ABSTRACT
Substitutions for Bi, Sr, and Ca were considered in BSSCO system in order to increase the superconducting properties and to increase the stability and formation of the superconducting phases [437,438]. High-Te oxides have a layered perovskite structure composed of 3+ (electric charge) ion, 2+ ion, and Cu oxides. The radii of 3+ and 2+ ions are much larger than that of the Cu ion. In the BSCCO system, the 3+ ions with a large ion radius that include Bi, Sb, In, and Tl can form a pervoskite structure. The 2+ ions that may be involved in a perovskite structure with Cu include alkaline earths and other 2+ ions such as Pb and Cd. In order to adjust the distance between Cu-Cu ions in the CuOz plane precisely, the coexistence of the 2+ ions such as Sr and Cll. Ba and Ca, Bi and Pb, and so on will be necessary. The distance between Cu-Cu ions in the CuOz plane plays an important role in Teo
Takano et al. [438] showed that partial substitution of Bi with lead had a drastic effect on the formation and properties of the 2223 phase. In the identification of Pb as a partial substitution for Bi, they were guided by two key observations. The first observation was the existence of many perovskite-type oxides containing Bi3+ and Pb2+ as their"A" ion, such as PbTi03 and BiFe03, that suggested easy substitution of Bi3+ with Pb2+ in the cupric oxide. Second, the substitution could increase the hole concentration in the CuOz sheets and thereby raise the Te. It was found that Pb substitution sharply increased the volume fraction of the 2223 phase in a sample with a composition of Bi:Pb:Sr:Ca:Cu = 1.4:0.6:2:2:3.6. Pbz+ has a larger ionic radius than Be+, and the Pbz+ substitution for Bi3+ contributes to lowering of the internal stress and to the enhancement of the 2223 formation rate. The partial melting range also becomes lower and wider with this substitution. Further work led to two different monophasic compositions: Bl.sPbo.•SrzCaZCu3'ZOZ and BI.92Pbo..aSrzCaZoZCu3.Z0Z'
Thus, partial substitution of Pb for Bi increased the stability of the 2223 phase and the Te was increased up to 120 K for optimized samples. The formation was accelerated by the presence of a molten phase during synthesis and the crystalline quality was improved. The Pb substitution also was found to improve the formation of single-phase 2212 and 1212 material. Figure 83 illustrates how the control of the sintering temperatures and lead contents can favor the formation of a single-phase high-Te material in the BSSCO system [439].
