HIGH-TEMPERATURE X-RAY DIFFRACTION STUDY OF PHASE EVOLUTION IN Ba2YCu3O6+x FILMS USING THE “BaF2 CONVERSION PROCESS”

In-situ high-temperature x-ray diffraction (HTXRD) was used to study the phase formation and reaction kinetics of the Ba2YCu3O6+x (Y-213) phase using the ex-situ “BaF2 conversion process”. Three sets of films on single crystal SrTiO3 substrates were prepared using the e-beam coevaporation technique of BaF2, Y and Cu targets. HTXRD studies were conducted on precursor films with thicknesses of 0.3 μm and 1.0 μm. The 0.3 μm films showed mainly (00l) texture whereas the 1.0 μm films showed a significant volume fraction of (h00) texture as well. A semiquantitative estimate of the texture fraction was obtained using the software TexturePlus. The growth of Y-213 and the consumption of BaF2 were found to be approximately linear in time. INTRODUCTION Today, the coated conductor technology holds tremendous promise for the superconductor industry. The key feature of the coated-conductor process is the deposition onto polycrystalline substrates (usually Ni or Ni-alloy) of superconductor films that are biaxially textured. Currently the principal techniques for preparing biaxially textured templates are: rolling-assisted bi-axially textured substrate (RABiTS) deposition [1], ion-beam assisted deposition (IBAD) [2], and inclined substrate deposition (ISD) [3]. The primary issue in the development of coated conductors for practical use is the application of this technology to the production of long-length tapes. The “BaF2” ex-situ process is currently one of the most promising methods for producing long-length tapes [4-7]. This 2-step process involves a low-temperature deposition of precursor layers using either high-rate e-beam deposition of Y, BaF2 and Cu onto a substrate, or open-air solution techniques, both of which are followed by a post-annealing at high-temperature in the presence of water vapor under reduced atmosphere. The process occurs between 700°C and 800°C. The overall reaction for the process can be written as: 2BaF2 + 1⁄2Y2O3 + 3CuO + 2H2O → Ba2YCu3O6.5 + 4HF. In order to control and optimize the BaF2 process, it is important to understand the detailed mechanism of the Y-213 phase formation. In the present work we report preliminary results of the high-temperature x-ray diffraction (HTXRD) studies aimed to understand the chemistry, mechanism, and kinetics involved in converting the BaF2-Y-Cu precursor films to Y-123 in the presence of H2O vapor. EXPERIMENTAL Film deposition The “BaF2” precursor films were prepared by electron beam evaporation of Cu and Y metal and BaF2 as described in [4]. The substrates were not intentionally heated during the deposition. Three sets of films, one with thickness of 0.3 μm and two with 1 μm, respectively, were Copyright©JCPDS International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 257