Distribution of Ag and Y₂BaCuO₅ in Superconducting YBCO/Ag Composite Fabricated by Unidirectional Solidification

Abstract

The distribution of 211(Y₂BaCuO₅) and Ag particles in the columnar 123(YBa₂Cu₃O_7−x) crystal of YBCO/Ag composites solidified unidirectionally was investigated to clarify the peritecto-monotectic solidification mechanism. The fabricated 123 crystal has a faceted cell and a band structure of 211 particles at the center of the cell, because of radial solute diffusion layer at the cell tip where 211 particles can’t sufficiently dissolve into the melt. Ag-free region was also formed at the same central region of the 123 cell by pushing of Ag particles, which sizes are under a critical radius (r_c), by the 123 growth interface. The relation between r₀ and growth rate R_i (: the following R_a or R_c) of faceted planes was given by the equation: R_i=0.52⁄r_c. In the Ag added hyperperitectic YBCO, the amount of the 211 particles in the 123 phase was larger in the Ag-rich region than in the Ag-free region because the amount of 123 phase in the Ag-rich region was smaller than that in the Ag-free region. Therefore, the distribution of 211 particles in the 123 phase takes a duplicated structure. The growth direction of the cell tip was obtained by analyzing the above band structures of Ag-free regions or 211-rich regions which correspond to the growth trace of cell edges. From the measured crystallographic orientation of the cell, the relation between R_a, R_c and R are R_a(R_b)=0.66R and R_c=0.54R, where R (μm/s), R_a, and R_c (μm/s) are macro-growth rate of each faceted plane, respectively.