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.

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).

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, Jc values of up to 2.1 MA/cm2 for 0.4 μm films were measured at 77 K, self-field. Using CeO2 buffered YSZ single crystals, Jc values of up to 5 MA/cm2 for 0.4 μm films and 2.5 MA/cm2 for 1.2 μm films can be obtained by such a solution-based YBCO process. Differences in the Jc performance of different metal substrate samples are possibly related to a-axial grain growth.

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 BaF2 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/cm2 at 77 K, 0 T.

A solution process was used to grow epitaxial La2Zr2O7 (LZO) buffer layers on roll-textured Ni (100) substrates to produce YBa2Cu3O7−δ (YBCO)-coated conductors. The LZO precursor solution was prepared by an all alkoxide sol–gel route using mixed metal methoxyethoxides in 2-methoxyethanol. The partially hydrolyzed solution was either spin-coated or dip-coated onto the textured Ni substrates. The amorphous thin film was then heat treated at 1150°C under (96%)Ar/(4%)H2 atmosphere for 1 h. X-ray diffraction (XRD) of the buffer layer indicated a strong c-axis orientation on the Ni (100) substrate. The LZO (222) pole figure revealed a single cube-on-cube texture. SEM images of the LZO buffer layer showed a dense microstructure without cracks. The YBCO deposited on the sol–gel LZO-buffered Ni substrates with sputtered YSZ and CeO2 top layers had a critical current density of 480,000 A/cm2 at 77 K and self-field.

Short segments of YBa2Cu3O7-y (YBCO) coated conductors were fabricated on rolling-assisted biaxially textured substrates (RABiTS) with a layer sequence of CeO2/YSZ/Ni using an ex situ BaF2 precursor process. Pulsed laser deposition (PLD) was used to deposit both YSZ and CeO2 layers. The YBCO films were grown using e-beam coevaporated Y–BaF2–Cu precursors followed by postannealing. An overall engineering current density, JE, of 28,000 A/cm2 and critical current, Ic, of 147 A/cm width at 77 K were achieved for a 1.6-μm-thick YBCO film. This result demonstrates the possibility of using both the ex situ BaF2 precursor approach and the RABiTS process for producing long lengths of high-JE coated conductors.

It is well established that in order for many large-scale bulk applications of HTS materials to be realized the cost/performance of HTS conductors needs to be optimized. From a superconducting performance standpoint, a long, flexible, single crystal-like wire is required. From a cost and fabrication standpoint, an industrially scalable, low cost process is required. Both of these critical requirements are met by a conductor fabrication technique referred to as Rolling assisted biaxially textured substrates (RABiTS). RABiTS employs simple, scaleabre, thermomechanical processing techniques to obtain a near single crystal-like, flexible metal substrate in arbitrary lengths on which epitaxial oxide buffer layers and superconductors are then deposited. High J(c)'s approaching 3 MA/cm(2), similar to the values obtained on single crystal ceramic substrates such as SrTiO3, have been achieved on epitaxially grown YBCO films on RABiTS using thermomechanically biaxially textured Ni as the starting template. This has been made possible by modifications to the thermomechanical processing to obtain a very sharp cube texture with FWHM of the in-plane texture of 7 degrees and out-of-plane texture of 6 degrees. Orientation image micrographs of an epitaxial YBCO film on a RABiT substrate with a J(c) of 1.6 MA/cm(2) at 77K, show that most of the film is percolatively connected within 2 degrees. For many large-scale applications, high purity Ni is not acceptable due to its magnetic nature and its low mechanical strength and modulus. This had been viewed until now as a possible fundamental limitation of this technique. Successful fabrication of single orientation (100%), biaxially textured, non-magnetic and strengthened Ni-Cr and Ni-V substrates has now been accomplished. Furthermore, significant progress has been made in depositing single orientation, epitaxial oxide buffer layers on cube-textured Ni using very low cost, non-vacuum techniques such as dip-coating of sol-gel precursors. High-J(c),, epitaxial YBCO firms have been made on RABiTS by heat treatment of a precursor deposited at room temperature, thereby enabling easy scaleup of all parts of the process. Six US companies are currently working with Oak Ridge National Laboratory to scaleup conductor fabrication based on the RABiTS technology.

In order for many large-scale bulk applications of high-temperature superconducting materials to be realized, the cost/performance of the superconductors needs to be optimized. From a performance standpoint, a long, flexible, single-crystal-like wire is required; from a cost-and-fabrication standpoint, an industrially scalable, low-cost process is required. Both of these critical requirements are met by rolling-assisted biaxially textured substrates, a conductor-fabrication technique that employs simple, scalable, thermomechanical processing techniques to obtain a near-single-crystal-like, flexible metal substrate in arbitrary lengths on which epitaxial oxide buffer layers and superconductors are then deposited.

In an effort to develop alternative single buffer layer architectures for YBCO (YBa2Cu3O7-y) coated conductors, we have studied RE2O3 (RE = Y, and rare earths) as candidate materials. High-quality Y2O3, Gd2O3 and Yb2O3 buffer layers were grown epitaxially on biaxially textured Ni (100) substrates using reactive electron beam evaporation. Using thermodynamic considerations for the formation of metal oxides, we employed both reducing atmospheres and water vapour to oxidize the film in situ to form stoichiometric RE2O3. We have also prevented NiO formation at the substrate-film interface during this process. Detailed x-ray studies have shown that the Y2O3, Gd2O3 and Yb2O3 films were grown with a single epitaxial orientation. The lattice mismatch between YBCO and Gd2O3 was small as compared with that of YBCO with other rare earth oxides. SEM micrographs indicated that ~0.5 µm thick Y2O3 films on rolled-Ni substrates were dense, continuous and crack free. A high Jc of 1.8 × 106 A cm-2 at 77 K and self-field was obtained on YBCO films grown on alternative buffer layers with a layer sequence of YBCO/Yb2O3 (sputtered)/Y2O3 (e-beam)/Ni.

A reel-to-reel, electron beam evaporation system has been developed to continuously deposit epitaxial CeO/sub 2/ and other oxide buffer layers on meter-long lengths of biaxially textured Ni tapes. The deposition system includes two interconnected electron beam evaporation chambers and a chamber in which as-rolled Ni tape is in situ annealed to develop biaxial texture. An integral reel-to-reel system with tension control enables motion of the tape with little or no plastic deformation. When depositing epitaxial oxides on Ni, the formation of unfavorably oriented NiO is difficult to avoid. Oxide free, {100}<100> oriented Ni tapes are prepared by control of the partial pressures of H/sub 2/, H/sub 2/O and O/sub 2/ during Ni annealing. X-ray /spl phi/-scans have been performed as a function of length to determine the crystallographic consistency of the epitaxial CeO/sub 2/ over length. Results of SEM examinations of the CeO/sub 2/ buffer layer microstructure are presented. Results for YBCO films deposited on short segments of these buffered substrates are summarized.

In continuation of our effort to develop single buffer layer architectures for YBCO (YBa 2 Cu 3 O 7- g ) coated tape conductors, we have studied RE 2 O 3 (RE = Y, and rare earths) as candidate materials. Three types of crystal structures including the preferred cubic phase are known for the rare earth oxides. High quality simple cubic RE 2 O 3 buffer layers were grown epitaxiahy on {100} textured Ni substrates using both reactive evaporation and sol-gel processing. Detailed X-ray studies have shown that the Y 2 O 3 , Eu 2 O 3 , Gd 2 O 3 , and Yb 2 O 3 were grown with a single epitaxial orientation. SEM micrographs indicated that both e-beam and sol-gel grown films were dense, continuous and crack free. High J c YBCO films were grown on RE 2 O 3 -buffered Ni substrates with sputtered cap layers. Two new alternative buffer layer architectures were developed. A high J c of 1.8 MA/cm 2 at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (pulsed laser deposition)/Yb 2 O 3 (sputtered)/Y 2 O 3 (e-beam)/Ni. Also, a high J c of over 1 MA/cm 2 at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (ex-situ BaF 2 process)/CeO 2 (sputtered)YSZ sputtered)/RE 2 O 3 (sol-gel or e-beam)Ni. The performance of sol-gel grown buffers approached the quality of e-beam grown buffers.

A reel-to-reel, electron beam evaporation system has been developed to continuously deposit epitaxial CeO2 and other oxide buffer layers on meter long-lengths of rolled Ni tapes. The deposition system includes two interconnected electron beam evaporation chambers and a chamber in which as-rolled Ni tape is in situ annealed to develop biaxial texture. An integral reel-to-reel system with tension control enables motion of the tape with little or no plastic deformation. When depositing epitaxial oxides on Ni, the formation of unfavorably oriented NiO needs to be avoided. Oxide-free, {100}[100]-oriented Ni tapes are prepared by control of the partial pressure of H-2 during Ni annealing. In situ annealed rolled Ni tape over 70 cm long and 1 cm wide has been deposited with an epitaxial CeO2 buffer layer. Critical current density as high as 700 000 A/cm(2) at 77 K in self-field has been achieved for TiBa2Cu3O7-x (YBCO) films deposited on short segments of these buffered substrates. X-ray in-plane phi-scans have been performed as a function of length to determine the crystallographic consistency of the epitaxial CeO2 over length. Results of scanning electron microscopy (SEM) examinations of the CeO2 buffer layer microstructure are presented. (C) 1999 Published by Elsevier Science B.V. All rights reserved.