| Compared with Nb3Sn,Nb3Al has higher superconducting transition temperature,upper critical field and more excellent stress/strain resistance.The most successful method for preparing high-performance Nb3Al superconducting materials is the rapid-heating quenching and transformation method(RHQT):the Nb/Al composite metal precursor wire is quickly heated to 2000?,and then quenched in the Ga bath to obtain bcc Nb(Al)sssupersaturation solid solution.Finally,through low temperature heat treatment process,bcc Nb(Al)ss is transformed into high-quality Nb3Al superconductor.The fluctuation of heating current,distance and tension in the process of RHQ will affect the stability of the wire temperature in the RHQ process,and it is necessary to accurately control the RHQ parameters to prepare a superconducting long wire.In addition,the RHQ process has strict requirements on the uniformity of the precursor wire,including the uniformity of the Nb/Al micro distribution,wire diameter and resistance distribution.The unevenness of the wire will make the mechanical strength of the wire vary greatly at high temperature,resulting increase the disconnection rate.Therefore,exploring the processing technology of the precursor line is the key to preparing uniform Nb3Al long wires.In response to the above problems,this study explored the static and dynamic RHQ preparation and phase treatment process of the JR Nb/Al precursor line used in the rapid-heating and quenching process,and studied the effect of ball milling energy on the phase formation.The uniform Nb3Al long wire laid the foundation for the experiment.The main research contents are as follows:1.Systematically studied the phase law and superconductivity of Nb3Al wire prepared by static RHQ and dynamic RHQ.Study different Joule heating currents(IRHQ):static RHQ(IRHQ,64?71 A)and dynamic RHQ(IRHQ,120?124 A)prepared Nb3Al wires,compare their phase formation characteristics,including phase formation behavior,micro-morphology,chemical composition and superconducting properties.It was found that during the static RHQ process,the IRHQ superconducting core wire is a single-phase A15 Nb3Al superconducting wire in the range of 67?69 A;in the dynamic RHQ process,the single-phase A15 structure Nb3Al superconducting core wire has a phase formation interval of 120?123 A.The static RHQT Nb3Al wire is 4.2 K,and the Jc at 6.5 T 1.5?3.4×104 A/cm2,the Jc of dynamic RHQT Nb3Al wire is 4.7?6.9×104 A/cm2,the dynamic RHQT Nb3Al has better Jc performance.The dynamic RHQT Nb3Al wire has a denser surface morphology and smaller pores than the static RHQT wire.The Al content in the RHQT Nb3Al sample is about 23 at%.When the applied magnetic field is about 0.2 Birr,the 120 A dynamic RHQT Nb3Al sample shows the highest Fp value at about 2.4 T,which is about 7.5×108 N/m3.Through fitting,the main magnetic flux pinning mechanism of Nb3Al superconductor is surface pinning.2.Two dynamic Nb(Al)ss supersaturated solid solution phases were prepared by the dynamic RHQ process,and the influence of phase formation temperature(700?1100?)on the superconductivity of Nb3Al was explored.The results show that the Nb3Al superconducting phase obtained with 272 A and 315 A RHQT,with the increase of post-annealing temperature,the Tc value of Nb3Al wire tends to increase and then decrease.272 A fast heating sample can get the best Tc=16.3 K and Jc=3.22×104 A/cm2@5 K,6 T at 800?annealing temperature,315 A fast heating sample needs 900?,the best Tc=16.5 K,Jc=4.1×103 A/cm2@5 K,6 T.The common law of annealing is that the annealing temperature is too low to transform the bcc Nb(Al)ss phase into the A15superconducting phase,but if the temperature is too high,the transformed A15 Nb3Al phase is decomposed into non-superconducting Nb2Al phase and the Al-poor Nb3Al weak superconducting phase reduces the Tc and Jc performance of the superconducting wire.Since the diffusion length of the Al layer is greater than 1?m,partial reaction of the 272 A annealed sample is insufficient.3.Select planetary ball milling and high-energy ball milling processes,and prepare a Nb/Al composite precursor wire with a diameter of?=1.60 mm through the powder tube(PIT)process to study the effect of ball milling energy,RHQ energy density,and diffusion distance on RHQT Nb3Al superconductivity influences.The results show that the powder is densely arranged and uniformly deformed after plastic processing.The main phase of the planetary ball milling sample after RHQT is Nb3Al phase,and its superconducting critical transition temperature rises first with the increase of energy density(5.9?8.9 J/mm3).The trend of decreasing after high,?Tc takes the minimum at 8.9 J/mm3,its Jc is 1×104 A/cm2@8 K,5 T,and its stoichiometric ratio of Nb3Al is about 3:1.The energy density(?7.4 J/mm3)of the high-energy ball milled samples cannot detect the presence of the Nb3Al phase,and the Tc reaches 17.1 K at 6.5 J/mm3.The excellent strain resistance of Nb3Al may be used in the future to prepare superconducting magnets by the reaction first winding process.This process will greatly simplify the magnet preparation complexity and reduce the process cost.Influencing factors,clarify the phase formation process and mechanism of Nb3Al,improve the magnetic flux pinning behavior of superconducting phase,and lay a solid foundation for the overall improvement of Nb3Al superconducting performance in the future. |
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