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ABSTRACT: Dense pure and doped (Mg1 – x
A
x
)B2 samples with A = Na, Ca, Cu, Ag, Zn, and Al were synthesized at high pressure–high temperature in a multi-anvils press (3.5–6 GPa, 900–1000C) for 0 x 0.20. They were studied by X-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by AC susceptibility, magnetization, and transport measurements. Only Al is really substituted on the Mg site. The other elements form secondary phases with B or Mg. No large effect is observed on the superconducting properties T
c, j
c critical current, H
irr, and H
c2.
Journal of Superconductivity 11/2002; 15(6):613-619.
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ABSTRACT: Superconducting properties of bulk, dense, pure MgB2 and doped (Mg1-xAx)B2 samples with A = Na, Ca, Cu, Ag, Zn and Al were studied for compositional ranges 0 < x < 0.20. The effect on pinning properties and critical current were investigated, particularly for A = aluminium. The samples were sintered and/or synthesised at high pressure - high temperature in a cubic multi-anvils press (typically : 3.5-6 GPa, 900-1000C). They were characterised by x-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by a.c. susceptibility, magnetisation (VSM and SQUID) and transport measurements under magnetic field. Only Al substitutes really on the Mg site. The other elements form secondary phases with B or Mg which do not act as pinning centres. No positive effect is observed on the superconducting properties of the bulk MgB2 samples added with these doping elements : Tc, jc critical current, Hirr and Hc2. For Al, the effect on Hc2 remains small, and the irreversibility line does not move, thus not improving the critical current of the Al-doped MgB2 samples. Comment: 32 pages, 12 figures, submitted to Superond. Sci. Technol. (Proceedings BOROMAG, Genova, Italy, June 17-19, 2002
07/2002;
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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.
04/2002;
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ABSTRACT: Highly dense monofilamentary MgB<sub>2</sub>/Ni and MgB<sub>2</sub>/Fe tapes and multifilamentary MgB<sub>2</sub>/Fe wire with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. High density of the MgB<sub>2</sub> core and a fine starting powder appear to be decisive factors for reaching high critical current densities. After annealing, we measured transport j<sub>c</sub> values up to 2.3×10<sup>5</sup> A/cm<sup>2</sup> at 4.2 K in a field of 1.5 T (corresponding to an I<sub>c</sub> of 300 A) in a MgB<sub>2</sub>/Ni tape and up to 10<sup>4</sup> A/cm<sup>2</sup> at 4.2 K in 6.5 T in a MgB<sub>2</sub>/Fe tape. For higher currents these monofilamentary tapes quenched due to insufficient thermal stability. Thus, j<sub>c</sub> at 4.2 K, 0 T can for instance only be extrapolated, yielding a value close to 1 MA/cm<sup>2</sup>. The recrystallization during annealing leads to a densification and to j<sub>c</sub> values which are more than a factor of ∼10 higher than those measured in the as-deformed tapes. Ni shows a limited reaction with MgB<sub>2</sub>, leading to the formation of Mg<sub>2</sub>Ni reaction layers between the filament and the matrix. On the other hand, Fe stays chemically inert and turns out to be an excellent candidate material for preparation of multifilamentary MgB<sub>2</sub> tapes. A transport j<sub>c</sub> value of 9×10<sup>4</sup> A/cm<sup>2</sup> at 4.2 K and 0.75 T in MgB<sub>2</sub>/Fe wire with 7 filaments produced by groove rolling was obtained.
IEEE Transactions on Appiled Superconductivity 04/2002; · 1.04 Impact Factor
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ABSTRACT: An overview of the current state of development of MgB2 wires and tapes prepared by several techniques is presented, based on the known literature data and our own results. We focus on the powder-in-tube processing method, using pre-reacted MgB2 powders surrounded by an Fe sheath (ex situ processing route). The study of the effect of the initial MgB2 particle size shows that after reducing the initial particles by ball milling to sizes of the order of 3 μm, the degree of texturing and the upper critical field, Bc2, remain unchanged, while both the critical current density, Jc, and the irreversibility field, Birr, show a considerable increase, followed by a decrease after longer milling times. After various recrystallization times and temperatures we show that a critical amount of impurities introduced during the particle reduction process is responsible for the observed maximum of Jc and Birr. Our analysis indicates that this is a grain boundary effect, thus pointing the way for further improvement of the transport properties. A preferential orientation along the c-axis has been observed in Fe/MgB2 tapes by X-ray diffraction. An anisotropy ratio of 1.3 for both Bc2 and Birr was found. The Fe/MgB2 tapes exhibit a very high n factor, which opens up the possibility for a persistent mode operation at 4.2 K and moderate fields. Transport Jc values above 104 A/cm2 are obtained in monofilamentary Fe/MgB2 tapes at 4.2 K and 6.5 T and at 25 K and 2.25 T. Multifilamentary tapes were found to exhibit lower Jc values due to the presently used deformation process yielding a lower density with respect to monofilaments. In all measured tapes, quenching was observed at the lowest applied fields. Improvement of the thermal stability of MgB2 tapes will be one of the major challenges in future developments.
Physica C: Superconductivity.
