Materials developments and applications of Bulk RE-Ba-Cu-O superconductors

Before the discovery of high-temperature superconductors (HTSs), the applications of bulk superconductors were generally deemed not practical mainly due to their low cryostability [1]. However, the discovery of HTS changed such a situation. The thermal stability of HTS is extremely high because of their large thermal capacity. The stability is also greatly enhanced when they are used at higher temperatures. The simplicity of one of the earliest demonstrations of HTS, the levitation of a permanent magnet over a bulk Y-Ba-Cu-O, as shown in Fig. 1, started researchers seeking for their potential applications. During the past fifteen years, researchers have made significant improvements in the materials properties of bulk HTSs and have investigated the feasibility of their use in various engineering applications. The combination of permanent magnets and HTS enabled us to construct magnetic bearings with a low-loss rotation that are installed in prototype highefficiency flywheels. Bulk HTSs have demonstrated the ability to trap magnetic flux, and some samples have exhibited surface magnetic fields approximately an order of magnitude higher than presently used permanent magnets. The performance of bulk HTS materials is given by the following equation: M =A J_c r (1) where M is magnetization and stands for the performance of bulk materials, A is a geometrical constant, J_c the critical current density, and r the radius of single domain or the radius of a current loop. Thus in order to improve the performance of bulk HTS, one needs to increase J_c or the size of bulk HTS.