Heat treatment control of Ag–Bi₂Sr₂CaCu₂Oxmultifilamentary round wire: investigation of time in the melt

It is well known that the critical current density J c of Ag-sheathed Bi 2 Sr 2 CaCu 2 O x (2212) varies strongly with heat treatment details, particularly the maximum processing temperature T max , but the mechanism for such J c variations and how the processing window can be widened remain unknown. We systematically measured the J c and electromagnetic properties of a powder-in-tube Ag-sheathed multifilamentary Bi 2 Sr 2 CaCu 2 O x (2212) round wire processed with the maximum processing temperature T max ranging from 887 to 900 °C and the time at the maximum temperature t max from 0 to 3 h using three representative heat treatment schedules. We found that J c correlates weakly to T max , but it correlates strongly to the time in the melt t melt , a processing parameter that has not been explicitly considered before. J c is rather insensitive to T max in the temperature range 887–900 °C and the true cause of J c declining with high T max appears to be the long t melt that leads to collapse of filament structure. By tuning t melt we were able to widen the T max window to 10 °C. The J c – t melt correlation, as well as quench studies, indicate that J c is controlled by complex diffusion processes occurring in the melt (filament bonding, bubble agglomeration, and perhaps Cu loss). Our findings highlight t melt as an important processing parameter for optimizing J c and may serve as a general guide for heat treating 2212 coils.