An ion-beam engineered microstructure for high-performance superconducting films
Authors:   , Ambadiyil Soman Arya 
Published: February 2024

<p><strong>High-temperature superconductors, distinguished by their exceptional properties, stand as leading candidates for a broad spectrum of applications addressing global challenges. However, the suitability of these materials depends upon the critical current performance under the specific conditions required for each application.</strong></p><p>The critical current of a superconductor is determined by flux pinning — the interaction between nanometre-scale structural defects and the vortex matter — and varies strongly with temperature, magnetic field, and field orientation. Different defect structures with distinct pinning characteristics can interact in ways that either strengthen or weaken the overall pinning effect, but not necessarily in a straightforwardly additive manner. The best possible critical current performance under particular operating conditions requires an optimal pinning landscape tailored to those conditions.</p><p>This thesis employs ion irradiation as the material modification tool to systematically probe how controlled populations of point (0D), linear (1D), planar (2D), and spherical (3D) microstructural defects combine to pin magnetic flux vortices at different orientations and densities in REBCO-coated conductors. Ion irradiation allows the incorporation of defects independently of the existing microstructure, enabling control over dimensionality, density, and orientation through selecting appropriate ion energy, fluence, and angle of incidence, respectively.</p><p>Critical current measurement studies on samples irradiated with different ions and energies showed that under a low temperature, high magnetic field regime, for example, at 20 K, 8 T, an optimal isotropic critical current enhancement factor in the range of 2 – 3 can be realised through different types of irradiations, including proton irradiation, irradiation with gold ions of energy 18 MeV, and silver ions with energies ranging from 50 MeV to 150 MeV. Critical current studies at higher temperatures revealed distinct regimes of defect dimensionality as the energy of the silver ions increased from 50 MeV to 150 MeV. It was found that at 65 K, 8 T, irradiation with silver ions in the energy range of 75 – 150 MeV, creating columnar tracks, offers a greater c-axis peak enhancement ranging from a factor 3 to 3.6, whereas 50 MeV irradiation creating spherical defects exhibited a nearly isotropic enhancement of a lower factor of ~2. A composite pinning landscape formed by triple irradiation using 75 MeV silver ions at 0°, +60°, and −60° angles, creating a combination of segmented columnar and spherical defects, demonstrated superior effectiveness in reducing critical current anisotropy and higher critical current performance compared to single or double irradiations at different angles. The extensive experimental data on individual incorporation of defects and different combinations of defects over a wide range of temperatures (20 – 77.5 K), magnetic fields (0 – 8 T), and field orientations (0° – 360°) provides significant materials engineering knowledge that will guide performance improvements in commercial superconducting wires and accelerate the widespread uptake of HTS technology.</p>