Search

The thickness dependence of critical current density (Jc) has been investigated in YBa2Cu3O7-x films with BaZrO3 (BZO) nanorods (YBCO/BZO) aligned along the film normal. With the presence of linear defects in the YBCO/BZO films, the thickness dependence of Jc becomes negligible at self field and weak applied magnetic field (H), suggesting these linear defects playing a key role to the elimination of the thickness dependence of Jc. The effect of temperature (T) and H on Jc thickness dependence have been investigated and comparisons have been made between YBCO and YBCO/BZO samples. Since BZO nanorods alignment is greatly influenced by growth temperature, the quantitative difference in terms of nanorod's density and average length is reflected on Jc angular dependence of H and Jc thickness dependence as thermally assisted flux motion (TAFM) becomes important above a threshold H determined by the BZO nanorod density. With further increasing H, a monotonically increasing Jc vs. thickness trend was observed in YBCO/BZO films, in contrast to an opposite trend when collective pinning is dominant. This result suggests the thickness dependence of Jc is dictated by the microstructure and hence pinning mechanism in YBCO films. [ABSTRACT FROM AUTHOR]

We report on the effects of different multifilamentary geometries on the dynamic current and field distributions in YBa2Cu3O7-δ thin films. Finite-element model simulations are presented that allow a direct comparison of the calculated magnetic flux and current profiles with experimental data obtained by time-resolved magneto-optical imaging. We find that increasing the distance between filaments reduces the magnetic coupling and decreases the ac losses. Screening and transport currents also redistribute more evenly between all filaments. The measured transport current density profiles are in good agreement with the modified critical state model. [ABSTRACT FROM AUTHOR]

We have studied the effects of localized defects in the YBCO coated conductors on the critical current. The artificial defects were introduced into 4, 10 and 12 mm wide tapes as cuts of various lengths made either by laser ablation or mechanical means. Transport measurements were carried out in an external variable magnetic field to obtain the I-V characteristics of the damaged areas. The distribution of the magnetic field in the vicinity of the defects has been mapped as well. The reduction of the critical current by the defects, with and without an external DC magnetic field are discussed and compared with existing theories. A criterion for determining the critical current in the area containing a defect is suggested. [ABSTRACT FROM AUTHOR]

When grown on miscut SrTiO3 substrates, significant microstructural changes are observed in BaZrO3-doped YBa2Cu3O7-x thin films when compared to those on non-vicinal substrates. Scanning Electron Microscopy indicates a surface morphology strongly influenced by the vicinal angle, and an accumulation of BaZrO3 particles is observed near the step edges. Cross-sectional Transmission Electron Microscopy reveals that while the columnar formations of BaZrO3 rods typically seen on non-vicinal substrates are present, a significant increase in planar defects in a 100 vicinal film are observed. The effects observed with increasing miscut angle indicate that the modulated surface provided by the vicinal substrate influences the crystalline quality of the YBCO matrix and BZO columnar formation through the thickness of the film. [ABSTRACT FROM AUTHOR]

The critical current density (Jc) of YBa2Cu3O7-z films doped with varying flux pinning nanoparticle additions was systematically studied, for applied magnetic fields of H = 0-9 T and operation temperatures T = 20-77 K. Films were prepared with pulsed laser deposition by (M/YBCO)N multilayer or (YBCO)1-xMx single-target methods, for different M phases including Y2O3, Y2BaCuO5 (Y211) green-phase, and BaZrO3. Very significant differences of Jc(H//c, 20 - 77 K) were measured for optimized M phase additions, that are difficult to model or predict at present. Multilayer films with Y211 and Y2O3 nanoparticle additions had the highest Jc(20 - 77K) for H < 4 T, and YBCO+BZO-nanorod samples had the strongest Jc(H) for H > 4 T and 65-77 K, however not for T < 50 K. Seemingly unusual Jc(H, T) properties were measured for (BZO/YBCO)N multilayer films when compared to YBCO and other doped films; Jc(H) was almost the same as YBCO at 77 K, however at 30 K Jc(H > 2 T) had the strongest properties increasing 70% compared to YBCO+nanoaddition films and increasing 400% compared to YBCO. [ABSTRACT FROM AUTHOR]

The superconducting properties of YBa2Cu3O7-δ (YBCO or 123) thin films doped with BaZrO3 (BZO) nanopartides by multilayer and single-target methods were studied and compared. Thin films of 123 + BZO were processed by pulsed laser deposition on LaA1O3 and SrTiO3 single crystal substrates. Multilayer (BZO0.6 nm-1.4/12315nm)19 structures were grown by alternating deposition from 123 and BZO targets, and BZO additions of 0–2 Vol% were deposited using (1231-xBZOx) single-targets. The multilayer and single-target methods of BZO addition caused significant differences of superconducting transition temperatures (Tc) measured by AC susceptibility, and critical current densities (Jc) measured by both magnetic and transport methods as a function of temperature (T), applied magnetic field (H) and angle of H field incidence (θ). Single-target films had almost linear decrease of Tc and self-field Jc with BZO vol% addition, and compared to multilayer films had lower Jc (77 K, H < 4 T) however had improved high-field properties for Jc (77 K, H > 4 T). Multilayer films had almost no decrease of Tc and self-field Jc for high BZO additions up to 10 Vol% and very strong peak of Jc(H//ab in — plane) and constant and high Jc(H,0 < θ < 80°). Single-target BZO-2 Vol% films had slightly enhanced Jc (H//c — axis), consistent with results by other authors. [ABSTRACT FROM AUTHOR]

Striation of wide YBCO coated conductor tapes is necessary to reduce the hysteresis losses. However, the processes used to striate the films may adversely affect the overall critical current of the tape not only by simple removal of superconducting material, but by introducing additional defects. We studied the transport properties of individual filaments of a filamentary YBCO tape on Hastelloy substrate striated by laser ablation. The tape consisted of 24 parallel filaments 0.5 mm wide, covered by thin Ag layer. I–V curves on 9 cm long filament sections as well as the changes of the critical current along the filament were measured at 77 K in magnetic fields up to 70 mT. 8 of 24 filaments were experimentally characterized by the mean value of the critical current and the associated n-value. The best filament had a critical current of ∼7 A at 1 microvolt/cm and B = 0. [ABSTRACT FROM AUTHOR]

The distributions of dc transport current in a copper stabilized mono-core YBCO coated conductor sample were compared with that of sinusoidal ac transport current, i(t) = Io · sin(wt), with frequencies up to 700 Hz. The current distribution was determined from the measurement of the magnetic field produced by the transport current using a small active area Hall probe. The ac magnetic field component perpendicular to the surface of the tape was measured to determine its variation across the tape. A triggering Data AcQuisition (DAQ) system, which had a capability of sampling frequency of up to 200 kHz, based on DAQ measuring cards was used. The current distribution at various phases within one ac transport current cycle for different frequency was studied. The change in frequency from ∼0 Hz up to 40 Hz had only a minor effect on the current distribution at any of the studied phases and the current distribution is practically the same as the one in dc regime. [ABSTRACT FROM AUTHOR]

A comparison study is given of a typical superconducting YBa2Cu3O7-d (Y123) film and a Y123 film containing a nonsuperconducting Y2BaCuO5 (Y211) phase nanoparticulate dispersion. The inclusion of the second phase nanoparticulates was for the express purpose of increasing superconducting film's magnetic pinning strength with the resultant improved in-field critical current density. LaAlO3 substrates were used and the Y123 and Y211 nanoparticulates were grown by pulsed laser deposition (PLD). The Y211 nanoparticulate dispersion in the Y123 resulted from multiple consecutive depositions by PLD of the respective targets. The Y123 phase maintained excellent epitaxy with high in-plane orientation with and without the Y211 inclusions. With the Y211 additions, the critical current densities of the films increased significantly in applied magnetic fields as compared to the high quality Y123 film with no Y211 additions. [ABSTRACT FROM AUTHOR]

To evaluate possible flux pinning enhancement in YBa2Cu3O7-x (Y123) films due to partial rare-earth ion substitutions, Ce and Tb doping are studied. Bulk ceramic targets of varying compositions (Y1-xRExBa2Cu3O7-x) were made with several doping levels (x = 0.00 1 to 0.1, RE = Ce or Tb) by using regular solid-state reaction and sintering procedures. These targets were used to deposit Ce and Tb doped YBCO films onto SrTiO3 single crystal substrates by pulsed laser ablation. Doped YBCO films were characterized for Tc, magnetic field dependence of Jc (at 77 K), microstructure, and other properties. The results are compared to undoped YBCO films processed in similar manner. [ABSTRACT FROM AUTHOR]

One of the methods to reduce magnetization loss of YBCO coated conductors in a perpendicular magnetic field is subdividing the YBCO layer into filaments and twisting them as a whole. A 10 mm wide multifilamentary YBCO coated conductor with 200 μm wide filaments was prepared by striation using the laser ablation technique. The number of filaments was 40. The sample length was varied from 100 mm to 25 mm, and their magnetization loss was measured at various frequencies. The measured magnetization losses were scaled using sample length, frequency, and field amplitude. This clarifies the magnetization loss characteristics of multifilamentary YBCO coated conductors and obtains empirical expressions for the magnetization loss. The measured loss was compared with the loss calculated numerically using a two dimensional FEM model. The experimentally confirmed effect of striation to reduce the magnetization loss was compared with theoretical predictions. [ABSTRACT FROM AUTHOR]

Magnetization vs. applied field measurements (M–H loops) were taken on short samples of YBa2Cu3O7−δ (YBCO) thin films which were divided into narrow filaments. The YBCO was deposited using pulsed laser deposition onto single-crystal LaAlO3 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 Tc and Jc due to the ablation process is discussed. [Copyright &y& Elsevier]

YBa2Cu3O7-δ high temperature superconductors can maintain fairly high critical current densities (Jc) with increasing magnetic field. This in-field performance can be further improved upon by incorporating nanoparticulate magnetic flux pinning centers into the superconductors. This short paper briefly discusses and compares recent efforts by the U.S. Air Force Research Laboratory to incorporate insulating nanoparticles into the YBCO superconducting thin films by pulsed laser deposition. [ABSTRACT FROM AUTHOR]

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 <0.5 T or in other cases at high fields >8 T 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 65 K, causing all films to have similar Jc(H) behavior at 65 K even though dramatically different at 77 K. [ABSTRACT FROM AUTHOR]

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 <0.5 T or in other cases at high fields >8 T 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 65 K, causing all films to have similar Jc(H) behavior at 65 K even though dramatically different at 77 K. [ABSTRACT FROM AUTHOR]