Atomic-resolution imaging of radiation-induced defects in high temperature superconductors for fusion applications

稀土氧化钡铜氧化物(REBCO)是唯一一种已被开发成商业导线的高温超导材料，其工程性能足以用于小型聚变托卡马克的高场磁体，如STEP计划(https://ccfe.ukaea.uk/research/step/)中正在设计的那种。在将这些昂贵的材料部署到聚变反应堆之前，我们必须了解的关键方面之一是，它们的超导性能如何受到高能中子和大量伽马射线的影响，以确保它们能够在磁体的寿命内保持足够的性能。裂变光谱聚变研究表明，随着离子和中子的辐照，超导转变温度随着注量的增加而降低，这归因于REBCO晶格中产生的点缺陷。原子分辨成像和电子显微镜中的光谱可以直接揭示与晶格缺陷相关的局域结构和成键排列。然而，我们已经证明，REBCO的光离子辐照对阳离子亚晶格的影响不大，即使在已经被足够高的注量照射到完全失去超导电性的样品中，使用高角度环状暗场(HAADF)成像模式的扫描电子显微镜(STEM)也很容易看到阳离子亚晶格。因此，人们认为氧亚晶格中的损伤是超导电性损失的主要原因。该项目将使用一种名为电子光刻的STEM技术，允许对氧原子进行成像，氧原子只对电子进行微弱散射，因此在HAADF模式下无法观察到。此外，电子能量损失谱(EELS)将揭示由于引入缺陷而对材料电子结构的破坏。该项目将包括先进的电子显微镜仪器的样品制备和操作，随后是数据处理和建模。该项目的主要目的是：1)研究使用电子光刻技术对原始和辐照的REBCO高温超导体中的氧亚晶格进行成像。2)研究了用原子分辨电子能谱测定未辐照和辐照后REBCO中氧含量的变化。3)发展量化STEM图像中缺陷和无序的方法，使不同样品之间能够进行比较。电子光刻技术是由Nellist小组开创的，以前还没有应用于REBCO超导体。将被研究的样本将是斯佩勒小组更大规模辐射活动的一部分，使多种不同的表征技术之间能够进行复杂的关联。该小组与英国原子能管理局(UKAEA)在这项工作相一致的更广泛的辐射损害项目上密切合作。该项目属于EPSRC能源和物理科学主题，特别是能源应用材料和超导研究领域。

Rare-earth barium copper oxides (REBCO) are the only class of high-temperature superconducting (HTS) materials that have been developed into commercial wires with an engineering performance good enough for use in the high field magnet for small fusion tokamaks like the one being designed in the STEP programme ( https://ccfe.ukaea.uk/research/step/ ). One of the critical aspects we must understand before deploying these expensive materials in a fusion reactor is how their superconducting properties are affected by exposure to high energy neutrons and a significant flux of gamma rays to ensure that they can retain adequate performance for the lifetime of the magnets. It is known from fission spectrum fusion studies that the superconducting transition temperature decreases as a function of fluence with both ion and neutron irradiation, and this has been attributed to point defects being created in the REBCO lattice.Atomic-resolution imaging and spectroscopy in an aberration-corrected electron microscope can directly reveal the local structure and bonding arrangement associated with lattice defects. However, we have shown that light ion irradiation of REBCO does not greatly affect the cation sublattices that are easily visible with scanning transmission electron microscopy (STEM) using the high-angle annular dark field (HAADF) imaging mode, even in samples that have been irradiated with sufficiently high fluences to lose superconductivity altogether. Therefore, it is believed that damage in the oxygen sublattice is largely responsible for the loss of superconductivity. This project will use a STEM technique known as electron ptychography that allows the oxygen atoms, which only weakly scatter electrons so cannot be observed in HAADF mode, to be imaged. Alongside this, electron energy-loss spectroscopy (EELS) will reveal the disruption to the electronic structure of the material due to the introduction of defects. The project will involve sample preparation and operation of advanced electron microscope instruments followed by data processing and modelling allow a full understanding to be developed.The main aims of this project are to:1) Investigate the use of electron ptychography to image the oxygen sublattice in pristine and irradiated REBCO high temperature superconductor. 2) Investigate the use of atomic resolution EELS to determine oxygen content variation in pristine and irradiated REBCO. 3) Develop methodologies for quantifying the defects and disorder in STEM images, enabling comparisons to be made between different samples.The electron ptychography technique has been pioneered by the Nellist group, and has not previously been applied to REBCO superconductor. The samples that will be studied will be part of a larger irradiation campaign by the Speller group, enabling sophisticated correlations to be made between multiple different characterisation techniques. The group collaborates closely with the United Kingdom Atomic Energy Authority (UKAEA) on the wider irradiation damage project that this work aligns with. This project falls within the EPSRC Energy and Physical Sciences themes, specifically the Materials for Energy Applications and Superconductivity research areas.