Jung-Woo Lee

RCE-DR, a novel process for coated conductor fabrication with high performance

We report in detail on SuNAMs reactive co-evaporation by deposition and reaction (RCE-DR) process. We have successfully fabricated a high performance GdBCO coated conductor (CC) with high throughput by the RCE-DR process, that consists of two steps for the deposition of elemental metal oxides and the conversion of cation oxides into the GdBCO superconducting phase. Constituting metals such as Gd, Ba and Cu were first deposited on LaMnO3 (LMO)-buffered IBAD-MgO templates at low temperatures and low pressures followed by a high temperature treatment step under high oxygen partial pressure for fast phase conversion. GdBCO CCs fabricated by RCE-DR showed excellent transport properties such as a critical current of 794 A cm−1 width at 77 K in self-field. With the RCE-DR process, we have achieved an overall processing speed of more than 120 m h−1 (in terms of a real process linear tape speed equivalent). SuNAMs RCE-DR technique showed great potential as the highest throughput fabrication process compared with other methods developed previously for second generation high temperature superconducting wires, meeting the current and future need of industry in terms of price and production speed.

Characteristics of High-

We report the characteristics of GdBa₂Cu₃O_7-δ (GdBCO) coated conductors (CCs) on LaMnO₃ (LMO)-buffered IBAD MgO template, produced by the Reactive Co-Evaporation Deposition & Reaction (RCE-DR) process. The transport and magnetic critical current density (<i>J</i>_c) values of GdBCO CC samples were 3.7 and 2.92 MA/cm² at 77 K in self-field for <i>B</i>//<i>c</i>, respectively. Analyses by transmission electron microscopy revealed that a large amount of elongated round Gd₂O₃ particles with 70-150 nm size and a small amount of the Cu-O phase were trapped in the GdBCO matrix, and a thick layer of the Cu-O phase was also found on the top surface of the GdBCO film, suggesting that the GdBCO film must be grown by the peritectic recombination reaction between Gd₂O₃ and a liquid phase. While both the GdBCO film and some Gd₂O₃ particles grown on the LMO-buffer layer were biaxially textured, the Gd₂O₃ particles fully trapped in the GdBCO matrix were randomly oriented. The GdBCO film by RCE-DR possessed slightly degraded value of exponent α ~ 0.84 in the power-law relationship <i>J</i>_c ~ <i>H</i>^-<i>a</i> and rather large anisotropy in field-orientation dependence of <i>J</i>_c, <i>J</i>_c(θ), and thus refinement of Gd₂O₃ particles may be required for stronger flux pinning.

J_{c}

High-quality epitaxial YBa₂Cu₃O_7-δ (YBCO) films were successfully fabricated on LaAlO₃ (LAO) (001) substrates by fluorine-free metal-organic deposition (MOD). YBCO powder was dissolved into a fluorine-free source to prepare the coating solution. The solution was dip-coated on LAO, dried at 80°C for 10 min, pyrolized at the temperature up to 400°C in humid O₂ atmosphere, subsequently fired to produce YBCO in an oxygen partial pressure of 300 ppm, and finally oxygenated in a pure O₂ atmosphere at 450°C for 1 h. While, in addition to <i>a</i>-axis oriented grains, the formation of randomly oriented polycrystalline grains was unavoidable when as-pyrolized films were simply fired at 825°C for 2 h, high-quality epitaxial YBCO films with only a small amount of <i>a</i>-axis oriented grains were obtainable when as-pyrolized films were heated up to 850°C at first and then cooled down to 825°C and held for 2 h at the firing stage. Although our best YBCO film had a <i>Tc</i>,zero value of 87.5 K and <i>Jc</i> value of 0.47 MA/cm² at 77 K in a self-field at the present stage, this fluorine-free MOD is a promising alternative to conventional TFA-MOD since further improvement in <i>Jc</i> is possible through the processing optimization.

GdBCO Coated Conductors Fabricated by the RCE-DR Process

We study depth profiling of the microwave surface resistance (R S) of GdBa2Cu3O7−δ (GdBCO) coated conductors (CCs) grown using the reactive co-evaporation by deposition and reaction (RCE-DR) method, a method enabling extremely high deposition rate. GdBCO CCs with the critical current (I C) of more than 790 A cm−1 at 77 K in self-field are used for the study. The R S of the GdBCO CCs is measured at temperatures of 10–80 K using a 8.5 GHz TE011-mode rutile resonator, which is compared with that of YBa2Cu3O7−δ films and GdBCO films epitaxially grown on single crystal substrates. It turns out that there is significant inhomogeneity in the R S over the thickness of the GdBCO layer, with the R S value of the top part at 30 K being almost two times higher than the corresponding one of the bottom part. A transmission electron microscopy study reveals that Gd2O3 grains coexist with GdBCO grains with the average Gd2O3 grain sizes being ~150 nm at the top and ~100 nm at the bottom of the GdBCO layer. We relate the inhomogeneity in the R S of the GdBCO layer with the positional dependence of the Gd2O3 grain size, for which effects of the dielectric losses from the Gd2O3 grains on the measured R S of the GdBCO layer are considered. Our results imply that the critical current density, another important transport property of superconductors, could be inhomogeneous over the thickness of the GdBCO layer grown using the RCE-DR method.

Fabrication of High-Quality Epitaxial YBCO Films Prepared by Fluorine-Free MOD

We report the flux pinning properties of YBa2Cu3O7-δ (YBCO) thin films with BaZrO3 (BZO) nanoparticles, fabricated by a fluorine-reduced trifluoroacetate-metal-organic deposition process. For this study, the Ba-deficient coating solutions (Y : Ba : Cu = 1 : 1.5 : 3) with various BZO contents ranging from 0 to 15 mol% were prepared by mixing Ba-trifluoroacetate with fluorine-free Y, Cu, and Zr sources. Precursor films were deposited on SrTiO3 (001) substrates by dip-coating, pyrolized in a humid oxygen atmosphere, and finally fired at a high temperature in a reduced oxygen atmosphere. Highly enhanced critical current densities (Jc) in magnetic fields were achievable from all BZO-doped samples. The optimally processed YBCO film with 7.5 mol% BZO nanoparticles exhibited the Jc values of 0.39 and 0.5 MA/cm2 at 77 K in 1 T and at 65 K in 3 T for H//c, respectively. The BZO nanoparticles of 10-50 nm size trapped in the YBCO matrix are considered to be responsible for enhanced flux pinning in BZO-doped YBCO films.

Depth profiling of the microwave surface resistance of high-JC GdBa2Cu3O7-.DELTA. coated conductors grown using the RCE-DR process

We report the effects of barium-based perovskite particle inclusions of BaCeO₃ (BCO), BaZrO₃ (BZO), and YBa₂SnO_5.5 (YBSO) on the thickness dependence of critical current density (J_c) in metal organic deposition (MOD)-processed YBa₂Cu₃O_7-δ (YBCO) films on the SrTiO₃ (STO) single-crystal substrate. With increasing film thickness, while BZO-doped YBCO films show a serious degradation in J_c, caused by the interlayer at the boundaries of multicoated layers, BCO- and YBSO- doped films exhibit a significant enhancement of J_c (77 K, self-field) over ~1 μm thickness even though BCO- or YBSO- doped YBCO films have degraded in-plane textures compared with undoped films, implying that these perovskite dopants affect not only the pinning properties but also the growth of multicoated layers. The highest critical current (I_c) value over 440 A/cm-width, corresponding to J_c value of 1.8 MA/cm², was obtained from the 2.4-μm-thick YBSO-doped film.

Enhanced Flux Pinning Properties of MOD-Processed

We investigated the effect of the growth temperature on the microstructures and superconducting properties of GdBa₂Cu₃O_7-δ (GdBCO) coated conductors (CCs) deposited by the Reactive Co-Evaporation Deposition and Reaction process on the LaMnO₃-buffered IBAD MgO template. For the growth of GdBCO films with ~1.5μm thickness at the oxygen pressure of 150 mtorr, we employed three different growth temperatures of 840, 860, and 880 °C. By decreasing the growth temperature, the critical current densities Jc at 65 and 77 K were improved due to enhanced pinning properties, which are attributable to refined Gd₂O₃ particles trapped in the GdBCO matrix. Analyses by transmission electron microscopy revealed that the average particle size of Gd₂O₃ is 126.5 nm in the 840 °C sample, which is smaller than that of 171.4 nm in the 860 °C sample and 217.8 nm in the 880 °C sample. Interestingly, although (103)-misoriented GdBCO grains existed together with biaxially textured grains in the film grown at 840 °C, they exhibited higher J_c values in magnetic fields compared with other films composed of only biaxially textured grains, suggesting that the increase in J_c due to pinning by refined Gd₂O₃ particles surpasses the degradation of Jc by (103)-oriented grains present at the film surface.

\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7 - \delta}

The in-field pinning properties of GdBa₂Cu₃O_7-δ (GdBCO) coated conductors (CCs) could be remarkably improved through the post-annealing process in a low oxygen pressure (PO₂). At the annealing temperature of 800°C in a low PO₂ of 300 mtorr, the T_c,zero value of as-prepared GdBCO CCs by SuNAM Co. via reactive co-evaporation deposition and reaction (RCE-DR) was increased from 89.9 to 93.7 K after 5 min annealing, whereas it was degraded after annealing longer than 10 min. The angular dependence of critical current density J_c at 77 K in 1 and 3 T and at 65 K in 3 T revealed that GdBCO CCs post-annealed for 5 min exhibited greatly improved pinning properties along the c-axis of the GdBCO film, which is attributable to piled-up stacking faults along the c-axis of the GdBCO matrix.

Thin Films With

We investigated the <i>ex-situ</i> conversion process of amorphous precursor films for SmBa₂Cu₃O_7-delta (SmBCO) coated conductors (CC). Amorphous precursor films were first deposited on the LaMnO₃/homoepi-MgO/ IBAD-MgO/Y₂O₃/Al₂O₃/Hastelloy substrate by e-beam co-evaporation and subsequently crystallized to the superconducting phase at various post-annealing temperatures in two different reduced oxygen pressures. In 100 ppm of O₂ in Ar, <i>c</i>-axis oriented SmBCO grains with minor a-axis oriented grains were grown at 825degC whereas polycrystalline SmBCO film with some <i>a</i> -axis oriented grains was formed at 800degC. In the temperature region of 825 ~ 850degC, protruded spots were observed on the sample surface. In 6 ppm of O₂ in Ar, SmBCO film composed of <i>a</i>-axis oriented SmBCO grains with partial c-axis oriented grains were obtained at 700degC but other phases were formed above 725degC. The SmBCO sample grown at 700degC exhibited <i>Tc</i> _,zero value of 90.2 K. However, its <i>Jc</i> value at 77 K was negligibly small due to the formation of <i>a</i>-axis oriented grains.