Growth Mechanism and Opmization of MOD CeO 2 Buffer Layers for TFA YBa 2 Cu 3 O 7 /CeO 2 Multilayers
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Growth Mechanism and Opmization of MOD CeO2 Buffer Layers for TFA YBa2Cu3O7/CeO2 Multilayers Felip Sandiumenge, Andrea Cavallaro, Mariona Coll, Jaume Gàzquez, Teresa Puig, Alberto Pomar, Neus Romà, Narcís Mestres, Xavier Obradors Institut de Ciència de Materials, C.S.I.C., Campus de la UAB, 08193 Bellaterra, Catalonia, Spain. ABSTRACT The evolution from a partially oriented granular microstructure to a dense epitaxial one in CeO2 buffer layers deposited by metallorganic decomposition (MOD) on single crystal Ystabilised ZrO2 (YSZ) substrates, has been investigated. CeO2 buffer layers were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Reducing atmospheric conditions inhibit grain growth resulting in a nanometric granular microstructure with a high concentration of C impurities decorating grain boundaries and porosity. Oxidation and elimination of C species promotes grain growth resulting in a dense epitaxial film, as well as stabilizes otherwise energetically prohibitive polar (001) planes. Trifluoracetate (TFA) derived MOD YBa2Cu3O7 (YBCO) films deposited on optimized CeO2 buffers exhibit a sharp interface while the undesired reaction giving BaCeO3 is minimized. Jc values of 1.5 MA/cm2 and 14 MA/cm2 at 77K and 5K, respectively, are achieved.
INTRODUCTION Chemical solution deposition (CSD) has emerged as a highly attractive non-vacuum methodology for producing ceramic films [1-3]. In particular, at present CSD routes are being considered as a feasible alternative to vacuum techniques for the fabrication of coated conductors (CCs). CCs consist of a metallic substrate, one or various buffer layers, and the YBCO film. Among a large number of candidates, CeO2 constitutes one of the most intensively investigated layers for CC architectures. To date, CSD growth of CeO2 buffers on metallic substrates has been achieved on ion beam assisted deposition (IBAD) grown YSZ templates deposited on stainless steel tapes (YSZIBAD/SS) [4], and biaxially textured Ni-W [5]. However, despite its potentiality as buffer layer, the growth mechanism of CeO2 films by MOD appears complex and it is difficult to achieve quality surfaces for subsequent deposition of YBCO films. Moreover, since ceria is prone to react with YBCO to give BaCeO3 [6], it is mandatory to achieve very low surface roughness values in order to minimise the interphase area. Unexpectedly, RHEED analysis of MOD CeO2 buffers with an “apparent” sharp in plane texture, as revealed by φ-scans, revealed a completely disordered surface [4]. On the other hand, on Ni-W tapes [5], the MOD CeO2 buffer was subsequently buffered with a double layer of YSZ and CeO2 deposited by rf-magnetron sputtering, and therefore its performance for the direct deposition of YBCO could not be assessed. A mandatory constraint imposed by the use of metallic substrates comes from the need to avoid substrate oxidation. Thus, MOD CeO2 b
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