SOLIDIFICATION OF YBA2CU3O7-DELTA FROM THE MELT

Nucleation and growth of 123 from the melt via a peritectic reaction i nto domains of aligned platelets is studied. Analysis of the microstru cture of well-formed domains indicates that there is no orientation di fference between adjacent platelets within a domain, suggesting that a domain grows from a single nucleus. The platelet boundaries are found to be filled-in with secondary phases that correspond to the liquid p hase at high temperature, suggesting that constitutional supercooling effects may be operative. Samples quenched from temperatures considera bly below the peritectic temperature contain only a few crystals, indi cating the presence of a large nucleation barrier. The above observati ons, coupled with extensive microstructural examination of quenched so lid-liquid interfaces, suggest that the 211 size, distribution and vol ume fraction not only control the growth rate of 123 along the fast gr owth ab-plane (by supply of yttrium), but also the growth rate along t he slow growth c-direction since the nucleation barrier is reduced at 211/123 intersections. At high cooling rates there is a distinct chang e in the nucleation and growth processes. Structures characteristic of sympathetic or autonucleation and spherulitic growth are observed. Th ese structures are distinct from the single crystal nature of well-for med domains. The growth mechanism which results in the formation of 12 3 domains and the final microstructure within a single domain, also ex plains the observed non-weak-link characteristics for current flow alo ng the a, b- and c-directions, as determined by direct transport and m agnetization measurements.