N. Musolino

Large transport critical currents in dense Fe- and Ni-clad MgB2 superconducting tapes

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.

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.

Transport and inductive critical current densities in superconducting MgB2

Abstract The critical current density of four MgB 2 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.

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.

High-pressure synthesis of pure and doped superconducting MgB2 compounds

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 effects on pinning properties and critical current were investigated, particularly for A = aluminium. The samples were sintered and/or synthesized at high pressure–high temperature in a cubic multi-anvil press (typically 3.5–6 GPa, 900–1000 °C). They were characterized by x-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by ac susceptibility, magnetization (VSM and SQUID) and transport measurements under a magnetic field. Only Al really substitutes 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 with these doping elements added: Tc, critical current jc, 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.

Properties of YBCO films at high current densities: fault current limiter implications

We have studied the properties of superconducting strip lines, based on epitaxial YBCO thin films, at high current densities. Experiments performed with short constant current pulses show that a quasi spontaneous highly dissipative state (HDS) appears at high current densities. By carefully measuring the temperature of the YBCO line, we found that its temperature is still below T/sub c/ when the HDS occurs, indicating that this state does not originate from a thermal runaway. Once initiated, this HDS starts to propagate with an initial velocity larger than 100 m/s, which is one order of magnitude higher than thermal velocities. We also applied constant voltage pulses to the YBCO line, therefore simulating a real short circuit. We found that the spatial extent of the HDS along the line, a few microseconds after the short circuit, depends linearly on the applied voltage. These results allow explanation of the fast switching properties observed in superconducting fault current limiters (SFCL) and the fact that the peak current is limited at, typically, 3 times the critical current.

Origin of the magnetization peak effect in the Nb 3 Sn superconductor

We report a pronounced peak effect in the magnetization and the magnetocaloric coefficient in a single crystal of the superconductor Nb3Sn. As the origin of the magnetization peak effect in classical type-II superconductors is still strongly debated, we performed an investigation of its underlying thermodynamics. Calorimetric experiments performed during field sweeps at constant temperatures reveal that the sharp increase in the current density occurs concurrently with additional degrees of freedom in the specific heat due to thermal fluctuations and a liquid vortex phase. No latent heat due to a direct first-order melting of a Bragg glass phase into the liquid phase is found which we take as evidence for an intermediate glass phase with enhanced flux pinning. The Bragg glass phase can however be restored by a small AC field. In this case a first-order vortex melting transition with a clear hysteresis is found. In the absence of an AC field the intermediate glass phase is located within the field range of this hysteresis. This indicates that the peak effect is associated with the metastability of an underlying first-order vortex melting transition.

Modulation-free phase in heavily Pb-doped (Bi,Pb)2212 crystals

Abstract We report the complete disappearance of the structural modulation in heavily lead-doped Bi 2− x Pb x Sr 2 CaCu 2 O 8+ δ crystals observed by transmission electron microscopy. Crystals with a nominal lead content of x =0.8, corresponding to an effective lead content of x =0.39, yield the non-modulated phase. The superconducting properties of this modulation-free phase (β phase) have been studied and compared to those of undoped crystals displaying the modulated phase (α phase). Magnetisation measurements reveal that the irreversibility field H irr ( T ) and relaxation rates are strongly improved within the β phase. Measurements of the lower critical field, H c1 , show that the anisotropy factor, ϵ , is considerably reduced in the modulation-free crystals. This is the signature of stronger coupling between CuO 2 layers which in turn deeply influences the effectiveness of the pinning. These measurements explain the enhanced pinning properties in moderately Pb-doped crystals in which the α phase and β phase coexist. The enhanced pinning is not only due to the α/β interfaces, which act as effective pinning centers: the emergence of modulation-free domains, characterized by a strongly reduced anisotropy, also significantly contribute to this effect.

Growth and superconducting properties of Bi2Sr2Ca2Cu3O10 single crystals

Single crystals of Bi2Sr2Ca2Cu3O10 (Bi-2223) have been grown using the travelling solvent floating zone technique in an image furnace. Annealing the crystals under high pressures of O2 increased their critical temperature to 109 K, and resulted in sharp superconducting transitions of ΔTc = 1 K. The superconducting anisotropy of Bi-2223 was found to be , from measurements of the lower critical field with the magnetic field applied parallel and perpendicular to the c-axis. The anisotropy of Bi-2223 is significantly reduced compared to that of Bi2Sr2CaCu2O8 (Bi-2212), and this accounts for the enhanced irreversibility fields in Bi-2223. Furthermore, Bi-2223 has a higher critical current density, and a reduced magnetic relaxation rate compared to Bi-2212, which are both signatures of more effective pinning in Bi-2223 due to its reduced anisotropy.

Superconductivity in high-pressure synthesized pure and doped MgB2 compounds

Dense pure and doped (Mg1 − xAx)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, 900–1000°C) 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 Tc, jc critical current, Hirr, and Hc2.