The effects of variable oxygen partial pressures during Bi-2223 tape processing
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The effects of variable oxygen partial pressures during Bi-2223 tape processing T. G. Holesinger and J. F. Bingert MST-6, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
J. O. Willis Superconductivity Technology Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
V. A. Maroni and A. K. Fischer Chemical Science and Technology, Argonne National Laboratory, Argonne, Illinois 60521
K. T. Wu State University of New York/Old Westbury Campus, Old Westbury, New York 11568 (Received 15 April 1997; accepted 17 July 1997)
The effect of changes in the oxygen partial pressure during the heat treatment of Bi-2223 tapes has been investigated. Distinct differences were observed in the phase assemblages, compositions, and critical current densities. Of particular importance in the correlation of transport properties and microstructure was the redistribution of lead in both the secondary phases and the Bi-2223 matrix. The highest Jc values were associated with tapes that contained Ca2 PbO4 as part of the phase assemblage coupled with lower concentrations of lead in the superconducting phase. The high Jc of 30.4 kAycm2 was obtained in tapes processed at 820 ±C where the oxygen partial pressure was increased from 10% O2yAr to 20% O2yAr after 25 h of each 50-h sinter cycle. Samples processed only in 10% O2yAr at 820 ±C did not contain Ca2 PbO4 , had slightly higher concentrations of lead in the superconducting phase, and had significantly lower Jc values. I. INTRODUCTION
The role of oxygen partial pressure in the processing and stability of (Bi, Pb)2 Sr2 Ca2 Cu3 Oy (Bi-2223) tapes has been an intensely investigated area in the past several years. Optimal Bi-2223 formation in fixed oxygen partial pressures is strongly coupled to the processing temperatures.1–4 However, a relatively unexplored, but important area of Bi-2223 oxide-powder-in-tube (OPIT) tape development, is the effect of changing the oxygen partial pressure during processing. The stabilities of the Bi-2223 and Bi-2212 phases as well as several alkalineearth cuprates and lead-containing phases are affected by changes in the oxygen partial pressure.5–11 Several groups have touched on this area by investigating the decomposition/reformation of Bi-2223 in oxygen, by post-annealing at lower temperatures in the same oxygen partial pressure, or by slow-cooling of OPIT tapes in a set P(O2 ).12–15 Interest in the area of variable P(O2 ) processing is also motivated by previous work which showed that the lead solubility in Bi-2223 is dependent upon both the temperature and oxygen partial pressure.16–19 The addition of lead is needed to improve the formation and stability of the Bi-2223 phase.20,21 In addition, there is evidence that suggests that the mode of lead addition influences the Bi-2223 formation mechanism as well as the development and control of transient or persistent liquid phases.22–25 It is widely believed that the 3046
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 11, Nov 19
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