Phase Formation Mechanism During Low-temperature Sintering Of High Performance Nb₃Al Superconductors And Microstructure Control

Nb₃Al superconductors with high critical current density?J_c?and less sensitive to stress-strain characters have been recognized as the promising materials using for in high magnetic fields.In order to achive the higher critical trainstion temperature?T_c?,the Nb₃Al superconductors are generally prepared through high-temperature heat treatment to approach the stoichiometric ratio nowadays.But the grains will be coarse so that the value of J_c descends because of high temperature perperation.Therefore,it is difficult to obtain the ideal stoichiometric ratio and form small size grains at the same time.Based on above research background,high performance Nb₃Al superconductors were fabricated by mechanical alloying and low temperature sintering in this paper.Firstly,the kinetic mechanism of Nb₃Al phase formation during sintering process was studied.On this basis,the influence of phase formation mechanism on superconductivity was investigated and the effects of sintering temperature and holding time on phase composition and microstructure of Nb₃Al superconductor were analyzed.Then,the relationship between microstructure and superconducting properties of Nb₃Al superconductor was further discussed.During the ball milling process,the Nb-Al local amorphous was formed first and amorphous transformed supersaturated solid solution Nb?Al?_ss gradually.In the subsequent sintering process,Nb-Al local amorphous crystallizes into Nb₂Al and Nb₂Al reacts with Nb to form Nb₃Al.The reaction mechanism is random nucleation and then instantaneous growth.While Nb?Al?_ss is directly transformed into Nb₃Al,and the reaction mechanism is similar,but the activation energy is lower.The formation route of Nb₃Al phase is closely related to its properties.The value of T_c of Nb₃Al bulk synthesized via reaction between Nb₂Al and Nb is about15.85 K.It is because is the stoichiometric ratio of Nb₃Al phase is more closed to3:1.On the other hand,the Nb₃Al phase transformed by Nb?Al?_ss has a higher critical current density and the value of J_c is over 10⁴ A/cm² at 4.2 K and 6 T.The less grain sizes cuased by long milling time lead to high the value of J_c.In addition,sintering temperature and keeping time effects the phase composition and microstructure of Nb₃Al superconductor.The kinetics conditions of8h-milled sample can be satisfied until the sinterring temperature up to 850°C,so that Nb₂Al and Nb can react to Nb₃Al completely.High sintering temperature and proper keeping time can ensure the grain growth integrity,and promote the improvement of microstructure uniformity and intergranular connectivity,so as to improve the superconductiviting properties of Nb₃Al superconductor.