| Nb3Al has higher superconducting properties and is less sensitive to strain than that of industrialized Nb3Sn superconductor,therefore it is an ideal candidate for next generation high field magnets.The bronze route process is not suitable for Nb3Al fabrication due to the formation of ternary non-superconducting Nb-Al-Cu compounds.Nb3Al superconductors fabricated by low temperature diffusion processes exhibit poor superconducting properties because of the formation of Al deficient A15 phase.High performance Nb3Al wires can be fabricated through the rapid heating,quenching and transformation(RHQT)process.In the RHQT process,Nb/Al precursor wire is rapidly heated to around 1900?in less than 2 s and is immediately quenched in a Ga bath then annealed below 1000?to obtain the Nb3Al A15 phase.To improve the superconducting properties of Nb3Al,various doping elements were studied:doping of Be,B,Sn,Ga,Ge,Cu to increase Tc and doping of Cu,Ag to stabilize the A15 phase in the RHQ process.In this work,(Nb1-xTax)3Al bulk samples with different Ta contents(x=0.01,0.03,0.05,0.10)were prepared by mechanical alloying method.Nb(Al)sswas obtained after high-energy balling,A15 Nb3Al was prepared by 925?post-annealing.The optimized doping amount was 3.0 at%Ta.Compared with pure Nb3Al(Tc=15.5 K,Jc=3.2×103A/cm2at 8 K,5 T),Birr=17.0 T at 4.2 K and 14.2 T at 6K),Tc,Jcof(Nb0.97Ta0.03)3Al dropped a little only(Tc=15.0 K,Jc=1.1×103A/cm2at 8K,5 T)),Birrof(Nb0.97Ta0.03)3Al was slightly enhanced at 4.2 K and 6.0 K(Birr=17.5 T at 4.2 K and 14.5 T at 6 K),however,Birrdecreased at 8.0 K and 10.0 K.No impurity phases were found in Ta-doped samples and flux pinning of the(Nb1-xTax)3Al bulks remains the surface pinning mechanism.Based on previous works,Powder-in-tube(Nb0.97Ta0.03)3Al precursor wire was prepared and then it was processed with the RHQT procedure.(Nb0.97Ta0.03)3Al with RHQ energy density of 8.09J/mm3performed well,whose Jc(2.6×102A/cm2at 12 K,5T)gradually exceeded pure Nb3Al(Jc=1.8×102A/cm2at 12 K,5 T,Birr=11.0 T at 8 K and 9.0 T at 10 K)as the external magnet going up to 5 T.Birrof this sample increased a little as well(Birr=12 T at 8 K and 9.5 T at 10 K).Powder-in-tube(PIT)Nb3(Al1-xCux)(x=0?0.04)superconducting wires were fabricated by rapid heating,quenching and transformation(RHQT)process.With the addition of Cu,disordered Nb3Al A15 phase with stoichiometric composition was formed in the quenched wires and atomic long range ordering occurred during the low temperature annealing process,resulting in improvement of Tc,Jcand Birrperformance compared to the pure Nb3Al sample.The highest Tcof 18.0 K,narrowest?Tcof 0.6K and best layer Jcof about 30481 A/cm2at 12 K,4 T was found in the Nb3(Al0.97Cu0.03)wire,which is nearly four times of pure Nb3Al sample.The Nb3(Al0.97Cu0.03)wire also shows the highest Birrvalue of 25.7 T at 8 K and 12.9 T at12 K,respectively.The formed A15 phase exhibit uniform microstructure and composition.Superconducting properties decreased as the doping amount going up to4.0 at%Cu,the optimized doping content lay between 2.0 at%Cu and 3.0 at%Cu.Flux pinning of the Nb3(Al1-xCux)wires follows the surface pinning mechanism,where stacking faults were considered as pinning centers.Powder-In-Tube 2.5 at%Cu Nb3Al single-filament wires were RHQT processed,post-annealing temperature were set to 800 with different annealing time of 10 h,20 h and 30 h.Superconducting properties of melting-region-temperature RHQ quenched wires were discussed.Optical morphology showed that the wires were quenched from a liquid phase.Diffraction peaks of A15 and bcc could be observed in all the samples after RHQT.Tcand?Tcshowed no obvious differences with the variation of annealing time,Pure Nb3Al showed a Tcof 17.6 K and a?Tcof 1.4 K.Prolonged annealing time could enhance Jc,20 h-annealed sample showed a Jcof 1.10×105A/cm2at 8 K,5 T,closed to the optimism Jcof our JR RHQT Nb3Al fabricated.Birrdecreased with prolonged annealing time.20 h-annealed sample exhibited slight decay in Birras well as enhancement in Jc,so the optimum annealing condition is 800?,20 h. |
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