B. W. Kang

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.

Comparative Study of Thickness Dependence of Critical Current Density of Yba 2 Cu 3 O 7–δ on (100) SrTiO 3 and on Rolling-assisted Biaxially Textured Substrates

We investigated the dependence of critical current density ( J c ) on thickness of Yba 2 Cu 3 O 7−δ (YBCO) films grown by pulsed laser deposition on (100) SrTiO 3 (STO) and on rolling-assisted biaxially textured substrates (RABiTS). The thickness of YBCO films varied from 0.19 to 3 μm. The highest J c s of 5.3 and 2.6 MA/cm 2 at 77 K, self-field were obtained for 0.19-μm YBCO films on STO and RABiTS, respectively. J c was found to decrease exponentially with YBCO thickness on both substrates. However, the results suggest different mechanisms are responsible for the J c reduction in the two cases. On STO, growth of a -axis grains within c -axis films and broadening of the in-plane texture were observed in thick films. On RABiTS, degradation in cube texture as well as development of a porous surface morphology were found to correlate with film thickness.

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

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).

Chemical Solution Deposition of Lanthanum Zirconate Buffer Layers on Biaxially Textured Ni–1.7% Fe–3% W Alloy Substrates for Coated-conductor Fabrication

Sol-gel processing of La 2 Zr 2 O 7 (LZO) buffer layers on biaxially textured Ni-1.7% Fe-3% W alloy substrates using a continuous reel-to-reel dip-coating unit has been studied. The epitaxial LZO films obtained have a strong cube texture and uniform microstructure. The effects of increasing the annealing speed on the texture, microstructure, and carbon content retained in the film were studied. On top of the LZO films, epitaxial layers of yttria-stabilized zirconia and Ceria (CeO 2 ) were deposited using rf sputtering, and YBa 2 Cu 3 O x (YBCO) films were then deposited using pulsed laser deposition. Critical current densities (J c ) of 1.9 MA/cm 2 at 77 K and self-field and 0.34 MA/cm 2 at 77 K and 0.5 T have been obtained on these films. These values are comparable to those obtained on YBCO films deposited on all-vacuum deposited buffer layers and the highest ever obtained using solution seed layers.

La0.7Sr0.3MnO3: A single, conductive-oxide buffer layer for the development of YBa2Cu3O7−δ coated conductors

Coated conductor applications in power technologies require stabilization of the high-temperature superconducting (HTS) layers against thermal runaway. Conductive La0.7Sr0.3MnO3 (LSMO) has been epitaxially grown on biaxially textured Ni substrates as a single buffer layer. The subsequent epitaxial growth of YBa2Cu3O7−δ (YBCO) coatings by pulsed laser deposition yielded self-field critical current densities (Jc) of 0.5×106 A/cm2 at 77 K, and provided good electrical connectivity over the entire structure (HTS+conductive-buffer+metal substrate). Property characterizations of YBCO/LSMO/Ni architecture revealed excellent crystallographic and morphological properties. These results have demonstrated that LSMO, used as a single, conductive buffer layer, may offer potential for use in fully stabilized YBCO coated conductors.

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.

Quantification and control of the sulfur c (2 × 2) superstructure on {100}〈100〉 Ni for optimization of YSZ, CeO 2 , and SrTiO 3 seed layer texture

We investigated the influence of a chemisorbed S template with c(2 × 2) structure on the epitaxial growth of different oxide buffer layers on {100}〈100〉 Ni. The sulfur superstructure spontaneously forms on the Ni surface during the texturing anneal as a consequence of segregation. However, depending on the initial S bulk concentration and/or specific annealing conditions, the S layer can cover less than the entire substrate’s surface. We show that an incomplete c(2 × 2) coverage causes degradation of the seed buffer layer texture as compared to the substrate texture. A simple step consisting of an H2S predeposition anneal can be used to control the superstructure coverage and optimize the seed layer texture.

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

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.

An all-sputtered buffer layer architecture for high-Jc YBa2Cu3O7-δ coated conductors

Abstract We report the development of an all-sputter-deposited buffer multilayer architecture for the growth of high critical current density ( J c ) YBa 2 Cu 3 O 7− δ (YBCO) films on metallic tapes. The structure comprises the layer sequence of CeO 2 /YSZ/LNO (YSZ: yttria stabilized zirconia, LNO: LaNiO 3 ) on biaxially textured Ni substrates. X-ray diffraction analysis of the base LaNiO 3 (LNO) layers indicated that well textured films can be obtained at deposition temperatures ( T dep ) between 450°C and 500°C, while higher T dep yielded polycrystalline films. The degree of LNO crystallinity was also influenced by the oxygen partial pressure during the growth. Scanning electron microscopy observations revealed a smooth, dense and crack-free surface morphology, independent of film thickness in the range of 50–300 nm. Subsequent YSZ and CeO 2 films were deposited to complete the buffer-layer structure. Epitaxial YBCO films grown by pulsed laser ablation on the CeO 2 /YSZ/LNO/Ni multilayer structure have self-field J c values as high as 1.3×10 6 A/cm 2 at 77 K. Effects of the LNO base-layer thickness on the quality of buffers and on the superconducting properties of YBCO films showed no remarkable differences. The present results demonstrate a viable approach for the production of long length YBCO superconducting wires.

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.