Improvement Of Nb₃Al Superconducting Properties By Doping Ti And Annealing Time

Compared to Nb₃Sn,Nb₃Al superconductor has better strain resistance,higher superconducting transition temperature（T_c）and upper critical field(H_c2),and better critical current density（J_c）superconductors in high fields,thus Nb₃Al superconducting material was considered as the optimal substitute of Nb₃Sn used in future fusion reactor magnet and the research on Nb₃Al superconducting material is of great significance.This thesis focuses on the improvement and optimization of the basic physical properties of Nb₃Al superconductor.Firstly,the effect of Ti doping on the structure and properties of high-energy ball milled Nb₃Al bulk was studied.Based on this,we studied the effect of Ti doping on the structure and properties of powder in tube（PIT）Nb₃Al superconducting wires.Finally,the effect of post-annealing time on the structure and properties of rapid heating and quenching（RHQ）Nb₃Al wire was studied.For the high-energy ball milled(Nb_1-xTi_x)₃Al（x=0～0.05）superconductors that were prepared at low temperatures,partial substitution of Nb atom by Ti atom may suppress the formation ofσ-Nb₂Al phase.For samples measured at 10 K,J_c value of the(Nb_0.98Ti_0.02)₃Al sample decays more slowly than the other samples at fields beyond 6 T,indicating appropriate doping content of Ti（～2 at%）into Nb₃Al can improve the J_c performance of Nb₃Al wires at high fields.Besides,with the increase of measured temperature,the irreversible field(B_irr)of(Nb_0.98Ti_0.02)₃Al is gradually superior to that of the pure sample,which may be due to the uniformity improvement of A15 superconducting phase.In the PIT RHQT(Nb_1-xTi_x)₃Al（x=0～0.03）superconducting wires,the T_cof pure Nb₃Al wire is 16.4 K and T_c of Ti doped Nb₃Al samples slightly increased except the sample with Ti doping amount of 1 at%.Among them,2 at%Ti doped sample shows the highest T_c value of16.7 K and 2.5 at%Ti doped sample shows the smallestΔT_c value of 0.5 K,indicating that appropriate amount of Ti doped into Nb₃Al can improve superconducting performance.Superconducting layer J_c at 5 T of pure Nb₃Al wire is about 1.32×10⁵ A/cm²,4.94×10⁴A/cm² and 5.84×10³ A/cm² at 8 K,10 K and 12 K,respectively,which are close to the optimism value of our jelly-roll（JR）RHQT Nb₃Al wires.The layer J_c of(Nb_1-xTi_x)₃Al（x=0.025）correspond to 2.63×10⁵ A/cm²,1.12×10⁵ A/cm² and 1.44×10⁴ A/cm² at 8 K,10 K and 12 K,respectively,which is twice of the pure Nb₃Al sample,indicating J_c improvement of Nb₃Al wires through appropriate amount of Ti doping（～2.5 at%）.In addition,the B_irr values of(Nb_1-xTi_x)₃Al（x=0.025）wire at 8 K,10 K,and 12 K respectively are 13 T,11.1 T,and 7.5T,which is 8.3%,15.6%and 7.1%higher than that of the pure Nb₃Al sample.For the RHQ Nb₃Al wires fabricated near the"T_c peak"current,the disorder degree of the Nb（Al）_ss phase changed when quenched under different RHQ current(I_RHQ),resulting in the variation of post-annealing time to obtain best J_c results of the wires.I_RHQ=272 A and I_RHQ=276 A correspond to the left and right sides of the"T_c peak"current.The atomic disorder degree of Nb（Al）_ss phase obtained by I_RHQ=276 A is higher,that takes longer post-annealing time to get the best J_cperformance than that of I_RHQ=272 A.For JR RHQT Nb₃Al wires,due to the large hardness difference between Nb and Al,the Nb and Al layers of the precursor wire have irregular deformation and uneven distribution during the extrusion and drawing process.With prolonging of the post-annealing time,the uniformity of Nb₃Al wire gradually increases.