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Flexible Y Ba2Cu3O7−δ-coated superconductor tapes on non-metallic substrates with spinon-glass and IBAD-YSZ buffer layers
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2005 Supercond. Sci. Technol. 18 381 (http://iopscience.iop.org/0953-2048/18/4/001) View the table of contents for this issue, or go to the journal homepage for more
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INSTITUTE OF PHYSICS PUBLISHING
SUPERCONDUCTOR SCIENCE AND TECHNOLOGY
Supercond. Sci. Technol. 18 (2005) 381–384
doi:10.1088/0953-2048/18/4/001
Flexible YBa2Cu3O7−δ -coated superconductor tapes on non-metallic substrates with spin-on-glass and IBAD-YSZ buffer layers S Gnanarajan and J Du Applied Quantum Systems Group, CSIRO Industrial Physics, PO Box 218, Lindfield NSW 2070, Australia E-mail:
[email protected]
Received 31 March 2004, in final form 10 November 2004 Published 1 February 2005 Online at stacks.iop.org/SUST/18/381 Abstract Non-metallic flexible YBa2 Cu3 O7−δ (YBCO)-coated superconductor tapes were fabricated using zirconia with 3 mol% yttria CERAFLEXTM (CF) substrates. The CF substrate surface was treated by spin-coating of spin-on-glass (SOG). Yttria-stabilized zirconia (YSZ) buffer layers were deposited on the SOG layer by ion beam assisted magnetron deposition (IBAD). The epitaxial YBCO films were grown on the buffer layer. The crystalline quality and degree of biaxial alignment were measured using the x-ray θ –2θ and φ-scans. The YBCO films were c-axis aligned and had an YBCO(103) φ-scan full width half maximum (FWHM) of 10.5◦ . Films with φ-scan FWHM of 19◦ had a zero field critical current density, Jc , of 0.25 MA cm−2 at 77 K as measured by the induction method.
1. Introduction Over the past decade, second-generation high-temperature superconductor (HTS)-coated tapes based on YBa2 Cu3 O7−δ (YBCO) films with critical current densities Jc of over 1 MA cm−2 have been demonstrated in metre lengths [1–4]. Nickel-based metal alloys, such as Hastelloy (HAS) are the substrate of choice for tapes due to their stability at temperatures in the range of 650–800 ◦ C required for the deposition of YBCO and the buffer layers. There are also some reports of depositing YBCO films and buffer layers on glass substrates [5, 6] with Jc of 0.1 MA cm−2 and ceramic SM 210 substrates [7] with Jc of 1.5 MA cm−2 . But these substrates are not flexible. However, one of the major problems encountered with the YBCO superconductors is the formation of weak links at the grain boundaries, which limit the Jc . This was overcome using biaxially aligned YBCO films with φscan FWHM of less than 10◦ , which removes the weak links. Biaxially aligned buffer layers, such as YSZ, were used as templates for the epitaxial growth of YBCO. Ion-beam assisted deposition (IBAD) is one of the techniques used to produce 0953-2048/05/040381+04$30.00 © 2005 IOP Publishing Ltd
biaxially aligned buffer layers. A highly polished substrate surface is a prerequisite for the IBAD process [1–3]. Although the flexible metal substrates can be used for most of the power applications, a flexible non-metallic substrate would be preferred for cryogenic applications in the temperature range of 20–77 K due to low-heat conduction loss [8]. The thermal conductivity of the CF (2.5 W m−1 K−1 ) is about one-fifth of that of Hastelloy (12.5 W m−1 K−1 ) at 20 ◦ C. AC loss measurements on Hastelloy-based YBCO tapes indicate that AC loss due to the eddy current in the metal substrate become dominant at higher frequencies [9]. Recently, YBCO-coated metal substrates have been used as the flux transformer of planar [10] and axial HTS firstorder gradiometers [11]. Here the pickup loop pattern was lithographically etched into the YBCO layer to form the desired structure with a standard HTS SQUID flip-chipped at the pickup loop. In this application, the metal substrates were found to increase the SQUID noise due to the formation of eddy currents in the Hastelloy tape creating a ‘self-field’ effect. Therefore, it is desired to have a non-metallic substrate to reduce the SQUID noise. To our knowledge there have been no reports of non-metallic flexible superconductor tapes.
Printed in the UK
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S Gnanarajan and J Du
Figure 1. SEM image of a polished CF substrate. The field of view is 15 µm across.
Zirconia with 3 mol% yttria, known commercially as CERAFLEXTM (CF), of 0.1 mm thickness can be bent with a 30 mm diameter and is stable up to 800 ◦ C. Polished CF materials have a surface roughness of the order of 100 nm [12], which is much higher than that of the polished thick ceramic substrates which have surface roughness of the order of 1 nm [7]. In the coated YBCO tapes with MgO as a buffer layer, silicon nitride was applied on polished Hastelloy substrate to reduce the surface roughness to the order of 4 nm [13]. In this work we have investigated the dependence of the YBCO film properties on the surface morphology of the CF substrate and developed a process using spin-on-glass (SOG) to planarize the CF surface and improve Jc by more than an order of magnitude.
2. SOG coating One-side-polished CF substrates of 200 mm × 10 mm size and 0.1 mm thickness were obtained from MarkeTech International, Inc [12]. Substrate pieces of areas 20 mm × 10 mm and 10 mm × 10 mm were coated with SOG (Accuglass AlliedSignal)1 using a standard spin coating process. The SOG is made of siloxane polymers with 10% organic content and solvents. The material becomes silica glass upon curing at high temperatures. The SOG possesses excellent gap-filling, smoothing and planarization capability, which fills narrow, high aspect ratio spaces (see footnote 1). SEM images of a typical polished surface of a CF substrate before and after the application of a SOG coating are shown in figures 1 and 2, respectively. These images show the improved planarization of the SOG-coated CF substrate surface at micrometre-length scales. The thickness of the SOG coating, which was in the range of 500–1000 nm, was adjusted and controlled by the spin speed during the application. The film was cured at 400 ◦ C for an hour and then annealed at 600 ◦ C for one hour. We have also used a 10 mm × 10 mm polished Hastelloy substrate with SOG for comparison. 1
AlliedSignal, Advanced Microelectronic Materials, 1349 Moffett Park Drive, Sunnyvale CA 94089, USA.
382
Figure 2. SEM image of an SOG-coated polished CF substrate. The field of view is 15 µm across.
3. YSZ and YBCO deposition We used an ion-beam assisted magnetron deposition system to deposit the IBAD-YSZ film [14, 15] on the substrates. The deposition system consisted of a UHV chamber fitted with a planar magnetron sputter gun to provide a physical vapour source and a 3 cm diameter Kaufman-type ion-beam source. The ion-beam source was operated with high-purity Ar gas (3 sccm) and its collimated beam was directed at an angle of 55◦ to the normal of the substrate plane to bombard the growing YSZ film with Ar + ions of energy 200–300 eV. The planar magnetron was fitted with a solid crystalline YSZ target and was operated at 200 W rf power (13.56 MHz) in a gas mixture with ratio O2 /Ar of 1/50 at a total pressure of 0.1 Pa. The substrates were initially coated with YSZ films of about 150 nm thickness without ion-beam assist and overcoated with IBAD YSZ films of about 200 nm. The substrates with the buffer layers were epitaxially coated with 20 nm CeO2 , 400 nm YBCO and protectively coated with 50 nm Au by THEVA GmbH [16]. The YBCO films were coated by co-evaporation of yttrium, barium, and copper metals from resistively heated boat sources in a high vacuum. The necessary oxygen was supplied by an oxygen shuttle, which consists of an oscillating oxygen shower with a Knudsen seal for local enhancement of the oxygen pressure. The substrate was kept at 650 ◦ C. The CeO2 film was coated by evaporation of Ce under similar conditions.
4. Crystallographic and transport measurements The crystalline structure of the YBCO films and the out-ofplane orientation were determined by x-ray diffraction θ –2θ scans. The in-plane biaxial alignment was measured by φscans using a Philips X’pert diffraction system. The critical current density, Jc , of the YBCO films was measured using an induction probe method by THEVA [17] and in-house using a standard four-point current transport technique [18]. 4.1. XRD results The θ –2θ scans of YBCO/CeO2 /YSZ/CF and YBCO/CeO2 / YSZ/SOG/CF samples are shown in figure 3. It is clear
Flexible YBCO-coated superconductor tapes on non-metallic substrates with spin-on-glass and IBAD-YSZ buffer layers
Table 1. φ-scan FWHM, Jc (77 K) using induction (I) and transport (T) and morphology of films for different substrates with and without SOG treatment. Sample
FWHM (deg)
YBCO/CeO2 /YSZ/CF YBCO/CeO2 /YSZ/SOG/CF YBCO/CeO2 /YSZ/SOG/CF YBCO/CeO2 /YSZ/HAS YBCO/CeO2 /YSZ/SOG/HAS
40 19 10.5 12 8.0
Jc (I) (MA cm−2 )
