A review and prospects for Nb3Sn superconductor development

Nb3Sn superconductors have significant applications in constructing\nhigh-field (> 10 T) magnets. This article briefly reviews development of Nb3Sn\nsuperconductor and proposes prospects for further improvement. It is shown that\nsignificant improvement of critical current density (Jc) is needed for future\naccelerator magnets. After a brief review of the development of Nb3Sn\nsuperconductors, the factors controlling Jc are summarized and correlated with\ntheir microstructure and chemistry. The non-matrix Jc of Nb3Sn conductors is\nmainly determined by three factors: the fraction of current-carrying Nb3Sn\nphase in the non-matrix area, the upper critical field Bc2, and the flux-line\npinning capacity. Then prospects to improve the three factors are discussed\nrespectively. An analytic model was developed to show how the ratios of\nprecursors determine the phase fractions after heat treatment, based on which\nit is predicted that the limit of current-carrying Nb3Sn fraction in\nsubelements is ~65%. Then, since Bc2 is largely determined by the Nb3Sn\nstoichiometry, a thermodynamic/kinetic theory was presented to show what\nessentially determines the Sn content of Nb3Sn conductors. This theory explains\nthe influences of Sn sources and Ti addition on stoichiometry and growth rate\nof Nb3Sn layers. Next, to improve flux pinning, previous efforts in this\ncommunity to introduce additional pinning centers (APC) to Nb3Sn wires are\nreviewed, and an internal oxidation technique is described. Finally, prospects\nfor further improvement of non-matrix Jc of Nb3Sn conductors are discussed, and\nit is seen that the only opportunity for further significantly improving Jc\nlies in improving the flux pinning.\n