Effect of PO 2 and Ag on the phase formation of the Bi(Pb)-2223 superconductor
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Effect of PO2 and Ag on the phase formation of the Bi(Pb)-2223 superconductor W. Wong-Ng and L. P. Cook Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
W. Greenwood Geology Department, University of Maryland, College Park, Maryland 20742 (Received 15 May 1998; accepted 11 January 1999)
The chemical reactions and the compositional characteristics of liquids which lead to the phase formation of the Bi(Pb)-2223 [(Bi, Pb) : Sr : Ca : Cu : O] superconductor have been studied for a precursor composition of Bi1.8 Pb0.4 Sr2 Ca2.2 Cu3 Ox . The combined techniques of quenching, powder x-ray diffraction, differential thermal/thermogravimetric analysis (DTA/TGA), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDS) were used to characterize the subsolidus phases and the presence of liquid. Samples were annealed under purified air and under a volume fraction mixture of 7.5% O2y92.5% Ar. The effects of Ag in both the pure air and the 7.5% O2y92.5% Ar experiments were also studied. Results are discussed with respect to their processing implications. I. INTRODUCTION
Among the three superconductors present in the Bi–Sr–Ca–Cu– O (BSCCO) system, the 110 K, 3-layer 2223 (Bi : Sr : Ca : Cu) phase is the most difficult to prepare relative to the 80 K, 2-layer 2212 (Bi : Sr : Ca : Cu) and the 20 K, 1-layer 2201 (Bi : Sr : Ca : Cu) phases (Refs. 1, 2 and references cited within). However, if Pb is doped in the precursor materials, long-term annealing often results in a high yield of the Pb-substituted 2223 [(Bi Pb) : Sr : Ca : Cu : O] phase.3–14 Wire and tape technologies utilizing the Pb-free 2212 or Pb-doped 2223 phases have demonstrated the feasibilities of these materials for large-scale applications.15–18 Prototypes of a variety of devices based on these technologies have been successful; examples include transmission cables, generators, motors, transformers, and magnets. A. Previous work
A substantial amount of information has been published concerning the sequence of reactions resulting in the formation of the 2223 phase.3–14 The literature prior to 1992 has been reviewed by Wong-Ng and Freiman.19,20 Mechanisms reported include disproportionation,21,22 dissolution-precipitation,23 intergrowth and reaction,24 mobile liquid droplet migration,25 peritectic transformation,26 diffusion/insertion,27,28 liquid-aided sintering,29,30 Lewis acid/base equilibrium,31 and consecutive addition reactions.32,33 In situ studies of 2223 phase formation were reported by Xu et al.,34 Merchant et al.,35 Drews et al.,36 and Thurston et al.37,38 However, despite these research efforts, the chemical reactions leading to the formation of the 2223 phase remain
controversial. Issues such as whether an intermediate liquid promotes the formation of the 2223 phase during long-term annealing, the nature of this liquid phase, the details of the reaction sequence, and the kinetics of 2223 phase formation, all of which have an impact on pr
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