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

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% O-2 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% O-2 the 2212 phase melted near 800 degrees C and subsequently recrystallized near 820 degrees 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 degrees C. The dramatic reactivity on silver was also highly dependent on heating rate; rapid heating in 7.5% O-2 to 825 degrees 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.