High-temperature x-ray diffraction studies of a precursor mixture for Pb-substituted Bi-2223 superconducting wires
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High-temperature x-ray diffraction studies of a precursor mixture for Pb-substituted Bi-2223 superconducting wires A. R. Drews, J. P. Cline, and T. A. Vanderah Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
K. V. Salazar Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 3 October 1996; accepted 29 May 1997)
High-temperature x-ray diffraction measurements of a (Bi, Pb)-2223 precursor mixture used to produce high-Jc superconducting tapes were conducted on silver and ZrO2 substrates. The precursor mixture consisted primarily of 2212, Ca2 PbO4 , and CuO. Phase evolution was markedly sensitive to oxygen partial pressure: In 10% O2 growth of the 2223 phase on silver was rapid, proceeded at the expense of the 2212 phase, and was preceded by the disappearance of the Ca2 PbO4 phase. When slowly heated on a silver substrate in 7.5% O2 the 2212 phase melted near 800 ±C and subsequently recrystallized near 820 ±C in a highly textured form, but with no detectable 2223 formation. Under similar conditions on a ZrO2 substrate, the mixture exhibited no marked changes in the XRD patterns up to 850 ±C. The dramatic reactivity on silver was also highly dependent on heating rate; rapid heating in 7.5% O2 to 825 ±C did not result in melting of the 2212 phase or appearance of the 2223 phase. In experiments leading to formation of 2223, the c-lattice parameter of the 2212 phase contracted just prior to the onset of formation of 2223. This result is consistent with the formation of an intermediate Pb-doped phase of 2212. A transient amorphous phase appeared briefly at the onset of formation of 2223. No evidence for intergrowth conversion of 2212 to 2223 was observed. I. INTRODUCTION
Considerable interest exists in improving the bulk superconducting properties of long-length wires using the 2212 and 2223 Bi–Sr–Ca–Cu–O high temperature superconductors (HTS). A major limitation is growth of impurity phases in the core of the wire during heat treatment. Wires and tapes are typically drawn from a billet of silver containing a core of partially reacted superconductor precursor mixture, wound, and then heattreated. The silver sheath material has been found to play a critical role in lowering the reaction temperature and enhancing grain alignment.1 The mechanism(s) by which the silver affects the growth and texturing is unknown, though it is widely believed to involve a transient liquid phase. Many investigators have studied the formation of 2223 and numerous growth mechanisms have been proposed. An extensive review of phase formation mechanisms will not be presented here, but such a review has been prepared by Wong-Ng and Frieman.2 Several models propose intermediate species such as 2201 or CaO, transient liquids formed from (but not limited to) Ca2 PbO4 , or intergrowth mechanisms wherein the 2212 is converted to 2223 by a stepwise insertion of calcium and copp
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