Influence of processing parameters of partial-melting process on phase assemblage, microstructure and superconducting properties of Bi-2212 bulk material

Partial-melting was studied as an alternative to the melt-cast process for the production of Bi-2212 blocks designed for the construction of fault-current limiters. Peritectic melting and solidification of Bi-2212:Ag precursor powder were investigated in situ by XRD in a synchrotron radiation facility. The result of this experiment allowed a design of heat treatment conditions for Bi-2212 block processing, including maximum processing temperature, holding time, cooling rate and annealing time. After each processing stage, microstructural and phase evolution were monitored by SEM/EDS and XRD. Also the release and uptake of oxygen during peritectic melting and solidification of Bi-2212 in the bulk form were monitored in oxygen atmosphere by thermo-gravimetric balance. Based on these results several Bi-2212 blocks were prepared with different processing conditions. Critical temperatures were measured by magnetic susceptibility and dc critical current (Ic) were determined in self-field at the temperature of 77 K. Two modes of peritectic melting of Bi-2212 phase in the temperature range 865–900 °C have been observed. This resulted in different microstructural and superconducting properties for the Bi-2212 blocks melted in the temperature ranges 870–890 and 890–900 °C.