STRUCTURAL AND TRANSPORT PROPERTIES OF SUPERCONDUCTORS SUITABLE FOR NUCLEAR FUSION MAGNETS BY COMPLEMENTARY EXPERIMENTAL TECHINIQUES

Abstract

Progressive research and constant development in technology lead to an increasing variety of possible applications of High-Temperature Superconductors (HTS). Although a remarkable progress in the understanding of the theoretical and practical features and mechanisms of HTS has been made, many aspects have not yet been studied well enough to be able to control the whole preparation and fabrication processes for a proper optimization of the HTS performances. Power applications, such as nuclear fusion magnets, demand very strong vortex pinning capable to yield high values of the critical current density (Jc) in high magnetic fields (H) and low temperatures (T). The lack of knowledge and experimental studies of the HTS fundamental properties in either intermediate temperature ranges (20 – 50 K) or extreme high field conditions at 4.2 K can be diminished by the continuous development of REBCO-based HTS (RE = rare Earth), including YBa2Cu3O7-x (YBCO). The aim of this thesis is a detailed experimental study of YBa2Cu3O7-x transport properties by combination of the two measurement techniques and vortex pinning investigation at the magnetic vortex regimes relevant for fusion applications of superconductors. The focus has been on the understanding at a microscopic level of the vortex dynamics in different regimes in YBa2Cu3O7-x - the HTS material most promising for fusion application regimes. The measurements have been performed at low temperatures (4 – 50 K) and high magnetic fields (up to 12 T). Two methods have been used to grow the samples - Pulsed Laser Deposition (PLD) and Metal Organic Decomposition (MOD) techniques. The thesis discusses the theoretical background of superconductivity with its basic theories with focus on the High Temperature superconductors; introduces the techniques used for the growth of the YBCO thin films and presents the structural investigation in these samples; describes the transport measurement techniques and presents the obtained characteristic data; and finally discusses in details some selected aspects of vortex flux pinning in YBCO thin films with and without introduced second phases: (i) the effect of pinning and channeling in MOD pristine sample and its absence in MOD YBCO with introduced Artificial Pinning Centres (APCs); (ii) the effect of different secondary phases (BZO and BYNTO) in PLD films and (iii) of different concentration of APCs (0%, 5% and 8%) in MOD films. The boundary between vortex pinning and vortex channeling regimes in the H – T diagram has been identified through the analysis of anisotropy in the pristine MOD sample. The comparison of the PLD samples with different second phases has shown that the combination of c-axis aligned BYNTO continuous columns provides the strongest pinning source for vortices among all the samples. The analysis of different amount of BZO doping in the MOD derived films has shown that at low temperatures the introduced nanoparticles act as strong pinning sites also at high magnetic fields up to 12 T, and that the pinning improves with increasing BZO content. Finally, the experimental correlation between the microwave and the DC parameters has been presented among all the studied samples grown by different methods, with or without APCs at different temperatures and magnetic fields.