M.S. Bhuiyan

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.

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.

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.

AC Losses in YBCO Coated Conductor With Inkjet Filaments

To achieve low ac losses in applied ac fields, YBa2Cu3Ox (YBCO) filaments were created on a RABiTS buffered substrate through solution inkjet deposition. A metal organic decomposition (MOD) solution was placed into an inkjet dispenser and filaments of widths of 100 mum and 0.8 mm were deposited on the substrate at a spacing of 50 to 100 mum. Each sample, which had a width of 1 cm and a nominal length of 4 cm, was placed in a perpendicular ac field and the ac losses were measured thermally as a function of the field strengths up to 100 mT and at frequencies between 60 Hz and 120 Hz. Samples with inkjet filaments had a high coupling loss. This coupling between filaments may extend along the entire sample length because removal of the conductor ends did not reduce the coupling loss contribution. Reduction in ac loss was observed in samples with laser-scribed filaments that were made from the same MOD solution.

Solution deposition approach to high J/sub c/ coated conductor fabrication

Great strides have been made in YBCO coated conductor fabrication using the RABiTS approach in the past few years and critical current densities (J/sub c/) of over 3 MA/cm/sup 2/ on 10 meter long tapes have been achieved. Solution deposition for buffer layer processing has the potential to reduce the process complexity and make the conductor fabrication more cost-effective. In our work, we have demonstrated that several of the standard buffer layers can be replaced by sol-gel processed lanthanum zirconium oxide (LZO) layer. A J/sub c/ of about 2 MA/cm/sup 2/ has been demonstrated on LZO films for pulsed laser deposited YBCO and J/sub c/ up to 1.5 MA/cm/sup 2/ have been demonstrated for MOD-YBCO using a sputtered CeO/sub 2/ cap layer on the sol-gel LZO films. Solution processed buffer layers have been found to have rapid growth kinetics which could potentially mean high rate processing of these buffer layers. Using simulated ex-situ YBCO annealing studies, it has been determined that the performance of 80-120 nm thick LZO films is comparable to the standard 3-layer vapor deposited CeO/sub 2//YSZ/Y/sub 2/O/sub 3/ buffer stack. Using a 120 nm thick LZO layers on NiW substrates, in collaboration with American Superconductor Corp., all-solution coated conductors with the stacking sequence MOD-YBCO/Solution CeO/sub 2//Solution LZO/NiW, critical currents of up to 140 A/cm has been measured. Such high critical currents on an all-solution conductor offers promise for cost-effective scale-up of coated conductor processing.

Growth of rare-earth niobate-based pyrochlores on textured Ni-W substrates with ionic radii dependency

Epitaxial films of rare-earth (RE) niobates, RE{sub 3}NbO{sub 7} with pyrochlore structures, were grown on biaxially textured nickel-3 at.% tungsten (Ni-W) substrates using a chemical solution deposition process. A precursor solution of 0.3--0.50 M concentration of total cations was spin coated on to short samples of Ni-W substrates, and the films were crystallized at 1050--1100 C in a gas mixture of Ar-4% H{sub 2} for 15 min. Detailed studies revealed that RE-niobates with ionic radius ratio R{sub RE}/R{sub Nb} (R = ionic radius) from 1.23 to 1.40 (i.e., Sm, Eu, Gd, Ho, Y, and Yb) grow epitaxially with the pyrochlore structure. X-ray studies showed that the films of pyrochlore RE niobate films were highly textured with cube-on-cube epitaxy. Scanning electron and atomic force microscopy investigations of RE{sub 3}NbO{sub 7} films revealed a fairly dense and smooth microstructure without cracks and porosity. The rare-earth niobate layers may be potentially used as buffer layers for YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} coated conductors.

Development of Modified MOD-TFA Approach for YBCO Film Growth

Low-cost coated-conductor fabrication methods are essential for various electric-power applications. Metal-organic-deposition (MOD) approach to grow both YBa2Cu3O7-delta (YBCO) and buffer layers on textured metal substrates is very promising towards fabrication of lower-cost second generation wires. YBCO coated conductors (CC) are being developed with high critical currents that should be sufficient for their extensive use in power applications. However, the present CC has high energy losses in ac magnetic field that are unacceptable. We have developed a modified MOD precursor route to deposit ~ 0.8 mum thick YBCO films in a single coat that requires less than one-fifth of the pyrolysis time compared to the traditional MOD approach. We have also developed a filamentization technique of CC using ink-jet printing to reduce ac losses due to applied ac fields. The preliminary results of YBCO films deposited on standard RABiTS template yielded an of 140 A/cm at 77 K and self- field. A modest reduction of ac loss was observed for the solution ink-jet printed filamentary conductor.

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.