Flux pinning in porous type II superconductors

The flux pinning mechanism in porous type II superconductors is investigated by measurements on sintered Nb samples with relatively small Ginzburg-Landau Parameter (kappa) values (kappa less than 5). By careful examination of the metallurgical microstructure it is shown that the porous samples can be regarded as metallurgical model structures. The volume pinning force P/sub v/ is measured as a function of B, T, and of the microstructure. The results are compared with theories for the pinning mechanism at phase boundaries. Agreement is found with V. V. Shmidt's theory as long as the pore diameter is greater than the superconducting penetration depth. Deviations from this theory are observed near T/sub c/. The dependence of P/sub v/ on the microstructure, especially on the specific surface S/sub v/ of pores and on the mean linear pore size d in the form P/sub v/ varies as S/sub v//d, is consistent with the proposed pinning defect model, in which the pinning centers are considered as polyhedrons