Effect of sintering temperature and cooling rate on microstructure, phase formation, and critical current density of Ag-
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Effect of sintering temperature and cooling rate on microstructure, phase formation, and critical current density of Ag-sheathed Bi1.8 Pb0.4 Sr2 Ca2 Cu3 Ox superconducting tapes J. P. Singh and N. Vasanthamohan Energy Technology Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received 10 June 1996; accepted 1 May 1997)
Silver-sheathed Bi–Pb–Sr–Ca–Cu–O (2223) superconducting tapes (with a starting composition of Bi1.8 Pb0.4 Sr2 Ca1 Cu2 O8 , calcium cuprate, and CuO) were fabricated by the powder-in-tube technique. The tapes were sintered at various temperatures to optimize the formation of Bi1.8 Pb0.4 Sr2 Ca2 Cu3 O10 phase within the tape. The results show that sintering within the temperature range of 815–825 ±C can produce tapes with high critical current density sJc d. The Jc of samples sintered at the higher temperature of 825 ±C, where more liquid is present, depended markedly on the rate at which tapes were cooled from the sintering temperature; samples sintered at lower temperatures did not exhibit such a cooling-rate effect. The optimum combination of phase purity and microstructure that yielded an average transport Jc of > 2.5 3 104 Aycm2 was obtained when the tapes were sintered at 825 ±C for 150 h and cooled at a rate of 25 ±Cyh from the sintering temperature. Quenching studies indicate that the Bi-2223 phase becomes unstable below 700 ±C during slow cooling. This result may have important implications for processing Bi–Sr–Ca–Cu–O tapes with high Jc . Addition of 15 vol. % Ag flakes to the monolithic core exerted no significant effect on Jc .
I. INTRODUCTION
The powder-in-tube (PIT) technique has proved to be a promising method for fabricating high-temperature superconductor wires and tapes, especially those in the Bi–Sr–Ca–Cu– O (BSCCO) system because their twodimensional microstructures are favorably oriented. Critical current density (Jc ) values over 60 kAycm2 have been reported by Sato et al.1 on short lengths of Bi1.8 Pb0.4 Sr2 Ca2.2 Cu3 Ox (Bi-2223) tapes. However, for practical applications, these results must be reproduced consistently and long lengths of superconductors must be fabricated. Efforts to obtain higher Jc values have been directed toward optimizing overall stoichiometry, initial phase assemblage, mechanical and thermal processing, sintering atmosphere, Ag additions, etc. Studies on Bi2 Sr2 Ca1 Cu2 Oy (Bi-2212) tapes revealed a strong correlation between phase purity and cooling rate.2 These tapes were melt-solidified and hence a faster cooling rate was necessary to prevent decomposition of the 2212 phase.2 Parrell et al.,3 who studied the effect of cooling rate on Bi-2223 tapes over a limited temperature range (725–825 ±C), observed improved grain connectivity and a ø 20–50% increase in Jc of slowly cooled tapes. In their study, these investigators first cooled the samples at 240 ±Cyh from the sintering temperature to room temperature, reheated them to the sintering temperature, and then held them for 1 h before cooling
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