Preparation and characterization of Bi-based single crystals of high Tc superconductors

The discovery of B i S r C a C u O oxide superconductors has attracted considerable attention because their superconducting transition temperatures exceed 100 K. It is reported [1] that there exist two phases, low-Tc phase (2212) and high-Tc phase (2223). Synthesis of (2223) phase is the more difficult. It has been established [2] that addition of lead helps in the formation of highTc superconducting phase but the exact mechanism of the process is not clear. According to some authors [3], a melt is made by means of lead which is favourable for the growth of (2223) phase prior to (2212). On the other hand there are a number of authors who have pointed out that the lead content is greatly reduced with an increase in the time of synthesis [4, 5]. Many studies have shown that the concentration of lead in superconducting systems drops to zero [6]. This indicates the necessity of additional research aimed at specifying the actual role of lead in the formation of (2223) phase. Takano et al. [4] synthesized a B i P b S r C a C u O superconductor containing lead atoms for which they found Tc (0) at 107 K and a relatively large fraction of high-Tc phase. Thus, in crystal growth of the B i P b S r C a C u O system, it is necessary to develop a method to control the composition of the grown crystals precisely. Therefore, the flux method has been accepted as an important synthetic technique for growing single crystals in the oxide superconductors. The method allows the growth of crystals with a wide range of composition. The reason for this development is the need for large and perfect crystals for the application and study of their physical properties. Single crystal growth using various compounds as fluxes have been reported but many problems remain unsolved, such as the extraction of crystals from the flux material, the relatively small size of the crystals, uncontrolled cooling rates, the choice of a suitable flux, etc. Solute preparation is particularly important. Polycrystalline samples were prepared from Bi203, PbO, SrCO3, CaCO3 and CuO powders. The purity of all the starting materials was 99.9%. The powders were mixed to give a nominal starting composition of (Bil.4Pb0.6Sr2Ca2Cu3Oy). The mixtures were calcined at 800 + 5 °C and sintered at 845 + 5 °C in the repeating step of 24 h. Resistivity measurements show that Tc (0) was 106 + 1 K. This information confirmed the formation of high-Tc.