Crystallization of colonies of locally aligned grains during thallination of spray-pyrolyzed TlBa₂Ca₂Cu₃O_x thick films

Recent microstructural studies of high-J_c, Tl-1223, spray-pyrolyzed thick films have shown that the high-J_c 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 Tl-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 (EBSD). 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. Furthermore, it is found that the film nucleates at the substrate on the side closest to the thallium source. The growth front then moves up through the thickness and along the length of the film. X-ray microdiffraction measurements show that local biaxial alignment or the formation of colonies starts to occur at the onset of 1223 crystallization.