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A revolutionary impact on the performance of many optical systems and components can come from the integrative design of multifunctional coatings. Such coatings should be mechanically robust, and combine user-defined optical and wetting functions with scalable fabrication formulations. By taking cues from the properties of some natural biological structures, we report here the formation of low-refractive index antireflective glass films that embody omni-directional optical properties over a wide range of wavelengths, while also possessing specific wetting capabilities. The coatings comprise an interconnected network of nanoscale pores surrounded by a nanostructured silica framework. These structures result from a novel fabrication method that utilizes metastable spinodal phase separation in glass-based materials. The approach not only enables design of surface microstructures with graded-index antireflection characteristics, where the surface reflection is suppressed through optical impedance matching between interfaces, but also facilitates self-cleaning ability through modification of the surface chemistry. Based on near complete elimination of Fresnel reflections (yielding >95% transmission through a single-side coated glass) and corresponding increase in broadband transmission, the fabricated nanostructured surfaces are found to promote a general and an invaluable ∼3–7% relative increase in current output of multiple direct/indirect bandgap photovoltaic cells. Moreover, these antireflective surfaces also demonstrate superior resistance against mechanical wear and abrasion. Unlike conventional counterparts, the present antireflective coatings are essentially monolithic, enabling simultaneous realization of graded index anti-reflectivity, self-cleaning capability, and mechanical stability within the same surface. The concept represents a fundamental basis for development of advanced coated optical quality products, especially where environmental exposure is required.

Significant efforts are being devoted to the development of semiconductor thin film and nanostructured material architectures as components of solar energy harvesting and conversion devices. In particular, nanostructured assemblies with well-defined geometrical shapes have emerged as possible highly efficient and economically viable alternatives to planar junction thin film architectures , , , . However, fabrication of inorganic nanostructures generally requires complicated and multiple step processing techniques, making them less suitable for large-scale manufacturing. Hence, innovative cell architectures and materials processing schemes are essential to large-scale integration and practical viability in photovoltaic devices. Here we present here a new approach towards nanostructured thin film solar cells, by exploiting phase-separated self-assembly , . Through a single-step deposition by rf magnetron sputtering, we demonstrate growth of an epitaxial, composite film matrix formed as self-assembled, well ordered, phase segregated, and oriented p-n type interfacial nanopillars of Cu2O and TiO2. The composite films were structurally characterized to atomic resolution by a variety of analytical tools, and evaluated for preliminary optical properties using absorption measurements. We find nearly atomically distinct Cu2O-TiO2 interfaces (i.e. a p-n junction), and an absorption profile that captures a wide range of the solar spectrum extending from ultraviolet to visible wavelengths. Thismore » work opens a novel avenue for development of simple and cost-effective optically active thin film architectures, and offers promise for significantly increased photovoltaic device efficiencies using nanostructured cells that can be optimized for both incident light absorption and carrier collection.« less

In the present work, composite LaMnO3:MgO (LMO:MgO) cap buffer layers with varying MgO contents 5 vol% up to 75 vol% have been grown on homo-epi MgO/IBAD(MgO) substrates to enhance the performance of YBa2Cu3O7-x (YBCO) films. Results showed formation of phase separated MgO nanocolumns within the LMO matrix. The impact of these nanocolumns on the superconducting properties of YBCO films deposited using pulsed laser deposition (PLD) on the nanostructured layer was investigated by electrical transport measurements. Such YBCO films showed better in-field performance compared to that of YBCO films on standard LMO cap films. In particular, measurements of the field-angle dependence revealed c-axis correlated pinning for YBCO films on these composite cap layers. The present results demonstrate a practical approach to obtain high performance superconducting wires.

Technological applications of high temperature superconductors (HTS) require high critical current density, Jc, under operation at high magnetic field strengths. This requires effective flux pinning by introducing artificial defects through creative processing. In this work, we evaluated the feasibility of mixed-phase LaMnO3:MgO (LMO:MgO) films as a potential cap buffer layer for the epitaxial growth and enhanced performance of YBa2Cu3O7-δ (YBCO) films. Such composite films were sputter deposited directly on IBAD-MgO templates (with no additional homo-epitaxial MgO layer) and revealed the formation of two phase-separated, but at the same time vertically aligned, self-assembled composite nanostructures that extend throughout the entire thickness of the film. The YBCO coatings deposited on these nanostructured cap layers showed correlated c-axis pinning and improved in-field Jc performance compared to those of YBCO films fabricated on standard LMO buffers. Microstructural characterization revealed additional extended disorder in the YBCO matrix. The present results demonstrate the feasibility of novel and potentially practical approaches in the pursuit of more efficient, economical, and high performance superconducting devices.

The dependence of the critical current density (Jc) on the orientation of an applied magnetic field was studied for a prototype (Gd,Y)Ba2Cu3O7−x (GdYBCO) coated conductor fabricated by MOCVD on an IBAD-MgO template. Additional rare-earth cations (Y and Gd) and Zr were incorporated into the superconducting film to form (Y,Gd)2O3 and BaZrO3 nanoparticles extended nearly parallel to the a–b planes and to the c-axis, respectively, to enhance the flux pinning. In-field measurement of Jc was carried out with electrical current flowing either along or perpendicular to the longitudinal axis of the tape, while a maximum Lorentz force configuration was always maintained. Details in the angular dependence of Jc were related to the unique structure of the film, specifically the tilt in the GdYBCO lattice and the tilts in the extended (Y,Gd)2O3 and BaZrO3 nanoparticles. XRD and TEM were used to study the structure of the coated conductor. The effect of the misalignment between the external field H and the internal field B on the angular dependence of Jc is discussed.

Following a review of heating induced chemical and structural changes in RBa2Cu3O7 (RBCO) fluoride precursors (R = rare earth of Y) deposited by metalorganic deposition (MOD) and physical vapor deposition (PVD), a modular process comprising successive, functionally distinct, brief annealing steps (modules) is introduced. By decoupling events that otherwise occur simultaneously, the modular process provides a framework for addressing the complex kinetics associated with the temperature ramp of the ex situ conversion anneal. Modules for modifying the F concentration, porosity and microstructure, and RBCO nucleation are described.

Previously, it has been well established that in an IBAD architecture for coated conductors, (1) LaMnO3 (LMO) buffer layers are structurally and chemically compatible with an underlying homo-epitaxial MgO layer and (2) high current density YBCO films can be grown on these LMO templates. In the present work, the homo-epi MgO layer has been successfully eliminated and a LMO cap layer was grown directly on the IBAD-MgO template. The performance of the LMO/IBAD-MgO samples has been qualified by depositing 1 mum-thick YBCO coatings by pulsed laser deposition. Electrical transport measurements of YBCO films on the standard (with homo-epi MgO) and simplified (without homo-epi MgO) IBAD architectures were carried out. The angular dependence of critical current density (J c) is similar for both IBAD architectures. XRD measurements indicate good, c-axis aligned YBCO films. Transmission electron microscopy (TEM) images reveal that microstructures of YBCO/LMO/IBAD-MgO and YBCO/LMO/homo-epi MgO/IBAD-MgO templates are similar. These results demonstrate the strong potential of using LMO as a single cap layer directly on IBAD-MgO for the development of a simplified IBAD architecture.

Ex situ conversion of e-beam co-evaporated precursors was studied in an effort to fabricate high critical current YBCO films using the BaF2 process. The precursors were deposited on CeO2 buffered single crystal YSZ substrates. It was shown that an intermediate oxygenation annealing prior to the conversion modifies the precursor crystallinity and promotes c-axis epitaxial growth while randomly-oriented film formation is suppressed. With the modified precursors, a critical current density (Jc, 77 K & 0 T) of 2.1 MA/cm2 measured by SQUID magnetometry was obtained in 1.8 mum thick YBCO films. The mechanism of the pre-annealing effect was investigated by characterization of the precursors, quenched films, and fully converted films using XRD, SEM, and TEM. Cross-sectional TEM was used to study the early nucleation of YBCO film at the precursor/substrate interface. The significant effect of the precursor modification indicated that, in addition to optimizing conversion processing parameters, modifying the precursor is an effective way to achieve the desired epitaxial film structure and to obtain higher critical currents, Ic.

The growth of epitaxial MgO/TiN multilayer films on (001) Cu has been investigated. In particular, epitaxial structures were grown on (001) Cu layers that were epitaxial on (001) SrTiO3. X-ray diffraction and reflection high-energy electron diffraction indicate that the multilayer structures are epitaxial on the (001) Cu surface. The motivation is the use of crystalline MgO/TiN multilayers as a diffusion barrier to both copper and oxygen. MgO/TiN multilayers are potentially useful as diffusion barriers for Cu interconnects on semiconductors as well as for superconducting wires based on the epitaxial growth of cuprate superconductors on biaxially textured copper.

Self-assembled nanodots of CeO{sub 2} on (1 0 0) LaAlO{sub 3} substrates, generated in situ by means of a pulsed laser deposition method prior to the deposition of YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} (YBCO) films, have been used to modify the superconducting properties of resulting YBCO films. Structural characterization has indicated that CeO{sub 2} layers grow via van der Merwe three-dimensional mode and the islands eventually acquire a pancake type of structure with lateral dimension several times larger than vertical dimension. The three-dimensional growth of CeO{sub 2} islands with (1 0 0) preferred orientation is believed to be associated with its surface energy anisotropy. The magnetization versus temperature and magnetization versus field measurements and analysis have suggested that CeO{sub 2} can affect the superconducting properties of YBCO films favorably or adversely depending on the density of CeO{sub 2} nanodots on the substrate surfaces prior to the deposition of YBCO films.

Simplification of the ion-beam-assisted deposition (IBAD) buffer architecture is one of the key issues for reduced manufacturing cost of second-generation superconducting wire production. In this work, we studied various radio frequency magnetron sputter deposition conditions for epitaxial growth of LaMnO3 (LMO) layers, with varying thicknesses, directly on IBAD-MgO without homo-epitaxial MgO layers. Performance of the simplified LMO/IBAD-MgO samples was qualified by pulsed-laser-deposited 1-μm-thick YBa2Cu3O7−δ (YBCO) coatings. Detailed property characterizations revealed that though the growth temperature has a substantial effect on the texture of LMO layers, neither LMO thickness nor different sputter gas compositions had a significant effect on the performance of YBCO films. The superconducting properties of YBCO on LMO/IBAD-MgO are found to be similar to those obtained on templates having homo-epitaxial MgO layers. The present results underscore the strong potential of LMO as a single cap layer directly on IBAD-MgO for the development of a simplified IBAD architecture.

ARTICLE I NFO The formation of oxidation-resistant buffer layers on (001) oriented Cu for coated high-temperature superconducting tape applications was investigated. The approach employed Cu/Mg multilayer precursor films that were subsequently annealed to form either Mg-doped fcc Cu or intermetallic Cu2 Mg. The precursor consisted of an Mg/Cu multilayer stack with 5 each of 25 nm thick Mg and 25 nm thick Cu layers which were grown at room temperature by sputter deposition. At annealing temperature of 400 °C, formation of the intermetallic Cu2 Mg was observed. X-ray diffraction showed that the Cu2 Mg (100) oriented grains were epitaxial with respect to the underlying Cu film, possessing a cube-on-cube orientation. In order to test oxidation resistance, CeO2 films were deposited at elevated temperature on Ni/(Cu,Mg)/Cu/MgO structures. In case of the CeO2 film on Ni/Cu/MgO, significant surface roughness due to the metal oxidation is observed. In contrast, no surface roughness is observed in the SEM images for the CeO2/Ni/(Cu,Mg)/Cu/MgO structure.

The formation of epitaxial (La, Sr)TiO3 on biaxially-textured Ni–W metal alloy tape was examined as a conductive buffer layer for YBa2Cu3O7 film growth. A TiN epitaxial nucleation layer served as an effective template layer for subsequent (La, Sr)TiO3 film growth on the Ni–W tape. The TiN/(La, Sr)TiO3 multilayer was epitaxial on the Ni–W substrate as confirmed via four-circle X-ray diffraction. A high temperature superconducting YBa2Cu3O7 film was deposited epitaxially on the (La, Sr)TiO3/TiN/Ni–W tape. The YBa2Cu3O7 film exhibited a superconducting transition temperature of 86 K and critical current density of 0.42 × 106 A/cm2 at 77 K.

Raman microprobe spectroscopy and scanning electron microscopy were used to study the initial nucleation and growth of YBCO in thick precursors by the BaF2 ex situ process. For quenched films of 2 mum thickness, the data indicate a low density of c-axis nuclei near the substrate, apparently due to a reduced oxygen concentration deep inside the precursor layer. Significant non c-axis growth was also observed; the majority of this material nucleates away from the substrate. Measurement of the conversion rate by in situ XRD for films in the range 0.2-2 mum suggest a weak thickness dependence.

Epitaxial YBa2Cu3O7-delta was formed by processing of laser-ablated, fluorine-free precursor films. The depositions were conducted at room temperature in low oxygen pressure on LaAlO3(LAO) single crystal substrates. Processing was done in the same deposition chamber by heating the precursor film to reaction temperatures of 750-850degC in a reducing gas ambient, and then raising the oxygen pressure to the conversion point. Typical processing times are a few minutes, corresponding to minimum YBCO growth rates of 1 nm/s. XRD analysis shows epitaxial growth and high crystallinity, although measured Tc values are somewhat suppressed at 88 K, with resulting critical current density, Jc of about 1 MA/cm2 at 77 K, as determined by magnetic hysteresis. Properties indicate that the materials lack appropriate level of defects needed both for rapid oxygenation and flux pinning.

We have demonstrated improved critical current density, Jc, in YBa2Cu3O7-delta (YBCO) films through a controlled study of substrate surface modifications with nano-sized second-phase pre-formed CeO2 particles. Nanoparticles were applied to single crystal SrTiO3 surfaces using suspension-based techniques prior to YBCO film growth. With the introduction of CeO2 nanoparticles, YBCO films showed more robust field dependence at intermediate fields (Jc prop B-prop), where a smaller power law exponent of alpha ~ 0.3 is obtained. The self-flefd Jc(77K, Bparc) values of the YBCO films on reference and CeO2 modified substrates are 1.1 and 1.9 MA/cm2 ; and at 1 Tesla 0.1 and 0.52 MA/cm2 , respectively. Consistent with this alpha-value, angular field dependent Jc and transmission electron microscopy studies indicate the presence of c-axis aligned correlated defects in these modified samples.

US efforts to develop and deploy “second generation” (2G) high-temperature superconducting (HTS) wires that use the compound Y 1 Ba 2 Cu 3 O x (YBCO) or other rare-earth (RE) superconducting materials are described. Wires have been demonstrated in 20 to >200-m lengths with the YBCO deposited using vapor deposition or wet chemical processes in ultra-thin layers onto textured, buffered metallic templates, which forces the grains of the RE-BCO into near perfect alignment. Critical currents for these pre-commercial wires are now within striking distance of those achieved for commercial BSSCO wires. One expected advantage of 2G wire is a 5-fold decrease in the price of wire compared with first generation wires. Another advantage of 2G wire is the intrinsic behavior of YBCO in the presence of a strong magnetic field. Enhancements in flux pinning of at least a factor of two have been demonstrated for MOCVD and MOD deposited YBCO films. US progress towards meeting the challenging goals for the year 2006, including current exceeding 300 A/cm width (77 K, self-field) in 100-m lengths and engineering current density exceeding 10,000 A/cm 2 (65 K, 3-T) is reported. In addition, initial efforts toward engineering the conductor for the mechanical and electrical properties needed for strong magnetic field applications are described.

The physical and electrical characteristics of oxide buffer layers for Cu-based coated conductors are reviewed, with emphasis on the need for conductive buffers and advanced Cu-based substrates. Methods for preventing Cu oxidation are discussed because it is an anticipated problem for applications using Cu-based rolling assisted biaxially textured substrates (RABiTS).

Chemical solution processing of Gd2Zr2O7 (GZO) thin films via sol-gel and metalorganic decomposition (MOD) precursor routes have been studied on textured Ni-based tape substrates. Even though films processed by both techniques showed similar property characteristics, the MOD-derived samples developed a high degree of texture alignment at significantly lower temperatures. Both precursor chemistries resulted in exceptionally dense, pore-free, and smooth microstructures, reflected in the cross-sectional and plan-view high-resolution scanning and transmission electron microscopy studies. On the MOD GZO buffered Ni–3at.% W (Ni–W) substrates with additional CeO2/YSZ sputtered over layers, a 0.8-μm-thick YBa2Cu3O7−δ (YBCO) film, grown by an ex situ metalorganic trifluoroacetate precursor method, yielded critical current, Ic (77 K, self-field), of 100 A/cm width. Furthermore, using pulsed-laser deposited YBCO films, a zero-field superconducting critical current density, Jc (77 K), of 1 × 106 A/cm2 was demonstrated on an all-solution, simplified CeO2(MOD)/GZO(MOD)/Ni–W architecture. The present study establishes GZO buffers as a candidate material for low-cost, all-solution coated conductor fabrication.

The development of biaxially textured, second-generation, high-temperature superconducting (HTS) wires is expected to enable most large-scale applications of HTS materials, in particular electric-power applications. For many potential applications, high critical currents in applied magnetic fields are required. It is well known that columnar defects generated by irradiating high-temperature superconducting materials with heavy ions significantly enhance the in-field critical current density. Hence, for over a decade scientists world-wide have sought means to produce such columnar defects in HTS materials without the expense and complexity of ionizing radiation. Using a simple and practically scalable technique, we have succeeded in producing long, nearly continuous vortex pins along the c-axis in YBa2Cu3O7?? (YBCO), in the form of self-assembled stacks of BaZrO3 (BZO) nanodots and nanorods. The nanodots and nanorods have a diameter of ~2?3?nm and an areal density ('matching field') of 8?10?T for 2?vol.% incorporation of BaZrO3. In addition, four misfit dislocations around each nanodot or nanorod are aligned and act as extended columnar defects. YBCO films with such defects exhibit significantly enhanced pinning with less sensitivity to magnetic fields H. In particular, at intermediate field values, the current density, Jc, varies as Jc~H??, with ?~0.3 rather than the usual values 0.5?0.65. Similar results were also obtained for CaZrO3 (CZO) and YSZ incorporation in the form of nanodots and nanorods within YBCO, indicating the broad applicability of the developed process. The process could also be used to incorporate self-assembled nanodots and nanorods within matrices of other materials for different applications, such as magnetic materials.

Nanodot arrays of Y2O3 were dispersed in thin films of YBa2Cu3O7−δ (YBCO) by growing alternating layers of these two species using a pulsed laser deposition method. As a result, critical current density Jc both in applied magnetic field and self-field is enhanced by as much as an order of magnitude, along with a significant increase in the irreversibility field Hirr. High-resolution scanning transmission electron microscopy (STEM) and Z -contrast STEM show that the nanoparticles are crystalline and coherent with the YBCO matrix. Whereas in most other studies pinning has been attributed to the strain fields around the nanoparticles, in this case pinning may actually be due to the nanoparticles themselves ,s incethe delineation between the two species is very sharp and STEM reveals no discernible strain fields in the superconducting material around the nanoparticles. (Some figures in this article are in colour only in the electronic version)

We have investigated the pinning of vortices in high-Jc films of polycrystalline MgB2, by studying the dependence of current density J on electric field E using both magnetic and transport methods. Precursor films of amorphous boron, deposited on sapphire substrates, were converted to 0.6??m thick MgB2 by post-annealing in the presence of Mg vapour at 890??C for 1?h. In magnetic studies, a SQUID magnetometer was used conventionally to determine the induced current density by the Bean model. The decay of J with time t was determined unconventionally with the sample fixed in position, by monitoring the SQUID feedback voltage versus time. The logarithmic decay rate S = ?d ln(J)/d ln(t) was found to be very low in the H?T phase space away from the irreversibility line. Complementary four-probe transport studies of E(J) were analysed as a power law dependence of the form and used to obtain the corresponding creep rate S = 1/(n?1). Effective values for n approach and often significantly exceed 100. From these results, we estimate the effective energy U0 for vortex pinning, as a function of magnetizing field H.

Two ex-situ techniques including metal-organic deposition using solution from trifluoroacetates (TFA-MOD) and electron beam co-evaporation using BaF/sub 2/ are regarded as promising candidates for fabricating long-length, uniform, high critical current density (J/sub c/) YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) superconducting films for practical applications. It is generally accepted that the precursors made by these two methods and their post-deposition processing are basically similar. Typical MOD-TFA and e-beam BaF/sub 2/ precursors were made and then annealed in our low-pressure processing system. It was found that the conditions required for the conversion of these two types of precursors are quite different. The precursors were compared in terms of the differences in their composition and pre-conversion structure. The conversion behaviors were studied by structure characterization and property measurement. The effect of composition, especially the fluorine content on the conversion reaction is discussed.

Implementation of improved processing for BaF/sub 2/ ex situ YBCO coatings from e-beam evaporated precursors enables faster conversion with rates up to 12 /spl Aring//s and critical currents I/sub c/ at 77 K greater than 350 A/cm-width on a RABiTS template. Details of the faster processing are described and compared to an earlier slower process. A linear relation between I/sub c/ and YBCO layer thickness provides evidence of new opportunities to further improve I/sub c/.

Several applications of high-temperature super-conducting wire require high currents at intermediate magnetic fields B and over a range of orientations; however, such conditions are at present achievable only at low temperatures (/spl sim/30 K). The goal of this study is to determine the feasibility of higher operating temperatures for these applications by investigating temperature dependent, low- and high-field pinning properties of YBCO coated conductor samples. The YBCO films were grown on RABiTS templates by a PVD ex situ BaF/sub 2/ process. Variations in pinning properties were induced by introducing excess yttrium (Y) in the precursor and controllably increasing the growth rate. The main result is a more uniform dependence of J/sub c/ over all orientations of B, along with high irreversibility field B/sub irr/ and high critical current densities J/sub c/. Results also show that for films with various pinning properties and processed under different conditions the self-field J/sub c/ at 77 K is an effective indicator of performance in the temperatures and fields of interest.

Sharpening of the substrate texture is key to obtain critical current densities approaching single crystal values in coated conductors. In particular, great improvements in J/sub c/ are obtained by narrowing the substrate texture down to values of 3-4/spl deg/ for both phi-scan and omega-scan FWHM. The best Ni-alloy substrates used today for RABiTS show FWHM's of 6-5/spl deg/. Although the majority of buffer layers deposited on these tapes by various techniques approximately duplicate the substrate's grain alignment, some materials have been found to develop much sharper out-of-plane texture. Here we report on growth and structural characterization of TiN seed layers on various textured metal tapes. TiN seed layers deposited by PLD have consistently shown tilting of the c-axis toward the direction of the sample's surface normal. We address the extent of such tilt and discuss feasibility of alternative RABiTS architectures that use a TiN seed layer to provide very sharp out-of-plane texture and serve as an effective metal-ion diffusion barrier.

For power applications of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) coated conductors, it is necessary to electrically stabilize the conductor. An economic way to achieve this, which also benefits the engineering J/sub E/, is to grow conductive buffer layers directly on textured Cu or Ni metal surfaces. However, due to poor oxidation resistance and high reactivity/diffusivity of Cu or Ni, an insulating oxide layer usually forms at the metal/substrate interface, degrading the electrical connectivity of the entire architecture. To overcome this problem, we have developed a new conductive, nonmagnetic buffer layer architecture of La/sub 0.7/Sr/sub 0.3/MnO/sub 3//Ir on textured Ni-based tapes. This structure serves as a barrier to both inward diffusion of oxygen and outward diffusion of metal cations. Using PLD to grow YBCO, we demonstrate ideal electrical coupling to the metal substrate. Critical current (I/sub c/) values for 1 /spl mu/m thick YBCO coatings exceed 100 A/cm-width at 77 K on a Ni-W RABiTS template.

A low-cost, nonvacuum, solution precursor route has been developed to produce epitaxial oxide buffer layers of Eu2O3 or La2Zr2O7 on biaxially textured Ni (100) tapes. A reel-to-reel continuous dip-coating unit consisting of a constant-tension tape transport system attached to a controlled atmosphere furnace was fabricated. Nickel tapes were pulled through a 2-methoxyethanol solution of europium methoxyethoxide/acetate or lanthanum zirconium methoxyethoxide. The double-sided dip-coated tapes were then annealed in a preheated furnace at 1000°–1100°C with a high flow rate of Ar/H2 (4%) gas. The dip-coated buffers were dense, continuous, crack-free, and epitaxial with a single cube texture. A critical current (Jc) of >1 MA/cm2 at 77 K and self-field was obtained for YBa2Cu3O7-δ (YBCO) films with a layer sequence of YBCO (ex situ BaF2 process)/CeO2 (sputtered)/YSZ (sputtered)/Eu2O3 (dip-coated)/nickel. We have produced 1–2 m lengths of epitaxial buffer layers on textured nickel substrates using a nonvacuum process for the first time.

We have developed a simple alternative buffer layer architecture for the rolling-assisted biaxially textured substrates (RABiTS) approach. Buffer layers with both oxygen and metal diffusion barrier properties are of interest. Cube textured magnesium oxide MgO buffers were grown directly on biaxially textured Ni and Ni–W3 at.% substrates using electron beam evaporation. We have also grown epitaxial MgO layers on 2 m long textured Ni–W3 at.% tapes in a reel-to-reel e-beam evaporation. Highly textured LaMnO3 (LMO) buffers were grown on MgO-buffered Ni substrates using rf sputtering. MgO and LMO buffers have been proved to be good oxygen diffusion barriers and Ni diffusion barriers, respectively. YBCO films with a Jc of 1 × 106 A cm−2 at 77 K and self-field were grown on this newly developed architecture of LMO/MgO/Ni using pulsed laser deposition.

There is interest in probing the feasibility and possible advantages of making uniform and cost-effective long-length coated conductors through a low-pressure processing approach. The low-pressure approach, eliminating the boundary layer and gas flow considerations, consumes much less of the processing gases and offers the possibility of improved uniformity and faster growth rate of superconducting films. Here, we have fabricated Y Ba2Cu3O7?? (YBCO) epitaxial films of thickness 0.1?1.0??m on SrTiO3 single crystal substrates using ex situ post-deposition processing of co-evaporated Y, BaF2 and Cu precursors in a controlled low-pressure gas mixture of oxygen and water vapour. Partial pressures of oxygen (PO2) and water vapour (PH2O) as low as 10 and 0.1?mTorr, respectively, were used. X-ray diffraction and scanning electron microscopy inspection were conducted for structure characterization of the films. High critical current densities (Jc) of at 77?K in self-field were obtained, yielding properties comparable to those of in situ films and ex situ films processed under atmospheric pressure condition.

A novel buffer layer architecture consisting of LaMnO3/MgO/TiN is proposed as a suitable structural and chemical template for the epitaxial growth of high-Tc superconductors on Cu metal surfaces. For the first time, high Jc values are reported for YBCO films grown by laser ablation on (001) Cu single crystals, textured Cu surfaces and textured Cu?48%?Ni?1%?Al alloys, without intervening metal coatings. The Jc for single-crystal-like substrates is as high as 3.5?MA?cm?2 and values of 2?MA?cm?2 were obtained on the Cu?alloy tape substrates.

The structure and transport properties of LaTiO3+x epitaxial thin films grown on (001) SrTiO3 by pulsedlaser deposition is examined. Four-circle X-ray diffraction indicates that the films possess the defect perovskite LaTiO3 structure when deposited in vacuum, with the higher X compounds forming at moderate oxygen pressures. The crystal structure of the LaTiO3 films is tetragonal in the epitaxial films, in contrast to the orthorhombic structure observed in bulk materials. A domain structure is observed in the films, consisting of LaTiO3 oriented either with the [110] or [001] directions perpendicular to the substrate surface. Z-contrast scanning transmission electron microscopy reveals that this domain structure is not present in the first few unit cells of the film, but emerges approximately 2 – 3 nm from the SrTiO3/LaTiO3 interface. Upon increasing the oxygen pressure during growth, a shift in the lattice d-spacing parallel to the substrate surface is observed, and is consistent with the growth of the La2Ti2O7 phase. However, van der Pauw measurements show that the films with the larger d-spacing remain conductive, albeit with a resistivity that is significantly higher than that for the perovskite LaTiO3 films. The transport behavior suggests that the films grown at higher oxygen pressures are LaTiO3+x with 0.4 < x < 0.5.

The transport and structural properties of (La,Sr)TiO3 epitaxial thin films grown by pulsed-laser deposition is presented. In particular, the potential use of (La,Sr)TiO3 as a conductive buffer layer for subsequent growth of high temperature superconducting films for coated conductors is discussed. Van der Pauw measurements of film resistivity as a function oxidation conditions show that, for undoped LaTiO3 films, the resistivity increases rapidly as background oxygen pressure is increased, which is consistent with the formation of the LaTiO3þx phase. Sr doping of LaTiO3 significantly enhances the conductivity of thin film materials when synthesized under oxidizing conditions. The transport behavior for Sr-doped LaTiO3 films correlates with structural data showing no significant shift in lattice spacing as oxygen partial pressure is increased during film growth. In addition, the epitaxial growth of (La,Sr)TiO3 on biaxially textured Ni alloy tapes is demonstrated. These results suggest that (La,Sr)TiO3 is a viable candidate as a conducting buffer for superconducting film growth on biaxially textured metal tapes. � 2003 Elsevier Ltd. All rights reserved.

In this paper a novel buffer layer architecture consisting of LaMnO3/MgO/TiN is proposed as a suitable structural and chemical template for the epitaxial growth of high-transition temperature (Tc) superconductors on Cu metal surfaces. Using techniques such as high-energy electron diffraction and scanning transmission electron microscopy, we present in situ and ex situ analyses of the buffer-layer and superconductor growth with focus on structural properties of the interfaces formed. While MgO is a good barrier to oxygen diffusion, we find that MgO alone is not a suitable buffer layer due to rapid Cu diffusion. Further, growth of MgO with a single epitaxy can be hindered by the presence of impurities such as S, which form strongly bonded superstructures on the metal surface. With the addition of a TiN layer as a barrier to Cu diffusion, oxide formation is suppressed, interfaces are clean, and a single cube-on-cube epitaxy is observed. While the Cu/TiN and TiN/MgO interfaces are rough, the MgO and LaMnO3 layers planarize the material, leading to growth of smooth YBa2Cu3O7−δ (YBCO). Residual strain in the YBCO film is 0.25% or less and does not lead to apparent cracking. The superconducting properties of the samples were investigated by electrical transport and magnetization measurements. For the first time, high critical current density (Jc) values are reported for YBCO films grown on (001) single-crystal and 100‹100›?textured Cu surfaces without intervening metal coatings. Jc on single crystal-like substrates is as high as 3.5 MA/cm2. Reduced Jc of approximately 1 MA/cm2 on rolled Cu tapes is limited by damage to the tape surface during the rolling process.

A single LaMnO3 buffer layer was developed for the growth of superconducting thick YBa2Cu3O7−δ (YBCO) films on polycrystalline Ni-alloy substrates where a biaxially textured MgO layer, produced by ion-beam assisted deposition (IBAD), was used as a template. Using pulsed laser deposition, a 1.65-μm-thick YBCO film with a critical current density of 1.4 × 106 A/cm2 in self field at 75 K was achieved on sputtered LaMnO3-buffered IBAD MgO substrates. This corresponds to a critical current (Ic) of 231 A/cm-width. This result demonstrates the possibility of using both LaMnO3 buffer and IBAD MgO template for producing high current density YBCO-coated conductors.

Epitaxial YBCO films with a thickness of 0.9 μm have been deposited by ex situ conversion of a trifluoroacetate-based metal organic decomposition precursor on both RABiTS TM (NiW) and MgO-IBAD Hastelloy substrates. These are potential processes for fabricating second generation coated conductor wires. I c values, at 77 K and self-field, as high as 202 and 194 A/cm-width have been achieved on the RABiTS TM and MgO-IBAD samples, respectively.

Single buffer layer of LaMnO/sub 3/ (LMO) has been epitaxially grown on biaxially textured Ni and Ni-alloy substrates by sputter deposition. The property characterizations revealed that LMO buffers can serve both as a chemical as well as a structural buffer for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films. Epitaxial YBCO films grown on LMO buffers using pulsed laser deposition (PLD) have self-field critical current densities (J/sub c/) exceeding 1 /spl times/ 10/sup 6/ A/cm/sup 2/ at 77 K. The present results have demonstrated the strong prospect of LMO for the practical development of YBCO coated conductors.

The current carrying capabilities of RABiTS are connected to the crystalline quality of the seed buffer layer and the stability of the metal/seed layer interface. Our study shows that the epitaxial growth of commonly used seed layers on textured Ni is mediated by a sulfur superstructure present on the metal surface. Many structural defects generated during seed layer growth (secondary orientations, in-plane rotation, incomplete cube texture) can be related to the S surface concentration and superstructure coverage. More generally, our results indicate that the epitaxial deposition of several classes of oxides (fluorite, perovskite, RE/sub 2/O/sub 3/) on several {100} fcc metals depends, in addition to chemical stability and lattice match, on the existence and optimization of S superstructures on the metal surface. On these bases, we discuss issues related to the growth of different oxides on Ni, Ni-alloys, and Pd surfaces having different chemical and structural properties.

High critical current density (high-Jc) YBa2Cu3O7–δ (YBCO) films were obtained by pulsed laser ablation on biaxially textured Cu substrates. To achieve epitaxy of LaMnO3 (LMO) on Cu, thin epitaxial Ni overlayers were deposited on Cu tapes. The structure comprises the layer sequence of YBCO/LMO/Ni/Cu. For 200-nm-thick YBCO, self-field Jc values exceeding 1 × 106 A/cm2 at 77 K were achieved. Characterization of these short prototype conductors revealed good structural and morphological properties. Magnetic analysis suggested that hysteretic loss due to the ferromagnetic Ni overlayer is minimal.

A nonvacuum 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-(3 at.% W-1.7 at.% Fe), defined as Ni-alloy, metal tapes. Because of its significance as a seed layer, optimum processing conditions (postannealing speed and temperature) of Gd 2 O 3 buffer layers have been studied. Highly textured films were obtained under reducing (96% Ar + 4% H 2 ) atmosphere at temperatures between 1100° and 1150°C; postannealing 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 pronounced 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-(3 at.% W-1.7 at.% Fe) conductors yielded self-field critical current densities (J c ) as high as 1.2 x 10 6 A/cm 2 at 77 K. Pure Ni tapes were used to assess 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 values of 6.25 x 10 5 A/cm 2 at 77 K. These results demonstrate the reproducible epitaxy of solution-derived seed layers on pure Ni and Ni-alloy tapes as well as underscore the viability of solution approaches for the production of long length YBCO-based coated conductors.

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.

Low-cost, non-vacuum bulk solution technique has been developed to deposit epitaxial Gd 2 O 3 and La 2 Zr 2 O 7 (LZO) buffer layers directly onto both textured Ni and mechanically strengthened biaxially textured Ni–W(3 at.%)–Fe(1.7 at.%) (hereafter referred to as Ni–W) substrates. A reel-to-reel continuous dip-coating unit was used to produce meter lengths of highly textured, crack-free Gd 2 O 3 and LZO buffers on Ni and Ni–W tapes. Auger electron spectroscopy analysis of the textured Ni–W substrates indicated the presence of sulfur segregation to the surface, which possibly facilitated the growth of solution seed layers directly on Ni–W substrates. On dip-coated seed layers, a reel-to-reel sputtering unit was used to deposit epitaxially 200 nm thick YSZ layers followed by 10 nm thick CeO 2 to complete the RABiTS architecture. Pulsed laser deposition was used to grow YBa 2 Cu 3 O 7− δ (YBCO) films on these tapes in short lengths. YBCO films with J c values of 1.2×10 6 and 1.9×10 6 A/cm 2 at 77 K were obtained on Gd 2 O 3 and LZO buffered Ni–W substrates, respectively. YBCO films grown by ex situ BaF 2 precursor process on 80 cm long, 1 cm wide CeO 2 /YSZ/Gd 2 O 3 /Ni tapes exhibited end-to-end J c of 6.25×10 5 A/cm 2 at 77 K and self-field. This demonstrates that the solution layers provide a very good crystallographic template for the growth of high current density YBCO films, with performance approaching that of vacuum-deposited seed layers.

Sol-gel processing of La2Zr2O7 (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 (CeO2) were deposited using rf sputtering, and YBa2Cu3Ox (YBCO) films were then deposited using pulsed laser deposition. Critical current densities (Jc) of 1.9 MA/cm2 at 77 K and self-field and 0.34 MA/cm2at 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.

Superconducting HgBa 2 CaCu 2 O 6 (Hg-1212) films have been fabricated on CeO 2 -buffered YSZ single-crystal substrates using a two-step cation-exchange process. X-ray diffraction analyses show that the Hg-1212 films are highly crystalline and grow epitaxially on these CeO 2 /YSZ substrates. Although the superconducting transition temperatures ( T c ) of these films are approximately 122 K, the critical current densities ( J c ) fall far below the expected values. The reason for these low- J c values is explained by the observations of a cracked surface morphology in the superconducting layers. The likely mechanism for formation of the cracks and implications for the development of Hg-based coated conductors are discussed.

The growth mechanisms of MgB2 films obtained by different methods on various substrates are compared via a detailed cross-sectional scanning electron microscopy (SEM) study. The analyzed films include (a) samples obtained by an ex-situ post-anneal at 900 degree of e-beam evaporated boron in the presence of an Mg vapor (exhibiting bulk-like Tc0 about 38.8 K), (b) samples obtained by the same ex-situ 900 degree anneal of pulsed laser deposition (PLD)-grown Mg+B precursors (exhibiting Tc0 ~ 25 K), and (c) films obtained by a low-temperature (600 - 630 degree) in-situ anneal of PLD-grown Mg+B precursors (with Tc0 about 24 K). A significant oxygen contamination was also present in films obtained from a PLD-grown precursors. On the other hand, it is clearly observed that the films obtained by the high-temperature reaction of e-beam evaporated B with Mg vapor are formed by the nucleation of independent MgB2 grains at the film surface, indicating that this approach may not be suitable to obtain smooth and (possibly) epitaxial films., 1 table, 4 figures, 17 references, 16 pages PDF file

Power applications of high-temperature superconducting (HTS) coated conductors will require stabilization against thermal runaway. We have developed conductive buffer layers to electrically couple the HTS layer to the underlying metal substrate. The structure comprises the layer sequence of SrRuO3 (SRO) on LaNiO3 (LNO) on biaxially textured Ni substrates. We report baseline investigations of compatibility of SRO/LNO multilayer structure with processing of Yba2Cu3O7−δ (YBCO) and demonstrate biaxially textured YBCO films on conductively buffered Ni tapes. These YBCO coatings exhibit self-field Jc values as high as 1.3 × 106 A/cm2 at 77 K, and the entire structure (HTS + conductive buffers + metal substrate) shows good electrical connectivity. These results demonstrate that SRO/LNO buffer layers may provide a basis for stabilized coated conductors.

Bulk polycrystalline Tl2Ba2CaCu2Ox materials were irradiated with 0.8 GeV protons to introduce randomly oriented columnar defects. Proton fluences up to 8.7×1020 m-2 were used to create defect arrays corresponding to a `matching field' of 1.5 T. Studies were conducted on the superconductive transition temperature, the Meissner fraction, the intragrain persistent current density against the magnetic field and temperature, and the equilibrium magnetization. The magnetization was modelled using London theory with the addition of vortex-defect interactions, yielding physically reasonable parameters.

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.

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.