The crystallization mechanism of superconducting phases in the (Bi,Pb)−Sr−Ca−Cu−O system was determined on the basis of the
results of DTA, DTG and TG studies, supplemented by X-ray examination of ceramic powders obtained by the sol-gel method.
It has been demonstrated that the factor determining the formation of superconducting phases: Bi2Sr2CaCu2Ox (low-Tc) and Bi2Sr2Ca2Cu3Ox (high-Tc) is the kinetics of reaction of calcium and strontium carbonates with molten CuBi2O4. As a result of the reaction of the bimetallic compound CuBi2O4 with SrCO3 in the liquid phase the compound Bi2Sr2CuO6 is formed. This compound, reacting with calcium and copper oxides, yields superconducting phases: the low-Tc and the high-Tc phase. It has been also observed that an increase in the volume fraction of high-Tc phase in powder subjected to thermal treatment takes place probably due to the repeated disproportionation of low-Tc phase and its repeated synthesis from Bi2Sr2CuO6, CuO and CaO.