Reduced ac-losses due to an interfacial resistivity between filaments and silver matrix in Bi-2223 multifilamentary tapes

Abstract

Time constant and ac-loss measurements were performed with specially prepared Bi-2223 multifilamentary tapes having a low superconductor filling factor. This assures that no superconducting interconnections between filaments exist. From measured time constants of decaying coupling currents we calculated values of effective transverse matrix resistivities. They were found to be highly anisotropic and, in the direction perpendicular to the broad face, well above the resistivity of the silver matrix. This is explained by the existence of an interfacial resistance between filaments and matrix. Experiments at 4.2 K, where the silver resistivity is a factor of ∼30 smaller than at 77 K, confirmed this explanation: measured time constants τ (and also coupling losses for ωτ≪1) are increased by a much smaller factor, compared to 77 K. The interfacial resistance therefore dominates the transverse resistivity and reduces the coupling losses below the value expected from the pure matrix resistivity. The beneficial effect of the interfacial resistivity is frequently masked by superconducting connections between filaments which may drastically decrease the effective transverse resistivity. Avoiding filament interconnections is therefore a more effective means of reducing losses than increasing the silver resistivity by alloying.