Microstructural engineering of superconducting materials for large scale applications

Low temperature superconducting materials are already deployed at scale in magnetic resonance imaging machines used for diagnosis purposes in hospitals and for magnets in big physics experiments such as particle accelerators and fusion reactors. However, the expected commercial impacts of high temperature superconductors discovered over 35 years ago are only just beginning to come to fruition, driven by the new compact fusion reactor market. The most important property for all large scale applications of superconductors is the critical current density of the material, which is strongly influenced by microstructural defects in the material. Therefore, this review focuses on understanding the effects of microstructure on the current carrying ability of technologically useful superconductors and the processing strategies that are used for optimising and tailoring the microstructure of commercial wires or tapes. After laying out some key fundamental concepts of type II superconductivity, the importance of flux pinnng by microscopic defects in the material is introduced and examples are given of how the flux pinning landscape is optimised in conventional low temperature superconducting wires of Nb-Ti, Nb3Sn and MgB2. Additional challenges associated with obtaining high current densities in high temperature superconductors, including grain boundary weak-link behaviour, anisotropy and flux creep, are outlined, along with the remarkable materials science that has enabled these challenges to be tamed in commercial conductors.