PHASE ASSEMBLAGE AND MORPHOLOGY DURING THE PARTIAL MELT PROCESSING OFBI-2212 THICK-FILMS

Bi2.2Sr2.05Ca0.95Cu2Ox (Bi-2212) thick films which are put on silver s ubstrates and heated in pure oxygen, decompose peritectically at 890 d egrees C. The decomposition products are the Bi-free compound Sr8.5Ca5 .5Cu24Ox (0<(14)under bar>x<(24)double under bar>), the Cu-free phase Bi9Sr11Ca5Oy (9<(11)under bar>50), a Bi-rich liquid and oxygen. Slight ly above T-solidus the solid peritectic phases occupy volume fractions of 15 to 16% each. The rest of the sample consists of liquid. At high er temperatures, the volume fractions of the solid phases decrease and their grains coarsen. Above 922 degrees C the Bi-free phase 0<(14)und er bar>x<(24)under bar> disappears and Bi-free grains with the composi tion 01x1, 02x1 and 01x0 are formed. During slow cooling at a rate of 5 K/h from the partially molten state, Bi-2212 precipitates directly f rom the melt at T less than or equal to 875 degrees C. Upon further co oling, more of the Bi-2212 phase is formed under consumption of the pe ritectic phases. At 85 degrees C, there are 5 vol% liquid and 5 vol% s olid peritectic phases left in the microstructure, even after an isoth ermal annealing of 48 h at 850 degrees C. The kinetics of the Bi-2212 formation from the melt during cooling are controlled by the uptake of oxygen from the atmosphere and at the same time by the dissolution of the solid peritectic phases in the melt delivering the needed cations for the Bi-2212 formation. (C) 1997 Elsevier Science B.V.