Commercialization of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) superconducting coated conductor composite (CCC) technology requires a cost-effective continuous manufacturing process. High critical current YBCO CCC wires with excellent uniformity over length have been fabricated using an all-continuous process. The conductor architecture consists of a metal organic derived YBCO layer, coated on a deformation-textured NiW alloy substrate buffered with Y/sub 2/O/sub 3//YSZ/CeO/sub 2/. Critical current at 77 K, self-field, of up to 118 A was achieved in 1 cm-wide tapes over 1.25 meter lengths, with a standard deviation of 3% measured on a 5 cm scale. The high uniformity and performance supports the feasibility of commercial long-length CCC wire based on deformation textured metal substrates and solution-based deposition of YBCO.

Commercialization of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) superconducting coated conductor composite (CCC) technology requires a cost-effective continuous manufacturing process. High critical current YBCO CCC wires with excellent uniformity over length have been fabricated using an all-continuous process. The conductor architecture consists of a metal organic derived YBCO layer, coated on a deform...

A series of multifilament composite conductors (tapes), (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223)/Ag, was studied by transmission synchrotron X-ray diffraction, including c-axis texture studies, and by critical current measurements. The diffraction measurements indicate that the tapes typically consist of ∼90% Bi-2223 phase. Rocking curve studies of the Bi-2223 (200) peak reveal full widths at half maximum (FWHMs) in the range 13–19°. A weak correlation between rocking curve FWHMs and critical current densities suggests that the critical current values in these tapes may not be limited by the Bi-2223 c-axis orientation.

Heat treatments of monocore and multifilament Ag-sheathed 2223 tape were carried out using overpressure (OP) processing in a static and a flow OP system between 125 and 180 bar. Mass density measurements and microstructural observations of the oxide core show that OP processing densified the core, achieving up to 95±1% of the theoretical density. Our first runs in the flow OP system yielded Jc as high as 41 kA/cm2 (0 T, 77 K), which is 89% of Jc in a fully processed tape with an optimized conventional heat treatment.

The connectivity and flux pinning components of the critical current density Jc in a high-Jc Ag-clad, 19-filament (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) composite throughout important steps in its thermomechanical treatment were investigated by mass density measurement, microstructural observation and extensive superconducting property characterization. The mechanical integrity of the filaments was greatest after the first heat treatment, because damage produced by intermediate rolling was not completely erased by subsequent heat treatment. The final relative filament mass density was less than 75%, even though Jc more than tripled to reach ~50 kA cm-2 (77 K, 0 T) after final heat treatment. Filament resistance and Jc data showed that the electrical connectivity improved significantly throughout the process, although ultrasonic fracturing, magneto-optical images and density measurements all showed that the mechanical and physical connectivity degraded. Our study suggests that the attainable Jc of Bi-2223 is determined by the electrical connectivity of each filament which is itself an uncertain compromise between minimizing the significant porosity produced by the Bi-2223 formation reaction and the extensive crack network that porosity reduction by intermediate rolling produces.

Using photolithography, links for transport measurement have been placed across individual Ni grain boundaries and within individual Ni grains on several coated conductor samples. The typical Ni grain size is similar to 50 mum, while the YBa2Cu3O7-x grains are submicron in size. It is found that the intragrain J(c)(0 T,77 K) can exceed 5 MA/cm(2), thus showing that present coated conductor J(c) values are not significantly limited by the intragrain J(c). Inter- and intragrain J(c) values ranged from one-half to more than four times full-width measured values, demonstrating that current percolates through the conductor. The misorientation angle dependence of J(c) fits well with previous studies of [001] tilt SrTiO3 bicrystals. (C) 2001 American Institute of Physics.

Solution-based YBCO and buffer layers on deformation–textured Ni or Ni-based alloys have been developed as a low-cost coated conductor technology for high temperature superconducting wire. For short samples, Jc values of up to 2.1 MA/cm2 for 0.4 μm films were measured at 77 K, self-field. Using CeO2 buffered YSZ single crystals, Jc values of up to 5 MA/cm2 for 0.4 μm films and 2.5 MA/cm2 for 1.2 μm films can be obtained by such a solution-based YBCO process. Differences in the Jc performance of different metal substrate samples are possibly related to a-axial grain growth.

The porosity and its effect on critical current density J/sub c/ of Ag-clad (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (Bi-2223) tapes throughout important steps in their thermomechanical treatment was investigated by using mass density measurement, microstructural observation, and superconducting property characterization. The relative mass density of the final filament for 19 filament tape was less than 75%, even though the J/sub c/ is /spl sim/50 kA/cm/sup 2/ (77 K, 0 T). The mass density in monofilaments reached 90%, because retrograde densification during the first heat treatment (HT1) was less or even absent. Well textured Bi-2223 grain growth could result in a significant densification during the first HT in the monofilament composite. Our results hint at considerable variability in this important property of Bi-2223 tape.

Solution-based techniques have been examined as potential low-cost processes for manufacturing YBCO coated conductors. YBCO films prepared from metal trifluoroacetate precursors have achieved performance levels equaling or exceeding that of vapor deposited films with the same thickness on CeO/sub 2//YSZ(sc) substrates. J/sub c/'s of 4.5 MA/cm/sup 2/ and 2 MA/cm/sup 2/ have been achieved in 0.4 /spl mu/m thick YBCO films on CeO/sub 2//YSZ(sc) and CeO/sub 2//YSZ/CeO/sub 2//Ni substrates, respectively. Textured Gd/sub 2/O/sub 3/ buffer layers have been deposited on deformation textured Ni substrates in a reel-to-reel process. The performance of YBCO films deposited on substrates containing the Gd/sub 2/O/sub 3/ seed layers is comparable in performance to YBCO films grown on all vacuum deposited buffer layers.

Progress in the development of an economically and technically viable YBCO coated conductor technology for HTS wire using deformation-textured buffered substrates and solution-deposited superconductor layers is reported. Biaxially textured Ni substrates have been fabricated using a deformation texturing process. Epitaxial oxide buffer layers have been deposited by a combination of e-beam deposition and magnetron sputtering. Epitaxial YBCO films have been grown using a trifluoroacetate solution precursor. The resulting composite conductors have critical current densities approaching 2 MA/cm2 at 77K in self-field. Critical current densities of 4.5 MA/cm2 have been also obtained on CeO2 buffered YSZ single crystal substrates, demonstrating performance parity with vacuum deposited YBCO films.

A detailed compositional analysis of high-critical-current-density ( J _c) (55 and 65 kA/cm^2 at 77 K) (Bi,Pb)_2Sr_2Ca_2Cu_3O_ y (Bi-2223) tapes was undertaken by energy dispersive spectroscopy in the transmission electron microscope. Structural features were coupled with characteristic compositions of the Bi-2223 phase. The average of all compositional measurements of the Bi-2223 phase was determined to be Bi_1.88Pb_0.23Sr_1.96Ca_1.95Cu_2.98O_ y . However, spatial variations in the Bi-2223 composition and differing phase equilibria were found throughout the filament structure. In particular, a considerable range of Bi-2223 compositions can be found within a single tape, and the lead content of the Bi-2223 phase is significantly depressed in the vicinity of lead-rich phases. The depletion of lead in the Bi-2223 phase around the 3221 phases may be a current-limiting microstructure in these tapes.

We have investigated the stability and microstructural transformability of the Bi-2223 phase in a silver-sheathed monofilament composite tape fabricated using fine grained Bi/sub 1.7/Pb/sub 0.3/Sr/sub 1.9/Ca/sub 2.0/Cu/sub 3.0/O/sub y/ (Bi-2223) as the precursor powder. The fully formed Bi-2223 precursor was prepared using established procedures. The purpose of this study was to explore the prospects for growing textured, large-grain-size Bi-2223 from the fine-grained precursor by process parameter perturbations. These perturbations included thermal ramp up variations, programmed heat treatment temperature and oxygen pressure fluctuations, and parameter manipulations during cool-down. Our results show that the types of heat treatments used in conventional oxide-powder-in-tube (OPIT) processing do not facilitate Bi-2223 grain growth when the precursor powder is preconverted Bi-2223. We also observed that the Bi-2223 partially decomposed during conventional thermal ramp-up in 0.075 atm O/sub 2/, but that this decomposition can be inhibited by ramping up in a reduced oxygen pressure. A pathway was found for back-reacting the fine-grained Bi-2223 (to Bi-2212, Bi-2201 and nonsuperconducting secondary phases), then reforming large-grained Bi-2223 in a colony microstructure having some distinct differences from that produced during conventional OPIT processing.

Many large-scale applications of high-temperature superconductors depend crucially on the ability to achieve high critical-current densities J c (of the order of 10 5 A cm −2 ). Existing silver-sheathed (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x (BSCCO) tapes have J c values that come within about 25% of this target 1 , 2 , 3 , 4 , these values being limited by the fact that thesupercurrent flows percolatively around barriers that occur over many length scales 5 . To elucidate the nature of these barriers, we have measured the transport properties of individual filaments extracted from very-high- J c multifilament tapes 6 . We find that J c for individual filaments reaches at least 8× 10 4 A cm −2 —about 50% higher than the average value over the whole cross-section of the wire. Although we injected the current to flow along the crystallographic a–b planes of the material, we found that all filaments possessed local characteristics of c -axis transport, indicating the presence of occasional nanometre- to micrometre-scale barriers at basal-plane-faced grain boundaries. An independent and much larger limiting influence on the critical current comes from unhealed cracks produced by deformation during the processing of the wires. These results provide direct evidence that better processing methods aimed at improving the c -axis alignment and at inhibiting residual cracks should raise the accessible J c values towards those needed for applications.

The rate at which Ag-clad (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes are cooled from their final reaction heat treatment influences both the intergranular connectivity and intragranular flux pinning strength of the polycrystalline filaments. As the cooling rate from 825 °C to 730 °C in 7.5% O 2 was decreased over a range of 5 °C/min to 0.005 °C/min, J c (77 K, 0 T) increased from ∼8 to ∼24 kA/cm 2 , and the irreversibility field increased from, ∼120 to, ∼200 mT. The J c (4.2 K, 0 T) increased in a similar fashion. Cooling slowly also sharpened the critical temperature transition and increased the critical onset temperature from 107 K to 109 K. These improvements in the superconducting properties occurred despite partial decomposition of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x phase into non-superconducting impurity phases during the slow cooling. A microstructural basis for these multiple effects is described.

Significant enhancements in critical current densities in rolled multifilamentary Bi-2223 HTS composite conductors have been achieved using the powder-in-tube (PIT) technique. At 77 K and self field, oxide critical current densities (J/sub c/) of 55 kA/cm/sup 2/, overall or engineering critical current densities (J/sub e/) of 15 kA/cm/sup 2/, and critical currents (I/sub c/) of 125 A have been achieved in different rolled multifilamentary composites. Progress in achieving such high electrical performance is believed to stem in part from an improvement of grain connectivity by reducing weak links. The J/sub c/ dependence on magnetic field (B) and the degree of c-axis texture of these high quality conductors have been investigated at various temperatures. Our results also demonstrate that the critical current retention in magnetic field can be independently controlled from the self field critical current density, suggesting that flux pinning improvements and weak link reductions can be separately engineered into Bi-2223 composites fabricated using manufacturable processes.

By manipulating the cooling rate from the final heat treatment, we have raised the 77 K, self-field critical current density (Jc) of multifilament (Bi,Pb)2Sr2Ca2Cu3Ox (2223) tapes by a factor of 3, and the irreversibility field (H*) by more than 50%. The Jc of samples cooled in 7.5% O2 from their reaction temperature of 825 °C increased from ∼8 to ∼24 kA/cm2 and H*(77 K) increased from ∼120 to ∼200 mT as the cooling rate was decreased from 5 to 0.016 °C/min. The results unambiguously show that the flux pinning properties of 2223 tapes can be improved by simple changes in wire processing.

The performance of high-temperature superconductor (HTS) composite conductors is rapidly advancing. Filament current densities of greater than 32,000 A/cm2 (77 K, self field, 1 μV/cm) have been achieved in multifilamentary composite conductors prepared with scaleable powder in tube techniques. This has allowed the fabrication of composite conductors with overall conductor current densities of 9100 A/cm2. These advances are being applied to the manufacture of composite conductors with lengths in excess of 1 km and filament currents densities of 8900 A/cm2 (77 K, self field, 1 × 10−11 Ω-cm). Recent advances in the development of high Jc composite conductors will be reviewed. The performance and characterization of long length conductors will be described and the integration of these conductors into practical applications will be reported.