Current transfer at superconducting Bi2Sr2CaCu2O8+δ-Ag joints

Composite reaction textured Bi2Sr2CaCu2O8 + delta superconducting samples were made by partial melt processing, with aligned MgO fibers as seeding and texturing interfaces. The electric contacts were in situ processed thick silver tapes of the same width as the sample. The voltage drop at the metal-superconductor joint was measured in liquid nitrogen during continuous current ramp in self fields and applied fields. The contact resistivity and current transfer length were estimated by applying a transmission line model for current transport across the joint. The calculated and estimated transfer length, corresponding to >95% current transfer, occurs over only 0.75 mm of the contact in the case of a 0.125-mm-thick Ag current lead. At currents, I, below critical, I-C, this approach was satisfactory. However, at higher currents nonohmic behavior is observed. The apparent contact resistance increased with increasing current, and with applied magnetic field. This contribution to the contact resistance arises from flux flow effects in the superconductor and is shown to vary nonlinearly with both increasing current and increasing applied field. Consequently the current transfer length increased progressively over the whole contact, approaching a rather uniform current distribution at about I similar to 12I(C). The results are compared with a nonlinear finite element model of current flow in an in situ current element; the agreement of the model with experiment is good. (C) 1999 American Institute of Physics. [S0021-8979(99)03614-2].