Formation of the (Bi,Pb)(2)Sr2Ca2Cu3O10+delta phase: reaction mechanism and kinetics

In order to elucidate the formation mechanism of the Bi,Pb(2223) phase, ext ensive investigations have been performed by means of combined differential thermal analysis and thermo-gravimetric measurements, X-ray diffraction as well as scanning electron microscopy and energy dispersive X-ray microanal ysis. Starting from co-precipitated oxalate powders, the transformations leading to the formation of the Bi,Pb(2223) phase were studied. Based on these inve stigations the Bi, Pb(2223) phase formation process, involving the Bi,Pb(22 12), Ca2PbO4 and alkaline-earth cuprate phases as precursors has been separ ated into elemental steps. A new model for the formation mechanism of the B i, Pb(2223) phase will be presented. The Bi, Pb(2223) phase formation kinetics has been studied using the Avrami relation for isothermal phase transformations. The analysis of the Bi,Pb(2 223) phase evolution revealed a marked change of the Avrami exponent during the course of the Bi, Pb;(2223) phase formation after about 12 hours sinte ring at 857 degrees C. The activation energy for the formation of the Bi, Pb(2223) phase has been determined under Various experimental conditions. The nominal composition o f the precursor powders, the temperature at which they were calcined as wel l as the size of their constituents were found to have a significant influe nce on the value of the activation energy. (C) 1999 Academic Press.