High-temperature x-ray diffraction studies of a precursor mixture for Pb-substituted Bi-2223 superconducting wires

Abstract

High-temperature x-ray diffraction measurements of a (Bi, Pb)-2223 precursor mixture used to produce high-J_c superconducting tapes were conducted on silver and ZrO₂ substrates. The precursor mixture consisted primarily of 2212, Ca₂PbO₄, and CuO. Phase evolution was markedly sensitive to oxygen partial pressure: In 10% O₂ growth of the 2223 phase on silver was rapid, proceeded at the expense of the 2212 phase, and was preceded by the disappearance of the Ca₂PbO₄ phase. When slowly heated on a silver substrate in 7.5% O₂ the 2212 phase melted near 800 °C and subsequently recrystallized near 820 °C in a highly textured form, but with no detectable 2223 formation. Under similar conditions on a ZrO₂ substrate, the mixture exhibited no marked changes in the XRD patterns up to 850 °C. The dramatic reactivity on silver was also highly dependent on heating rate; rapid heating in 7.5% O₂ to 825 °C did not result in melting of the 2212 phase or appearance of the 2223 phase. In experiments leading to formation of 2223, the c-lattice parameter of the 2212 phase contracted just prior to the onset of formation of 2223. This result is consistent with the formation of an intermediate Pb-doped phase of 2212. A transient amorphous phase appeared briefly at the onset of formation of 2223. No evidence for intergrowth conversion of 2212 to 2223 was observed.