PHASE REGIONS AND KINETICS OF FORMATION OF (BI,PB)(2)SR2CA2CU3OX IN AG-SHEATHED TAPE

Ag-sheathed (Bi,Pb)(2)Sr2Ca2Cu3Ox (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 similar to 810 degrees and similar to 837 degrees C. The Avrami equation was applied to describe the kinetics of 2223 formation from a mixture of Bi2Sr2CaCu2Ox and nonsuperconducting phases, mainly Ca2PbO4 and CuO. The calculated Avrami exponent, n approximate to 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 similar to 2900 kJ/mol from similar to 817 degrees to similar to 825 degrees C and similar to 890 kJ/mol from similar to 825 degrees to similar to 837 degrees C. A temperature-time-transformation diagram was constructed based on the kinetic data; it describes the transformational behavior of this particular system.