Biaxially-textured Substrates Research Articles

Prefabricated Metal Nanorods on Biaxially-Textured Templates on Flexible Substrates for REBCO Superconductors

As an alternative to the introduction of flux pinning centers in RE-Ba-Cu-O (REBCO, RE = rare earth) films by the self-assembly process, we have explored prefabrication of metal nanorods on biaxially-textured templates on flexible substrates followed by REBCO film deposition. This approach provides an opportunity to control nanorod features such as size, shape, density, and orientation, which are difficult to achieve using self-assembly of nanoscale defects during in-situ growth. Successful growth of various metal nanorods such as Ni and Co on biaxially-textured substrates has been accomplished using high-energy ion bombardment of polycarbonate films for template formation followed by metal electrodeposition and polycarbonate removal. Ni nanorods of length up to 2 μm, diameter as small as 10-20 nm, variable orientation and density as high as 5 × 1010/cm2have been grown successfully on biaxially textured templates on flexible metal substrates. In order to provide a suitable interface between the metal nanorods and the subsequently deposited REBCO film, a strontium titanate (SrTiO3) film was deposited to the nanorods by solution spin coating, which was also (200) textured on the substrate surface.

Solderability study of RABiTS-based YBCO coated conductors

The solderability of commercially available YBa2Cu3O7−x (YBCO) coated conductors that were made from Rolling Assisted Biaxially Textured Substrates (RABiTS)-based templates was studied. The coated conductors, also known as second-generation (2G) high temperature superconductor (HTS) wires (in the geometry of flat tapes about 4mm wide), were laminated with copper, brass, or stainless steel strips as stabilizers. To understand the factors that influence their solderability, surface profilometry and scanning electron microscopy were used to characterize the wire surfaces. The solderability of three solders, 52In48Sn, 67Bi33In, and 100In (wt.%), was evaluated using a standard test (IPC/ECA J-STD-002) and with two different commercial fluxes. It was found that the solderability varied with the solder and flux but the three different wires showed similar solderability for a fixed combination of solder and flux. Solder joints of the 2G wires were fabricated using the tools and the procedures recommended by the HTS wire manufacturer. The solder joints were made in a lap-joint geometry and with the superconducting sides of the two wires face-to-face. The electrical resistances of the solder joints were measured at 77K, and the results were analyzed to qualify the soldering materials and evaluate the soldering process. It was concluded that although the selection of soldering materials affected the resistance of a solder joint, the resistivity of the stabilizer was the dominant factor.

Strain characteristics of critical current in RABiTS/Y-Ba-Cu-O coated conductors

The strain dependence of critical current I c of a commercial RABiTS (Rolling-Assisted Biaxially Textured Substrates) YBCO coated conductor tape with Cu stabilizing layers on both surfaces has been studied by two kinds of bending apparatus and a tensile apparatus at 77 K and 293 K. According to the voltage measurements using multi-taps in the longitudinal direction, local I c degradation was also observed. In the continuous bending-tension test at 77 K, in which YBCO layer was subjected to the tensile strain, I c began to decrease rapidly beyond the strain of 0.6% after initial moderate degradation. In the continuous bending-compression tests, the I c began to decrease rapidly beyond the strain of 0.4%. The strain for the beginning of degradation of I c obtained by tensile tests at 77 K was almost the same as that by the bending-tension tests. Bending test at RT using FRP holders showed I c degradation not so different from that by the continuous bending tests at 77 K. In all tests, the I c initially degraded locally irrespective of the position in the specimens. Furthermore, I c degradation by repeated bending was also studied.

Formation of High-Quality, Epitaxial La2Zr2O7Layers on Biaxially Textured Substrates by Slot-Die Coating of Chemical Solution Precursors

Crystallization studies were performed of epitaxial La{sub 2}Zr{sub 2}O{sub 7} (LZO) films on biaxially textured Ni-3at.%W substrates having thin Y{sub 2}O{sub 3} (10 nm) seed layers. LZO films were deposited under controlled humid atmosphere using reel-to-reel slot-die coating of chemical solution precursors. Controlled crystallization under various processing conditions has revealed a broad phase space for obtaining high-quality, epitaxial LZO films without microcracks, with no degradation of crystallographic texture and with high surface crystallinity. Crack-free and strong c-axis aligned LZO films with no random orientation were obtained even at relatively low annealing temperatures of 850-950 C in flowing one atmosphere gas mixtures of Ar-4% H2 with an effective oxygen partial pressure of P(O2){approx}10{sup -22} atm. Texture and reflection high-energy electron diffraction analyses reveal that low-temperature-annealed samples have strong cube-on-cube epitaxy and high surface crystallinity, comparable to those of LZO film annealed at high temperature of 1100 C. In addition, these samples have a smoother surface morphology than films annealed at higher temperatures. Ni diffusion rate into the LZO buffer film is also expected to be significantly reduced at the lower annealing temperatures.

High in-field critical current densities in epitaxialNdBa2Cu3O7−δ films on RABiTS by pulsed laser deposition

We report the epitaxial growth and superconducting properties forNdBa2Cu3O7−δ (NdBCO) films grown on rolling-assisted, biaxially-textured substrates (RABiTS)by pulsed laser deposition. At the optimum deposition temperature of760 °C, the criticalcurrent densities, Jc, at 77 K, self-field, of NdBCO films ranging in thickness from 0.13 to0.25 µm were found tobe in the range 2.2–3.4 MA cm−2, with a lower thickness corresponding to a higherJc. Comparedto epitaxial YBa2Cu3O7−δ (YBCO) films, these NdBCO samples are found to have superior field and angular dependences ofJc. In an applied field of 1 T, NdBCO of the same thickness has more than twice theJc of YBCO films with a significantly enhancedJc peak for a fieldapplied parallel to the c-axis. These results suggest the presence of an increased density ofc-axis correlated pinning centres within NdBCO films compared to YBCO films.

Mechanical properties of pure Ni and Ni-alloy substrate materials for Y–Ba–Cu–O coated superconductors

Mechanical properties of rolling-assisted, biaxially-textured substrates (RABiTS) and substrates for ion-beam assisted deposition (IBAD) coated superconductors are measured at room temperature, 76, and 4 K. Yield strength, Young’s modulus, and the proportional limit of elasticity are determined, tabulated and compared. Results obtained are intended to serve as a database of mechanical properties of substrates having the same anneal state and texture as those incorporated in the general class of RE–Ba–Cu–O coated conductor composites (RE = rare earth). The RABiTS materials measured are pure Ni, Ni–13at.%Cr, Ni–3at.%W–2at.%Fe, Ni–10at.%Cr–2at.%W, and Ni–5at.%W. The IBAD substrate materials included Inconel 625 and Hastelloy C-276. The Ni alloys are substantially stronger and show higher strains at the proportional limit than those of pure Ni. Substrates fully coated with buffer layers, ≈1 μm of Y–Ba–Cu–O, and 3–5 μm of Ag have similar mechanical properties (at 76 K) as the substrate alone. Somewhat surprisingly, plating an additional 30–40 μm of Cu stabilizer onto high-yield-strength (690 MPa) Hastelloy coated conductors ∼100 μm thick, reduces the overall yield strength of the composite structure by only about 10–12% at 76 K and 12–14% at room temperature; this indicates that the Cu layer, despite its relatively soft nature, contributes significantly to the overall strength of even high-strength coated conductors.

High Critical Current YBa2Cu3O7−δ Thick Films on Improved Rolling‐Assisted Biaxially Textured Substrates (RABiTS™)

YBa2Cu3O7−δ (YBCO) films with thicknesses ranging from 1.0 to 6.4 μm were deposited by pulsed laser deposition on rolling‐assisted biaxially textured substrates (RABiTS). The RABiTS were of the configuration CeO2/YSZ/Y2O3/Ni–3 at.% W. As the YBCO film thickness increased, Ic continued to increase and reached ∼300 A/cm width for a 4.3 μm‐thick YBCO film. Commonly observed mechanisms for Jc decrease with increasing YBCO film thickness were not observed. Homogeneous microstructures obtained in even the thickest YBCO films, suggest that the Ic/width can still be enhanced considerably.

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.

Identification of a self-limiting reaction layer in Ni–3 at.% W rolling-assisted biaxiallytextured substrates

Analytical transmission electron microscopy was used to identify the presenceof a self-limiting reaction layer developed within rolling-assisted biaxiallytextured Ni–3 at.% W substrates during the pulsed laser deposition of thickYBa2Cu3O7−δ (YBCO) films. Improvements in YBCO film quality and physicalproperties were attributed in part to the development of aNiWO4 layer at the buffer–substrate interface. The formation ofNiWO4 between NiO and the Ni–3 at.% W substrate was observed to restrict the growth of NiO withinthe coated conductor during YBCO deposition at elevated temperatures. A 5–8 nm thickNiWO4 layer, identified through both electron diffraction and energy dispersive spectroscopy,was found to limit NiO growth to between 20 and 25 nm in thickness. TheNiWO4 layer was found to have a [100] orientation relationship to the substrate normal, withmultiple variants observed.

The RABiTS Approach: Using Rolling-Assisted Biaxially Textured Substrates for High-Performance YBCO Superconductors

AbstractThis article provides an overview of the fabrication of epitaxial, biaxially aligned buffer layers on rolling-assisted biaxially textured substrates (RABiTS) as templates for YBCO films carrying high critical current densities.The RABiTS technique uses standard thermomechanical processing to obtain long lengths of flexible, biaxially oriented substrates with smooth surfaces.The strong biaxial texture of the metal is conferred to the superconductor by the deposition of intermediate metal and/or oxide layers that serve both as a chemical and a structural buffer.Epitaxial YBCO films with critical current densities exceeding 3 106A/cm2at 77K in self-field have been grown on RABiTS using a variety of techniques and demonstrate magnetic-field-dependent critical current values that are similar to those of epitaxial films on single-crystal ceramic substrates.The RABiTS architecture most commonly used consists of a CeO2(sputtered)/YSZ (sputtered)/Y203(e-beam)/Ni-W alloy.The desired texture of the base metal has been achieved in 100 m lengths and 10cm widths.Scaleable and cost-effective techniques are also being pursued to deposit the epitaxial multilayers.The results discussed here demonstrate that this technique is a viable route for the fabrication of long lengths of high-critical-current-density wire capable of carrying high currents in magnetic fields and at temperatures accessible by cooling with relatively inexpensive liquid nitrogen (up through the 77K range).

Transverse compressive stress effect in Y-Ba-Cu-O coatings on biaxially textured Ni and Ni-W substrates

Electromechanical properties of yttrium-barium-copper-oxide (YBCO) coatings on both pure Ni and Ni-5at.%W alloy rolling-assisted, biaxially-textured substrates (RABiTS) were investigated. The effect of transverse compressive stress on transport critical-current densities (J/sub c/) was measured on samples at 76 K and self magnetic field. Transverse compressive stress can significantly degrade J/sub c/ in YBCO deposited on pure Ni RABiTS unless sufficient frictional support is provided to the sample or the substrate is given a work-hardening treatment. On the other hand, results obtained for YBCO on Ni-5at.%W alloy RABiTS show that the tolerance to transverse stress of these conductors is significantly improved. These electromechanical properties are interpreted with scanning-electron micrographs of the microstructure of the samples after electromechanical testing, as well as stress-strain characteristics measured on RABiTS substrates at 76 K. The tensile yield strength, Young's modulus, and proportional limit of elasticity of candidate RABiTS substrate materials are tabulated and compared.

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).

Optimization of buffer layers on rolled-Ni substrates for high current YBCO and Tl,Bi-1223 coated conductors using ex-situ precursor approaches

High current YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) and Tl/sub 0.78/Bi/sub 0.22/Sr/sub 1.6/Ba/sub 0.4/Ca/sub 2/Cu/sub 3/O/sub 9-y/ (Tl,Bi-1223) coated conductors were fabricated on Rolling-Assisted Biaxially Textured Substrates (RABiTS) with a layer sequence of CeO/sub 2//YSZ/CeO/sub 2//Ni. The top and bottom CeO/sub 2/ (Cerium oxide) layers were deposited epitaxially on textured-Ni [100] substrates using reactive evaporation of Ce metal. The thickness of the CeO/sub 2/ films was 200-400 /spl Aring/. The YSZ (Yttria Stabilized Zirconia) layers were also deposited epitaxially on CeO/sub 2/-buffered Ni substrates either by rf magnetron sputtering or e-beam evaporation. The thickness of the YSZ films was typically 3000-9000 /spl Aring/. The e-beam CeO/sub 2/ films were dense, crack-free and columnar. The microstructure of sputtered YSZ was dense and the e-beam deposited YSZ was porous. To understand the stability of these buffer layers, the as-grown CeO/sub 2/-buffered YSZ (both sputtered and e-beam)/CeO/sub 2//Ni substrates were annealed in a controlled oxygen furnace at various temperatures. RBS studies indicate that the YSZ sputtered films were quite stable up to 780/spl deg/C and 200-mTorr oxygen. For e-beam YSZ films, there was an indication of diffusion of oxygen through these buffers into the Ni substrate. The Tl,Bi-1223 films were grown on all e-beam buffers using pulsed laser ablation of precursor films followed by post-annealing. The YBCO films were grown on e-beam/sputtered buffers using e-beam co-evaporated Y-BaF/sub 2/-Cu precursors followed by post-annealing.

The Effects of Surface Contamination on the Biaxially Textured Substrate for YBCO Thick Film Deposition

The epitaxial growth of YBa 2 Cu 3 O 7-x (YBCO) films on biaxially textured substrates is one of the most promising technique for the fabrication of high current superconducting tapes operating at high temperature. Ni is very attractive as substrate because it easily develops a (100)[001] cubic texture. The low oxidation resistance represents the main drawback of the Ni substrate. In order to better assess the role of oxygen on the Ni substrates, a surface physics technique as Auger spectroscopy has been used. It has allowed to evaluate the amount of impurities for different Ni processing and exposure to the air. The results demonstrate that the surface contamination can be efficiently removed by RF sputtering before buffer layer deposition. This procedure allows to obtain CeO 2/Pd/Ni architecture by laser ablation with a good epitaxy both of Pd and CeO 2 films. On the contrary, when CeO 2 is directly deposited on Ni a low epitaxy is obtained. The Auger analysis confirms that the formation of (111) NiO at the Ni - CeO 2 interface hampers the epitaxial growth of the ceria film.

Epitaxial YBa 2Cu 3O 7 films on rolled-textured metals for high-temperature superconducting applications

The epitaxial growth of high temperature superconducting (HTS) films on rolled-textured metal represents a viable approach for long-length superconducting tapes. Epitaxial, 0.5 μm thick YBa 2Cu 3O 7 (YBCO) films with critical current densities, J c, greater than 1 MA cm −2 have been realized on rolled-textured (001) Ni tapes with yttria-stabilized zirconia (YSZ)/CeO 2 oxide buffer layers. This paper describes the synthesis using pulsed-laser deposition (PLD) of epitaxial oxide buffer layers on biaxially-textured metal that comprise the so-called rolling-assisted biaxially-textured substrates (RABiTS™). The properties of the buffer and YBa 2Cu 3O 7 films on rolled-textured Ni are discussed, with emphasis given to the crystallographic and microstructural properties that determine the superconducting properties of these multilayer structures.

Epitaxial Growth of YBa2CH3O7-δ Films on Rolling-Assisted Biaxially Textured Substrates (RABiTS) Produced by Pulsed Laser Ablation at Low Temperatures

AbstractEpitaxial YBa2Cu3O7-δ (YBCO) films were fabricated using laser ablation of YBCO onto rolling-assisted-biaxially-textured-substrates (RABiTS) at a temperature of 650°C. The configuration of the sample was YBCO/YSZ/CeO/Ni. The CeO2 and YSZ buffer layers were deposited using electron-beam evaporation and sputtering, respectively. Both of these techniques are considered to be industrially scalable. SEM images of the YBCO reveal a range of structures ranging from jumbled a-axis to dense c-axis morphologies depending upon the deposition conditions. X-ray θ-2θ scans show a similar c-axis dependence on the deposition conditions.

Epitaxial superconductors on rolling-assisted biaxially-textured substrates (RABiTS): a route towards high critical current density wire

Abstract Fabrication of epitaxial, high- J c , biaxially aligned YBCO thick films on rolling-assisted biaxially-textured substrates (RABiTS) is summarized. The RABiTS technique utilizes standard thermomechanical processing to obtain long lengths of flexible, biaxially oriented substrates with smooth surfaces (rms ∽50 nm). The strong biaxial texture of the metal (in-plane 6–7° FWHM) is conferred to the superconductor by deposition of intermediate metal and/or oxide layers which serve both as a chemical as well as a structural buffer. Epitaxial YBCO films grown using laser ablation on RABiTS™ have critical current densities exceeding 10 6 A cm −2 at 77 K in zero-field and have field dependences similar to epitaxial films on single crystal ceramic substrates. The texture of the base metal has been achieved in lengths over 1 m and scaleable techniques are being pursued to deposit epitaxial multilayers. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high- J c wire capable of carrying high currents in high magnetic fields and at elevated temperatures.