Advances in Bi-Based High-T c Superconducting Tapes and Wires
- PDF / 165,102 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 63 Downloads / 153 Views
Advances in
Bi-Based High-Tc Superconducting Tapes and Wires
Hitoshi Kitaguchi and Hiroaki Kumakura Introduction Since the discovery of high-Tc superconductors (HTSs), especially the Bi-based oxide superconductors (Bi-HTSs) in 1989, much effort has been concentrated on the fabrication of Bi-HTS wires and tapes. Bi-HTSs are interesting materials from the viewpoint of practical applications. One of the interesting applications of Bi-HTSs is the cryogen-free conduction-cooled magnet. Besides having a high transition temperature (Tc), Bi-HTSs have high upper critical fields (Bc2) or high irreversibility fields (Birr) at temperatures below 30 K. Because 30 K can be efficiently maintained by a cryocooler, a Bi-HTS magnet cooled with a cryocooler has great potential in many technological applications. Another promising application of Bi-HTSs is the high-field magnet. When the temperature is reduced to 5 K, Bi-HTSs show a much higher Bc2 or Birr than do conventional metallic superconductors (low-temperature superconductors, LTSs) such as Nb3Sn. This indicates that Bi-HTSs also have great potential for use in a highfield superconducting magnet if the magnet is operated at low temperature. The utilization of a superconducting material in a magnet requires winding a sufficiently flexible tape or wire conductor into a coil. Because oxide superconductors are intrinsically brittle, as is the case with conventional intermetallic superconductors, a special technique must be developed for making tape or wire conductors. Another problem of high-Tc oxide superconductors is the “weak coupling” of grains, which significantly reduces current transfer across the grain boundaries.1 In order to improve the coupling of grains, and hence to in-
MRS BULLETIN/FEBRUARY 2001
crease current-carrying capacity, alignment of the grain orientation is needed. In the case of Bi-HTSs, single-axis (c axis) grain alignment is sufficient to increase the current-carrying capacities up to a practical level, while for the YBa2Cu3Ox (Y-123) system, biaxial grain alignment is required. In the case of Bi-HTSs, c-axis grain alignment can be easily obtained due to the high two-dimensionality of the system, and high critical current density (Jc) values are attained. For the Bi2Sr2CaCu2Oy (Bi-2212) system, the so-called meltsolidification process is applied in order to obtain a grain-oriented microstructure.2 For the Bi2Sr2Ca2Cu3Oz (Bi-2223) system, on the other hand, this melt-solidification process is not applicable. For Bi-2223, a
c-axis grain-oriented microstructure can be obtained by a combination of cold rolling and subsequent sintering heat treatment.3 There are two major methods for fabricating Bi-HTS tapes and wires: the socalled powder-in-tube (PIT) method3 and the surface-coating method.4 The PIT method employs a Bi-based oxide powder and a Ag tube. The Ag/oxide-composite tube is cold-worked into wire or tape and then heat-treated in order to form superconducting particles and/or to obtain their excellent electrical grain connectivity. The typical
Data Loading...
