No abstract

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.

The various fabrication and reaction processes leading to multifilamentary Bi,Pb(2223) tapes with high transport are described. In air, the Bi,Pb(2223) phase in pressed powders is found to be formed by nucleation, following the complete decomposition of the Bi,Pb(2212) intermediate phase. In this work, the precise formation conditions of the Pb-free phase Bi(2223) are described for the first time. It is shown that the presence of Pb leads to a lowering of the melting temperature from 879 to and of that of the `transient liquid' . The latter is formed after decomposition of the 2-layer compound and is necessary to the formation of the 3-layer compound. Thus Pb is found to have no influence on the formation mechanism of the 3-layer compound, which always starts by a nucleation process. Nucleation processes are also found to occur in the first phase of reaction inside Ag sheathed tapes, intercalation being possible at later stages. The highest values of the critical current density in Bi,Pb(2223) tapes reported at the present day are compared, taking into account the analysed tape lengths. The variation of in mono- and multifilamentary Bi,Pb(2223) tapes as a function of the applied magnetic field at 77 and at 4.2 K is described. A new process including deformation by a four-roll machine is presented, and the distribution of inside various tape configurations with similar critical current densities is analysed: for monofilamentary tapes, a maximum of is found at the borders, the ratio being 2.4, while for conventional multifilamentary tapes the maximum is found at the tape centre (, ratio 1.6). For the new four-rolled tapes, a ratio of only 1.1 was measured, indicating a more homogeneous current distribution. In order to lower the AC losses due to the highly conductive Ag matrix, the new concept of `oxide barriers' is introduced. A barrier of surrounds each filament, thus leading to an enhanced radial resistivity, which in turn reduces AC losses. Finally, the current transport mechanism inside Bi,Pb(2223) tapes and the various current limiting mechanisms are discussed.

The reaction of the precursor powders necessary for the formation of t h e final Bi(2223) phase was investigated with and without the presence of an Ag sheath by means of differential thermogravimetry (DTWDTG). The nature of t h e endothermic peaks observed by D T A ~~ well as the influence of Ag on their positions are discussed. By means of x-ray diffraction and energy dispersive x-ray analysis (EDX) on oil-quenched Ag-sheathed tapes it is shown that the Bi(2212) phase originally present in the precursor powders undergoes a structural modification which can be associated with the dissolution of Pb. This transformation occurs at a temperature near 836°C in Agsheathed tapes. This work h a s been essentiallyfocused on t h e role of Pb and its effects on the Bi(2212) phase i n the first stages of transformation.

A new route has been found for producing the Bi,Sr,CaCu,O, phase with Pb substituted for part of the Bi (denoted Bi,Pb(2212)). By this novel route, Bi,Pb(2212) has been prepared in air at 860°C. For comparison, this phase tias also been prepared in argon at 740°C by the previously known route. Bi,Pb(2212) transforms from a tetragonal to an orthorhombic structure as the Pb content grows. Once this phase was obtained with a nominal composition of Bi,-,Pb>r,CaCu,O, with Pb contents x = O . 0.2, 0.4 and 0.6, the Bi( 2223) phase was formed starting from Bi(2212) and Bi,Pb(2212). Single-phase samples were obtained after two heat treatments at 849OC with Bi,Pb(2212) and a proportion of Pb of x = 0.4. This proportion corresponds to the one used previously by the authors to obtain high critical current carrying tapes in Ag-clad Bi(2223) tapes.