Stefanie Kaesche

The Pb solubility of the Bi-based high-Tc superconductors “Bi2Sr2CaCu2O8” and “Bi2Sr2Ca2Cu3O10” as a function of temperature

Abstract The Pb solubility of the 2212 and 2223 phase has been determined as a function of the temperature, and phase relations within the quasiquinary system Bi 2 O 3 -PbO-SrO-CaO-CuO with emphasis on the high-temperature superconducting compounds 2212 and 2223 have been obtained. The Pb solubility exhibits a distinct maximum at about 860°C and 840°C for the 2212 and 2223 phase, respectively. With increasing and decreasing temperature the Pb content of both phase decreases resulting in a charge of the content of the superconducting phases. The Pb content of the 2223 phase has no influence on the T c of the phase.

Phase diagram studies in the system Ag-“Bi2Sr2CaCu2O8”

Abstract The phase relations in the system of Ag and the high-temperature superconducting phase “Bi 2 Sr 2 CaCu 2 O 8 ” (2212) are studied up to a Ag content of 50 mol percent in air. The most significant observation is that the temperature of the complete melting of 2212 decreases significantly from about 895°C to some 865°C with increasing Ag content. Therefore, at constant sinter temperature between 895°C and 865°C the amount of liquid in the sample depends significantly on the Ag concentration.

The increase of pinning in (Bi,Pb)2Sr2Ca2Cu3O10+d bulk ceramics

Considering the phase equilibrium diagram of the Bi2O3-PbO-SrO-CaO-CuO system, ceramics of the 2223 phase with the composition Bi1.8Pb0.4Sr2Ca2Cu3O10+d have been transformed by a simple annealing procedure into multi-phase samples. The transformation results in a four-fold increase of the critical current density at 2 T and 30 K (three-fold at 1 T and 50 K), which is believed to express an enhanced pinning capacity of the material. The nature of the pinning centres produced is not yet clarified.

2√2 ap × 2√2 ap phase in superconducting ceramics

Abstract By means of electron diffraction the 2√2 a p × 2√2 a p phase, well-known in YBa 2 Cu 3 O 7−δ , was observed in two other perovskite-based materials (Y 0.75 Ce 0.25 ) 2 (Sr 0.85 Y 0.15 ) 2 AlCu 2 O 9 and Bi 1.8 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+ x . Highly correlated ordering is observed in the ab -plane, the correlation along the c -direction being weak. The plane group of the superstructure symmetry elements was determined on the basis of observed reflection conditions in diffraction patterns. Our results unambiguously rule out oxygen ordering as a possible origin of the superstructure. Experimental evidence points out that the superstructure is associated with the CuO 2 layers, that are the only structural elements common to the three compounds studied. A model is proposed where the CuO 2 sheet is displacively modulated. Experimental evidence suggests a correlation between adjacent CuO 2 sheets. Comparison of simulated and experimental [001] zone diffraction patterns strongly supports our model.

Phase relations and homogeneity region of the high temperature superconducting phase (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+d

For the nominal composition of Bi2.27−xPbxSr2Ca2Cu3O10+d, the lead content was varied from x < 0.05 to 0.45. The compositions were examined between 800 and 890‡C which is supposed to be the temperatue range over which the so-called 2223 phase (Bi2Sr2Ca2Cu3O10+d) is stable. Only compositions between x < 0.18 to 0.36 could be synthezised in a single phase state. For x <0.36, a lead-containing phase with a stoichiometry of Pb4(Sr,Ca)5CuOd with a small solubiliy of Bi is formed, for x > 0.18 mainly Bi2Sr2CaCu2O8+d and cuprates are the equilibrium phases. The temperature range for the 2223 phase was found to be 800 to 890‡C but the 2223 phase has extremely varying cation ratios over this temperaure range. Former single phase 2223 samples turn to multiphase samples when annealed at slightly higher or lower temperaures. A decrease in the Pb solubility with increasing as well as decreasing temperature with a maximum at about 850‡C was found for the 2223 phase.