V. Selvamanickam

Recent Developments in 2G HTS Coil Technology

Recent developments in 2G HTS coil technology are presented highlighting the ability of 2G HTS wire to function under difficult operating conditions without degradation. The challenges of use in various coil constructions and applications are discussed. Several applications where the conductor is subjected to high stress levels include high field insert coils and rotating machinery. While these applications present different challenges, the ability of the conductor to operate under high stress levels has been demonstrated in both direct sample measurement and test coils. The high winding current density that is available with SuperPowers thin 2G HTS wire was utilized in a high field insert coil demonstration generating central fields in excess of 26.8 T . The ability of the wire to be tailored (stabilization, insulation, ac losses) to fit various operating parameters will also be discussed.

Stability and quench protection of coated YBCO "Composite" tape

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.

Joining of high current bulk Y-Ba-Cu-O superconductors

A new fabrication method which makes it possible to enlarge the size of small, but high quality, grain‐aligned superconductors has been developed. This method involves solid state joining of high current density bulk YBa2Cu3Ox samples prepared by melt‐processing. The joined samples exhibit superconductivity above 90 K and are found to be capable of carrying currents in excess of 150 A at 77 K. The interface of the joined samples is found to sustain current densities (Jc) in excess of 6000 A/cm2 at zero field and in excess of 4000 A/cm2 at 1.5 T. Also, the magnetic field dependence of Jc of the joined samples is similar to that of the individual samples. Since this joining method is not limited by the size of the superconductors that can be joined, the favorable transport and magnetic properties render this process suitable for applications requiring high temperature superconductors in bulk forms.

Recent progress in high-temperature superconductors at Intermagnetics General Corporation

Abstract Recent progress achieved at Intermagnetics General Corporation (IGC) is presented on high-temperature superconductors. The transport critical current density ( J c ) has been measured at various applied magnetic fields and temperatures up to liquid nitrogen for Y 1 Ba 2 Cu 3 O x (YBCO)-coated conductors by metal–organic chemical vapor deposition (MOCVD) process. J c >1.4 MA/cm 2 in self-field at 77 K have been achieved in the YBCO samples. In addition, good progress has been achieved in the multifilament BiSrCaCuO powder-in-tube wires and tapes. Development of strengthened sheath designs and improvement in filament uniformity in production lengths up to 300 m is reported. The transport J c of 348,786 A/cm 2 in self-field at 4.2 K is obtained for round multifilament Bi 2 Sr 2 Ca 1 Cu 3 O x (BSCCO-2212) wires.

Quench detection method for 2G HTS wire

2G HTS conductors are increasingly used in various commercial applications and their thermal and electrical stability is an important reliability factor. Detection and prevention of quenches in 2G wire-based cables and solenoids has proven to be a difficult engineering task. This is largely due to a very slow normal zone propagation in coated conductors that leads to formation of localized hotspots while the rest of the conductor remains in the superconducting state. We propose an original method of quench and hotspot detection for 2G wires and coils that is based upon local magnetic sensing and takes advantage of 2G wire planar geometry. We demonstrate our technique experimentally and show that its sensitivity is superior to the known voltage detection scheme. A unique feature of the method is its capability to remotely detect instant degradation of the wire critical current even before a normal zone is developed within the conductor. Various modifications of the method applicable to practical device configurations are discussed.

Ion-beam texturing of uniaxially textured Ni films

The formation of biaxial texture in uniaxially textured Ni thin films via Ar-ion irradiation is reported. The ion-beam irradiation was not simultaneous with deposition. Instead, the ion beam irradiates the uniaxially textured film surface with no impinging deposition flux, which differs from conventional ion-beam-assisted deposition. The uniaxial texture is established via a nonion beam process, with the in-plane texture imposed on the uniaxial film via ion beam bombardment. Within this sequential ion beam texturing method, grain alignment is driven by selective etching and grain overgrowth.

Pre-fabricated nanorods in RE–Ba–Cu–O superconductors

Pre-fabrication of metallic nanorods on biaxially textured templates has been explored in this study to introduce flux pinning centers in RE–Ba–Cu–O (REBCO, RExa0xa0=rare earth) based superconductors. Pt nanorods were deposited by an electron beam assisted deposition method on LaMnO3-capped biaxially textured IBAD-(ion beam assisted deposition) substrates. Well-controlled nanorods with varying diameter (50–120xa0nm), length (up to 1xa0μm), orientation and unit cell size were grown over an area of 120–150xa0μm2. The nanorod-decorated samples were then deposited with Gd–Y–Ba–Cu–O ((Gd, Y)BCO) by metal organic chemical vapor deposition (MOCVD). The Pt nanorods remain in their positions during MOCVD and become embedded in the (Gd, Y)BCO matrix, although they suffer creep-induced shape deformation due to exposure to elevated temperature. Higher unit cell size, longer nanorods, and nanorods oriented at an angle to the substrate normal adversely affect the epitaxy of the (Gd, Y)BCO film due to formation of a-axis grains. The observed current-carrying capacity of the Pt nanorod sample is lower than its corresponding reference sample without any nanorods and processed under identical conditions, but it decreases at a slower rate with increasing magnetic field. Potential routes to improve the performance while retaining the desirable characteristics of controlled nanorod direction and density are discussed.

Single crystalline gallium arsenide photovoltaics on flexible metal substrates

Combining the unsurpassed performance of GaAs based multi-junction technologies with a conventional roll to roll processing standards of thin film industry could lead to paradigm-shifting reduction of the cost of solar electricity and increase of specific efficiencies. But thus far, attempts toward the direct deposition of GaAs and related compounds on metal foils have yielded to poorly performing polycrystalline films and devices Here we report on the fabrication of single crystalline GaAs-based epilayers and solar cells on thin (50 microns) flexible polycrystalline Ni-based metallic substrates. A rapid (1m/hour) roll to roll ion beam assisted deposition is used to deposit oxide based adaptation buffers on the metal substrates followed by a growth of highly textured thin Ge films and the subsequent growth of GaAs epilayers by molecular beam epitaxy. RHEED, X-ray diffraction and transmission electron microscopy analysis confirm the (001) orientation and the single crystalline nature of the GaAs films. The fabricated samples exhibit strong photoluminescence response attesting the optoelectronic quality of the fabricated films and analysis of near band edge excitons confirms minimal (or no) thermoelastic/lattice mismatch strain in GaAs epilayer.

Analysis of Quench Initiation in YBCO Coated Conductors Using Optical Interferometric Techniques

Quench initiation in homogeneous YBa2Cu3O7-delta (YBCO) coated conductors in nitrogen vapor has been analyzed using digital speckle pattern interferometry (DSPI) and measurements of the temporal evolution of the electric field along the conductor. These optical experiments can detect sample surface displacements smaller than 100 nm in the perpendicular direction. In these quench experiments, displacements are caused by dilatation effects associated with heat generation during the transition to the normal state. DSPI detects a multispot origin of quench initiation in these homogeneous materials. This confirms that, in addition to I c distributions, there are other factors as inhomogeneities in thermal stabilizations that strongly determine sample quench behavior. The knowledge of the origin of this behavior is important for the design of different applications. The technique has been improved in order to characterize samples immersed in liquid nitrogen. The first results in 0.025 mm-thick Cu foils immersed in liquid nitrogen are presented. In addition to sample heating, DSPI allows the visualization of liquid nitrogen convention movements.

Development of Tl-1223 conductors for 77 K HTS applications

Tl-1223 conductors have been fabricated by the powder-in-tube (PIT) and surface-coated conductor (thick film) techniques. Current densities (J/sub c/) of 20,000 A/cm/sup 2/ and 12,000 A/cm/sup 2/ have been achieved at 77 K in short monofilament tapes and meter-long 37-filament tapes respectively. A large diameter test pancake coil was successfully fabricated with 17 m long PIT tapes. A new process has been developed to achieve grain alignment in PIT Tl-1223. Critical currents of 18 A and a J/sub c/ of 19000 A/cm/sup 2/ has been achieved at 77 K in surface-coated Tl-1223 conductor fabricated by an industrial process.