Isothermal study of the formation mechanism of 2223 phase in BPSCCO and BPSCCO-10 wt% Ag precursors

M.T. MALACHEVSKY; D.A. ESPARZA

THERMOCHIMICA ACTA

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1995 vol. 265 p. 197 - 207

0040-6031

Isothermal heat treatments were employed to investigate the transformation 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 phase composition did not include these superconducting phases. Silver was added at 10wt% to observe modifications in the reaction. The sintering conditions were chosen to enhance the ll0K phase formation: 840°C for undoped and 825°C for doped precursors in an atmosphere of 7% oxygen in nitrogen. With the aid of the Rietveld refinement, the fractional conversion of 2223 phase with the annealing time was calculated. Silver addition was observed to stabilize the PbCa20 4 phase and to slow down the initial formation of the 2212 phase, consequently delaying the transformation to 2223 phase. SEM analysis of the samples showed that in both cases, with and without silver, the 110 K phase 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 liquid.