[en] A reel-to-reel furnace equipped with seven transverse flow modules has been developed to continuously process ex situ BaF₂ precursor coated conductors. Experimental results have revealed that as the precursor thickness increases, the processing parameters need to be adjusted in order to obtain high quality YBCO films with J_c' s greater than 1 MA cm^-2 at 77 K and self-field. For thin precursors, a relatively aggressive conversion environment of high P(H₂O) can be used to obtain epitaxial YBCO with an extremely small amount of undesirable random or a-axis grains. Thin 0.3 μm YBCO was converted on a 2.3 m long by 1 cm wide Ni RABiTS substrate, yielding an end-to-end I_c of 18.9 A (J_c = 0.63 MA cm^-2). 16 cm sectional measurements showed that the tape performance is uniform, with a mean I_c of 19.1 A (0.64 MA cm^-2) and a 5.4% SD. In thick precursors, however, a less aggressive environment in the form of low P(H₂O) is better suited for epitaxial YBCO nucleation during the incubation period. By using a low initial P(H₂O) with progressively higher values during later stages of conversion, an end-to-end I_c of 92 A (1.02 MA cm^-2) was obtained in 1.1 m long by 1 cm wide 0.9 μm YBCO on better-textured Ni-W RABiTS. This conductor possesses uniform I_c according to 1 cm sectional measurements with a mean I_c of 94.4 A (1.05 MA cm^-2) and only 4.3% SD. Further adjustment in the P(H₂O) schedule resulted in a 45 cm long sample with end-to-end I_c of 111.8 A (1.24 MA cm^-2), a mean I_c of 114.8 A (1.28 MA cm^-2) and 4.8% SD. The highest I_c of 138 A (1.53 MA cm^-2) was obtained in a short 0.9 μm YBCO sample on a pre-treated Ni-W RABiTS substrate

[en] 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 Ba₂Cu₃O_7-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 J_c 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

[en] 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 deg. and 7.5 deg., 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, J_c, 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

[en] Use of many Ni-based and Cu-based alloys requires deposition of a Ni overlayer prior to the deposition of the seed layer in order to obtain better epitaxy. In this work, Ni overlayers have been deposited by DC sputtering on biaxially textured NiW and Cu substrates. Films were characterized by x-ray diffraction and atomic force microscopy as a function of temperature, film thickness and deposition rate. The optimum temperature range for deposition was found to be around 400-500 deg. C, which results in relatively smooth films with a good cube texture on both substrates

[en] The nucleation of (001)-oriented yttria-stabilized zirconia (YSZ) directly on the (001) Ni surface is realized via nucleation on an oxygen-terminated nickel surface using pulsed-laser deposition. Under conditions where the nickel surface is either oxygen free or substantially covered with NiO, a mixed orientation of YSZ occurs. The epitaxial YSZ layer grown on a biaxially textured Ni(001) surface was used as a single buffer layer for a high temperature superconducting coated conductor architecture, yielding superconducting YBa₂Cu₃O₇ films with high critical current densities, J_c. This architecture eliminates the necessity for a multilayer buffer architecture, since high J_c superconducting films are achieved with no intermediate buffer layer between the (001) YSZ and the biaxially textured metal. (c) 2000 American Institute of Physics

[en] Second generation, high-temperature superconducting wires are based on buffered, metallic tape substrates of near single crystal texture. Strong alignment of adjacent grains was found to be necessary from previous work that suggested large angle, YBa₂Cu₃O_7-δ [001]-tilt boundaries reduce J_c exponentially with increasing misorientation angle (θ). We pursue the low-θ regime by evaluating single grain boundaries (GB) and biaxially aligned polycrystalline films utilizing both the rolling-assisted biaxially textured substrates and ion-beam assisted deposition coated conductor architectures. Analysis concludes that an exponential dependence on J_c is applicable for θ(greater-or-similar sign)4 degree sign , where the spacing between the periodic disordered regions along the GB become smaller than a coherence length. (c) 2000 American Institute of Physics

[en] Pulsed electron deposition (PED), an ablation-based film growth technique similar to pulsed-laser deposition, is a relatively new method for the physical vapour deposition (PVD) of thin films. This paper describes the implementation of PED in a reel-to-reel apparatus for the room-temperature deposition of fluorine-based precursors onto buffered and textured Ni-W tapes (RABiTS). These precursors have been converted into superconducting YBa₂Cu₃O_7-x films with high critical currents. The influence of the PED parameters, the background gas pressure, and the target composition on the resulting material is analysed. Ion-probe measurements are used to monitor the deposition process and aid in the determination of the optimum deposition conditions. Special emphasis is placed on issues related to throughput and reliability. The strengths of the PED approach are discussed together with a careful analysis of the open issues and limiting factors that determine whether this method can be used for the commercial fabrication of coated conductors

[en] The growth of 0.9-1.0 μm thick Y Ba₂Cu₃O_7-δ (YBCO) films on biaxially textured Ni-3 at.% W (NiW) substrates using the BaF₂ ex situ process was investigated at reduced pressures. By varying the water vapour pressure (P_H2O), Y-BaF₂-Cu-O (YBFCO) precursor films deposited by e-beam co-evaporation were converted at a reduced total pressure (P_total) of 50-55 Torr and conversion temperature (T_S) of 740 deg. C for a wet conversion time (t_W) of 60 min. Critical current density (J_C) values greater than 1 MA cm^-2 for the thick YBCO films were obtained under the condition of varying P_H2O from low pressure to 10 Torr. The transition temperatures (T_C) of the samples were over 90 K with ΔT_C = 1.8-2.5 K. Pre-heat treatment of the precursor films on CeO₂/Y SZ/Y₂O₃/Ni/NiW substrates under an O₂ atmosphere condition before the conversion resulted in smooth surfaces without large secondary phase particles embedded in the films