Microstructural dynamics of Bi-2223/Ag tapes annealed in 8% O₂

The microstructural dynamics of Bi-2223 tapes are investigated in situ during annealing in 8% O-2 by means of 100 keV x-ray diffraction. A green mono- and a green multi-filamentary tape are annealed at 829.5 degreesC. During ramp-up (Ca,Sr)(2)PbO4 decomposes above 750 degreesC, resulting in an incorporation of Pb in Bi-2212.

The associated grain growth of Bi-2212 is the main cause of the strain relief and the c-axis grain alignment of the Bi containing phases. Above 825 degreesC the Bi-2212 partly dissociates into (Ca,Sr)(14)Cu24Ox and a liquid phase. The linewidth of Bi-2212 is constant during the transformation to Bi-2223, indicating no strain or finite-size broadening.

The most probable transformation mechanism is found to be nucleation and growth with a fast decomposition of the individual Bi-2212 grain, followed by a growth of Bi-2223 from the Bi-2212 melt reacting with (Ca,Sr)(14)Cu24Ox. The multi-filamentary tape transforms faster and exhibits a much better degree of c-axis alignment.

Results are compared to similar studies in air. During a final annealing of a fully converted tape no change is found in the average concentration, stoichiometry and grain misalignment. A post-annealing experiment at 650 degreesC on quenched tapes shows a reaction over 3 weeks, whereby randomly oriented 3222 grains are formed and Bi-2223 is depleted of Pb.

Simultaneously, the critical current decreases by a factor of 2.