Effect of secondary phases in the precursor powders on the transformation to the (Bi,Pb)-2223 phase

摘要

The (Bi,Pb){sub}2Sr{sub}2Ca{sub}2Cu{sub}3O{sub}y system [also know as (Bi,Pb)-2223] has shown promise for use in superconducting tapes and wires due to its high transition temperature and high critical current density. The tapes and wires are usually fabricated by the powder-in-tube method, after which they are subjected to thermal and mechanical treatments. Depending on the nature of the heat treatment, phase transformations occur in the powder and it is of paramount importance to understand the response of the powders to different processing conditions. In this study, we have synthesized a precursor powder of the nominal composition Bi{sub}1.8Pb{sub}0.33Sr{sub}1.87Ca{sub}2Cu{sub}3O{sub}y by spray pyrolysis. The powder was then calcined under controlled conditions between 760 and 800°C in an atmosphere between 0 and 21 % O{sub}2 for 2 to 24 hours with controlled heating and cooling, such that different amounts of Pb are incorporated into the majority phase. The resultant powders were then converted to the (Bi,Pb)-2223 phase with suitable heat treatments. Using powder X-ray diffraction and magnetization measurements, the volume fractions of the secondary phases in the precursors were estimated and the effect of the phases on the conversion to the (Bi,Pb)-2223 phase was studied. We show that the ratio of the intensity of the (020) and (115) reflections of the (Bi,Pb)-2212 phase can be used to tune the precursor powders for optimal conversion. While a lower fraction of the secondary phases such as CaO and CuO is desirable in the precursor powders, a higher volume fraction of Ca{sub}2PbO{sub}4 seems to help in the rapid conversion to the (Bi,Pb)-2223 phase.