Preperation Of Nb₃Al Superconducting Materials By Powder In Tube Method

With the International Thermonuclear Experimental Reactor?ITER?proposed,Nb₃Al superconducting materials have been known as excellent alternatives to Nb₃Sn with excellent superconductivity and stress-strain resistance properties in magnet coil applications,therefore,Nb₃Al has attracted extensive research.The stoichiometric ratio of Nb₃Al can only be obtained at about 2000? and Nb₃Al wires are coated with Cu stabilization layer after the high temperature heat treatment,which increasing the complexity of the process.The high temperature and continuouly heating have a high requirement on the uniformity of the wire,which has a high requirement for the preparation of the precursor wire,thus restricting the preparation of long Nb₃Al superconducting wire.In this thesis,the Nb₃Al wires and tapes were prepared by in-situ powder in tube method?in-situ PIT?,and the effects of hot-pressing sintering process on the formation of Nb₃Al phase and its superconducting properties were studied.After optimizing the heat treatment process,the Nb₃Al tapes with good superconducting properties were obtained under the heat treatment temperature of less than 1000?.Series Nb₃Al bulks were fabricated by using hot pressing technique,and the milling time of Nb-Al powder,sintering temperature and sintering pressure were optimized.The milling of precursor Nb-Al mixed powders with 2-3 h could refine the powder particles and ensure Nb₃Al phase with good quality in the heat treatment process.The heat treatment temperature over 1000? would lead to the formation of large amounts of Nb2Al.The results showed that the superconducting transition temperature of Nb₃Al was insensitive to the sintering pressure during sintering process,but the sintering pressure could improve the superconducting transition properties of Nb₃Al and increase its critical current density.The superconducting transition temperature of Nb₃Al bulks prepared by hot-pressing sintering heat treatment was 15.7 K,and its critical current density was 106 A/cm2?8 K,0 T?and 104 A/cm2?8 K,7 T?.Based on the research of Nb₃Al bulks prepared by hot-pressing sintering process,Nb₃Al wires and tapes were prepared by in-situ PIT method with Nb-Al powder mixed by high energy ball milling,and the effects of hot pressing and pressureless sintering on the microstructure and superconducting properties of Nb₃Al tapes were investigated.The results showed that the connectivity of the powder particles in the core of the Nb₃Al tapes prepared by hot-pressing sintering process could be effectively enhanced,which leads to the improvement of the critical current density.The superconductivity transition properties and critical current density of Nb₃Al tapes prepared by hot-pressing sintering were improved significantly,which was consistent with the results of Nb₃Al bulks prepared by hot-pressing sintering process.At a temperature of 8 K and an applied magnetic field of 7 T,the J.value of Nb₃Al tapes prepared by atmospheric heat treatment was 9.6 x 102 A/cm2,and the Jc value of those prepared by hot pressing sintering increased to 2 x 104 A/cm2.The critical current density of Nb₃Al tapes fabricated with in-situ PIT method and then heat treated at less than 1000? was close to the wires heat treatment with RHQT method,moreover,the Cu layer was not required to be corroded at the temperature below 1000?,which greatly simplified the preparation process of the Nb₃Al superconducting wires.