M. Bindi

Transport AC Losses in YBCO Coated Conductors

Modeling and design of superconducting power devices (e.g. fault current limiters), based on second generation HTS tapes, requires accurate evaluation and prediction of AC losses. Transport AC losses measurements have been performed on samples of YBCO coated conductors at 77 K, as a function of current. The results have been compared with the classical analytical model for self field AC losses calculation, taking into account the role of the magnetic substrate

Continuous deposition of thermally co-evaporated YBCO/CeO/sub 2//Ni coated conductors

By using a simple single-pass reel-to-reel system, we demonstrate a simple and robust in-situ route for the continuous deposition of 1-2 m long Coated Conductors (CC) consisting of 1 /spl mu/m thick superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films deposited by thermal co-evaporation onto 100 nm thick CeO/sub 2/ buffered cube textured Ni tapes. The buffer layer was deposited by either e-beam or thermal evaporation using respectively ceria or metallic cerium. The structural and morphological properties of the samples were investigated by X-ray /spl thetav/-2/spl thetav/ Bragg-Brentano, and pole figure diffraction, Nomarsky optical microscopy and Scanning Electron Microscopy (SEM) analysis. The 123 composition was controlled during deposition by means of an atomic absorption system. The optimized buffer layer and YBCO layer are grown at 690/spl deg/C in respectively forming gas and oxygen partial pressures of 5/spl times/10/sup -5/ mbar. These conditions ensure the growth of dense and crack-free layers with good biaxial texture, as indicated by FWHMs values in the 5-7/spl deg/ range for both in- and out-of-plane orientations. Zero-resistance critical temperatures T/sub c/=86-87 K with transition widths /spl Delta/T/sub c/<3 K are reproducibly obtained. End-to-end critical current densities J/sub c/ at 77 K and self-field typically are in the 1.5-2.5 MA/cm/sup 2/ range, as measured by transport measurements. It is found that tape motion does not significantly affect these properties.

Progress on Single Buffer Layered Coated Conductors Prepared by Thermal Evaporation

Coated conductors with a single CeO2 buffer layer architecture have been developed in recent years by various research groups owing to the simplicity and cost effectiveness. The main challenge of this architecture is the modest thickness (~100 nm) of crack-free CeO2 layers. In this work, we report on the successful deposition of thicker crack-free single CeO2 buffer layers on biaxially textured Ni-5 at%W substrates by e-beam evaporation thanks to the use of dopants, such as Sm, Yb or Zr, introduced in pure CeO2 targets. We have minimized the mechanical stress at the CeO2/substrate interface, that is responsible for the cracking, by optimizing the dopant concentration. X-Ray diffraction rocking curve analyses show that such doped Ceria layers have a strong out-of-plane c-axis orientation with FWHM values of 5deg. Further analysis by scanning electron microscopy shows dense and crack-free layers. These results indicate no sizeable degradation of the high epitaxial quality of the layers induced by the doping. We finally report on the deposition of high-quality superconducting YBCO films on such improved single buffer layer structures using thermal co-evaporation assisted by a novel oxygenation method based on a supersonic oxygen gas beam.

Superconducting and structural properties of YBCO/CeO2/NiCr14 tapes prepared by thermal co-evaporation☆

Abstract We report on the in situ growth and characterisation of c -axis oriented superconducting YBa 2 Cu 3 O 7− δ /CeO 2 structures deposited on biaxially textured Ni–Cr alloy tapes by respectively thermal and e-beam evaporation. We have studied the effect of growth temperature on the degree of texturing of both CeO 2 and YBCO layers. We find that the highest degree of texture of both layers and best superconducting properties with sharp transitions and zero resistance T c in excess of 85 K are obtained for substrate temperatures during deposition above 700 °C.

Progress in the Continuous Depostition of YBCO Coated Conductors by Thermal Co-Evaporation

We report on recent progress towards the continuous deposition of YBCO Coated Conductors (CC) by thermal co-evaporation. This is an attractive vacuum technique thanks to the simplicity, low cost and intrinsic uniformity over large areas. Recently, we published the in situ preparation route of 1 μm thick superconducting YBCO films deposited onto CeO2 buffered Ni biaxially textured tapes using a reel-to-reel system; end-to-end critical current densities Jcs at 77 K and self-field, measured by transport measurements are in the 1-2 MA/cm2 range for 1 m. long samples, with zero-resistance Tc= 87 K and transition widths DTc<3 K. In spite of the very good CC’s performances reported by a number of laboratories all over the world, several steps must be optimized in order to limit the CC production costs, in particular concerning the complexity of the CC architecture and the choice and optimization of the YBCO deposition technique. We specifically address the following critical points regarding: 1) the in situ oxidation of the YBCO layer using a novel supersonic nozzle, 2) the deposition by thermal or e-beam evaporation of thick crack-free CeO2 buffer layers and 3) the scaling-up of the deposition process using a new multichamber system.

In Situ Oxidation of Superconducting YBCO Films by a Supersonic

We report on the successful implementation of a supersonic molecular oxygen (O2) beam for the in situ oxidation of superconducting YBCO films. The beam is produced by a specially designed array of conical nozzles that were laser machined in a platinum foil. The array was mounted in proximity of the deposition area of a thermal co-evaporation system for YBCO film deposition. After a brief description of the basic concepts underlying the physics of supersonic beams, we describe the design of the nozzle implemented in our system. Then, we illustrate and discuss the preliminary results obtained by varying a number of key parameters of the supersonic beam. Most important parameters turn out to be the input O2 pressure and the nozzle-film distance. We show that excellent electrical properties with zero-resistance superconducting critical temperatures, Tc, in excess of 90 K are reproducibly obtained over the entire 20 cm X 20 cm deposition area. These results were obtained using various types of substrates relevant to coated conductor fabrication, including Zr-doped CeO2 buffered cube-textured Ni-W and MgO buffered hastelloy substrates.

{\rm O}_{2}

We recently reported on a simple thermal co-evaporation route based on a reel-toreel system, that enables the production of 2 m-long superconducting tapes with Ic values up to 120 A/cm-width (Jc = 2 MA/cm2) and Tc = 88 K. This work describes the development of a new multi-chamber system designed for the continuous production of these tapes suitable for an industrial process. The system consists in three distinct vacuum chambers connected one with another by two specially designed 50 cm long slits. The length and cross-section of these slits have been calibrated in order to achieve a pressure difference up to 5 orders on magnitude across adjacent chamber. The system is originally conceived for the continuous production of CeO2 buffered RABiTS Ni-based tapes that require a pre-treatment of the bare metallic tapes in forming gas, followed by CeO2 deposition and a post-treatment in oxygen. To improve the robustness of the whole tape production process, we have designed and built a novel device based on a supersonic oxygen gas expansion for the continuous in situ oxygenation of YBCO during deposition. Here, we present preliminary results demonstrating the effectiveness of this device. Specifically, thanks to the enhancement of the number of collisions of the O2 molecules with the substrate due to the focused supersonic beam, we find that the oxygen pressure in the vicinity of the substrate can be up to 3 orders of magnitude higher than the background pressure in the chamber. The main advantage of this supersonic device is the insensitivity of the effective pressure to the substrate-nozzle distance, in the 2-5 mm range, which can be easily controlled during a continuous production process.

Beam

The challenge for the commercialization of YBCO Coated Conductor s (CC) is the development of a low cost manufacturing process to allow for a cheap, fast and continuous deposition of superconducting coatings with high electrical performance. We are currently investigating 2 ways to reduce the CC production costs: i) reducing the co mplexity of the CC architecture, by growing a single buffer layer based on doped CeO2, and ii) utilizing a new reel-to-reel apparatus for long length CC processing, equipped with a cheap and reliable deposition system (PED, Pulsed Electron Deposition). In this work we report on the successful continuous deposition of very thick (up to 700 nm) doped-CeO2 single buffer layers on biaxially textured Ni-5at%W substrates by PED. XRD patterns display complete orientation and very good texture quality of our samples (FWHM out-of-plane values of ≈ 6°), over 20 cm length. Optical and electron microscopy show a dense and crack-free film surface and dielectric strength measurement confirms excellent insulating properties. Preliminary results indicate that the simplified single buffer layer structure could be a reliable solution for t he reduction of HTS CC production costs.

Multi-chamber deposition system for continuous production of YBCO coated conductors by thermal co-evaporation

A market penetration of HTS-based applications requires the development of a low-cost continuous production system for long HTS conductors. We report on deposition and characterization of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///CeO/sub 2/ structures on metallic substrates prepared by a thermal co-evaporation technique. X-ray diffraction analysis reveals good biaxial orientation of both layers with inand out-of-plane FWHM = 8/spl deg/ and 9/spl deg/ respectively on biaxially textured Ni-Fe or Ni-W and on polycrystalline Hastelloy C276 substrates. YBCO films are metallic and superconducting with T/sub c,onset/ /spl sim/ 88 K. Work is in progress to scale up the process for long tapes.