Research On The Growth Interface Characteristic And Phase Transformation Mechanism Of YBCO

The anisotropy crystal structure and atomic bonding type of YBa₂Cu₃O_7-δ (YBCO) endow it with unique solidification characters, so, the preparation of YBCO with the ab-crystal face of it well orientated by unidirectional solidification method is much more complex and challenging than the preparation of metal materials by the same technique. Primary powders of micron dimension diameter have been prepared through multi-step sintering procedure and batch type grinding process, and temperature field with proper temperature gradient has also been established. Unidirectional solidification process and PMP synthesis method have been employed together during the sample preparation process. YBCO rods with the angle between ab-crystal plain and axial direction within 5° have been successfully prepared via adjusting the processing parameters, such as the maximum heat processing temperature and pulling rate, et al. Besides, quenching samples and samples containing NdBa₂Cu₃O_7-δ seed have also been fabricated.The interface structures of different crystal face of YBCO under semi-melt state have been observed by SEM and the Jackson factors of them have also been calculated, and based on which, the ab crystal face and ac/bc of YBCO have been determined to be facet and non-facet respectively. Based on the above research results, it has been confirmed that the growth process of YBCO along c-axis direction is controlled by both of solute diffusion and atomic deposit kinetics, thus the growth velocity of YBCO along c-axis direction is lower than that along a- and b- axis direction, they can be described by different velocity equation. The ac/bc crystal faces of YBCO tend to branch off to cellular-like structure affected by the constitution supercooling factor and thus lead to component segregation in the micro zone ahead the YBCO growth front. Based on the above analysis, an appropriate mode has been established at last to interpret the common texture structure of YBCO made by melt-textured process.The nucleation and growth of YBCO as well as the growth of Y₂BaCuO₅ during the non-equilibrium phase transformation process have also been studied by means of elementary analysis based on the solute diffusion viewpoint. Judging by both the morphology observation and theoretical analysis results, it has been confirmed that the YBCO phase prior to heterogeneously nucleates on the surface of 211 grain when there exist 211 particles in the system. And then, the Y³⁺ yields from the solution of 211 grains will diffuses through the liquid phase to the growth front of YBCO to contribute...