  Nb3Sn superconductors have significant applications in constructing
high-field (> 10 T) magnets. This article briefly reviews development of Nb3Sn
superconductor and proposes prospects for further improvement. It is shown that
significant improvement of critical current density (Jc) is needed for future
accelerator magnets. After a brief review of the development of Nb3Sn
superconductors, the factors controlling Jc are summarized and correlated with
their microstructure and chemistry. The non-matrix Jc of Nb3Sn conductors is
mainly determined by three factors: the fraction of current-carrying Nb3Sn
phase in the non-matrix area, the upper critical field Bc2, and the flux-line
pinning capacity. Then prospects to improve the three factors are discussed
respectively. An analytic model was developed to show how the ratios of
precursors determine the phase fractions after heat treatment, based on which
it is predicted that the limit of current-carrying Nb3Sn fraction in
subelements is ~65%. Then, since Bc2 is largely determined by the Nb3Sn
stoichiometry, a thermodynamic/kinetic theory was presented to show what
essentially determines the Sn content of Nb3Sn conductors. This theory explains
the influences of Sn sources and Ti addition on stoichiometry and growth rate
of Nb3Sn layers. Next, to improve flux pinning, previous efforts in this
community to introduce additional pinning centers (APC) to Nb3Sn wires are
reviewed, and an internal oxidation technique is described. Finally, prospects
for further improvement of non-matrix Jc of Nb3Sn conductors are discussed, and
it is seen that the only opportunity for further significantly improving Jc
lies in improving the flux pinning.
