Properties of superconducting MgB₂wires:in situversusex situreaction technique

Abstract

We have fabricated a series of iron-sheathed superconducting wires prepared by the powder-in-tube technique from (MgB 2 ) 1−x :(Mg+2B) x initial powder mixtures taken with different proportions, so that x varies from 0 to 1. It turned out that ex situ prepared wire (x = 0) has considerable disadvantages compared to all the other wires in which in situ assisted (0 < x < 1) or pure in situ (x = 1) preparation was used due to weaker inter-grain connectivity. As a result, higher critical current densities J c were measured over the entire range of applied magnetic fields B a for all the samples with x > 0. Pinning of vortices in MgB 2 wires is shown to be due to grain boundaries. J c (B a ) behaviour is governed by an interplay between the transparency of grain boundaries and the amount of 'pinning' grain boundaries. Differences between thermo-magnetic flux-jump instabilities in the samples and a possible threat to practical applications are also discussed.

Transport and magnetic Jc of MgB2 strands and small helical coils

“…This might be expected since the primary role of SiC is to increase the H irr and H c2 . Nevertheless some increases in pinning have also been seen [14][15][16][17] , but not in the present case at higher temperatures and lower fields.. …”

Overcritical state in superconducting round wires sheathed by iron

“…As we have already mentioned previously, 15 for MgB 2 wires prepared by the in situ method the presence of the iron sheath can lead to stronger thermomagnetic flux-jump instabilities. 16 Indeed, as can be seen in Fig.…”

Large upper critical field and irreversibility field in MgB2 wires with SiC additions

“…20 with some increases seen. Hydride-based MgB 2 powder with SiC dopants was also investigated, and seen to give B c2 values of Ϸ23 T. 21 Dou et al [7][8][9][10] showed improved high-field critical current results for SiC, and similar wires measured in this laboratory showed improvements in the apparent irreversibility field as might be extrapolated from transport results. 11 In this work, strands of similar construction were investigated systematically, and high-field resistive transitions were used to demonstrate relatively large values of 0 H irr and B c2 with various kinds of SiC dopants under various heating conditions.…”

“…2͒, and more generally high-field properties in wires with SiC. [7][8][9][10][11] Bulk samples which had been exposed to excess Mg vapor showed high B c2 values as well, with a B c2 ͑0͒ estimate of 29 T.…”