Evolution of the High T c Phase in Rapidly Solidified Bi-Ca-Sr-Cu Oxides
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EVOLUTION OF THE HIGH TC PHASE IN RAPIDLY SOLIDIFIED Bi-Ca-Sr-Cu OXIDES A. ASTHANA*, P.D. HAN*, Z. XU*, L. CHANG*, D.A. PAYNE* AND P.J. GILBERT** * Department of Materials Science and Engineering, Materials Research Laboratory, and Science and Technology Center for Superconductivity, University of Illinois at UrbanaChampaign, Urbana, IL. 61801, USA. ** Division of Ceramics, School of Materials, University of Leeds, Leeds, LS2 9JT, UK. ABSTRACT Materials in the system Bi-Ca-Sr-Cu-O were prepared by rapid solidification. Emphasis was placed on optimizing processing conditions for enhancement of the high Tc (2223) phase. A number of starting compositions and heat-treatments were investigated. The effect of dopants (such as, Pb, Sb, Ge, Sn) on the stabilization of the high Tc phase was evaluated. Small amounts of Pb and Sb were effective in significantly increasing the 2223 yield. Improvements in superconducting properties as a function of 2223 evolution are discussed with reference to SEM, TEM, XRD and SQUID results. INTRODUCTION The Bi-superconductor family has three related perovskite-like layered phases in the homologous series, Bi2Can-lSr2CunO2n+4. They are referred to as 2021 (n=l, Tc - 12 K), 2122 (n=2, Tc - 80 K), and 2223 (n=3, Tc - 110 K). To-date, it has not yet been possible to obtain the high Tc (2223) phase in pure form since most existing processes yield multi-phase material. Hence, recent research efforts have attempted to enhance the volume fraction of the high Tc phase in ceramic specimens. Some of the strategies used for preferential development of the high Tc phase, include: (a) addition of Pb [11, (b) use of Ca and Cu-rich starting compositions [21, (c) prolonged heat-treatment at temperatures close to the melting point [3,4], and (d) controlled oxygen atmospheres [5]. Rapid solidification techniques which involve quenching to an amorphous state, followed by crystallization of superconducting phases, have shown promise [6-9]. Earlier, we reported on melt-quenched materials which yielded significant fractions of 2223 [10,11]. Phase development of 2223 from 2122 was reported. In this paper, we give further information on the enhancement of the 2223 phase from melt-quenched ceramics, together with their improved superconducting properties. EXPERIMENTAL PROCEDURE Powders of Bi203, CaCO3, SrCO3 and CuO, and additives such as PbO, Sb203, GeO2, Sn02, etc., were mixed in a mortar and pestle, and fused between 1100-1200 C in Pt crucibles. The equilibrated melts were quenched between Cu slabs, which were rapidly contacted together. Strong, shiny plates, several cms in dimension and 0.5-1 mm thick were obtained. The specimens were heat-treated under various conditions. SEM, TEM and XRD methods were used to characterize the microstructure and phase composition. A SQUID magnetometer (operated at H=50 Oe) was used to determine magnetic transitions. RESULTS AND DISCUSSION In an attempt to increase the 2223 yield, three starting compositions (given by Bi:Ca:Sr:Cu=2:2:2:3, 2:3:2:4, and 2:4:2:5) were initially selec
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