Effects of PtO2 and CeO2 additives on the microstructures of the quenched melts of Y-Ba-Cu-O materials

The microstructures of the quenched melts of samples of Y123 and Y123 +- 15 -20 mol% Y211 with PtO2 and CeO2 additives have been examined with optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spe ctrometry (EDS) and X-ray Diffractometry (XRD). Significantly higher temper atures are required for the formation of dendritic or lamellar eutectic pat terns throughout the samples with PtO2 and CeO2 additives as compared to sa mples without additives. The BaCuO2 (BCl) phase appears first in solid form and, instead of rapidly melting, is slowly dissolving or decomposing in th e oxygen depleted melt. PtO2 and CeO2 additives slow down or shift to highe r temperatures the dissolution or decomposition process of BCl. A larger fr action of BCl in solid form explains why samples with additives have higher viscosities and hence lower diffusivities than samples without additives. There is also a reduction in the Y solubility to about half the value in sa mples without additives. The mechanism that limits the Ostwald ripening of the Y211 particles is correlated to the morphology of the quenched partial melt. It is diffusion controlled for a finely mixed morphology and interfac e-controlled when the melt quenches into dendritic or lamellar eutectic pat terns. The change in the morphology of the Y211 particles from blocky to ac icular is related to an equivalent undercooling of the Y-Ba-Cu-O partial me lt, particularly through the crystallization of BCl. (C) 2000 Elsevier Scie nce B.V. All rights reserved.