Microstructure-Based Model for Current Flow in Bi-2212 Round Wire Conductors

The high-temperature superconductor Bi₂Sr₂ CaCu₂O_y (Bi-2212) is a viable candidate for low-temperature, high-field (>; 20 T) superconducting magnet applications due to its high irreversibility field and ability to be formed into a high-current, round multifilamentary wire. However, after over 20 years of research, a clear understanding of current flow within these round wires remains elusive. We present here a model for current flow in these round wire conductors based on a microstructural feature that is continuous along the wire axis. Scanning and transmission electron microscopy along with micro X-ray computed tomography were used to define key microstructural features and establish relationships between the structure and superconducting properties. A model was developed based on the layer of well-formed Bi-2212 that is found next to the silver sheath. This model correctly predicts observed trends with increasing J_c with smaller filament diameters and provides a basis for developing new experiments to understand and improve this important high-temperature superconducting conductor.