Study On The Fabrication And Superconducting Properties Of Nb₃Al Superconducting Wires With RHQT Heat-treatment

Since Nb3Al has better stress/strain tolerance,higher upper critical field,Hc2 and larger critical current density,J,under high field than those of Nb3Sn,it is promising to replace Nb3Sn superconducting material in the next-generation high-field superconducting magnets application.However,high performance Nb3Al superconductor must be stoichiometric,which is only stable at temperatures higher than 1800 ℃,and the range of its phase formation is quite narrow in the equilibrium phase diagram.As a result,it is very hard to fabricate Nb3Al superconductor with high superconducting properties.Rapid Heating-Quenching and Transformation(RHQT)method is believed to be the most promising way to prepare Nb3Al long wires.But the researches on the fabrication and phase formation control of Nb3Al superconducting wire by RHQT method are still insufficient.In this thesis,we focus on the study of Nb3Al wire fabrication by RHQT method.A new Rapid-Heating-Quenching(RHQ)apparatus is successfully designed and constructed.The fabrication techniques of Rod-in-Tube(RIT),in-situ Powder-in-Tube(in-situ PIT),and Jelly-Roll(JR)precursor wires are studied.All these precursor samples are successfully heat-treated by the RHQ apparatus.RHQ parameters,such as maximum heating temperature,dwelling time,and heating times are systematically optimized.The superconducting properties are tested by transport method and magnetization method.The micro-structures of the samples are investigated by SEM and EDS.The relationship between superconducting properties and RHQ parameters is also investigated.The RHQ apparatus designed by other groups are studied and compared.A new rapid heating-quenching structure is proposed,in which the sample is parallel to the surface of Ga bath.This new design ensures the cooling rate in the whole sample being constant.The control circuit is designed based on C8051F350 single-chip,which can heat and quench the sample within 0.1 s.A radiation thermometer is fabricated based on a charge coupled device(CCD),which can effectively monitor the temperature of precursor wires without affecting its temperature.156-filament RIT precursors with 0,6 and 0.8 mm diameter are fabricated utilizing the hot-extrusion method.The relationship between the maxmium heating temperature of RHQ and superconducting properties is investigated.The highest onset critical temperature,Tc(Tc-onset)of RIT samples are 16.9～17.2 K.EDS results show that near-stoichiometric A15 Nb3Al phase is obtained.SEM images show that the cross-sections of RIT samples are honeycomb-shaped with high-performance thin A15 phase existing on the inside wall of each tube.The size of the honeycomb-shaped structure can be decreased by reducing the diameter of the Nb/Al filaments.In-situ PIT precursor with Ta as barrier is prepared,within which pure Nb and Al powders are adopted as raw materials.The Nb-Al powder reaction during the rapid heating and slow cooling process is studied.The relationship between RHQ parameters and superconducting properties is studied.It is found that the temperature plateau exists only in the heating process and disappears in the following heating cycles.The optimalTc-onset is 16.8 K while the heating and quenching time is 20 ms during RHQ process.The A15 phase is slightly off-stoichiometric.The A15 phase of samples that are RHQ twice is also off-stoichiometric.The RHQ process for in-situ PIT precursor should be further optimized.A simple-structured 18-filament JR precursor is designed and fabricated using the hydrostatic-extrusion technique.The reaction of Nb/Al foils is studied by monitoring the temperature during rapid heating and slow cooling processes.Temperature plateaus exist in both heating and cooling processes.With the increase of heat cycles,the temperature plateau in cooling process tends to disappear,and the Kirkendall voids tend to conbine and grow larger.The influences of RHQ parameters on superconducting properties of JR precursors are investigated.It is found that the optimized superconducting propertis appear when the RHQ heating time is 40 ms.The highest Tc-onset is 17.5 K.The A15 phase is dense and homogenous and stoichiometric according to EDS results.At 11 K,self field,the best transport J,is 140 A/mm2.Based on the M-H measurement,the calculated Je is 1100 A/mm2 at 10 K,2 T according to Bean model.