Currents In YBa2Cu3O7 Research Articles

Growth of all-chemical high critical current YBa2Cu3O7−δ thick films and coated conductors

The achievement of high critical currents in ‘all-chemical’ YBa2Cu3O7−δ thick films from low cost and versatile chemical solution deposition (CSD) methodology is still an open issue. Here we report a study of the nucleation and growth conditions to achieve YBa2Cu3O7−δ films in excess of 1 micron using single pass inkjet printing or multideposition of low-fluorine metalorganic precursors. Growth conditions of thick YBa2Cu3O7−δ layers are first investigated on LaAlO3, where there is no interfacial chemical reactivity. The second architecture investigated is an ‘all-chemical’ CSDCe0.9Zr0.1O2(CZO)/YSZ multilayer on single crystal substrates which serves as model system for coated conductors. Finally, the ‘all-chemical’ coated conductor architecture CSDYBa2Cu3O7/CSDCZO/ABADYSZ/stainless steel, where ABAD stands for alternating beam assisted deposition, is investigated. The nucleation conditions of YBa2Cu3O7−δ films on top of CΖΟ cap layers have been selected to minimize the formation of the BaCeO3 phase at the interface. We demonstrate that by combining the use of Ag additives in the starting YBCO solution and processing conditions leading to low supersaturation (high water pressure and low temperature) we can achieve ∼1 μm thick YBa2Cu3O7−δ films and coated conductors with high critical currents of = 390 and 100 A/cm-w, respectively, at 77 K and self-field. The achieved control of the interfacial reactivity with CeO2 cap layers opens a route for further increasing film thickness and critical currents in ‘all-chemical’ YBa2Cu3O7−δ coated conductors.

Maximum allowable currents in YBa2Cu3O7 superconducting tapes as a function of the coating thickness, external magnetic field induction, and cooling conditions

Maximum allowable (ultimate) currents stably passing through an YBa2Cu3O7 superconducting current-carrying element are determined as a function of a silver (or copper) coating thickness, external magnetic field induction, and cooling conditions. It is found that if a magnetic system based on yttrium ceramics is cooled by a cryogenic coolant, currents causing instabilities (instability onset currents) are almost independent of the coating thickness. If, however, liquid helium is used as a cooling agent, the ultimate current monotonically grows with the thickness of the stabilizing copper coating. It is shown that depending on cooling conditions, the stable values of the current and electric field strength preceding the occurrence of instability may be both higher and lower than the a priori chosen critical parameters of the superconductor. These features should be taken into account in selecting the stable value of the operating current of YBa2Cu3O7 superconducting windings.

Dependence of the critical current ofYBa2Cu3O7−δ coated conductors on in-plane bending

A new method to measure the effect of in-plane bending on the critical current ofYBa2Cu3O7−δ coated conductors is presented. Such a bending mode can be important in transmissioncables, saddleback magnets, and double-pancake windings. A linear strain distribution overthe width of the conductor develops in this bending mode, where one half of the conductoris under axial compressive strain and the other half is under axial tensile strain. Areversible reduction in critical current of up to 5% is measured in 4 mm wide conductors ata critical bending radius of 0.25–0.28 m. The critical current degrades irreversibly forbending radii less than this because the strain at the edge of the conductor that is undertension irreversibly damages the conductor. The results are described by use ofa model that calculates the critical current as a function of in-plane bendingradius by taking into account the strain gradient over the width of the sampleand the measured dependence of the critical current on axial strain. A similarapproach can be used to calculate the degradation of the critical current n ofother deformation modes, such as torsion, or other more complex geometries.

Enhanced flux pinning and critical currents in YBa2Cu3O7−δ films by nanoparticle surface decoration: Extension to coated conductor templates

Interfacial engineering via nanoparticle substrate surface decoration has been extended to coated conductor templates. Preformed BaTiO3 and BaZrO3 nanoparticles were applied to substrate surfaces, prior to YBa2Cu3O7−δ (YBCO) deposition, by using a scalable and inexpensive technique of solution-based suspension. Compared to untreated reference samples, nanodecoration yields improved in-field critical current density (Jc) as well as strong correlated pinning along the c-axis direction of the YBCO film. Accordingly, a much reduced falloff of Jc with magnetic field strength was observed in all of the modified samples. In addition, scaling behavior of the normalized volume pinning force density (Fp) with respect to temperature provided insight as to the differences in flux pinning mechanisms dependent on the decoration technique. Finally, with these results our earlier proof-of-concept demonstrations on nanoparticle modified single crystal substrates were replicated on technological substrates, pointing to the versatility of this simple technique for realization of enhanced high temperature superconductor performance.

Atomically flat MOD La0.7Sr0.3MnO3 bufferlayers for high critical current YBa2Cu3O7 TFA films

Superconducting YBa2Cu3O7 (YBCO) thin films were grown by the trifluoroacetates route onto metallorganic depositedLa0.7Sr0.3MnO3 (LSMO) buffer layers. The growth conditions of the LSMO films have been studied in detail andfully strained epitaxial layers with atomically flat surfaces have been obtained with local roughness∼1 nm, as determined by atomic force microscopy. We prove that this surface morphology is a keyparameter for achieving YBCO/LSMO multilayers with high critical current densities in self-field,Jcsf (77 K) = 1.8 MA cm−2. Angular pinning studies demonstrate that the defect structure is modifiedwith respect to standard TFA–YBCO films as a pronounced peak inJc is observed for at high temperatures. This indicates the presence of anisotropic defects playing animportant role in vortex pinning even at the irreversibility line stressing the suitability ofthese all-chemical TFA–YBCO/LSMO multilayers as conductive coated conductorarchitectures.

Precursor Evolution and Nucleation Mechanism of YBa2Cu3Ox Films by TFA Metal−Organic Decomposition

We describe the conversion of yttrium, barium, and copper trifluoracetate-derived solid precursors to epitaxial YBa2Cu3Ox superconducting ceramics on (001)-oriented LaAlO3 substrates. Transmission electron microscopy, electron energy loss spectroscopy, energy-dispersive X-ray analysis, and X-ray diffraction are used to characterize the reaction path and nucleation mechanism yielding high critical current YBa2Cu3O7. Our results show that the pyrolysis of the trifluoracetate solutions yields a nanostructured, partially amorphous Ba1-xYxF2+x matrix having a Ba/Y ratio close to 2, with homogeneously dispersed CuO nanoparticles. Upon heating, the chemical trajectory of the fluoride matrix and the overall microstructural evolution of the ceramic precursor prior to YBa2Cu3O7 nucleation is driven by the decomposition and oxidation of this solid solution. The Y solid solubility decreases with temperature yielding Y2O3 which reacts with the CuO particles forming Y2Cu2O5 at about 700 °C. In addition, electron energy loss spectroscopy reveals a high oxygen concentration and almost no Y in the matrix quenched from 795 °C, at a stage where the YBa2Cu3Ox phase still forms disconnected 50−100 nm thick islands spaced by 1−2 μm. The observed evolution from Ba1-xYxF2+x to a barium oxyfluoride mostly occurs prior to the heteroepitaxial nucleation of YBa2Cu3Ox at about 700 °C. Hence, a microstructural scenario is defined which favors competitive nucleation growth between heteroepitaxial YBa2Cu3Ox and bulk Y2Cu2O5. X-ray diffraction pole-figure analysis reveals that the oxyfluoride phase is heavily textured, exhibiting two epitaxial relationships with the (001)-LaAlO3 substrate: (001)OF//(001)LaAlO3, [110]OF//[100]LaAlO3 and (111)OF//(001)LaAlO3, [110]OF//[100]LaAlO3 (OF stands for oxyfluoride). High-resolution observations of the growth front support that (111)-oriented oxyfluoride regions provide low-barrier nucleation sites for c-axis-oriented YBa2Cu3Ox on the buried (001)-LaAlO3 substrate. However, owing to its high mismatch, this orientation only represents roughly 15% percent of the total OF volume. Considering that the nucleation of YBa2Cu3Ox is confined to those regions, this would lead to an anomalously large internuclei spacing, as is indeed observed, favoring the formation of large YBa2Cu3Ox grains and films with a low mosaic spread.

Bend strain tolerance of YBa2Cu3O7−x-coated conductors fabricated by inclined substrate deposition

The effect of tensile bend strain on the critical current ofYBa2Cu3O7−x (YBCO)-coated conductors was evaluated as a function of YBCO film thickness. TheYBCO-coated conductors were fabricated on Hastelloy C276 metal substrates templatedwith MgO by inclined substrate deposition. It was observed that a conductor with0.2 µm thick YBCO retained 90% of the unstrained critical current until a strain of≈0.4%, whereas for aconductor with 0.5 µm thick YBCO film the critical strain value was≈0.2%. At strains higher than the critical strain, the critical current dropped precipitously. Theinverse dependence of bend strain tolerance on the YBCO film thickness is in reasonableagreement with analytical prediction. This inverse dependence was used to approximate thefracture toughness of the YBCO film, which was found to be in general agreement with theliterature.

Fast calcination of purified Trifluoroacetate metal-organic precursors for high critical current YBa2Cu3O7 thin films

A new processing schema is proposed to fasten the metal-organic decomposition step required to grow high critical current YBa2Cu3O7 thin films based on Trifluoroacetates (TFA). AFM and optical microscope study of quench samples at different temperatures have been performed to determine the temperature window where strong stresses are generated. The thickness evolution with the temperature has been observed and linked to the final microstructure. The influence of the gas flow and temperature annealing ramp for two different types of solution has been investigated. We demonstrate that high critical current YBa2Cu3O7 thin films, i.e. Jc = 3MA/cm2, with thickness of 300 nm can be grown with a new fast calcination process lasting only 60 min. This fastened calcination process allows to enhance the coated conductor production rate without the need of any modification of the metal-organic precursor solutions.

Enhancement of flux pinning and critical currents in YBa2Cu3O7−δ films by nanoscale iridium pretreatment of substrate surfaces

We have acquired positive results in a controlled study to investigate the effects of substrate surface modification on the growth-induced flux-pinning nanostructures in YBa2Cu3O7−δ (YBCO) films. Nanoscale iridium (Ir) particles were applied to single-crystal SrTiO3 substrate surfaces using dc-magnetron sputtering. Superconducting properties of YBCO films grown on the Ir-modified substrates, measured by transport and magneto-optical imaging, have shown substantial improvement in the critical current densities (Jc) at 77 K over those on untreated, control substrates. Results also show a nearly uniform enhancement of Jc over all orientations of magnetic field. Present results are found to be consistent with cross-sectional transmission electron microscopy investigations. Ultimately, the objective of this approach is to produce enhancements in the properties of coated conductors by a simple pretreatment of the substrate surface.

Improving the ratio of the grain-boundary and bulk critical currents in YBa2Cu3O7−δ film

Magneto-optical images of flux patterns in a ring of epitaxial YBa2Cu3O7−δ film with an artificial 10° grain boundary are used to determine the ratio of the grain-boundary critical current to the bulk critical current. This ratio which first increases slowly with temperature is found to make a sharp upturn above 77 K showing that the grain-boundary critical current quickly converges to the bulk value at higher temperatures. From the measured ratio and the grain-boundary critical current obtained from magnetization, the temperature dependence of the bulk critical current is recovered.

High critical current YBa2Cu3O7 artificial joints using Ag foils as welding agent

A new high quality joining technique of melt processed YBCO has been developed based on interfacial melting induced by a metallic silver foil inserted between two single domain YBCO tiles. This simple technique allows joining large monoliths to form the complex-shaped pieces required for many power applications. Hall probe magnetic flux profile analysis under magnetic field and high field magnetoresistance measurements demonstrate that the superconducting behaviour of the joint is indistinguishable from the bulk.

Critical currents in YBa2Cu3O7−x high-temperature superconducting thin films irradiated by 4-MeV electrons

A decrease in the critical current density Jc of a YBa2Cu3O7−x thin film is observed after its irradiation by 4-MeV electrons. It is shown that the temperature dependence of Jc agrees with the idea of a granular structure of the film, with intergranular contacts of the superconductor–metal–insulator–superconductor type.

The Relation Between Barrier Structure and Current Uniformity in YBCO Josephson Junctions

Electromagnetic transport measurements were combined with high-resolution electron microscopy observations to study the relation between structure and local critical currents in YBa2Cu3O7−x (YBCO) Josephson junctions. The spatial variation of the critical current J(x) along the length of the boundary for interface engineered Josephson junctions and bicrystal grain boundary Josephson junctions was determined using a phase retrieval algorithm. The current distribution solutions were found to be highly uniform along the length of interface engineered junctions in contrast to solutions for grain boundary junctions. The latter showed significant spatial oscillations in the critical current as well as areas along the boundary that carried no current. Microstructural evaluation of interface engineered junctions fabricated using identical processing parameters to the junctions used for transport measurements suggest that the uniform current distribution is controlled by a highly uniform barrier layer formed between the superconducting electrodes. Microstructural evaluation of grain boundary junctions similar to the junctions used for transport measurements show considerable variations of the grain boundary structure within a single junction.

Biaxially textured YBCO coated tape prepared using dynamic magnetic grain alignment

A new magnetic grain alignment technique has been applied to produce biaxially aligned YBCO coated tapes. A biaxially aligned dispersion of orthorhombic Y2Ba4Cu7O15 (Y-247) powder was settled on untextured silver substrates. The Y-247 tapes were then melt processed to achieve high critical current YBa2Cu3O7 (Y-123) tapes with CuO as a secondary phase. The biaxial alignment is preserved after the densification process and a clear enhancement of Jc relative to identically prepared untextured or uniaxially textured samples is obtained. Critical current densities of up to 5000 A cm-2 at 77 K in self-field and 1500 A cm-2 in 0.5 T magnetic field at 65 K were obtained in films from 20 to 40 µm thick. Problems were experienced in achieving fully densified thick films while retaining biaxial texture. The initial grain size distribution was found to have a major influence on the final microstructure. Provided significant improvements in Jc can be obtained this method offers an alternative to coated tape processes based on epitaxial growth which has the advantage that it does not require textured substrates. The biaxial alignment technique described here intrinsically acts on the bulk material rather than at surfaces. This offers the possibility of texturing without thickness limitations.

Origin of high critical currents in YBa2Cu3O7−δ superconducting thin films

Origin of high critical currents in YBa2Cu3O7−δ superconducting thin films

Influence of micro-structure in the low temperature critical currents of YBa2Cu3O7?? thin films

Epitaxial YBa2Cu3O7−δ films nucleate in c-axis oriented single-crystalline islands. The surface of the single-crystalline SrTiO3 substrates exhibit steps of one third of the YBa2Cu3O7−δ c-axis. These steps generate a mismatch in the island boundaries between the CuO2 superconducting blocks. We show that these defect regions are strong candidates for being the pinning centers responsible for the large critical currents observed in Laser Ablated and Sputtered thin films.

High current YBa2Cu3O7−δ thick films on flexible nickel substrates with textured buffer layers

High current YBa2Cu3O7−δ (YBCO) thick films on flexible nickel substrates with textured buffer layers were fabricated. Highly textured yttria-stabilized-zirconia buffer layers were deposited by using ion beam assisted deposition (IBAD). Pulsed laser deposited YBCO films were not only c-axis oriented with respect to the film surface but also strongly in-plane textured. The in-plane mosaic spread of YBCO films was ∼10°. A critical current density of 8×105 A/cm2 was obtained at 75 K and zero field for thin YBCO films. It was also demonstrated that thick YBCO films with a high critical current and excellent magnetic field dependence at liquid nitrogen temperature can be obtained on flexible nickel substrates by using the textured buffer layers. The result indicates that thick film technology in combination with IBAD buffer layers could be a viable method for fabricating YBCO tapes in long lengths.

The role of intergrain junctions in transport critical current irreversibility

The magnetic field dependence of voltage at constant measuring currents of YBa2Cu3O7−δ bulk ceramics and Bi(Pb)SrCaCuO screen printed films were studied at different temperatures so as to obtain information on critical current irreversibility. Two ranges of these curves were found to show sensitivity to magnetic field. They suggest that the two types of superconducting medium, i.e., the Josephson and Abrikosov medium, result in two different voltage generation mechanisms. It is shown that the Josephson medium plays a role in both low and high magnetic field critical current irreversibility.

Morphology and Defect Structure of Sputtered High-Quality In-Situ YBa2Cu3O7−δ Films

AbstractTransmission Electron Microscopy has been used to study the morphology and defect structure of sharp superconducting transition, high (2-6 ×107 A/cm2) critical current YBa2Cu3O7−δ films on MgO substrates. These were oriented such that the unit cell axes of the film aligned with those of the substrate, with some domains obeying a second orientation relationship rotated by 45° in the plane of the film, i.e. film <110> parallel to substrate <100>. The latter is not expected from simple lattice matching considerations. A strong influence of substrate surface topography on film microstructure was noted, leading to a high density of out-of-phase, low-angle tilt, and other boundaries near the substrate-film interface, which decreased with increasing distance from the substrate. Finally, the effects on film microstructure of two variables of specific interest in our sputtering system were investigated: the thickness of the deposited film, and the temperature at which a high oxygen pressure (500 torr) is introduced after deposition is complete. Increases in film thickness resulted in longer, more widely spaced twins, whereas lower oxygenation temperatures resulted in shorter twins.

Flux-creep-limited critical currents in YBa2Cu3O7−δ ceramics

Critical currents of YBa2Cu3O7−δ bulk-sintered samples were measured at and above 77 K in the presence of magnetic fields. In moderate magnetic fields a H−n dependence with n<1 was observed. Transport measurements give n≂0.5, indicating that critical currents are limited by flux creep rather than by field quenching of Josephson junctions.