Effects of Wire Diameter and Filament Size on the Processing Window of Bi-2212 Round Wire

High engineering critical current density ( JE of 1300 A/mm2 at 4.2 K and 15 T) in Bi-2212 round wire has been achieved through a partial melt, overpressure heat treatment process. JE varies strongly with processing conditions, particularly the maximum heat treatment temperature ( T max). Increasing T max results in longer time in the melt (defined as the time between when Bi-2212 melts on heating and when Bi-2212 begins to form on cooling), more bridging between the filaments, lower JE , and higher ac losses. A wide processing window with a large range of T max that has a nearly constant JE is desired for processing large coils with large thermal mass and significant thermal time constants that may make precise control over the desired temperature – time profiles uncertain. Accordingly, we wanted to explore broadening the T max window by controlling the Bi-2212 powder melting or wire architecture design. Here we report on studies of the performance variation with T max for two production wires with a filling factor of about 20% and 85 × 18 filaments where filament size was varied by changing the wire diameter, a process which also shortens the distance between filaments. We found that wires with smaller filament diameter (9 to 11 μm) showed a peak JE at the low end of T max and also a JE that was more sensitive to T max. A JE – T max plot showed a plateau JE (4.2 K, 5 T) of ∼1100 A/mm2 between T max of 886 and 894 °C for 1.0 and 1.2 mm wires, where JE is less sensitive to the wire diameter and T max. This JE plateau range is a preferred processing window for achieving high JE in coils.