Superconducting Properties of (Bi,Pb)-Sr-Ca-Cu-O Ceramics

Abstract

              Sixteen specimens of four initial nominal composition ceramics were synthesized by solid state reaction for a given initial cation ratio (Bi:Pb)/Sr = (1.5:0.5)/2.0, and the varied cation ratio Ca/Cu = 2.5/4.0 , 2.5/4.5 , 3.0/4.0 and 3.5/4.5.
             SEM images of each specimen showed different grain sizes with porous structure. The grain size increased with sintering time. The specimen synthesized by reground-repressed process and sintered for 50 h , possessed smallest grain size. Three different grain structures were observed. The platelike dark gray grain , flake-pellet bright grain and sphere-like bright grain corresponded to high TC phase, low TC phase and Ca₂PbO₄, respectively. The platelike grain changed to granular after the second grinding.
              XRD spectra showed high TC phase coexisting with low TC phase and Ca₂PbO₄ in each specimen. The ratio of lattice parameter of high T_C and low T_C phases , c/a , were 6.84-6.88 and 5.70-6.02, respectively. Magnetic-susceptibility measurement showed that T_C was significantly affected by the composition changed. Each specimen revealed nearly the same onset critical temperature, T_C onset , at 107.0-108.5 K. T_C and H_C2 increased when the sintering time was extended from 50 h to 100 h. An intermediate ground-pressed process enhanced T_C but depressed H_C2 at 90 K. The optimum atomic ratio leading to the formation of the Bi-2223 phase was Ca/Cu = 0.75. The highest T_C=106.0 K belonged to the specimen which was sintered for 100 h by an intermediate ground-pressed process, the critical field at 90 K was 427 A/m. The highest H_C2= 539 A/m at 90 K belonged to the specimen which was sintered for 100 h by reground-repressed process,and T_C=105.2 K.