Showing papers by "Zhifeng Ren published in 1994"

Superconducting epitaxial (Tl,Bi)Sr1.6Ba0.4Ca2Cu3O9−δ film with high critical current in magnetic field

Abstract: High‐quality 1‐μm‐thick (Tl,Bi)Sr1.6Ba0.4Ca2Cu3O9−δ films were synthesized by a simple two‐step procedure. A prefilm of TlxBiySr1.6Ba0.4Ca2Cu3Oy was first deposited on the (100) surface of LaAlO3 by laser ablation at 300–500 °C. This prefilm was placed on a gold plate sitting between two similarly placed unfired Tl0.95Bi0.22Sr1.6Ba0.4Ca2Cu3O8.76 pellets, wrapped in silver foil, and subsequently heated in air at 840–870 °C for 25–60 min. The resulting glossy film exhibited at 77 K a transport Jc of 2×106 A/cm2 at 0 T and 1.5×106 A/cm2 at 0.5 T with magnetic field parallel to the film. These observed values show the absence of weak links effect. X‐ray diffraction (XRD) reflections reveal the existence of only 1223 phase with nearly perfect c‐axis alignment. The half‐width of rocking angle monitored at (006) reflection is only 0.365°. XRD φ scan also shows no trace of intergranular misalignment along the a and b axes.

Relationship between grain connectivity, alignment, and critical current density in high-temperature superconducting bulk materials

Abstract: Uniaxial hydraulic pressure was successfully employed to partially reorient the c -axes of the highly anisotropic superconducting grains in bulk Tl 0.78 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 9−δ samples. The partial reorientation was confirmed by X-ray diffraction pattern, rocking curves, and SQUID magnetization measurements. Samples made from the superconducting powder with partial alignment in c -axis direction and samples made from reaction mixture (in-situ synthesis method) with random orientation should have a similar grain connectivity since both were made by the same fabrication procedure. Both types of samples were found to exhibit about the same J c value of 15 000–17 000 A/cm 2 at 77 K. By contrast, another pair of samples with the same c -axis alignment as shown by X-ray diffraction, rocking curve, and SEM measurements, but with different grain connectivity due to the different compression and heat treatment procedure used, showed a significant difference in J c . The just rolled tape had a J c of 13 300 A/cm 2 , whereas the rolled and subsequently compressed tape had a J c of 19 500 A/cm 2 at 77 K. Based on the results of these and other related experiments, we have to conclude that grain connectivity, not lattice alignment, is the decisive factor in determining the J c of Tl based 1223 bulk superconducting materials and silver-sheathed superconducting tapes made by the powder-in-tube method.

Superconducting thallium oxide films via an electro-deposited precursor

Abstract: Electro-deposited precursor films of BaCaCuO (BCCO) doped with Ag were prepared on Ag foil. The electrodeposited films were processed using a two-zone anneal, which employs a Tl source zone and a separate sample zone, both within an O 2 ambient. The annealed films showed c -axis oriented 1223 as the major phase. At 77 K and no magnetic field, the critical current density was 4.42×10 4 A/cm 2 using the field criterion of 1 μV/cm. The field dependence for the Tl-1223 film was equally impressive, showing 8.2×10 3 A/cm 2 at 5.5 T (77 K) for the field oriented parallel to the ab plane ( H ‖ ab ) and parallel to the applied current.

Microstructural development and critical currents in long-length (Tl, Bi)-(Sr, Ba)-Ca-Cu-O wires

Abstract: Synthesis of long Tl−Ba−Ca−Cu−O wires has been achieved byin situ synthesis techniques. Substitutions of Bi for Tl and of Sr for Ba were found to greatly enhance formation of the superconducting phase. Detailed microstructural characterization was performed on wires processed under various conditions in order to elucidate the reaction mechanisms. In addition, microstructural studies were carried out to investigate the beneficial effects of uniaxial compression on critical current density. It was found that thein situ reaction follows a “natural” two-powder process with early formation of Tl-1212. This reaction pathway provides a mechanism for sintering and crack healing and protects existing Tl-1223 for regrowth. Intermediate heat treatment allows more complete advantage to be taken of the above processes, while subsequent pressing increases the density and promotes grain alignment with minimum damage. Further heat treatment sinters grains and allows completion of the transformation to Tl-1223. Elements of this process have been incorporated in a sandwich-type geometry using plate-like grains to promote additional grain alignment.