Phase Regions and Kinetics of Formation of (Bi,Pb)2Sr2Ca2Cu3Ox in Ag-Sheathed Tape

Ag-sheathed (Bi,Pb)₂Sr₂Ca₂Cu₃O, (2223) tapes were made by the oxide-powder-in-tube method. Tapes were heat-treated isothermally at several different temperatures in 7.5% O₂/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 Bi₂Sr₂CaCu₂O _x and nonsuperconducting phases, mainly Ca₂PbO₄ 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.