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Solution Processing of Lanthanum Zirconate Films as Single Buffer Layers for High Ic YBCO Coated Conductors Srivatsan Sathyamurthy, M. Parans Paranthaman, Hong-Ying Zhai, Sukill Kang, Hans. M. Christen, Claudia Cantoni, Amit Goyal, and Patrick. M. Martin.
Abstract—Sol-gel processing of lanthanum zirconate (La2 Zr2 O7 – referred to as LZO) buffer layers on biaxially textured nickel and Ni-3 at.% W alloy substrates using spin coating and a continuous reel-to-reel dip-coating unit has been studied. The epitaxial LZO films obtained have a strong cube texture and uniform microstructure. This coating and annealing process was repeated to get the desired buffer layer thickness. On these all-solution single buffer layer substrates, YBCO films were grown using pulsed laser deposition process. Critical current density about 2 MA cm2 at 77 K and self-field has been obtained on these samples. Continuous processing of these substrates and processing of high YBCO films on them will be discussed. Index Terms—All-solution buffer layers, coated conductors, lanthanum zirconate, single buffer layer.
I. INTRODUCTION Fig. 1. XRD patterns of multiple coats of LZO on textured Ni substrates.
A
focus of research in the area of high-temperature superconductivity (HTS) in recent years has been the development of second-generation wires also known as coated-conductors [1]–[3]. One of the leading processing approaches for the fabrication of coated-conductors is the rolling assisted biaxially textured substrates (RABiTS) approach [2]. In this approach, cube textured nickel substrates, obtained by cold rolling and recrystallization, act as a template for the epitaxial deposition of buffer layers and the YBCO superconductor. The buffer layers, apart from providing a structural template, also act as a chemical barrier between the metal substrate and the HTS coating. Using such architecture, sufficient biaxial texturing of the HTS layer has been obtained to avoid problems associated with weak-linked, high-angle grain boundaries [4]. To date, in the processing of high current coated conductors using RABiTS, the best results have been obtained reproducibly using two or three layer buffer architectures like
Manuscript received August 6, 2002. The U.S. DOE, Division of Materials Sciences, Office of Science and Office of Power Technologies-Superconductivity Program, and Office of Energy Efficiency and Renewable Energy sponsored this research. This research was performed at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC for the U.S. DOE under Contract DE-AC05-00OR22725. S. Sathyamurthy and M. P. Paranthaman are with the Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA (e-mail:
[email protected]). H-Y. Zhai, H. M. Christen, and C. Cantoni are with Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA. S. Kang, A. Goyal, and P. M. Martin are with Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA. Digital Object Identifier 10.1109/TASC.2003.811940
CeO /YSZ/CeO /Ni [5], [6]. The fabrication of this multi-layered buffer architecture, however, may present significant roadblocks to the scale-up of the process to long-lengths. Typically, deposition of the buffer layer stack would involve a combination of vacuum and nonvacuum deposition techniques coupled with the exposure of the samples to thermal cycling and ambient environment. These requirements could lead to the degradation of the individual buffer layers and add to the complexity, and cost of the over-all process. However, if a single buffer layer deposited using a scaleable technique is developed, it would significantly decrease the processing time and make the process simpler and more conducive to scale-up to long lengths. II. EXPERIMENTAL OUTLINE Stoichiometric quantities of lanthanum isopropoxide and zirconium-n-propoxide were dissolved in 2-methoxyethanol and refluxed in a Schlenk-type apparatus to obtain the precursor solution with 0.25 M cation concentration. All-solution buffered substrates were prepared by spin coating the precursor solution on textured Ni and Ni-3at. % W (NiW) substrates, and heat treating the films at 1100 C for 1 h in Ar/4%H atmosphere. The process was repeated to get thicker coating of the buffer layer. The YBa Cu O (YBCO) was deposited using pulsed laser deposition (PLD) at 790 C in 120 mTorr oxygen with average laser energy of 400-410 mJ. Resistivity and transport critical current density, , were measured using a standard four-point probe technique. The
U.S. Government work not protected by U.S. copyright.
SATHYAMURTHY et al.: SOLUTION PROCESSING OF LANTHANUM ZIRCONATE FILMS AS SINGLE BUFFER LAYERS
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Fig. 2. SEM microstructure of multiple coats of LZO on textured Ni substrates.
Fig. 4.
SEM microstructure of YBCO on LZO buffered Ni substrate.
Fig. 5.
RBS analysis of YBCO/LZO/Ni coated conductor.
Fig. 3. XRD pattern of YBCO on 60 nm all-solution LZO buffered Ni substrates.
phase purity of the samples was analyzed using X-ray diffraction (XRD). The film thickness and compositional homogeneity were analyzed using Rutherford Backscattering Spectroscopy (RBS) and the microstructural analysis of the samples was performed using a scanning electron microscope (SEM). III. RESULTS AND DISCUSSION A typical XRD pattern obtained from multiple coatings of LZO on Ni substrates is shown in Fig. 1. The presence of little or no detectable LZO (222) peak clearly evident from this figure shows that it is possible to coat thick films of LZO on nickel substrates using multiple coating. In Fig. 2, the microstructure of the multiple coated LZO film on Ni substrate is illustrated. This figure shows that the LZO films are fine-grained, dense and crack free with a smooth surface. Such a dense-smooth surface is essential for a buffer layer to act as an effective barrier layer. The XRD pattern obtained from YBCO deposited using PLD on multiple coated LZO films on Ni substrates is shown in Fig. 3. Along with a good -axis texture of the YBCO film, the figure also shows that there is no detectable amount of NiO in the sample. This shows that the LZO is indeed acting as a barrier layer and protecting the Ni substrate. A scanning
electron micrograph of the YBCO film on all-solution LZO buffered Ni substrate is shown in Fig. 4. This microstructure is typical of PLD-YBCO on metal substrates. Using RBS, the films were analyzed for thickness, composition and the quality of the interfaces. The RBS data and simulation of PLD-YBCO on all-solution LZO buffered Ni substrate is shown in Fig. 5. From this analysis, the thickness of the YBCO and LZO films were found to be 200 nm and 60 nm respectively, and both YBCO and LZO were found to be stoichiometric. Also, the good agreement between the data and simulation suggests that the interfaces and the surface of the film are smooth. The field dependence of the critical current density of the all-solution buffered samples on Ni and NiW substrates is compared with the corresponding performance of three layer buffers on similar substrates in Fig. 6. It is clearly evident from this figure that the performance of all-solution LZO buffers is comparable to that of the three layer buffers. Critical current density of over 1 MA cm on Ni substrates and about 2 MA cm on NiW substrates has been measured.
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about 75 A/cm width were measured for both Ni and NiW substrates. XRD analysis of these samples revealed very little NiO formation. This suggests that the 60 nm LZO buffer layer can indeed act as a good barrier layer even for thick YBCO deposition. However, the XRD also revealed that there was a significant portion -oriented YBCO in the thick films. This suggests that better optimization of the PLD-YBCO deposition for these buffer layers to control the fraction of -oriented YBCO could lead to higher critical currents on these all-solution buffers. IV. CONCLUSION
Fig. 6. Comparison of the J Vs B performance of YBCO on all-solution LZO buffers that of YBCO on tri-layer buffer architectures for Ni and NiW substrates.
Using a solution-based approach to buffer layer processing, coated conductors with a single, all-solution buffer have been fabricated. Critical current densities over 1 MA cm on Ni substrates and up to 2 MA cm on NiW substrates have been measured. Using XRD, it has been confirmed that the LZO films can act as good barrier layers and prevent the metal substrates from oxidation. On thick YBCO films processed on 60 nm thick LZO buffer layers on Ni and NiW substrates, critical current densities up to 75 A/cm have been measured. With better optimization of the PLD-YBCO, higher may be possible on thin all-solution LZO buffer layers. ACKNOWLEDGMENT The authors with sincere acknowledgment is extended to Oak Ridge Associated Universities for making this work possible. REFERENCES
Fig. 7. buffer.
Performance of thick YBCO films on 60 nm thick all-solution LZO
The performance of 60 nm thick LZO buffer layers on Ni and NiW substrates for thick YBCO deposition using PLD was evaluated. Fig. 7 shows the variation of and of these samples with YBCO thickness for NiW substrates. Similar dependence was observed for Ni substrates as well. Critical current
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