Fabrication And Optimization Of FeSe-based Superconducting Materials

FeSe-based superconducting material is considered to have practical prospects due to their high current capacity,strong flux pinning properties,as well as the non-toxicity and low cost.In this paper,the superconductivity of FeSe-based superconducting materials is optimized by element doping.The influences of chemical doping on the phase composition,lattice structures,microstructures and superconducting properties were systematically analysed,and low-oxygen annealing technique was optimized.Leading to FeSe-based superconducting tapes with high current capacity was prepared.The main research contents are listed as follows:F-doping at Se sites on the phase composition and superconductivity of FeSe superconducting bulks were systematically studied.It is found that F-ions entered into the lattice of superconducting tetragonal β-FeSe phase,which influenced the phase evolution mechanism of the binary system and led to the change of superconducting phase content.Both the higher sintering temperature of 950 ℃ and the optimal F-ions doping content of x=0.02 were beneficial to the superconducting properties of FeSe bulks.Thus the maximum critical temperature,Tc of 8.3 K was obtained.However,based on the researches of Cu+,Cu2+and Co3+ions doping,it could be concluded that the doping of cations tended to lead to a severe inhibitory effect on the superconductivity of the FeSe-based superconducting system,which could lead to the vanishing of superconductivity.Fe:Se ratio on the superconductivity of FeSe system during low oxygen annealing was explored,showing that the best Fe:Se ratio was 1.15.Based on the optimization of Fe:Se ratios,Fei.isSe showed the optimized superconducting properties,the optimized annealing heat treatment conditions for Fe1.15Se superconducting samples was decided as 400 ℃ to 5%O2-10 h.The effects of different sheath materials including Fe,Ag and Nb on the phase composition and properties of FeSe-based tape were studied.Due to the inevitable reaction between both Ag and Nb sheath with Se,Fe sheath was more suitable for the preparation of FeSe system.Furthermore,the content of superconducting phase in Fe shealth FeSe tapes was increased by introducing a high-energy ball milling-aided sintering process during the precursor powder preparation process.As a result,Fe-sheathing FeSe superconducting tape with a critical current density of 340 A/cm2 has obtained.In conclusion,in this study,both the chemical composition and fabrication techniques of FeSe based superconductors have been systematically optimized.The research of this paper has led to a completely fresh orientation for the development of high-performance FeSe-based superconducting materials and practical wire preparation process.Therefore,this study provided a steady base in theoretical and experimental study of FeSe based superconductors.