BCO Films Research Articles

RETRACTED ARTICLE: Significantly enhanced critical current density and pinning force in nanostructured, (RE)BCO-based, coated conductor

High-temperature superconducting wires have many large-scale, niche applications such as commercial nuclear fusion as well as numerous other large-scale applications in the electric power industry and in the defense, medical and transportation industries. However, the price/performance metric of these coated conductor wires is not yet favorable to enable and realize most large-scale applications. Here we report on probing the limits of Jc (H, T) possible via defect engineering in heteroepitaxially deposited high-temperature superconducting thin-films on coated conductor substrates used for long-length wire fabrication. We report record values of Jc (H, T) and pinning force, Fp (H, T) in (RE)BCO films with self-assembled BaZrO3 nanocolumns deposited on a coated conductor substrate. A Jc of ~190 MA/cm2 at 4.2 K, self-field and ~90 MA/cm2, at 4.2 K, 7 T was measured. At 20 K, Jc of over 150 MA/cm2 at self-field and over 60 MA/cm2 at 7 T was observed. A very high pinning force, Fp, of ~6.4 TN/m3 and ~4.2 TN/m3 were observed at 7 T, 4.2 K and 7 T, 20 K respectively. We report on the highest values of Jc and Fp obtained to date for all fields and operating temperatures from 4.2 K to 77 K. These results demonstrate that significant performance enhancements and hence far more favorable price/performance metrics are possible in commercial high-temperature superconducting wires.

Nucleation and epitaxy growth of high-entropy REBa2Cu3O7–δ (RE= Y, Dy, Gd, Sm, Eu) thin films by metal organic deposition

High-entropy REBa2Cu3O7–δ (RE = Y0.7 + Dy0.2 + Gd0.2 + Sm0.2 + Eu0.2) (HE-REBCO) superconducting films doped with multiple rare earth elements were successfully fabricated with thickness up to 800 nm by a trifluoroacetate-metal organic deposition (TFA-MOD). The enhanced entropy change ΔS of the HE-REBCO system promotes the c-axis growth of REBCO thin film in the competition with a/b-axis growth. The microstructure and element distribution were investigated by the transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The results show that HE-REBCO films have a great amount of stacking defects, lattice distortions and various rare earth oxides. Furthermore, the microstrain of HE-REBCO films increases significantly compared to the undoped YBCO and (Y, Dy)BCO films. Finally, the superconducting critical temperature (Tconset) of HE-REBCO films is about 93 K, and the in-field properties are significantly improved, especially at the temperature of about 77 K. The maximum pinning force density (Fp,max) of the HE-REBCO samples at 77 K is 1.7 times higher than that of the undoped YBCO films.

Properties on Yttrium-Doped/Undoped Barium Cerate and Barium Zirconate Thin Films Formed by E-Beam Vapor Deposition

As electrolyte materials for proton conductive fuel cells, perovskite-type materials such as barium cerates and barium zirconates have received a lot of attention due to their high protonic conduction at intermediate temperatures. Yet, the crystalline structure and the microstructure of the electrolyte layers are of the utmost importance that define the resulting protonic conductivity. The aim of this research was to investigate the formation of doped/undoped BCO and BZO thin films using e-beam vapor deposition and to analyze the influence of the formation parameters on the microstructural and crystallographic properties. Crystalline structure and microstructure were investigated by X-ray diffractometer and scanning electron microscope, while the elemental composition of the resulting thin films was analyzed by an energy-dispersive X-ray spectroscope. It was found that the formed thin films were highly dense and consisted of the oriented columnar grains. The crystallinity of the thin films was strongly expressed with the predominant crystallographic orientations for undoped/doped barium cerates. Yttrium dopant had an influence on the lattice parameters and crystallite sizes. With the chosen technological parameters allowed to both, barium cerates and barium zirconates did not form carbonates and did not experience the degradation process.

An advanced electrical heating technique for double-sided Y(Gd)BCO coated conductor: gaining high engineering current density based on MOCVD

An advanced electrical heating technique was proposed and adopted for the reel-to-reel deposition of double-sided Gd x Y1−x Ba2Cu3O7−δ (Y(Gd)BCO) films on the surface of LaMnO3/epitaxial-MgO/IBAD-MgO/Y2O3/Al2O3/Hastelloy tapes based on the metal organic chemical vapor deposition process. In this technique, heating current is introduced into alloy tape to produce heat through the electric brushes. The use of thin Hastelloy tapes is an effective method to obtain a high engineering current density. However, the reduction of the substrate thickness will directly attenuate its mechanical strength, which will lead to the deformation of tapes at high temperature based on original electric heating device. More seriously, the electrical contact between the alloy substrate and the brush will deteriorate, which could cause ignition and ablation at the edge of the tapes. Therefore, in order to improve mechanical and electrical stability, we redesigned a novel electrical heating device to deposit Y(Gd)BCO films. Furthermore, through adopting the multiple-deposition process based on the new electrical heating device, the J e of Y(Gd)BCO film can reach 900 A mm−2 (at self-field, 77 K), which has been significantly improved compared with the J e before optimization.

Dynamic behavior of reversible oxygen migration in irradiated-annealed high temperature superconducting wires

We use atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy to determine the atomic-scale structural, chemical and electronic properties of artificial engineered defects in irradiated-annealed high temperature superconducting wires based on epitaxial Y(Dy)BCO film. We directly probe the oxygen vacancy defects in both plane and chain sites after irradiation with 18-meV Au ions. The plane site vacancies are reoccupied during post-annealing treatment. Our results demonstrate the dynamic reversible behavior of oxygen point defects, which explains the depression and recovery of self-field critical current and critical temperature in irradiation-annealing process. These findings reveal the strong effect of oxygen vacancies in different sites on the superconductivity properties of irradiated Y(Dy)BCO film, and provide important insights into defects engineering of 2G HTS coil wires.

Manipulation of G1Ba2Cu3O7-x Film Properties by Simply Changing Growth Temperature with RF Sputtering Method

We have investigated the relationship between Jc ( the critical current density) and H (applied magnetic field magnitude) for Gd1Ba2Cu3O7-x((Gd)BCO) films grown from 750 °C to 850 °C by RF sputtering. Measurements of JC(H, θ) (the magnetic-field angular dependence of the critical current density) reveal that film B (800 °C) shows enhanced pinning compared with C (850 °C), A (750 °C) (Gd)BCO films. The film C (850 °C) is the most anisotropic. We speculate that a lot of stacking faults exist in film C (850 °C), based on a board peak along ab-plane and no peak along c axis from the Jc angular measurements together with the small anisotropy parameter γ = 3. The speculation is proved by angular dependence measurement of film C (850 °C) after high temperature process in oxygen.

Development of cryogenic tensile testing apparatus for lattice strain measurement using synchrotron radiation for REBCO composite conductors

REBa2Cu3O7−δ (REBCO) composite tapes are candidate conductors for future accelerator magnets. Given that REBCO is a brittle oxide film, stress/strain management is important in the mechanical design of a magnet. Therefore, the strain state of a superconducting film should be predicted under various operating conditions. A quantum beam is a useful tool in evaluating the strain state of a superconducting material in a composite conductor. Although various cryogenic tensile testing machines, in combination with neutron beam and synchrotron radiation, have been developed, they are not necessarily optimized for measuring highly textured materials such as REBCO films. This study reports a novel cryogenic tensile testing apparatus that can be used at the beamline of a synchrotron radiation facility. In this system, a superconducting tape is cooled using a Gifford–McMahon (GM) cryocooler through 2.5 m long thermal links made of 5 N high-purity aluminum. This enables a compact cryogenic loading system that can be mounted on the goniometer of a beamline. Lattice strain measurements using synchrotron radiation were conducted at 40 K, 77 K, and 300 K for a commercially available (Y,Gd)BCO coated conductor at BL28B2 in SPring-8. The onset strain of film fracture determined from the lattice strain measurement at 77 K agreed well with the irreversible strain of critical current at the same temperature. The residual and fracture strain of the (Y,Gd)BCO film at 77 K was evaluated as well.

Improved Performance of CSD-Grown Y1- x Gd x Ba2Cu3O7-BaHfO3 Nanocomposite Films on Ni5W Substrates

Y1- x Gd x Ba2Cu3O7-BaHfO3 (YGBCO-BHO) nanocomposite films containing 12 mol% BHO nanoparticles and different amounts of Gd, x , were grown by Chemical Solution Deposition (CSD) on Ni5W substrates in order to investigate the impact of the rare-earth stoichiometry on the structure and superconducting properties of these films. For Gd contents x > 0.5, epitaxial YGBCO-BHO films with an approximate thickness of 270 nm self-field critical current density J c at 77 K ∼ 1,5 MA/cm² were obtained. The field dependence of the critical current density J c( B ) shows a much larger accommodation field and lower exponents α in J c ∼ B −α values compared to pristine YBCO films. This is both due to the high amount of individual nanoparticles in the matrix as observed in TEM images and the higher critical temperatures T c. The results show that the CSD is a potential candidate for the preparation of RE BCO films in long-length coated conductors.

Preparation of high performance YGdBCO films by low fluorine TFA-MOD process

YBCO films doped with different contents of gadolinium (Gd) were prepared by the low-fluorine (low-F) trifluoroacetate metal-organic deposition (MOD) method. The effects of flow rate and holding time of the firing (crystallization) stage on the superconducting properties of YxGd1–xBa2Cu3O7–δ (YGdBCO) films were investigated. The phase formation and texture were characterized by the X-ray diffraction (XRD), which indicate that severe degradation of the microstructure will be induced with the inappropriate flow rate. The surface morphology and element distribution were investigated by the scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that increasing the holding time of the firing stage is effective for the further decomposition of residual impurity phase on the surface. The mechanisms of the phase and surface evolution are also discussed. Finally, a high critical current density (Jc) value of 5.4 MA/cm2 was achieved in the Y0.9Gd0.1BCO film fabricated by the cooperative control of the flow rate and holding time of the firing stage, which are contributed to the formation of excellent texture, homogeneous microstructure and dense surface of the YGdBCO films.

Optimization of MOCVD Hardware Configuration and Process for Growth of High Quality (Gd,Y)BCO Superconducting Tapes Based on CFD Modeling

A systematic computational fluid dynamic (CFD) study was performed to investigate the effects of hardware configuration and process parameters on flow field and temperature profile above the film growth surface for (Gd,Y)BCO superconducting tapes in a metalorganic chemical vapor deposition (MOCVD) reactor. Significant improvement in the uniformity of flow field and temperature profile was achieved by optimizing the gap between showerhead and susceptor, showerhead length, gas flow rate, and chamber pressure. As results of the improvement in hardware configuration based on CFD modeling, high quality (Gd,Y)BCO superconducting films were achieved and the limit on growth rate was increased. The improvement was not only in the surface morphology (achieving more clean and smooth surface with less outgrowth grains and impurity phase particles), but also in the cross sectional microstructure view (more uniform from the bottom to the top) for thick films processed by the reel-to-reel MOCVD. A 1.76-μm-thick (Gd,Y)BCO film in a 284-m-long tape yielded a self-field critical current of 769 A/12 mm width at 77 K.

Enhancement of the in-field critical current density of trifluoroacetate metal organic deposition derived (Y0.77Gd0.23)Ba2Cu3Oy films by annealing of CeO2 buffered R-Al2O3 substrates

We investigate the superconducting properties of (Y0.77Gd0.23)Ba2Cu3Oy [(Y,Gd)BCO] films on as-grown and annealed (600 °C–1000 °C) CeO2 buffered R-plane sapphire (R-Al2O3) substrates using trifluoroacetate metal organic deposition (TFA-MOD). The annealing was found to improve the crystallinities of not only the CeO2 buffer layers but also the (Y,Gd)BCO films. Although the anneal had no apparent effect on the pinning centers of the (Y,Gd)BCO films, the in-field Jc of the (Y,Gd)BCO films grown on the CeO2 buffered substrates improved with increasing annealing temperature. The present results are discussed in terms of the crystallinities and Jc values of the (Y,Gd)BCO films.

Trifluoroacetate metal organic deposition derived (Y0.77Gd0.23)Ba2Cu3Oy films on CeO2 buffered R-plane Al2O3 substrates

(Y0.77Gd0.23)Ba2Cu3Oy [(Y,Gd)BCO] films were grown on CeO2 buffered R-plane sapphire (R-Al2O3) substrates using trifluoroacetate metal organic deposition (TFA-MOD). Annealing of the CeO2 buffered R-Al2O3 substrates was performed to control the crystallinity and surface morphology of the CeO2 buffer layer. The annealing treatment led to a significant improvement in the crystallinity and surface morphology of the CeO2 buffer layer. A (Y,Gd)BCO film grown on the CeO2 buffer layer with high crystallinity and an atomically flat surface exhibited high self-field (at 77 K) and in-field (at 20 K, 9 T, μ0H∥c) critical current densities (Jc). Annealing of the CeO2 buffer layer thus enabled enhancement of not only the self-field Jc for the (Y,Gd)BCO film but also the in-field Jc.

Effect of Yb Substitution on Microstructure and Superconducting Properties of Y1−xYbxBa2Cu3O7−δ Films

The purpose of this study was to improve the critical current density (Jc) of YBa2Cu3O7−δ (YBCO) superconducting films at high magnetic fields. Thin Y1−xYbxBa2Cu3O7−δ (Y1−xYbxBCO) superconducting films on LaAlO3 substrates were prepared by modified low-fluorine metal organic deposition method (LF-MOD) in order to investigate the effect of Yb substitution on the microstructure and superconducting properties of a YBCO-coated conductor. In this study, single-phased, c-axial-aligned Y1−xYbxBCO films were obtained. Yb doping had a little effect on the critical temperature (Tc) and transition width (ΔT) values of Y1−xYbxBCO films. An abnormal phenomenon was observed that the Jc of Y1−xYbxBCO films at low field was smaller than that of the pure YBCO film, but the Jc of Y1−xYbxBCO films at high field was much greater than that of the undoped YBCO film. Compared to pure YBCO film, the Jc values of Y0.75Yb0.25BCO and Y0.25Yb0.75BCO films were enhanced by a factor of 8 at 8 T, 65 K, which meant that chemical substitution of Yb into the Y site could improve flux pinning significantly under the high magnetic field in Y1−xYbxBCO coated conductors.

Improvement of flux pinning in GdBa2Cu3O7−δ thin film by nanoscale ferromagnetic La0.67Sr0.33MnO3 pretreatment of substrate surface

The present paper presents the effects of ferromagnetic La0.67Sr0.33MnO3 (LSMO) nanoparticles on the pinning characteristics of an epitaxial GdBa2Cu3O7−δ ((Gd) BCO) film deposited on top. LSMO nanoparticles with the size between 10 and 20nm were obtained on a (001) STO substrate by RF sputtering method. The analyses of magnetic measurements revealed that the presence of a complex vortex pinning mechanism within the (Gd) BCO film deposited on the undecorated substrate. With respect to a reference (Gd) BCO film, two additional pinning effects in LSMO decorated (Gd) BCO film were observed. One is the effect of the threading dislocations due to LSMO nanoparticles, and the other one is the LSMO nanoparticles itself. Like other nanosized particles, the LSMO nanoparticles on substrate will induce threading dislocations passing through the (Gd) BCO film. These threading dislocations serve as correlated pinning centers along the c-axis, which lead to the improvement of flux pinning properties in LSMO decorated (Gd) BCO film. According to the ferromagnetic property of LSMO material, a magnetic pinning mechanism was proposed to explain the significant increase of Jc and Fp values. The flux pinning abilities of LSMO nanoparticles were evaluated. The enhanced pinning characteristics in the high field region and high temperature together with the movement of H Fpmax (the corresponding field of Fpmax) also suggest a magnetic pinning mechanism for LSMO nanoparticles existed in LSMO decorated (Gd) BCO film.

Structural study of commercially produced (RE)BCO films

Various deposition techniques used in the coated conductor industry result in the formation of different structural inhomogeneities within the superconducting film, having an impact on its electromagnetic properties. In this paper, an attempt for complex characterization of the Y(Gd)Ba 2 Cu 3 O 7-x [(RE)BCO] structure in tapes from two major producers was performed. Scanning and transmission electron microscopy techniques were used for the characterization of the fine structure of (RE)BCO thin films in the cross-sectional and planar views. The examination was focused on structural inhomogeneities, e.g., outgrowths, precipitates, stacking faults, nanocolumns, and their mutual correlations. The structural study was correlated to critical current dependence on magnetic field and temperature, where the effect of particular structural inhomogeneities on functional properties was evaluated.

Microstructure Characteristics of High Lift Factor MOCVD REBCO Coated Conductors With High Zr Content

We report the microstructural characteristics of high levels of Zr-added REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-x</sub> (RE = Gd, Y rare earth) coated conductors fabricated by Metal Organic Chemical Vapor Deposition (MOCVD). The enhancements of the lift factor defined as a ratio of the in-field (3 T, B ∥ c-axis) critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) at 30 K and self-field J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> at 77 K have been achieved for Zr addition levels of 20 and 25 mol% via optimization of deposition parameters. The presence of strong flux pinning is attributed to the aligned nanocolumns of BaZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and nanoprecipitates embedded in REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-x</sub> matrix with good crystal quality. A high density of BZO nanorods with a typical size 6-8 nm and spacing of 20 nm has been observed. Moreover, the high Zr content was found to induce a high density of intrinsic defects, including stacking faults and dislocations. The correlation between in-field performance along the c-axis and microstructure of (Gd,Y)BCO film with a high level of Zr addition is discussed.

Pre-fabricated nanorods in RE–Ba–Cu–O superconductors

Pre-fabrication of metallic nanorods on biaxially textured templates has been explored in this study to introduce flux pinning centers in RE–Ba–Cu–O (REBCO, RE =rare earth) based superconductors. Pt nanorods were deposited by an electron beam assisted deposition method on LaMnO3-capped biaxially textured IBAD-(ion beam assisted deposition) substrates. Well-controlled nanorods with varying diameter (50–120 nm), length (up to 1 μm), orientation and unit cell size were grown over an area of 120–150 μm2. The nanorod-decorated samples were then deposited with Gd–Y–Ba–Cu–O ((Gd, Y)BCO) by metal organic chemical vapor deposition (MOCVD). The Pt nanorods remain in their positions during MOCVD and become embedded in the (Gd, Y)BCO matrix, although they suffer creep-induced shape deformation due to exposure to elevated temperature. Higher unit cell size, longer nanorods, and nanorods oriented at an angle to the substrate normal adversely affect the epitaxy of the (Gd, Y)BCO film due to formation of a-axis grains. The observed current-carrying capacity of the Pt nanorod sample is lower than its corresponding reference sample without any nanorods and processed under identical conditions, but it decreases at a slower rate with increasing magnetic field. Potential routes to improve the performance while retaining the desirable characteristics of controlled nanorod direction and density are discussed.

Texture and morphology developments of Yttria-stabilized zirconia (YSZ) buffer layer for coated conductors by RF sputtering

Yttria-stabilized zirconia (YSZ) films are deposited on CeO2/NiW tapes by RF sputtering for (Gd) BCO-coated conductors. Surface morphology and texture developments are investigated as the O2:Ar ratio and sputtering power increase. Both the grain size and the roughness of YSZ films increase as sputtering power increases. A simple model proposed by Bartelt et al. is used to explain the results. The strain relaxation mechanism plays a major role in the sudden increase in roughness for YSZ film deposited at a sputtering power of 50W. It is observed that low O2:Ar ratio favors the growth of (200) orientation for YSZ film, while high O2:Ar ratio favors the growth of (111) orientation. The preferential orientation (200) of the YSZ film with low O2:Ar ratio arises from template effect of CeO2 buffer layer. The (111) orientation of YSZ film with high O2:Ar ratio is due to thermodynamic mechanism aiming for the lowest surface energy. The grain size and roughness of our optimal YSZ film are 10–15nm and 0.7nm, respectively. The out-of-plane texture of our optimal YSZ film is 4.05°. The in-plane texture is 6.35°. The plan view and cross-section SEM images of YSZ films on CeO2/NiW tapes show a flat, dense and no micro-cracks morphology. Based on these results, (Gd) BCO films are deposited by RF sputtering, achieving self-field Jc of 4.0 MA/cm2 at 77K.

Power-law index and penetration depth of (NdxSmxGd1−2x)Ba2Cu3O7−δ films studied by AC susceptibility

Superconducting (NdxSmxGd1−2x)Ba2Cu3O7−δ films with x=0, 0.1, 0.25, 0.33 were grown by PLD on STO single crystal substrates. The power-law index n and penetration depth λ are studied by AC susceptibility. During cooling, n in films with x ≠ 0 increases much slower compared with the films with x = 0. The films with x ≠ 0 also tend to have a longer penetration depth. These properties might be related to the higher possibility for disorder in the mixed (Nd,Sm,Gd)BCO films.

Growth and Property of Y&lt;SUB&gt;0.5&lt;/SUB&gt;Yb&lt;SUB&gt;0.5&lt;/SUB&gt;BCO Films Prepared by MOD Method

Growth and Property of Y&lt;SUB&gt;0.5&lt;/SUB&gt;Yb&lt;SUB&gt;0.5&lt;/SUB&gt;BCO Films Prepared by MOD Method