Paola Lezza

Superconducting properties of MgB2 tapes and wires

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.

Improved critical current densities in B4C doped MgB2 based wires

An improvement of the transport critical current density, Jc, of MgB2 wires was obtained after the addition of 10 wt% B4C powders, after reaction at 800 °C: Jc values of 1 × 104 A cm−2 at 4.2 K and 9 T were obtained for wires of 1.11 mm diameter in a Fe matrix. The starting mixture of Mg and B was doped with submicrometric 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 3.0854 to 3.0797 A 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.

Improved transport critical current and irreversibility fields in mono- and multifilamentary Fe/MgB2 tapes and wires using fine powders

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 influence of Ti doping methods on the high field performance of (Nb,Ta,Ti)/sub 3/Sn multifilamentary wires using Osprey bronze

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.

Transport properties and exponential n-values of Fe/MgB2 tapes with various MgB2 particle sizes

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 4.2 K, values of 5.3 × 104 and 1.2 × 103 A/cm2 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 10 T, 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.

Critical current versus strain measurement up to 21T and 1000A of long length superconducting wires and tapes

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.

Quantitative study of the inhomogeneous distribution of phases in Fe-sheathed ex situ?MgB2 tapes

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.

Transport critical current densities and n factors in mono- and multifilamentary MgB/sub 2//Fe tapes and wires using fine powders

Mono- and multifilamentary MgB/sub 2//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/ has been investigated. At 25 K and 1 T, j/sub c/ values close to 10/sup 5/ A/cm/sup 2/ were measured, while j/sub c/ of 10/sup 6/ A/cm/sup 2/ were extrapolated for 4.2 K/0T in our monofilamentary tape. MgB/sub 2//Fe tapes exhibit high exponential n factors for the resistive transition: n/spl ap/80 and 40 were found at 5 T and 7 T, respectively. The highest transport j/sub c/ values obtained so far in MgB/sub 2//Fe wires with 7 filaments were 1.1/spl times/10/sup 5/ A/cm/sup 2/ at 4.2 K and in a field of E2 T, which is still lower than for monofilamentary tapes. The function F/sub p//spl prop/b/sup p//spl middot/(1-b)/sup q/ has been established over the whole field range, and exhibits a maximum at F/sub p//spl cong/0.18. Improved deformation and recovering processing is expected to lead to higher j/sub c/ values.

Texture gradients in Fe-sheathed ex situ produced MgB2 tapes

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.

Specific Heat and Magnetic Relaxation Analysis of

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.