Fabrication and characterization of La2Zr2O7 films on different buffer architectures for YBa2Cu3O7−δ coated conductors by RF magnetron sputtering
© Xu et al.; licensee Springer. 2013
Received: 17 December 2012
Accepted: 17 February 2013
Published: 27 February 2013
La2Zr2O7 (LZO) films were grown on different buffer architectures by radio frequency magnetron sputtering for the large-scale application of YBa2Cu3O7−x (YBCO)-coated conductors. The three different buffer architectures were cerium oxide (CeO2), yttria-stabilized zirconia (YSZ)/CeO2, and CeO2/YSZ/CeO2. The microstructure and surface morphology of the LZO film were studied by X-ray diffraction, optical microscopy, field emission scanning electron microscopy, and atomic force microscopy. The LZO films prepared on the CeO2, YSZ/CeO2, and CeO2/YSZ/CeO2 buffer architectures were preferentially c-axis-oriented and highly textured. The in-plane texture of LZO film on CeO2 single-buffer architecture was ∆ φ = 5.5° and the out-of-plane texture was ∆ ω = 3.4°. All the LZO films had very smooth surfaces, but LZO films grown on YSZ/CeO2 and CeO2/YSZ/CeO2 buffer architectures had cracks. The highly textured LZO film grown on CeO2-seed buffered NiW tape was suitable for the epitaxial growth of YBCO film with high currents.
KeywordsLZO Buffer layer YBCO Texture
In the productive process of the second-generation high-temperature superconducting (HTS) strips, the epitaxial growth of highly textured buffer layer is important for the fabrication of YBa2Cu3O7−x (YBCO) superconducting film . Several technologies have been used to fabricate biaxially textured YBCO-coated conductors on metallic substrates, including inclined substrate deposition , ion beam-assisted deposition , and rolling-assisted biaxially textured substrate (RABiTS) . Among them, the RABiTS approach appears to be one of the most promising routes for scale-up processing of the second-generation HTS strips due to its easily controlled buffer growth, highly textured substrates, and cost-effective processing techniques such as chemical solution deposition (CSD) [5–7]. A wide variety of oxide materials, such as cerium oxide (CeO2), yttria-stabilized zirconia (YSZ), yttrium oxide (Y2O3), and La2Zr2O7 (LZO), have been successfully used as potential buffer layers for the preparation of YBCO-coated conductor [8, 9]. Among them, CeO2 (cubic, a = 5.41 Å, lattice mismatch CeO2/NiW = 8.2%, and YBCO/CeO2 = 0.52%) is a preferred and well-examined buffer layer that grows nicely due to its chemical stability and lattice match with the NiW substrate and YBCO superconducting layer . Unfortunately, epitaxial CeO2 films crack extensively when the thickness of CeO2 film exceeds 100 nm. Therefore, a stack of CeO2/YSZ/CeO2 or CeO2/YSZ/Y2O3 is commonly used as an effective buffer architecture satisfying the epitaxial growth of YBCO-coated conductors.
LZO films have been applied effectively as a buffer layer for YBCO-coated conductors prepared by various methods. From the results of previous studies, Ying et al. reported that they prepared CeO2/LZO and single LZO buffer layers for YBCO films by pulsed laser deposition (PLD) [11, 12]. Knoth et al. reported that they fabricated LZO buffer layer by CSD with the out-of-plane texture Δω = 7.2° and the in-plane texture Δφ = 6.9° . Wee et al. reported that they obtained LZO films by slot die coating of CSD with the out-of-plane texture of Δω = 5.7° and the in-plane texture of Δφ = 6.7° . However, the low texture and rough surface morphology of LZO film cannot satisfy the requirements of the epitaxial growth of high-performance YBCO film. Therefore, it is necessary to prepare an LZO film with high in-plane and out-of-plane textures and smooth surfaces in order to achieve an YBCO film with high critical current density (J c ).
In the present work, we fabricate highly textured LZO films on the CeO2, YSZ/CeO2, and CeO2/YSZ/CeO2 buffered NiW tapes under optimal conditions by radio frequency (RF) magnetron sputtering. The microstructure and surface morphology of LZO film are investigated. YBCO-coated conductors are prepared on the LZO/CeO2, LZO/YSZ/CeO2, and LZO/CeO2/YSZ/CeO2 buffer architectures, and we also discuss the superconductivity of YBCO-coated conductors.
Highly textured RABiTS tapes were used for the subsequent preparation of CeO2 seed layer, YSZ buffer layer, CeO2 cap layer, LZO film, and YBCO-coated conductor. The RABiTS tape was provided by evico magnetics GmbH in Dresden, Germany . The in-plane and out-of-plane textures of RABiTS tape used in this study were evaluated by the full width at half maximum (FWHM) of the φ-scan and ω-scan as ∆ φ = 6° to 7° and ∆ ω = 5° to 6°, respectively. The RABiTS tape was approximately 80 μm in thickness, and the average roughness value of surface roughness was less than 5 nm. A long RABiTS tape was cut into several short samples, which were 10 cm in length and 10 mm in width. Before the preparation of LZO film, all the CeO2 seed layer, YSZ buffer layer, and CeO2 cap layer were fabricated on these short samples by PLD. A KrF excimer laser (LPX220, Lambda Physik Inc., Fort Lauderdale, FL, USA) with a wavelength of 248 nm was used for CeO2, YSZ, and YBCO film deposition, and the incident angle between the laser beam and the target surface was 45°. Detailed experiments were reported in other works [16, 17]. From previous experiments , we obtained the samples of CeO2, YSZ/CeO2, and CeO2/YSZ/CeO2 buffered NiW tapes. We then fabricated LZO films on the CeO2, YSZ/CeO2, and CeO2/YSZ/CeO2 buffered NiW tapes by RF magnetron sputtering in Ar gas of 20 sccm at a substrate temperature of 600°C. Deposition pressure and applied RF power were fixed at 20 Pa and 100 W, respectively. The distance between the target and the substrate was 5 cm. Finally, we fabricated the YBCO films on the LZO/CeO2, LZO/YSZ/CeO2, and LZO/CeO2/YSZ/CeO2 buffer architectures at the substrate temperature of 800°C by PLD. The oxygen partial pressure was 50 Pa. The laser energy was 200 mJ, and the laser repetition rate was 50 Hz. After deposition, YBCO films were quickly cooled to room temperature and then annealed at 500°C in pure O2 gas for 1 h. More details can be found elsewhere [18, 19].
The structure and texture of LZO film were measured by a general area detector diffraction system (D8 Discover with GADDS, Bruker AXS, Inc., Fitchburg, WI, USA) with Cu-Kα radiation operated at 40 mA and 40 kV. The surface morphologies of LZO films were observed by optical microscopy (OM, BX51M, Olympus Corporation, Shinjuku-ku, Japan), high-resolution field emission scanning electronic microscopy (FEI Sirion 200, FEI Company, Hillsboro, OR, USA) operated at 5 kV, and tapping mode atomic force microscopy (AFM, Multimode 8, Bruker AXS, Inc., Fitchburg, WI, USA). The critical current (I c ) of YBCO-coated conductor was evaluated by the conventional four-probe method at 77 K and self field using a criterion of 1 μV/cm.
Results and discussion
Texture analysis data of LZO films grown on three different buffer architectures
Out-of-plane texture ∆ ω(deg)
In-plane texture ∆ φ(deg)
LZO (004) + CeO2(002)
LZO (222) + CeO2(111)
LZO films were grown on CeO2, YSZ/CeO2, and CeO2/YSZ/CeO2 buffered RABiTS tapes by RF magnetron sputtering. As a result, LZO films prepared on the single CeO2 and CeO2/YSZ/CeO2 buffer architectures were preferentially c-axis-oriented and highly textured. Only small LZO (222) peak was observed in the LZO film fabricated on YSZ/CeO2 buffered NiW tape. Both in-plane and out-of-plane textures of LZO film on the CeO2-seed buffered NiW tape were ∆ φ = 5.5° and ∆ ω = 3.4°. LZO films had very smooth surfaces, but microcracks were observed in LZO films grown on the YSZ/CeO2 and CeO2/YSZ/CeO2 buffer architectures. From the results discussed above, LZO film on CeO2-seed buffered NiW tape had the smoothest surface with the smallest RMS value and best in-plane and out-of-plane textures. The highly textured LZO film grown on CeO2-seed layer with smooth surface satisfied the requirements of epitaxial growth of YBCO-coated conductors with high currents.
Atomic force microscopy
chemical solution deposition
full width at half maximum
critical current density
pulsed laser deposition
rolling assisted biaxially textured substrate
root mean square
This research is sponsored by the Ministry of Science and Technology of China (under 863 project grant no. 2009AA032402), the Youth Fund of Natural Science Foundation of China (grant no. 11204174), the International Thermonuclear Experimental Reactor (ITER) Plan (grant. no. 2011 GB113004), and the Shanghai Science and Technology Committee (grant nos. 09DZ206000 and 11DZ1100402).
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