THE MICROSTRUCTURE OF INTERFACES IN MELT-TEXTURED YBA2CU3O7-X - MECHANISMS OF FORMATION

Microstructures of textured YBa2Cu3O7-x ceramics were examined by transmission electron microscopy. The samples were synthesized under a residual thermal gradient by using modified melt textured growth on a Y oxide substrate. Defects running parallel to stacking (ab) planes are in fact growth defects associated with 211 trapped particles. Microanalysis of such areas proves the presence of solidified liquid in these defects. The growth is cellular or dendritic and the texture has many characteristics typical of a single crystal. Interfaces formed between 211 and 123 were studied and analysed with reference to the crystallization mechanism. Two types of interface were evidenced: (A) those for which the matrix is deformed and (B) those for which structural (and chemical) modifications of the inclusions are the main characteristics. The interfaces belonging to group A may be classified according to the types of defect they create in the 123 matrix. Most coherent interfaces (interface planes parallel to (010)(123), observed when {110}(123) are parallel to {001}(211)) induce very few defects in the matrix; interfaces with ordinary coincidences give rise to strongly disordered areas. Interfaces with no planes in coincidence with the matrix are characterized by microprecipitates, which may be viewed as an alternative way for stress relaxation. Type B interfaces are found for small 211 particles; these induce more strain in the matrix than larger ones, and their composition and structure are also more defective.