Effects Of Mg And Ge Doping On Phase Formation And Superconducting Properties Of Rapid Heating And Quenching Nb₃Al Wire

Compared to Nb₃Sn,Nb₃Al superconductor has higher superconducting transition temperature（T_c）,upper critical field(H_c2),critical current density（J_c）and better strain resistance under high field.Thus Nb₃Al superconducting material was considered as the optimal choice for future magnetic confined nuclear fusion reactor material,and the research on Nb₃Al superconducting material is of great significance.The Japanese NIMS team studied Mg and Ge doped Nb₃Al superconductors by Rod-In-Tube and Jelly-Roll.The main purpose of Mg doped into Nb₃Al system is to improve the workability of the wire.But there are few studies on its performance and phase formation.Doping Ge into Nb₃Al multifilamentary superconducting wires could improve the superconducting performance.However,due to the formation of brittle A15 phase after heat treatment,it is difficult to fabricate long wires and coils.In this paper,Mg and Ge doped Nb₃Al superconductors were prepared by Powder-In-Tube method.Through PIT method,doping elements can be easily mixed into the Nb₃Al system.The process is simple and convenient to study the correlation between Nb₃Al phase formation and superconducting properties,which has guiding significance for future large-scale industrial production.The main research conclusions of this paper are as follows:Nb₃(Al_1-xMg_x)（x=0.01～0.05）superconducting wires with different Mg doping contents were prepared by PIT method.3 at%Mg doped Nb₃Al sample retained Nb（Al）ss bcc phase after RHQ treatment.The 3 at%Mg doped Nb₃Al sample has the highest T_c（17.3 K）and smallest（35）T_c（0.7 K）,and these are better than pure sample.The superconducting layer Jc of the 3 at%Mg doped sample reaches 3.4×10⁵ A/cm² at 8 K,5 T,which is about twice of the optimal pure Nb₃Al J_c value（2.0×10⁵ A/cm²）,which proves that the performance of the sample is improved after the doping of Mg into the Nb₃Al system.The B_irrvalues of 3 at%Mg doped sample at 8 K and 12 K were 15.7 T and 8.5 T respectively,which were higher than those of pure samples.After rapid heating/quenching and transformation annealing treatment,the surface of 3 at%Mg doped Nb₃Al sample is smooth and the element composition ratio is close to the standard dose ratio of Nb₃Al 3:1.The Nb₃(Al_0.95Ge_0.05)superconducting wires prepared with different particle size Nb powders.The precursor wire prepared by 5μm Nb powder obtained A15phase Nb₃Al after RHQ,while the precursor wire prepared by 45μm Nb powder retained bcc phase.After low temperature annealing,the two kinds of superconducting wires received A15 phase.The T_c of all Nb₃(Al_0.95Ge_0.05)superconductors prepared by RHQT are exceed 18.2K,which is higher than pure Nb₃Al（17.2k）.The minimum（35）T_c of Nb₃(Al_0.95Ge_0.05)superconductor is 0.7K.The J_c and B_irr values of A15 Nb₃(Al_0.95Ge_0.05)superconductor obtained by bcc phase transition are better than those obtained by A15 phase ordering after quenching.The probable reason is that the high density stacking fault formed during the bcc→A15 phase transition as the magnetic flux pinning center.After rapid heating and quenching（RHQ）treatment,the Nb₃(Al_0.95Ge_0.05)wire core has a compact and smooth surface morphology,which indicates that the sample has fine grain connectivity.The surface scanning results showed that the element distribution of Nb₃(Al_0.95Ge_0.05)sample with an energy density of 10.56 J/mm³ was relatively uniform,and the composition ratio was close to the standard chemical dose ratio of the pure Nb/Al ratio of 3:1.