The synthesis and characterization of HgBa₂Ca₂Cu₃O_8+δ superconductors with substitution of Hg by Pb

We have successfully fabricated ceramic superconductors of Hg-1223 with substitution of Pb for Hg, i.e. Hg_1-xPb_xBa₂Ca₂Cu₃O_y, by using encapsulation and short time annealing techniques. Samples fabricated in this manner may have nearly single phase structure and some enhancement of T_c can be achieved through this method. The materials of this ceramic with x ranging from 0 to 0.5 exhibit high-temperature superconductivity at temperatures falling in the range from 103 to 135 K. The sample with optimum doping of Pb of x = 0.34 and ° = 0.40 has a highest T_c of 135 K, and its diamagnetic onset temperature T_c^dia = 143 K. The composition of this sample has been found experimentally and is specified by a formula of Hg_0.66Pb_0.34Ba₂Ca_1.98O_8.4. The valence number for the copper constituent is given by V_Cu = 2.30. X-ray diffraction (XRD), transmission electron microscope (TEM) and electron probe microanalyzer (EPMA) analyses have been carried out for detailed investigation. Results indicate that the critical temperature for samples which were post-annealed with either oxygen flow or in high pressure of oxygen gas was increased, while that for those annealed in argon flow was decreased. Furthermore, the as-prepared samples were slightly in the under-doped state with respect to the vacancy. It was shown that T_c parabolically depends on the valence state of copper. © 1995.