Tape Development: Microstructure and Critical Current Densities

The morphological transformations occurring in the precursor powders during the formation of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ phase were investigated by scanning electron microscopy performed on the same location on the surface of a pellet, which underwent several heat treatments at 852 • C for various times. Following the evolution of a given group of grains we have found that the formation of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ crystals occurs simultaneously with a gradual decomposition of the intermediate (Bi, Pb) 2 Sr 2 CaCu 2 O 8+δ phase. This result was confirmed by microprobe analysis as well as x-ray diffraction performed on the same samples. As a main result, we conclude that in the observed powder mixtures, the (Bi, Pb) 2 Sr 2 Ca2Cu 3 O 10+δ phase does not form from the pre-existing (Bi, Pb) 2 Sr 2 CaCu 2 O 8+δ crystallites but through a nucleation and growth process. Based on the present observations, a model for the phase formation mechanism of (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ is presented.

Section: Resultssupporting

confidence: 92%

Visualization of the formation of the phase

Grivel¹,

Flükiger²

1996

“…The Pb to Bi and the Sr to Ca ratio of the 451 phase in equilibrium with the 2212 and 2223 phase is about 4:1 and 1:1, respectively [43]. Due to the fact that the phase exhibits also a slightly varying (Pb + Bi):(Sr + Ca):Cu ratio it is also named 3221 phase [45][46][47][48][49]. The XRD patterns of both phases are very similar (figure 12) and therefore, the phases can only be distinguished by microprobe analysis.…”

Section: The System Bi 2 O 3 -Pbo-sro-cao-cuomentioning

confidence: 99%

Phase diagram studies in the system Bi - Pb - Sr - Ca - Cu - O - Ag

Majewski

1997

110

Phase diagram studies with emphasis on the high temperature superconducting phases Bi 2+x Sr 2−y Ca 1+y CuO 8+d (2212 phase) and (Bi, Pb) 2+x Sr 2−y Ca 2+y CuO 10+d (2223 phase) are reviewed in this article. The phase relations of the Bi 2 O 3 -SrO-CaO-CuO and the homogeneity regions of the 2212 and 2223 phases have been studied in detail at various temperatures and oxygen partial pressures. However, the Pb-doped system has been investigated less comprehensively. The studies of this system are mostly focused on the 2212 and 2223 phase and the experimental work regarding the 2223 phase is often concentrated on the processing of 2223 samples with high critical current density and not on the preparation of single phase 2223 ceramics.The investigations of the Ag-containing system have just started. Nevertheless, first results show that Ag significantly influences the phase relations and the temperature dependence of the cation solubilities of the 2212 and 2223 solid solutions. These results indicate that the obtained phase diagrams of the system Bi 2 O 3 -PbO-SrO-CaO-CuO cannot be applied without caution when Ag is present. Therefore, as Ag is widely used as a sheath material for the processing of 2212 and 2223 wires and tapes, the investigation of the entire Ag-containing system is inevitable.

“…More seriously, microcracks may be introduced during the sausage formation, limiting J c or even blocking the current completely. Since there are strong indications that 0953-2048/97/060371+17$19.50 c 1997 IOP Publishing Ltd Ag promotes the alignment of the Bi-2223 crystals along the interface between the Ag sheath and the oxide core [12,13], a wavy interface may have an undesired influence on the alignment. Moreover, sausaging also introduces inhomogeneities in the mechanical properties of the tape, reducing the tolerances of bending and tension.…”

Section: Introductionmentioning

confidence: 99%

The mechanical deformation of superconducting BiSrCaCuO/Ag composites

Han

Skov-Hansen

Freltoft

1997

107

The mechanical deformation of BiSrCaCuO/Ag composites made by the powder-in-tube method is a multi-step process. The main difficulty is that the mechanical properties of the ceramic powder are very different from those of the Ag sheath. A key parameter is the core density, which changes during mechanical deformation. In this review, basic concepts of the classical mechanical deformation theory are briefly discussed. Simple descriptions of deformation processes like pressing, rolling, drawing and extrusion are also presented. The term 'freedom parameter', f , is introduced to illustrate the influence of various constraint factors on the mass-flow behaviour. Simple pictures including mass redistribution and the powder-flow model are presented for interpreting the plastic deformation process of the composites. Experimental results are reviewed and our proposed pictures and models are applied for discussion.