Alignment of Carbon Nanotube Additives for Improved Performance of Magnesium Diboride Superconductors

The rapid progress in developing MgB2 superconductors since its relatively recent discovery has made this material a strong competitor to other lowand high-temperature superconductor (HTS) materials for technological applications, especially in niche markets such as magnetic resonance imaging (MRI). Thanks to the lack of weak links and the two-gap superconductivity of MgB2, [2,3] a number of different additives have been successfully used to enhance the critical current density, Jc, and the upper critical field, Hc2. [4–12] Carbon nanotubes (CNTs) have unusual electrical, mechanical, and thermal properties, and hence are ideal components for inclusion in composites to improve composite performance. To take advantage of the extraordinary properties of CNTs, it is important to align the CNTs in the composites. Here, we report a method for aligning CNTs in CNT–MgB2 superconductor composite wires through a readily scalable drawing technique. The aligned-CNT-doped MgB2 wires show an enhancement in magnetic critical current density, Jc(H), by more than one order of magnitude under high magnetic fields compared to the undoped samples. The CNTs also significantly improve heat transfer and dissipation. CNTs have mainly been used in structural materials, but here the advantages of their use in functional composites are demonstrated, which has wider ramifications for other functional materials. High in-field Jc of MgB2 superconductors is a major requirement for large-scale applications. In addition, MgB2 wires need to exhibit good mechanical properties and thermal stability. The additives studied so far have mainly been used to improve Jc(H) by introducing effective pinning sites in MgB2. Not much attention has been focused on the mechanical and thermal properties of the MgB2 wire core. Among the additives that have been studied, CNTs have the potential to additionally improve the mechanical and thermal properties of the MgB2 wires, since they have been used as reinforcing components in a number of different composites. Fossheim et al. have reported an enhanced flux pinning in Bi2Sr2CaCu2O8 + x superconductors with embedded CNTs. [19] Yang et al. have discovered a significant enhancement in Jc(H) for HTSs by introducing nanorods as columnar pinning centers in the composites. We previously reported that CNT doping enhanced Jc under magnetic fields for bulk MgB2. [12,23] However, in that case the CNTs were randomly dispersed in bulk MgB2. Inspired by the previous reports, in this communication we report a technique for aligning CNTs in Fe-sheathed MgB2 wires. The advantages of doping with CNTs rather than other additives are: 1) Multiwalled CNTs can carry current densities up to 10–10 A cm (compared to a typical value of 10–10 A cm for superconductors), and remain stable for extended periods of time; CNT doping can thus improve the current path and connectivity between the grains in MgB2. 2) The thermal conductivity for isolated multiwalled CNTs is estimated to be about 3000 W mK, and hence could enhance heat dissipation and thermal stability of the MgB2 wires. 3) Bundled CNTs have a very high axial strength and stiffness, approaching values for an ideal carbon fiber. If CNTs could be aligned along the longitudinal axis of the MgB2 wires, it is expected that they would significantly improve the mechanical properties of the CNT–MgB2 composite wire. 4) Finally, CNTs are one-dimensional (1D) materials with very large aspect ratios and can act as line-pinning sites instead of the point-defect-pinning sites provided by other additives. We demonstrate that a high degree of CNT alignment can be readily achieved by mechanical drawing in the powderin-tube (PIT) process. The aligned CNT-doped MgB2 wires show a significant enhancement in flux pinning and heat transfer. Figure 1 shows Jc(H) calculated from magnetization hysteresis loops with the magnetic field, H, applied both parallel C O M M U N IC A IO N S