金属系超電導線材 [7] ― MgB2 線材―

Since the discovery in 2001, MgB2 has gained much interests as a new hope of superconductors for its applicability at temperatures of liquid H2 (~20 K) or liquid Ne (~27 K). Then large efforts have been dedicated to the processing of MgB2 wires. Ex situ and in situ techniques are two major techniques for the fabrication of MgB2 wires. The ex situ process uses previously reacted MgB2 powders filled into appropriate sheath tubes which are then fabricated into a wire. In the in situ process a mixture of Mg and B powders is encased in a sheath tube, and after the wire fabrication, the MgB2 core is synthesized in situ by the heat treatment. C or SiC nano-powders are effective dopants to enhance the high-field performance of MgB2 wires. Wires a few kilometers long have been produced via both processes. The diffusion process starting from a composite of Mg rod or pipe and B powder is another promising process in MgB2 wire fabrication. The connectivity of MgB2 grains is still poor due to the insufficient packing factor of powders, voids caused by the volume contraction during MgB2 formation, and the envelope of MgO and other insulating phase around MgB2 grains. This poor connectivity reduces the critical current density of MgB2 wires. Improved powder quality, fabrication techniques, and dopants may appreciably improve the performance of MgB2 wires. Presently, the main target of MgB2 wire application is direct-current use at 20 K, such as for MRI magnets. In this article, material aspects, fabrication process, superconducting performance and future possibilities of MgB2 wires are briefly surveyed.