Formation of Colonies of Locally Aligned Grains During Thallination of Spray-Pyrolyzed Ba2Ca2Cu3Ox Thick Films

Recent microstructural studies of high’Jc, TI-1223, spray-pyrolyzed thick films have shown that the high Jc and reduced weak-link behavior can be ascribed to the presence of a unique microstructure, wherein the film is divided into ’’colonies" of biaxially aligned regions. In an effort to understand the nucleation and growth of TI-1223 as well as the formation of the “colony" microstructure, several sets of films quenched from various stages of the high-temperature thallination process were examined using electron microscopy, X-ray diffraction and Electron backscatter diffraction (EBSP). It is found that the crystallization of the 1223 phase is associated with formation of a liquid phase which contains significant amounts of all oxides. X-ray micro-diffraction measurements show that the 1223 phase crystallizes in an aligned manner. Studies of films very early into the thallination process show that local biaxial alignment or the formation of colonies starts to occur at the onset of 1223 crystallization. In addition, it appears that the structure within colonies is a function of the processing conditions employed. This presents the possibility for further optimization of the microstructure to attain improved superconducting properties.