OPTIMIZATION OF Y2BACUO5 PHASE MORPHOLOGY FOR THE GROWTH OF LARGE BULK YBCO GRAINS

YBCO samples were prepared using different types of precursor powders: quenched, or YBa2Cu3O7-x and Y2BaCuO5 commercial powders, with and wi thout Pt addition. Two stoichiometries, corresponding to 0 and 40% mol ar excess of the Y2BaCuO5 phase, were adopted. The behavior of the Y2B aCuO5 phase through thermal treatments, typical of melt processes, was observed by quenching the samples at various stages. YBa2Cu3O7-x grai n growth kinetics was enhanced and liquid phase losses were limited by optimizing the morphology of the Y2BaCuO5 phase in the partially melt ed state, with the simultaneous presence of round particles less than or equal to 1 mu m and needle-like particles with very high aspect rat io. The optimized processing conditions were adopted, together with a seeding technique, to grow YBCO samples. At 77 K, a magnetic levitatio n force of 2.5 kg was measured for a sample with 18 mm diameter, and m agnetization critical current densities over 10(4) A/cm(2) were reache d at 1 T.