Influence of heat-treatment schedules on the transport current densities of long and short segments of superconducting MgB2 wire

Various MgB2/sheath composite strands have been tested using transport and magnetic measurement techniques. The strands had bimetallic sheaths of (1) Fe/monel, (2) Fe/Cu and (3) Cu/Cu. In strands (1) and (2) the Fe played the role of diffusion barrier. Transport current densities of more than 5 x 105 A/cm(2) at 4.2 K in self-field and 2.7 x 10(4) A/cm(2) at 5 T were obtained on short lengths of Fe/monel strand. In coils, 6 x 101 A/cm(2) in 1 T at 4.2 K for Cu/Cu sheath strands and 2.2 x 10(5) for Fe/monel sheath strands have been measured. The strands had diameters of 0.8-1.1 mm and superconducting fractions ranging from 27% to 40% of the total wire cross-section. They were heat-treated (HT) at temperatures of from 675 to 900 degreesC under various heat-treatment schedules. The best results were obtained at 675 degreesC for Cu/Cu and 700 degreesC for Fe/monel strands. (C) 2004 Elsevier B.V. All rights reserved.