The formation of superconducting oxide phases in the (Bi,Pb)−Sr−Ca−Cu−O System

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: Bi₂Sr₂CaCu₂O_x (low-T _c ) and Bi₂Sr₂Ca₂Cu₃O_x (high-T _c ) 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-T _c and the high-T _c phase. It has been also observed that an increase in the volume fraction of high-T _c phase in powder subjected to thermal treatment takes place probably due to the repeated disproportionation of low-T _c phase and its repeated synthesis from Bi₂Sr₂CuO₆, CuO and CaO.