Effect of CO2 on the Processing of Y-Ba-Cu-O Superconductors

The superconducting properties of YBa2Cu3O6+x reacted with various known ratios of O2/CO2 gas mixtures during sintering at different temperatures were studied. Jc was found to decrease drastically upon reaction with CO2, becoming zero at certain CO2 activities. The stability region for the 123 superconductor, as a function of CO2 activity and temperature, was empirically formulated as follows: log pCO2 \u3c (−45,000)/T + 33.4. The grain boundaries in sintered samples with Jc = 0 were investigated with HRTEM in conjunction with EDS. Two distinct types of grain boundaries were observed. Approximately 10% of the grain boundaries were wet by a thin layer of a second phase, deduced to be BaCuO2. The remaining boundaries were sharp grain boundaries. The grain structure near the sharp grain boundaries was tetragonal. These two types of grain boundaries are thought to be responsible for Jc being zero

Methodology for evaluation of alternative technologies applied to nuclear fuel reprocessing

An analytic methodology has been developed to compare the performance of various nuclear fuel reprocessing techniques for advanced fuel cycle applications including low proliferation risk systems. The need to identify and to compare those processes, which have the versatility to handle the variety of fuel types expected to be in use in the next century, is becoming increasingly imperative. This methodology allows processes in any stage of development to be compared and to assess the effect of changing external conditions on the process. (7 tables, 15 figs

Degradation of properties of YBa sub 2 Cu sub 3 O sub x superconductors sintered in CO sub 2 -containing atmosphere

Stability of the YBa{sub 2}Cu{sub 3}O{sub x} (YBCO) superconductor toward reacting with CO{sub 2} in CO{sub 2}/O{sub 2} gas mixtures during sintering was investigated as a function of the partial pressure of CO{sub 2} and temperature. The transport critical current density, J{sub c}, of the superconductor decreased drastically with increasing partial pressure of CO{sub 2} was increased, J{sub c} became zero (at 77 K) even though the major phase of the sample was still a superconductor as determined by magnetic susceptibility measurements. Microstructures and compositions of the samples were investigated by transmission electron microscopy in conjunction with energy-dispersive x-ray spectroscopy. Two types of grain boundaries were observed: {approximately}10% of the grain boundaries contained a second phase; the regions near the remaining grain boundaries were tetragonal. At high partial pressures of CO{sub 2}, the YBCO completely decomposed to BaCO{sub 3}, Y{sub 2}BaCuO{sub 5}, and CuO. 11 refs., 3 figs

Effects of atmosphere and heating rate during processing of a ceramic superconductor

Properties of ceramic superconductors depend strongly on the temperature, heating rate, pressure, and atmosphere used during synthesis and fabrication. We have developed a process for synthesizing orthorhombic YBa{sub 2}Cu{sub 3}O{sub x} (123) superconducting powders by calcining the precursor powders under reduced total oxygen pressure. The resultant 123 powders are mixed with organics, and wires and coils are fabricated by extrusion. The wires and coils are fired at a reduced total pressure in flowing O{sub 2} to reduce the concentrations of CO{sub 2}, CO, and H{sub 2}O and thus prevent decomposition of the 123. Transport critical current density of the superconductor decreases drastically with increasing concentrations of CO{sub 2} in the gas mixture. Transmission electron microscopy of materials sintered in O{sub 2} atmospheres containing various levels of CO{sub 2} clearly shows the extent of grain boundary degradation. 29 refs., 9 figs., 2 tabs