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.     

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.     

Dependence of Microstructure and <Emphasis Type="Italic">J</Emphasis><Subscript>c</Subscript> of YBCO Films on CeO<Subscript>2</Subscript> Cap Layers Deposited by Multi-plume PLD

In order to achieve high-performance YBa₂Cu₃O_7?x (YBCO)coated conductors (CCs) fabricated in industrial scale, it is necessary to enhance the transport properties and production speed of the CCs for use in various application forms. The transport performance of CCs depends upon the inner structure of the conductors, which make it important to analyze the microstructure and transport properties. The thickness of the buffer layer is a factor in improving speed. In this work, we deposited YBCO films on CeO₂ cap layers with different thicknesses ranging from 21 to 563 nm by multi-plume pulsed laser deposition (PLD) and investigated the dependence of the microstructure and superconducting properties of YBCO film on the thickness of CeO₂ films. The crystalline structure and surface morphology of YBCO films are systematically characterized by means of XRD, AFM, SEM and TEM. The critical current of YBCO film was measured by the conventional four-probe method at 77 K, in self-field. The results showed that the microstructure and superconducting performance of YBCO film were strongly dependent on the thickness of CeO₂ films. At the optimal CeO₂ layer thickness of 221 nm, the YBCO film exhibited a sharp in-plane and out-of-plane texture of full width at half maximum (FWHM) values of 1.5° and 2.4°, respectively, and smooth morphology of root mean square (RMS) value as low as 4.0 nm. The sharply biaxially textured YBCO films with the critical current density as high as 4.7 × 10⁶ A/cm² (77 K, in self-field) were obtained on CeO₂/MgO/Y₂O₃/Al₂O₃/C276 architecture.     

Effect of PrBa2Cu3O7−x buffer layer thickness on the properties of YBa2Cu3O7−x thin films grown on sapphire by laser ablation

Superconducting YBa₂Cu₃O_7?x (YBCO) thin films were deposited onr-plane A1₂O₃ substrates with PrBa₂Cu₃O_7?x (PBCO) buffer layer by XeCl excimer laser ablation. The thickness of PBCO buffer layer was systematically changed to investigate the superconducting properties of YBCO thin films on sapphire. The structure and surface morphology of the films were characterized by X-ray diffraction and scanning electron microscopy (SEM). Superconducting transition temperatures were varied depending on the buffer layer thickness. Interdiffusion between laser-ablated YBCO thin films and A1₂O₃ substrates had been studied by Auger electron spectroscopy (AES). The results of this study show that diffusion does not occur between the YBCO thin film and the substrate even with 20 Å thick PBCO buffer layer.     

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.     

Fabrication of YBCO Film Using a New Copper Precursor by Chemical Solution Method

YBCO films were fabricated by using a modified precursor solution, which was prepared by mixing yttrium trifluoroacetate, barium trifluoroacetate, and a fluorine-free copper precursor. Crack-free films have been obtained in a rapid pyrolysis process. The texture, microstructure and superconducting properties of films were characterized by x-ray diffraction, scanning electron microscopy, and the inductive method, respectively. The effect of the heating rate on the phase formation, texture and critical properties (J _c) of YBCO films was investigated. The texture and critical current density varied significantly with the heating rate. Moreover, the J _c of 1.7?MA/cm² was obtained for YBCO films prepared at the heating rate of 3?K/min.     

Superconducting YBa2Cu3O7?δ Thin Film Detectors for Picosecond THz Pulses

Ultra-fast THz detectors from superconducting YBa₂Cu₃O_7?δ (YBCO) thin films were developed to monitor picosecond THz pulses. YBCO thin films were optimized by the introduction of CeO₂ and PrBaCuO buffer layers. The transition temperature of 10 nm thick films reaches 79 K. A 15 nm thick YBCO microbridge (transition temperature—83 K, critical current density at 77 K—2.4 MA/cm²) embedded in a planar log-spiral antenna was used to detect pulsed THz radiation of the ANKA storage ring. First time resolved measurements of the multi-bunch filling pattern are presented.     

Influence of Water Vapor Pressure on Superconducting Properties of YBCO Film by TFA-MOD

We fabricated YBa₂Cu₃O_7−x (YBCO) films on LaAlO₃ (LAO) substrates using metal organic deposition of precursor solutions containing metal trifluoroacetates (TFA-MOD). The effect of water vapor pressure on the microstructure, degree of texture, and critical current of prepared YBCO films was evaluated. The firing process was performed in the water vapor pressure range from 7 to 31 kPa. According to the results of the XRD, SEM and X-ray ω scan analyses, the causes of critical properties’ change with the different water pressure were explained. The YBCO film prepared at the lower water pressure showed the low critical current because of the a–b axis grain. The films fired at 20 kPa showed the highest critical current of 3.8 MA/cm².     

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.     

YBa2Cu3O7 − δ thin films with enhanced film properties grown on sapphire using Y2O3/CeO2 bi-layer buffer

Highly epitaxial, microcrack-free thin films of YBa₂Cu₃O_7 − δ (YBCO) have been grown by pulsed laser deposition on sapphire substrates with a double buffer of Y₂O₃/CeO₂. The Y₂O₃ layer, which has excellent compatibility with CeO₂ and YBCO, has been found to be beneficial in reducing the surface porosity of YBCO films as well as inhibiting a-axis-oriented epitaxial growth. The reduction in porosity is attributed to the presence of the Y₂O₃ layer which acts as a suitable barrier for the chemical reaction occurring at high deposition temperatures between YBCO and CeO₂. Due to the improvement in film quality and surface morphology, enhancement of the self-field critical current density (J_c, 77.3 K) by as high as 30% was obtained for a 650-nm thick YBCO film deposited on Y₂O₃/CeO₂ bi-layer buffer compared to simultaneously-deposited YBCO film on CeO₂ single-layer buffer only.     

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.     

Effect of Firing Temperature on the Structure and Properties of YBa2Cu3O7−x Films by TFA-MOD Method

YBa₂Cu₃O_7−x (YBCO) films were fabricated on LaAlO₃ (LAO) substrate under various firing temperatures (760–870 °C) in the crystallization process by metalorganic deposition (MOD) method using trifluoroacetates. The effect of firing temperature on the structure and properties of YBCO films was systematically investigated. According to the XRD and SEM images, the films fired at low temperature (760–800 °C) showed poor electrical performance due to rough surfaces and impurity phases. However, the films fired at 850 °C showed the highest critical temperature of 90 K and the highest J _c of 3.1 MA/cm² which attribute to the formation of a purer YBCO phase, fewer pores, and stronger biaxial texture.     

Influence of CeO2-Cap Layer on the Texture and Critical Current Density of YBCO Film

YBa₂Cu₃O₇ (YBCO) thin films have been fabricated on different textured CeO₂-cap layers by pulsed laser deposition (PLD). The texture and critical current density J _c of YBCO thin films have been systematically investigated. Both in-plane and out-of-plane textures of YBCO films and CeO₂-cap films were characterized by X-ray diffraction (XRD). And the critical currents of YBCO films were measured by the conventional four-probe method. It was found that the texture and J _c of YBCO films were largely dependent on the texture of CeO₂-cap layers under the optimized deposition conditions. With increasing the degree of in-plane and out-of-plane texture of CeO₂-cap layers, J _c of YBCO films decreased from 4.23×10⁶ A/cm² to 0.47×10⁶ A/cm². The FWHM values of the omega scan rocking curves of YBCO films decreased from 3.71 to 1.84° and the phi scan rocking curves from 6.68 to 4.91° with improvement of CeO₂-cap layer texture. Our results indicated that the fabrication of high texture quality of CeO₂-cap layer was necessary for the epitaxial growth of high-J _c YBCO films. The high-quality YBCO films which are comparable with those grown on single crystal substrates could be achieved on high textured CeO₂-cap layers buffered metal substrates.     

Improved critical current densities in thick YBa2Cu3O7 − δ multilayer films interspaced with non-superconducting YBa2Cu3Ox nanodots

We have fabricated and studied quasi-multilayered thick YBa₂Cu₃O_7 − δ (YBCO) films composed of several YBCO layers interspaced with quasi-layers of non-superconducting YBa₂Cu₃O_x nanodots, grown by Pulsed Laser Deposition on SrTiO₃ (100) substrates. Magnetization J_c(B) at 77.3 K for these thick films showed significant improvement as compared to pure YBa₂Cu₃O_7 − δ films of same or even smaller thickness. A high J_c(B) in our quite thick films (1 μm to 6 μm) provides a very high total critical current per centimetre of the film width, I_c − w. Critical current as high as 830 A per cm width in self field and 77.3 K was achieved in 5 μm thick quasi-multilayer film with non-superconducting YBa₂Cu₃O_x nanodots. Frequency-dependent susceptibility measurements showed also an increase in the pinning potential. The angular dependence of I_c − w at 86.5 K, in 3 T shows a clear indication of anisotropic pinning centres aligned along the c-direction.     

Buffer layers for high-T c thin films on sapphire

Buffer layers of various oxides including CeO₂ and yttrium-stabilized zirconia (YSZ) have been deposited onR-plane sapphire. The orientation and crystallinity of the layers were optimized to promote epitaxial growth of YBa₂Cu₃O_7?δ (YBCO) thin films. An ion beam channeling minimum yield of ～3% was obtained in the CeO₂ layer on sapphire, indicating excellent crystallinity of the buffer layer. Among the buffer materials used, CeO₂ was found to be the best one for YBCO thin films onR-plane sapphire. HighT _c andJ _c were obtained in YBCO thin films on sapphire with buffer layers. Surface resistances of the YBCO films were ～4 mω at 77 K and 25 GHz.     

Localized Pair Breaking Effect in the Quasi-multilayers of YBa2Cu3O7?δ /La0.88Ca0.12MnO3

Quasi-multilayers consisting of superconducting YBa₂Cu₃O_7??? (YBCO) and ferromagnetic La_0.88Ca_0.12MnO₃ (LCMO), namely p×(YBCO(m)/LCMO(n)) (p: repetition periods, m: YBCO pulse number, and n: LCMO pulse number) are prepared on single crystal LaAlO₃ (LAO) by pulsed laser deposition (PLD). X-ray diffraction measurements reveal that both in-plane and out-of-plane textures of the quasi-multilayers are as good as in the pure YBCO films. Nevertheless, the present quasi-multilayer system shows significant modifications in the superconducting properties, as evidenced by the suppressed superconducting transition temperature as well as the critical current density. This indicates that a special pair breaking effect emerges at YBCO/LCMO interfaces, and it is believed to arise from the spin scattering by the local exchange field due to the insulating nature of the ferromagnetic LCMO.     

Effect of Ba/Y Molar Ratio on Superconducting Properties of BaTiO<Subscript>3</Subscript>-Doped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Films

YBa₂Cu₃ O _7?δ (YBCO) films with BaTiO₃ (BTO) nanostructures were prepared by using the precursor solutions with different cationic molar ratios of Y:Ba:Cu = 1.0:1.6–2.0:3.0 in the TFA-MOD process. These YBCO films were deposited on (00 l)-oriented LaAlO₃ single-crystal substrates using a spin coater. The high superconducting critical current density (J _C) (77 K, self-filed) of more than 10 MA/cm² for the final BTO-doped YBCO film was obtained. Moreover, the effect of different Ba/Y molar ratios in the precursor solution on superconducting properties of BTO-doped YBCO films was investigated. Compared with the BTO-doped YBCO film deposited by using the precursor solutions with Ba/Y molar ratio of 2.0, an enhancement of J _C in a magnetic field for the film from the solution with Ba/Y molar ratio of 1.9 was achieved. For Ba/Y molar ratios of 1.6 and 1.7, a reduction of J _C in a magnetic field occurred. The J _C enhancement may be mainly ascribed to the enhanced flux pinning by the Y₂Cu₂ O ₅ nanostructures with the optimal number dispersing in YBCO matrix.     

The Influence of the Microstructure and Morphology of CeO 2 Buffer Layer on the Properties of YBCO Films PLD Grown on Ni Tape

YBa₂Cu₃O_7?δ films were deposited on CeO₂-buffered nickel substrates, with different buffer thickness. Full width at half maximum of rocking curve, Δω, of CeO₂ and yttrium barium copper oxide (YBCO), as well as the critical temperature, T _c, of YBCO were shown to be strongly dependent on buffer thickness. They behave similarly but not proportional to the buffer thickness increase. This and the fact that Δω vs. buffer thickness and T _c vs. buffer thickness for YBCO behave similar with RMS roughness vs. thickness of CeO₂ indicates that the surface peculiarity of buffers is responsible for YBCO properties. More precisely, the surface of CeO₂ films prepared by the chemical solution route based on propionic acid is prone to agglomerate (de-wet) and the degree of agglomeration depends in an intricate way on buffer thickness. We showed that nor RMS roughness neither (00 l) texture degree can define alone the surface suitable for c-axis YBCO nucleation. The {111} faceted grains (even in the case of high (00 l) texture) and other defects generated by agglomeration supply a low fraction of (00 l) flat terminations of buffer that affect the nucleation of c-axis-oriented YBCO phase. Moreover, the thermal instability of the surface morphology of CeO₂ buffers (further development of de-wetting process, {111} faceted grains, etc. during superconducting layer deposition) influence the quality of YBCO films.     

Epitaxial Growth and Characterization of RF-Sputtered LaMnO3 Cap Layers on Homo-Epi MgO/IBAD–MgO Templates

Epitaxial LaMnO₃ (LMO) films, serving as the cap layer for high performance coated conductors, have been successfully fabricated on homo-epi MgO/Ion-Beam Assisted Deposition (IBAD)–MgO templates on polycrystalline hastelloy by radio-frequency magnetron sputtering. Crystalline structure and morphology analysis revealed good texture, and a smooth, dense, and crack-free surface. Highly epitaxial 500 nm thick YBa₂Cu₃O_7?x (YBCO) films with the full width at half-maximum values around 3.8^° and 1.2^° for the ?-scan of (103) and rocking curve of (005) YBCO, respectively, exhibited the self-field critical current density (J _c) of 1.2 MA/cm² at 77 K.     

Influence of Interfacial LSMO Nanoparticles/Layer on the Vortex Pinning Properties of YBCO Thin Film

YBa₂Cu₃O_7?δ (YBCO) thin films have been deposited on bare and La_0.67Sr_0.33MnO₃ (LSMO) modified single crystal SrTiO₃ (STO) substrates. The effect of randomly distributed ferromagnetic LSMO nanoparticles and a complete LSMO layer, present at STO/YBCO interface, on the superconducting properties of YBCO thin films has been investigated by temperature dependent magnetization studies. The YBCO thin film on LSMO nanoparticles decorated STO substrate shows significant improvement in the critical current density and pinning force density as compared to the YBCO thin film deposited on bare STO substrate and this improvement is more significant at higher applied magnetic field. However, the LSMO/YBCO bilayer showed the improved flux pinning properties only up to a magnetic field of 1.5 T above which it deteriorates. In the case of LSMO/YBCO bilayer, the underlying LSMO layer gives rise to magnetic inhomogeneities due to domain structure, which leads to improved flux pinning properties limited to lower field. However, in the case of LSMO nanoparticles decorated substrate, the presence of LSMO nanoparticles at YBCO/STO interface seems to introduce magnetic inhomogeneities as well as structural defects, which might be acting as correlated pinning sites leading to improved flux pinning properties of the YBCO thin film over a wide range of applied magnetic field.