Atom Probe Tomography analysis of BaFe2As2 superconductors for high field applications

该项目将成为牛津大学和佛罗里达应用超导中心（https://nationalmaglab.org/）合作的一部分。这项研究的背景是，国际上正在努力为大型物理机器（如欧洲核子研究中心的未来圆形对撞机）所需的磁体选择超导材料。超导绕组载流能力的目标值要求非常高，在4.2 K、16 T条件下，Jc = 1500 A/mm2，很少有超导材料能达到这个值。其中一种正在探索的材料是基于化合物BaFe2As2（122）的材料，这种材料可以由廉价的起始原料制成，并且至少在单晶中具有高场磁体应用所需的超导特性。然而，用于制造磁体所需的多丝导体的工艺导致多晶122丝的性能大幅下降，目前制造的主要挑战是改进工艺以增加跨晶界的电流。最重要的技术问题是什么会降低122中的GB连接性，如何避免这种降低，以及该解决方案是否适用于负担得起的制造过程？最近的研究表明，最先进的122多晶中的GB连通性主要是由晶界处的杂质导致的，因此将纳米化学与制造路线和超导性能相关联是国际上该领域研究的重点。我们在美国的项目合作伙伴，佛罗里达州立大学的Fumitake Kametani教授是这类超导体合成和性能方面的专家。他最近获得了美国能源部的资助，研究在超清洁条件下制造122种材料。该项目将使用原子探针断层扫描作为主要技术来分析在这些精心控制的条件下制备的122个独特样品的晶界局部化学。APT是唯一一种提供纳米晶粒尺寸的微观结构中晶界化学分布的高分辨率3D分析技术，学生将有机会成为最通用的高分辨率分析技术之一的专家用户，并成为团队的一员，探索切实可行的方法来提高最有前途的新型超导化合物之一的性能。该项目涵盖EPSRC能源主题（能源材料，制造技术）和工程主题（制造技术，材料工程，工程设计）中的关键主题。

This project will form part of a collaboration between Oxford and the Applied Superconductivity Center in Florida (https://nationalmaglab.org/). The context of the research is that there is a strong international effort to select superconducting materials for the magnets needed for large physics machines like the Future Circular Collider at CERN. The target values for current carrying capacity of the superconducting windings are extremely demanding, Jc = 1500 A/mm2 at 4.2 K in 16 T, and rather few classes of superconducting material can possibly achieve this value. One of materials being explored for this and similar high-field applications is the class of materials based on 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 manufacturing challenge at the present time is improving the processing to increase current flow across grain boundaries. The most important technological questions are what degrades the GB connectivity in 122, how this degradation can 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 primarily by impurities at the grain boundaries, and so correlating the nano-scale chemistry with the manufacturing route and superconducting performance is the key focus of international research in this area.Our project partner in the USA, Prof. Fumitake Kametani of Florida State University is an expert in the synthesis and performance of this class of superconductor. He has recently received funding from the DoE to study the manufacturing of 122 materials under ultra-clean conditions. This project will use Atom Probe Tomography as the primary technique to analyse the local chemistry of grain boundaries in a unique set of 122 samples prepared under these 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 high resolution analysis techniques and become a member of the team exploring practical ways to improve the performance of one of the most promising new classes of superconducting compounds.This project spans key topics in the EPSRC Energy Theme (Materials for Energy, Manufacturing Technology) and the Engineering Theme (Manufacturing Technology, Materials Engineering, Engineering Design).