W. Wischert

New family of superconducting copper oxides: Bi cuprates of type 1212

Abstract Bi cuprates of type 1212 have been synthesized above 970°C from the proper starting materials. The oxides of idealized composition (Bi 0.5 Cu 0.5 )Sr 2 YCu 2 O 7− z crystalline in the tetragonal space group P4/mmm ( a=3.815(5), c=11.73(1) A ) and show solid solution behaviour. The as-prepared materials are non-superconducting above 5 K. However, post-treatment in flowing oxygen at moderate temperature (≈500°C) introduces superconductivity. The highest superconducting transition temperature is situated at 68 K for a sample of nominal composition (Bi 0.5 Cu 0.5 )ue5f8Sr 2 Y 0.8 Cu 2.2 O 6.95 .

Material processing of Ca/Y substituted single (Bi, Pb) O-layered 1212

Abstract The superconducting properties of bulk (Bi, Pb)-1212 material are strongly influenced by the chemical starting composition, the employed temperature/time schedule and gas atmosphere. It is shown that via the application of melt texturing (Bi, Pb)-1212 material with improved intragrain and intergrain properties is obtained. The best materials are formed for a Y/Ca ratio of 0.7/0.4 with Bi:Pb:Sr:Y:Ca:Cu = 0.4:0.45:1.9:0.7:0.4:2.25 after application of 1030°C as melting temperature and a cooling rate of 35°C/h to 985°C/12 h. The transition temperature is situated at 94 K with a critical current density j cm = 7.4 × 10 5 Acm −2 (5 K, 0 T) and 2.8 × 10 3 Acm −2 (77 K, 0 T).

Superconductivity and substitution of Bi-1212

Abstract For superconductors of type Bi-1212 the influence of substitution of (i) Pb for Bi (ii) rare earth elements (Ln) for Y and (iii) Ca for Y was studied. It is shown that a partial Pb→Bi substitution stabilizes the 1212 type and an incommensurate superstructure develops (λ≈4.8). Similar to the Y-123 case a Ln→Y substitution is working without destroying the superconductivity. The highest transition temperature of 90 K is obtained with a combined Ca→Y and Pb→Bi substitution in materials of composition (Bi, Pb, Cu) Sr2 (Y, Ca) Cu2O7-z. Bi-based 1212 powdered materials are characterized by non-superconducting grain boundaries, resulting in a pronounced weak link behavior. This situation can be improved by introducing an intermediate step of partial or total melting in the conditions of preparation.

Superconducting properties of the lead-containing 2212 phase Bi1.75Pb0.25Sr2CaCu2O8 + z with Tc in the range 70–98 K

Abstract The lead-containing, nearly single-phase 2212 material of nominal composition Bi 1.75 Pb 0.25 Sr 2 CaCu 2 O 8 + z has a T c of about 70 K after air quenching. By controlling post-treatment, the transition temperature can be raised to about 98 K. Since no significant changes in the oxygen content are observed, we conclude that the function of post-treatment is to produce a more favorable ordering of oxygen in the lattice. Accordingly, all dimensions are influenced by the post-treatment. Additional results for air-quenched 2212 material with variations in the Bi:Pb:Sr:Ca ratio show that differences in the position of T c are also reflected in variations of the cell dimension, thus pointing to an extended field of existence for the 2212 phase in the system Biue5f8Pbue5f8Srue5f8Caue5f8Cuue5f8O with phase boundaries which are still unknown.

High-resolution transmission electron microscopy of (1212) Bi-cuprate superconductor materials

Abstract Powders as well as ion-thinned compressed powder specimens in the form of pellets are investigated by 300 keV high-resolution transmission electron microscopy and selected area electron diffraction techniques. They reveal, as the majority and superconducting phase, an oxide with the bulk composition Bi0.5Cu0.5Sr2Y0.8Cu2.2O6.9, as previously determined. High-resolution electron micrographs are compared with contrast simulation, based on the dynamical theory of electron diffraction. This comparison reveals the position of the metal atoms within the unit cell. The lattice parameters of the tetragonal unit cell, consistent with previous X-ray measurements, are a = 3.75 ± 0.08 A and c = 11.53 ± 0.2 A , as measured in selected area electron diffraction patterns. The crystal structure deduced from the high-resolution micrographs corroborates the 1212-structure type suggested on the basis of X-ray powder data (Ehmann et al., 1992). The material is virtually free of planar defects.

Superconductivity and processing of single Bi−O layered cuprates in the (Bi, Pb)−Sr−(Ca, Y)−Cu−O system

The effects of improved materials processing on single Bi−O layered cuprates in the (Bi, Pb)−Sr−(Ca, Y)−Cu−O system have been investigatged. For Bi-1212 we have improvedTc to 102 K. The bulk nature of superconductivity is confirmed by the presence of superconducting volume fractions (χ ZFC) around 30–40%. The critical current density is 2×106 Acm−2 at 5 K and 0 T. Moreover, indications for the presence of a second phase probably Bi-1223 with a transition to superconductivity in the range of 115–150 K have been found.

Substitution and magnetic properties of Pb-1212

Abstract A combined study of X-ray and neutron powder diffraction on (Pb 0.75 Cu 0.25 )Sr 2 Y 0.8 Ca 0.2 Cu 2 O z indicates that the rocksalt-like AO monolayers between the perovskite blocks are strongly distorted. By variation of the carrier concentration in the perovskite blocks via a substitution Ca 2+ → Y 3+ in the system (Pb 0.75 Cu 0.25 )Sr 2 Y 1 − x Ca x Cu 2 O z superconductivity is introduced with T c around 40 K. Doping of the AO layers via a substitution Bi 3+ → Pb 4+ in the system (Pb 0.75 − y Bi y Cu 0.25 )Sr 2 Y 0.8 Ca 0.2 Cu 2 O z has a more pronounced effect. Transition from a non-superconducting material with y = 0 to a bulk superconductor with y = 0.3 and T c = 96 K is observed. Post-treatment in flowing Ar shifts T c for y = 0.3 to 100 K. A simultaneous variation of the carrier concentration in the perovskite blocks in the system (Pb 0.75 − y Bi y Cu 0.25 )Sr 2 − Y 1 − x Ca x O z is of subordinate effect.

Melt texturing and thermomechanical processing of (Bi,Pb)-1212 superconductors

Abstract The conditions of melt texturing of bulk (Bi,Pb)-1212 were improved by adaption of (i) the chemical composition of the precursor, (ii) the temperature/time schedule and (iii) the method of post-treatment. It is shown that especially an increase of the Ca and Pb content is suitable and well textured materials are obtained. In melt textured materials the best values are T c, χ =94 K and j c, χ =7.8×10 5 A cm −2 (5 K, 0 T). However, thermomechanical processing has failed to induce a pronounced grain alignment in (Bi,Pb)-1212 PIT tapes and the superconducting properties are still dominated by weak intergrain superconducting couplings.

Crystal structure and superconductivity of Bi cuprates of type 1212 and 0212

Abstract Improved material processing of single BiO layered 1212 raises T c to 102K with a superconducting volume fraction around 30–50% and a critical current density of 2×10 6 Acm −2 (5K, OT). Moreover, strong indications for the existence of Bi-1223 are present. Bi-0212 is realized for the first time in the cuprate family (Bi, Sr) 2 YCu 2 O 6+z , and easily obtained for a partial substitution of Fe → Cu. According to neutron diffraction profile refinement at RT and 10K the crystal structure is tetragonal (space group 14/mmm) with double layers of (Cu,Fe)O pyramides. Additionally, the existence of isotypic Pb-0212 is reported.

Stacking polytypes in the systems Bi-Pb-Sr-Ca-T-O with T = Mn, Fe, Co, Ni, Cu

The influence of chemical substitution on the properties of the stacking polytypes (AO) m M 2 B n−1 T n O y (A, B, M = (Bi, PB), Sr, Ca) with T = Mn, Fe, Co, Ni, Cu is studied for the systems Cu-2212 (Bi 1 6 Pb 0.4 Sr 2−x Ca 1−y Nd x+y Cu 2 O 8+z Bi 2−x Pb x Sr 2 Ca 1−y Gd y Cu 2 O 8+z ); T-2201 (BiPbSr 2 TO 6+z TO 6+z ; T=Fe, Co, Ni) as well as for the series T-2212 and T-3212 with T = Mn, Fe.