A two-powder process for Bi-2223 precursors
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A two-powder process for Bi-2223 precursors T. G. Holesinger, K. V. Salazar, D. S. Phillips, B. L. Sargent, J. K. Bremser, J. F. Bingert, J. O. Willis, and D. E. Peterson Superconductivity Technology Center, MS K763, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 21 February 1995; accepted 28 September 1995)
A two-powder process is described for the production of uniform, fine-grained Bi2 Sr2 Ca2 Cu3 Oy (Bi-2223) powders. One powder is the Bi2.12x Pbx Sr1.92y Cay Oz (2 : 2 Cu-free) phase. The other is a multi-phase powder of approximate overall composition SrCaCu3 Oy . The 2 : 2 Cu-free is one of the first Bi-containing phases to form from a nominal Bi-2223 mixture of oxides and carbonates. This precursor route was chosen for investigation because (1) the powders have very similar particle morphologies and (2) the mixing volumes are closely matched. Both of these characteristics facilitate the milling and blending process. This precursor mix was found to be stable in that explosive grain growth of undesirable phases was not observed during sintering. Critical current densities up to 26,900 Aycm2 in self field at 75 K were obtained in tapes.
I. INTRODUCTION
Oxide powder-in-tube (OPIT) processing is currently the most promising approach for the development of long lengths of superconducting Bi2 Sr2 Ca2 Cu3 Oy (Bi-2223) conductors.1–4 Additions of lead, multiple thermo-mechanical deformation steps, and sintering in low oxygen partial pressures are well-known approaches to the processing of highly textured Bi2223 in tape form.5–9 However, before the powder is loaded into the tube, another critical issue, the synthesis of finely divided, homogenous precursor powders, must be addressed. Of prime importance is the existence of a controlled phase assemblage in the tape prior to heat treatment. It has been shown that an ill-defined phase balance results in a multiplicity of competing reactions, many of which favor non-superconducting phases.10 Progress toward controlling the starting phase assemblage in OPIT tapes came with the development of the so-called “twopowder” process.11,12 This process was based on the knowledge that the formation of Bi-2223 was preceded by Bi2 Sr2 CaCu2 Oy (Bi-2212). Use of a mixture of Bi-2212 and “CaCuO2 ” as opposed to simple oxides and carbonates increased the conversion rate to Bi2223. The initial two-powder mix actually consisted of Bi-2212 plus a multi-phase mixture of Ca2 CuO3 (2 : 1), CuO, and PbO. Further improvements in the two-powder process were made by incorporating lead into the Bi-2212 phase.13,14 The intent of the latter studies was to eliminate as much as possible the presence of PbO and Ca2 PbO4 in the precursor mix by replacing bismuth with lead in the Bi-2212 phase. It was found that nearly single-phase Bi-2223 could be produced in as little as 50 h using the latter synthesis route. 28
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J. Mater. Res., Vol. 11, No. 1, Jan 1996
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