In this thesis, first two chapters give brief introduction to several aspects of superconductivity, including the concepts, history and applications of superconductivity, and fabrication techniques for long and uniform high-temperature superconductors tapes/wires. Last four chapters present detail information about my research in developing c-axis textured high-Tc YBCO superconducting thick films on unoriented metallic substrates Previous work in the development of YBCO superconducting wires and tapes has been focused on the deposition of YBCO on buffered metallic substrates. Although such an approach has proved successful in terms of achieving grain texturing and high transport current density, critical issues involving continuous processing of long-length conductors and stabilization of the superconductor have not yet been entirely settled. In this work, a novel process Direct Peritectic Growth (DPG) approach was used to directly deposit c-texture YBCO thick film onto an unoriented silver alloy. No buffer layer is employed between the YBCO superconducting film and the metallic substrate. The textured YBCO grains have been obtained through peritectic solidification over a wide range of temperature and time on Ag10%Pd alloy substrate. No observable reaction of the Ag10%Pd substrate was found with the YBCO melt at the maximum processing temperatures near the peritectic point (from 950 °C to 1030°C). In order to investigating YBCO thick film growth dynamics, quenching experiments were carried out near the peritectic temperature for YBCO thick films on the Ag10%Pd substrate using DPG method. The initial YBCO morphology exhibits a column-like grain structure as a result of rapid a-growth when quenched from 1000 °C (the sample was pre-melted at 1030°C). A waffle-like structure was observed on the surface of the silver alloy substrate as the quenching temperature was lowered to 950°C providing a much greater driving force. We found that a grain-oriented substrate may not be required to achieve the grain texturing in the peritectic-reaction controlled process. During solidification, the YBCO grains will parallely nucleate on the surface of the silver alloy to minimize its surface energy, and grow along the a-axis rapidly resulting in a textured film. To identify the underlying mechanism of grain texturing, extensive transmission electron microscopy (TEM) experiments were carried out in this study. A thin “buffer” of 300 nm thickness was observed between the textured YBCO and the silver alloy substrate. This “buffer” was identified to be essentially the YBCO structure, however lacking of the superlattice. Initially randomly oriented, the grains of the buffer gradually self-organize into a preferred orientation and eventually grow into a single crystal YBCO. The underlying mechanism of crystal evolution on an unoriented substrate is discussed in chapter 5. To conclude, we have found a structural evolution from the randomly oriented silver alloy substrate that eventually leads to a large grain of YBCO. Within each large grain the in-plane texture is nearly perfect. The transport Jc has reached a respectable value of 8´10 4 A/cm 2 at 77 K and in zero magnetic field. This self-orientation mechanism suggests that the formation of ordered YBCO crystal lattices may not require a single crystal substrate in the peritectic solidification. Such a film growth mechanism is fundamentally different from the epitaxial growth in the vapor deposition. Based on such a phenomenon, it is possible to induce growth along a specific orientation that is essential in the development of textured YBCO film on randomly oriented substrates. Based on the experimental results in this work, We show that the DPG method offers an effective alternative for the fabrication of long-length YBCO conductors. Also reported is a physical explanation of the texturing mechanism on the metal substrate.
|School:||University of Cincinnati|
|Department:||Engineering : Materials Science|
|School Location:||United States -- Ohio|
|Source:||MAI 57/06M(E), Masters Abstracts International|
|Keywords:||Direct peritectic growth, Superconductor, Thick film, Ybco|
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