Materials aspects of the high-temperature superconductors in the system Bi 2 O 3 -SrO-CaO-CuO
- PDF / 1,471,419 Bytes
- 17 Pages / 612 x 792 pts (letter) Page_size
- 22 Downloads / 145 Views
Materials aspects of the high-temperature superconductors in the system Bi2O3–SrO–CaO–CuO Peter Majewski Max-Planck-Institut fu¨r Metallforschung, Pulvermetallurgisches Laboratorium, D-70569 Stuttgart, Germany (Received 19 July 1999; accepted 28 January 2000)
Ten years after the discovery of high-Tc superconductivity in the system Bi–Sr–Ca–Cu–O, the superconducting compounds have been identified, and their structures, crystal chemistry, phase equilibria, and properties have been extensively investigated. In this review, the results of studies of crystal chemistry and phase equilibria are presented to give materials scientists a comprehensive insight into the phase equilibria and crystal chemistry of these challenging materials. I. INTRODUCTION
II. THE STRUCTURE OF THE HIGH-TC PHASES
One of the most exciting developments in recent years has been the discovery of superconducting oxides with critical temperatures (Tc) well above the boiling point of liquid nitrogen (T ⳱ 77 K) (Fig. 1). The discovery of the high-temperature superconducting phases in the Bi–Sr– Ca–Cu–O system 1 (Bi 2+x Sr 2−y Ca 1+y Cu 2 O 8+d , 2212 phase, Tc 艋 95 K and Bi2+xSr2−yCa2+yCu3O10+d, 2223 phase, Tc 艋 110 K) has stimulated worldwide research activities aimed at understanding and controlling these exciting materials. Only thirteen years after the discovery of these new compounds, prototypes of superconduting high-electriccurrent cables have been manufactured using the 2223 phase, which carries electric currents up to 3000 A at 77 K. In addition, prototypes of superconducting magnets have been processed based on the 2212 phase, which generates magnetic fields up to 2 tesla at about 20 K. Key to the development of these technologically interesting devices is the knowledge of the fundamental materials aspects underlying the preparation of these challenging compounds. Great efforts have been put into the processing of the complex oxides to optimize the superconducting properties (Tc, Jc, and Hc). Numerous papers have been published on the phase equilibria, and the preparation of suitable bulk, wire, tape, and thin and thick film samples. The emphasis has been on optimizing the amount of superconducting phase in the multiphase samples. Also of interest is the characterization of the microstructure, texture, impurity phases, and the chemical composition of the superconducting compounds with respect to anions and cations.
Immediately after the discovery of high-Tc superconductivity in the system Bi–Sr–Ca–Cu–O, the superconducting phases were identified as members of the homologous series Bi2Sr2Can−1CunO2n+4 (n ⳱ 1, 2, 3) and their crystal structure was determined2–6 (Table I). However, recent reinvestigations of the structure of the 2212 phase has resulted in very different conclusions7 and, therefore, the crystal structure of the phases is still a matter of discussion. Taking into account the solid solution character of the members of the homologous series, the chemical composition can be expressed more precisely with the formula Bi2+x+zSr2−x−yC
Data Loading...
