Second-Generation HTS Conductors
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2005 PREFACE
The discovery of high temperature superconductors (HTS) in 1986 by two IBM scientists led to an unprecedented explosion of research and development efforts world-wide because of the significant potential for practical applications offered by these materials. However, the early euphoria created by the exciting prospects was dampened by the daunting task of fabricating these materials into useful forms with acceptable superconducting properties. Progress towards this goal has been hindered by many intrinsic materials problems, such as weak-links, flux-creep, and poor mechanical properties.
The earliest studies of critical current density Jc in HTS materials revealed that for a polycrystalline material containing a distribution of grain boundaries is much lower than that for a single crystal. High angle grain boundaries act as Josephson coupled weak-links leading to a significant field-dependent suppression of the supercurrent across the boundary. For clean stoichiometric boundaries, the grain boundary critical current density depends primarily on the grain boundary misorientation. The dependence of Jc(gb) on misorientation angle has been determined YBa2O3Cu7-δ(YBCO) in boundary types which can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case high angle boundaries were found to be weak-linked. These experiments have also been extended to artificially fabricated [001] tilt bicrystals in Tl2Ba2CaCu2O8, Tl2Ba2Ca2Cu3Ox, TlBa2Ca2Cu3Ox (Tl-1223) and Nd1.85Ce0.15CuO4. In each case it was found that, as in YBCO, Jc depends strongly on grain boundary misorientation angle. Data on current transmission across artificially fabricated grain boundaries in Bi-2212 also indicate that most large angle [001] tilt and [001] twist boundaries are weak links. It is likely that the variation in Jc with grain boundary misorientation is similar in all high-Tc superconductors. Hence, the low Jc observed in randomly oriented polycrystalline HTS can be understood on the basis that the population of low angle boundaries is small and that frequent high angle boundaries impede long-range current flow. Using conventional processing techniques, three HTS materials were successfully fabricated in polycrystalline form with modest Jc’s These are the Bi-2223 powder-in-tube conductors, the Tl-1223 spray-pyrolyzed films and the Bi-2212 melt-processed thick films. These three types of conductors comprised the First-Generation HTS conductors or wires. Since bicrystal studies using most HTS compounds show that high angle boundaries are weakly-linked, it was important to determine how the current flows in these materials in order to further increase the properties. In this case one must talk about the grain boundary misorientation distribution (GBMD) and its relation to the measured critical current density. In the last ten years, significant progress has been made to experimentally determine the distribution of misorientation angles in high-Jc superconductors.
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CONTENTS
A. Methods to Produce Biaxially Textured Substrates
1. IBAD Template Films for HTS Coated Conductors
2. Epitaxial Superconductors on Rolling-Assisted-Biaxially-Textured-Substrates (RABiTS)
3. Inclined Substrate Deposition
4. ISD by Thermal Evaporation
B. Methods of YBa2Cu3O7-δ Deposition and Related Issues
5. Pulsed Laser Deposition of YBa2Cu3O7-δ FOR Coated Conductor Applications: Current Status and Cost Issues
6. Methods of HTS Deposition: Thermal Evaporation
7. Sputtering of Y1Ba2Cu3O7-δ
8. Pulsed Electron-Beam Deposition of High Temperature Superconducting Films for Coated Conductor Applications
9. BaF2 Post-Deposition Reaction Process for Thick YBCO Films
10. Issues and Progress Related to the Continuous ex-situ BaF2 Processing of Long-Length YBCO Coated Conductors
11. Solution Deposition of YBa2Cu3O7-δ Coated Conductors
12. Non-Fluorine Based Bulk Solution Techniques to Grow Superconducting YBa2Cu3O7-δ Films
13. Jet Vapor Deposition for Continuous, Low Cost Manufacture of High Temperature Superconducting Tape
14. Processing of Long-Length Tapes of High-Temperature Superconductors by Combustion Chemical Vapor Deposition
15. MOCVD Growth of YBCO Films for Coated Conductor Applications
16. LPE Processing for Coated Conductor
C. Deposition of Other HTS Materials
17. Ex-situ Processing of Tl-Containing Films
18. Epitaxy of Hg-based High-Tc Superconducting Thin Films