Physicochemical causes for the microstructure of melt-textured YBa2Cu3O7-delta/Y2BaCuO5 composites

We report semiquantitative results for a physicochemical model intended to describe the multigrain growth of YBa2Cu3O7-deltaN2BaCuO5 (123/211) composites under isothermal undercooling. This model takes into account various ingredients supposed to be controlling the growth of such superconducting compounds, i.e. the presence of secondary phases. Moreover, a possible dynamical interaction between the growing 123 grains and the 211 solid particles is also included. The numerical investigations were performed for a model bivariate-like 211 particle distribution. The complete chemical dissolution of the small particles is allowed but the large ones are only partially dissolved in the first step which can be followed when chemically possible by a complete dissolution. The model is restricted to a two-dimensional square lattice. Various microstructural morphologies are obtained as a function of the initial composition of the melt and the initial size distributions of the 211 particles. Results are in quite good agreement with experimental observations. The quantity of 123 phase is predicted depending on the initial conditions. The optimal situation when a minimum fraction of liquid phase segregates at the gain boundaries is found to occur for 20 % excess of 211 phase in the initial melt.