P. Toulemonde

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.

Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2

Abstract The two-gap structure in the superconducting state of MgB 2 gives rise to unusual thermodynamic properties which depart markedly from the isotropic single-band BCS model, both in their temperature- and field dependence. We report and discuss measurements of the specific heat up to 16 T on ceramic, and up to 14 T on single crystal samples, which demonstrate these effects in the bulk. The behavior in zero field is described in terms of two characteristic temperatures, a crossover temperature T c,π ≅13 K, and a critical temperature T c = T c,σ ≅38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field μ 0 H c2,π ≅0.35 T, and an anisotropic upper critical field with extreme values μ 0 H c2,σ, c ≅3.5 T and μ 0 H c2,σ, ab ≅19 T, where the indexes π and σ refer to the three-dimensional and two-dimensional sheets of the Fermi surface. Irradiation-induced interband scattering tends to move the gaps toward a common value, and increases the upper critical field up to ∼28 T when T c ≅30 K.

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.

Specific heat of MgB2 after irradiation

We studied the effect of disorder on the superconducting properties of polycrystalline MgB2 by specific-heat measurements. In the pristine state, these measurements give a bulk confirmation of the presence of two superconducting gaps with 2Δ0/kBTc = 1.3 and 3.9 with nearly equal weights. The scattering introduced by irradiation suppresses Tc and tends to average the two gaps although less than predicted by theory. We also found that by a suitable irradiation process by fast neutrons, a substantial bulk increase of dHc2/dT at Tc can be obtained without sacrificing more than a few degrees in Tc. The upper critical field of the sample after irradiation exceeds 28 T at T → 0.

Bi,Pb(2223) equilibrium and decomposition: in situ high-temperature neutron diffraction study

Abstract The non-equilibrium reaction path which is commonly used to form the Bi,Pb(2223) phase inside Ag-clad tapes involves a small amount of a local transient liquid. New high-temperature reaction paths involving a larger amount of a stable liquid that are closer to equilibrium are needed to improve the Bi,Pb(2223)–Ag tape performance and achieve their high expected potential. In order to understand the reactions involved at high temperature during the Bi,Pb(2223) decomposition, an in situ neutron diffraction study has been carried out. Both Bi,Pb(2223) pellets and Ag-clad tapes have been melted and slowly cooled at different rates under various oxygen partial pressures. A phase analysis from the full pattern profile refinement has been performed. Bi,Pb(2223) decomposes into a Bi,Pb-rich liquid and (Ca,Sr)2CuO3 which is found to be the main phase at high temperature regardless of the oxygen partial pressure. Direct crystallisation of Bi,Pb(2223) from the melt has been observed in Ag-sheathed tapes and the critical current can be partially recovered after melting and slow cooling.

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.

Specific heat of ceramic and single crystal MgB2

Abstract The two-gap structure of MgB 2 gives rise to unusual thermodynamic properties which depart markedly from the single-gap BCS model, both in their temperature- and field-dependence. We report measurements of the specific heat up to 16 T on ceramic and single crystal samples, which demonstrate these effects in bulk. The low-temperature mixed-state specific heat reveals a field-dependent anisotropy, and points to the existence of unusually large vortices, in agreement with local density-of-states measurements by scanning tunneling spectroscopy. It is finally shown that a suitable irradiation process nearly doubles H c2 in the bulk.