Control of YBa sub 2 Cu sub 3 O sub 7 minus x degradation during heat treatment

The importance of closely controlling the furnace atmosphere during processing of YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) was examined. The stability of YBCO during sintering was studied as a function of CO{sub 2} partial pressure in CO{sub 2}/O{sub 2} gas mixtures. The zero-field critical current density, J{sub c}, decreased with increasing CO{sub 2} partial pressure, and ultimately reached zero, even though Meissner effect measurements showed that the bulk of the samples with zero transport J{sub c} remained superconducting. Examination of the microstructure and composition of the samples by TEM, AEM, and SIMS showed the presence of BaCuO{sub 2} and BaCu{sub 2}O{sub 2} at a minority of the grain boundaries. Near other grain boundaries, where second phases were not readily evident, the structure was found to be tetragonal for several tens of nm up to the grain boundaries, whereas the grain interiors were found to be orthorhombic. At high partial pressures of CO{sub 2}, YBCO completely decomposed to BaCO{sub 3}, Y{sub 2}BaCuO{sub 5} (211), and CuO. In the firing of plastically processed superconductors, such as extruded YBCO coils, it was found that the use of reduced total pressure prevents the decomposition of YBCO. 13 refs., 5 figs., 2 tabs.