Thesis (Ph. D.)--Lehigh University, 1999. / Includes vita. Bibliography: leaves 145-153.

Thesis (Ph. D.)--Lehigh University, 1999. / Includes vita. Bibliography: leaves 144-149.

Thesis (M. Sc.)--University of Alberta, 2009. / Title from PDF file main screen (viewed on Oct. 20, 2009). A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science in Materials Engineering, Department of Chemical and Materials Engineering, University of Alberta. Includes bibliographical references.

Thesis (Ph.D.)--Brown University, 2005. / Vita. Thesis advisor: Clyde L. Briant. Includes bibliographical references (leaves 95-99). Also available online.

This dissertation describes the investigation of the normal-state properties of high-angle grain boundaries in YBa2Cu3O7-d (YBCO) and Y1-xCaxBa2Cu3O7-d (calcium-doped YBCO). YBCO is a high-temperature superconducting material with a superconducting transition temperature up to 93 K. Grain boundaries are interfaces between two crystals or grains, and severely reduce the attainable currents in practical, polycrystalline material. A grain boundary is characterised by the misorientation angle between the two adjacent crystals, which determines the atomic structure of the interface. The structure of low-angle grain boundaries (misorientation angles < ~7°) is well understood; it consists of a regular array of dislocations. For higher misorientation angles the dislocations merge and form a continuously distorted zone. The structure of these high-angle grain boundaries, and, hence, the mechanism for charge transport across the interface, is less well understood. The current-voltage behaviour of grain boundaries below the transition temperature of the YBCO has been investigated extensively, but less data is available of the resistive behaviour of the grain boundary in the normal state above the transition temperature. The doping of YBCO with calcium is known to decrease its transition temperature, but it can simultaneously improve the charge transport properties of grain boundaries in polycrystalline material. YBCO and calcium-doped YBCO thin films were fabricated on bicrystalline substrates. The grain boundaries had misorientation angles between 18° and 45°. The films were processed in order to obtain microscopic devices that made it possible to determine the resistance of the grain boundary below and above the transition temperature. A measurement system was used with which the voltage across the grain boundary can be measured as a function of applied current between 5 K and room temperature. A detailed model for charge transport by tunnelling across a grain boundary was used to interpret the results of the measurements of grain boundaries. An algorithm based on this model was formulated that made it possible to calculate a shape for the potential barrier at the grain boundary from the temperature dependence of its resistance. The microstructure of the grain boundary was investigated using Transmission Electron Microscopy. It was found that the grain boundary can show considerable deviations from a straight path in line with the substrate grain boundary, but there are also locations where such deviations are not observed. Extensive measurements showed that the resistance of the grain boundary decreases with increasing temperature above the transition temperature, and that resistance and the extent of resistance variation increases with misorientation angle. The resistance below the transition temperature was for certain misorientation angles observed to be independent of temperature at voltages sufficiently high to exclude the influence of the superconducting behaviour of YBCO. A reduction of the oxygen content of YBCO (higher value of d) increases the resistance and the temperature dependence of the resistance. The doping of YBCO with calcium decreases the resistance of the grain boundary below and above the transition temperature. The shape of the potential barrier at the grain boundary was calculated on the basis of most resistance-temperature measurements. The shape and size of potential barriers are used to explain the variation of the grain boundary resistance with misorientation angle, oxygen content and calcium doping percentage. The model shows little validity for grain boundaries with a higher misorientation angle and resistance, which indicates that charge transport across the grain boundary does not necessarily take place according the tunnelling with the assumptions made.

No description available.

Thesis (M.S.)--West Virginia University, 2001. / Title from document title page. Document formatted into pages; contains vii, 64 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 62-64).

No description available.

Thesis (Ph. D.)--Oregon Graduate Center, 1988.

Grain boundaries form the basis of an important Josephson junction technology in the cuprates and also limit the superconducting critical currents attainable in practical, polycrystalline materials. An improved understanding of these defects is therefore important for applications. The status of the current understanding of cuprate grain boundaries is summarised and experimental investigations are presented, focusing on the less well understood high angle boundaries. Measurements of the capacitance of grain boundaries in the overdoped superconductor Y1-xCaxBa2Cu3O7-8, were performed as a function of the calcium content, using the Josephson coupling across the boundaries. Particular care was taken to eliminate the effects of heating and stray capacitance due to the substrate. The effect of thermal noise was also assessed. These measurements provide important information about the area and the width of the grain boundaries, that highlights their inhomogeneous nature. A new technique was applied to measure the normal state properties of YBa2Cu3O7-8 grain boundaries above the critical temperature. Since the resistance of the adjacent material at high temperatures is comparable to, or greater than, that of the grain boundary a compensating Wheatstonebridge structure was used. The errors involved in this technique are carefully assessed and quantified. The normal state resistance of a number of different grain boundary orientations was measured from room temperature to the critical temperature. Detailed characterisation of the grain boundaries, including measurements of the critical current and the current voltage characteristics at low temperatures, was performed. The results obtained are used to assess the validity of the various theories for the grain boundary electrical structure. A tunneling model that accounts for the band structure of the material is developed and applied to potential barriers consistent with a band bending model. This theory is shown to provide a convincing account of the experimental results presented in this thesis.