Large-area quantification of BaCeO 3 formation during processing of metalorganic-deposition-derived YBCO films
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A method is described for the quantification of BaCeO3 formation during the growth of YBa2Cu3O7–x (YBCO) films on CeO2 buffer layers. The method is based on the selective etching of BaCeO3 followed by inductively coupled plasma (ICP) excitation spectroscopy. A 10% HNO3 solution, at room temperature, dissolved BaCeO3 and YBCO in minutes but did not significantly etch CeO2 films. ICP excitation spectroscopy was used to quantify the extent of the reaction over macroscopic areas of film (∼1 cm2). BaCeO3 peak areas were measured by x-ray diffraction (XRD) and calibrated to the ICP excitation spectroscopy results. XRD and ICP excitation spectroscopy results indicated that BaCeO3 formation through a metalorganic deposition (MOD)-derived CeO2 layer follows the parabolic growth law. Almost the entire ceria cap layer was consumed by the growth of BaCeO3 after 2 h at 760 °C in the MOD process examined. BaCeO3 growth was substantially slower at 725 °C; only 25 ± 3% of the ceria layer reacted. I. INTRODUCTION
YBa2Cu3O7–x (YBCO)-coated conductors require oxide buffer layers to protect the superconducting layer from the metal substrate. Numerous buffer-layer architectures have been proposed and demonstrated in coated conductors.1 Physical vapor deposition (PVD) and, recently, chemical solution deposition ceria layers have frequently been used as the “cap” layer in highperformance coated conductors.2–6 Ceria is a good choice for a cap layer because it has a good lattice match to YBCO, does not diffuse into the superconductor, and is a zirconia diffusion barrier.7 Ceria is, however, known to react with the barium in YBCO at elevated temperatures. CeO2 has little solid solubility for Ba, and the compound BaCeO3 is stable up to 1440 °C.8 X-ray diffraction (XRD),9 transmission electron microscopy (TEM),7,10–12 and Raman13 spectroscopy studies of quenched samples have long shown that this compound forms at rather low temperatures during the heat treatment of YBCO. Some yttrium may dissolve in BaCeO3 that is in contact with YBCO.8 There has been some discussion about whether BaCeO 3 forms before or after the nucleation of YBCO.7,10–12 Studies by the National Institute of Standards and Technology9 (NIST) have demonstrated that BaCeO3 follows parabolic growth kinetics during YBCO formation over a wide range of temperatures (700– 800 °C). a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0125 J. Mater. Res., Vol. 22, No. 4, Apr 2007
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TEM studies have noted that BaCeO3 grows into the CeO2 layer, showing that the diffusion of Ba controls the reaction.7 Most assessments have concluded that BaCeO3 must form after YBCO nucleates because the material is not textured and would serve as an off-axis nucleation site.7,11,12 No practical method for quantifying the total amount of BaCeO3 that forms during YBCO processing has been developed, despite the array of analytical techniques for detecting its presence. TEM and spectroscopy studies suffer from sma
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