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 values (<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 forceP_v is measured as a function ofB, 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 nearT_c . The dependence ofP_v on the microstructure, especially on the specific surfaceS_v of pores and on the mean linear pore sized in the formP_v S_v/d, is consistent with the proposed pinning defect model, in which the pinning centers are considered as polyhedrons.This paper is based on thesis D 26, Universität Giessen, 1975. It was presented in part at the International Discussion Meeting on Flux Pinning in Superconductors, Sonnenberg, Germany, 1974.