Monofilamentary MgB2 Wires Research Articles

Critical current density defined at low electric field criterion and energy margin of superconducting MgB2 wires in wide temperature and magnetic field range

High-temperature superconductors (HTS) and MgB2 may potentially improve the usability of superconducting magnets dramatically owing to their large energy margin. When HTS and MgB2 wires are used for magnets operated in the persistent current mode, such as in magnetic resonance imaging (MRI) scanners, the electric field generated in the wires must be lower than 10−10 V m−1. In this paper, critical current density, J c, defined at an electric field criterion of 10−10 V m−1 is evaluated from a magnetisation decay measurement for state-of-the-art monofilamentary MgB2 wires. By using the obtained J c, a critical line of our multifilamentary MgB2 wire is shown on the temperature-magnetic field plane. Here, the critical line is defined as a line on which an electric field of 10−10 V m−1 is generated at a coil current density of 150 A mm−2. The area inside the critical line is demonstrated to be large enough to fulfil the requirement of 1.5 T MRI scanners operated at 10K–15K. In addition, the iso energy-margin lines are shown on the temperature-magnetic field plane and compared with those of NbTi wires. The MgB2 wire has an order of magnitude greater energy margin than the NbTi wires in most of the area inside the critical line. This suggests that the MgB2 wire is highly unlikely to be quenched due to mechanical disturbances.

Effect of the premixing of MgB2 powder on microstructures and electromagnetic properties in PIT-processed MgB2 wires

Nowadays, in situ- and ex situ-processed MgB2 superconducting wires are commercially available. A premix process is a hybrid of the in situ and ex situ processes, in which a mixture of magnesium, boron, and MgB2 powders is used as a precursor. Hence, the premix process should cause less trouble in the manufacturing of practical wires. In this study, to clarify the potential regarding the critical current density of the premix process, the mono-filamentary MgB2 wires are prepared at premix ratios, xp, of 0.25, 0.50, and 0.75 by drawings with a large total area reduction ratio of 99.92%, and their electromagnetic properties and microstructures are investigated. Here, xp is defined as the mass ratio of MgB2 to the total mixture; that is, the composition of the mixture is Mg: B: MgB2 = 1 − xp: 2(1 − xp): xp on a molar basis. Although a high MgB2 packing factor is obtained by the premix process, it does not lead to an increase in electrical connectivity of MgB2 filaments because of the poor sinterability of the premixed MgB2 powder. On the other hand, strong electron scattering, which is introduced to the premixed MgB2 particles in the pulverisation process, leads to the enhancement of the upper critical field in the low temperature region. As a result, the critical current density in a low temperature and high magnetic field region is improved in wires prepared at xp = 0.50.

An innovative technique to synthesize C-doped MgB2 by using chitosan as the carbon source

Here, we report a new technique to synthesize carbon-doped MgB2 powder. Chitosan was innovatively used as the carbon source during the synthesis of boron from boron oxide. This allowed the introduction of local defects, which later on served as pinning centers in MgB2, in the boron lattice itself, avoiding the traditional and time consuming ways of ex situ MgB2 doping (e.g. ball milling). Two volume percentages of C-doping have been tried and its effect on the superconducting properties, evaluated by magnetic and transport measurements, are discussed here. Morphological analysis by scanning electron microscopy revealed nano-metric grains’ distribution in the boron and MgB2 powder. Mono-filamentary MgB2 wires have been fabricated by an ex situ powder-in-tube technique by using the thus prepared carbon-doped MgB2 and pure MgB2 powders. Transport property measurements on these wires were made and compared with MgB2 wire produced using commercial boron.

High critical current density in MgB2/Fe wires

High-quality monofilamentary MgB2wires with Fe sheath were fabricated by the powder-in-tube (PIT) method at ambient pressure. The phase compositions, microstructure features and flux pinning properties are investigated by using x-ray diffractometer, SEM, TEM and a standard four-probe technique. It is found that MgB2 has no reaction with Fe. The results indicate that MgB2 wires are dense and only MgB2 and few MgO phases exist in the core area. MgB2/Fe wires exhibit very high transport critical current densities of 1.43 × 105 A cm−2 (4.2 K, 4 T), 2.8 × 104 A cm−2 (4.2 K, 6 T), 3.72 × 104 A cm−2 (15 K, 4 T) and 2.34 × 104 A cm−2 (25 K, 3 T). The maximum flux pinning force shows a peak at 4 T at 4.2 K, suggesting strong pinning at high fields. The reasons for the large Jc in the sample was also discussed.

Mechanically reinforced MgB 2 wires and tapes with high transport currents

Monofilamentary MgB 2-wires with a 2- or 3-component sheath containing mechanical reinforcing stainless steel (SS) were prepared and characterized. In direct contact to the superconductor Nb, Ta or Fe was used. For a selection of samples with a Fe and Fe/SS sheath, we investigated the transport critical current behaviour in magnetic fields changing systematically the geometrical shape from a round wire to a flat tape. A strong increase of the current densities in flat tapes was observed and possible reasons for this are discussed. Reinforcing the sheath in the outer layer with different amounts of stainless steel leads to a systematic field dependent decrease of the transport critical current density with increasing steel amount. This is an indication for a pre-stress induced degradation of the critical currents in MgB 2 wires and first I c–stress–strain experiments seem to confirm this observation and interpretation.

High transport currents in mechanically reinforced MgB2 wires

We prepared and characterized monofilamentary MgB2 wires with amechanically reinforced composite sheath of Ta(Nb)/Cu/steel, which leads todense filaments and correspondingly high transport currents up toJc = 105 A cm-2 at 4.2 K under self-field. The reproducibility of themeasured transport currents was excellent and did not depend on the wirediameter. Using different precursors, commercial reacted powder or anunreacted Mg/B powder mixture, a strong influence on the pinning behaviour andthe irreversibility field was observed. The critical transport current densityshowed a nearly linear temperature dependency for all wires being still52 kA cm-2 at 20 K and 23 kA cm-2 at 30 K. Detailed data forJc(B,T) and Tc(B) were measured.