TEM STUDY OF THE (BI,PB)(2)SR2CA2CU3OX PHASE-FORMATION IN (BI,PB)(2)SR2CA2CU3OX SILVER-SHEATHED TAPES

The phase formation of (Bi,Pb)(2)-Sr-2-Ca2Cu3Ox (denoted as Bi/Pb(2223 )) in Bi/Pb(2223) silver-sheathed tapes has been investigated using tr ansmission electron microscopy (TEM). In partly reacted Bi2Sr2CaCu2Ox (Bi(2212)) grains, spatial transitions from Bi(2212) layers to Bi/Pb(2 223) layers along the basal plane are observed, bur they occur very ra rely. These transitions cause considerable lattice distortions. Amorph ous regions, which correspond to a liquid phase present during the hea t treatment, have slightly higher Bi and Pb contents than the adjacent Bi(2212) grains. An analysis of stacking sequences demonstrates that the majority of the layers is surrounded by layers of the same type an d that the conversion rate of the Bi(2212)-to-Bi/Pb(2223) transformati on decreases rapidly. These observations support the idea that the Bi/ Pb(2223) phase primarily forms via a nucleation-and-growth process, wh ich is orientation-controlled by the unreacted Bi(2212) grains serving as a substrate for the growing Bi/Pb(2223) phase. (C) 1998 Elsevier S cience B.V.