Limitation of Critical Current Density by Intermetallic Formation in Fine Filament Nb-Ti Superconductors

Abstract

Two experiments have been performed to investigate the role that the intermetallic reaction between the copper matrix and the Nb-Ti filaments plays in limiting the critical current density (J_c) of Nb 46.5 wt% Ti composites. The first experiment involved composites which were industrially extruded. It was found that as the number of heat treatments increased, the J_c declined, the resistive transition broadened and the filaments sausaged. The filament sausaging was initiated by intermetallic particles at the filament matrix interface. A series of many heat treatment procedures were then applied to composites fabricated in our own laboratories without extrusion. Very high J_c values were obtained at filament sizes of 20 μm. When the same heat treatment procedures were applied to 4 - 5 μm conductors, extensive sausaging and degraded J_c values resulted. This degradation was also found to be due to the formation of Cu-Nb-Ti intermetallic compounds. It is concluded that a reliable filament diffusion barrier technology is necessary to permit full flexibility in the heat treatment of 2 - 5 μm filament Nb-Ti composites.