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 Title: Chemical diffusion of oxygen in yttrium barium(2) copper(3) oxygen(6+x) Author(s): LaGraff, John Robert Doctoral Committee Chair(s): Payne, David A. Department / Program: Materials Science and Engineering Discipline: Materials Science Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Chemistry, Inorganic Physics, Condensed Matter Engineering, Materials Science Abstract: This thesis is primarily concerned with a quantitative investigation of the chemical diffusion of oxygen in YBa$\sb2$Cu$\sb3$O$\sb{\rm 6+x}$. The research also investigates the optimization of crystal growth and the interrelationship between oxygen mobility and ferroelastic characteristics. Oxygen jump mechanisms in YBCO were also proposed and evaluated using an oxygen interaction model. Crystal growth mechanisms in YBCO are highly anisotropic, with extremely slow growth in the c-axis direction. However, the understanding and optimization of these mechanisms have led to the successful growth of large, high quality crystals often in the range of $\sim10\times10\times2{\rm mm}\sp3$. These single crystal specimens in conjunction with polycrystalline specimens with varying microstructures were investigated to distinguish between intrinsic diffusion and microstructure-dependent behavior.Resistance measurements as a function of heating and cooling rates provided data regarding oxygen in-diffusion and out-diffusion behavior. The different onset temperatures for oxygen out-diffusion between ceramic single crystals ($\sim$500$\sp\circ$C) and polycrystalline specimens ($\sim$350$\sp\circ$C) suggest that grain boundary diffusion becomes active near 350$\sp\circ$C. Isothermal electrical resistance measurements on single crystals revealed that the intrinsic rate of oxygen in-diffusion was comparable to, if not slower than out-diffusion, which was contrary to measurements on polycrystalline specimens. This type of behavior was attributed to the formation of a highly oxygenated shell during in-diffusion which behaved as a high conductivity pathway as well as a barrier to bulk oxygen in-diffusion. The use of single crystals has enabled these effects to be clearly distinguished.Values of electrical resistance were determined for YBCO single crystals and polycrystals as a function of temperature (450-850$\sp\circ$C) when the specimens were in equilibrium with oxygen partial pressures ranging from 0.001-1atm. A clear p-type $\to$ n-type conductivity transition was observed near 750$\sp\circ$C in 0.001 atm of oxygen (i.e., x = 0.1). Diffusion measurements were also made as a function of the initial oxygen content of the specimen. The chemical diffusivity was found to increase with decreasing oxygen content typical of a vacancy diffusion mechanism. However, the magnitude of the dependence suggests that other factors (e.g., correlation effects or the thermodynamic factor) are responsible for the concentration dependence. Issue Date: 1992 Type: Text Language: English URI: http://hdl.handle.net/2142/20901 Rights Information: Copyright 1992 LaGraff, John Robert Date Available in IDEALS: 2011-05-07 Identifier in Online Catalog: AAI9305593 OCLC Identifier: (UMI)AAI9305593
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