ISOTHERMAL STUDY OF THE FORMATION MECHANISM OF 2223-PHASE IN BPSCCO AND BPSCCO 10-WT-PERCENT AG PRECURSORS

Isothermal heat treatments were employed to investigate the transforma tion mechanism from 2212 phase to 2223 phase in the (Bi, Pb)-Sr-Ca-Cu- O system. Precursors prepared by a citrate route were annealed; the ph ase composition did not include these superconducting phases. Silver w as added at 10 wt% to observe modifications in the reaction. The sinte ring conditions were chosen to enhance the 110K phase formation: 840 d egrees C for undoped and 825 degrees C for doped precursors in an atmo sphere of 7% oxygen in nitrogen. With the aid of the Rietveld refineme nt, the fractional conversion of 2223 phase with the annealing time wa s calculated. Silver addition was observed to stabilize the PbCa2O4 ph ase and to slow down the initial formation of the 2212 phase, conseque ntly delaying the transformation to 2223 phase. SEM analysis of the sa mples showed that in both cases, with and without silver, the 110K pha se grew by deposition of a viscous liquid in layers over the platelets . We assigned this mechanism to the coherent superficial nucleation of 2223 phase on 2212 plates, the rate of formation being controlled by nuclei growth at a speed related to the velocity of migration of the l iquid.