Phase composition and grain alignment in partial melt processed Bi-2212 thick films

The melting and solidification of Bi2.2Sr2.05CaO0.95Cu2O8+delta (Bi-2212) thick films on silver substrates and bulk material during the partial melting process was investigated. The Bi-2212 formation via a liquid phase is known as a peritectic reaction resulting in a multiphase microstructure after. cooling. Therefore, special emphasis was laid on the examination of the Bi-2212 phase reformation during processing. The three crucial steps of the heat treatment were heating up to a maximum temperature above the solidus (875 degrees C), cooling with a defined rate to 850 degrees C and annealing at 850 degrees C, all done in pure oxygen (p(O-2)=1 atm). This lead to a microstructure consisting of Bi-2212 with minor amounts of a Bi-free and a Cu-free secondary phase of the stoichiometries Sr14-xCaxCu24Oy (014x24) and Bi9Sr11Ca5Ox, (91150). The one-layer compound Bi11Sr9Cu5O6+delta (Bi-11905) was also present as minority phase or as intergrowths in Bi-2212 grains. The microstructure of the material depending on the processing steps was investigated. The absolute volume fraction of the four solid phases was only slightly changed when varying the processing parameters. However, an inhomogeneous microstructure resulted when heating higher than 10 degrees C above the solidus upon partial melting and cooling slower than 5 degrees C/h. Macroscopic areas of distortions increased in size and number that could not be eliminated by post-processing long-time annealing. In some areas of the samples, liquid disappeared due to capillary forces acting on the framework of liquid, Bi-free and Cu-free secondary phases in the partially molten state. In the liquid poor areas, the network of the secondary phases could not resolve upon cooling due to the lack of liquid but remained as an open porous structure. The overall phase composition was independent of the sample thickness. The grains were aligned with their (a, b) planes parallel to the lateral sample dimensions only in films thinner than 150 mu m. In mu m thick samples, an average misalignment of +/-5 degrees was found. This degree of texturing got lost when increasing the thickness. Above a sample thickness of 200 mu m, the grains-were randomly distributed. (C) 1997 Elsevier Science Ltd.