C. Beneduce

University of Geneva, Genève, GE, Switzerland

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Publications (18)18.14 Total impact

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    ABSTRACT: Mono- and multifilamentary MgB<sub>2</sub>/Fe tapes and wires with high transport critical current densities have been prepared using the powder-in-tube (PIT) process. The fabrication details are described. The effect of powder grain sizes and recrystallization temperature on j<sub>c</sub> has been investigated. At 25 K and 1 T, j<sub>c</sub> values close to 10<sup>5</sup> A/cm<sup>2</sup> were measured, while j<sub>c</sub> of 10<sup>6</sup> A/cm<sup>2</sup> were extrapolated for 4.2 K/0T in our monofilamentary tape. MgB<sub>2</sub>/Fe tapes exhibit high exponential n factors for the resistive transition: n≈80 and 40 were found at 5 T and 7 T, respectively. The highest transport j<sub>c</sub> values obtained so far in MgB<sub>2</sub>/Fe wires with 7 filaments were 1.1×10<sup>5</sup> A/cm<sup>2</sup> at 4.2 K and in a field of E2 T, which is still lower than for monofilamentary tapes. The function F<sub>p</sub>∝b<sup>p</sup>·(1-b)<sup>q</sup> has been established over the whole field range, and exhibits a maximum at F<sub>p</sub>≅0.18. Improved deformation and recovering processing is expected to lead to higher j<sub>c</sub> values.
    IEEE Transactions on Applied Superconductivity 07/2003; · 1.20 Impact Factor
  • Physica C Superconductivity 05/2003; · 0.72 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 03/2003; · 0.72 Impact Factor
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    ABSTRACT: Mono- and multifilamentary Fe/MgB2 tapes and wires with high transport critical current densities have been prepared by the powder-in-tube technique using fine powders. The influence of the initial MgB2 grain size on critical current density, upper critical and irreversibility fields has been studied. After reducing the MgB2 grains to micrometer size by ball milling, the critical current density, Jc, was enhanced, while the upper critical field, μ0Hc2, remained unchanged. The anisotropy ratio between the upper critical fields parallel and perpendicular to the tape surface was determined to be 1.3, reflecting a deformation induced texture. A good agreement has been found between resistive and inductive Jc values, measured at various temperatures between 4.2 and 25 K. On monofilamentary tapes, Jc values close to 105 A cm−2 were measured at 25 K/1 T, while Jc values ≈106 A cm−2 were extrapolated for 4.2 K/0 T. Fe/MgB2 tapes exhibit high exponential n factors for the resistive transition: n values of 60 and 30 were found at 4 T and 6 T, respectively. Multifilamentary wires (with seven filaments) show slightly lower Jc values, 1.1 × 105 A cm−2 at 4.2 K/2 T. The improvement of thermal and mechanical stability of MgB2/Fe tapes and wires appears clearly as a challenge for future developments.
    Superconductor Science and Technology 01/2003; 16(2):264. · 2.76 Impact Factor
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    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2003; 387(3):419-419.
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    ABSTRACT: One of the parameters of power in tube (PIT) processed Bi,Pb(2223) tapes which still leaves room for improvement is the density of the filaments, presently reported to be around 80–90%. To some extent this can be addressed with optimised intermediate deformation methods, such as periodic pressing, but this solution is limited by the necessity to heal the damage after deformation. The synthesis of the Bi,Pb(2223) phase whilst applying a hot isostatic pressure (HIP) has been found to be an attractive alternative route to obtain higher ceramic density. In this work we compare the properties of tapes processed under various isostatic pressure up to 1000 bar. High pressure was found to fasten the kinetics of phase formation. SEM observation showed an increased density of the ceramic core in HIP processed tapes. 20 h annealing under 100 bar improved the critical current density by 30% compared to the standard first heat treatment in PIT process.
    Physica C Superconductivity 08/2002; 372:980-983. · 0.72 Impact Factor
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    ABSTRACT: The level of AC losses is still too high in Bi,Pb(2223) tapes to allow their use in many applications. Therefore the reduction of AC losses in Bi,Pb(2223) is a key factor for their economical viability. The reduction of AC losses can be achieved by twisting of the tapes, alloying of the sheath or introducing a resistive barrier between the filaments. Up to now, the lowest value ever reported for a Bi,Pb(2223) tape in a parallel magnetic field of 0.1 T at 50 Hz is P=0.33 mW/Am, which is near the requirements for practical applications. On the other hand, when the applied field is perpendicular to the tape surface, the level of the AC losses is still too high and the critical current densities of such tapes is too low. We developed square wires, which allow the use of smaller twist pitches than in conventional tapes and have reduced anisotropy. We report on the effect of this new configuration on the level of AC losses.
    Physica C Superconductivity 07/2002; 372-376:1814-1817. · 0.72 Impact Factor
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    H. L. Suo, C. Beneduce, X. -D Su, R. Flukiger
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    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. Comment: 13 pages, 7 figures
    Superconductor Science and Technology 05/2002; 15:1058-1062. · 2.76 Impact Factor
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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 Applied Superconductivity 04/2002; · 1.20 Impact Factor
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    ABSTRACT: Highly dense monofilamentary MgB2/Ni and MgB2/Fe superconducting tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. High density of the MgB2 core and a fine starting powder appear to be decisive factors for reaching high critical current densities. After annealing, we measured transport jc values up to 2.3×105 A/cm2 at 4.2 K in a field of 1.5 T (corresponding to an Ic of 300 A) in a MgB2/Ni tape and up to 104 A/cm2 at 4.2 K in 6.5 T in a MgB2/Fe tape. For higher currents these monofilamentary tapes quenched due to insufficient thermal stability of filament thickness above 50 mum. Thus, jc at 4.2 K, 0T can only be extrapolated, yielding a value close to 1 MA/cm2. The recrystallization during annealing leads to a densification and to jc values which are more than a factor of ~10 higher than those measured in the as-deformed tapes. Ni shows a limited reaction with MgB2, leading to the formation of MgB2Ni 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 multifilamentary tapes with self-field jc values at 4.2 K in excess of 1 MA/cm2. .
    AIP Conference Proceedings 01/2002; 614B.
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    ABSTRACT: We report on the preparation of dense monofilamentary MgB2/Ni and MgB2/Fe tapes with high critical current densities. In annealed MgB2/Ni tapes, we obtained transport critical current densities as high as 2.3×105 A/cm2 at 4.2 K and 1.5 T, and for MgB2/Fe tapes 104 A/cm2 at 4.2 K and 6.5 T. An extrapolation to zero field of the MgB2/Fe data gives a critical current value of ∼1 MA/cm2, corresponding to a critical current value well above 1000 A. The high jc values obtained after annealing are a consequence of sintering densification and grain reconnection. Fe does not react with MgB2 and is thus an excellent sheath material candidate for tapes with self-field jc values at 4.2 K in excess of 1 MA/cm2. © 2001 American Institute of Physics.
    Applied Physics Letters 11/2001; 79(19):3116-3118. · 3.52 Impact Factor
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    ABSTRACT: Control of precursor density during the deformation process of Bi,Pb(2223) tapes is crucial in order to achieve a good homogeneity of the filaments. When an excessive density is reached too early, the ceramic grains cannot longer slide properly, which results in an more pronounced sausaging. Two-axes rolling is known to yield a higher homogeneity among the filaments. On the other hand, the ceramic density increases much faster than during conventional deformation processes. Therefore, we used the combination of two-axis rolling and longitudinal stress. We present density, texture and transport measurements both during the deformation process and after the heat treatment. Results clearly show an improvement in the control of microstructure resulting in higher critical current densities in tapes deformed under longitudinal stress compared to tapes deformed without stress.
    Physica C Superconductivity 01/2001; 357:1119-1122. · 0.72 Impact Factor
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    ABSTRACT: We present magnetization data on a series of Bi2.2-xPb x(2212) single crystals with x varying from 0.0 to 0.8, confirming for the first time and also extending earlier reports on drastic improvements of the flux pinning with lead doping. The crystals were grown by slow cooling a melt in a temperature gradient. The spontaneous formation of a solid `capping' layer prevented the evaporation of Pb, which was verified by monitoring weight losses during the whole heat treatment. X-ray and microprobe analysis confirm that Pb is incorporated in the 2212 structure up to a solubility limit of x=0.8. The magnetization loops measured at various temperatures show a systematic improvement of the flux pinning and the critical current density with doping level, especially at temperatures above T=35 K. At T=70 K, this increased pinning leads to irreversibility fields which are a factor >50 higher in the x=0.8 crystals than in the undoped crystals, with the appearance of a peak effect all the way up to Tc . The marked difference between the pinning behavior at lower and higher temperatures seems to correspond to two distinct pinning regimes
    IEEE Transactions on Applied Superconductivity 01/2001; 11(1):3643-3646. · 1.20 Impact Factor
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    ABSTRACT: The critical current density of four MgB2 samples was measured inductively and for one of them also by transport. Pure phased and dense bulk samples yield a critical current density which in value, as well as in its magnetic field and temperature dependence, is essentially the same as the intra-granular current density measured in a dispersed powder. Also the correspondence between the inductive and transport data indicates that the grain boundaries in the bulk samples are totally transparent for the current. The current–voltage relation becomes rapidly shallow in the vicinity of a depinning line, well below the second critical field. Between the depinning line and the second critical field the material is ohmic and shows a marked magneto-resistivity, indicative of a flux-flow regime.
    Physica C Superconductivity 01/2001; · 0.72 Impact Factor
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    ABSTRACT: The control of precursor density during the deformation process of Bi,Pb(2223) tapes is of vital importance in order to achieve good homogeneity of the filaments. Above a certain density, the ceramic grains cannot slide anymore during deformation, leading to an increased sausaging. Two axial rolling is known to yield a higher homogeneity among the filaments. However, the ceramic density reaches a relatively high level after a few deformation steps. Therefore, we used a new combination of two axial rolling and longitudinal tension, in order to control the density of the tapes. We present the evolution of density and degree of texture, both during the deformation process and after the heat treatment. Results clearly show an improvement in the control of microstructure resulting in higher critical current densities in tapes deformed under longitudinal tension compared to tapes deformed without stress. A correlation was established between the level of longitudinal tension and the transport jc
    IEEE Transactions on Applied Superconductivity 01/2001; 11(1):3744-3747. · 1.20 Impact Factor
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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, μ0Hirr, while the upper critical field, μ0Hc2, remained unchanged. The anisotropy ratio of μ0Hc2 between magnetic fields parallel and perpendicular to 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.
  • H. L. Suo, C. Beneduce, M. Dhalle