STUDY OF THE MECHANISM OF FORMATION AND DEVELOPMENT OF BI2SR2CA2CU3O10 FROM A LYOPHILIZED PRECURSOR

The synthesis of the stoichiometric compound, without Pb, by sintering of a lyophilised precursor has been previously described (1). We are now going to study its mechanisms of formation and development. Severa l techniques of characterisation: D.T.A., X-ray diffraction, X-ray mic roanalysis and resistivity measured as a function of temperature, for quenched materials or not, before or after sintering, were used to fol low the synthesis, by varying the parameters of the thermal treatment: temperature, time, partial pressure of oxygen. As shown by a majority of authors, the 2223 phase is indeed formed by incongruent fusion of 2212 and not by precipitation from a liquid phase. This phase can only develop when the phase (CaSr)3Cu5Oy which is rich in Sr and directly linked to the partial oxygen pressure, is absent. This observation lea ds to suggest that there is a critical concentration of Sr in 2212, tr ansmitted to 2201, which does not prevent the formation of 2223, but i ts development.