Materials, characterization, and applications for high T_c superconducting permanent magnets

A program for the development of materials and applications for high temperature superconducting permanent magnets has been pursued in this Institute. We have found that mapping of the magnetic field, plus use of a simple but accurate current model, permits easy measurement/calculation of J_c. This characterization method also shows grain structure simply and clearly, and is applicable to large size samples. The current model indicates that the maximum trapped magnetic field, B_T,max, is approximately given by B_T,max 8 J_cf(d), where f(d) is a monotonically increasing function of the diameter, d, of a single grain. Guided by this resultl our program aims to increase d, and J_c, in order to increase the field. We report on our progress. Currently non-irradiated materials with J_c 13,000 A/cm², and irradiated materials with J_c up to 85,000 A/cm² at 77 K are being applied, and developed further. Presently single grain tiles of ∼ 2 cm diameter in the a,b plane are produced. Tests these are in progress on the newest and best materials, and tests have been completed on earlier materials of smaller d, and lower J_c. Using these earlier materials, magnetic fields of 6,400 G have been retained on single grain irradiated tiles 1.2 × 1.2 × 0.4 cm³ and 2,200 G on single grain non-irradiated tiles of 2 cm diameter, at 77 K. A mini-magnet of dimensions approximately 1.2 × 1.2 × 1.2 cm, made up of 5 proton irradiated tiles, retained a field of 14,200 G at 77 K. A mini-magnet 1.2 × 1.2 × 0.6 cm retained a field of almost 2 T at 77 K. Creep has been studied for long term behavior in saturated samples, for unsaturated samples, and as a function of T, and irradiation. Tiles have been used to provide magnetic field for a motor with 19 Watt output.