Void and phase evolution during the processing of Bi-2212 superconducting wiresmonitored by combined fast synchrotron micro-tomography and x-ray diffraction

C Scheuerlein,J Jiang,A Malagoli,D C Larbalestier,F Kametani,M Di Michiel,E E Hellstrom,M Scheel

doi:10.1088/0953-2048/24/11/115004

Abstract

Recent study of the current-limiting mechanisms in Bi-2212 round wires has suggestedthat agglomeration of the residual Bi-2212 powder porosity into bubbles offilament-diameter size occurs on melting the Bi-2212 filaments. These pores introduce amajor obstacle to current flow, which greatly reduces the critical current density (Jc). Here we present an in situ non-destructive tomographic and diffraction study ofthe changes occurring during the heat treatment of wires and starting powder,as well as a room temperature study of ex situ processed wires. The in situthrough-process study shows that the agglomeration of residual porosity is morecomplex than previously seen. Filament changes start with coalescence of thequasi-uniform and finely divided powder porosity into lens-shaped defects at about850 °C when the Bi-2201 impurity phase decomposes before the Bi-2212 starts to melt. Theselens-shaped voids grow to bubbles of a filament diameter on melting of the Bi-2212 andcontinue to lengthen and then to agglomerate across multiple filaments while the filaments arein the liquid state. The experiment makes clear why melt processing is vital to developing highJc and also shows how rearrangement of the residual filament porosity on meltingimposes a strong longitudinal inhomogeneity in each filament. Reducingthe bubble density is clearly an important path to reaching much higherJc values in Bi-2212 round wires. Synchrotron micro-tomography is an exceptionally powerfultechnique for studying the spatial extent of the porosity on a scale of about 2 µm and larger.

Highlights

Conductors based on Bi2Sr2CaCu2Ox (Bi-2212) high temperature superconductor are unique in that they can attain high critical current densities in round wire shapes at very high magnetic fields
Filament changes start with coalescence of the quasi-uniform and finely divided powder porosity into lens-shaped defects at about 850 ◦C when the Bi-2201 impurity phase decomposes before the Bi-2212 starts to melt
The diffraction peaks formed when Bi-2212 melts are possibly characteristic for alkaline earth cuprate (AEC) and/or copperfree (CF) phases [17], they are not yet clearly identified by comparison with reference diffraction patterns

Summary

Introduction

Conductors based on Bi2Sr2CaCu2Ox (Bi-2212) high temperature superconductor are unique in that they can attain high critical current densities in round wire shapes at very high magnetic fields. The round wire geometry makes the Bi2212 conductors especially interesting for high energy physics applications where Rutherford cables are preferred for race track windings [2, 3]. At this time the best conventionally processed Bi2212 wires attain engineering current densities which exceed ∼400 A mm−2 at 4.2 K and 12 T (∼1400 A mm−2 in the superconductor) [4], about a factor of two below Nb3Sn conductors for which the superconductor Jc can attain 2500– 3000 A mm−2. Slow cooling appears to enhance the connectivity of the filament bundle by forming large Bi-2212 grains. Variations of Jc of 3–5 appear to occur without any change in Hirr(T ), pointing to variations of the filament connectivity as the dominant control variable for Jc

Results

Discussion

Conclusion