Phase Evolution Of Bi-2223High-Temperature Superconducting Tapes During Heating Treatment

Bi-based superconductor can be considered as an important branch of the high-temperature superconductor. Especially, Bi-2223high-temperature superconductors, with high Tc, esay processing and strong current-carring capacity have become one of the most important practical high temperature superconducting materials. The improvement of Jc is closely related to the content of2223phase, the texture of2223crystals and the vestigital second phases. While the residual second phases which confine the texture growth of2223phases and lower the2223phase content have an obviously effect on the current capacity performance. Therefore, this thesis is based on exploring the phase evolution process of the2223tapes during heat treatment and investigating the involved phase transition mechanism. The results of this thesis can provide a basis to limite the second phase and solve the problems of2223phase formation in tape.This paper takes two-powder process as a framework which starts with the co-precipitation preparation of2212powder and CaCuO2powder. During the powder preparation process, phase composition, phase associated and phase evolution progress of the2212powder in each sintering stages have been systematically studied. Optimized heat treatment temperature and atmosphere have been applied and the content of characteristic phases, such as (Bi,Pb)-2212,2201,Ca2PbO4,14:24AEC, etc, have been precisely controlled. The final powders after calcination contains (Bi,Pb)-2212more than95%, Ca2PbO4content of<1%, no14:24AEC and very low carbon content. Meanwhile, CaCuO2powder (Ca2CuO3and CuO mixed powders), in which Ca and Cu nominal content are1:1, has alse been synthesized.(Bi,Pb)-2223tapes are prepared using PIT method. By using XRD, SEM, EDX and other analytical methods, the evolution of phase composition are systemically studied during HT1, HT2and post-processing. In order to study the phase evolution of tapes in the first heat treatment process,2223tapes have been quenched in air at different heat treatment process, during both heating and dwell ptocess. The results show that the Pb-rich phases melted and decompose during heating process, Pb ions enter into2212, and the (Bi,Pb)-2212phase which can be easily converted into2223is formed. At the same time, Ca2CuO3and CuO convert into smaller14:24AEC and2:1AEC with smaller size. During the dwell process,(Bi,Pb)-2212and AEC gradually react and2223phase content increase with increasing time. As the result, the formation mechanism of2223is proposed:during sintering progress2223phase is formed by multiphase solid-state reaction,2212phase participated in the solid sate-reaction. Ca2CuO3and CuO first convert to the AEC phase (including2:1the AEC and14:24the AEC phase). Then2212reacts with AEC phase to form the2223phase.The working rate in intermediate rolling progress has greater effects on the current capacity performance of the final tapes. After the first heat treatment, tapes are rolled into different thicknesses, and then the critical current is measured by the four-probe method. The results show that smaller or larger working rate decrease the Jc values of2223tape. Smaller working rate cannot disrupt the AEC packed by the2223phases and2223parcel layers, therefore influence the2223formation progess at the second heat treatment process, and also show difficulties to reduce the holes density in the tapes. While larger working rate tend to cause larger cracks, which cannot be easily healed during the second heat treatment. The optimum working rate has been decided to be45%(tape thickness of0.23mm), the critical current density of the tape reaches5400A/cm2.In order to further increase the content of2223phase, different samples have been obtained by quenching at different time during the dwell process of the second heat treatment, and then the evolution of phase composition is analyzed. The results show that in HT2stage the disrupted AEC phase continously forms the2223phase, meanwhile bridging the cracks. After HT2the content of2223obtains more than90%in the tapes. In the post annealing-processing stage in air at800℃, the amorphous phases are transformed into3321phases which have a good wettability to the2223phases. The final heat treatment process can greatly increase the contact area between the2223layer, thereby improving the connectivity between the2223grains in the tapes and thus the critical current density.