF criteria: a recent thermo-hydraulic simulation confirmed it. In the future, this study, completed by AC losses measurements on cabled strands, should help to optimise the strands performances below the ITER PF security margins. (C) 2002 Published by Elsevier Science B.V

Niobium addition effect in molds at last cooling step on EN-GJL250 gray cast iron: Microstructural changes and electrochemical behavior

International audienc

Dépôt chimique en phase vapeur d'hétérostructures YBa2_2Cu3_3O7−x_{7-x}/PrBa2_2Cu3_3O7−x_{7-x}

A reactor which allows chemical vapor deposition from metalorganic precursors (MOCVD) has been specially designed to synthesize alternate layers of " superconductor/insulator " type. The YBa$_2$Cu$_3$O$_{7-x}$/PrBa$_2$Cu$_3$O$_{7-x}$ studied system is suitable for heteroepitaxial multilayer structures. Thin films, deposited between 750 and 900 $^{\circ}$C on MgO $\{100\}$, SrTiO$_3$ $\{100\}$ and LaAlO$_3$ $\{012\}$ are obtained by thermal decomposition of precursors (tetramethylheptanedionates of Y, Ba, Pr and Cu). YBa$_2$Cu$_3$O$_{7-x}$ layers grown in this reactor have critical current densities of some 10$^6$ A.cm$^{-2}$ at 77 K. The temperature dependence of resistivity in the PrBa$_2$Cu$_3$O$_{7-x}$ films shows semiconducting behavior. Growth temperature effect on the interdiffusion at the multilayer interfaces will be studied in the case of two samples elaborated at different temperatures of deposition (750 and 900 $^{\circ}$C). Although the element diffusion becomes larger with the temperature of deposition, the superconducting properties are not damaged.Un réacteur de dépôt chimique en phase vapeur à partir de précurseurs organométalliques (EPVOM) a été spécialement conçu pour la synthèse de couches alternées du type supraconducteur/isolant. Le système étudié (YBa$_2$Cu$_3$O$_{7-x}$/PrBa$_2$Cu$_3$O$_{7-x}$) permet la réalisation de structures épitaxiées. Les films minces, déposés entre 750 et 900 $^{\circ}$C sur MgO $\{100\}$, SrTiO$_3$ $\{100\}$ et LaAlO$_3\{012\}$, sont obtenus à partir de la décomposition thermique des précurseurs (tétraméthylheptanedionates de Pr, Y, Ba et Cu). Les couches d'YBa$_2$Cu$_3$O$_{7-x}$ déposées dans ce réacteur ont des densités de courant critique à 77 K de quelques 10$^6$ A.cm$^{-2}$. La variation de la résistivité des films de PrBa$_2$Cu$_3$O$_{7-x}$ en fonction de la température montre un comportement semiconducteur. L'effet de la température de croissance sur l'interdiffusion aux interfaces des multicouches sera étudié dans le cas de deux échantillons élaborés à des températures de dépôt différentes (750 et 900 $^{\circ}$C). Malgré que la diffusion des éléments s'amplifie avec la température de dépôt, les caractéristiques supraconductrices ne sont pas altérées

Interlayer tunneling spectroscopy of layered CDW materials

We have measured intrinsic tunneling spectra of charge density wave (CDW) materials NbSe$_3$ and o-TaS$_3$ in sub-micron scale mesa structures. Beyond the interband tunneling across the CDW gap, $2\Delta$, we observe intragap states at the voltage $V\approx 2\Delta/3$ which we associate with creation of the amplitude soliton. The onset of the tunneling occurs only above a threshold voltage $V_t\approx 0.1\Delta$ followed by a quasi-periodic staircase spectrum. We interpret that behavior as the CDW phase decoupling between neighboring layers via the formation of a grid of dislocation lines. In NbSe$_3$, the application of a high magnetic field (up to 27 T) yields a strong suppression of the tunneling density of states within the CDW gap indicating a possible field induced metal-insulator transition

Vortex phases in YBa2Cu3Oy (y=6.5) for B parallel to CuO: impact of regular and random pinning arrays

Using high angular resolution (up to 10-4 degree) resistive measurements we studied a YBa2Cu3O6.5 single crystal for the magnetic field (B&lt;28 T) direction closely parallel (within the lock-in angle of 0.3°) to the crystal CuO-planes. Our data show that the vortices become confined between the CuO-planes below a well-defined temperature Tcf. Using a pseudo-flux transformer geometry for electrical transport measurements we demonstrate directly a smectic-like vortex movement for temperatures below TS. Our measurements reveal a rich phase diagram for magnetic field orientation BCuO, originating from the inter-play between pinning by the CuO-planes, point-like disorder from oxygen vacancies and temperature fluctuations. Commensurability between the inter-vortex distance and the CuO-layer period causes a pronounced oscillatory behaviour dominated by in-plane vortex movement.<br/