T Melišek

The role of MgO content in ex situ MgB2 wires

An experimental study of the effect of MgO content in the MgB2 powder used for ex situ made composite wires was carried out. Two single-core MgB2/Fe/Cu wires were made using commercial MgB2 powders from Alfa Aesar containing different fraction of MgO. Critical temperature and critical currents of as-deformed and heat-treated wires were measured. The differences between the wires are discussed and correlated with the MgO content. It was found that by increasing the amount of MgO, the inter-grain connectivity worsens, but well distributed and low size MgO particles improve flux pinning.

Improvement of the current carrying capability of ex situ MgB2 wires by normal particle additions

A wide variety of powders, including Nb, Ti, Zr, Hf and W, metal oxides like Nb2O5, Ti2O5, V2O5 and other compounds like SiC, SrCO3 and boron nitride, have all been used at the 10 wt% level as additions into commercial MgB2 powder for the fabrication of single-core, ex situ wires using groove and two-axial rolling deformation and final heat treatment at 950 °C /0.5 h in Ar. Transport current measurements have shown that metal particle addition leads to an especially significant improvement in current carrying capacity. The presence of normal particles influences both the resistivity and also the thermal conductivity of the MgB2 core, and the improved internal stability may be responsible for the increased critical current values. This observation may be very important for future development of practical MgB2 composite superconducting wires.

Relation between critical current and exponent n in Bi(2223)/Ag tapes

Transport critical currents at and exponents n of the power law characteristics of single-filament Bi(2223)/Ag tapes were measured at 77 K in the self-field and in external magnetic fields up to 1.2 T. The effects of tape orientation in a transverse magnetic field from to (related to the basic state at -axis) were investigated as well. The values of the transport critical current and exponent n as well their mutual relation reflect the quality of the Josephson-linked current network connectivity at ( is the cross-over field at which the weak-link currents cease to contribute to the transport current) and the ability of pinning centres to pin flux lines in the whole magnetic field region. The weak-link connectivity and the pinning potential are directly related to the structural state of the samples resulting from their different thermomechanical treatments. The better the structural quality, the higher the ratio. This ratio seems to reflect the effective number of intergrain connections and pinning centres.

Transport currents of two-axially rolled and post-annealed MgB2/Fe wires at 4.2 K

Single- and four-core MgB2 wires have been made by the powder-in-tube (PIT) method using a commercial MgB2 powder (Alfa Aesar) and two-axial rolling deformation in an Fe sheath. The deformed wires were subjected to annealing at temperatures from 950 ?C up to 1100 ?C for 30 minutes in argon. The interface layer thickness of 10?25.4 ?m caused by inter-diffusion and reaction between the MgB2 core and the Fe sheath has been observed in all annealed wires. Transport currents of as-deformed and post-annealed wires were measured at the temperature 4.2 K and external magnetic field B = 0?5.5 T. The presence of a ferromagnetic sheath in MgB2 composite wire influences the Ic(B) characteristics leading to Ic hysteresis and Ic anisotropy, which is more pronounced for those that have not been annealed (weakly linked grains) in comparison to well-connected MgB2 grains after recrystallization. A comparison of the overall current densities versus the external magnetic field of NbTi, Nb3Sn and Bi-2223 superconductors with four-core MgB2 wire is also presented.

Properties of stabilized MgB2 composite wire with Ti barrier

Stabilized four-filament in situ MgB 2 /Ti/Cu/Monel composite wire was produced by the rectangular wire-in-tube (RWIT) technique. 10 wt% of nanosize SiC was added into the Mg-B powder mixture, which was packed into the Ti/Cu and Monel tubes, respectively. The assembled composite was two-axially rolled into wire and/or tape form and sintered at temperatures of 650-850 °C/0.5 h. Stabilized MgB 2 wire with Ti barrier is studied in terms of field-dependent transport critical current density, effects of filament size reduction and thermal stability.Stabilized four-filament in situ?MgB2/Ti/Cu/Monel composite wire was produced by the rectangular wire-in-tube (RWIT) technique. 10?wt% of nanosize SiC was added into the Mg?B powder mixture, which was packed into the Ti/Cu and Monel tubes, respectively. The assembled composite was two-axially rolled into wire and/or tape form and sintered at temperatures of 650?850??C/0.5?h. Stabilized MgB2 wire with Ti barrier is studied in terms of field-dependent transport critical current density, effects of filament size reduction and thermal stability.

Influence of the winding geometry on the critical currents and magnetic fields of cylindrical coils made of Bi(2223)Ag anisotropic tapes

A theoretical model for the calculation of the critical currents of cylindrical magnets made of Bi(2223)Ag tapes has been developed. The model takes into account the real angle dependence of the critical current versus external magnetic field of a short sample, which enables to estimate the critical currents of individual turns of the winding. As a result figures in a 3-D representation showing the position of the weak spots in the winding were obtained. Consequently, a detailed analysis of the influence of the winding geometry on the critical currents and magnetic fields of the cylindrical magnets formed from the set of pancake coils was performed. It is shown that when optimizing the magnet geometry with respect to the inner bore diameter and the overall tape length, there exists only one winding configuration for which the central magnetic field has a maximum. An example of the optimum magnet design, utilizing the data measured on a 55 filament Bi(2223)Ag tape of Vacuumschmelze GmbH production, is presented.

Behaviour of filamentary MgB2 wires subjected to tensile stress at 4.2 K

Different filamentary MgB2 wires have been subjected to tensile stress at 4.2 K. Stress–strain and critical current versus stress and strain characteristics of wires differing by filament architecture, sheath materials, deformation and heat treatment were measured and compared. It was found that the linear increase of critical current due to the pre-compression effect (ranging from 5% up to ≈20%) is affected by thermal expansion and the strength of used metallic sheaths. The values of irreversible strain eirr and stress σirr depend dominantly on the applied outer sheath and its final heat treatment conditions. Consequently, the strain-tolerance of MgB2 wires is influenced by several parameters and it is difficult to see a clear relation between Ic(e) and σ(e) characteristics. The lowest eirr was measured for Monel sheathed wires (0.3–0.6%), medium for GlidCop® sheath (0.48–0.6%), and the highest eirr = 0.6–0.9% were obtained for MgB2 wires reinforced by the stainless steel 316L annealed at temperature between 600 and 800 ° C. The highest eirr = 0.9% and σirr = 900 MPa were measured for the work-hardened steel, which is not considerably softened by the heat treatment at 600 ° C/2.5 h.

Dependence of the critical current in ex situ multi- and mono-filamentary MgB2/Fe wires on axial tension and compression

The transport critical current Ic of single- and four-core MgB2/Fe wires under tensile and compressive axial strain e was measured with a U-shaped spring set up. In all wires, Ic increases linearly and reversibly with applied tensile strain, up to a sample-dependent reversible strain limit eirr. The same reversible and linear Ic(e) relation is found when applying compressive strain. This shows how the Ic(e) increase with tension can be understood as a gradual release of the thermal pre-compression strain induced in the MgB2 filaments of the composite. The value of the reversible strain limit eirr depends mainly on the degree of pre-compression, but also on the shape of the composite wire. It is highest in a square wire and lowest in the flattest tape. Tensile strain levels above eirr cause an irreversible degradation of Ic. This Ic(e) degradation, due to filament cracking, is significantly steeper in heat-treated wires than in as-deformed ones. The different Ic(e) regimes are analysed and the mechanical behaviour of the MgB2/Fe composite is compared with that of typical Nb3Sn wires and Bi,Pb(2223) tapes.

Structure, grain connectivity and pinning of as-deformed commercial MgB2 powder in Cu and Fe/Cu sheaths

Single-core MgB2 wires and tapes have been made by the powder-in-tube (PIT) method using commercial MgB2 powder (Alfa Aesar). Composites have been made using the two-axial rolling process in Cu and/or Fe/Cu sheaths. Alternative deformations by wire drawing, rotary swaging and cold isostatic pressing have been applied to PIT wires and tapes. Current–voltage characteristics and transport current densities in the self-field and in the external field were measured. It was found that the grain connectivity of ex situ MgB2 is affected by the applied sheath and the mode of deformation. Two-axial rolling has generated the highest powder density resulting in the best grain connectivity. The highest transport current densities of 8700 A cm−2 and 55 830 A cm−2 were measured for Cu and Fe/Cu sheathed square wires, respectively. Cold isostatic pressing at 1.5 GPa has increased current density and n-exponent, which suggests an improvement in grain connectivity. It was found that the external pressure improves the inter-grain connectivity but decreases the pinning in MgB2 cores.

BSCCO/Ag tapes made by a tape-in-rectangular tube process

Multi-core tape composites with various filament configurations have been made by a tape-in-rectangular tube (TIRT) process. This TIRT process is based on the deformation of multifilamentary composites assembled from a rectangular Ag (Ag-alloy) tube filled by pre-deformed single-core BSCCO/Ag tapes. Two-axial rolling induces texture in the filaments in two rolling planes, with the filament planes oriented parallel or perpendicular to the tape width, thus reducing the high Ic anisotropy which can degrade the total current in coils working at high temperatures (77 K). The homogeneity of the filaments was studied by a magneto-optical technique. Roll-sintered samples were measured at 77 K in an external magnetic field up to 1 T and at constant fields (0.1 T and 0.5 T) at various orientations (between parallel and perpendicular) to the tape surface. The results demonstrate the possibility of using these tapes for selected parts of a coil winding to reduce the apparent effect of the radial field component on the reduction of the total coil current.