SILVER-ACCELERATED PHASE-TRANSFORMATION FROM YBA2CU3O7-X TO YBA2CU4O8

The effects of silver on the phase transformation from YBa2Cu3O7-x (the 123 compound) to YBa2Cu4O8 (124) via either 123 + CuO --> 124 (the synthesis reaction) or 123 --> 124 + Y-Ba-Cu oxides (the decomposition reaction) were investigated, by using SEM, EDX and quantitative XRD analyses and T(c) measurements, under the conditions (temperature: 810-880-degrees-C, oxygen pressure: 1-50 atm) where 124 is the thermodynamically stable phase. It was found that both reactions were greatly accelerated by Ag. In particular, the transformation rates increase tremendously at high oxygen pressures where Ag melts. These results suggest that the liquid Ag is much more effective in accelerating the reactions than the solid. The Ag-accelerated synthesis reaction produces 124 as the sole product phase, while the decomposition reaction, which is slower than the synthesis one, results, in addition to 124, in Y2BaCuO5 and an amorphous Ba-Cu-0 phase. EDX indicated no Ag within the 124 grains resulting from both reactions. The 124-Ag composites thus prepared have a dense structure with Ag filling the voids existing between the neighboring oxide grains and exhibit bulk superconductivity with T(c)s typically at 78-79 K and 75-80 K for those prepared by the synthesis and decomposition reactions, respectively.