Soon-Mi Choi

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.

Improved dielectric properties of BaTiO3-added CaCu3Ti4O12 polycrystalline ceramics

The effects of the BaTiO3 (BTO) additive on the electrical properties of CaCu3Ti4O12 (CCTO) polycrystalline ceramics were systematically investigated. Various amounts of BTO powder up to 15 mol. % were added to CCTO powder. Each batch was ball-milled, pressed into pellets, and finally sintered at 1060°C for 12 h in air. Compared with pure CCTO sample (ɛr ∼ 52,000 and tan δ ∼ 0.38 at 100 kHz), BTO-added CCTO samples commonly showed significantly reduced dielectric losses although their dielectric constants were decreased approximately by one order of magnitude (for instance, ɛr ∼ 4,075 and tan δ ∼ 0.02 at 100 kHz for 5 mol. % BTO-added CCTO sample). In addition, the breakdown voltages of BTO-added CCTO samples were much higher than that of pure CCTO sample, and thus the leakage currents were greatly reduced at the applied voltage above ∼ 10 V. A large reduction in the dielectric losses and leakage currents is attributed to the secondary phases segregated at the CCTO grain boundary which are composed of CaTiO3, Ba4Ti12O27, and unreacted BTO.

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}

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.

GdBCO Coated Conductors Fabricated by 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}

Epitaxial growth of CeO₂ buffer layers on the ion-beam assisted deposition (IBAD)-textured MgO templates could be successfully achieved by an annealing heat treatment prior to CeO₂ deposition by pulsed laser deposition (PLD). The annealing process was performed in a tube furnace (<italic>ex-situ</italic> annealing) or in a PLD chamber (<italic>in-situ</italic> annealing). Analyses by x-ray photoelectron spectroscopy on the surface of <italic>ex-situ</italic> annealed MgO single crystals revealed that Mg(OH)₂ formed on the MgO surface in air was gradually decomposed into MgO with increasing annealing temperature and period. The CeO₂ layer on the IBAD MgO template <italic>ex-situ</italic> annealed for 6 h at 750 °C was only <italic>c</italic>-axis oriented in comparison with that on the nonannealed IBAD MgO template. However, some 45°-rotated grains were observed in the CeO₂ layer on <italic>ex-situ</italic> annealed IBAD MgO template. In contrast, <italic>in-situ</italic> annealing for 1 h at 800 °C was found effective to grow CeO₂ layer with a cube-on-cube orientation, suggesting that Mg(OH)₂ and MgCO₃ phases on the MgO surface were fully decomposed into MgO phase with (001) (a//[100]) orientation. TEM analyses supported that a high degree of biaxial alignment of CeO₂ layer was deposited on <italic>in-situ</italic> annealed IBAD MgO template.

Thin Films With

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.

\hbox{BaZrO}_{3}

Significantly enhanced low-field magnetoresistance (LFMR) and maximum dMR/dH {(dMR/dH) max } values were successfully achieved from La 0.7 Sr 0.3 MnO 3 (LSMO)-manganese oxide composite samples prepared by liquid phase sintering, compared with those of the same composites prepared by solid state reaction. For this study, pure LSMO and LSMO-manganese oxide composites with various nominal compositions of (1-x)LSMO?xMn₂O₃ (x = 0.1, 0.2, 0.3, 0.4, and 0.8) were sintered at 1450°C, above the eutectic temperature of 1430°C, for 1 h in air. The highest LFMR value of 1.28% with the highest (dMR/dH) max value of 21.1% kOe ?1 was obtained from the composite sample with x = 0.3 at 290 K in 500 Oe. This enhancement of LFMR and (dMR/dH) max values is ascribed to efficient suppression of magnetic disorder at the LSMO grain boundary, by forming a characteristic LSMO-manganese eutectic structure.

Nanoparticles Using a Ba-Deficient Coating Solution

We tried to optimize the BaSnO₃ (BSO) doping content in GdBa₂Cu₃O_7-δ (GdBCO) coated conductors (CCs) on CeO₂-buffered ion-beam assisted deposition (IBAD)-MgO templates. The BSO-doped GdBCO CCs with various x vol% ( x=0, 2, 3, 4, 5, and 6) relative to GdBCO were prepared at 800°C with the oxygen pressure of 300 mtorr by pulsed laser deposition (PLD), and their pinning properties were characterized by measuring magnetic hysteresis curves for the applied field parallel to the c-axis of GdBCO (B//c) up to 5 T and field-orientation dependence of transport critical currents for 1, 3, and 5 T at 65 and 77 K, respectively. Among all samples, a 5 vol% BSO-doped GdBCO CC exhibited the highest maximum pinning force density (F_p,max) values of 32.5 GN/m³ near 4 T at 65 K and 6.5 GN/m³ near 3.2 T at 77 K, for B//c (θ = 180 ^°), and also the highest minimum transport critical current density (Jc,min) of 0.39 MA/cm² at 77 K in 1 T and 0.68 MA/cm² at 65 K in 3 T, for θ = 120^°, evidencing that 5 vol% is the optimum BSO content. Observation of BSO-doped GdBCO CCs by transmission electron microscopy revealed that the effective flux pinning of BSO-doped GdBCO CC was attributed to the BSO nanorods of ~ 10-nm diameters roughly aligned along the c-axis of the GdBCO matrix.