Superconducting Transition Of Nb₃Sn Crystal Under High-pressure

A15 superconducting Nb₃Sn has shown great promise for applications in the International Thermonuclear Experimental Reactor(ITER),and high energy physics.The mechanical deformation induced critical performance degradation of superconducting Nb₃Sn is one of the basic issues involved in high-field magnet manufacturing process.The study of superconducting transition of Nb₃Sn crystal under high pressure is valuable to understand the strain sensitivity of Nb₃Sn.In this paper,on the basis of the experimental observations of Nb₃Sn microstructure,the lattice structure of single crystal Nb₃Sn is constructed by the method of molecular dynamics(MD)simulations.And with the aid of Voronoi tessellate,the microstructure model of Nb₃Sn ploycrystal with periodic boundary condition is established for MD simulations.The reliability of empirical potential function for Nb₃Sn is verified by comparing the MD simulation results of the elastic constants and lattice constant of Nb₃Sn with the experimental results as well as the first-principle simulation results.Following this analysis,the high-pressure induced atomic scale deformation of Nb₃Sn is carefully studied.The results show the structural variations of Nb₃Sn single-crystal and ploycrystal.The stress distribution of atom Nb and atom Sn,and the grain boundary response of polycrystal Nb₃Sn to high pressure are revealed by using MD simulations.Based on the MD simulations,a superconducting transition model of single crystal Nb₃Sn is established by considering the T²(square of temperature)dependence of Nb₃Sn normal-state resistivity in the temperature region ranging from T_c to 44K.And the model is extended to describe the superconducting transition behavior of Nb₃Sn polycrystal under high pressure.The model predictions of superconducting phase transition and critical temperature degradation of Nb₃Sn crystal under high pressure are qualitatively consistent with the experimental observations.The effect of grain boundary deformation on the critical temperature degradation response of Nb₃Sn superconductor is revealed by comparing with the experimental results.The study give the details of experimental measurements on the mechanical deformation and the electro-mechanical coupling behavior of Nb₃Sn superconducting under the environment of very high pressure and extremely low temperature.It is helpful to improve the understanding of the deformation mechanism of Nb₃Sn and to quantitatively reveal the role of grain boundary deformation in the electro-mechanical coupling behavior of superconducting Nb₃Sn.