Defects, Flux Pinning, and Changes of Copper Stoichiometry in Superconducting Yttrium Barium Copper Oxide

The purpose of this research is to understand the mechanism by which phase changes related to CuO plane (Cu-O chain layer) occupation initiate and propagate in superconducting YBaCuO. Phase changes of this type, where only the number of CuO layers in a unit cell changes, lead to a family of related crystal structures with stoichiometries Y_2Ba_4Cu _{rm 6 + x}O _{14 + x}, where the number of CuO planes per Ba-Y-Ba sequence is (x + 2) /2. Three superconducting phases, corresponding to x = 0 (single CuO layer), x = 1 (alternating single and double CuO layers), and x = 2 (double CuO layer), are known to exist in this system. A barrier to the development of high-Tc superconductor devices is limited current carrying capacity, and so synthesis of YBaCuO with enhanced critical current density is an important practical development. Because the above microstructure is associated with the 1-2-4/1-2-3 (x = 2 to x = 0) phase transformation, mixed-phase YBaCuO material was synthesized by "partially converting" pure YBa_2Cu _4O_8 to structures of lower copper content under a variety of temperature and pressure conditions. Micrographs reveal dislocations and stacking faults associated with the diffusion of copper and oxygen as transformation progresses. Based on these experimental images, as well as published studies of diffusion in YBa_2Cu_3O _7, an atomic mechanism involving the intercalation and removal of extra CuO planes by partial dislocation climb is developed for changes in the layered YBaCuO crystal structure. An intercalation model is consistent with the known path for oxygen diffusion in YBa_2 Cu_3O_7, and explains observed transformation behavior without invoking diffusion along the c-axis of the crystal structure. The size of the strain fields surrounding stacking faults in YBaCuO suggests that such structures may arise as a result of relatively long-range (~100A interactions), which would make them equilibrium phases in an appropriate temperature range. Investigation of YBaCuO thin films also reveals the presence of many CuO stacking faults, and numerous other crystallographic defects as well. A type of screw dislocation is seen which indicates the films have grown via a spiral growth mechanism. Both the screw dislocations and CuO stacking faults have favorable geometry for flux pinning, and this would explain high critical current values found in most YBaCuO films. (Abstract shortened by UMI.).