| Nb3Al owns an extremely high critical current density(Jc)and its Jc is less sensitive to stress-strain so that it is considered to be a desirable superconductor material to be applied in high magnetic field.In recent years,mechanical alloying(MA)and subsequent low temperature sintering is used to prepare Nb3Al.However,there are always inevitable micropores in the sintered body,which obviously degrades the superconducting performance.On the other hand,ball milling time is an important parameter to obtain Nb3Al with excellent superconducting property.The Nb/Al ratio of Nb3Al phase effects superconducting property.Based on these backgrounds,this paper firstly combines hot isostatic pressing technology with mechanical alloying to eliminate sintered micropores and improve superconducting performance.On this basis,the parameter of ball milling time and Nb/Al ratio were further optimized to prepare Nb3Al with excellent superconducting property and analysis the influence of two different phase evolution routes of Nb-Al system on superconducting property.The value of Tc and Jc of Nb3Al superconductors are both improved with the increase of sintering pressure.The Tc of 180 MPa sintered sample reached nearly 16.2K,and its Jc was above 1.0x104 A/cm2(4.2K,5T),which is an order of magnitude higher than the Jc value of the ambient sintered sample.Based on the detailed analysis of phase composition and microstructure,the Nb3Al phase prepared by isostatic pressure sintering has higher crystallinity,better phase uniformity,and its Nb/Al ratio is closer to the stoichiometric ratio of 3:1.Therefore,the Tc values significantly improve as the isostatic sintering pressure increases.At the same time,with the isostatic sintering pressure increasing,the sintering density is gradually improved,and the Nb3Al grains are continuously refined.These two reasons work together to make the Jc value greatly improved.What's more,according to the calculation of theoretical model,it is found that the magnetic flux pinning mechanism of the isostatic pressure sintering sample is a combination of grain boundary pinning and defect pinning.In order to further improve the superconducting property of Nb3Al,this paper optimized ball milling time and original Nb/Al ratio.It was found that when the milling time is less than 16h,the Nb3Al is formed via the reaction between the Nb2Al that crystallizes from Nb-Al amorphous phase and residual Nb during subsequent sintering process.Whereas the milling time exceeds 16h,the Nb3Al transforms directly from Nb-Al supersaturated solid solution(Nb(Al)ss)during subsequent sintering process.Due to this difference in phase formation route,the grain size of M16h sintered sample is larger than that of M12h and M20h sintered samples.Besides,the intergranular connectivity of M16h sintered sample is the poorest among them.These factors make the critical current density(Jc)of M16h sample abnormally lower than that of both M12h and M20h samples.According to these results,the highest Jc(about 1 x104 A·cm-2 at 4.2K,6T)is obtained for the sample prepared by20h ball milling and subsequent sintering at 850?for 5h.Meanwhile,the properties of Nb3Al can be further optimized by adjusting the content ratio of Al element in the original powder.When Nb3Al is formed via the reaction between the Nb2Al and Nb,the Jc value of Nb3Al can be improved since original Nb/Al ratio of Nb:Al=2.85:1 can optimize the intergranular connectivity of Nb3Al phase.The change of Al content in the original ratio has little effect on Jc value of Nb3Al transformed by(Nb(Al)ss).These results in our work are helpful to understand the internal relationship between phase formation,microstructure and Jc,and provide guidance for optimizing the mechanical alloying process to prepare Nb3Al with high Jc. |
PDF Full Text Request