Manufacturing And Properties Of Ge,Sn Co-Doped Nb₃Al Superconducting Wires

Since Nb₃Al has a higher upper critical field(H_c2)and a larger strain allowance than Nb₃Sn,while Rapid-Heating,Quenching and Transformation（RHQT）method for long-length wire manufacturing of such material has been well developed worldwide,the conductor of this material has good potential in large-scale magnet application scenarios such as fusion,accelerator and nuclear magnetic resonance（NMR）in the future.In this paper,on the basis of the RHQT method,the superconducting properties of Nb₃Al powder-in-tube（PIT）wire including T_c,H_c2and low&medium field J_cparameters were further optimized by doping with Sn and Ge elements.The effects of Sn and Ge element single/co-doping on the phase stability,microstructure and composition distribution of supersaturated solid solution Nb（Al）_ss obtained by Rapid-Heating and Quenching（RHQ）treatment were also investigated.Finally,the ideas and conjectures for continuing to optimize the RHQT method are given.The main research results of this paper are as follows:For in-situ Sn doping,the phase stability of Nb（Al）_ss is more sensitive to RHQ temperature.The RHQ treatment peak temperature for single-phase Nb（Al）_ss is increased from 1940°C for 4%Sn doping（nominal substitution of Al site,at.%）to2270°C for 16%Sn doping.However,for in-situ Ge doping,the phase stability of Nb（Al）_ss is more sensitive to the doping amount.After the RHQ treatment of2100～2150℃,only samples with≤2%Ge doping amount formed single-phase Nb（Al）_ss.With the increase of Sn doping amount,the spacing between non-equilibrium stripes generated by RHQ treatment in the liquid-phase temperature region becomes wider and Al composition fluctuation is suppressed.With the increase of Ge doping amount,stripes spacing first decreased and then increased,and the minimum spacing occurs at the critical doping amount（2%Ge）to keep the Nb（Al）_ss phase stable,and excess Ge precipitates from the grain boundaries.The T_c onset of the manufactured 12%Sn-doped Nb₃Al short wire reaches 17.0 K,its B_c2^*（8 K）reaches 19.4 T,J_c（8 K,5 T）reaches 2.2×10⁵ A/cm²and J_c（8 K,10 T）reaches 6.5×10⁴ A/cm².The T_conset monotonically increases with the Ge doping amount,the T_conset of the manufactured 12%Ge-doped Nb₃Al short wire reaches 18.9 K and its B_c2^*（8 K）reaches 20.6 T.The co-doping of Sn and Ge is not conducive to the stability of the Nb（Al）_ss phase,and single-phase Nb（Al）_ss is only obtained in the 1%Sn 1%Ge co-doped sample.In the1%Sn 1%Ge and 2%Sn 2%Ge co-doped samples,non-equilibrium stripes with short spacing appear,and the spatial frequency of the Al composition fluctuation increases and the amplitude decreases.The T_c onset of the manufactured 12%Sn 4%Ge co-doped Nb₃Al short wire reaches 17.5 K and its B_c2^*（8 K）reaches 17.1 T.The highest low-field J_c appears in the 1%Sn 1%Ge co-doped Nb₃Al short wire which keeps Nb（Al）_ss stable after RHQ treatment,and its J_c（8 K,5 T）reaches 3.0×10⁵ A/cm².The J_c（8 K,10 T）of the manufactured 1%Sn 1%Ge and 2%Sn 2%Ge co-doped Nb₃Al short wires both reach5.0×10⁴ A/cm².The conjecture about the relationship between phase stability and superconducting properties is to further broaden the metastable range of the Nb（Al）_ss phase obtained by RHQ treatment and make it tend to the perfect non-equilibrium point(This means that a well-structured A15 lattice lattice is generated after Nb（Al）_ss transformation),or explore the effect of the transformation process on the metastable stability boundary for the final generated A15 phase.This idea will help continue to improve the superconducting properties of Nb₃Al wires prepared by the RHQT method.