Two-stage sintering of dense superconducting YBa₂Cu₄O₈ ceramics

Dense YBa₂Cu₄O₈ (124) ceramics were prepared by the following two-stage sintering: compressed 124 powders were heated to 1203 K for decomposition to YBa₂Cu₃O_7-x (123) and CuO, and then cooled down to 1073 K and annealed for the re-formation of 124 single phase. The relative densities thus obtained were 93-95%, as high as those prepared under high oxygen pressure. The densification was considered to take place due to the liquid phase. In the sintered 124, the island-like fine grains of CuO were observed within the 123 matrix. As a result, it was considered that the re-formation of 124 proceeded in each grain. The geometrical phase distribution in the grains is discussed on the basis of superconducting properties from resistivity measurements.