CRITICAL CURRENTS OF INSITU FORMED MULTIFILAMENTARY CU-NB3SN COMPOSITES

Critical current densities of in situ formed Cu-Nb3Sn composites with discontinuous filaments were measured as a function of superconducting volume fraction, matrix resistivity, area reduction ratio, and applied magnetic field. In agreement with recent modelling by Tinkham and co-workers, the effective superconducting volume fraction in a given composite was found to be field-dependent, necessitating the distinction between microstructural and electrical percolation. In composites with a low filament volume fraction, proximity effect coupling, controlled by matrix resistivity, was found to be the dominant factor determining both the composite remnant resistivity and the critical current density. For sufficiently high filament volume fractions and area reduction ratios, the remnant resistivities fall below the level of detection, as predicted by theory, and critical current densities become comparable to those of continuous filament composites. SEM, TEM, and STEM analysis reveal a dense distribution of submicron, ribbon-like Nb3Sn filaments in relatively pure Cu matrix. The microstructure of the filaments is equi-axed with an average grain size of ∼ 400Å, ensuring effective flux pinning. © 1979 Chapman and Hall Ltd.