B. Maiorov

Strongly enhanced current densities in superconducting coated conductors of YBa2Cu3O7-x + BaZrO3.

There are numerous potential applications for superconducting tapes based on YBa2Cu3O7–x (YBCO) films coated onto metallic substrates1. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields2. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material3. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5–5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.

Angular-dependent vortex pinning mechanisms in YBa2Cu3O7 coated conductors and thin films

We compare the angular-dependent critical current density (Jc) in YBa2Cu3O7 films deposited on MgO templates grown by ion-beam-assisted deposition (IBAD), and on single-crystal substrates. We identify three angular regimes in which pinning is dominated by different types of correlated and uncorrelated defects. Those regimes are present in all cases, but their extension and characteristics are sample dependent, reflecting differences in texture and defect density. The more defective nature of the films on IBAD turns into an advantage as it results in higher Jc, demonstrating that the performance of the films on single crystals is not an upper limit for the IBAD coated conductors.

Overcoming the barrier to 1000A∕cm width superconducting coatings

Remarkable progress has been made in the development of YBa2Cu3O7−δ (YBCO)-based coated conductors, and the problems of continuous processing of commercially viable tape lengths are being rapidly solved by companies around the world. However, the current carried by these tapes is presently limited to about 100A for a 1-cm-wide tape, and this is due to a rapid decrease of critical current density (Jc) as the coating thickness is increased. We have now overcome this problem by separating relatively thin YBCO layers with very thin layers of CeO2. Using this multilayer technology, we have achieved Jc values on metal substrates of up to 4.0MA∕cm2 (75K, self-field) in films as thick as 3.5μm, for an extrapolated current of 1400A∕cm width.

Self-assembled, rare earth tantalate pyrochlore nanoparticles for superior flux pinning in YBa2Cu3O7-δ films

Addition of pyrochlore rare earth tantalate phases, RE3TaO7 (RTO, where RE = rare earth, Er, Gd and Yb) to YBa2Cu3O7?? (YBCO) is shown to vastly improve pinning, without being detrimental to the superconducting transition temperature. The closely lattice matched to RTO phase provides a lower interfacial energy with YBCO than BaZrO3 (BZO) and produces very fine (~5?nm) particles with high linearity in their self-assembly along c. Critical current densities of 0.86, 0.38?MA?cm?2 at 1 and 3?T (for fields) parallel to the c axis were recorded at 77?K in 0.5?1.0??m thick films and a transition temperature of 92?K was observed even in the highest level doped sample (8?mol%).

Systematic enhancement of in-field critical current density with rare-earth ion size variance in superconducting rare-earth barium cuprate films

Enhanced in-field critical current densities (Jc’s) have been obtained in epitaxial superconducting (RE1,RE2)Ba2Cu3O7−x (RE1=rare‐earthion 1, and RE2=rare earth ion 2) films grown on both single crystal and buffered metallic substrates. For a constant average RE ionic radius (equal to that of yttrium), there is a systematic dependence of the in-field Jc on the RE ion size variance, with a small, but nonzero, variance being optimum. Compared to the standard YBa2Cu3O7−x composition, a factor of two improvement in Jc(75.5K) is reproducibly observed at 0.2T(‖c) for the composition Dy1∕3Ho2∕3Ba2Cu3O7−x on both single crystal and buffered-metallic substrates. Angular dependent magnetic field studies and transmission electron microscopy indicate the presence of additional pointlike random defects.

Improved flux pinning in YBa2Cu3O7 with nanorods of the double perovskite Ba2YNbO6

We report significantly enhanced critical current densities (Jc) and flux pinning forces (Fp) in applied magnetic fields for YBa2Cu3O7 (YBCO) films with embedded Ba2YNbO6 (BYNO) nanorods. The films were grown by pulsed laser deposition with a target consisting of YBa2Cu3Oy with five molar per cent additions of BaNbOy and Y2O3. With this composition, deposited films were found to contain a high density of BYNO nanorods that frequently traversed the entire thickness of the film (up to 1? ?m), depending upon the deposition conditions. Enhanced Jc performance occurs primarily for applied field orientations near the c-axis of the YBCO, which is nominally along the growth direction of the BYNO nanorods. The threading nanorod density of one film of the present work was measured by plan-view transmission electron microscopy to be 710?850 nanorods??m ? 2. For approximately 1??m thick films, typical Jc(75.6?K, sf) and values were ~ 4.5?MA?cm ? 2 and 1.3?1.5?MA?cm ? 2, respectively. For a 0.5??m thick film, was achieved, and values of Fp in excess of 30 and 120?GN?m ? 3 were achieved at 75.5?K and 65?K, respectively.

Pseudoisotropic Upper Critical Field in Cobalt-Doped SrFe2As2 Epitaxial Films

We present resistivity measurements of the complete superconducting upper critical field (H{c2}) phase diagram as a function of angle (theta) and temperature (T) for cobalt-doped SrFe2As2 epitaxial films to 0.5 K and 50 T. Although H{c2}(theta) at 10 K is indistinguishable from that derived from a single-band anisotropy model, the apparent anisotropy H{c2}{ perpendicularc}/H{c2};{ parallelc} linearly decreases to 1 at low T, with H{c2}(0)=47 T. The data are well described by a two-band model with small, opposing anisotropies for the bands. This unusual relationship is confirmed by the observation of a local maximum for H{c2};{ parallelc} at low T.

Identification of intrinsic ab-plane pinning in YBa/sub 2/Cu/sub 3/O/sub 7/ thin films and coated conductors

The angular-dependent critical current density J/sub c/ in YBa/sub 2/Cu/sub 3/O/sub 7/ films grown by pulsed laser deposition exhibit a sharp peak for magnetic field orientations near the ab plane, which arises from the combined effects of intrinsic pinning and extended defects parallel to the planes. An analysis of the temperature and field dependence of the height and width of this peak allows us to distinguish both contributions. We find that, in a film on single crystal substrate, the peak at low fields is due primarily to the extended defects, but at high fields it is dominated by intrinsic pinning. We compare these results with those of coated conductors with a larger density of ab-oriented correlated defects. We show a novel effect consisting in an inverse correlation between J/sub c/ and the power law exponent (N) of the I-V curves that only occurs in the intrinsic-pinning dominated regime, and we present an interpretation of its origin.

Progress in Performance Improvement and New Research Areas for Cost Reduction of 2G HTS Wires

Second-generation (2G) HTS wires are now being produced routinely in kilometer lengths using Metal Organic Chemical Vapor Deposition (MOCVD) process with critical currents of 300 A/cm. While this achievement is enabling several prototype devices, in order to reach a substantial commercial market, the cost-performance metrics of 2G HTS wires need to be significantly improved in device operating conditions. Zr-doping has been found to be an effective approach to improve in-field critical current performance of MOCVD-based HTS wires. In this work, we have explored modifications to the Zr-doped precursor compositions to achieve three and two-fold increase in deposition rate in research and production MOCVD systems respectively. Production wires made with modified Zr-doped compositions exhibit a self-field critical current density of 50 MA/cm2 at 4.2 K and a 55 to 65% higher performance than our previous wires with Zr-doping, over magnetic field range of 0 to 30 T. We have also developed an alternate, low-cost technique, namely electrodeposition, to deposit silver overlayer on superconducting film. Wires made with electrodeposited silver are able to sustain the same level of overcurrent as sputtered silver layers. This process has been successfully scaled up to 100 m lengths.

Correlated enhancement of Hc2 and Jc in carbon nanotube doped MgB2

The use of MgB2 in superconducting applications still awaits the development of a MgB2-based material where current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, Jc, is in the range 2.5–10 at.% depending on field and temperature. Record values of the upper critical field, Hc2(4 K) = 41.9 T (with extrapolated Hc2(0)≈44.4 T), are reached in a bulk sample with 10 at.% DWCNT content. The measured Hc2 versus T dependences for all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich and co-workers.