A novel Pre-HIP two-step annealing process for enhancing the critical current of Ba1-xKxFe2As2 superconducting tapes

Iron-based superconductors (IBSs) are compelling candidates for high-field magnet applications due to their excellent intrinsic properties. The hot isostatic pressing (HIP) effectively enhances their critical current density by increasing core density. However, IBS tapes are highly stress-sensitive after heat treatment. This necessitates fabricating magnets prior to the HIP process. Consequently, the requirement for large-volume HIP furnaces for full-scale magnets presents significant technological and cost barriers. To circumvent this manufacturing bottleneck, this paper introduces a novel “pre-HIP” two-step annealing process. The principle involves an initial, low-temperature HIP (pre-HIP) heat treatment on IBS tapes to achieve densification. Subsequently, the tapes are wound into the desired magnet or conductor geometry, followed by a final heat treatment at atmospheric pressure (AP). This strategy allows large-scale magnets to be treated in conventional, cost-effective furnaces. We systematically evaluated the effect of the pre-HIP temperature (370-450 °C at 50 MPa) on the transport properties and bend performance of 7-filamentary Ba 1-x K x Fe 2 As 2 (Ba-122) tapes. Results show that pre-HIP significantly enhances the critical current. However, a critical trade-off was identified: higher pre-HIP temperatures yield superior critical performance but induce local grain growth, causing severe degradation during bending. Samples pre-HIP at 370 °C retained 78% of their initial performance after bending to 10 mm diameter, whereas the 450 °C samples degraded significantly. This study confirms that pre-HIP is a cost-effective method to improve IBS magnet performance when the optimal temperature is selected.