Fabrication And Optimization Of Bi-2223 Superconducting Materials

(Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223)high temperatur^e superconducting(HTS)tapes,called as the First generation HTS tapes,have the most sophisticated fabrication techniques.Therefore,it has been adopted in many practical superconducting application fields.As a kind of oxide ceramic material with complex chemical composition,due to its complex phase formation mechanism,many secondary phases are involved in the reaction during the Bi-2223 phase formation process,which all have great impacts on the final performance of Bi-2223 tapes.In this work,both the fabrication technique of precursor powders and the cold def'ormation process have been optimized,combining with the optimization of heat treatment process.The influence mechanisms of key parameters on the current capacity of Bi-2223 tapes were systematically discussed,and we obtained the research results as follow.First of all,the fabrication technique of precursor powders based on double powder coprecipitation process was optimized,by tuning the particle size of CaCuO2 powders as well as the Sr content in Bi-2212 powders.The optimal CaCuO2 particles size was achieved as less than 750 nm,with corresponding coprecipitation pH value of 3.0.On the other hand,the optimal Sr content of the double powder was determined to be 1.85.Finaly the maximum critical current of 128 A was achieved,corresponding to the critical current density value of 21 kAcm-2.Secondly,the fabrication technique of precursor powders based on single powder coprecipitation process was developed and optimized.The precursor powders with the chemical compositions based on the reports by American superconducting company(AMSC)and Merck were both fabricated with coprecipitation process.Based on the characterization of both powders and tapes,the influences of powder composition on microstructure,phase composition and current-carrying property were clarified,and Bi-2223 tapes with higher critical current density were achieved.Lastly,the cold working process involving with the powder in tube(PIT)process,including powder packing process,drawing process and rolling process were all systematically optimized.The research showed that the powder packing atmosphere with lower oxygen partial pressure was suitable for the achievement of higher Bi-2223 phase content and better texture structures.During the drawing process,reducing the annealing times was beneficial to the better coordination deformation between Ag sheath and Bi-2223 precursor powders.And the reduction rates per pass during the initial rolling process and intermediate rolling process were both optimized.Due to the increase of filament density and enhancement of texture structures,the current capacity was effectively enhanced.Based on the above study,several key parameters involved in the precursor powder fabrication process,cold deformation process as well as the heat treatment process have been optimized systematically.The results of this work can bult a solid foundation for the development of Bi-2223 fabrication technique.