Reversible melting and equilibrium phase formation of (Bi,Pb)2Sr2Ca2Cu3O10+d

The decomposition and the reformation of the (Bi,Pb)2Sr2Ca2Cu3O10+d (?Bi,Pb(2223)?) phase have been investigated in-situ by means of High-Temperature Neutron Diffraction, both in sintered bulk samples and in Ag-sheathed monofilamentary tapes. Several decomposition experiments were performed at various temperatures and under various annealing atmospheres, under flowing gas as well as in sealed tubes, in order to study the appropriate conditions for Bi,Pb(2223) formation from the melt. The Bi,Pb(2223) phase was found to melt incongruently into (Ca,Sr)2CuO3, (Sr,Ca)14Cu24O41 and a Pb,Bi-rich liquid phase. Phase reformation after melting was successfully obtained both in bulk samples and Ag-sheathed tapes. The possibility of crystallising the Bi,Pb(2223) phase from the melt was found to be extremely sensitive to the 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 Pb losses are responsible for moving away from equilibrium and therefore hinder the reformation of the Bi,Pb(2223) phase from the melt. Thanks to the full pattern profile refinement, a quantitative phase analysis was carried out as a function of time and temperature and the role of the secondary phases was investigated. Lattice distortions and/or transitions were found to occur at high temperature in Bi,Pb(2223), Bi,Pb(2212), (Ca,Sr)2CuO3 and (Sr,Ca)14Cu24O41, due to cation diffusion and stoichiometry changes. The results indicate that it is possible to form the Bi,Pb(2223) phase from a liquid close to equilibrium conditions, like Bi(2212) and Bi(2201), and open new unexplored perspectives for high-quality Ag-sheathed Bi,Pb(2223) tape processing.Comment: 45 pages (including references,figures and captions), 13 figures Submitted to Supercond. Sci. Techno

Effects of the fast discharge system on the ITER poloidal and toroidal field circuits

The paper deals with the criteria adopted to design the ITER Fast Discharge Resistors and considers the effects on the machine as well as induced current in the passive structures (Vacuum Vessel and Back Plate) and overvoltage across converters; in particular, an analytical, method to design the discharge resistors for a mutually coupled coil system is presented. The effect of a coil short circuit during Fast Discharge has been also considere

Microstructure, texture and critical current of Ag-sheathed 2223 multifilament tapes

An Ag-sheathed 2223 multifilament tape was produced by the powder-in-tube method. The various parts of the tape were heat treated at different temperatures under reduced oxygen partial pressure. The microstructure and the texture were characterized by synchrotron x-ray diffraction and SEM and correlated with Jc. In the low temperature range (<826 °C), the 2223 fraction and the c-axis alignment of 2223 grains increased with increasing temperature. A significant increase of Jc (from 1 to 41 kA cm-2) was observed in this range, indicating that the phase purity and the texture were the major controlling factors. In the medium temperature range (826-830 °C), the 2223 fraction and the grain alignment tend to saturate, and Jc remains nearly constant at a level of 40 kA cm-2. In the high temperature range (830-836 °C), the 2223 fraction and the grain alignment remained unchanged but Jc decreased with increasing temperature. The drop in Jc was related to the presence of an amorphous phase and a small amount of 2201 phase, indicating that the grain connectivity has become the major current-limiting factor. The variations in the filament shape, density and alignment within the multifilament tape were characterized. The influence of the inhomogeneous structure on Jc is discussed