Study On Preparation Technics And Properties Of Superconducting Materials Nb₃Al System

Superconducting technology, which is full of strategic significance, vast development potential and market prospect, is becoming a high-tech of the 21st century, and superconducting materials is the focus of current research in this area. At the end of the last century 70’s, researchers have already proved Nb₃Al has very high critical current density （J/c） and good resilience at high magnetic field, which can be used instead of Nb₃Sn for high-field and large-scale applications, such as in power systems, thermonuclear fusion, superconducting maglev train, military, medical and other fields. However, the main factors restricting the large-scale application of Nb₃Al superconducting materials are complicated and expensive preparation techniques.The paper aims to explore preparation techniques of Nb₃Al with low cost, high performance, practical technology, easy popularization. Meanwhile, a further study will be made on the rich electrical and magnetic properties of Nb₃Al. The paper mainly involves the exploitation and application of preparation techniques for Nb₃Al with high energy ball milling method, and its process optimization, and the influence of the different chemical elements doping on the material crystal structure, microstructure and superconductivity. In addition, this paper has a detailed comparison and analysis about the superconducting properties of Nb₃Al wires prepared by the high energy ball milling with the ex-situ method, thermal diffusion method and rapid-heating-quenching-treatment （RHQT） method, respectively.The first chapter mainly discusses the paper’s research background and significance, study contents and the structure. At the same time, the world’s scientific research in the field of superconducting materials research situation were reviewed in this section, and also including the summary of superconductivity and the research progress of superconducting material in this section. In addition, this chapter focus on the research status of Nb₃Al superconductor, and the existing problems, especially the technical route and the traditional preparation process of Nb₃Al superconductor as well as the advantages and disadvantages of them.The second chapter mainly elaborates on the experiment and characterization methods. On the one hand, this section has the detailing methods about the preparation of the Nb₃Al superconductors, including rapid heating and quenching treatment （RHQT）, diffusion and high-energy ball milling method. While, on the other hand, the characterization methods of crystal structure, microstructure, electromagnetic properties are also introduced.In the third chapter, the process factors that affecting the excellent performance of Nb₃Al prepared by high-energy ball milling are explored and optimized. By combining experimental with analysis mechanism, we explored that, how the milling initialization stoichiometric material ratio, milling time, sintering holding temperature, and sintering holding time effect on the Nb₃Al crystal structure, morphology and superconducting properties, respectively. At last, the superconducting transition temperature（T_c）, the superconducting critical current density Jc（4.2 K,7 T）, and the superconducting critical field H_c2 （0） of the optimized Nb₃Al samples are 15.7 K,6.3x104 A/cm² and 28.1 T, respectively.The fourth chapter focuses on the effect of chemical elements Ge and Sn doping on the superconductivity of Nb₃Al. By the high energy ball milling method, Mb3(Al_1-xGex) and Nb₃(Al_1-xSnx) samples were pepared sucessfully. Through the experiment combining with mechanism analysis, we explored that, how the doping content x of Ge and Sn, sintering holding temperature, and sintering holding time effect on the crystal structure, morphology and superconducting properties of Nb₃Al, respectively. Finally, with the optimization of preparation process, the Nb₃(Al_1-xGex) samples T_c=17.7 K, and the Jc（4.2 K,7 T）= 8.3× 104 A/cm². The overall superconducting performance has been significantly improved.The chapter 5 focuses on the investigation of the effect of chemical elements Mg and Ir. The doping of Mg and Ir is not only improving the mechanical properties but also influencing the superconductivity of Nb₃Al. By the high energy ball milling method, Nb₃(Al_1-xMg_x) and Nb₃(Al_1-xIr_x) samples were pepared sucessfully. By the combination with theoretical knowledge, we explored that, how the factors such as doping content x of Mg and Ir, effect on the crystal structure, morphology and superconducting properties of Nb₃Al.Finally, the sixth chapter mainly focuses on improveing the superconductivity of Nb₃Al from the comparison of different preparation technologies. Nb₃Al wires were pepared sucessfully by thermal diffusion method, high energy ball milling method and RHQT method. We invetigate that how the different preparation process effect on the properties of Nb₃Al wires from the microstructure and superconducting properties. On the basis of the previous research, the paper proposed the technology plan for the further preparation of Nb₃Al wires in the later stage.