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This issue of MRS Bulletin provides an overview of the current status of research and development in the area of high-temperature superconductor (HTS) wires. High-temperature oxide superconductors, discovered in the late 1980s, are moving into the second generation of their development.The first generation relied on bismuth strontium calcium copper oxide, and the second generation is based on yttrium barium copper oxide, which has the potential to be less expensive and to perform better.The potential uses of HTS wires for electric power applications include underground transmission cables, oil-free transformers, superconducting magnetic-energy storage units, fault-current limiters, high-efficiency motors, and compact generators.Wires of 10–100 m in length can now be made, but material and processing issues must be solved before an optimized production scheme can be achieved.This issue covers a range of processing techniques using energetic beams, rolling, and laser and chemical methods to form wires with good superconducting properties.
This article reports on the development of second-generation Y1Ba2Cu3O7–(YBCO)-coated conductors deposited on biaxially textured MgO templates fabricated using ion-beam-assisted deposition (IBAD).The materials system architecture and processing techniques used to achieve high critical supercurrents on flexible superalloy substrates is described.The texturing of YBCO films on metal substrates approaches that of films deposited on single-crystal oxide substrates.Critical currents and critical current densities of YBCO films on metal substrates are also equivalent to YBCO films deposited on single-crystal oxides.
This article provides an overview of the fabrication of epitaxial, biaxially aligned buffer layers on rolling-assisted biaxially textured substrates (RABiTS) as templates for YBCO films carrying high critical current densities.The RABiTS technique uses standard thermomechanical processing to obtain long lengths of flexible, biaxially oriented substrates with smooth surfaces.The strong biaxial texture of the metal is conferred to the superconductor by the deposition of intermediate metal and/or oxide layers that serve both as a chemical and a structural buffer.Epitaxial YBCO films with critical current densities exceeding 3 106A/cm2at 77K in self-field have been grown on RABiTS using a variety of techniques and demonstrate magnetic-field-dependent critical current values that are similar to those of epitaxial films on single-crystal ceramic substrates.The RABiTS architecture most commonly used consists of a CeO2 (sputtered)/YSZ (sputtered)/Y203 (e-beam)/Ni-W alloy.
The desired texture of the base metal has been achieved in 100 m lengths and 10cm widths.Scaleable and cost-effective techniques are also being pursued to deposit the epitaxial multilayers.The results discussed here demonstrate that this technique is a viable route for the fabrication of long lengths of high-critical-current-density wire capable of carrying high currents in magnetic fields and at temperatures accessible by cooling with relatively inexpensive liquid nitrogen (up through the 77K range).
Ion-beam-assisted deposition (IBAD) has been used to prepare biaxially textured templates necessary to realizing high superconducting performance in coated conductors.The IBAD method is characterized by the direct deposition of sharply aligned templates on nontextured metal substrates using fluorite-like oxide films (yttrium-stabilized zirconia, Gd2Zr2O7, etc.).Recent progress in vacuum technology for IBAD has made it possible to reproducibly fabricate long lengths (100 m) of IBAD-GZO templates.Continuous deposition of YBCO films on these templates has been achieved by pulsed laser deposition (PLD).Furthermore, a new approach to improving the texture of IBAD templates was found by using CeO2 films grown by PLD.Trifluoroacetate-based metalorganic deposition has also been used instead of PLD to produce superconducting layers on IBAD buffered tapes, aiming at a low-cost process.This article reviews progress in the research and development of biaxially textured templates produced using IBAD and their subsequent use in fabricating superconducting tapes and wires by means of several processes.
Metalorganic deposition (MOD) is an attractive process for low-cost, high-rate deposition of YBa2Cu3O7– (YBCO) films on continuous lengths of biaxially textured metallic templates for second-generation (2G) high-temperature superconductor (HTS) wires.MOD of YBCO films involves four steps:precursor synthesis, coating, decomposition, and reaction.The final films must meet stringent requirements, including high critical current, uniformity across the width and along the length of the textured substrate, and excellent mechanical properties.Achieving these properties has required the development of a metalorganic precursor that produces an intermediate BaF2-based film, which in turn is converted to a high-quality YBCO film.Understanding and controlling the deposition of the metalorganic precursor and its conversion to YBCO are critical to reproducibly manufacturing uniform, high-performance, HTS wires required for commercial applications.This article reviews the issues that must be addressed in the use of MOD for low-cost YBCO film fabrication and summarizes the performance of 2G HTS wires prepared by this manufacturing process.
Metalorganic chemical vapor deposition (MOCVD) is a well-developed deposition process that shows great promise for scaling up the production of high-temperature superconductors (HTSs) to quickly fabricate useful lengths of superconducting tapes and wires.The primary advantage of MOCVD is its potential for high tape throughput, a key factor in determining the cost of second-generation HTS tapes.This article details progress in long-length HTS tape fabrication, high-throughput processing, and techniques to improve critical current levels in high magnetic fields.
High-temperature superconductors of the second generation—coated conductors—are based on an architecture of YBCO films deposited on a well-textured substrate tape. The deposition technique used in the processing of YBCO films is responsible not only for both the resulting critical currents in the conductors and the cost efficiency of the employed production route, but also for the ultimate viability of the chosen technology. This article describes an advanced deposition method for YBCO films using high-rate pulsed laser deposition (HR-PLD).An elaborate variable azimuth ablation allows target roughening to be considerably reduced in the course of continuous deposition, and as a result, the integral deposition speed and speed stability can be increased to technologically interesting high values.Well-selected process parameters have been demonstrated to yield high currents of up to 480A/cm-width in short tapes and 360A/cm-width in 6-m-long tapes.Together with quasi-equilibrium heating, the HR-PLD method allows the processing of long-length YBCO-coated conductors and offers a cost-efficient route for their production on an industrial scale.