EFFECTS OF Y2BACUO5 AND PLATINUM ADDITIONS ON THE UNIDIRECTIONAL SOLIDIFICATION OF YBA2CU3O7-DELTA

The effects of 211 and/or Pt additions on the solidification of YBaCuO according to the horizontal Bridgman method have been studied. It was found that mixing the pure 123 powder with 20 wt% very fine (1 mu m) and regular in size 211 particles gave rise to a strong increase in th e viscosity of the peritectic liquid and to a dramatic refinement of t he distribution of the 211 particles finally trapped into the 123 matr ix of the samples. Moreover, it was observed that the extra addition o f 0.5 wt% Pt refined further this distribution, which we interpret as due to a Pt induced hindrance of the growth and the coarsening rates o f the 211 particles in the peritectic liquid. The effects of 211 and P t additions on the size of the 123 grains were also studied: it was co ncluded that the former increased the 123 grain size because it made t he constitutional supercooling smaller, whereas the latter increased i t due to heterogeneous nucleation of 123 grains on globules of a Pt co mpound. To quantify the various observed effects, the variations of th e mean diameter of particles, their area fraction and their density we re measured as a function of the distance from the first solidified en d. Results are also presented for samples prepared from unreacted prec ursors which were simultaneously synthesized and melt textured without experiencing the peritectic melting.