High Critical Current Density MgB₂,Nb₃Al Superconducting Wires

MgB₂is a very attractive practical magnet-grade superconductor,since it has a lot of advantages,like as moderate superconducting transition temperature T_c（39 K）,high current-carrying capability,simple manufacturing process and lower raw materials cost.Therefore,it is thought to possess the potential at the application of next-generation MW-grade wind turbine generator,high-capacity DC superconducting cable,moveable small-size magnetic resonance imaging（MRI）and so on.Both Nb₃Al and Nb₃Sn are A15 phase superconductor,and both have excellent critical current density J_cunder high magnetic field.Compared with Nb₃Sn,the superconducting properties of Nb₃Al are not sensitive to the lattice integrity,and the stress and strain have little effect on the superconducting current density.At 12 T magnetic field,the critical current of Nb₃Sn dropped to only 60%of its original value after applying an inherent strain of 0.3%,while Nb₃Al maintained 95%of its original current under the same conditions.Therefore,Nb₃Al wire is being widely studied as a future substitute for Nb₃Sn.This paper takes MgB₂and Nb₃Al superconducting wires as the research objects,and the main contents are summarized as follows:（1）The effects of Mg powder particle size on filament-homogeneity,microstructure and superconducting properties of kilometer-length 18-and 30-filament MgB₂long wire has been detailed discussed.Comparing to the MgB₂wires with 10μm Mg powder size,the 18-filament wires by using the 5μm Mg powder have a transport critical current density（J_c）improvement by 32%and the 30-filament wires have achieved the J_cincreasement by 11%at 4.2 K in the magnetic fields from 2 T to 4 T.The30-filament MgB₂wire with the heat-treatment of 580℃for 4 h has the optimal in-field critical current density,6.5×10⁵A/cm²at 4.2 K and 2 T.（2）In order to further increase the percentage of MgB₂area in the cross-sectional area,useing cassette roller die（CRD）to draw 18-and 36-filament MgB₂superconducting wire.The MgB₂area in the 18-filament wire accounts for 25.6%of the cross-sectional area of the wire,while the 36-filament wire reaches 21.49%.By analyzing the J_cof the18-and 36-filament sample annealed at different temperatures,it was found that the current density increases with the number of filaments while the diameter of the wire is the same.The 36-filament MgB₂wire with the heat-treatment of 580℃for 2 hours has the best performance,and the current density reaches 4.23×10⁵A/cm²at 4.2 K and 2 T.（3）The manufacturing process of 30-filament Nb₃Al precursor wire and the effect of RHQ（RHQ:rapid hating quenching）heat-treatment on the microstructure and J_cproperties of the Nb₃Al superconducting wires are discussed.For the Dia.1.30 mm Nb₃Al wire,its critical current,I_cat 4.2 K,12 T reaches 1078 A with optimized RHQ heat-treatment conditions.Furthermore,the finer wire with Dia.0.72 mm has the layer and non-Cu J_cof up to 3770 and 924 A/mm²at 4.2 K,12 T,respectively.（4）At present,Nb₃Al wires are manufactured by hydrostatic extrusion,which is complicated and expensive process,involving numerous and expensive instruments.The36-filament Nb₃Al wire with Ta barrier layer is made by CRD,which solves the problem of magnetic flux jumping under magnetic field.In order to further increase the superconducting area in the cross-section,we design new wire structures.For the sample at the V_RHQof 9.5 V,at 4.2 K@12 T,the non-copper J_creachs 653 A/mm².