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

Abstract

High engineering critical current density (<italic>J_E</italic> of 1300\xa0A/mm² at 4.2\xa0K and 15 T) in Bi-2212 round wire has been achieved through a partial melt, overpressure heat treatment process. <italic>J_E</italic> varies strongly with processing conditions, particularly the maximum heat treatment temperature (<italic>T</italic>_max). Increasing <italic>T</italic>_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 <italic>J_E</italic>, and higher ac losses. A wide processing window with a large range of <italic>T</italic>_max that has a nearly constant <italic>J_E</italic> 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 <italic>T</italic>_max window by controlling the Bi-2212 powder melting or wire architecture design. Here we report on studies of the performance variation with <italic>T</italic>_max for two production wires with a filling factor of about 20% and 85\xa0×\xa018 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\xa0μm) showed a peak <italic>J_E</italic> at the low end of <italic>T</italic>_max and also a <italic>J_E</italic> that was more sensitive to <italic>T</italic>_max. A <italic>J_E</italic> – <italic>T</italic>_max plot showed a plateau <italic>J_E</italic>(4.2\xa0K, 5\xa0T) of ∼1100\xa0A/mm² between <italic>T</italic>_max of 886 and 894\xa0°C for 1.0 and 1.2\xa0mm wires, where <italic>J_E</italic> is less sensitive to the wire diameter and <italic>T</italic>_max. This <italic>J_E</italic> plateau range is a preferred processing window for achieving high <italic>J_E</italic> in coils.