NONSUPERCONDUCTING PHASES AT GRAIN BOUNDARIES IN YBa2Cu3Ox SUPERCONDUCTORS

The morphology, composition and phases of grain boundaries in YBa2Cu3Ox superconductors have been investigated by SEM, TEM and AES. The results show that some impure phase particles present at the grain boundary in the samples sintered at 940°C for 26 hours, which grow to be a shell around the grain as the sintering temperature increases to 970°C. No such particle has been found at the grain boundaries in the sample sintered at 920°C for 26 hours by SEM, Jc is the highest in our studied samples. AES and TEM analyses indicate that the grain boundaries are complex multilayer structures consisting of barium-rich, copper-rich, carbon-rich, yttrium and oxygen-deficient impure phases. The dominant impure phases harmful to Jc at the boundary are the mixtures of BaCO3, Cu2O, BaC2, Y2C3 and an amorphous material.