The processing of (Bi, Pb)2223 tapes has been investigated extensively since the first OPIT tapes were made in 1989. The formation of Bi-2223 is via a Pb-induced liquid phase, which is known to assist the Bi-2223 formation. However, there are many unsolved microstructural inhomogeneities and defects in current OPIT tape; consequently research has begun to focus on the role of the liquid phase. In this study differential thermal analysis results on green (Bi, Pb)2223 tape in 7.5% pp O2 give direct evidence for two phase transitions, with onset temperatures of 800 and 820 °C. XRD phase analysis on tapes heat treated for prolonged periods show two distinct secondary phase formation regions: between 800 and 820 °C; and above 820 °C. A link is drawn between the phase transitions in the initial stages of Bi-2223 formation and the long-term secondary phase formation. The relevance of these results to transport properties is discussed.

Recent differential thermal analysis studies on green Bi-2223 tapes showed two phase transitions, with onset temperatures 800 and 820 °C. From microstructural examination of the long-term phase formation in the tapes the higher temperature endotherm was associated with a partial melt. In this study it is demonstrated how control of the heating rate can promote the partial melt (characterized by DTA), and thereby improve the transport current, Ic. XRD, SEM and transport measurements in field show microstructure features typical of an increased volume of liquid phase: a reduction in secondary phase volume and pores with a corresponding increase in c-plane texture.

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