Preparation Of Nb₃Al By High-Energy Ball Milling Method And The Research On Its Superconducting Properties

NbTi and Nb3Sn are the most widely used superconducting materials currently. With the development of fabrication technology,their properties have been gradually improved. However, they can not meet the demand for large superconducting magnets in the future for its brittlenes, limited upper critical field and critical current density. It is necessary for us to find a higher-performance superconductor as substitute. The Nb3Al has been considered as an alternative to Nb3Sn and they have the same crystal structure, and its maximum of critical temperature, the upper critical field and the critical current density are 18.9K,32T(4.2K),and 1000 A/mm2(16T,4.2K) respectively. At the same time,Nb3Al inherently has an excellent strain tolerance in critical current performance. Therefore, Nb3Al will one of be the ideal choice for large-scale superconducting magnets in the future, such as in the magnetic confinement fusion reactors.However,Nb3Al with stable stoichiometric composition can be only owned at tempertures above 1800℃.At present, the RHQT method is the only way to prepare Nb3AI with A15 stoichiometry in many preparation methods. Nevertheless, there are still many problems to be solved in preparation methods of Nb3Al,such as the lower level of industrialization, the technical problems and so on.High energy ball milling method can not only refine the grain size, but also can effecti-vely control the formation of microstructure, and can dircet alloying or solid solution in the solid state. Its maximum advavtage is that it can avoid the high temperature treatment in RHQT method. It made it possible to prepare Nb3Al in low temperature. In this paper, We used high energy ball milling method for Nb3Al,optimize process parameters(heat treament conditions, milling medium and et al),and analyse the influence of Ge and Ta doping on the superconducting properties of Nb3Al.In the first chapter,the research background and significance, research progress of Nb3Al superconducting material and the research content and structure of this paper have been mainly introduced.In the second chapter, the fabrication of Nb3Al superconducting,the experimental design of this paper prepared by high-energy ball milling method prepared Nb3Al,and measurement method of the samples have been briefly introduced.In the third chapter, the process parameters of Nb3Al prepared by high energy ball milling has been optimized. The study found that:the superconductivity of Nb3Al depends on the Nb/Al stoichiometry and heat treatment conditions.Using different ball grinding medium (hardened steel, zirconia) has little effect on the formation of Nb3Al phase. Sintering adding Al piece found that the superconducting transition temperature of Nb3Al were not improved, still 15 K. the influences of stearic acid can be considered in many aspects,such as high powder yield, uniform dispersed powder. But at the same time,it introduce more impurities lead to the superconducting transition temperature decrease. Finally,we obtian the optimum process parameters:the content of Al is 26 at.%, the heat treatment temperature is 900 C, holding time is 5h, milling time is 3h,grinding medium is hardened steel,ball-to-powder weight ratio is 10:1,milling atmosphere is high purity argon and at the moment Tc is15.1K.In the fourth chapter, the main effects of Ge and Ta doping on the superconducting properties of Nb3Al.The study found that high energy ball milling method can really make a solid solution of Ge into the Nb matrix. But perhaps because of the crystal structure change of Mb3Al for the addition of Ge,or increase of Al vacancy,or introduce more impurities phase, the superconducting properties of Ge doped Nb3Al were not good. The addition of Ta made serious Al loss, so it made the formation of Nb3Al phase very hard.