34 research outputs found
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 YBaCuO/PrBaCuO
A reactor which allows chemical vapor deposition from
metalorganic precursors (MOCVD) has been specially designed to synthesize
alternate layers of " superconductor/insulator " type. The
YBaCuO/PrBaCuO studied system is suitable for
heteroepitaxial multilayer structures. Thin films, deposited between 750 and
900Â C on MgO , SrTiO
and LaAlO
are obtained by thermal decomposition of precursors (tetramethylheptanedionates
of Y, Ba, Pr and Cu). YBaCuO layers grown in this reactor have
critical current densities of some 10Â A.cm at 77 K. The temperature
dependence of resistivity in the PrBaCuO 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 900Â 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
(YBaCuO/PrBaCuO) permet la réalisation de
structures épitaxiées. Les films minces, déposés entre 750 et 900 C
sur MgO , SrTiO
et LaAlO, sont obtenus Ã
partir de la décomposition thermique des précurseurs
(tétraméthylheptanedionates de Pr, Y, Ba et Cu). Les couches
d'YBaCuO déposées dans ce réacteur ont des densités de courant
critique à 77 K de quelques 10 A.cm. La variation de la résistivité
des films de PrBaCuO 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 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 and o-TaS in sub-micron scale mesa structures. Beyond the interband
tunneling across the CDW gap, , we observe intragap states at the voltage
which we associate with creation of the amplitude soliton. The
onset of the tunneling occurs only above a threshold voltage
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, 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<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/