A. Knoll

Massachusetts Institute of Technology, Cambridge, MA, United States

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Publications (9)7.3 Total impact

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    ABSTRACT: A peel test was used to study the adhesion strength of a commercial grade 2G HTS wire which features a characteristic multilayer structure with the rare earth-based MOCVD superconducting film deposited on an IBAD-MgO template. The peel test could be carried out at various peeling angles (from 90° to 180°) and the peel strength of a wire was defined as the steady-state peeling load determined from a load–displacement curve. The test results had good reproducibility and accuracy, making the test a reliable and useful method for studying the adhesion strength of the wire. By characterizing the peeled surfaces the weakest interface in a wire could be identified. The peel strength data of the wire was analyzed together with the performance of the experimental magnet coils fabricated using the wire. The effect of the silver contact layer annealing on the peel strength is discussed.Highlights► A peel test is developed for measuring the adhesion strength of 2G HTS wires. ► The test results have good reproducibility and accuracy. ► The test is used to investigate the wire delamination issue. ► Performance of wet-wound coils is dependent on the peel strength of the wires.
    Physica C Superconductivity 02/2012; 473:41-47. DOI:10.1016/j.physc.2011.11.013 · 1.11 Impact Factor
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    ABSTRACT: REBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-delta</sub> (REBCO) coated conductor, namely second generation high-temperature (2G HTS) wire, holds enormous promise for many real world applications such as electrical power transmission cables, superconducting magnets, energy storage, fault current limiters, motors, generators and medical devices. SuperPower is developing and manufacturing high-performance 2G HTS wire based on two high-throughput processes: IBAD-MgO for texture buffer and MOCVD for the HTS layer. Tremendous progress has been made in 2G HTS wire technology development and manufacturing in the past few years. In this paper, we report our overall achievements in kilometer long wire fabrication reaching a minimum I<sub>c</sub> of 282 A/cm-w over 1,065 m, practical conductor development for AC application and recent achievement in high-field coils and quench detection technique.
    Applied Superconductivity and Electromagnetic Devices, 2009. ASEMD 2009. International Conference on; 10/2009
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    ABSTRACT: SuperPower is focused on scaling up second-generation (2-G) high-temperature superconductor (HTS) technology to pilot-scale manufacturing. The emphasis of this program is to develop R&D solutions for scale-up issues in pilotscale operations to lay the foundation for a framework for large-scale manufacturing. Throughput continues to be increased in all process steps including substrate polishing, buffer and HTS deposition. 2-G HTS conductors have been produced in lengths up to 100 m. Process optimization with valuable information provided by several unique process control and quality-control tools has yielded performances of 6000–7000 A m (77 K, 0 T) in 50–100 m lengths using two HTS fabrication processes: metal organic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD). Major progress has been made towards the development of practical conductor configurations. Modifications to the HTS fabrication process have resulted in enhanced performance in magnetic fields. Industrial slitting and electroplating processes have been successfully adopted to fabricate tapes in width of 4 mm and with copper stabilizer for cable and coil applications. SuperPower�s conductor configuration has yielded excellent mechanical properties and overcurrent carrying capability. Over 60 m of such practical conductors with critical current over 100 A/cm-width have been delivered to Sumitomo Electric Industries, Ltd. for prototype cable construction.
    Physica C Superconductivity 10/2005; 426-431:849-857. DOI:10.1016/j.physc.2005.03.071 · 1.11 Impact Factor
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    ABSTRACT: The paper presents results, experiment and simulation, of quench/recovery study of coated YBCO "composite" test samples, cooled by boiling liquid nitrogen and subjected to an over-current pulse. To operate stably and be protected from damage under adverse operating conditions of a real device, the YBCO tape must be a "composite", loaded with normal metal of a sufficient thickness that significantly increases the thickness of the original YBCO tape.
    IEEE Transactions on Applied Superconductivity 07/2005; DOI:10.1109/TASC.2005.849238 · 1.32 Impact Factor
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    ABSTRACT: YBa<sub>2</sub>Cu<sub>3</sub>O<sub>x</sub> (YBCO) coated conductors have been produced in 100 m lengths in pilot scale facilities established at SuperPower. In addition to scaling up coated conductors to long lengths with high critical current, we have modified the basic conductor for enhanced in-field performance and low ac losses. In addition, our coated conductors have been produced in practical configurations in 4 mm widths and with a surround copper stabilizer. The conductors were slit first to a 4 mm width and then subjected to electroplating to apply the copper stabilizer. Mechanical and electrical properties of the complete conductor have been elaborately tested. 61 m of 4 mm wide, surround copper stabilized conductor was provided to Sumitomo Electric Industries who fabricated a 1 m cable using the conductor. The cable exhibited the lowest ac losses reported with HTS conductors. SuperPower also fabricated a pancake coil with 7.4 m of coated conductor, which generated a maximum magnetic field of 0.28 T at 77 K.
    IEEE Transactions on Applied Superconductivity 07/2005; DOI:10.1109/TASC.2005.847664 · 1.32 Impact Factor
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    ABSTRACT: On the basis of previously reported 10 m long YBCO tape with over 100 A/cm performance, we are working toward scaling up the YBCO coating process to 100 m lengths. We are using a high tape speed of 15 m/h for fabrication of long lengths. To understand the mechanism of high growth rates of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-x</sub> (YBCO) on buffered metal substrates, the relationship between critical current density and YBCO film thickness has been systematically investigated. When YBCO thickness was below one micrometer, the critical current density (J<sub>c</sub>) was over 2 MA/cm<sup>2</sup>. As YBCO thickness was increased, J<sub>c</sub> decreased gradually. However, J<sub>c</sub> still remained 0.91 MA/cm<sup>2</sup> at a YBCO thickness of 4.2 μm. Critical current (I<sub>c</sub>) was 380 A across 1 cm wide tape at 77 K. To scale up the YBCO coating process, the deposition rate was increased to a growth rate of 70 nm/s, corresponding to a tape speed of 15 m/h. The average phi-scan rocking curve of YBCO films was ∼3° on 11° buffered metal tape. The omega scan rocking curve (FWHM) was just 1.5°. J<sub>c</sub> of 2 MA/cm<sup>2</sup> at 77 K and self-field was achieved at a high deposition rate of 70 nm/s and a tape speed of 15 m/h by optimizing deposition parameters. The high rate process has been transferred to long lengths and results will be discussed in this manuscript.
    IEEE Transactions on Applied Superconductivity 07/2005; DOI:10.1109/TASC.2005.848209 · 1.32 Impact Factor
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    ABSTRACT: SuperPower has been scaling up YBa2Cu3Ox-based second-generation superconducting tapes by techniques such as pulsed laser deposition (PLD) using industrial laser and metal organic chemical vapor deposition (MOCVD). Both techniques offer advantage of high deposition rates, which is important for high throughput. Using highly-polished substrates produced in a reel-to-reel polishing facility and buffer layers deposited in a pilot ion beam assisted deposition facility, meter-long second-generation high temperature superconductor tapes have been produced. 100 A class, meter-long coated conductor tapes have been reproducibly demonstrated in this work by both MOCVD and PLD. The best results to date are 148 A over 1.06 m by MOCVD and 135 A over 1.1 m by PLD using industrial laser.
    Physica C Superconductivity 10/2003; DOI:10.1016/S0921-4534(03)00789-5 · 1.11 Impact Factor
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