Showing papers by "Dominic F. Lee published in 2001"

Recent progress in the fabrication of high-Jc tapes by epitaxial deposition of YBCO on RABiTS

Abstract: Progress made in the fabrication of rolling assisted biaxially textured substrates (RABiTS) and epitaxial deposition or formation of HTS on such substrates is reported. Significant progress has been made in understanding the role of meso-scale defects such as grain boundaries on long-range current flow of HTS conductors made using the RABiTS approach. Both experimental and theoretical calculations suggest that in well-textured samples these commonly present defects do not provide an intrinsic barrier to current flow in long-length conductors. Significant progress has also been made in the reel-to-reel deposition of oxide buffer layers and in the fabrication of long-length superconductors using the ex situ BaF 2 technique. Finally, non-magnetic, mechanically strengthened, biaxially textured metal templates have been fabricated with high quality oxide buffer layers. Epitaxial formation of YBCO on such substrates yields critical current densities over 1 MA/cm 2 at 77 K, 0 T.

Low cost Y-Ba-Cu-O coated conductors

Abstract: Solution-based techniques have been examined as potential low-cost processes for manufacturing YBCO coated conductors. YBCO films prepared from metal trifluoroacetate precursors have achieved performance levels equaling or exceeding that of vapor deposited films with the same thickness on CeO/sub 2//YSZ(sc) substrates. J/sub c/'s of 4.5 MA/cm/sup 2/ and 2 MA/cm/sup 2/ have been achieved in 0.4 /spl mu/m thick YBCO films on CeO/sub 2//YSZ(sc) and CeO/sub 2//YSZ/CeO/sub 2//Ni substrates, respectively. Textured Gd/sub 2/O/sub 3/ buffer layers have been deposited on deformation textured Ni substrates in a reel-to-reel process. The performance of YBCO films deposited on substrates containing the Gd/sub 2/O/sub 3/ seed layers is comparable in performance to YBCO films grown on all vacuum deposited buffer layers.

Fabrication of long lengths of YBCO coated conductors using a continuous reel-to-reel dip-coating unit

Abstract: A low-cost, non-vacuum, solution precursor route has been developed to produce epitaxial Gd/sub 2/O/sub 3/ and Eu/sub 2/O/sub 3/ buffer layers and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) superconductors on biaxially textured metal substrates. On sol-gel Eu/sub 2/O/sub 3/ seed layers with sputtered YSZ and CeO/sub 2/ top layers, a YBCO film with a J/sub c/ of over 1 MA/cm/sup 2/ at 77 K was obtained. On all solution buffer layers (CeO/sub 2//Eu/sub 2/O/sub 3//Ni), YBCO film with a J/sub c/ of 200,000 A/cm/sup 2/ at 77 K was grown using pulsed laser deposition (PLD). Meter lengths of epitaxial and crack-free Gd/sub 2/O/sub 3/ buffer layers were fabricated on cube textured Ni-W (3 at.%) substrates for the first time. High quality YBCO films were deposited on Rolling-Assisted Biaxially Textured Substrates (RABiTS) using a trifluoroacetate (TFA) precursor approach. The precursors were either spin-coated or dip-coated and decomposed in a newly developed fast 3-hour burn-out step followed by post-annealing. In a stationary burn-out route, we have produced 40 cm long crack-free YBCO TFA precursors on RABiTS. On short segments, YBCO films with a J/sub c/ of over 500,000 A/cm/sup 2/ at 77 K were grown on all PLD buffered-Ni substrates (CeO/sub 2//YSZ/CeO/sub 2//Ni).

Inter- and intragrain transport measurements in YBa2Cu3O7-x deformation textured coated conductors

Abstract: Using photolithography, links for transport measurement have been placed across individual Ni grain boundaries and within individual Ni grains on several coated conductor samples. The typical Ni grain size is ∼50 μm, while the YBa2Cu3O7−x grains are submicron in size. It is found that the intragrain Jc(0 T,77 K) can exceed 5 MA/cm2, thus showing that present coated conductor Jc values are not significantly limited by the intragrain Jc. Inter- and intragrain Jc values ranged from one-half to more than four times full-width measured values, demonstrating that current percolates through the conductor. The misorientation angle dependence of Jc fits well with previous studies of [001] tilt SrTiO3 bicrystals.

Progress in solution-based YBCO coated conductor

Abstract: Solution-based YBCO and buffer layers on deformation–textured Ni or Ni-based alloys have been developed as a low-cost coated conductor technology for high temperature superconducting wire. For short samples, J c values of up to 2.1 MA/cm 2 for 0.4 μm films were measured at 77 K, self-field. Using CeO 2 buffered YSZ single crystals, J c values of up to 5 MA/cm 2 for 0.4 μm films and 2.5 MA/cm 2 for 1.2 μm films can be obtained by such a solution-based YBCO process. Differences in the J c performance of different metal substrate samples are possibly related to a -axial grain growth.

Magneto-optical imaging of transport currents in YBa/sub 2/Cu/sub 3/O/sub 7-x/ on RABiTS/sup TM/

Abstract: Two YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) on RABiTS/sup TM/ conductors with high J/sub c/ (0 T, 77 K) values of 2.3 and 2.4 MA/cm/sup 2/ were scribed by laser cutting to constrict supercurrent flow. On one of these tracks, magneto-optical (MO) imaging of the self-field produced by a transport current directly demonstrates the percolative nature of the current flow. Using backscattered electron Kikuchi pattern (BEKP) analysis, we have found that Ni grain boundaries (GBs)>5/spl deg/ generally introduce GBs in the YBCO that have lower J/sub c/ values than the intra-grain.

Electron beam co-evaporation of Y-BaF2-Cu precursor films for YBa2Cu3O7-y coated conductors

Abstract: Y-BaF2-Cu precursor films for Y-Ba2Cu3O7-y (YBCO) coated conductors have been deposited on both uncoated nickel and Rolling Assisted Biaxially Textured Substrate (RABiTSTM) tapes by electron beam coevaporation. The water partial pressure was kept constant at 1×10-5 Torr during the deposition in order to control the oxygen content in the precursor films. Rutherford backscattering spectroscopy measurements indicate an average composition of Y:Ba:Cu of 1.05:2.10:3.0 with a standard deviation of less than 2.0%. This result was obtained for both moving and stationary tapes. The thickness variation of such precursor films is less than 2.5% over lengths of up to 1 m. Films up to 5 µm thick have been deposited on a stationary tape. An end-to-end Jc of 100 000 A cm-2 from a 1 m length of RABiTSTM sample has been obtained with the highest Jc of 774 000 A cm-2 in a short section within the tape. The present research demonstrates a possible route for industrial scale-up using electron beam coevaporation of YBCO precursors.

Continuous deposition of ex situ YBCO precursor films on rolling-assisted biaxially textured substrates by electron beam evaporation

Abstract: A reel-to-reel electron beam evaporation system has been developed to continuously deposit Y–BaF2–Cu precursor for ex situ YBa2Cu3O7−x (YBCO) processed films. The quality of a YBCO film on biaxially textured, oxide buffered, Ni tape strongly depends on the condition of the precursor deposit. For continuous precursor deposition, a uniform cation stoichiometry (i.e., Y:Ba:Cu=1:2:3) over the entire length of precursor film is essential. Rutherford backscattering spectrometry studies indicated that the partial pressure of water vapor during the deposition has a significant effect on cation deposition rates and the oxygen content in the precursor film. The oxygen content of the precursor is increased and the precursor stability upon exposure to air is improved by introduction of water vapor during precursor deposition. High quality precursor films with a thickness of 300 nm have been deposited on lengths of rolling-assisted, biaxially textured substrates in the reel-to-reel electron beam evaporation system. In this paper, long length precursor deposition processing will be discussed. Properties of some post-annealed YBCO films will also be presented.

High-Jc YBCO coatings on reel-to-reel dip-coated Gd2O3 seed buffer layers epitaxially fabricated on biaxially textured Ni and Ni-(3at%W-1.7at%Fe) alloy tapes

Abstract: A low-cost, non-vacuum reel-to reel dip-coating system has been used to continuously fabricate epitaxial Gd 2 O 3 buffer layers on mechanically strengthened, biaxially textured Ni- (3at.%W-1.7at%Fe), defined as Ni-alloy, metal tapes. X-ray diffraction analysis of the seed Gd 2 O 3 layers indicated that well textured films can be obtained at processing temperatures (T p ) between 1100 and 1175°C. Processing speed did not significantly affect the crystalline quality of the Gd 2 O 3 . Scanning electron microscopy revealed a continuous, dense and crack-free surface morphology for these dip-coated buffers. The Gd 2 O 3 layer thickness led to remarkable differences in the growth characteristics of the subsequent YSZ and CeO 2 layers deposited by rf magnetron sputtering. Epitaxial YBCO films grown by pulsed laser deposition on the short prototype CeO 2 /YSZ/Gd 2 O 3 /Ni-(3at%W-1.7at%Fe) conductors yielded self-field critical current densities (J c ) as high as 1.2×10 6 A/cm 2 at 77 K. Pure Ni tapes were used to asses the viability of dip-coated buffers for long length coated conductor fabrication. The YBCO films, grown on 80 cm long and 1 cm wide CeO 2 /YSZ/Gd 2 O 3 buffered Ni tapes by the industrially scalable ex-situ BaF 2 precursor process, exhibited end-to-end self-field J c of 6.25×10 5 A/cm 2 at 77 K.