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Degree

PhD, University of Cincinnati, Engineering : Materials Science, 2004.

Abstract

Highly textured superconducting YBCO films are prepared through the TMAP-MOD route by coating, burnout and annealing. Films deposited either on single crystal substrates of LAO, STO and YSZ/CeO₂, or on RABiTS with buffers of CeO₂/YSZ/CeO₂ and Y₂O₃/YSZ/CeO₂. In both cases high current density over 10⁶ A/cm² and Tc(R=0) around 90K have been demonstrated. XRD data and TEM/SAD analysis confirmed YBCO phase epigrowth from interface. This is the first demonstration of high J_c over 1 MA/cm² in a non-fluorine precursor. We found water is the key factor that helps the growth of c-axis oriented grains with a-b in plane alignment in terms of forming a liquid phase of Ba(OH)₂. While the dendritical growth of YBCO phase in the melt results in none uniformity and a rough surface, which degrades the properties of the YBCO film. Intermediate phases associated with Y, Ba, and Cu are discussed into detail especially for the copper and barium contained phases. Mechanisms of the liquid phase (melt) formation are proposed with regarding to the function of humidity, the core-shell model has been used to explain the copper oxidation process, and the stress state of burnout films are analyzed by introducing the strain energy release rate in the Griffith model. Based on the proposed key chemical reactions, the energy process in terms of barriers and driving force is calculated. It shows that the driving force comes mainly from three factors of ΔT, P_co2 and P_o2, and which are partially confirmed by experimental results. Consequently, an energy controlled nucleation modes have been suggested and factors of avoiding body nucleation as well as adjusting interfacial nucleation rate have been discussed. Finally, A dynamic process of rejecting impurities either to the grain boundaries or to the top surface is considered. As a result, the lateral growth mechanism has been proposed and factors based on the free energy and compositional availability that could block the dynamic growth front of YBCO phase are specified. A new growth mode by reducing driving force after nucleation is suggested, which could form small grains with less impurity segregation and weak links.

Keywords

YBCO; HTS; coated conductor; film; epitaxial growth

Committee / Advisors

Dr. Donglu Shi (Chair)
Dr. Relva C. Buchanan
Dr. Rodney Roseman
Dr. Amit Goyal

Pages

302p.

Document number: ucin1075992286
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=ucin1075992286

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