High-resolution microchemistry and structure of grain boundaries in bulk Y1Ba2Cu3O7-x

A simple chemical thinning process has been developed which allows high-quality TEM specimens to be prepared from bulk Y1Ba2Cu3O7-x (YBCO) material. This enabled the microchemistry of a statistically significant number of grain boundaries, with a wide range of misorientations, to be studied using TEM, STEM and EDX techniques. In YBCO samples where the overall stoichiometry is exactly 1:2:3 no unreacted CuO, BaCO3 or Y2O3 are present as grain boundary phases. Significant traces of segregated impurity elements (Mo, Zr and Sr in particular) were detected, and the presence of these impurities resulted in the formation of non-stoichiometric phases containing mostly Y, Ba and Cu. Segregation of these impurities was found to occur most often at grain boundaries where no high-symmetry relationship exists between the two grains. There is a direct correlation between the observed amount of impurity-induced grain boundary phases, the grain boundary orientations and the measured values of Jc.