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

稀土钡铜氧化物（REBCO）是唯一一类已经开发成商业导线的高温超导（HTS）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计划中设计的高场磁体（https://ccfe.ukaea.uk/research/step/）。在将这些昂贵的材料部署到聚变反应堆中之前，我们必须了解的一个关键方面是，它们的超导特性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。裂变谱聚变研究表明，超导转变温度随着离子和中子辐照能量密度的增加而降低，这是由于在REBCO晶格中产生了点缺陷。原子分辨率成像和像差校正电子显微镜中的光谱学可以直接揭示与晶格缺陷相关的局部结构和键合排列。然而，我们已经表明，REBCO的轻离子辐照不会极大地影响阳离子亚晶格，这些阳离子亚晶格使用高角度环形暗场（HAADF）成像模式用扫描透射电子显微镜（STEM）很容易看到，即使在已经用足够高的能量密度辐照以完全失去超导性的样品中。因此，人们认为，氧亚晶格中的损伤是超导性丧失的主要原因。该项目将使用一种称为电子重叠关联的STEM技术，该技术允许对氧原子进行成像，氧原子只能微弱地散射电子，因此无法在HAADF模式下观察到。除此之外，电子能量损失谱（EELS）将揭示由于引入缺陷而导致的材料电子结构的破坏。该项目将涉及样品制备和先进的电子显微镜仪器的操作，然后进行数据处理和建模，以便全面了解。该项目的主要目标是：1）研究使用电子重叠关联成像技术对原始和辐照REBCO高温超导体中的氧亚晶格进行成像。2)研究使用原子分辨率EELS来确定原始和辐照REBCO中氧含量的变化。3)开发用于量化STEM图像中缺陷和无序的方法，从而能够在不同样品之间进行比较。电子重叠关联技术由Nellist小组开创，以前从未应用于REBCO超导体。将被研究的样品将是Speller集团更大规模辐照活动的一部分，使多种不同表征技术之间能够建立复杂的相关性。该小组与联合王国原子能管理局（UKAEA）密切合作，开展与这项工作相一致的更广泛的辐射损害项目。该项目福尔斯属于EPSRC能源和物理科学主题，特别是能源应用和超导材料研究领域。

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.