THE SYNTHESIS AND CHARACTERIZATION OF HGBA2CA2CU3O8+DELTA SUPERCONDUCTORS WITH SUBSTITUTION OF HG BY PB

We have successfully fabricated ceramic superconductors of Hg-1223 with substitution of Pb for Hg, i.e. Hg1-xPbxBa2Ca2Cu3Oy, 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 delta = 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 Hg0.66Pb0.34Ba2Ca1.98Cu2.97O8.4. The valence number for the copper constituent is given by ($) over bar 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.