The rapid progress on MgB 2 superconductor since its discovery [1] has made this material a strong competitor to low and high temperature superconductors (HTS) for applications with a great potential to catch the niche market such as in magnetic resonant imaging (MRI). Thanks to the lack of weak links and the two-gap superconductivity of MgB 2 [2,3] a number of additives have been successfully used to enhance the critical current density, J c and the upper critical field, H c2 . [4][5][6][7][8][9][10][11][12] Carbon nanotubes (CNTs) have unusually electrical, mechanical and thermal properties [13][14][15][16] and hence is an ideal component to fabricate composites for improving their performance. To take advantages of the extraordinary properties of CNTs it is important to align CNTs in the composites.Here we report a method of alignment of CNTs in the CNT/MgB 2 superconductor composite wires through a readily scalable drawing technique. The aligned CNT doped MgB 2 wires show an enhancement in magnetic J c (H) by more than an order of magnitude in high magnetic fields, compared to the undoped ones. The CNTs have also significantly enhanced the heat transfer and dissipation. CNTs have been used mainly in structural materials, but here the advantage of their use in functional composites is shown and this has wider ramifications for other functional materials.

The influence of heating rates and annealing temperatures on the transition temperatures (T c ) and critical current densities (J c ) of pure MgB 2 , carbon nanotube-and nano-SiC-doped in situ monofilamentary MgB 2 /Fe wires was investigated. It was found that higher J c was obtained for pure MgB 2 samples when heat treated with slower heating rates. SiC-doped samples also have higher J c with slower heating rates, but the J c is less sensitive to annealing temperatures. However, the J c of the carbon nanotube-doped wire was found to be insensitive to heating rates. The variation in T c and J c with heating rate, and the different behaviors of differently doped MgB 2 /Fe wires, make it essential to carefully select the optimum heating rates for heat treatment.

A combined process of a mechanical ball milling and liquid glycerin (C 3 H 8 O 3 ) treatment of boron (B) powder has been conducted to enhance the superconducting properties of MgB 2 . The individual aims of the mechanical milling and the glycerin treatment were to reduce the grain size of the MgB 2 and to achieve homogeneous carbon (C) incorporation into the MgB 2 , respectively. Four kinds of B powders of as-received, glycerin treated, 2 h milled and 2 h milled + glycerin treated were prepared. MgB 2 bulks were fabricated by an in situ process using the prepared B powders. According to our experimental results, the mechanical ball milling was effective for grain refinement, and a lattice disorder was easily achieved by glycerin addition. It was found that the critical current density (J c ) values were enhanced in the samples with milled B or glycerin-treated B only. In the MgB 2 bulk prepared with both milled and glycerin-treated B, the J c was further increased to 1.27 × 10 4 A cm −2 at 5 K and 8 T due to a higher grain boundary density and a greater C substitution. The upper critical field and irreversibility field were also further enhanced by a combined treatment process of the B powder.

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