Controlled decomposition and reformation of the 2223 phase in Ag-clad (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes and its influe
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Controlled decomposition and reformation of the 2223 phase in Ag-clad (Bi, Pb)2 Sr2 Ca2 Cu3 Ox tapes and its influence on the microstructure and critical current density J. A. Parrell and Y. Feng Applied Superconductivity Center and Materials Science Program, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, Wisconsin 53706
S. E. Dorris Argonne National Laboratory, Energy Technology Division, Argonne, Illinois 60439
D. C. Larbalestier Applied Superconductivity Center and Materials Science Program, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, Wisconsin 53706 (Received 14 June 1995; accepted 15 November 1995)
The decomposition of almost fully reacted (Bi, Pb)2 Sr2 Ca2 Cu3 Ox (BSCCO-2223) tapes caused by heating in 1 atm of pure O2 at 825 ±C has been studied. It was found that partially decomposing 2223 tapes to a mixture of Bi2 Sr2 Ca1 Cu2 Oy , (Ca, Sr)2 PbO4 , and other secondary phases reduced the critical current density (77 K, 0 T) from ,20 kAycm2 to nearly zero. Reheating the tapes in 7.5% O2 restored the 2223 phase and, while there was some degradation of the 2223 grain alignment due to residual secondary phase growth, the critical current density was also restored to nearly its original value. We hypothesize that such a decomposition/reformation process can be useful in increasing the connectivity and relative density of polycrystalline 2223, by encouraging the formation of a liquid phase which heals residual cracks in the BSCCO core.
I. INTRODUCTION
The fabrication of Ag-clad (Bi, Pb)2 Sr2 Ca2 Cu3 Ox tapes requires a process of one or more heat treatment, mechanical deformation, and subsequent heat treatment cycles. This process has developed empirically, and many of its details are still not understood. It is generally believed that the deformation steps are necessary to develop the alignment of the plate-like 2223 grains.1 They also increase the density of the core,2–4 thus increasing its connectivity. Both effects correlate to an increased critical current density sJc d. An additionally proposed benefit of the deformation steps is that they break up the sintered structure and bring the reactants into more intimate contact, facilitating greater conversion of the precursor powder to 2223.3 For these multiple reasons, deformation is a beneficial and vital step in the fabrication of high-performance 2223 tapes. However, if the deformation causes cracks in the oxide core that do not heal during the final reaction heat treatment,4–6 the active cross section of the core will be reduced, resulting in a decrease in the effective conductor Jc 3,4,7 (recall that Jc is conventionally defined as the critical current sIc d divided by the entire BSCCO cross section, irrespective of how much of the cross section is carrying current). This potentially deleterious effect of deformation is difficult to quantify because only large cracks (,5 mm or longer) are obvious in polJ. Mater. Res., Vol. 11, No. 3, Mar 1996
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