New strategies for chemical solution deposition of YBa2Cu3O7-δ film towards fusion applications

The applications of superconducting materials are very wide-ranging and concern energy technologies, medical and analytical instruments, fundamental and applied research. In the field of nuclear fusion, superconductors are defined as enabling technology because the creation of a tokamak fusion machine for application purposes such as commercial power plant would not be possible without the use of superconducting materials to effectively confine the hot plasma generated in the reactor. The development of increasingly high-performance machines requires excellent material properties at high magnetic fields operating conditions. For this reason, attention is turning to high temperature superconductors (HTS). The design of new tokamak machines containing HTS is therefore promoting the research and development of materials that are more and more efficient in the presence of high magnetic fields. On an industrial level, the HTS production mainly takes place through physical methods, e.g. pulsed laser deposition or reactive co-evaporation. Chemical solution deposition (CSD) techniques are gradually spreading thanks to the process affordability and simplicity. However, the chemically derived HTS still cannot achieve the results at high magnetic fields of materials prepared by physical methods. My Ph.D. activity was devoted to YBa2Cu3O7-δ (YBCO) compound, one of the most studied HTS materials. The work was carried out through two principal lines. Firstly, an in-depth study of some chemical aspects that regulate the CSD process for YBCO thin film preparation was accomplished. In fact, despite the first article on YBCO film by CSD dates from 1988, some topics still needed to be deepened. In particular, the solution reactivity and stability, as well as the thermodynamic and kinetic aspects guiding the epitaxial growth of YBCO film, were investigated to improve the film properties. The second part of my Ph.D. work aimed at enhancing the YBCO superconducting performances at the operating conditions of the nuclear fusion process, i.e. high magnetic fields and low temperatures. With this purpose, two different strategies for the introduction of nanosized defects in YBCO film were explored and compared. Extensive and detailed film characterisations allowed understanding if the added dopants could act as artificial pinning centres and prevent transport properties degradation when an external magnetic field is applied. This study was performed within the framework of the EUROfusion Consortium that supports and funds research activities on nuclear fusion on behalf of the European Commission’s Euratom programme. The activity received partial funding from the Euratom programme 2014-2018 and 2019-2020 under grant agreement N° 633053.