MOD-YBCO Films Research Articles

Development of surface planarization process using MOD-Gd-Zr-O layer

REBa2Cu3Oy (RE: rare-earth element, REBCO) coated conductors (CCs) have a high critical current density (Jc) under a magnetic field at the liquid nitrogen temperature and a potential performance for high-magnetic field applications. In order to prepare a biaxially highly oriented REBCO, a highly-textured buffer layer is required. The ion-beam assisted deposition (IBAD) technique is one of the promising techniques for preparation of the highly-textured layer even on the non-textured metallic substrate such as Hastelloy™. However, excellent surface smoothness and uniformity are indispensable to the non-textured metallic substrate using the IBAD technique. Therefore, an inexpensive surface planarization process has been required for fabrication of REBCO CCs with low-cost. In this study, we have investigated the surface planarization using the metal-organic deposition (MOD) method for the IBAD-MgO texturing. The Gd-Zr-O (GZO) planarization layer was prepared by MOD method on roughly-polished Hastelloy™ C-276 substrate. The surface roughness values (Ra) of MOD-GZO planarization layer became smaller than that of the Hastelloy™ C-276 with increase in coating times. Minimum Ra of the MOD-GZO planarization layer achieved less than 2 nm, which is smooth enough for obtaining the highly-textured IBAD-MgO film. The in-plane grain alignment of CeO2 film deposited on sputter-LaMnO3 / IBAD-MgO / MOD-GZO / Hastelloy™ C-276 substrate by the pulsed laser deposition was 4°, and high Jc value of 2.5 MA/cm2 (1.8 μm-t) was obtained for MOD-YBCO film fabricated on it. The CCs using MODGZO planarization layer provided the delamination strength over 70 MPa, which was compatible with conventional structure of CCs.

Thermodynamics and Kinetics Analysis of MOD-YBCO Heat Treatment Process Using in situ Resistance Measurement Method

During the fabrication of $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ (YBCO) superconducting films via metal organic deposition (MOD) method, the crystallization temperature $(T)$ is an important parameter for YBCO nucleation and growth. In this paper, in situ resistance measurements were performed to investigate the influence of $T$ on the growth rate $(R)$ and saturation nuclei density $(N)$ of the YBCO film. Based on the reaction–diffusion model, the specific equation of $R$ as a function of $T$ was established, and the thermodynamics variables, including standard entropy change, standard enthalpy change, and standard Gibbs free energy change of YBCO formation reaction, were estimated. The influence of $T$ on $N$ was analyzed, according to the “capture zone” kinetics mechanism, and the average activation energy for surface diffusion during YBCO nucleation was estimated. The thermodynamics and kinetics analysis in this paper could help to obtain more understanding on the heat treatment process of MOD-YBCO film fabrication.

Effects of thickness on superconducting properties and structures of Y2O3/BZO-doped MOD-YBCO films**Project supported by the National Natural Science Foundation of China (Grant No. 51272250), the National Basic Research Program of China (Grant No. 2011CBA00105), the National High Technology Research and Development Program of China (Grant No. 2014AA032702), and the Beijing Natural Science Foundation, China (Grant No. 2152035).

We report the thickness dependence of critical current density (Jc) in YBa2Cu3O7−x (YBCO) films with BaZrO3 (BZO) and Y2O3 additions grown on single crystal LaAlO3 substrates by metalorganic deposition using trifluoroacetates (TFA-MOD). Comparing with pure YBCO films, the Jc of BZO/Y2O3-doped YBCO films was significantly enhanced. It was also found that with the increase of the thickness of YBCO film from 0.25 μm to 1.5 μm, the Ic of BZO/Y2O3-doped YBCO film increased from 130 A/cm to 250 A/cm and yet Jc of YBCO film decreased from 6.5 MA/cm2 to 2.5 M A/cm2. The thick BZO/Y2O3-doped MOD-YBCO film showed lower Jc, which is mainly attributed to the formation of a-axis grains and pores.

Enhanced flux pinning in MOD-YBCO films with co-doping of BaCeO3 and Y2O3 nanoparitcles

Enhancing the critical-current density of YBCO films is essential to gain a deeper understanding of the vortex pinning mechanisms and enable commercial applications of high-temperature superconductivity. Combined BaCeO3 and Y2O3 nanoparticles have been achieved to be co-doped in YBa2Cu3O7-x (YBCO) films by metalorganic deposition using trifluoroacetates (TFA-MOD). The formation of integrated nanoparticles increases the critical current density (Jc) of Y2O3/BaCeO3 doped-YBCO films while keeping the critical transition temperature (Tc) close to that in the pure YBCO films. YBCO film containing BaCeO3 and Y2O3 showed Tc value of 91 K and Jc value of 5 MA/cm2 at self-field (0 T, 77 K). The strongly enhanced flux pinning over a wide range of magnetic field may be attributed to the combined BaCeO3 and Y2O3 created by optimized TFA-MOD conditions.

Enhanced Jc of MOD-YBCO Films by Modifying Surface States of CeO2 Buffer Layers on Sapphire Substrates

Abstract Simple and easy treatments for the CeO2 buffer layers on sapphire single crystal substrates were demonstrated to enhance the superconducting properties of the metalorganic YBa2Cu3O7-y films using (i) acid solution dipping, (ii) UV light irradiation, and (iii) thermal annealing. When the CeO2 buffer layers were exposed to UV light, water contact angles diminished drastically, however, an apparent effect on the superconducting properties was not observed. On the other hand after the thermal annealing at 750°C under the mixture of dry-air and N2 gas, superconducting current density of 1.4 MA/cm 2 at 77.3K was achieved independent of treatment time.

Enhanced flux pinning in MOD YBa2Cu3O7−δ films by ion milling through anodic alumina templates

A straightforward and reproducible method has been developed for creating < 100 nm wide, 400–600 nm long c-axis columnar defects in fluorine-free MOD YBCO films (∼700 nm thick) using ion milling through free-standing nanoporous anodic aluminum oxide membranes positioned in contact with the films. At 77.3 K, a moderate Jc increase of ∼50% was achieved in self-field, whereas at 1 T a more than doubling of Jc was observed along ab, part of this enhancement coming from uncorrelated pinning effective over all field directions. X-ray analysis confirmed an additional in-plane microstrain component which can be attributed to oxygen disorder induced during the ion milling process.

Enhanced flux pinning in MOD-YBCO films with co-doping of BaZrO 3 and Y 2O 3 nanoparticles

A combined BaZrO 3 (BZO) and Y 2O 3 nanoparticles has been achieved in YBa 2Cu 3O 7− x (YBCO) films by metalorganic deposition using trifluoroacetates (TFA-MOD). The formation of integrated nanoparticles increases the critical current density ( J c) of Y 2O 3/BZO doped-YBCO films while keeping the critical transition temperature ( T c) close to that in the pure YBCO films. The YBCO film with 7.0 mol.% BZO and 7.0 mol.% Y 2O 3 showed T c value of 90 K and J c value of 6.5 mA/cm 2 at self-field (0 T, 77 K). The strongly enhanced flux pinning over a wide range of magnetic field can be attributed to the combined different crystal structures of BZO and Y 2O 3 created by optimized TFA-MOD conditions.

Development of Solution Buffer Layers for RABiTS Based YBCO Coated Conductors

The main objective of this research is to find a suitable alternate solution based seed layer for the standard RABiTS three-layer architecture of physical vapor deposited CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cap/YSZ barrier/Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> seed on Ni-5%W metal tape. In the present work, we have identified CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> buffer layer as a potential replacement for Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> seeds. Using a metal-organic deposition (MOD) process, we have grown smooth, crack-free, epitaxial thin films of CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (pure and Zr, Cu and Gd-doped) directly on biaxially textured Ni-5W substrates in short lengths. Detailed XRD studies indicate that a single epitaxial CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> phase with slightly improved out-of-plane texture compared to the texture of the underlying Ni-W substrates can be achieved in pure, undoped CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> samples. We have also demonstrated the growth of YSZ barrier layers on pure CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> seeds using sputtering. Both sputtered CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cap layers and MOD-YBCO films were grown epitaxially on these YSZ-buffered MOD-CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ni-5W substrates. High critical currents per unit width, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 264 A/cm (critical current density, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 3.3 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) at 77 K and 0.01 T was achieved for 0.8 μm thick MOD-YBCO films grown on MOD-CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> seeds. These results indicate that CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films can be grown directly on Ni-5W substrates and still support high performance YBCO coated conductors. This work holds promise for a route for producing low-cost buffer architecture for RABiTS based YBCO coated conductors.

Homoepitaxial growth of MOD-YBCO thick films on evaporated and MOD templates

We have prepared metal organic deposition (MOD)-YBCO thick films by repeating the coating-pyrolysis-crystallization procedure onto ∼100-nm-thick evaporated and MOD templates. Surface morphology of the template was found to strongly affect the homoepitaxial growth of MOD-YBCO layers on the template; namely, the epitaxial growth of MOD-YBCO on the evaporated template was much easier than that on the MOD template. A 220-nm-thick epitaxial MOD-YBCO film was successfully prepared on the 100-nm-thick evaporated-YBCO template to obtain a 320-nm-thick YBCO film, which exhibited Jc=2.44MA/cm2 and Ic=78A/cm. The Ic value has significantly increased from 37A/cm for the evaporated-template.

Advanced development of TFA-MOD coated conductors

American Superconductor is manufacturing 2G wire for initial commercial applications. The 2G wire properties satisfy the requirements for these initial projects; however, improvements in the critical current, field performance and cost are required to address the broad range of potential commercial and military applications. In order to meet the anticipated the performance and cost requirements, AMSC’s R&D effort is focused on two major areas: (1) higher critical current and (2) enhanced flux pinning.AMSC’s current 2G production wire, designed around a 0.8μm thick YBCO layer deposited by a Metal Organic Deposition (MOD) process, carries a critical current in the range of 200–300A/cm-w (77K, sf). Achieving higher critical current requires increasing the thickness of the YBCO layer. This paper describes recent progress at AMSC on increasing the critical current of MOD-YBCO films using processes compatible with low-cost, high-rate manufacturing.

Enhanced flux-pinning in fluorine-free MOD YBCO films by chemical doping

YBCO films without and with dilute cobalt and zinc doping were prepared on (00l) LaAlO3 substrate by non-fluorine metal organic deposition method. Effects of dilute cobalt and zinc doping on biaxial texture, microstructure and flux-pinning properties of YBCO films were investigated. The surface density and smoothness of the doped YBCO films have been distinctly improved compared with that of the pure film. Dilute cobalt- and zinc-doped YBCO films exhibit significantly enhanced Jc values in the magnetic field. The best result is achieved in the cobalt-doped YBCO film. At 77K, Jc values of cobalt-doped film are 1.7 and 5.4 times higher than that of pure film in 0.5T and 1.5T, respectively. These results strongly suggest that dilute cobalt and zinc doping is a promising way to increase the current carrying capability of YBCO films.

Texture analysis of RF-sputtered CeO2 buffer layers and superconducting MOD-YBCO films

CeO2 buffer layers were deposited on YSZ single-crystal substrates using an RF-sputtering method. The development of crystalline textures of sputtered CeO2 films at different sputtering pressure and their effects on YBCO films, deposited by Metal Organic Deposition (MOD), were investigated. Both CeO2 and subsequent YBCO films grew well epitaxially. The relative XRD peak intensities of CeO2 (200) to substrate YSZ (200) increased with deposition pressure in the range of 3–5mTorr and were inversely proportional to the θ–2θ scan FWHM values of CeO2 (200). Also, the reaction layers of BaCeO3 were thicker in the samples with lower CeO2 (200) intensities and poor out-of-plane alignment when CeO2 were deposited at the lower pressure of 3.3mTorr. It is noted, however, that the superconducting layer grew well epitaxially on these BaCeO3 layers, possibly due to the epitaxial relation between CeO2 and YBCO. The superconducting critical currents of MOD-YBCO films showed an increasing tendency as both the Δ2θ (CeO2) and BaCeO3 peak intensities decreased.

The effect of water pressure on the texture and morphology of MOD-YBCO films onbuffered metal substrates

The influence of water pressure in the metal–organic deposition (MOD) method on the texture and morphologyof YBa2Cu3O7−X (YBCO) films grown on buffered metal substrates was investigated. The water pressure wasvaried from 4.2% to 10.0%, with the other process parameters, such as annealing temperatureand oxygen pressure, kept constant. In this work, a YBCO precursor solution was preparedusing a fluorine-free Y and Cu precursor solution with the addition of Sm. MOD-YBCOfilms were fabricated by continuous slot-die coating and calcination, followed by hightemperature annealing. Using x-ray diffraction analysis, unreacted phase peaks, such asBaF2 peaks, were found in the samples made at a water pressure of 4.2%; however, theBaF2 peak intensity was much reduced as the water pressure was increased. The higher waterpressure of about 10.0% in this experiment led to the poor crystallinity of MOD-YBCOfilms, possibly due to the fast reaction with the supplied water vapor. Meanwhile, themorphologies of the YBCO films were not much different from each other in therange of water pressure of this work. The maximum critical current density of3.8 MA cm−2 was obtained when the sample was made at the water pressure of 6.2% and the annealing temperatureof 780 °C.

Flux pinning in MOD YBCO films by chemical doping

A novel nanomaterial synthesis technique has been developed to introduce 0D (particles),1D (columnar defects) and 3D (domains) nanoscale pinning centres in MODY1Ba2Cu3O7 (YBCO) coated conductors. We have succeeded in introducing nanoscale Y enriched particles, nanoscale90° rotated Y1/3Sm2/3Ba2Cu3O7 domains and nanoscale Zr enriched columnar defects into YBCOlayers by different chemical doping. The pinning force density inY2O3-doped YBCO film is found to be larger than that of pure YBCO film atall fields. Also it was found that YBCO films with Sm substituting for Yhave yielded improved critical current density characteristics over a widerange of magnetic fields. Maximum pinning force densities exceeding 7 and8 GN m−3 are obtained in 5% BZO-doped and Sm substituted YBCO films, respectively. Additionally,TEM studies revealed nanoscale Zr enriched columnar defects distributing in the matrix of thec-oriented YBCO film throughout the whole cross section. This indicates that chemicaldoping is a promising fabrication technique to create specific pinning landscapes in YBCOcoated conductors.

MOD Buffer/YBCO Approach to Fabricate Low-Cost Second Generation HTS Wires

The metal organic deposition (MOD) of buffer layers on RABiTS substrates is considered a potential, low-cost approach to manufacturing high performance Second Generation (2G) high temperature superconducting (HTS) wires. The typical architecture used by American Superconductor in their 2G HTS wire consists of a Ni-W (5 at.%) substrate with a reactively sputtered Y2O3 seed layer, YSZ barrier layer and a CeO2 cap layer. This architecture supports critical currents of over 300 A/cm-width (77 K, self-field) with 0.8 mum YBCO films deposited by the TFA-MOD process. The main challenge in the development of the MOD buffers is to match or exceed the performance of the standard vacuum deposited buffer architecture. We have recently shown that the texture and properties of MOD - La2Zr2Ogamma (LZO) barrier layers can be improved by inserting a thin sputtered Y2O3 seed layer and prepared MOD deposited LZO layers followed by MOD or RF sputtered CeO2 cap layers that support MOD-YBCO films with Ic's of 200 and 255 A/cm-width, respectively. Detailed X-ray and microstructural characterizations indicated that MOD - CeO2 cap reacted completely with MOD YBCO to form BaCeOs. However, sputtered CeO2 cap/MOD YBCO interface remains clean. By further optimizing the coating conditions and reducing the heat-treatment temperatures, we have demonstrated an Ic of 336 A/cm with improved LZO layers and sputtered CeO2 cap and exceeded the performance of that of standard vacuum deposited buffers.

High Critical Current YBCO Films Prepared by an MOD Process on RABiTS Templates

The metal organic deposition (MOD) of YBCO high temperature superconducting films on RABiTS (rolling assisted biaxially textured substrates) templates has been developed at American Superconductor as a low-cost, scalable manufacturing process for the commercialization of the second generation (2G) HTS wire. The MOD process is based on the deposition of a triflu-oroacetate (TFA) based metal organic precursor film which is converted, in an ex-situ process, to the superconducting YBCO film. A major goal of the development has been achieving high critical currents. This paper reports the preparation and characterization of MOD-YBCO films with critical currents exceeding 500 A/cm-w (77 K, self-field) using a scaleable thick film approach on RABiTS templates. The high critical current films were obtained through optimization of the precursor composition, nucleation and growth conditions. The through-thickness dependence of the critical current density of MOD Alms as a function of film thickness and a correlation of the through-thickness transport properties and microstructure of the thick MOD/RABiTS samples is reported.

Properties of MOD-YBCO films at low pressure annealing process

The method of low pressure annealing was adopted to enhance the removal of HF gas from the surface of the MOD-YBCO films, hence to increase the growth rate of YBCO films. In this work, the total pressure in annealing process was varied from 700 Torr to 1 Torr and its effect on the growth of YBCO films was compared with atmospheric pressure. The lower pressure was found effective to increase the growth rate and to control the pore size of the YBCO films in MOD method. A fast growth of MOD-YBCO films was realized with high critical current density over 1 MA/cm 2 using low pressure annealing. Large pores, usually observed at atmospheric pressure in MOD method, disappeared and also the number of pores was reduced.

All MOD buffer/YBCO approach to coated conductors

RABiTS based metal-organic deposition (MOD) buffer/YBa2Cu3O7−δ (YBCO) approach has been considered as one of the potential, low-cost approaches to fabricate high performance second generation coated conductors. The most commonly used RABiTS architectures consisting of a starting template of biaxially textured Ni–W (5 at.%) substrate with a seed layer of Y2O3, a barrier layer of YSZ, and a CeO2 cap. In this three layer architecture, all the buffers are deposited using physical vapor deposition (PVD) techniques. Using these PVD deposited templates, 0.8-μm thick MOD-YBCO films with an Ic (critical current) of 250A/cm have been achieved routinely in short lengths. We have developed a low-cost, non-vacuum, MOD process to grow epitaxial buffer layers on textured Ni–5W substrates. The main challenge in this effort is to match the performance of MOD templates to that of PVD templates. We have recently shown that the properties of MOD-La2Zr2O7 (LZO) layers can be improved by inserting a thin Y2O3 seed layer. Using MOD-CeO2 cap layers, we have demonstrated the growth of high performance MOD-YBCO films with an Ic of 200A/cm-width on MOD-La2Zr2O7/Y2O3/Ni–5W substrates. This approach could potentially decrease the overall cost of the coated conductor fabrication.

Structural aspect of high-Jc MOD-YBCO films prepared on large area CeO2-buffered YSZ substrates

YBa2Cu3O7 (YBCO) film was prepared by fluorine-free metalorganic deposition (MOD) on a 2-inch-diameter CeO2-buffered yttria-stabilized zirconia (CbYSZ) substrate. The MOD-YBCO films on a flat CbYSZ prepared using a metal acetylacetonate-based coating solution demonstrated high critical current density (Jc), the average being higher than 4 MA/cm2 at 77.3 K and in a self field using an inductive method. The Jc distribution was uniform in the whole area of the MOD-YBCO films on the large size substrates. Cross sectional transmission electron microscopic images of the MOD-YBCO film exhibited a very smooth CeO2/ YSZ and CeO2 buffer layer/ YBCO film interfaces. Both the buffer and the YBCO layers were dense; however, a small amount of (100)-oriented YBCO occurred inside the matrix of (001)-oriented YBCO layers. Also, some distinct dislocations were observed at the CeO2/ YBCO interface. From around the stepped surface of the CeO2 buffer layer there seemed to grow a slightly distorted (001)-oriented YBCO domain, however, the domain restored its (001)-oriented periodicity in the region at some distance above the steps. Such growth mode is considered to correlate the high Jc of the MOD-YBCO film with very small average roughness of CeO2 buffer layer.

Improved YBCO Coated Conductors Using Alternate Buffer Architectures

The Rolling-Assisted Biaxially Textured Substrates (RABiTS) process has been identified as one of the leading candidates for the fabrication of high performance YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) coated conductors. The RABiTS process uses standard thermomechanical processing to obtain long lengths of flexible, biaxially oriented substrates with smooth surfaces. The strong biaxial texture of the metal is then transferred to the superconductor by the deposition of intermediate oxide buffers that serve both as a chemical and structural buffer. The typical three-layer RABiTS architecture consists of an e-beam Y/sub 2/O/sub 3/ seed, sputtered YSZ barrier and a sputtered CeO/sub 2/ cap layer. Chemical solution deposition of buffer layers offers potential cost advantage relative to physical vapor deposition (PVD) processes. Our main goal of this study is to develop simplified buffer architectures and demonstrate high J/sub c/ Metal-Organic Deposition (MOD)-YBCO films on all-MOD buffers. La/sub 2/Zr/sub 2/O/sub 7/ (LZO)/CeO/sub 2/ buffers have been identified as potential candidates for this study. MOD-YBCO films with a critical current, I/sub c/ of 212 A/cm have been achieved on MOD-LZO seeds with sputtered YSZ and CeO/sub 2/ cap layers. In addition, MOD-YBCO films with a critical current, I/sub c/ of 140 A/cm have been achieved on all MOD buffers of LZO/CeO/sub 2/ for the first time. This offers a potential toward fabrication of lower cost YBCO coated conductors.