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

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/mm 2 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.

In situ x-ray and neutron diffraction investigation of Bi-2212 in multifilamentary wires during thermal treatment

“…Finally, a square wire is obtained, having a 0.7 × 0.7 mm 2 cross-section, an average size of the single filament of about 20-25 μm and a superconducting fill factor of about 22%. In the above described process, the new concept for the densification of powders before the partial melt process is used [9], namely a proper alternation of drawing and groove-rolling steps that drastically reduces the porosity and leads to a powder density inside the filaments much higher than that obtained by the standard process with just a drawing deformation. Figure 1 shows the crosssection of a multifilamenatry wire.…”

“…The samples obtained by the OP process show engineering critical current density (J e ) values of 800-900 A/mm 2 at high fields, well above the application require-ments (J e =400-600 A/mm 2 ). As an alternative route to the OP process, an innovative mechanical deformation has been proposed by our group [9] [10] [11] with the aim to realize dense wires with superconducting properties that could satisfy the application requirements through a more attractive process for an industrial scaling-up. In such a context, significant insights for J c improvement in this material can still be obtained by an accurate analysis of the evolution of the structural and microstructural properties during the so called partial-melt process (PMP), a heat treatment needed to improve grain connectivity and therefore gain high J c .…”

The CERN FCC Conductor Development Program: A Worldwide Effort for the Future Generation of High-Field Magnets

“…However, this has been achieved through over-pressure processing (50-100 bar), which would be challenging to implement for the manufacture of large magnets. At CNR-SPIN, an alternative groove rolling process is under development [20]. In the present project, the aim is to demonstrate a Je of 500 A mm -2 at 16 T and 4.2 K through optimization of the wire architecture, deformation processing and heat treatment.…”

Investigation of inter-grain critical current density in Bi2Sr2CaCu2O8+δsuperconducting wires and its relationship with the heat treatment protocol

“…(3) in [ 30 ]), with suitable boundary conditions at the SN interface and at asymptotic distance from it (see eqs. (4)(5)(6)(7)(8) in [ 30 ]). In these equations and boundary conditions, two key parameters describe the junction, namely B and M.…”

“…Yet, to further maximize inter-grain current in Bi-2212 wires, the main issues to investigate and improve are the presence of voids due to bubbles grown during the melt stage and the formation of secondary and offstoichiometric phases within filaments (mainly either Bi2Sr2CuOx (Bi2201) grains or Bi-2201 intergrowths within Bi-2212 grains) and at the grain boundaries. The former problem, which is responsible for the degradation of the supercurrent over long lengths, can be addressed either by applying an over pressure during the heat treatment [ 1,4,5 ] or by the use of a suitable alternation of groove rolling and drawing to diminish the porosity prior to the heat treatment [ 6 ]. On the other hand, the latter problem requires fine tuning of fabrication protocol, and there may still exist an edge of improvement in this direction.…”