Fabrication And Electro-magnetic Properties Of Superconducting Nb₃Al Strand For High Field Magnets

It's considered that the most effective way to solve the energy crisis of future is to develop magnetic confinement fusion technology,and the innovation of superconducting wire preparation is the key to promote the development of magnetic confinement fusion.In the ongoing International Thermonuclear Experimental Reactor(ITER)project,which China participated in,the highest field of the magnet for plasma confinement is 11.8T.In the upcoming China Fusion Engineering Test Reactor(CFETR)program,the maximum field strength is 15T,and the existing superconducting wire cannot meet the requirements.Therefore,it is urgent to develop the preparation technology of high performance superconducting wire for 15T magnet.Among the tens of thousands of superconductors that have been found,there are only two of them,Nb₃Al and Nb₃Sn,that can be used in high field above 10T,and Nb₃Al has been considered as an alternative to Nb₃Sn for high-field and large-scale applications for its extremely high critical current density and excellent strain tolerance.Therefore,superconducting Nb₃Al wire is the best choice for high field magnet system.However,the wire preparation technology is still blank in China,which seriously restricts the development of magnetic confinement fusion.In this thesis,we focus on the preparation and heat treatment of superconducting Nb₃Al wire for high field magnet,and deeply study the superconducting properties of Nb₃Al wire at low temperature.It is found that Nb₃Al wires cannot be prepared by the method similar to Nb₃Sn wires from the phase diagrams of Nb-Al and Nb-Al-Cu system.High performance superconducting Nb₃Al wire can be obtained by rapid heating,quenching and transformation(RHQT)method,which makes Al dissolve in Nb to form supersaturated solid solution of Nb(Al)_ss,and then followed by transformation annealing heat treatment.The effective diffusion distance of Al during this process is in the order of submicron.Therefore,the preparation of superconducting Nb₃Al wire with high-performance is to obtain Nb₃Al precursor composite wire with submicron and high uniformity firstly,and then to undergo stable RHQT treatment.In order to get Nb₃Al precursor wire with high uniformity,we prepared the wire starting with the Rod-in-Tube method by using Al rod and Nb tube as raw materials,combing with hot-extrusion,drawing and rolling process,and consequently high-performance superconducting Nb₃Al wires had been made,which stood up a rapid heating,quenching(RHQ)process by using the home-made RHQ equipment up to 2000?and low-temperature annealing process.The results suggest that the critical current density(J_c)reaches 859.5 A/mm²at 4.2 K and 15 T,which is promising to develop the next-generation high-performance superconducting Nb₃Al wires for high-field magnet application.The optimum manufacturing process of the long Nb₃Al precursor wires were obtained through investigating the workability of wires with different numbers of filaments.Superconducting Nb₃Al wire was obtained through Rod-in-Tube method,however the size of Al filament is too large to form Nb₃Al fully,and then the long Nb₃Al precursor wires with mono-filament,18 filaments and 24 filaments structure were prepared through Jelly-roll method,combing with hydrostatic-extrusion,drawing and rolling process.We also discussed the relationship between superconducting performance of Nb₃Al and heat treatment conditions including low temperature diffusion heat treatment and RHQ heat treatment.We obtained the best cold working technology for 18 filaments and 24 filaments structure long Nb₃Al precursor wires by comparing the processing characteristics of wires with different filaments and different cold processing technologies.The RHQ heat treatments were held on mono-filament and multi-filament wires,and the results suggest that the T_cof superconducting Nb₃Al wire with 24 filaments structure is 17.7K and the critical current density(J_c)reaches 1587 A/mm²at 4.2 K and 12 T because the diffusion distance in multi-filament wire is smaller than it in mono-filament.And then we focus on preparing the long Nb₃Al precursor wires through the new way by combing with high energy ball milling and powder-in-tube process.We discussed the relationship between the superconducting Nb₃Al phase purity and milling time,Al powder contents of the original mix-powder,heat-treatment conditions under normal and high pressures,and found that the best condition to prepare Nb₃Al superconductor is milling the Nb26at.%Al mix-powder with milling time of 2.5 h and then heat-treated at 800?for 10h.Furthermore,mono-filament and multi-filament Nb₃Al wires were prepared through powder-in-tube process with the Nb(Al)_sspowder.After heat-treated at 800?for 10h,the critical current density(J_c)of the mono-filament Nb₃Al wire reaches 10⁴A/cm²at 4.2 K and 12 T.Meanwhile,we discussed the effect of sintering under high pressure on the superconductivity of Nb₃Al,and found that sintering under high pressure can significantly improve the density and grain connectivity of superconductors,and then increase the critical current density,but it has little effect on T_c.Finally,we wind the high-field magnet using the superconducting Nb₃Al wire with 18filaments structure prepared by the Jelly-roll process.After wound,the coil was heat-treated at 850?for 20h with outside diameter of 65 mm and inner diameter of 20 mm.The magnetic field reaches 15.2 T in the center of the magnet,which directly proved that the high performance superconducting Nb₃Al wire can be used in 15 T magnets.Finally,we succeeded in fabricating long Nb₃Al precursor wires with different structures through Rod-in-Tube and Jelly-roll methods and holding reel-to-reel RHQ heat treatment on multi-filament Nb₃Al wires.Meanwhile,a high-field magnet was wound using the 18 filaments wire combing with low temperature diffusion heat treatment for the first time in China.A new way by combing with high energy ball milling and powder-in-tube process to fabricate long Nb₃Al precursor wires may be an alternative to the RHQ heat treatment,for the process is simple.