New concept for the development of Bi-2212 wires for high field applications

TL;DR

This paper introduces a novel deformation technique combining groove-rolling and drawing to densify Bi-2212 superconducting filaments before heat treatment, significantly improving critical current in long-length wires without complex additional processes.

Contribution

The study presents a new deformation method that enhances filament densification prior to melt, enabling high critical currents in long Bi-2212 wires without overpressure treatments.

Findings

Achieved critical current values in long wires comparable to short wires.

Densification prior to melt stage improves superconducting performance.

Method is straightforward and applicable to long-length wires.

Abstract

The first step towards high critical currents in Bi-2212 wires was the comprehension that the supercurrent is blocked over long lengths by filament-diameter bubbles grown during the melt stage, which cause expansion of the wire diameter and dedensification of the superconducting filaments. Whereas the previous successful approach to reduce the problem of voids related to bubbles was based on the application of a high overpressure during the heat treatment, we fabricated Bi-2212 wires by applying a new concept of suitably alternating groove-rolling and drawing techniques with the aim of densifying the phase already during the working procedure prior to the heat treatment. We here for the first time were able to reach in wires reacted with closed ends - i.e. with gas trapped in the wire as it happens in long-length wires - the very same values of critical current shown in short wires reacted with open ends. This is the irrefutable evidence that, only by acting on the deformation technique, we were able to raise the critical current by properly densifying the superconducting powder inside the filaments already before the melt stage. Whole-conductor current densities in our long length simulation wires already reach 400 A/mm2 at 4.2 K and 5 T, which can be still easily increased through architecture optimization. The actual breakthrough is that the densification is optimized without further complex treatments through a technique which can be straightforwardly applied to long-lengths wires.