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州应用超导中心(https://nationalmaglab.org/).)合作的一部分这项研究的背景是，国际上正在进行一项强有力的努力，为大型物理机(如欧洲核子研究中心的未来环形对撞机)所需的磁体选择超导材料。超导绕组的载流能力目标值要求非常高，在4.2K、16T的温度下，Jc=1500 A/mm2，相当少的几类超导材料可以达到这个值。为这种和类似的高场应用而探索的材料之一是基于化合物BaFe2As2(122)的一类材料，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性能。然而，用于制造磁体所需的多丝导体的工艺导致性能显著退化的多晶122丝，而目前的主要制造挑战是改进工艺以增加流经晶界的电流。最重要的技术问题是是什么降低了122中的GB连接，如何避免这种降级，以及该解决方案是否适用于负担得起的制造工艺？最近的研究表明，最先进的122多晶中的GB连接性主要是由于晶界上的杂质而退化的，因此将纳米级化学与制造路线和超导性能联系起来是这一领域国际研究的关键焦点。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。他最近接受了美国能源部的资助，研究在超清洁条件下制造122种材料。这个项目将使用原子探针断层扫描作为主要技术，在这些精心控制的条件下制备的一套独特的122个样品中分析晶界的局部化学。APT是唯一一项对纳米颗粒微结构中晶界化学分布进行非常高分辨率3D分析的技术，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，并成为团队中的一员，探索实用方法来提高最有前途的新型超导化合物的性能。本项目涵盖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)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).