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Mode of lead addition and its effects on phase formation and microstructure development in composite conductors

An extensive examination has been made of silver-sheathed (Bi-2223) composite conductors produced by the two-powder processing method, wherein single-phase (Bi-2212) is combined with a mix of Pb-Ca-Cu-O phases to give an overall stoichiometry of . The principal variable is the proportioning of lead between the Bi-2212 phase and the Pb-Ca-Cu-O phases. X-ray diffraction measurements and scanning electron microscopy analyses have revealed that the length of the induction period prior to the onset of Bi-2223 formation decreases monotonically as the initial lead fraction in the Bi-2212 is increased up to a value of 0.3. The observed decrease in critical current density of fully processed composites with decreasing lead fraction in the Bi-2212 phase appears to be more closely related to the quantity of unconverted Bi-2212 than to the quantity of residual alkaline earth cuprate secondary phases. Transmission electron microscopy shows that the frequency of Bi-2212 intergrowths declines with increasing lead fraction in the starting Bi-2212 component.