NUCLEATION AND GROWTH MECHANISMS OF TEXTURED YBACUO AND THE INFLUENCE OF Y2BACUO5

123 texturation has been achieved with success on Y2O3 without application of an external thermal gradient for the purpose of a better understanding of nucleation and growth mechanisms. We have studied the effects of thermal parameters such as the maximal applied temperature and crystallization speed. Their respective contributions to the peritectic recrystallization (211 consumption), and to the textured domain size have been made precise by using X-ray data and semi-quantitative analysis on the micrographs. The interface reaction between the Y2O3 substrate and 123 has been analyzed. The substrate promotes a seeded type growth of 123 owing to an interfacial 211 layer. The mechanism of crystallization has been analyzed in the light of existing theoretical models. Our data confirm a crystallization of 123 directly from a liquid as in a peritectic reaction where a major part is played by the yttrium diffusion in the liquid. Y atoms are supplied by the dissolution of 211 particles. According to Uhlmann-Chalmers-Jackson (UCJ) theory, coarsened 211 grains are trapped by the solidification front in contrast to particles smaller than a critical radius that are consumed in the peritectic reaction. (This process explains why such large 211 particles are obtained in MTG contrary to QMG or MPMG methods.) Moreover, EPR results show a preferential orientation of 211 inclusions according to the 123 matrix which could be favorable to flux pinning in the superconducting state. 211 oriented inclusions make possible, to some extent, heterogeneous nucleation at platelet-211 junctions. On the other hand, microstructural studies show that the liquid/solid interface is rather unstable being frequently cellular or dendritic, consistent with a model proposed by Alexander et al.