Effect of rapid-heating process on the preparation of Nb3Sn superconductor and establishment of critical current density model

In this work, bulk-Nb 3 Sn superconductors were prepared by the powder metallurgy route under the heat treatment conditions of rapid-heating and short-holding and the effect of Cu content and rapid-heating process on the microstructure and superconducting properties of bulk-Nb 3 Sn superconductors were systematically investigated. The results showed that both of critical temperature ( T c ) and critical current density ( J c ) of bulk samples got decreased with increasing Cu content. The rapid-heating and short-holding process exhibited a 'healing' effect on the holes in Nb 3 Sn layers, significantly improving the compactness of the bulk samples. Moreover, the bulk sample, which was heat treated at 600 °C for 50 h, then induction heated to 1000 °C and isothermally held for 20 s, exhibited the highest J c,non-Cu values of 2.38 × 10 9 A/m 2 (at 4.2 K & 6 T). And this could be attributed to the small average grain size, high content of Nb 3 Sn superconducting phase, and the Cu(Sn) artificial pinning centers in the Nb 3 Sn layer. Besides, Nb 3 Sn wire by internal-tin method was processed using the same heat treatment schedule to validate the effectiveness of the rapid-heating process. The critical current ( I c ) of the wire sample, measured via V–I characteristic curve at 4.2 K & 6 T, was determined to be 96 A, corresponding to a J c,layer value of 4593 A/mm 2 . Moreover, a J c,non-Cu model ( J c , n o n − C u = 4985 · x 3.5 + 0.466 [ − 10 − 30 exp ( y 0.00348 ) + 577 y ] 0.5 + 45 · e − z 175 ) concerning with the microstructural parameters ( x : superconducting phase content, y : average Sn content in Nb 3 Sn layer, and z : grain size of Nb 3 Sn) was proposed, providing a semi-quantitative reference to describe the J c,non-Cu of bulk-Nb 3 Sn superconductors. Finally, it should be noted that the relevant conclusions in this work regarding the kinetics of rapid-heating and microstructural evolution of bulk samples may require adjustment or further validation when applied to the structure of wires.