Fabrication And Properties Of Bi-2223/AgAu Superconducting Tape

Large scientific devices,such as thermonuclear fusion reactor,particle accelerator and superconducting magnetic energy storage are important equipment reflecting the advanced technology of science and technology.These devices need to be operated under extremely low temperature conditions.Therefore,current lead connecting the cryogenic superconducting magnet and room temperature circuit becomes the key component to ensure the stable operation of the equipment at low temperature and reduce the refrigeration cost.Because of the low thermal conductivity of Ag Au alloy,current leads prepared by Ag Au sheathed Bi₂Sr₂Ca₂Cu₃O_x（Bi-2223）high temperature superconducting tapes can effectively reduce the system heat leakage,which is currently the key material for preparing superconducting current leads.However,studies have shown that the critical current of Bi-2223/Ag Au tape is generally lower than that of other alloy sheathed Bi-2223/Ag M（M is Mg,Cu,Mn,etc.）tapes,and the related mechanism has not been thoroughly studied.At the same time,oxalate co-precipitation method,as the common powder preparation technology,has complex process and micro scale inhomogeneity,which can not meet the needs of engineering application.This paper aims to improve critical current and homogeneity of Bi-2223/Ag Au tapes,and introduces spray pyrolysis method instead of traditional co-precipitation method.The precursor powder with uniform distribution of chemical composition and phase composition was obtained,and the key parameters of spray pyrolysis method,powder sintering,tape rolling and tape heat treatment were optimized.The phase evolution mechanism of spray pyrolysis powder during the heat treatment process,the texture evolution of tape rolling process,and the limiting effect of Au element on the critical current were clarified.Finally,Bi-2223/Ag Au tape with high current current was obtained.The main achievements are as follows:The powders prepared by spray pyrolysis and oxalate co-precipitation method were compared systematically.It was proved that the powder prepared by spray pyrolysis method had higher reactivity,lower element loss rate,smaller particle size and more uniform distribution of non-superconducting phase.These characteristics contributed to the tapes with highly uniform current-carrying performance.The main technological parameters of the spray pyrolysis method were optimized,and the best preparation parameters were obtained,and the high quality Bi-2223 precursor powder was successfully prepared.The influence of spray pyrolysis temperature and solution concentration on the phase composition and morphology of the final powders is obvious among the various process parameters.The spray pyrolysis temperature is directly proportional to the decomposition and pre-reaction degree of the powder,and the optimum content and size of（Sr,Ca）_xCu_yO_z（AEC）phase and Cu O phase can be obtained at the decomposition temperature of 800℃.The solution concentration mainly affected the particle size and phase composition of the powder,and finally tuned the number of holes and cracks and the size of AEC phase in the tape.The results showed that the precursor powder had the best phase distribution and the corresponding tape exhibited the highest critical current when the solution concentration was 1.5mol/L.The phase evolution of Bi-2223 precursor powder during heat treatment was systematically elucidated.With the increase of heat treatment temperature,the nitrate in the spray powder decomposed and a pre-reaction occurred between some components.When the temperature reaches 566℃,Bi₂Sr₂Cu O_z（Bi-2201）phase began to form in the powder.At775℃,the Bi-2201 phase reacted with other second phases to form Bi-2212 phase,and the phase composition was（Bi,Pb）-2212,2:1AEC,1:1AEC,14:24AEC and Cu O,which was a suitable phase composition of Bi-2223 precursor powder.However,when the temperature further increased to 813℃,the liquid phase began to appear,and Bi₂Sr₂Ca Cu₂O_z（Bi-2212）phase melted and decomposed.In the previous study of Bi-2223 phase formation kinetics,it was found that the oxygen content in the sintering atmosphere had a significant effect on the phase formation process.Therefore,the sintering atmosphere was optimized first.By changing the oxygen partial pressure in the sintering atmosphere,it was found that the highest critical current of Bi-2223 tape could be obtained when the precursor powder was sintered in the atmosphere with 0.1%oxygen content.While increasing the oxygen content in the sintering atmosphere,the activity of the spray powder decreases,and Pb gradually precipitated from the（Bi,Pb）-2212 phase to form Ca₂Pb O₄.At the same time,although the content of 2:1AEC,1:1AEC and Cu O phases in the precursor powder gradually decreased,the grain size of Bi-2223 also decreased,which reduced the intergranular connectivity of this system.Then,the sintering temperature and sintering time were optimized under the fixed oxygen content of 0.1%.Finally,the optimal sintering process of the precursor powder was obtained at 770℃for 12h,and the highest critical current（I_c～110A@77K,0T）and superconducting transition temperature（T_c～110.45K）could be obtained for corresponding Bi-2223 tape.The rolling process of green tapes was optimized,and the relationship between the rolling process and the density,flatness and texture of filaments was obtained.Research on the rolling process of five pass reductions（10%,20%,40%,52%and 75%）showed that with the increased of pass reduction,the core density of the tape gradually increases,and it approached the maximum value when the reduction of the pass reached 20%.However,the flatness of the filaments decreased and the overlap between the filaments become more and more serious with increasing reduction per pass..Considering that the grain texture was the result of the combined effect of filament flatness,filament density and density uniformity,the Bi-2223/Ag Au tape prepared with the rolling process of 20%pass reduction had the highest grain texture and critical current.Due to the different morphology between the powders fabricated with the spray pyrolysis process and the conventional co-precipitation process,it was necessary to systematically optimize the sintering parameters of Bi-2223/Ag Au tapes.The results showed that when the content of Bi-2223 phase in the first heat treatment process（HT1）was about 80%,there were smallest microcracks and holes and the best interface smoothness between silver and superconductor.The effects of cooling rate at HT1 stage and heat treatment temperature on the critical current were mainly reflected with the content of Sr rich 14:24AEC phase,（Pb,Bi）₃Sr₂Ca₂Cu O_x（Pb-3221）phase,Bi-2223 superconducting phase content and superconducting transition width.Based on the above process optimization,the critical current of Bi-2223/Ag Au tape has been enhanced to 92.8A（77K,0T）.The factors limiting the increase of critical current of Bi-2223/Ag Au tape are described.The large hardness differences between Ag Au matrix material and superconducting core resulted in the more deformation of Ag Au matrix during intermediate rolling.So there were more holes,cracks,and low-density areas in the core,and the overall density of the wire was lower than that of Bi-2223/Ag Mg Ni tape,which was considered as one of the factors that limit the increase of critical current.The doping of Au element changed the oxygen diffusion behavior during the Bi-2223 formation process,which in turn changed the phase formation kinetics of Bi-2223 in Ag Au sheath from other alloy matrix tapes,which thus could be recognized as another factor limiting the increase in critical current.In addition,the Au element diffused into the superconducting core during heat treatment,which changed the phase forming kinetics of Bi-2223.It resulted in the decrease of 1:1AEC and 14:24AEC phase in Bi-2223/Ag Au tapes,the decrease of Bi-2223 superconducting phase content and the increase of superconducting transition width.These changes become another factor limiting the increase of critical current of Bi-2223/Ag Au tape.