Atom Probe Tomography analysis of BaFe2As2 superconductors for high field applications

This project will form part of a collaboration between Oxford and the Applied Superconductivity Center in Florida (https://nationalmaglab.org/). There is a strong international effort to select superconducting materials for the magnets for large physics machines like the Future Circular Collider at CERN where the target values for current carrying capacity are extremely demanding, Jc = 1500 A/mm2 at 4.2 K in 16 T. One of materials being explored for these applications is the compound BaFe2As2 (122) which can be made from inexpensive starting raw materials, and at least in single crystals possesses the superconducting properties needed for high-field magnet applications. However the processes used to manufacture the multi-filamentary conductors needed for magnets results in polycrystalline 122 filaments with substantially degraded properties, and the major challenge at the present time is increasing current flow across grain boundaries. The most important questions are what degrades the GB connectivity in 122, how can the degradation be avoided, and is this solution applicable in an affordable manufacturing process? Recent studies suggest that the GB connectivity in state-of-the-art 122 polycrystals is degraded by impurities at the grain boundaries, and correlating the nano-scale chemistry with the manufacturing route and superconducting performance is now required.

This project will use Atom Probe Tomography as the primary technique to analyse the local chemistry of grain boundaries in 122 samples prepared under carefully controlled conditions. APT is the only technique that offers very high resolution 3D analysis of chemical distributions in grain boundaries in microstructures with nanometer grain sizes, and the student will have the opportunity to become an expert user of one of the most versatile analysis techniques and become a member of the team exploring ways to improve the performance of one of the most promising superconducting compounds.

TEM of nanocrystalline BaFe2As2

A transmission electron micrograph of nanocrystalline BaFe2As2 (image courtesy of Fumitake Kametani)

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