Understanding High Critical Currents in YBa2Cu3O7 Thin Films and Coated Conductors

The main challenges for the success of high temperature superconducting wires, the YBa₂Cu₃O₇ (YBCO) coated conductors (CC), are to avoid the the weak-link problem through the production of biaxially textured films, and to increase the critical current density (J _c) through the introduction of large densities of appropriate defects. To that end, it is essential to understand the pinning mechanisms and their correlation with the microstructure of the CC. We first present a brief overview of the main methods currently used to produce YBCO CC, and we describe the architecture of the YBCO on IBAD fabricated at Los Alamos, summarizing the recent improvements of their structural and superconducting properties. Then, we analyze some aspects of the J _c dependence on temperature and magnetic field (orientation and intensity) for the best CC available, and we compare and contrast the results with those of YBCO thin films on single crystal substrates, in order to determine if the defects controlling the pinning mechanisms are the same in both cases. Our results indicate that over large field and angular ranges J _c on CC is higher than J _c in thin films on SCS.     

Processing of Y1Ba2Cu3O x Films by Solution Techniques Using Metal-Organic Decomposition

Processing of YBCO films by decomposition of metal trifluoroacetate precursors and its application to the development of coated conductors is investigated. The technique involves preparation of the solution, deposition, and a two-stage heat treatment. A stoichiometric mixture of the acetates of Y, Ba, and Cu and trifluoroacetic acid are used as the starting materials. The glassy residue of the trifluoroacetates of Y, Ba, and Cu formed by drying the above mixture is redispersed in a fixed amount of methanol to provide the starting solution of a given concentration. This solution is then deposited on the substrate surface by the spin coating technique and heat treated in two stages. X-Ray characterization shows that the transformation to 123 is complete in less than 1 h. Pole figure analysis of films deposited on (100) SrTiO₃ and LaAlO₃ single crystal substrates show that these films have a very high degreee of alignment with the substrate both out-of-plane (<0.5°) and in-plane (<1.5°). These chemically derived films were also found to have critical current densities well above 5 × 10⁵ at 77 K for 0.5 μm thick films. The applicability of this technique to the production of thick films of YBCO for the development of coated conductors will be discussed.     

Magnetic properties and critical currents of epitaxial YBa2Cu3O7-x films

The authors report results of magnetic and transport measurements on thin epitaxial films of YBa₂Cu₃O_7-x which show critical current densities of 10⁷ A/cm² at 4.2 K. They exhibit well-formed symmetrical hysteresis loops and flux-trapping effects and linear susceptibilities at low fields. Magnetic and transport critical currents are in good agreement at low temperatures. The above properties are attributed to strong pinning from point defects which are suggested to be more numerous in films than in bulk single crystals. Diamagnetic shielding effects can be very large and are proportional to the critical current at zero field; however, there is a large penetration of H_a at all field values. Field-cooled magnetization is always very small, being only a few percent of the diamagnetic shielding. This small value is attributed to a balance between trapped flux and expelled flux in the cooling process. The strong pinning in attributed to a high density of defects in the film     

The Effects of Pinning on Critical Currents of Superconducting Films

Using molecular dynamics simulations, we analyze the effects of artificial periodic arrays of pinning sites on the critical current of superconducting thin films as a function of vortex density. We analyze two types of periodic pinning array: hexagonal and Kagomé. For the Kagome pinning network we make calculations using two directions of transport current: along and perpendicular to the main axis of the lattice. Our results show that the hexagonal pinning array presents higher critical currents than the Kagomé and random pinning configuration for all vortex densities. In addition, the Kagomé networks show anisotropy in their transport properties.     

High Critical Current Densities in MgB2 Films Grown on Hastelloy Tape by Hybrid Physical-Chemical Vapor Deposition

We report on the high critical current densities in MgB₂ films directly grown on Hastelloy tapes without any buffer layer by using the hybrid physical-chemical vapor deposition method. MgB₂ films were formed by reaction of Mg metal vapor with the incoming B₂H₆ gas on the heated substrates. In MgB₂ films grown for 10 min at 500 °C in total working pressure 100 Torr with gas mixing ratio H₂:B₂H₆=70:30, we observed the transport critical current density (J _c) was approximately 10⁶ A/cm² at 4 T and 20 K in magnetic fields applied parallel to the substrate plane. This value is higher than those observed in epitaxial MgB₂ films on sapphire substrates grown by using the same method. Magnetic field dependence of J _c of this sample was well explained by the grain-boundary pinning model. Our result opens up a possibility that the coated conductors made of MgB₂ films have a strong potential for high current applications.     

Strain scaling law for flux pinning in practical superconductors. Part 1: Basic relationship and application to Nb3Sn conductors

Critical current and flux pinning densities have been determined for a series of Nb₃Sn, V₃Ga, Nb₃Ge, and NbTi conductors as a function of uniaxial tensile strain in magnetic fields ranging from 4 to 19 T. An empirical relationship has been found at 4.2 K that describes these data over the entire range of field under both compressive and tensile strain. The pinning force F has been found to obey a scaling law of the form $\text{F = [B}{}_{\mathrm{c2}}\text{1(?)]}{}^{\mathrm{n}}\text{f(b)}$, where B_c2¹ is the strain-dependent upper-critical field determined from high-field critical-current measurements and f(b) is a function only of the reduced magnetic field $\text{b  B/B}{}_{\mathrm{c2}}\text{1}$. The detailed shape of f(b) depends on the super-conducting material and reaction conditions, but n was found to be nearly constant for a given type of superconductor. For Nb₃Sn conductors n = 1 ± 0.3, for multifilamentary V₃Gan?1.3, for CVD-Nb₃Ge tape n?1.6, and for multifilamentary NbTi n?3.3. The importance of this relationship is that, for these conductors at least, it is possible to measure F at one strain and then immediately be able to predict F (and thus the critical current) at other strain levels simply by scaling the results by [B_c21(?)]ⁿ. Part I of this paper presents the basic uniaxial-strain scaling relationship and focuses on its application to Nb₃Sn conductors. The strain scaling law with n = 1 ± 0.3 was found to hold for all Nb-Sn based conductors examined thus far, including commercial-multifilamentary conductors, extremely fine-filament composites, partially-reacted specimens, ‘insitu’ conductors, and Nb-Hf/Cu-Sn-Ga conductors. The detailed dependence of B_c21 on strain was-found to be nearly universal for highly-reacted commercial Nb₃Sn specimens, greatly simplifying the application of the scaling law to this group of practical superconductors. These results are discussed within the context of flux pinning models and a general scaling relation is proposed which unifies the usual temperature-scaling relation with this strain-scaling relation.     

Improved Critical Current Densities in YBa2Cu3O7?�� Multilayer Films Interspaced with Palladium Nano-dots

Power applications of superconducting coated conductors in high magnetic fields require thick films with high critical current density J _c and strong artificial pinning centers. Here, we report on the artificial pinning centers induced in YBCO quasi-multilayer films interspaced with palladium (Pd) nano-dots. Quasi-multilayered (QM) YBa₂Cu₃O_7??? (YBCO) films composed of YBCO layers interspaced with quasi-layers of palladium nano-dots were grown by pulsed laser deposition on SrTiO₃(100) substrates. DC magnetization and frequency-dependent measurements showed high J _c comparable with best YBCO films in thin quasi-multilayers and significant improvement of J _c in thick quasi-multilayers. TEM study shows regions of planar defects, stacking faults, and pore formations suitable for immobilizing vortices. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films.     

Processing of Y1Ba2Cu3Ox Films by Solution Techniques Using Metal-Organic Decomposition

Processing of YBCO films by decomposition of metal trifluoroacetate precursors and its application to the development of coated conductors is investigated. The technique involves preparation of the solution, deposition, and a two-stage heat treatment. A stoichiometric mixture of the acetates of Y, Ba, and Cu and trifluoroacetic acid are used as the starting materials. The glassy residue of the trifluoroacetates of Y, Ba, and Cu formed by drying the above mixture is redispersed in a fixed amount of methanol to provide the starting solution of a given concentration. This solution is then deposited on the substrate surface by the spin coating technique and heat treated in two stages. X-Ray characterization shows that the transformation to 123 is complete in less than 1 h. Pole figure analysis of films deposited on (100) SrTiO₃ and LaAlO₃ single crystal substrates show that these films have a very high degreee of alignment with the substrate both out-of-plane (<0.5°) and in-plane (<1.5°). These chemically derived films were also found to have critical current densities well above 5 × 10⁵ at 77 K for 0.5 m thick films. The applicability of this technique to the production of thick films of YBCO for the development of coated conductors will be discussed.     

Progress in Nanoengineered Microstructures for Tunable High‐Current,High‐Temperature Superconducting Wires

High critical current densities (J_c) in thick films of the Y₁Ba₂Cu₃O_7–δ (YBCO, T_c ≈ 92 K) superconductor directly depend upon the types of nanoscale defects and their densities within the films. A major challenge for developing a viable wire technology is to introduce nanoscale defect structures into the YBCO grains of the thick film suitable for flux pinning and the tailoring of the superconducting properties to specific, application‐dependent, temperature and magnetic field conditions. Concurrently, the YBCO film needs to be integrated into a macroscopically defect‐free conductor in which the grain‐to‐grain connectivity maintains levels of inter‐grain J_c that are comparable to the intra‐grain J_c. That is, high critical current (I_c) YBCO coated conductors must contain engineered inhomogeneities on the nanoscale, while being homogeneous on the macroscale. An analysis is presented of the advances in high‐performance YBCO coated‐conductors using chemical solution deposition (CSD) based on metal trifluoroacetates and the subsequent processing to nano‐engineer the microstructure for tuneable superconducting wires. Multi‐scale structural, chemical, and electrical investigations of the CSD film processes, thick film development, key microstructural features, and wire properties are presented. Prospects for further development of much higher I_c wires for large‐scale, commercial application are discussed within the context of these recent advances.     

High Critical Current Density YBa2Cu3O x Tapes Using the RABiTs Approach

Progress in the fabrication of epitaxial, high-J _c, biaxially aligned YBCO thick films on Rolling-assisted biaxially textured substrates (RABiTs) is reported. RABiT substrates comprise a biaxially textured metal substrate with epitaxial oxide buffer layers suitable for growth of superconductors. Oxide buffer layers have been deposited using three techniques: laser ablation, electron-beam evaporation, and sputtering. Epitaxial YBCO films grown using laser ablation on such substrates have critical current densities approaching 3 × 10⁶ A/cm² at 77 K in zero field and have field dependences similar to epitaxial films on single crystal ceramic substrates. Critical current densities in excess of 0.2 MA/cm² have been obtained on stronger, nonmagnetic substrates. In addition, samples with J _e of 12.5 kA/cm² at 77 K have been fabricated. The highest strain tolerence obtained so far is 0.7% in compression and 0.25% in tension. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high-J _c wire capable of carrying high currents in high magnetic fields and at elevated temperatures.     

High Critical Current Density YBa2Cu3Ox Tapes Using the RABiTs Approach

Progress in the fabrication of epitaxial, high-J _c, biaxially aligned YBCO thick films on Rolling-assisted biaxially textured substrates (RABiTs) is reported. RABiT substrates comprise a biaxially textured metal substrate with epitaxial oxide buffer layers suitable for growth of superconductors. Oxide buffer layers have been deposited using three techniques: laser ablation, electron-beam evaporation, and sputtering. Epitaxial YBCO films grown using laser ablation on such substrates have critical current densities approaching 3 × 10⁶ A/cm² at 77 K in zero field and have field dependences similar to epitaxial films on single crystal ceramic substrates. Critical current densities in excess of 0.2 MA/cm² have been obtained on stronger, nonmagnetic substrates. In addition, samples with J _e of 12.5 kA/cm² at 77 K have been fabricated. The highest strain tolerence obtained so far is 0.7% in compression and 0.25% in tension. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high-J _c wire capable of carrying high currents in high magnetic fields and at elevated temperatures.     

Enhanced Flux Pinning Properties of BaZrO3-doped YBCO Films Grown by Pulsed Laser Deposition on LaAlO3(100) Substrates Decorated with Y2O3 Nanoislands

In this work, we intend to investigate the interaction between two types of nanoscaled artificial pinning centers and their pinning properties in YBCO thin films grown by pulsed laser deposition technique. The two types of artificial pinning centers were prepared in different processes, (1) Y₂O₃ nanoislands decorated on substrates prior to the deposition of YBCO thin film, and (2) BaZrO₃ nanoparticles self-assembled within YBCO matrix during the deposition of YBCO thin film. We compared the transport characteristics of the YBCO thin films containing these two types of artificial pinning centers with those of pure YBCO thin films grown on decorated substrates and BZO-doped YBCO thin films grown on undecorated substrates. It was found that these two types of artificial pinning centers, which are simultaneously present, acted constructively to enhance the pinning properties of YBCO thin films.     

A critical current-margin design criterion for high performance magnet stability

A ‘critical-current-margin. (CCM) design criterion for superconducting magnet stability is presented. It is applicable to high-performance magnets that operate at high current densities; these magnets have cooled conductor surfaces, but their calculated heat fluxes are well beyond recovery values. For stability of a magnet based on this CCM design criterion, disturbances must be limited in size but need not be eliminated nor localized.The CCM criterion is compared with the MPZ/Cold-End Theory. It is shown that this criterion offers a much higher confidence level of stability than the MPZ/Cold-End Theory. Furthermore, and unique to this theory, stability is independent of current density in the stabilizer.The CCM criterion is based on two magnet winding elements: heat transfer and composite conductor. In this criterion heat transfer must always remain in the nucleate boiling region throughout the operation of a magnet. The design emphasis is on more superconductor and less stabilizer. It is also demonstrated that the CCM theory works particularly well with conductors such as cabled conductors that have high ratios of cooled perimeter to cross-section.Experimental results are presented supporting this CCM design criterion.     

High Energy Milled Ex Situ MgB2 as Precursor for Superconducting Tapes Without Critical Current Anisotropy

One lane of the present MgB₂ research is focused on scale-up of the conductor preparation for magnet applications. One limitation is the deformation of long length powder in tube conductors since the composite structure leads to a couple of complications. Therefore, a combination of various methods is commonly used: swaging, drawing, and flat rolling. In dense tapes deformed via distinct routes, a critical current anisotropy with respect to an external magnetic field is observed. The in situ method (unreacted Mg+B) is preferably used for conductor preparation, with the advantage offering more doping possibilities for the precursors to create flux pinning centers and to enhance the upper critical field and supporting a dense filament. In this work, we show in concurrence to the commonly preferred route, the possibility and potential of ex situ conductor preparation schemes, with the option of carbon doping, using high energy milling. Long multifilament tapes with 20 hours milled powder without carbon and with 5?wt% C were successfully deformed to wires and tapes. Tapes with 21 cores show critical current densities with J _c=10⁴?A/cm² at B=8.8?T without any current anisotropy in different field-direction different to the case of the in situ conductors.     

Superconducting properties of Nb3Ge thin films prepared by cosputtering

Nb₃Ge films with thicknesses between 0.5 and 1.5 m have been deposited on heated sapphire substrates by dc sputtering in a pure argon atmosphere. With a maximum superconducting onset temperature T _c= 22.7 K, these high-T _csamples crystallize in a single-phase A15 structure with lattice parameters down to 5.14 ». Critical current densities and upper critical magnetic inductions have been measured as a function of magnetic field orientation, temperature, and film thickness. The anisotropies of the critical currents and fields depend on the microstructure of the samples and show maxima for the field orientation normal to the sample surface in those films that exhibit columnar growth of the crystallites normal to the substrate. The temperature, field, and angular dependences of the pinning forces in these films are compared to existing pinning theories.Dedicated to Prof. Dr. B. T. Matthias on the occasion of his 60th birthday.Supported by Deutsche Forschungsgemeinschaft.     

Nb5+掺杂对低氟MOD法制备的YBCO薄膜性能的影响

本研究通过低氟MOD法成功地制备了Nb⁵⁺掺杂的YBa₂Cu₃O_7-x(YBCO)薄膜, Nb⁵⁺在薄膜中生成了大小在20~30 nm之间的纳米颗粒; 纳米颗粒的生成不会对 YBCO薄膜的织构和临界温度(T_c)构成明显的影响。由于纳米颗粒的引入, 掺杂后薄膜的临界电流密度(J_c)在整个磁场范围内都要高于纯的YBCO, 自场下的Jc更是达到了3.4 MA/cm²。掺杂薄膜的钉扎力(F_p)也远远大于纯YBCO, 最大钉扎力达到了3.25 GN/m³, 有效地提高了YBCO在外加磁下的超导性能。     

Measurement of epitaxial misorientations and related effects in thin films of YBa2Cu3O7-δ grown on nominally (001)MgO substrates by pulsed laser deposition

Heteroepitaxial YBa₂Cu₃O_7-δ thin films have been deposited onto nominal (001)MgO substrates by pulsed laser deposition. For some of these films, Rutherford backscattering spectrometry analysis revealed anomalous variations in the backscattered yield at the substrate level during [001] YBa₂Cu₃O_7-δ axial channelling. A novel angular-dependent ion channelling technique has been developed to investigate these effects. It has been shown that the anomalies arise from an epitaxial misorientation between the lattices of the substrate and deposit. Strong correlations were found between the magnitudes of the misorientations, the densities of spiral features on the deposit surface and the critical current densities exhibited by the deposits. It is proposed that the misorientation arises due to vicinal offcut of the substrate surface, which leads to a lower density of threading dislocations and hence less flux pinning.     

Vortex matter in superconductor/ferromagnet hybrids

Vortex pinning in type-II superconducting films can be effectively controlled by combining these films with different ferromagnetic nanostructures. In this article an overview is presented of different types of ferromagnetic pinning centers. The investigated hybrid structures consist of Pb films that are deposited on top of arrays of ferromagnetic dots with in-plane magnetization (IMP) or out-of-plane magnetization (OPM), ferromagnetic films with IPM or OPM that contain arrays of submicron holes (antidots), or continuous films with OPM and a magnetic domain structure. Interesting effects such as field-polarity dependent vortex pinning and the dependence of the pinning strength on the domain structure of the ferromagnet are observed. Our experiments demonstrate that vortex pinning in superconductors is strongly influenced by the magnetic properties of the different ferromagnetic pinning centers.     

Velocity distribution from flux flow simulation experiments

Magnet needles floating in a cupric sulphate solution through which a strong current is passed are used to simulate flux flow in superconducting thin films. Pinning barriers are provided at regular intervals by cross-magnetized strips. The applicability of the model is limited by the presence of a dissipative term proportional to the square of the velocity and by a mass term. Single needles show the expected roughly sinusoidal velocity dependence on distance. Lattice type arrays of needles lose this behaviour already at low densities. The pinning current decreases drastically as the density of the needles is increased.     

Vortex Matter in Superconductor / Ferromagnet Hybrids

No Heading Vortex pinning in type-II superconducting films can be effectively controlled by combining these films with different ferromagnetic nanostructures. In this article an overview is presented of divergent types of ferromagnetic pinning centers. The investigated hybrid structures consist of Pb films that are deposited on top of arrays of ferromagnetic dots with in-plane magnetization (IPM) or out-of-plane magnetization (OPM), ferromagnetic films with IPM or OPM that contain arrays of submicron holes (antidots), or continuous films with OPM and a magnetic domain structure. Interesting effects such as field-polarity dependent vortex pinning and the dependence of the pinning strength on the domain structure of the ferromagnet are observed. Our experiments demonstrate that vortex pinning in superconductors is strongly influenced by the magnetic properties of the different ferromagnetic pinning centers.PACS numbers: 74.25.Qt, 74.25.Ha, 74.78.Na