Partial melting and HIP processing of Bi(2223): bulk and tapes

A detailed investigation of the equilibrium between the Bi,Pb(2223) phase and the melt was carried out by in-situ high temperature neutron powder diffraction both on sintered bulk samples and Ag-clad tapes. Stability, decomposition and re-formation of Bi,Pb(2223), as well as the evolution of secondary phases, were studied and the effect of the oxygen partial pressure was investigated. Bi,Pb(2223) melts incongruently to (Sr,Ca)₁₄Cu₂₄O₄₁, (Sr,Ca)₂CuO₃, and a Bi,Pb-rich liquid, and no precipitation of Bi(2212) was observed at this stage. Direct formation of the Bi,Pb(2223) phase from the melt was observed, opening up the possibility of processing Ag-sheathed tapes from a partial melting route. The possibility of reforming the Bi,Pb(2223) phase from the melt proved to be extremely sensitive to temperature and strongly dependent on the Pb-losses. The study of the mass losses due to Pb-evaporation was complemented by thermogravimetric analysis which proved that the Pb-losses are responsible for moving away from the equilibrium and therefore hinder the Bi,Pb(2223) to reversibly form from the melt. Hot isostatic pressure has proved to be an effective remedy for preventing volatile elements from evaporating and for increasing the density of the tapes. The first annealing stage of the Ag-Bi,Pb(2223) processing was performed under isostatic pressure after making the Ag-sheath airtight. Positive effects of pressure on filament density, formation kinetics and critical current were observed at 10 MPa. We have extended our search for an alternative processing route to the Pb-free Bi(2223) phase. High purity Bi(2223) bulk samples and Ag-sheathed tapes were successfully prepared from pre-reacted powders. Very large Bi(2223) grains (up to ∼500 μm) were observed to grow at the Ag-ceramic interface.