FORMATION AND COARSENING BEHAVIOR OF Y2BACUO5 FROM PERITECTIC DECOMPOSITION OF YBA2CU3O7-X

Formation and coarsening behavior of the Y2BaCuO5 (211) phase has been examined in samples produced by peritectic decomposition of pure YBa2Cu3O7-x (123), resulting in 211 crystals and the liquid phase [BaCuO2-CuO]. Through various temperature (1020-degrees-C-1060-degrees-C) and time (0.25 h-10 h) studies, the fundamental coarsening behavior was determined. At 1040-degrees-C, 211 crystals coarsen significantly over a 10 h period. The acicular crystals can be modeled by the diffusional ripening law, rBAR - r0BAR = (Kt)1/3, where rBAR = V1/3BAR. However, the log-normal distributions of the lengths, widths, and volumes for each coarsening run are much wider than general ripening theory would predict. Results from coarsening studies at 1020-degrees-C and 1060-degrees-C for 3 h reveal that the 211 crystal volume increases with increasing superheat. Prior coarsening of the 123 grain size yields much larger 211 particles, suggesting that the 211 crystals must nucleate at the 123 grain boundaries during peritectic decomposition, and this nucleation governs the size of the 211 crystals for short coarsening times. Addition of properitectic 211 (15 mole %) to pure 123 before peritectic decomposition strongly influences the particle habit of the resulting 211 crystals. Without any additions, acicular or needle-like 211 crystals result from the melting of 123. However, when equiaxed properitectic 211 is added to the 123, the resulting 211 is faceted, but still equiaxed. If acicular 211 is added to the starting composition, the resulting 211 is needle shaped. These results will be discussed in terms of 123 melt-texturing and directional solidification processing.