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

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.