Influence of temperature on stability of trapped flux magnets

Trapped flux in YBCO plates has an inherent dependence on temperature. The electro-magnetic current density and thermal specific heat are both highly dependent upon the temperature. Modeling and experimental data investigate the nature of a YBCO sample that has a heat pulse forced upon a trapped magnetic field by measuring the change in the magnetic field before and after the energy input. The experimental data suggest interesting trends that are further explained with models. An analytical approach and FEA simulations are performed to discover the physics behind the experimental data. By comparing the data, the temperature dependence on the YBCO sample is further investigated and illustrated.     

Analysis of the local current in GdBCO coated conductors using low-temperature scanning laser and Hall probe microscopy

The distribution of the local current in GdBCO coated conductors was investigated using low-temperature scanning laser and Hall probe microscopy (LTSLHPM). We prepared GdBCO coated conductors to study the spatial distribution of the current density in a single bridge. Inhomogeneity in the bridge was analyzed using the experimental results of scanning laser microscopy near the superconducting transition. The local transport and screening currents in the bridge were also obtained by scanning Hall probe microscopy. From the experimental results of scanning laser and Hall probe microscopy, we have observed redistribution of the current caused by defects of the coated conductors.     

Investigation of the Transport Properties of Coated Conductors and Comparison to Those of YBCO Films Deposited on Single-Crystal Substrates

The existence of non-superconducting zones in YBa₂Cu₃O_7??? (YBCO) films below the transition temperature to the non-resistive state can limit the performances of YBCO devices, especially of coated conductors for power transportation. In this work, we present detailed measurements of the temperature dependence of the surface critical current density of commercial YBCO-coated conductors. We compare our measurements to those carried out by other authors on the same types of samples and to measurements carried out on films deposited on SrTiO₃ single crystals. The existence of a T _c gradient along the YBCO films?? c-axis, probably linked to a gradient in the oxygen content of the films, seems established for all the investigated samples.     

Thermal peculiarities of the electric mode formation of high temperature superconductors with the temperature-decreasing n-value

Formation peculiarities of the static thermo-electric modes that may be observed in high temperature superconductors at the DC current are studied taking into consideration the temperature-decreasing dependence of the power exponent (n-value) of the voltage-current characteristic of a superconductor. The used models are based on the steady zero-dimensional models. The study is made for the conduction-cooling and liquid coolant conditions at different operating temperatures. It allows us to investigate the non-isothermal voltage-current characteristic of Bi2223 and YBCO that rooted the development of their stable and unstable thermo-electric regimes. It is shown that the temperature dependence of an n-value may lead, first, to usually observed static voltage-current and temperature-current characteristics having one stable and unstable parts and, second, to unexpected thermo-electric states in the temperature range that is closed to its transition into the normal state. It is a result of the formal temperature variation of the differential resistivity of a superconductor depending on its temperature-decreasing critical current density and n-value. As a result, to describe correctly thermal stability conditions and analyze theoretically the quench processes in high temperature superconductors, the temperature dependences of their critical current density and n-value must be restored before the transition of a superconductor into the normal state as carefully as possible.     

Improvement of Pinning Force and Critical Current Density in Thick YBa 2 Cu 3 O 7−δ Films Grown on SrTiO 3 Substrates Decorated with LaNiO 3 Nanodots

We have investigated by DC magnetization measurements and frequency-dependent AC susceptibility the critical current density (J _c), pinning force (F _p) and pinning potential in thick (1.3–1.6 μm) YBa₂Cu₃O_7−δ (YBCO) films grown by Pulsed Laser Deposition on SrTiO₃ substrates decorated with LaNiO₃ nanodots deposited by a few (5–15) laser pulses, in comparison with those of a 1 μm thick YBCO reference sample. Experiments show that the highest improvement of superconducting properties was achieved for films grown on substrates decorated with 10 laser pulses on the LaNiO₃ target, which have, at 77.3 K, a J _c of 40–125% higher than in pure YBCO in fields between 1 and 2 T, and F _p increased by 40%. These results could be important for further improvement of current-carrying capability of coated conductors for in-field power applications.     

Improvement of the High-Magnetic-Field Critical Current Density of the <Emphasis Type="Italic">Ex-Situ</Emphasis> Annealed MgB<Subscript>2</Subscript> Thick Films by Oxygen Doping

This paper presents a very simple way to synthesis MgB₂ thick films with high critical current density in a magnetic field by ex-situ annealing precursor B films in air with excessive Mg in a sealed quartz tube. The films show a significant improvement of critical current density in a magnetic field compared to the high purity films annealed in vacuum, while its zero-resistance transition temperature T _c ^zero and normal state resistivity still maintain about 38 K and 17 μΩcm. The results demonstrate MgB₂ thick films have great potential applications in superconducting coated conductors.      

High Quality YBCO Film Growth on SrTiO_3-Buffered LaAlO_3 Substrate by Full Solution Method

A full solution method has been developed as a low cost process of YBa2Cu3OT-x （YBCO） coated conductor fabrication. In this study, highly biaxially textured SrTiO3 （STO） buffer layers were fabricated on LaAlO3 （LAO） single crystal substrates by sol-gel method using metal alkoxides as the staring precursor materials. High quality YBCO superconducting film was then fabricated on STO-buffered LAO substrate by trifluoroacetic metalorganic deposition （TFA-MOD） method. For the YBCO superconducting film, only （001） diffraction peaks can be detected by XRD （X-ray diffraction） analysis with no other phases detectable. Especially, Inplane texture of YBCO film is improved compared to that of STO buffer layer from phi scans analysis, which indicates the self-epitaxy phenomenon explained by considering interracial energy. STO and YBCO films both show c-axis oriented grains growth and have uniform surface microstructure. A critical transition temperature, TC （R=0） of 89.5 K and a critical current density of 2 mA/cm2 （77 K, self-field） were obtained for a 0.2μm thick YBCO film on STO-buffered LAO substrate. No reaction between YBCO and STO was detected by XRD analysis. This full solution process may provide a promising low cost fabrication route for YBCO coated conductors on metal tape.     

Percolation and the resistive transition of the critical temperature Tc of Nb3Sn

The relationship between the distribution of the critical temperature, the percolation function, and the resistive transition of the critical temperature is explored for polycrystalline Nb₃Sn. In the neighborhood of the critical temperature, Nb₃Sn is assumed to be a random mixture of superconducting and normal grains. Percolation concepts are applied to a study of the resistivity. A general analysis is made showing that the onset and shape of the resistive transition for composite conductors are determined by the percolation function and the distribution of the critical temperature. An approximate form of the percolation function is determined based on a linear FEM analysis. Example resistive transitions are calculated for an assumed normal distribution of the critical temperature. An argument is presented that relates grain orientation and strain dependence in Nb₃Sn. It is noted that a dependence of the distribution of T_c with strain, in addition to the usual shift in T_c with strain, would be the result of a strain dependence that is a function of grain orientation. The analysis shows the extent to which the slope of the resistive transition is a measure of the distribution of the critical temperature, and therefore a measure of the grain orientation strain sensitivity. Finally, a method is described to determine the percolation function experimentally.     

Single cerium zirconate buffer layer on biaxially textured metal substrates for high performance coated conductors

Textured cerium zirconate (Ce _x Zr_1−x O₂) films were deposited on biaxially textured Ni-5at.%W substrate by direct-current (dc) reactive magnetron sputtering for low cost production of high performance YBa₂Cu₃O_7−δ (YBCO) coated conductors. Film composition was controlled by modulating dc power applied to the Ce metal target. X-ray diffraction analysis shows that all the samples exhibit epitaxial growth, with c-axis perpendicular to the substrate surface. The YBCO film deposited directly on the Ce_0.32Zr_0.68O₂ layer for optimized lattice matching shows a transition temperature T _c and critical current density J _c (75.5 K, self field) of 90.4 K and 1.3 MA/cm². The in-field dependence of J _c is similar to the standard CeO₂/YSZ/CeO₂ buffered samples. These results demonstrate that a single Ce _x Zr_1−x O₂ buffer layer, instead of CeO₂/YSZ/CeO₂ multi-buffer layers for the fabrication of YBCO coated conductors, provides advantages such as simplified architecture and potentially reduced cost due to the reduced fabrication steps.     

Enhancement of critical current density of YBa2Cu3O7 − δ thin films by self-assembly of Y2O3 nanoparticulates

YBa₂Cu₃O_7 − δ (YBCO) thin films, possessing high critical current density (J_c), have been synthesized by embedding a homogeneous array of Y₂O₃ non-superconducting nanoclusters/nanoparticles using a pulsed laser deposition technique. The size, interparticle spacing, and density of Y₂O₃ nanoparticles in YBCO thin films were tailored by varying the number of laser pulses in order to determine the optimum size for effective immobilization of vortices. Scanning transmission electron microscopy with atomic number contrast and X-ray diffraction techniques were used to determine the size and structure of the nanoparticles. Both techniques indicate that the Y₂O₃ particles are epitaxial with respect to the surrounding YBCO matrix. The information about pinning of vortices by the nanoparticles was obtained by investigating the behavior of critical current density as a function of temperature and applied field, which in turn determines the vortex density in the sample. The superconducting transition temperature (T_c) of YBCO films with the inclusion of nanoparticles was observed to remain almost the same or decrease marginally (1-2 K) with respect to T_c of pure YBCO films deposited under identical conditions. However, J_cs of YBCO films embedded with self-assembled nanoparticles were found to be significantly higher than that of pure YBCO films. The J_c enhancement was up to five times in high magnetic field, which is a key requirement for practical application of high-T_c materials.     

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.     

Preparation and Characterization of TiN Seed Layer in All-Conductive Multilayer Structure for Coated Conductors

TiN films were successfully prepared on biaxially textured Ni-5 at.%W substrates by pulsed laser deposition (PLD), serving as a seed layer of an all-conductive architecture, i.e., SrRuO₃(SRO)/TiN, for coated conductors. The structure and surface morphology of the TiN films were noticeably affected by the substrate temperature and pulse repetition rate. The subsequent conductive SRO and superconducting YBCO layer were deposited on the best sample of TiN buffered Ni-5 at.%W substrates. X-ray diffraction analysis confirmed that the biaxial textures were transferred from the TiN seed layer to the SRO cap layer and YBCO film with excellent out-of-plane and in-plane textures. The superconducting transition curves and the temperature-dependent resistivity of YBCO films on all-conductive buffer and on the traditional insulated CeO₂/YSZ/Y₂O₃ were also investigated.     

Fabrication and Analysis of Tri-layer YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Thick Films by Low-Fluorine MOD Method

Improving the thickness of superconducting layer in coated conductors is an effective way to enhance its critical current. In this work, tri-layer YBCO/YBCO/YDyBCO films were successfully deposited on buffered Hastelloy substrate using the multi-coating lowfluorine metal-organic decomposition (LF-MOD) method and the thickness of the films can be up to 2.4 μ m. The effects of high-temperature annealing time on microstructures and superconducting properties of the films were systematically studied. Energy dispersive X-ray spectroscopy (EDS) results reveal that there remains a large amount of F element in the upper layer of the film when the annealing time is too short. With increasing the annealing time, the fluoride-containing precursor converts to YBCO grains completely. But the coarsening of grains appeared, and the critical current density (J _c) of the film dropped slightly when the annealing time is too long. The cross-sectional scanning electron microscope (SEM) image and EDS plane analysis were applied to investigate the microstructure and element distribution of the final triple-layer YBCO films, respectively. The critical current of the final YBCO superconducting film could reach 316 A (77 k, self-field) for 1.2-cm-wide tapes with the optimal annealing conditions.     

Growth of thick chemical solution derived pyrochlore La2Zr2O7 buffer layers for YBa2Cu3O7−x coated conductors

The preparation of several 100 nm thick La₂Zr₂O₇ (LZO) buffer layers on biaxially textured Ni-5 at.%W substrates using chemical solution deposition is studied. This oxide material is currently of great interest for the fabrication of YBa₂Cu₃O_7−x (YBCO) coated conductors. Buffer layers for these coated conductors are required to have thicknesses greater than 100 nm in order to guarantee a sufficient barrier function against metal diffusion from the substrate. In this work, single LZO buffer layers with thicknesses exceeding 200 nm have been prepared. Detailed investigations were carried out in order to study the texture development with increasing thickness as well as the microstructure of these layers. Independent of the thickness, high quality buffer layers showing a distinct biaxial texture up to the surface, smooth surfaces, and a sufficient barrier function against Ni diffusion from the substrate have been reproducibly obtained. The high performance of these chemical solution derived LZO buffer layers was confirmed by a YBCO critical current density J_c of 1.0 MA/cm² (77 K, 0 T) achieved for a coated conductor sample with a layer sequence YBCO/CeO₂/LZO(CSD)/Ni-5 at.%W where CeO₂ and YBCO were deposited by pulsed laser deposition.     

Direct Peritectic Growth of c-Axis Textured YBa2Cu3Ox on a Flexible Metallic Substrate

Previous work in the development of YBa₂Cu₃O _x (YBCO) superconducting wires and tapes has been focused on the deposition of YBCO on buffered metallic substrates. Although such an approach has proved successful in terms of achieving grain texturing and high transport current density, critical issues involving continuous processing of long-length conductors and stabilization of the superconductor have not yet been entirely settled. We have developed a novel process, the so-called direct peritectic growth (DPG), in which textured YBCO thick films have been successfully deposited directly onto a silver alloy substrate. No buffer layer is employed in the film deposition process. The textured YBCO grains have been obtained through peritectic solidification over a wide range of temperatures and times. The substrate materials have not demonstrated any observable reaction with the YBCO melt at the maximum processing temperature near 1010°C. The transport J _c has reached a respectable value of 10⁴ A/cm² at 77 K and zero magnetic field. Based on the experimental results in this work, we show that the DPG method offers an effective alternative for the fabrication of long-length YBCO conductors. Also reported is a physical explanation of the texturing mechanism on the metal substrate.     

Fabrication and Properties of Coated Conductors with All Fluorine-Free CSD Route

A fluorine-free chemical solution deposition (CSD) method has been developed to fabricate YBCO epitaxial films and SmBiO₃ or Sm-doped CeO₂ (SCO) buffer layers on crystal substrates and textured Ni5%W metal substrates. The critical current density, J _c , is over 4 MA/cm² for YBCO deposited on crystal substrates, over 3 MA/cm² on SmBiO₃-buffered single crystal substrates, and it reaches 1 MA/cm² on a (Sm,Ce)O₂-buffered metal substrate, demonstrating the feasibility of an all-CSD method for fabricating high quality coated conductors.     

Reel-to-Reel PLD Fabrication of YBCO Coated Conductor by Single and Multi-coating Processes

YBa₂Cu₃O_7?δ (YBCO) coated conductors have been fabricated on CeO₂/YSZ/Y₂O₃ buffered Ni-5at%W tapes by pulsed laser deposition (PLD) using the reel-to-reel process. A multi-coating process for YBCO film was employed, and single and multi-coating methods for YBCO films are compared. X-ray diffraction texture measurements showed good in-plane and out-of-plane crystalline orientations for the YBCO films. Magnetic measurements were carried out on the samples, and the critical current density as a function of the magnetic field was investigated. The results showed that the superconducting properties of YBCO films fabricated by the multi-coating process were better than those prepared by the single coating process.     

Fabrication of low cost YBCO coated conductors using Ag-clad Hastelloy substrates

Mechanically reinforced Ag-clad Hastelloy tapes were fabricated as the inexpensive substrates for coated conductors without any buffer layer. The clad substrate has good adhesion between Ag and Hastelloy layers. Superconducting YBa₂Cu₃O₇ (YBCO) films were directly deposited on Ag-clad Hastelloy substrates by chemical vapor deposition in high magnetic fields. YBCO films were highly oriented along the c-axis perpendicular to the substrate with a zero resistance transition temperature of 86.2 K and a transport J_c value of 10⁴ A/cm² at 77 K and zero magnetic field. These results indicate that the present work appears to be a promising way for the development of YBCO films for large-scale applications.     

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.     

Determining the absolute thermo emfs of conductors using a Y<Subscript>1</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> rod

The voltage drop on a 110-mm-long Y₁Ba₂Cu₃O_{7 − δ} (YBCO) rod with a diameter of 7 mm bearing a current of 1 A has been studied as a function of the temperature. It is shown that YBCO rods can be used to measure the absolute thermo emf of conductors in a range of temperatures up to the onset of breakage of the superconducting state. The thermo emf of Ni, Cu, and W conductors was determined.