PHYSICOCHEMICAL CAUSES FOR THE MICROSTRUCTURE OF MELT-TEXTURED YBA2CU3O7-DELTA Y2BACUO5 COMPOSITES/

We report semiquantitative results for a physicochemical model intende d to describe the multigrain growth of YBa2Cu3O7-deltaN2BaCuO5 (123/21 1) composites under isothermal undercooling. This model takes into acc ount various ingredients supposed to be controlling the growth of such superconducting compounds, i.e. the presence of secondary phases. Mor eover, a possible dynamical interaction between the growing 123 grains and the 211 solid particles is also included. The numerical investiga tions were performed for a model bivariate-like 211 particle distribut ion. The complete chemical dissolution of the small particles is allow ed but the large ones are only partially dissolved in the first step w hich can be followed when chemically possible by a complete dissolutio n. The model is restricted to a two-dimensional square lattice. Variou s microstructural morphologies are obtained as a function of the initi al composition of the melt and the initial size distributions of the 2 11 particles. Results are in quite good agreement with experimental ob servations. The quantity of 123 phase is predicted depending on the in itial conditions. The optimal situation when a minimum fraction of liq uid phase segregates at the grain boundaries is found to occur for 20% excess of 211 phase in the initial melt.