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

Abstract

High engineering critical current density (JE of 1300 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 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 (Tmax). Increasing Tmax 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 Tmax 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 Tmax window by controlling the Bi-2212 powder melting or wire architecture design. Here we report on studies of the performance variation with Tmax 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 Tmax and also a JE that was more sensitive to Tmax. A JE - Tmax plot showed a plateau JE(4.2 K, 5 T) of ~1100 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> between Tmax of 886 and 894 °C for 1.0 and 1.2 mm wires, where JE is less sensitive to the wire diameter and Tmax. This JE plateau range is a preferred processing window for achieving high JE in coils.