React-Wind-Sinter Processing of High Superconductor Fraction Bi$_2$Sr$_2$CaCu $_{2}$O$_{\rm x}$ /AgMg Round Wire

Bi$_2$Sr$_2$CaCu $_{2}$O$_{\rm x}$ (Bi2212) conductor technology has advanced significantly but the development of magnets is still hampered by difficulties associated with the partial-melt process (for wind&react magnets) and strain limitations (for react& wind magnets). To avoid these problems, the React-Wind-Sinter (RWS) approach has been proposed. Here we report on experiments that investigate three split processes that are based on the conventional partial-melt process within the RWS concept. The partial-melt process was interrupted at ${\rm T}_{1}$, ${\rm T}_{1}-10^{\circ}{\hbox{C}}$ and ${\rm T}_{\rm S}$. After cooling to room temperature, the conductor is bent to a series of diameters (40 mm–100 mm), replicating magnet construction. The heat treatment process is then resumed on the bent samples from the split point and the heat treatment completed. The critical current is measured at 4.2 K in self-field using the four-probe method and the microstructure and phase composition of the Bi2212/AgMg wire are examined with scanning electron microscopy. For the split processes, the critical current after full heat treatment is as high as those from conventionally processed short samples, and in at least one case it is increased by 40% relative to conventional processing. These results show that a split process is a promising approach to improved Bi2212 conductors and magnets, and more broadly shows that conventional Bi2212 partial-melt processing is far from optimized.