Enhanced critical current density in the low-temperature sintered Nb3Al superconductor with Sn doping

The Nb 3 (Al 1-x Sn x ) superconductors (x = 0.05, 0.15, 0.25, 0.35) were prepared by mechanical alloying and subsequent low-temperature sintering. It was found that the critical current density ( J c ) of the obtained Nb 3 (Al 1-x Sn x ) sample is enhanced significantly compared to undoped Nb 3 Al prepared using the same method. The maximum value of the J c reaches up to 5.10 × 10 4  A·cm −2 (4.2 K, 5 T) for Nb 3 (Al 0.85 Sn 0.15 ) sample. Combined with the phase composition analysis and microstructure observation, it is indicated that Sn enters the A15 crystal structure of Nb 3 Al forming the Nb 3 (Al 1-x Sn x ) phase after low-temperature sintering. What's more, Sn doping improves the grain connectivity as well as refines the grain size of Nb 3 Al, which are the main reasons for the enhancement of J c in the Sn doped samples. Based on the exploration of the phase evolution of Nb–Al–Sn system during the mechanical alloying and subsequent low-temperature sintering process, it is also confirmed that Nb 3 Sn forms firstly during the sintering process and then can serve as the perfect nucleus of Nb 3 Al to increase its nucleation rate, leading to the significant decrease of the grain size of Nb 3 Al.