| Popis: |
Thin films of the high temperature superconductor (HTS) YBa2Cu3O7 (Y123) are of great potential in a wide range of applications, including low-loss microwave cavities and filters, bolometers, various superconducting terminal devices, flux transformers, and dc and rf superconducting quantum interference devices (SQUIDs). They also have the potential to give insight into the fundamental mechanisms governing high temperature superconductivity. Y123 coated conductors, the so-called second generation superconducting tapes, which are based on Y123 thin film technology, also have great applications in carrying large superconducting currents. However, some important challenges or fundamental problems hindering their practical applications need to be solved scientifically. These include: finding the most effective and economic approaches to enhance the superconducting critical current density in high magnetic fields; reducing the fabrication cost with high reproducibility; arriving at a profound understanding of the relationship between microstructure and superconducting performance, etc. The work described in this thesis deals with these challenges, with an emphasis on fundamental studies on supercurrent enhancement through nanotechnology and nanoengineering using advanced thin film fabrication methods, including pulsed laser deposition and spin coating, and on the relationship between microstructure and supercurrent performance.An overview of the research on HTS thin films in the period from 2000 to the end of 2005 is given in Chapter I. Attention was paid to both Y123 films and coated conductors fabricated using both PLD and chemical methods, on both single crystal and metallic substrates. The related electrical and magnetic properties are also reviewed. In the first part of the thesis work, Y123 thin films with and without Ag nanoparticle inclusions were grown by pulsed laser deposition (PLD) on YSZ (100), SrTiO3 (100), and LaAlO3 (100) single crystal substrates. A discontinuous layer of Ag nanodots was deposited on the substrates prior to the deposition of Y123 films. The Y123 films grown on such surfaces modified with Ag nanodots were extensively characterised by atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), AC susceptibility and DC magnetisation. The effects of the density of Ag nanodots, which was controlled by the numbers of PLD shots, on the microstructures and resultant critical current density Jc have been studied systematically. Results showed that under fixed physical deposition conditions Jc increased monotonically with the number of Ag shots, n, for films grown on both STO and LAO substrates. At 77 K, the Jc increased from 106 to 3.2 ×106 A/cm2 for LAO and from 8×105 to 3.5×106 A/cm2 for STO as n increased from 0 to 150. At 5 K, the Jc was enhanced by approximately four times at both low and high fields. These values obtained due to nano-Ag inclusions are comparable to the best results achieved by other prestigious research groups around the world. However, for films grown on YSZ substrate, Jc increased from 2×105 to 2×106 A/cm2 as Ag shots increased from 0 to 30, and decreased to 9×105 for n ³ 60. Detailed microstructure investigations indicated that the crystallinity and ab alignment gradually improved as the number of Ag-nano-dots increased. In the second part of the thesis work, YBa2Cu3Oy films were grown on single crystalline YSZ, SrTiO3, and MgO, and on polycrystalline Ag substrates using non-fluorine sol-gel and spin coating methods. The effects of heat treatment conditions on the phase evolution and microstructures were investigated using optical microscopy, X-ray diffraction, and atomic force microscopy. A detailed study was performed on the phase formation, degree of grain orientation, formation of cracks, and surface morphologies. It was found that sintering temperature and roughness of substrate surfaces are two key factors in controlling crack morphologies. With several important advantages, including precursor solution stability, improved film density, and elimination of HF during processing, high-quality YBCO films have been achieved on single crystal substrates with transport critical current densities up to 106 A/cm2. An extensive study was carried out on the alteration of precursor solution stoichiometry and its effects on superconducting properties. Fluorine-free sol-gel–derived films on the LAO substrate exhibited epitaxial growth with excellent in- and out-of-plane texture. Experimental details are given on the sol-gel synthesis chemistry and XRD and TEM characterization of the YBCO thin films.The phase evolution of YBCO films prepared by the fluorine-free sol-gel method was also systematically investigated using in-situ high temperature optical microscope observations and x-ray diffraction. The conversion sequences and the final resultant products have been determined for barium, copper and yttrium containing precursors, respectively. It was found that those conversions are strongly dependent on the experimental conditions such as water partial pressure. The formation of YBCO starts at a temperature of around 710 oC and lasts up to 800 oC over about 15 min. Depending on the barium containing phases and experimental conditions. It is suggested that a- and caxis YBCO grains are governed by different reactions. |
