D.S. Phillips

Development of Tl-1223 superconducting tapes

Abstract A superconducting tape based on the TlBa 2 Ca 2 Cu 3 O 9 (Tl-1223) phase sheathed in silver was fabricated using the powder-in-tube method. Optimal conditions to synthesize powder ( T c =110 K ) and anneal composite tapes were examined. Microstructural examination indicates that cracks are easily healed during a 3 h anneal at the relatively low temperature of 775°C. A transport critical current density J c of 6200 A/cm 2 at 75 K was measured on short lengths of the annealed rolled and pressed 0.20 mm thick tape. The J c value drops to about 250 A/cm 2 at 75 K and 1 T due to the presence of weak links; however, a J c of 1000 A/cm 2 at 45 K is maintained constant to above 6 T. Measurements of magnetic hysteresis loops indicate that the Tl-1223 tape exhibits a high degree of flux pinning. Very little texturing of the superconducting core was evident.

Isothermal melt processing of Bi-2212 thick films

Abstract An alternative melt-processing technique has been developed for the fabrication of Bi 2 Sr 2 CaCu 2 O y (Bi-2212) thick films at temperatures over 100°C lower than those employed in conventional Bi-2212 melt processing. Isothermal melt processing combines the melting and solidification steps at the same temperature [T.G. Holesinger et al., Appl. Phys. Lett. 63 (1993) 982]. This technique is possible due to the large drop in the solidus temperature with decreasing oxygen partial pressure. High-quality thick films were processed at temperatures as low as 770°C. Such films were found to be well-textured, contain relatively few secondary phases, and possess critical current densities over 1 × 10 5 A/cm 2 at 4 K in self-field.

Isothermal melt processing of Bi2Sr2CaCu2Oy round wire

Abstract Isothermal melt processing (IMP) has been used to process oxide-powder-in-tube (OPIT) Bi 2 Sr 2 CaCu 2 O y (Bi-2212) round wire with high critical current densities. Isothermal melt processing is a process in which the melting and solidification steps are carried out at the same temperature by melting in an inert atmosphere and solidifying in a high oxygen partial pressure. For each particular oxidizing atmosphere used during IMP, the highest critical currents were found in wires processed in the temperature range between 790°C to 820°C. The highest critical current densities obtained in 1.0, 0.75, and 0.5 mm diameter wires at 4 K and self-field were 4.3 × 10 4 , 6.0 × 10 4 , and 6.0 × 10 4 A/cm 2 , respectively. Transport measurements on a 0.75 mm wire revealed that critical current densities in excess of 3 × 10 4 A/cm 2 could be achieved at 4 K in applied magnetic fields of 9 T. Anisotropy measurements and SEM analysis were used to examine the radial alignment of the Bi-2212 grain colonies within the wires.

Atomic redistribution and mass transport in the formation of Bi1.80Pb0.43Sr1.71Ca2.14Cu3O10+x

Abstract A sample of nominal Bi 1.80 Pb 0.43 Sr 1.71 Ca 2.14 Cu 3 O 10+ x (Bi-2223) composition consisting primarily of fine grains of Bi 2 Sr 2 CaCu 2 O 8+ x (Bi-2212) and Ca 2 PbO 4 was synthesized under pure oxygen by a solid-state reaction. This assemblage was then converted to large grains of the 110 K superconducting Bi-2223 phase by annealing at 860°C in air. Utilizing scanning electron microscopy and wavelength dispersive X-ray spectroscopy we show that a redistribution of Ca and Pb occurs throughout the matrix during the formation of Bi-2223. These results indicate that the driving for Pb doped Bi-2223 formation may be due to the large change in entropy caused by melting of one or more of the precursor phases and mixing of the liquid constituents. Melting may be driven by the dependence of the conversion Pb 4+ ↔Pb 2+ on the oxygen partial pressure, with Pb 4+ in Ca 2 PbO 4 and Pb 2+ in the Bi-2223 phase.

A two-powder process for Bi-2223 precursors

A two-powder process is described for the production of uniform, fine-grained Bi{sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub {ital y}} (Bi-2223) powders. One powder is the Bi{sub 2.1{minus}{ital x}}Pb{sub {ital x}}Sr{sub 1.9{minus}{ital y}}Ca{sub {ital y}}O{sub {ital z}} (2:2 Cu-free) phase. The other is a multi-phase powder of approximate overall composition SrCaCu{sub 3}O{sub {ital y}}. The 2:2 Cu-free is one of the first Bi-containing phases to form from a nominal Bi-2223 mixture of oxides and carbonates. This precursor route was chosen for investigation because (1) powders have very similar particle morphologies and (2) the mixing volumes are closely matched. Both of these characteristics facilitate the milling and blending process. This precursor mix was found to be stable in that explosive grain growth of dundesirable phases was not observed during sintering. Critical current densities up to 26,900 A/cm{sup 2} in self field at 75 K were obtained in tapes. {copyright} {ital 1996 Materials Research Society.}

Synthesis of Ag-clad (Bi,Pb)-2223 tapes by an alternative precursor route

Abstract An alternative two-powder process for the synthesis of superconducting (Bi,Pb)-2223/Ag clad wires by the powder-in-the-tube (OPIT) process is described. The first powder, of nominal stoichiometry Bi 1.8 Pb 0.4 Sr 2 CaCu 2 O 8 , is a mixture of (Bi,Pb)-2212 and Ca 2 PbO 4 . The second powder, of nominal stoichiometry CaCuO 2 , is a solution-synthesized mixture of Ca 0.45 Cu 0.55 O 2 and CaO. Using these oxide precursor mixtures, we obtain tapes with well-aligned grains, and reproducible critical current densities J c in the range of 20 000–26 000 A/cm 2 at 75 K in self-field after anneals at 815°C in 10% O 2 in less than 200 h. The decrease in annealing time, relative to other two-powder processes, may be due to the increase in reactivity and possible reaction paths of the Ca 0.45 Cu 0.55 O 2 precursor. Results on tapes prepared using other processing conditions are also reported.

Processing factors for high critical current density in Bi-2223/Ag tapes

Abstract Ten factors relating to Bi-2223/Ag tape production and thermomechanical processing have been examined in a statistically designed study to determine optimal parameter values for high critical current density J c in rolled tapes. The study indicates that low fill factor, narrow tapes processed for long times yield the best J c values, and that strain rate, as was expected, is not a significant factor. The major microstructural conclusion is that macrocracking, even at the early stages of tape processing, should be avoided, as these cracks cannot easily be healed.

CHARACTERIZATION OF SUPERCONDUCTING (TL,BI)SR2CACU2OY

Abstract The superconducting phase with mixed Tl and Bi, namely (Tl0.55Bi0.45)Sr2CaCu2Oy, was synthesized and characterized. Synthesis conditions were studied, and this phase, (Tl,Bi)-1212, was found to form preferentially to (Tl,Bi)-1223 when Sr was present with no Ba present. X-ray powder diffraction with Rietveld refinement provided structural data. The phase is tetragonal with a = 3.7988(4) A and c = 12.076(1) A , with space group symmetry P4/mmm, and with a platelet crystal morphology. Magnetic-susceptibility data gave an onset of superconductivity Tc at 94.9 ± 1.3 K with a Tc mid of 86.0 ± 1.1 K. Magnetic-hysteresis measurements showed that substantial flux pinning occurs in this at phase high temperatures.

Transport and magnetization critical densities in TlBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tapes

The powder-in-tube process was used to produce silver-sheathed tapes of TlBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub 8+x/ (Tl-1223). The powder was produced by thalliating a precursor powder mixture to produce the Tl-1223 phase and then heating to drive off excess Tl and reach the Tl-2223 stoichiometry. The tapes were rolled and pressed, each step followed by a 3-h sintering. The 200- mu m-thick tapes show little sign of texturing; however, the critical current shows a small ( approximately 50%) dependence on the direction of the applied magnetic field. Both transport and magnetization measurements indicate relatively strong pinning at high temperatures. The 75 K self-field critical current density is 62 MA/m/sup 2/. Transport measurements reveal the presence of weak links at all temperatures, but with a relatively weak field dependence above approximately=0.1 T.<<ETX>>

Bi-2223/Ag-sheathed tape processing studies

The production of high critical current density Jc Bi-2223/Ag sheathed conductors is a complex process involving interactions among many different parameters. The effects of three factors: 1) powder production path, 2) the first sinter temperature, and 3) the subsequent sinter temperatures were investigated. Statistical methods were used to design the experiment and interpret the results. Transport Jc was the main response for the analysis, but microstructural results were also used to assess the physical basis for the differences in performance. The powder variable had the largest main effect with only very weak main effects for the other factors.