Is Jcin Nb3Sn limited by grain-boundary flux-shear?

It is postulated that flux-pinning by grain-boundaries may be avoided if flux can shear and flow down paths lying in grain-boundaries. The difference in flux-pinning behaviour between NbTi,
F_{p}\alpha b(1-b)
, and solid state reacted Nb
3
Sn,
F_{p}\alpha b^{\frac{1}{2}} (1-b)^{2}
, arises from the different morphologies of their grain-structures. The boundaries, drawn out parallel to filament axes and direction of current flow in NbTi act as effective barriers to flux motion. The boundaries of grain produced by reaction in Nb
3
Sn provide paths along which flux can flow by grain-boundary shearing. An expression is developed for grain-boundary shearing which gives good agreement with experimental results.