Keith J. Leonard

Reverse micellar synthesis of cerium oxide nanoparticles

Cerium oxide, CeO2, nanoparticles were prepared using reverse micellar synthesis, using cerium nitrate as a starting material, sodium hydroxide as a precipitating agent, n-octane as the oil phase, cetyl trimethyl ammonium bromide (CTAB) as the surfactant, and 1-butanol as the co-surfactant. Using x-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), the average size of the nanoparticles obtained was found to be around 3.7 nm, and the particles had a well defined polyhedral shape. The particles also showed strong UV absorption and room temperature photoluminescence. The photoluminescence peak was sensitive to the particle concentration and showed a blue-shift upon dilution.

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

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 Ics 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.

Enhanced flux pinning and critical currents in YBa2Cu3O7−δ films by nanoparticle surface decoration: Extension to coated conductor templates

Interfacial engineering via nanoparticle substrate surface decoration has been extended to coated conductor templates. Preformed BaTiO3 and BaZrO3 nanoparticles were applied to substrate surfaces, prior to YBa2Cu3O7−δ (YBCO) deposition, by using a scalable and inexpensive technique of solution-based suspension. Compared to untreated reference samples, nanodecoration yields improved in-field critical current density (Jc) as well as strong correlated pinning along the c-axis direction of the YBCO film. Accordingly, a much reduced falloff of Jc with magnetic field strength was observed in all of the modified samples. In addition, scaling behavior of the normalized volume pinning force density (Fp) with respect to temperature provided insight as to the differences in flux pinning mechanisms dependent on the decoration technique. Finally, with these results our earlier proof-of-concept demonstrations on nanoparticle modified single crystal substrates were replicated on technological substrates, pointing to the...

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.

Assessment of chemical solution synthesis and properties of Gd2Zr2O7 thin films as buffer layers for second-generation high-temperature superconductor wires

Abstract : Chemical solution processing of Gd2Zr2O7 (GZO) thin films via sol-gel and metalorganic decomposition (MOD) precursor routes have been studied on textured Ni-based tape substrates. Even though films processed by both techniques showed similar property characteristics, the MOD-derived samples developed a high degree of texture alignment at significantly lower temperatures. Both precursor chemistries resulted in exceptionally dense, pore-free, and smooth microstructures, reflected in the cross-sectional and plan-view high-resolution scanning and transmission electron microscopy studies. On the MOD GZO buffered Ni-3at.% W (Ni-W) substrates with additional CeO2/YSZ sputtered over layers, a 0.8-micron-thick YBa2Cu3O 7-delta (YBCO) film, grown by an ex situ metalorganic trifluoroacetate precursor method, yielded critical current, Ic (77 K, self-field), of 100 A/cm width. Furthermore, using pulsed-laser deposited YBCO films, a zero-field superconducting critical current density, Jc (77 K), of 1 x 10(exp 6) A/sq cm was demonstrated on an all-solution, simplified CeO2(MOD)/GZO(MOD)/Ni-W architecture. The present study establishes GZO buffers as a candidate material for low-cost, all-solution coated conductor fabrication.

Reel-to-reel x-ray diffraction and Raman microscopy analysis of differentially heat-treated Y–BaF2–Cu precursor films on metre-length RABiTS

Reel-to-reel x-ray diffraction (XRD) and Raman micro-spectroscopy are being evaluated as potential diagnostic tools for on-line feedback in the manufacturing of long-length coated conductors. To facilitate this evaluation, a procedure based on differentially heat-treated Y–BaF2–Cu precursors exposed to time-synchronized phase composition gradients has been developed. Two time-gradient-processed Y Ba2Cu3O7−x (YBCO) tapes of different thicknesses were fabricated using this procedure. The two techniques (used in combination) provided detailed phase and microstructure information as a function of temperature and heat treatment time and identified the same optimum processing time domain windows. More importantly, these deduced optimum times were found to be in close agreement with transport Jc measurements on replicate tapes. In addition, Raman data provided unambiguous identification of key intermediate phases such as BaF2, CuO, Y2Cu2O5, and barium cuprates. Using these results, a hypothetical Y–BaF2–Cu to YBCO reaction mechanism is proposed.

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.

An approach for electrical self-stabilization of high-temperature superconducting wires for power applications

Electrical and thermal stability of high-temperature superconducting (HTS) wires∕tapes are essential in applications involving efficient production, distribution, and storage of electrical energy. We have developed a conductive buffer layer structure composed of bilayer La0.7Sr0.3MnO3∕Ir on a textured Ni–W alloy metal tape to functionally shunt the HTS layer to the underlying substrate. The key feature is the Ir layer, which serves as a barrier to both inward diffusion of oxygen and outward diffusion of metal substrate cations during fabrication. Electrical and microstructural property characterizations of YBa2Cu3O7-δ films on short prototype conductors demonstrate self-field critical current density values, Jc, exceeding 2×106A∕cm2 at 77K and excellent electrical coupling to the underlying metal substrate, with no unwanted insulating oxide interfaces. Implementing this approach in power technologies would significantly increase the engineering current density of the conductor and reduce overall process c...