Fluxoid pinning by grain boundaries in bicrystals and polycrystals of niobium

In niobium bicrystals of various preassigned misorientations, the superconducting critical current, I/sub c/ has been measured as a function of the applied magnetic field, H, and the azimuthal angle, phi, between the grain boundary and H. Frequently, peaks in I/sub c/(phi) with half width Δphi 1/2 approx. 300, as well as high pinning force, f/sub p/ up to 10-4 Nm-1 at 0.2 T have been found for H both parallel and perpendicular to the grain boundary plane. Using f/sub p/ and Δphi 1/2, obtained from bicrystals, the critical current density, J/sub c/, has been calculated for polycrystalline samples. The results are in good agreement with experimental measurements both in magnitude and grain in size dependence