SEEDED CRYSTAL-GROWTH OF YBA2CU3O6.5 IN SEMISOLID MELTS

Melt textured single crystals of YBa2Cu3O6.5 (123) superconductors wer e produced by isothermal solidification from a semisolid melt using si ngle crystal NdBa2Cu3O6.5 or SmBa2Cu3O6.5 seeds. The microstructure wi thin the single crystals shows an inhomogeneous segregation of Y2BaCuO 5 (211) particles trapped in the 123 crystals during solidification. T he concentration of 211 particles varies with the crystal axes in 123 crystals produced from precursors with compositions of 80 wt. % 123 po wder and 20 wt. % excess 211. The 211 particle concentration along the c-axis in the crystal is much lower than that along the a- or b-axes during initial crystallization. This concentration increases in both d irections as the crystal grows larger. The 211 concentration along the c-axis increases more quickly than the concentration along the other axes during solidification, which allows the 211 concentration to appr oach that on the other axes as the solidification continues. 211 parti cle segregation in stoichiometric 123 samples formed ''X''-shaped trac ks instead of the variations in 211 concentration described above. A s ingle crystal growth model of 123 is proposed and employed to interpre t these experimental observations. Quenched samples were prepared to i nvestigate the volume fraction of 211 particles in the liquid phase. A constant distribution of 211 particles was observed in the Liquid, ex cept very near the crystal interface, where the 211 concentration decr eased rapidly. Copper oxide content in the liquid was also measured. I t is found that the copper content is lower at the (001) interface com pared with (100) or (010) interfaces.