The Phase Evolution In Nb-Al Ball Milled System And Its Influence On The Properties Of Superconducting Phase After Sintered

As the core of the ultimate solution to human energy crisis,superconducting materials draw people's attention around the world.Nb₃Al superconducting materials with superior allowable stress characteristics,higher critical transition temperature and critical current density are ideal long-distance transmission materials.The studies on preparation of Nb₃Al superconducting materials by mechanical alloying are beneficial to the process of industrial production.But some experiments have shown that the superconducting property of Nb₃Al phase obtained by mechanical alloying method is usually lower the ideal value.Previous researches showed that both the phase evolution in mechanical alloying process and the doping of the third element have significant influence on the superconductivity of Nb₃Al.On this background,this paper firstly studies the evolution of phase and microstructure in Nb-Al system during ball milling process.Then to improve superconductivity,the third element Sn was doped into the Nb-Al system and the critical current density of Nb₃Al was successfully improved after low-temperature sintering.The main conclusions of this study are as follows:In this part,the phase evolution of ball milling process was analyzed by means of experimental analysis combined with interface thermodynamics calculation.The results showed that the amorphous layer of Nb-Al was generated around Nb nanocrystals in the ball milling process and it was only stable in a certain stage.With the extension of milling time,these layers gradually crystallized into Nb?Al?_ss solid solution when the critical thickness arrived at about 2.2 nm.The critical thickness observed in experiment was consistent with the predicted value of interface thermodynamics calculation within the error range.This crystallization process promoted the solid solution of Al atoms into Nb lattice and improved the solid solubility of Nb?Al?_ss.In this paper,the maximum solid solubility was about8.0at.%.In order to further improve the superconductivity of Nb₃Al,studies on the Sn doping were also carried out in this paper.The effect of Sn on the superconductivity of Nb₃Al was comprehensively analyzed combining the microstructure morphology and superconductivity.The results showed that the Sn doped pre-alloy powder did not form Sn-riched second phase particles after sintering,but formed Nb₃(Al_1-xSn_x)phase.The Sn doping improved the critical transition temperature T_c of Nb₃Al by promoting the integrity of A15 superconducting structure.When x=0.15,the maximum T_c can reach 16 K.What's more,Sn atom doping greatly improved the nucleation rate of superconducting phase.And it significantly reduced the grain size of superconducting phase from undoped sample's 80-200nm to 25-40nm of x=0.15sample.The critical current density J_c was finally increased to 1.6×10⁵A·cm^-2?4.2K,5T?due to the increase of grain boundary and the number of pinning centers.In addition,the phase transition temperature of Nb₃(Al_1-xSn_x)phase after doping was also lower than that of Nb₃Al in the sintering process,indicating that the Sn doping promoted the formation of A15 superconducting phase.