Timothy J. Haugan

Addition of nanoparticle dispersions to enhance flux pinning of the YBa2Cu3O7-x superconductor

Following the discovery of type-II high-temperature superconductors in 1986 (refs 1, 2), work has proceeded to develop these materials for power applications. One of the problems, however, has been that magnetic flux is not completely expelled, but rather is contained within magnetic fluxons, whose motion prevents larger supercurrents. It is known that the critical current of these materials can be enhanced by incorporating a high density of extended defects to act as pinning centres for the fluxons. YBa2Cu3O7 (YBCO or 123) is the most promising material for such applications at higher temperatures (liquid nitrogen). Pinning is optimized when the size of the defects approaches the superconducting coherence length (∼ 2–4 nm for YBCO at temperatures ≤77 K) and when the areal number density of defects is of the order of (H/2) × 1011 cm-2, where H is the applied magnetic field in tesla. Such a high density has been difficult to achieve by material-processing methods that maintain a nanosize defect, except through irradiation. Here we report a method for achieving a dispersion of ∼8-nm-sized nanoparticles in YBCO with a high number density, which increases the critical current (at 77 K) by a factor of two to three for high magnetic fields.

Flux pinning enhancement in YBa2Cu3O7−x films with BaSnO3 nanoparticles

Nanoparticles of BaSnO3 were incorporated into YBa2Cu3O7?x (YBCO) films on LaAlO3 substrates for magnetic flux pinning enhancements. More than an order of magnitude improvement in the high field magnetization Jc at 6?T at 77?K was observed as compared to regular YBCO films. The irreversibility field (Hirr) was increased to 8.5?T at 77?K and to 13.4?T at 65?K. The in-field transport current measurements confirmed an order of magnitude improvement in high fields. The angular dependence of the Jc data at 1?T showed that is 1.3 times higher than indicating the presence of c-axis correlated defects. Transmission electron microscopy studies revealed the presence of a large density of uniformly distributed ~10?nm sized BaSnO3 precipitates and strain fields around them. A dual sector pulsed laser deposition target is used to produce the films, thus eliminating reactions between BaSnO3 and YBCO during the target preparation stage, but may allow the BaSnO3 to react locally and create defects that act as pinning centres.

Hysteretic loss reduction in striated YBCO

Abstract Magnetization vs. applied field measurements ( M–H loops) were taken on short samples of YBa 2 Cu 3 O 7− δ (YBCO) thin films which were divided into narrow filaments. The YBCO was deposited using pulsed laser deposition onto single-crystal LaAlO 3 substrates, with a range of film thicknesses from 0.25 to 0.33 μm. Using a YAG laser, the thin films were patterned into linear striations by removing strips of the superconductor by laser ablation. The resulting striated filamentary structure serves to reduce the effective width of the YBCO films and hence the hysteresis loss in the superconducting samples. The magnetization measurements were taken over the temperature range of 4.2–77 K in applied fields of 0–17 kOe using a vibrating sample magnetometer. The measured hysteresis losses show a highly linear relationship between superconductor filament width and hysteresis loss as anticipated. However, the laser ablation process did result in the redeposition of YBCO along the edges of individual filaments. Degradation of T c and J c due to the ablation process is discussed.

Island growth of Y 2 BaCuO 5 nanoparticles in (211 ∼1.5 nm /123 ∼10 nm )× N composite multilayer structures to enhance flux pinning of YBa 2 Cu 3 O 7−δ films

A controlled introduction of second-phase Y 2 BaCuO 5 (211) nanoparticles into YBa 2 Cu 3 O 7 - δ (123) thin films was achieved for the first time for the purpose of increasing flux pinning. The island-growth mode of 211 on 123 was utilized to obtain an area particle density >10 1 1 cm - 2 of 211 thick-disk-shaped nanoparticles in individual layers. Composite layered structures of (211 y nanoparticles/123 z )xN were deposited by pulsed laser deposition on LaAlO 3 substrates, with N bilayers = 24 to 100, y thickness = 1 to 2 nm, and z thickness = 6 to 15 nm (assuming continuous layer coverage). With 211 addition, the critical current densities at 77 K were higher at magnetic fields as low as 0.1 T and increased as much as approximately 300% at 1.5 T. The superconducting transition temperature was reduced by approximately 2 to 4 K for 211 volume fraction <20%. Reinitiation of 123 growth after every 211 layer resulted in a smooth and flat surface finish on the films and also greatly reduced surface particulate formation especially in thicker films (∼1 μm).

Flux pinning behavior of incomplete multilayered lattice structures in YBa2Cu3O7−d

Magnetization results of YBa2Cu3O7−d films processed with interlayers of CeO2 inclusions are presented. Unexpected flux pinning results that are different from previous observations with nanoparticulate layered inclusions were observed. Flux pinning was found to be in some cases either slightly improved at either low fields 8T although degraded, sometimes severely, at interim magnetic fields. Most unexpectedly, the pinning performance of the various samples rapidly converges as the temperature is reduced from 77 to 65K, causing all films to have similar Jc(H) behavior at 65K even though dramatically different at 77K.

The effect of strain on grains and grain boundaries in YBa2Cu3O7−δ coated conductors*

The role of grains and grain boundaries in producing reversible strain effects on the transport current critical current density (Jc) of YBa2Cu3O7−δ (YBCO) coated conductors that are produced with metal–organic deposition (MOD) was investigated. The strain (e) dependence of Jc for full-width coated conductors is compared with that for samples in which the current transport was limited to a few or single grain boundaries by cutting narrow tracks with a laser or focused ion beam, as well as with thin films deposited on bicrystalline SrTiO3 substrates by use of pulsed-laser deposition (PLD). Our results show that the dependences of Jc on e for the grains and for the grain boundaries from the two kinds of YBCO samples can be expressed by the same function, however with a greater effective tensile strain at the grain boundaries than in the grains. The really striking result is that the grain boundary strain is 5–10 times higher for grain boundaries of in situ PLD grown bicrystals as compared to the aperiodic, meandered, nonplanar grain boundaries that develop in ex situ grown MOD-YBCO in the coated conductor of this study.

Minute doping with deleterious rare earths in YBa2Cu3O7−δ films for flux pinning enhancements

To enhance the critical current density of YBa2Cu3O7−δ films, flux pinning centers are intentionally added to inhibit flux flow in applied magnetic fields. Here we provide an initial demonstration that the incorporation of very minor additions (⩽1% of Y as opposed to the 10%–40% in standard substitutions) of typically deleterious rare earths into high quality YBa2Cu3O7−δ thin films provides significant improvement of the film’s in-field current density. This is accomplished without reoptimization of the deposition parameters. Instead of site substitution for Y as might be expected, the deleterious rare earths potentially result in the formation of nanoparticulates.

Controlled introduction of flux pinning centers in YBa2Cu3O7−x films during pulsed-laser deposition

To introduce controlled random inclusion of nanometer-sized nonsuperconducting particulates in YBa2Cu3O7−x films for flux pinning enhancement, a special pulsed-laser-ablation YBa2Cu3O7−x target with a Y2BaCuO5 sector was made and the films were deposited on LaAlO3 substrates. Initial results showed that the films consist of 10–20 nm-sized precipitates. In a 0.5μm thick film, a transport critical current density (Jc)>3MA∕cm2 at 77 K in self-field was measured. Magnetization Jc at 77 and 65 K showed significant improvements in these films with fine precipitates as compared to regular YBa2Cu3O7−x films (>10 times increase at 9 T, 65 K).

Control of BaZrO3 nanorod alignment in YBa2Cu3O7−x thin films by microstructural modulation

The alignment of BaZrO3-nanorods (BZO-NRs) in YBa2Cu3O7−x was studied using vicinal substrates to modulate the microstructure. As the vicinal angle was increased to 10°, the angular splay of BZO-NRs increased. Correspondingly, the vortex pinning along the c-axis increased slightly, a possible consequence of enhanced vortex entanglement. Up to 10°, an increasing density of planar defects was observed, while at ∼20° an orthogonal reorientation of the BZO-NRs along the a-b planes occurs. This suggests that the modulated microstructure introduces a competing effect on the BZO-NR formation and alignment, and beyond a critical vicinal angle, c-axis alignment is no longer favorable for BZO-NRs.

On the through-thickness critical current density of an YBa2Cu3O7−x film containing a high density of insulating, vortex-pinning nanoprecipitates

Using sequential ion milling the authors have studied the thickness dependence of the critical current density Jc(H) of a single crystal 1μm thick YBa2Cu3O7−x thin film containing ∼5vol% of insulating Y2BaCuO5 (Y211) nanoparticles in order to better understand how to obtain high critical currents in thick films. Except very near the interface where the defect density was enhanced, Jc(H) in the body of the film was uniform and independent of thickness with a high maximum pinning force of 8.8GN∕m3 at 77K. The authors conclude that the nanoscale Y211 precipitates result in strong, three-dimensional pinning characterized by a pin spacing of ∼30nm, much smaller than the film thickness.