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Connectivity and Critical Currents in Polycrystalline MgB2  [PDF]
M. Eisterer,J. Emhofer,S. Sorta,M. Zehetmayer,H. W. Weber
Physics , 2008, DOI: 10.1088/0953-2048/22/3/034016
Abstract: Current transport in polycrystalline magnesium diboride is highly non-uniform (percolative) due to the presence of secondary phases and also due to the intrinsic anisotropy of the material. The influence of secondary phases on the transport properties of MgB2 was investigated. Bulk samples were prepared from a mixture of MgB2 and MgO powders by the ex-situ technique in order to vary the MgO content systematically. The samples were characterized by resistive and magnetization measurements. The reduced MgB2 fraction is modeled by a reduced effective cross section (connectivity), which was assessed directly by the experiments. The presence of MgO also increases the percolation threshold, which reduces the zero resistivity (or irreversibility) field.
Critical currents and vortex dynamics in super-conducting MgB2  [PDF]
Y. Bugoslavsky,G. K. Perkins,X. Qi,L. F. Cohen,A. D. Caplin
Physics , 2001,
Abstract: The recently-discovered MgB2 super-conductor has a transition temperature Tc approaching 40K, placing it intermediate between the families of low and high temperature super-conductors (LTS and HTS). In practical applications, super-conductors are permeated by quantised magnetic flux vortices, and when a current flows there is dissipation unless the vortices are "pinned" in some way, and so inhibited from moving under the influence of the Lorentz force. This vortex motion sets the limiting critical current density Jc in the super-conductor. Vortex behaviour has proved to be more complicated in the HTS than in LTS materials. While this has stimulated extensive theoretical and experimental research, it has impeded applications. Clearly it is important to explore vortex behaviour in MgB2; here we report on Jc, and also on the creep rate S, which is a measure of how fast the "persistent" currents decay. Our results show that naturally-occurring grain boundaries are highly transparent to supercurrent, and suggest that the steep decline in Jc with increasing magnetic field H reflects a weakening of the vortex pinning energy, possibly because this compound forms naturally with a high degree of crystalline perfection.
Critical currents in weakly textured MgB2: Nonlinear transport in anisotropic heterogeneous media  [PDF]
M. Eisterer,W. Hassler,P. Kovac
Physics , 2009, DOI: 10.1103/PhysRevB.80.174516
Abstract: A model for highly non-linear transport in heterogeneous media consisting of anisotropic particles with a preferred orientation is proposed and applied to the current transport in weakly textured magnesium diboride, MgB2. It essentially explains why, unlike in conventional superconductors, a significant macroscopic anisotropy of the critical currents can be induced by the preparation of MgB2 tapes. The field and angular dependence of the critical current is calculated for various degrees of texture and compared to experimental data.
Upper critical fields of MgB2 thin films  [PDF]
V. Ferrando,C. Bernini,P. Manfrinetti,D. Marre',M. Putti,C. Tarantini,C. Ferdeghini
Physics , 2003, DOI: 10.1016/j.physc.2004.02.059
Abstract: Critical fields of four MgB2 thin films with a normal state resistivity ranging from 5 to 50 mWcm and Tc from 29.5 to 38.8 K were measured up to 28 T. Hc2(T) curves present a linear behavior towards low temperatures. Very high critical field values have been found, up to 24 T along the c-axis and 57 T in the basal plane not depending on the normal state resistivity values. In this paper, critical fields will be analyzed taking into account the multiband nature of MgB2; we will show that resistivity and upper critical fields can be ascribed to different scattering mechanisms.
Anisotropic Connectivity and its Influence on Critical Current Densities, Irreversibility Fields, and Flux Creep in In-Situ-Processed MgB2 Strands  [PDF]
Z. X. Shi,M. A. Susner,M Majoros,M. D. Sumption,X. Peng,M. Rindfleisch,M. J. Tomsic,E. W. Collings
Physics , 2010, DOI: 10.1088/0953-2048/23/4/045018
Abstract: The anisotropy of the critical current density (Jc) and its influence on measurement of irreversibility field (Birr) has been investigated for high quality, in-situ MgB2 strands. Comparison of transport and magnetization measurements has revealed the onset of a regime where large differences exist between transport and magnetically measured values of the critical current density and Birr. These effects, initially unexpected due to the lack of crystalline texture in these in-situ processed strands, appear to be due to a fibrous microstructure, connected with the details of the wire fabrication and MgB2 formation reactions. Scanning electron micrographs of in-situ-processed MgB2 monocore strands have revealed a fibrous microstructure. Grains (~100 nm) are randomly oriented, and there is no apparent local texture of the grains. However, this randomly oriented polycrystalline material has a fibrous texture at a larger length scale, with stringers of MgB2 (~ 60 {\mu}m long and ~5 {\mu}m in diameter) partially separated by elongated pores -- the spaces previously occupied by stringers of elemental Mg. This leads to an interpretation of the differences observed in transport and magnetically determined critical currents, in particular a large deviation between the two at higher fields, in terms of different transverse and longitudinal connectivities within the strand. The different values of connectivity also lead to different resistive transition widths, and thus irreversibility field values, as measured by transport and magnetic techniques. Finally, these considerations are seen to influence estimated pinning potentials for the strands.
Fabrication and transport critical currents of multifilamentary MgB2/Fe wires and tapes  [PDF]
H. L. Suo,C. Beneduce,X. D. Su,R. Flukiger
Physics , 2002, DOI: 10.1088/0953-2048/15/7/312
Abstract: Multifilamentary MgB2/Fe wires and tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. After annealing, we measured transport jc values up to 1.1 * 105 A/cm2 at 4.2 K and in a field of 2 T in a MgB2/Fe square wire with 7 filaments fabricated by two-axial rolling, and up to 5 * 104 A/cm2 at 4.2 K in 1 T in a MgB2/Fe tape with 7 filaments. For higher currents these multifilamentary wires and tapes quenched due to insufficient thermal stability of filaments. Both the processing routes and deformation methods were found to be important factors for fabricating multifilamentary MgB2 wires and tapes with high transport jc values.
Large transport critical currents in unsintered MgB2 superconducting tapes  [PDF]
G. Grasso,A. Malagoli,C. Ferdeghini,S. Roncallo,V. Braccini,M. R. Cimberle,A. S. Siri
Physics , 2001, DOI: 10.1063/1.1384905
Abstract: The Powder-In-Tube process has been employed to fabricate tape-like conductors with a strong metallic sheath and based on the newly discovered MgB2 superconducting phase. Long superconducting tapes have been prepared by packing reacted MgB2 powders inside pure Ag, Cu and Ni tubes which are then cold worked by drawing and rolling. Such tapes have shown transport properties as good as bulk MgB2 samples sintered in high pressure and high temperature conditions. At 4.2 K, the highest critical current density of 100000 A/cm2 has been achieved on Nickel-sheathed single-filament conductors. A direct correlation between the sheath tensile strength and the critical current of the unsintered tape has been observed.
Mechanically reinforced MgB2 wires and tapes with high transport currents  [PDF]
R. Nast,S. I. Schlachter,S. Zimmer,H. Reiner,W. Goldacker
Physics , 2001, DOI: 10.1016/S0921-4534(02)00982-6
Abstract: Monofilamentary MgB2-wires with a 2- or 3-component sheath containing mechanical reinforcing stainless steel (SS) were prepared and characterized. In direct contact to the superconductor Nb, Ta or Fe was used. For a selection of samples with a Fe and Fe/SS sheath, we investigated the transport critical current behaviour in magnetic fields changing systematically the geometrical shape from a round wire to a flat tape. A strong increase of the current densities in flat tapes was observed and possible reasons for this are discussed. Reinforcing the sheath in the outer layer with different amounts of stainless steel leads to a systematic field dependent decrease of the transport critical current density with increasing steel amount. This is an indication for a pre-stress induced degradation of the critical currents in MgB2 wires and first Ic-stress-strain experiments seem to confirm this observation and interpretation.
Enhancement of the critical current density in FeO-coated MgB2 thin films at high magnetic fields  [cached]
Andrei E. Surdu,Hassan H. Hamdeh,I. A. Al-Omari,David J. Sellmyer
Beilstein Journal of Nanotechnology , 2011, DOI: 10.3762/bjnano.2.89
Abstract: The effect of depositing FeO nanoparticles with a diameter of 10 nm onto the surface of MgB2 thin films on the critical current density was studied in comparison with the case of uncoated MgB2 thin films. We calculated the superconducting critical current densities (Jc) from the magnetization hysteresis (M–H) curves for both sets of samples and found that the Jc value of FeO-coated films is higher at all fields and temperatures than the Jc value for uncoated films, and that it decreases to ~105 A/cm2 at B = 1 T and T = 20 K and remains approximately constant at higher fields up to 7 T.
Transport critical current, anisotropy, irreversibility fields and exponential n factors in Fe sheathed MgB2 tapes  [PDF]
C. Beneduce,H. L. Suo,P. Toulemonde,N. Musolino,R. Flukiger
Physics , 2002,
Abstract: The influence of the initial MgB2 grain size on critical current density, upper critical fields and irreversibility has been studied on Fe sheathed monofilamentary MgB2 tapes prepared by the Powder-In-Tube technique. The effect of the reduction of MgB2 grain size by ball milling was mainly to enhance both the critical current density, jc, and the irreversibility field, while the upper critical field remained unchanged. The anisotropy ratio of the upper critical field between magnetic fields parallel and perpendicular the tape surface was determined to 1.3, reflecting a deformation induced texture. A good agreement has been found between resistive and inductive jc values, measured at various temperatures. At 25K and 1 T, jc values close to 105 A/cm2 were measured. The exponential n factor of the resistive transition was found to be quite high at low fields, and decrease linearly from 60 at 4T to 10 at 8.5T.
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