Filament to filament bridging and its influence on developing high critical current density in multifilamentary Bi₂Sr₂CaCu ₂O_x round wires

Increasing the critical current density (J_c) of the multifilamentary round wire Ag/Bi₂Sr₂CaCu ₂O_x(2212) requires understanding its complicated microstructure, in which extensive bridges between filaments are prominent. In this first through-process quench study of 2212 round wire, we determined how its microstructure develops during a standard partial-melt process and how filament bridging occurs. We found that filaments can bond together in the melt state. As 2212 starts to grow on subsequent cooling, we observed that two types of 2212 bridges form. One type, which we call Type-A bridges, forms within filaments that bonded in the melt; Type-A bridges are single grains that span multiple bonded filaments. The other type, called Type-B bridges, form between discrete filaments through 2212 outgrowths that penetrate into the Ag matrix and intersect with other 2212 outgrowths from adjacent filaments. We believe the ability of these two types of bridges to carry inter-filament current is intrinsically different: Type-A bridges are high- J_c inter-filament paths whereas Type-B bridges contain high-angle grain boundaries and are typically weak linked. Slow cooling leads to more filament bonding, more Type-A bridges and a doubling of J_c without changing the flux pinning. We suggest that Type-A bridges create a 3D current flow that is vital to developing high J_c in multifilamentary 2212 round wire.