C. Thieme

Progress in high temperature superconductor coated conductors and their applications

Second generation (2G) high temperature superconductor (HTS) wires are based on a coated conductor technology. They follow on from a first generation (1G) HTS wire consisting of a composite multifilamentary wire architecture. During the last couple of years, rapid progress has been made in the development of 2G HTS wire, which is now displacing 1G HTS wire for most if not all applications. The engineering critical current density of these wires matches or exceeds that of 1G wire, and the mechanical properties are also superior. Scale-up of manufacturing is proceeding rapidly, with several companies already supplying the order of 10 km annually for test and demonstration. Coils of increasing sophistication are being demonstrated. One especially attractive application, that relies on the specific properties of 2G HTS wire, is fault current limitation. By incorporating a high resistivity stabilizer in the coated conductor, one can achieve high resistance in a quenched state during a fault event and at the same time provide significant heat capacity to limit the temperature rise. A test of a 2.25 MVA single phase system at 7.5 kV employing such wire by the Siemens/AMSC team has demonstrated all the key features required for a cost-effective commercial system. A novel approach to providing fault current limiting functionality in HTS cables has also been introduced.

Low-cost YBCO coated conductor technology

Deformation-textured, non-silver substrates, and solution-based deposition of buffer and superconductor layers offer routes to a low-cost YBCO coated-conductor technology for high-temperature superconducting wire. Several significant steps towards such a technology are reported here: a solution-based Gd2O3 seed buffer layer was deposited by a web-coating technique over a metre-length tape of deformation-textured nickel with excellent texture and uniformity. Also, short full-stack samples with YBCO performance up to 0.8 MA cm-2 at 77 K were prepared at Oak Ridge National Laboratory (ORNL) and American Superconductor (ASC) using a CeO2/YSZ/CeO2 buffer sequence on textured nickel and a trifluoroacetate (TFA) precursor YBCO process; in this case the buffers are deposited by e-beam and magnetron sputtering.

Uniform performance of continuously processed MOD-YBCO-coated conductors using a textured Ni?W substrate

Second-generation coated conductor composite HTS wires have been fabricated using a continuous reel-to-reel process with deformation-textured Ni–W substrates and a metal-organic deposition process for YBa2Cu3O7−x. Earlier results on 1 m long and 1 cm wide wires with 77 K critical current performance greater than 100 A cm−1 width have now been extended to 7.5 m in length and even higher performance, with one wire at 132 and another at 127 A cm−1 width. Performance as a function of wire length is remarkably uniform, with only 2–4% standard deviation when measured on a 50 cm length scale. The length-scale dependence of the deviation is compared with a statistical calculation.

YBCO coated conductors by an MOD/RABiTS/spl trade/ process

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.

The Development of Second Generation HTS Wire at American Superconductor

Development of the second generation (2G) YBCO high temperature superconducting wire has progressed rapidly and its performance is approaching, and in some areas exceeding, that of first generation (1G) HTS wire. American Superconductors approach to the low-cost manufacturing of 2G wire is based on a wide-strip (4 cm) process using a metal organic deposition (MOD) process for the YBCO layer and the RABiTS (rolling assisted biaxially textured substrate) process for the template. In addition, the wide-strip RABiTS/MOD-YBCO process provides the flexibility to engineer practical 2G HTS wires with architectures and properties tailored for specific applications and operating conditions through slitting to custom widths and laminating with custom metallic stabilizers. This paper will review the status of the 2G manufacturing scale up at AMSC and describe the properties and architecture of the 2G wire being developed and tested for various applications including in cables, coils and fault current limiters. Performance of 100 meter class, 4 mm wide wires at 77 K, self-field has reached 100 A (250 A/cm-width) with single-coat YBCO and 140 A (350 A/cm-width) with double-coat YBCO. A 5 cm inner diameter coil fabricated from the latter wire achieved 1.5 T at 64 K, confirming the capability of the wire for coil applications.

Low cost Y-Ba-Cu-O coated conductors

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.

YBCO coated conductors by an MOD/RABiTS™ process

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.

Reversible axial-strain effect in Y–Ba–Cu–O coated conductors*

The critical-current density J/sub c/ of an yttrium-barium-copper-oxide (YBCO) coated conductor deposited on a biaxially-textured Ni-5at.%W substrate was measured at 76.5 K as a function of axial tensile strain /spl epsiv/ and magnetic field B applied parallel to the YBCO (a,b) plane. Reversibility of J/sub c/ with strain was observed up to /spl epsiv//spl sime/0.6% over the entire field range studied (from 0.05 to 16.5 T), which confirms the existence of an intrinsic strain effect in YBCO coated conductors. J/sub c/ vs. /spl epsiv/ depends strongly on magnetic field. The decrease of J/sub c/(/spl epsiv/) grows systematically with magnetic field above 2-3 T, and, unexpectedly, the reverse happens below 2 T as this decrease shrinks with increasing field. The pinning force density F/sub p/=J/sub c//spl times/B scaled with field for all values of strain applied, which shows that F/sub p/ can be written as K(T,/spl epsiv/)b/sup p/(1-b)/sup q/, where p and q are constants, K is a function of temperature and strain, b=B/B/sub c2//sup */ is the reduced magnetic field, and B/sub c2//sup */ is the effective upper critical field at which F/sub p/(B) extrapolates to zero.

Growth and characterization of oxide buffer layers for YBCO coated conductors

Metal oxide films were grown on single crystal oxide substrates and deformation textured metal substrates by a metal organic deposition technique using metal alkoxides as the starting precursor materials. The crystallinity, grain alignment, and morphology of the oxide films depend on the process conditions and the substrate properties. Epitaxial oxide films were grown under a range of oxygen partial pressures and temperatures required for film formation on technologically important metal substrates. YBCO films grown on epitaxial LaAlO/sub 3/ buffer layers on single crystal SrTiO/sub 3/ had J/sub c/s of 2.2 MA/cm/sup 2/ (77 K, self-field) demonstrating the quality of the MOD derived oxide films.

Control of Flux Pinning in MOD YBCO Coated Conductor

Two different types of defect structures have been identified to be responsible for the enhanced pinning in metal organic deposited YBCO films. Rare earth additions result in the formation of nanodots in the YBCO matrix, which form uncorrelated pinning centers, increasing pinning in all magnetic field orientations. 124-type intergrowths, which form as laminar structures parallel to the ab-plane, are responsible for the large current enhancement when the magnetic field is oriented in the ab-plane. TEM studies showed that the intergrowths emanate from cuprous containing secondary phase particles, whose density is partially controlled by the rare earth doping level. Critical process parameters have been identified to control this phase formation, and therefore, control the f 24 intergrowth formation. This work has shown that through process control and proper conductor design, either by adjusting the composition or by multiple coatings of different functional layers, the desired angular dependence can be achieved.