Irradiation damage in REBa2Cu3O7-x superconductors
Keywords: damage cascade
coated conductor
superconductor
neutron irradiation
ion irradiation
REBCO
Transmission Electron Microscopy (TEM); neutron irradiation

Future designs of fusion power reactors will require superconducting magnet windings that, in service, will be subjected to high fluxes of particles, in particular 14 MeV neutrons generated in the fusion reaction. It is an engineering priority and challenge to generate the highest magnetic fields possible to reduce the size of the reactor and increase its power output, which can be achieved by the use of high temperature superconducting tapes. Despite efforts to understand the irradiation effects associated with the impact of energetic particles on the complex structure of high temperature superconductors, little is known about the microstructural evolution under irradiation. Understanding the mechanism of irradiation damage is thus of great importance for the development of fusion reactors.
The characterisation work used Transmission Electron Microscopy as the primary tool together with imaging processing techniques and analytical tools to assess subtle changes of the structural parameters following irradiation. An in-reactor sample of superconducting tape was irradiated by neutrons to a fluence of 3.3 × 1018 cm-2, and to validate the findings, results were compared with a pristine reference sample without neutron irradiation. In order to further study the mechanisms of radiation damage on the REBCO matrix, extensive in-situ ion irradiation experiments in a TEM were taken place using different energies, temperatures, and incident ions at the MIAMI2 facility at the University of Huddersfield.
The first section of the experimental results presented in this thesis is a qualitative evaluation of selected irradiated samples. The impact of both ion and neutron irradiation is illustrated, and HRTEM images compared at the pre- and post-irradiation stages. Both neutron and proxy ion irradiation using heavy ions (Kr, Xe) also show small (2-10 nm) damage cascades. Lighter ion irradiation using He presents clear evidence for homogenous lattice degradation. The effects of temperature, irradiation energy, and incident particle mass are also discussed.
The second results section uses HAADF/STEM atomic-resolution images as an input in order to fit the lattice atomic locations. These locations are then used to extract the structural parameters of the Ba1-RE-Ba2 sublattice, that are then discussed as in terms of the associated damage mechanisms. Using the precise locations of the atomic columns, I was able to extract a wide-array of structural parameters from every unit cell and pool these together to a meaningful statistical results, insights that could not be replicated using traditional approaches such as XRD.
In the last results section, I report for the first time on image processing technique that allows a precise quantification of the irradiation damage compared relative to reference states of unirradiated and amorphous material. The performance of these entropy measurements in the frequency domain were proved robust and accurate by analysing more than 1000 HRTEM images irradiated under different conditions. I illustrate the direct, linear-like, correlation of the expected lattice damage, as experimentally observed in the first section, to the entropy values extracted. I also elaborate on the underlying reasons why the observed correlation exists. By using a combination of a large dataset and new analytical tools, this thesis sheds light on the irradiation mechanisms in high temperature superconductors. The data analysis conducted is also relevant for application in the study of other irradiated materials.