TEM and SEM studies on the formation of superconducting phases in the Bi-based system

We present the formation mechanism and structure of the superconducting pha ses in the Bi-Sr-Ca-Cu-O system by TEM and SEM observations. Oxalate coprec ipitation was used as preparation method. The average particle size, distri bution and the presence of different phases and shapes of the particles wer e evaluated. All samples were well crystallized, showing grains in submicro nic range. After thermal treatments, the complex samples showed mica-like c rystals uniform dispersed and also an orientation tendency. The superlattic e was composed of building blocks and successive planes. The TEM observatio ns were in agreement with the diffraction patterns that revealed a mixture of 2212 and 2223 superconducting phases. With the building blocks in the sy stem, it is possible to form many incommensurate phases that destroy the sy mmetry. Careful control of the preparation conditions is essential to produ ce a well-ordered commensurate phase, suitable for the precise determinatio n of the complete superstructure by X-ray diffraction. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.