Phase Regions and Kinetics of Formation of (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x in Ag‐Sheathed Tape

Ag‐sheathed (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O, (2223) tapes were made by the oxide‐powder‐in‐tube method. Tapes were heat‐treated isothermally at several different temperatures in 7.5% O 2 /Ar, then quenched into oil to retain the phase assemblages at the reaction temperatures. 2223 formed between ∼810° and ∼837°C. The Avrami equation was applied to describe the kinetics of 2223 formation from a mixture of Bi 2 Sr 2 CaCu 2 O x and nonsuperconducting phases, mainly Ca 2 PbO 4 and CuO. The calculated Avrami exponent, n ∼ 1, indicated that the kinetics in this system could be described as a diffusion‐controlled, two‐dimensional nucleation and growth process. The apparent activation energy for forming 2223 was ∼2900 kJ/mol from ∼817° to ∼825°C and ∼890 kj/mol from ∼825° to ∼837°C. A temperature‐time‐transformation diagram was constructed based on the kinetic data; it describes the transformational behavior of this particular system.