Melt textured single crystals of YBa2Cu3O6.5 (123) superconductors were produced by isothermal solidification from a semisolid melt using single crystal NdBa2Cu 3O6.5 or SmBa2Cu3O6.5 seeds. The microstructure within the single crystals show s an inhomogeneous segregation of Y2BaCuO5 (211) particles trapped in the 123 cr ystals during solidification. The concentration of 211 particles varies with th e crystal axes in 123 crystals produced from precursors with compositions of 80 wt.% 123 powder and 20 wt.% excess 211. The 211 particle concentration along th e c-axis in the crystal is much lower than that along the a- or b-axes during in itial crystallization. This concentration increases in both directions as the c rystal grows larger. The 211 concentration along the c-axis increases more quic kly than the concentration along the other axes during solidification, which al lows the 211 concentration to approach that on the other axes as the solidificat ion continues. 211 particle segregation in stoichiometric 123 samples formed "X "-shaped tracks instead of the variations in 211 concentration described above. A single crystal growth model of 123 is proposed and employed to interpret thes e experimental observations. Quenched samples were prepared to investigate the volume fraction of 211 particles in the liquid phase. A constant distribution o f 211 particles was observed in the liquid, except very near the crystal interfa ce, where the 211 concentration decreased rapidly. Copper oxide content in the liquid was also measured. It is found that the copper content is lower at the ( 001) interface compared with (100) or (010) interfaces.
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©CPRL, 1995