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.     

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.     

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.     

Direct Peritectic Growth of c-Axis Textured YBa2Cu3O x 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.     

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.     

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.     

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.     

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.     

Effect of Melt Processing Programme on Microstructure and Properties of YBCO Thick Films

The screen printing technique was used in the fabrication of YBa₂Cu₃O_7−σ (YBCO) superconducting thick films on yttria stabilised zirconia (YSZ) substrates. Different slow cooling rates were used in the preparation of YBCO thick films after fast cooling from the melt processing temperature. The effects of the melt processing programme on texturing, microstructure and superconducting properties of the melt processed YBCO films were studied. Slow cooling rates between 1005 and 990 °C were effective in increasing the interaction of viscous molten with reduced film/substrate, and hence a relatively large grain size has been obtained. Moreover, different c-axis texturing ratios and grain morphologies were observed.     

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.     

YBCO Superconducting Film Coated on LaAlO<Subscript>3</Subscript> Substrate by TFA-MOD Process

Epitaxial YBCO film was coated on (001) LaAlO₃ single-crystal substrate by metalorganic deposition of metal trifluoroacetate precursors. From XRD and EDS analysis, it was observed that the film contains CuO or Cu₂BaO₂ segregation, but transport measurement found the segregation structure does not severely prevent the percolation of supercurrent-carrying material. The YBCO films performance is up to 2.4 MA/cm² with I_c of 137 A/cm-width at 77 K and 0 T, and superconducting transition temperature T_c reaches around 91.3 K with sharp transition temperature T_c of 0.36 K.     

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.     

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.      

Nanostructural characterization of YBCO films on metal tape with textured buffer layer fabricated by pulsed-laser deposition

Thick YBa₂Cu₃O_7-x (YBCO) films with high critical current density (J_c) values were deposited by pulsed-laser deposition (PLD) on Hastelloy with a textured CeO₂/Gd₂Zr₂O₇ buffer layer. Both cross-sectional and plan-view TEM specimens of the YBCO films were prepared, and then the nanostructural characterization of the films was performed by transmission electron microscopy (TEM). The YBCO films less than 1 μm thick were predominantly composed of c-axis-oriented grains, however, many a-axis-oriented grains, which grew larger with the increase of the thickness of the YBCO film, were formed beyond about 1 μm from the CeO₂ interface. We found Y₂O₃ and copper oxides between a- and c-axes-oriented grains. In particular, Y₂O₃ grains were formed between the {001} plane of an a-axis-oriented grain and the {100} or {010} plane of a c-axis-oriented grain. The orientation relationships between Y₂O₃ and YBCO are found to be; (001)YBCO//(001)Y₂O₃ and (100)YBCO//(110)Y₂O₃. In addition, we also found gaps between YBCO grains. Since a-axis-oriented grain growth and the formation of Y₂O₃, copper oxides and the gaps are considered to reduce the J_c values of the YBCO film, it is important to determine the optimum process conditions to suppress the nucleation of a-axis-oriented grains, impurity oxides and gaps.     

Preparation of the YBCO/SmBiO3/NiO/Ni–W New Structure for Coated Conductors

A new structure basis of SmBiO₃ (SBO) potential buffer layer for low-cost coated conductors fabrication methods has been introduced in this paper, and studied with XRD, SEM and AFM analyses. It is found that highly in-plane and out-of-plane oriented, dense, smooth, and crack-free SBO buffer layer has been prepared on NiO-buffered Ni-5 %W (NiW) tapes via a CSD route. YBCO epitaxial film was obtained on CSD-YBCO/CSD-SmBiO₃/SOE-NiO/NiW by using F-free CSD method which shows a new route to search for SmBiO₃ new buffer layer material for coated conductors.     

Influence of Heat Treatment on Structure and Superconducting Properties of YBCO Film by Chemical Solution Deposition Method

YBCO films have been fabricated on a (00l) LaAlO₃ single-crystal substrate via self-developed fluorine-free polymer-assisted metal organic deposition (PA-MOD) method. The influence of heat treatment on texture, microstructure and superconducting properties of YBCO films has been investigated. After a pyrolysis process ranging from 145 °C to 500 °C with different heating rates, the samples were fired at 760–780 °C in Ar and O₂ mixture gas followed by annealing at 450 °C in pure O₂. The results indicate the film fired at 770 °C after decomposition at the rate of 0.5 °C/min showed the highest T _c of 90.4 K and J _c (77 K, 0 T) over 2 MA/cm². According to the XRD patterns, phi-scan and omega-scan curves as well as SEM images, the good properties may be attributed to better biaxial texture and purer YBCO phase as well as better grain connectivity.     

High-Quality YBa2Cu3O7?x Films with CeO2/YSZ Buffer Layers on 2-Inch Si Wafers Deposited by Pulsed Laser

The pulsed laser deposition (PLD) process is shown for in situ reproducibly fabricating YBa₂Cu₃O_7–x (YBCO) superconducting films with yttrium-stabilized zirconia (YSZ) and CeO₂ buffer layers, nonsuperconducting crystalline YBa₂Cu₃O_7–x (YBCO*) passivation layer, and silver contact film on 2-inch silicon wafers. Variations of less than ±7% in film thickness have been obtained for this multilayer growth over the whole wafer. The YBCO films on 2-inch silicon wafers have homogeneous superconducting properties with zero resistance temperature T _c0 from 88.4 K to 88.9 K. and critical current density J _c at 77 K and zero field from 2.5 × 10⁶ to 7× 10⁶ A/cm². The YSZ, CeO₂ and YBCO layers grow epitaxially on silicon wafers. Full widths at half maximum (FWHMs) of (113) reflections of 40 nm thick YBCO layer from -scan patterns are only 1.71° and 1.85° corresponding to the center and edge of the wafer, respectively. These results are very promising for developing high-quality high-T _c superconducting devices on large-area silicon wafers.     

Study of Critical Current and <Emphasis Type="Italic">n</Emphasis>-Values of 2G HTS Tapes: Their Magnetic Field-Angular Dependence

Since many applications of YBCO tapes operate in external magnetic fields, it is necessary to investigate the magneto-angular dependence of critical current and n-values in coated conductors. In this paper, five commercial YBCO tapes with different microstructures produced by three different manufacturers are chosen. The selected samples have a width of 2.0, 4.0, 4.8, 6.0 or 12 mm, with copper, brass or stainless steel laminations. The critical current density dependence J_c(B, θ) and n-values characteristics n(B, θ) of the tapes are comprehensively measured under various magnetic fields and orientations. Afterwards, the obtained experimental data sets are successfully fitted using a novel multi-objective model which considers the material anisotropy. By using this approach, a fitting function I_c(B, θ) can always be obtained to accurately describe the experimental data, regardless of the fabrication and width differences of the superconducting tapes. Moreover, our experiment shows that when subject to different external magnetic fields, the angular dependence of n-values characteristics is directly correlated with the corresponding critical current profiles. Our results are helpful to predict the critical current of electromagnetically interacting 2G HTS wires, thereby improving the design and performance of the devices made from YBCO tapes.     

The Influence of Temperature Distribution on YBCO Coated Conductor Deposited by Reel-to-Reel PLD Method

It is known that the tape’s temperature controlled by a close substrate heater will still deviate after the deposition process. In this work, we deposited YBa₂Cu₃O_7−δ (YBCO) films on RABiTS substrates with multilayer CeO₂/YSZ/CeO₂ buffer layers by pulsed laser deposition (PLD) under a different distribution of temperature deviation in order to improve it. Thermocouples were applied to exactly measure the temperature. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to inspect c-axis oriented crystals and surface morphology of YBCO films. Through this work, we found that the even shape is best. Under the temperature deviation with the best shape, a transport J _c of 3.2×10⁶ A/cm² and I _c =240 A was obtained for a 0.75 μm thick YBCO film on CeO₂/YSZ/CeO₂-bufferd RABiTS Ni–W alloy.