STUDY OF (BI,PB)-2223 PHASE-FORMATION IN REACTION COUPLES

Reaction couples of either Bi1.8Pb0.3Sr1.8Ca0.7Cu1.7O8+delta ((Bi,Pb)- 2212) single crystals with 'Ca2CuO3+CuO' powder or Bi2.1Sr1.9Ca1.0Cu2. 0OB+delta (Bi-2212) single crystals with '(1/2)Ca2PbO4+CuO' powder wer e used to study the formation reaction of (Bi,Pb)-2223. The samples we re annealed between 800 degrees C and 840 degrees C, in air, for 65 h. Microstructural observations were made of the different geometrical i nterfaces. No (Bi,Pb)-2223 platelets were formed below 820 degrees C. Above 820;degrees C, the (Bi,Pb)-2212 and (Bi-2212) crystals were foun d to break up near the interfaces with the powders, and (Bi,Pb)-2223 p latelets were formed adjoining the (Bi,Pb)-2212 and (Bi-2212) crystall ites. No evidence for large quantitites of liquids was found, although small, (< 2 mu m) rounded particles having a liquid droplet appearanc e of a Bi/Pb rich phase of stoichiometry near the (Bi,Pb)(3)Sr2Ca2CuOy (3221) phase were observed on many of the (Bi,Pb)-2223 platelet surfac es. The experiments suggest that the formation of (Bi,Pb)-2223 occurs simultaneously with decomposition of (Bi,Pb)-2212 and the presence of liquid droplets, providing further support for the dissolution-repreci pitation model for phase formation.