Sukill Kang

High-Performance High-Tc Superconducting Wires

We demonstrated short segments of a superconducting wire that meets or exceeds performance requirements for many large-scale applications of high-temperature superconducting materials, especially those requiring a high supercurrent and/or a high engineering critical current density in applied magnetic fields. The performance requirements for these varied applications were met in 3-micrometer-thick YBa2Cu3O7-δ films epitaxially grown via pulsed laser ablation on rolling assisted biaxially textured substrates. Enhancements of the critical current in self-field as well as excellent retention of this current in high applied magnetic fields were achieved in the thick films via incorporation of a periodic array of extended columnar defects, composed of self-aligned nanodots of nonsuperconducting material extending through the entire thickness of the film. These columnar defects are highly effective in pinning the superconducting vortices or flux lines, thereby resulting in the substantially enhanced performance of this wire.

MOD approach for the growth of epitaxial CeO2 buffer layers on biaxially textured Ni-W substrates for YBCO coated conductors

We have grown epitaxial CeO2 buffer layers on biaxially textured Ni–W substrates for YBCO coated conductors using a newly developed metal organic decomposition (MOD) approach. Precursor solution of 0.25 M concentration was spin coated on short samples of Ni–3 at%W (Ni–W) substrates and heat-treated at 1100 °C in a gas mixture of Ar–4%H2 for 15 min. Detailed x-ray studies indicate that CeO2 films have good out-of-plane and in-plane textures with full-width-half-maximum values of 5.8° and 7.5°, respectively. High temperature in situ XRD studies show that the nucleation of CeO2 films starts at 600 °C and the growth completes within 5 min when heated at 1100 °C. SEM and AFM investigations of CeO2 films reveal a fairly dense microstructure without cracks and porosity. Highly textured YSZ barrier layers and CeO2 cap layers were deposited on MOD CeO2-buffered Ni–W substrates using rf-magnetron sputtering. Pulsed laser deposition (PLD) was used to grow YBCO films on these substrates. A critical current, Jc, of about 1.5 MA cm−2 at 77 K and self-field was obtained on YBCO (PLD)/CeO2 (sputtered)/YSZ (sputtered)/CeO2 (spin-coated)/Ni–W.

Enhancement of flux pinning and critical currents in YBa2Cu3O7−δ films by nanoscale iridium pretreatment of substrate surfaces

We have acquired positive results in a controlled study to investigate the effects of substrate surface modification on the growth-induced flux-pinning nanostructures in YBa2Cu3O7−δ (YBCO) films. Nanoscale iridium (Ir) particles were applied to single-crystal SrTiO3 substrate surfaces using dc-magnetron sputtering. Superconducting properties of YBCO films grown on the Ir-modified substrates, measured by transport and magneto-optical imaging, have shown substantial improvement in the critical current densities (Jc) at 77 K over those on untreated, control substrates. Results also show a nearly uniform enhancement of Jc over all orientations of magnetic field. Present results are found to be consistent with cross-sectional transmission electron microscopy investigations. Ultimately, the objective of this approach is to produce enhancements in the properties of coated conductors by a simple pretreatment of the substrate surface.

Chemical solution deposition of lanthanum zirconate barrier layers applied to low-cost coated-conductor fabrication

Epitaxial lanthanum zirconate (LZO) buffer layers have been grown by sol-gelprocessing on Ni–W substrates. We report on the application of these oxide films asseed and barrier layers in coated conductor fabrication as potentially simpler, lowercost coated-conductor architecture. The LZO films, about 80–100-nm thick, werefound to have dense, crack-free surfaces with high surface crystallinity. Using 0.2- mYBCO deposited by pulsed laser deposition, a critical current density of 2 MA/cm

Fabrication of high J/sub c/YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// tapes using the newly developed lanthanum manganate single buffer layers

High J/sub c/YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films have been fabricated on LaMnO/sub 3/ buffered ion-beam assisted deposition (IBAD) MgO template layers. A LaMnO/sub 3/ buffer layer is compatible with MgO surfaces and also provides a good template for growing high current density YBCO films. LaMnO/sub 3/ buffer layers were deposited using rf magnetron sputtering. On LaMnO/sub 3/-buffered-MgO [100] single crystal substrates, YBCO films with a J/sub c/ of over 4 M/cm/sup 2/ were grown using both pulsed laser deposition (PLD) and ex-situ BaF/sub 2/ process. Using PLD, J/sub c/ of 1.8 MA/cm/sup 2/ at 77 K was obtained on 200-nm-thick YBCO films grown on LaMnO/sub 3/-buffered MgO IBAD/Ni-alloy substrates. In addition, an I/sub c/ of 230 A/cm-width was obtained for 1.65-/spl mu/m-thick YBCO films grown on LaMnO/sub 3/-buffered MgO IBAD/Ni-alloy substrates using PLD at Los Alamos National Laboratory. This performance is comparable to the best single-layer results achieved on IBAD-MgO substrates.

Thickness dependence of microstructure and critical current density of Yba2Cu3O7−δ on rolling-assisted biaxially textured substrates

The change in microstructure associated with the decrease in critical current density ( J c ) of Yba 2 Cu 3 O 7−δ (YBCO) films with increasing thickness was examined. Samples of pulse laser deposited YBCO films varying in thickness from 0.19 to 3.0 μm on rolling-assisted biaxially textured substrates with an architecture of CeO 2 /YSZ/CeO 2 /Ni were prepared by tripod polishing for cross-sectional electron microscopy. More randomly oriented grains in the upper portion of the YBCO film surface were observed with increasing film thickness, resulting in less cube texture. In addition, increases in mismatch across the boundaries of the c -axis grains with increasing time during deposition, along with the development of BaCeO 3 and Y 2 BaCuO 5 phases at the YBCO/CeO 2 interface, contributed to the degradation of film properties. Surface outgrowths of the YBCO film were examined as well as the defect structures and second-phase formations within the films.

Strong enhancement of flux pinning in YBa2Cu3O7−δ multilayers with columnar defects comprised of self-assembled BaZrO3 nanodots

Multilayer structures comprising YBa2Cu3O7−δ (YBCO) films with columns of self-assembled BaZrO3 (BZO) nanodots with interlayers of CeO2 or pure YBa2Cu3O7−δ were grown on rolling-assisted biaxially textured substrates (RABiTSs) using pulsed laser deposition. A significant enhancement of the critical current density (Jc) was observed for the multilayers compared with a single layer of YBCO + BZO. Jc varies as Jc~H−α with α of 0.27 for single layer of YBCO + BZO and 0.34 for both multilayered films. Enhancement of pinning in the multilayers is attributed to the presence of columnar defects comprised of self-assembled nanodots of BZO as well as planar CuO-type stacking defects arising as a result of interfacial reactions in the multilayers.

Microstructural characterization of thick YBa 2 Cu 3 O 7−δ films on improved rolling-assisted biaxially textured substrates

The microstructural changes associated with the reduced dependence of critical current density (J c ) versus thickness of thick, epitaxial YBa 2 Cu 3 O 7 - δ (YBCO) films on rolling-assisted biaxially textured substrates (RABiTS) were investigated. Pulsed laser deposited YBCO films varying in thickness from 1.0 to 6.4 μm on RABiTS with an architecture of Ni-3 at.% W/Y 2 O 3 /yttrium-stabilized-zirconia/CeO2/YBCO were prepared for cross-sectional transmission electron microscopy studies. Dramatic improvements in physical properties and microstructural quality were observed resulting from the use of Ni-3 at.% W substrates, which provided a sharper texture over earlier Ni substrates, and replacement of CeO 2 with Y 2 O 3 as the seed layer within the buffers. The YBCO films showed exceptional orientation up to 6.4 μm thickness, with no misoriented grains or dead layers observed and only limited reaction between the YBCO and CeO 2 cap layer. The high quality of the films was also attributed in part to the formation of a tungsten oxide layer forming at the top of the Ni-3% W substrate, limiting the growth of deleterious NiO into the conductor.

Identification of a self-limiting reaction layer in Ni–3 at.% W rolling-assisted biaxially textured substrates

Analytical transmission electron microscopy was used to identify the presence of a self-limiting reaction layer developed within rolling-assisted biaxially textured Ni–3 at.% W substrates during the pulsed laser deposition of thick YBa2Cu3O7−δ (YBCO) films. Improvements in YBCO film quality and physical properties were attributed in part to the development of a NiWO4 layer at the buffer–substrate interface. The formation of NiWO4 between NiO and the Ni–3 at.% W substrate was observed to restrict the growth of NiO within the coated conductor during YBCO deposition at elevated temperatures. A 5–8 nm thick NiWO4 layer, identified through both electron diffraction and energy dispersive spectroscopy, was found to limit NiO growth to between 20 and 25 nm in thickness. The NiWO4 layer was found to have a [100] orientation relationship to the substrate normal, with multiple variants observed.

Low-Cost Approaches for Flux-Pinning Enhancements in YBCO Films Using Solution Processing

Nanoparticles of several oxides have been synthesized using reverse micelle process. Microemulsions containing n-octane as the oil phase, cetyl trimethylammonium bromide and 1-butanol as surfactants, and an aqueous solution of metal nitrates and sodium hydroxide were used as the reaction medium. The nanoparticles obtained were characterized using differential thermal analysis, X-ray diffraction, and transmission electron microscopy. The application of these particles for flux-pinning enhancements has been studied.