Preparation And Heat Treatment Mechanism Of SPS-Nb₃Al Superconducting Materials

Compared with Nb₃Sn,Nb₃Al has higher critical current density（J_c）and remarkable stress-strain resistance under high field,which enables it to carry larger superconducting current at high magnetoelectric stress,showing a great potential in large-scale high-field magnet applications.Due to the complexity of preparation technology and phase formation mechanism of Nb₃Al,the commercial production of Nb₃Al superconducting long wires cannot be realized at present.Therefore,a lot of research is still needed to find a more efficient preparation process to accelerate the commercial application of Nb₃Al.Spark plasma sintering（SPS）has been applied to the research of various materials as a novel sintering process,which can achieve the dense sintering of materials in a short time.This thesis explored the preparation of Nb₃Al superconducting bulk by SPS technology.Firstly,the phase formation mechanism and superconducting properties of pure Nb₃Al prepared by SPS were investigated and compared with other common preparation methods（especially powder metallurgy）.On this basis,the effects of Ge doping on the phase formation,crystal structure and superconductivity of SPS-Nb₃Al by were investigated.In addition,the effect of heat treatment on the properties of SPS-Nb₃Al samples was also investigated in this thesis.The main results obtained are as follows.In the research on the preparation of pure Nb₃Al by SPS technology,it was found that SPS technology can quickly prepare Nb₃Al superconducting materials with high-density,close grain connection and nano-particle structure in the temperature range of 1200-1400°C,and the transition temperature were about 16 K.Due to their tight and clean grain connection,the SPS-Nb₃Al samples shows a high current-carrying capacity in self-field.The study clarified that the phase formation mechanism of Nb₃Al during SPS preparation is similar to that directly formed from high-temperature liquid phase rather than the transformation from bcc phase to A15 phase.Such a unique phase formation mechanism results in very few structural defects existing inside the SPS-Nb₃Al samples,which leads to rapid decay of J_c with the applied magnetic field.After post-annealing,the flux pinning force and superconducting performance of the SPS-Nb₃Al samples are significantly improved,thereby enhancing their performance in the applied magnetic fields.Nonetheless,the defect density is still lower than that generated during the phase transition from the bcc phase to the A15 structure,so the pinning force is low.On this basis,this paper attempts to introduce high-density structural defects in Nb₃Al by chemical doping to further improve the superconducting properties of SPS-Nb₃Al samples.Based on the study of pure SPS-Nb₃Al samples,this paper proposed to dope the Al site with Ge element to form Nb₃(Al_1-xGe_x)alloy,and studied the effects of Ge doping on the phase formation,crystal structure and superconductivity of SPS-Nb₃Al superconducting materials.It was found that an appropriate amount of Ge substituted for Al atoms can significantly promote the formation of Nb₃Al phase in SPS-Nb₃Al samples and improve the T_c of SPS-Nb₃Al samples.When the Ge doping amount is 10 at%,the T_cof the sample is increased to 17.05 K form the undoped 15.4 K.Annealing the sample at910°C can further improve the T_c of sample,which is 17.5 K,and its pinning force is significantly enhanced.It can be seen that Ge doping is an effective method to introduce high-density structural defects in SPS-Nb₃Al,enhance the magnetic flux pinning force,and improve superconductivity.In addition,this paper further analyzed the advantages and disadvantages of SPS method to prepare Nb₃Al through a detailed comparison with powder metallurgy（PM）.The results show that the SPS-Nb₃Al samples have a larger lattice constant compared to the PM-Nb₃Al,and the local high temperature generated by plasma heating during the SPS process accelerates the formation of Nb₃Al phase.Due to the short reaction time and fast cooling make the SPS-Nb₃Al samples grains small and tightly connected,which is completely different from the microscopic morphology of PM-Nb₃Al samples.Meanwhile,compared with the PM-Nb₃Al samples,the Al content in SPS-Nb₃Al is closer to the stoichiometric ratio,thus exhibiting better superconducting property.In summary,the results of this paper show that SPS technology has unique advantages in the preparation of high-performance Nb₃Al superconducting materials,and has important reference value for further improving the preparation technology and improving the performance of Nb₃Al materials.