JC DEGRADATION OF YBA2CU3O7-X SUPERCONDUCTORS SINTERED IN CO2/O2

The stability of YBa2Cu3O7-x (YBCO) towards reactions with CO2 in CO2/O2 gas mixtures has been studied during sintering of phase-pure YBCO powders as a function of CO2 partial pressure. The zero-field critical current density, Jc, in the sintered samples decreased with increasing CO2 partial pressure in the mixtures. Up to certain amount of CO2 partial pressure, the Jc values dropped to zero while the bulk of the samples was still largely superconducting, as determined by Meissner effect measurements. The microstructure and composition of the samples was investigated by TEM, AEM, and SIMS. Microbeam SIMS indicated segregation of C to grain boundaries (GBs). TEM images showed second phases at a minority of the grain boundaries. These second phases were identified as BaCuO2 and BaCu2O2. Near some grain boundaries without obvious second phases, the structure was tetragonal within several tens of nm to the boundaries, while the interior of the grains remained orthorhombic. At high CO2 partial pressure, the YBCO superconducting phase was completely decomposed to BaCO3, Y2BaCuO5, and CuO. The Jc degradation will be discussed in terms of the dominant GB microstructures. © 1990.