KINETICS AND MECHANISMS OF FORMATION OF THE BI2SR2CUOX SUPERCONDUCTOR

The solid state reactivity in the Bi, Cu, Sr/O pseudoternary system has been investigated using monodimensional diffusion experiments on selected reacting couples and differential thermal analysis, thermogravimetry, X-ray, powder diffraction, optical and scanning electron microscopy and electron microprobe analysis on binary and ternary powder mixtures during isothermal or polythermal reactions. The results show that only three binary compounds are relevant for 221 formation kinetics. They are Bi2CuO4, Bi2Sr2O5 and the wide range solid solution with rhombohedral symmetry and composition close to Bi4SrO7. The last compound shows the fastest formation kinetics from almost any couple of Bi- and Sr-containing phases, but does not eventually lead to the superconducting material, and therefore acts as a spurious phase or as a source of spurious phases in many ternary mixture reactions. Bi2Sr2O5 is formed at a reasonable rate from Bi4SrO7 but does not significantly react with CuO. Attention should be given also to some particular features of the ternary system, such as: (i) Bi2O3 sublimation (which makes more complex some solid state reaction by providing alternate mechanisms); (ii) the very slow reactivity of SrO in comparison to its precursors (so that conditions favouring precursor decomposition should be avoided); and (iii) the equilibrium between the superconducting phase and its insulating polymorph. A promising way towards the 221 superconductor involves: (i) Bi2CuO4 formation from the parent oxides and (ii) its reaction with a Sr-containing phase. SrO2 precursor, low temperature and P(O2) = 10-20 atm are required for the latter step.