Development of coated conductors by inclined substrate deposition

doi:10.1016/S0921-4534(03)00790-1

Physica C: Superconductivity

Volumes 392–396, Part 2, October 2003, Pages 806-814

https://doi.org/10.1016/S0921-4534(03)00790-1 Get rights and content

Abstract

Inclined substrate deposition (ISD) offers the potential for rapid production of high-quality biaxially textured buffer layers suitable for YBa₂Cu₃O_7−δ (YBCO)-coated conductors. We have grown biaxially textured magnesium oxide (MgO) films on Hastelloy C276 (HC) substrates by ISD at deposition rates of 20–100 Å/s. Scanning electron microscopy of the ISD MgO films showed columnar grain structures with a roof-tile-shaped surface. X-ray pole figure analysis revealed that the c-axis of the ISD MgO films is titled at an angle ≈32° from the substrate normal. A small full-width at half maximum of ≈9° was observed for the φ-scan of MgO films. YBCO films were grown on ISD MgO buffered HC substrates by pulsed laser deposition and were determined to be biaxially aligned with the c-axis parallel to the substrate normal. The orientation relationship between the ISD template and the top YBCO film was investigated by X-ray pole figure analysis and transmission electron microscopy. A transport critical current density of J_c=5.5×10⁵ A/cm² at 77 K in self-field was measured on a YBCO film that was 0.46-μm thick, 4-mm wide, 10-mm long.

Introduction

YBa₂Cu₃O_7−δ (YBCO) thin-film superconductors and coated conductor wires have many applications, including high-power transmission cables, high-field magnets, generators, fault-current limiters, magnetic shields, and large-scale microwave devices [1], [2], [3], [4]. YBCO can readily be deposited on single-crystal substrates to form biaxially textured thin films that carry high critical current density (J_c). However, for coated conductor applications, YBCO films must be deposited onto flexible polycrystalline metallic substrates and also be able to carry high J_c. Biaxially textured template films are needed for successful deposition of textured YBCO films to overcome weak links at grain boundaries and, therefore, to achieve high J_c in the YBCO films fabricated on these polycrystalline metallic substrates [5], [6], [7]. Significant efforts have been made in the past few years to accelerate processing, fabrication, and manufacturing of high-temperature superconducting coated conductors to meet the needs of the US electric power industry. Several fabrication techniques, including ion-beam-assisted deposition (IBAD) [8], [9], [10], [11], rolling-assisted biaxially textured substrates [4], [12], and inclined-substrate deposition (ISD) [13], [14], [15], were developed. Compared to the first two processes, ISD magnesium oxide (MgO) produces textured template films at high deposition rates (20–100 Å/s) and is independent of the recrystallization properties of the metallic substrates. It is also simpler and easier to accomplish; no assisting ion source or complicated heat treatment required.

We grew biaxially textured MgO thin films on mechanically polished Hastelloy C276 (HC) substrates by ISD with an electron beam (e-beam) evaporation system. Yttria-stabilized zirconia (YSZ) buffer layers, ceria cap layers, and YBCO films were subsequently deposited on ISD-MgO-buffered metallic substrates by pulsed laser deposition (PLD). Surface morphology was investigated by scanning electron microscopy (SEM), and surface roughness was measured by atomic force microscopy (AFM). The crystalline orientation of the films was studied by transmission electron microscopy (TEM). X-ray pole figures, as well as φ- and ω-scans, were used to analyze texture. In this paper, we discuss the growth mechanism, microstructure, and dependence of the biaxial alignment of ISD MgO thin films on film thickness; we also report the orientation relationship and superconducting properties of YBCO fabricated using ISD MgO architecture on polished HC substrates.

Section snippets

Experimental procedure

HC coupons (≈5 mm wide and 10 mm long) were mechanically polished to a mirror finish with 0.25-μm diamond paste for use as substrates. A surface roughness of ≈3 nm was measured by AFM. A schematic illustration of the experimental setup is given in Fig. 1. MgO thin films were grown from an MgO source by e-beam evaporation. Fused lumps of MgO (Alfa Aesar, 99.95% metals basis, 3–12 mm pieces) were used as the target material. The substrates were mounted on a tiltable sample stage above the e-beam

Results and discussion

Plan-view SEM revealed (Fig. 3a) a roof-tile structure for the ISD MgO film deposited at room temperature with α=55°. Columnar grains nearly perpendicular to the substrate surface were observed on the cross-sectional fracture surface (Fig. 3b). The MgO grain size increased when the film grew for the first 0.5-μm thickness; it then became stabilized at ≈0.2 μm, without noticeable change in size when the film grew thicker. To reduce surface roughness, an additional thin layer of MgO was deposited

Conclusions

Biaxially textured MgO films were successfully grown by the ISD method, which is much more time efficient for fabrication of buffer layers than is the IBAD YSZ process. MgO films grown by the ISD process contained columnar grains that were terminated at the surface by (0 0 2) planes. Plan-view SEM revealed a roof-tile structure. The surface roughness and biaxial texture of the ISD MgO thin films were significantly improved by deposition of an additional thin layer of MgO at elevated temperature.

Acknowledgements

SEM/TEM analysis was performed in the Electron Microscopy Center for Materials Research at Argonne National Laboratory. This work was supported by the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, as part of a DOE program to develop electric power technology, under Contract W-31-109-Eng-38.

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^☆: Work is supported by the US Department of Energy, Energy Efficiency and Renewable Energy, as part of a program to develop electric power technology, under Contract W-31-109-Eng-38.

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Development of coated conductors by inclined substrate deposition☆

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgements

Physica C

Physica C

Physica C

Physica C

Appl. Supercond.

Physica C

Supercond. Sci. Technol.

Science