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The simultaneous addition of various carbon based additives (in the present case B4C + SiC) to Mg and B powders has been introduced as a new concept in view of enhancing the superconducting parameters Bc2, Birr and Jc values of in situ Fe/MgB2 wires. A series of Fe sheathed monofilamentary wires of 1.1 mm diameter with a MgB2 core of 600 mum diameter was prepared with various B4C : SiC ratios, the relation being 2.5:7.5, 5:5, 7.5:2.5 and 7.5:7.5 (values in wt.%). After reaction of 1 hour at 760degC, the wire containing 7.5 wt.% B4C and 2.5 wt.% SiC powders exhibited a Jc value of 1 times 104 A/cm2 at 11.3 T and 4.2 K. Although only 2.5 wt.% SiC were added, these values are considerably above those of ternary wires with B4C additions, where the same Jc value is obtained at 10 T. The slope Jc vs. B for the B4C + SiC wires is steeper than for SiC additives, the Jc values at 4.2 K being superior at fields below 9 T. The lattice parameters a of the B4C + SiC added wires exhibit lower values than ternary wires with the same nominal C content, suggesting a higher C content in the MgB2 phase. The "disorder" in the MgB2 structure has been characterized as a partial substitution of B by Carbon. A reduction of the domain sizes in the c direction as well as in-plane to 250 Aring and 124 Aring has been determined from the FWHM and the breath values of the (110) and (002) peaks. With the simultaneous introduction of B4C + SiC, a strong improvement of Jc and Birr has been obtained with respect to B4C additions. A further enhancement of Jc is expected when using different combinations of additives with and without carbon, aiming for a further raise of Jc in wires with multiple additives, as a result of the combination of different mechanisms.

A correlation between the anisotropy of the critical current density J c in ex situ monofilamentary MgB2/Fe tapes and the degree of texturing of the MgB2 filaments has been found. After removing the Fe sheath, X-ray diffraction measurements were performed at various depths of MgB2 filaments. A texture gradient was detected, the degree of texturing being highest for the MgB2 grains close to the Fe interface, the grains at the centre being almost untextured. The J c anisotropy Gamma = J c(par)/J c(perp), was found to be highest for the larger initial particle grain size (average 4 mum), a strong decrease being observed for the smaller ones (1 mum). The texturing of MgB2 grains (and thus the anisotropy) is caused by the contact with Fe sheath surface during the cold deformation of the tapes, while in hot rolled tapes, a random orientation of the MgB2 grains was observed in the whole filament. From our data, it follows that almost no J c anisotropy would be expected in ex situ Fe/MgB2 tapes with submicron particle sizes and a sharp distribution. An anisotropic behavior of the irreversibility field B irr was also found by inductive measurements, the highest value of B irr being observed for the smallest particle diameters. The pinning force was found to scale only for measurements performed in perpendicular field. Using a simple model which takes into account the observed texture gradient, i.e. the filament volume consisting of a textured and an untextured part, scaling was also obtained in the case of fields parallel to the tape surface.

Ta and Ti are the most widely used additions for technical Nb3Sn multifilamentary superconductors. These elements are known to influence grain growth, grain morphology and chemical composition in the A15 layer, hence the current carrying properties of the wires over a wide magnetic field range. So far only few studies tried to compare systematically Ta and Ti doped and undoped Nb3Sn wires in the frame of the same work, down to a nanometric scale. We present an investigation on several multifilamentary (Nb, Ta, Ti)3Sn bronze route wires, fabricated at a laboratory scale, with various amounts of additives. The wires consist of fine filaments embedded in a Cu-Sn or Cu-Sn-Ti Osprey bronze with > 15 wt.% Sn and an external Cu stabilization. Microstructural observations are compared with the results of Jc and n values measured up to 21 T at 4.2 and 2.2 K, and for longitudinal strains up to 0.5%. Non-Cu Jc values up to 300 Amm−2 and n values up to 50 at 17 T and 4.2 K show clearly that wires with Ti addition to the bronze have a better performance with respect to wires with Ti additions to the filaments.

Superconducting Fe-sheathed MgB2 monofilamentary tapes have been fabricated by the powder-in-tube technique, varying the particle size of the starting MgB2 powder and applying either cold or hot rolling during the last deformation process. Measurements of the critical current density Jc with the magnetic field applied parallel or perpendicular to the tape surface revealed a pronounced anisotropy for the cold rolled tapes, which was found to increase with increasing particle size and magnetic field strength. The microstructural origin of the Jc anisotropy was confirmed by means of x-ray diffraction performed on the filaments after mechanical removal of the sheath. The local texture was studied in a series of diffraction patterns collected at different distances from the filament centre, removing each time some 25 µm of the filament thickness. In the cold rolled tapes, the average orientation of the MgB2 grains was found to approach a preferred orientation with the crystallographic c-axis perpendicular to the tape surface, near the interface with the sheath; however, the misalignment angle increased towards the centre of the tape. In the hot rolled tapes, for which no Jc anisotropy was observed, the grains were found to be approximately randomly oriented. Roughness measurements performed on the side of the Fe sheath in contact with the MgB2 filament are in agreement with the difference in texture observed for cold and hot rolled tapes.

We report a study of the pinning properties of Fe/MgB/sub 2/ tapes performed by means of ac and dc magnetic measurements. In particular, the third harmonic response of the ac susceptibility has been measured as a function of the dc magnetic field up to 9 T, at different temperatures, for various frequencies and amplitudes of the ac field. The irreversibility line has been determined using the third harmonic onset criterion for samples made with different ball milled MgB/sub 2/ powders. The study of the harmonic response allows to probe the nature of the vortex pinning and to analyze its influence on the enhancement of the irreversibility field in strongly ball-milled samples. We have investigated the dynamic regimes governing the vortex motion in the tapes by comparing the experimental curves with numerical simulations of the nonlinear diffusion equation for the magnetic field. Furthermore, the pinning mechanism governing the electrical transport properties has been determined by studying the magnetic field dependence of the volume pinning force F/sub P/=J/sub c//spl times/B, calculated by extracting the critical current density J/sub c/(B,T) from magnetic hysteresis loops. Scaling of the F/sub P/(B) curves at different temperatures allows us to establish that the pinning centres are confined to the grain boundaries.

A-priori knowledge of the normal zone development in MgB/sub 2/ conductors is essential for quench protection of applications. Therefore the normal zone propagation in a monofilament MgB/sub 2//Fe conductor under near-adiabatic conditions at 4.2 K has been measured and simulated. The results show normal zone propagation velocities up to several meters per second. In addition, by including the voltage-current relation into the computational model, the influence of the n-value on the normal zone propagation is determined. The simulations show that lower n-values suppress the normal zone propagation velocity due to lower heat generation in the MgB/sub 2/ filaments.

We have analyzed the influence of the two different Ti doping methods on the superconducting properties of bronze processed (Nb,Ta,Ti)/sub 3/Sn multifilamentary wires with identical configuration, consisting of a Nb7.5wt.%Ta alloy processed to 14641 filaments of 4.5 micrometer size embedded in an Osprey bronze and externally stabilized by Cu. The first doping method introduces NbTi rods into NbTa filaments in a Cu15.4Sn Osprey bronze and has been applied to two wires containing 1 and 2 wt.%Ti in the filaments, respectively. The second method uses a Ti added Cu15.5Sn0.25Ti Osprey bronze, while the filaments of two thereby fabricated wires contain 0 and 0.5wt.%Ti, respectively. A reference wire containing no Ti addition was also prepared. All five wires were manufactured by three hot extrusion steps, cold drawing and several intermediate anneals. Wire samples with round cross section have been prepared and reacted between 600 and 730/spl deg/C. The residual Sn content in the bronze after reaction and the residual niobium ratio of the filaments were determined by means of EDX and SEM analysis, respectively. The grain size of the A15 layer has been compared by FE-SEM and the local composition profile has been observed by systematic quantitative EDX measurements made on a TEM. Non-Cu J/sub c/ values up to 300 Amm/sup -2/ and n values up to 50 at 17 T and 4.2 K reveal clearly the Ti added bronze as more performing.

The anisotropy of the critical current density of Fe-sheathed ex situ MgB/sub 2/ monofilamentary tapes has been measured at various magnetic fields and temperatures and has been found to be influenced by the particle size of the starting material. Already reacted powders were milled in order to obtain different starting particle sizes. In order to understand the origin of the anisotropy, we have undertaken an X-ray study on the degree of texturing in the filaments after mechanical removal of the Fe sheath. The degree of texturing of the filaments at the interface Fe/MgB/sub 2/ and at the center of the filaments has been determined. A moderate texture has been found, being slightly higher at the interface.

Quantitative phase analyses have been performed on Fe-sheathed ex situ MgB2 tapes after the deformation process and after annealing at 700, 850 and 920 °C in vacuum. The techniques used were x-ray diffraction, differential thermal analysis and scanning electron microscopy. All superconducting cores were found to contain certain amounts of MgO, the largest amounts being observed near the sheath, and after annealing at the highest temperature. Small amounts of MgB4 were detected in the inner part of the core, presumably formed by the decomposition of MgB2. At the MgB2/Fe interface, a reaction layer was observed, which was identified as the boride Fe2B. In addition, tapes annealed at 700 °C revealed the presence of the metastable phase Fe23B6. The interaction of the MgB2 filament with the Fe sheath and the presence of trapped oxygen limit the possibilities of the current tape fabrication techniques.

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.

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.

The effects of SiC and Carbon doping on the superconducting properties of MgB2 polycrystalline samples have been analysed by means of specific heat and magnetic relaxation measurements. It is known that the addition of nanometric powders of SiC and C leads to the enhancement of Birr and Jc. However, the underlying physical mechanism is not completely understood. Magnetic relaxation measurements did not show detectable effects of both the additions on the pinning properties of MgB2. It follows that doping acts mainly introducing disorder into the superconductor and thus raising Bc2. In the case of MgB1.9C0.1, specific heat measurements show that the C substitution on the B sites modifies the low temperature shoulder related to the second gap. This effect is not visible in the sample doped with SiC. From the distribution of Tc determined from the deconvolution of the calorimetric data, we argue that SiC leads to an inhomogeneous distribution of C.

An improvement of the transport critical current density, Jc, of MgB2 wires was obtained after addition of 10 wt.% B4C powders, after reaction at 800 C: Jc values of 1.10^4 A/cm2 at 4.2 K and 9T were obtained for wires of 1.11 mm diameter in a Fe matrix. The starting mixture of Mg and B was doped with sub-micrometric B4C, the ratio being Mg : B : B4C = 1: 2 : 0.08, corresponding to 10 wt.% B4C. For T > 800 C, a decrease of Jc was found, due to the reaction with the Fe sheath. In order to investigate the origin of the improvement of the transport properties for heat treatments up to 800 C, X-ray diffraction measurements were performed. A decrease of the lattice constant a from 0.30854 nm to 0.30797 nm was found, thus suggesting an effect of the substitution of Carbon on the properties of the wires. A comparison with the literature data shows that the addition of B4C powders leads to the second highest improvement of Jc reported so far after SiC, thus constituting an alternative for future applications., 18 pages, 8 figures, 1 table

We found that the reduced pinning forces of ex situ MgB2 /Fe monofilamentary tapes do not scale with temperature if the external magnetic field is applied parallel to the tape surface. A model is proposed to explain the behavior of the volume pinning forces in parallel and perpendicular orientations of the magnetic field, accounting for the observed texturing gradient between the center of the filament and the MgB2 /Fe interface. The degree of texturing of the sample, as determined by x-ray diffraction, is confirmed by modeling the pinning force in parallel field as the superposition of two components, the first one due to a highly textured region on the external part of the filament and the latter due to a low texturing region in the core.

Hysteresis and magnetic relaxation measurements have been performed on MgB2/Fe tapes in parallel and perpendicular magnetic field in order to investigate the correlation between the anisotropy of the transport current density and the texturing recently analysed by Lezza et al.. Volume pinning forces FP have been obtained from the current densities Jc as a function of the dc magnetic field up to 9 T in the temperature range 5–35 K. The reduced pinning forces do not scale with temperature in the parallel field configuration. It has been shown that this could be explained considering that the texturing occurs mainly at the surface of the sample. The scaling is recovered in perpendicular field for temperature higher than 20 K and the scaled pinning curves have been compared to theoretical predictions. Furthermore, the anisotropy of the pinning properties has been investigated using the present relaxation measurements. The magnetization was found to decrease linearly with ln(t). This allows calculation of the pinning potential energy U0 by using the relationship U0 = kT/{dM/[M0 dln(t)]}. The resulting variation of U0 in field and temperature is not related to the field orientation. The relaxation rate of MgB2 has been compared to other superconducting compounds, i.e. Bi‐2223 and Nb3Sn.

8 pages, 9 figures, 1 table., The thermal stability of different Fe- and Ni-sheathed MgB2 conductors has been studied experimentally and numerically, focusing on the estimation of the quench propagation velocities, vp, minimum quench energies (MQEs) and minimum propagating zones (MPZs). The measurements have been done at self-field and under adiabatic conditions, at variable temperatures and transport currents. Energy pulses were deposited to the conductor by passing rectangular current pulses through a graphite-based epoxy heater. The measured voltage around the heater together with numerical simulation allows the estimation of the minimum propagating zone. Moreover, vp was obtained by measuring multiple voltage taps and thermocouples attached along the conductor. The effect of the current sharing temperatures, Ic(T), of the superconductor, and thermal and electrical properties of the metal sheath have been analysed. The experimental results are in qualitative agreement with the simulated ones, obtained by solving the one-dimensional heat balance equation of the system., This work was supported by the EU FP6 project NMP3- CT2004-505724 and by the SpanishMinistry of Education and Science, MAT-2002-04121-C03-02. EM would like to thank the ‘Ramón y Cajal’ program (MEC) for her contract.

Fe/MgB2 tapes have been prepared starting with pre-reacted binary MgB2 powders. As shown by resistive and inductive measurements, the reduction of particle size to a few microns by ball milling has little influence on Bc2, while the superconducting properties of the individual MgB2 grains are essentially unchanged. Reducing the particle size causes an enhancement of Birr from 14 to 16 T, while Jc has considerably increased at high fields, its slope Jc(B) being reduced. At 4.2K, values of 5.3*10^4 and 1.2*10^3 A/cm^2 were measured at 3.5 and 10 T, respectively, suggesting a dominant role of the conditions at the grain interfaces. A systematic variation of these conditions at the interfaces is undertaken in order to determine the limit of transport properties for Fe/MgB2 tapes. The addition of 5% Mg to MgB2 powder was found to affect neither Jc nor Bc2. For the tapes with the highest Jc values, very high exponential n factors were measured: n = 148, 89 and 17 at 3.5, 5 and 10T, respectively and measurements of critical current versus applied strain have been performed. The mechanism leading to high transport critical current densities of filamentary Fe/MgB2 tapes based on MgB2 particles is discussed., Presented at ICMC 2003, 25-28 May 2003

Mono- and multifilamentary MgB2/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 jc has been investigated. At 25K and 1 T, jc values close to 105 A/cm2 were measured, while jc of 106 A/cm2 were extrapolated for 4.2K/0T in our monofilamentary tape. MgB2/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 jc values obtained so far in MgB2/Fe wires with 7 filaments were 1.1 * 105 A/cm2 at 4.2 K and in a field of 2 T, which is still lower than for monofilamentary tapes. Improved deformation and recovering processing is expected to lead to higher jc values., 4 pages, 8 figures; Submitted to ASC 2002

A device has been developed to measure the critical current of long length superconductors under uniaxial strain up to 1000A and up to 21T. The latter is based on a modified Walters spring (WASP) where zero applied strain can be precisely controlled. The length of the investigated conductor is on the order of 1m with a typical gauge length between the voltage taps of about 0.5m. This facilitates the measurement of critical currents at an electric field criterion as low as 0.01μV∕cm, which is particularly important for superconductors to be used in magnets which are operated in the persistent mode. The operation of the WASP was simulated by finite element calculations indicating that the radial stress (contact pressure) of the superconductor is less than 5% of the axial stress. The performance of the probe is demonstrated for several Nb3Sn conductors with round and rectangular cross section, respectively, as well as for a Bi-2223 tape.