MgB₂ 超伝導バルク磁石の現状と展望

抄録

We have studied the trapped field properties of the MgB₂ bulk superconductors fabricated by various methods. The MgB₂ bulks were magnetized by a filed-cooled magnetization (FCM) method using a superconductor coil and a pulsed field magnetization (PFM) method using a copper coil. We firstly obtained a trapped field of 1.5 T at 14 K by FCM for an MgB₂ disc bulk (30 mm diameter and 7 mm thickness) with a filling factor of about 50 %, which was fabricated by an in-situ capsule method using a home-made capsule. The trapped field of MgB₂ bulk by FCM was enhanced up to 2.9 T by the densification and the enlargement. Such dense MgB₂ bulks were fabricated by an in-situ hot isostatic pressing under the pressure of up to 980 MPa or an ex-situ spark plasma sintering by applying uniaxial stress of 50 MPa. An in-situ infiltration method without the physical pressure also produced the dense MgB₂ bulks, which trapped a 3 T-class magnetic field. The Ti-doping improved considerably the trapped field by FCM up to 5.6 T at 11.3 K for the triple-stacked Ti-doped MgB₂ bulks. The PFM method also produced a Tesla-class MgB₂ bulk magnet. The trapped field of 1.1 T at 13 K was obtained for the HIP-processed MgB₂ disc bulk (22 mm diameter and 20 mm thickness). A magnetic field of 1.6 T was trapped at 20 K in the MgB₂ bulk composite, consisting of two ring-shaped MgB₂ bulks sandwiched by four thin copper plates and an inserted soft-iron yoke cylinder, using double PFM using a split-type coil with softiron yokes. We have shown the potential of the MgB₂ bulk magnets for the practical superconducting applications.