Current Flow and Flux Pinning Properties of YBCO Foam Struts

The current flow and the flux pinning properties on struts of superconducting YBa ₂ Cu ₃ O _x foams are analyzed in detail in the temperature range 60 K ≤ T ≤ T _c . For this purpose, magnetization loops were measured on foam struts taken from various positions of a 5 × 2 × 2 cm ³ large foam sample prepared at RWTH Aachen. From these data, the critical current densities, j _c , and the flux pinning forces, F _p = j _c × B, were calculated and pinning force scaling diagrams F _p /F _p ,max versus h = H _a /H _irr were established. The scaling in the temperature range 60 K < T < 90 K was found to be well developed for all samples with peak positions, h ₀ , above 0.4, which is an indication of δT _c -pinning. The shape of the pinning functions is found to be completely different from all other high-T _c materials and varies only slightly with position. This specific dome shape cannot be described by an addition of several pinning functions, and the parameters p and q do not fit to the description of Dew-Hughes. Therefore, we employ Kramer plots to obtainmore information on the flux pinning mechanism.