Diffusion Mechanism of Cations and Anions in Cation-Exchange Process for Fabrication of High-T c Superconducting HgBa 2
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Diffusion Mechanism of Cations and Anions in Cation-Exchange Process for Fabrication of High-Tc Superconducting HgBa2CaCu2O6+δ Films Y.Y. Xie1, J.Z. Wu1, T. Aytug2, D.K. Christen2 1 Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045 USA 2 Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA ABSTRACT We have investigated the diffusion mechanism of Hg and Tl cations in cation-exchange process that has been used successfully for fabrication of Hg-based high-Tc superconducting films. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) was employed to map the distribution of Tl and Hg cations in films quenched at different stages of the cation exchange process. SEM/EDS mappings showed that the nonuniform distribution of Hg is visible in micrometer size on the surface of samples quenched after short-time (~10min) Hg-annealing, but this nonuniformity disappears for longer-time (~45 min) Hg-annealing samples. This change could be ascribed to different stages in Hg diffusion -- the former is the early stage when Hg is concentrated in the channels and the latter Hg-cations have already diffuse to grains. Our experimental results hence suggest that Hg-cations channel through defects in the films then diffuse into grains along a-b planes and vise versa for Tl-cations. The diffusion mechanism of anions (oxygen) in post annealing has also been discussed. Fluorine-doped Hg-1212 films were post annealed in flowing oxygen at 300 oC for several hours. Magnetic measurement has shown these samples are comprised pure overdoped Hg-1212 phase with smooth and sharp transition below 120 K, but resistivity vs. temperature measurement shows a kink at ~123 K. This implies that the optimally doped Hg-1212 phase with higher Tc might be surrounded by overdoped Hg1212 phase and their contribution to magnetization is minimized, thus the possible diffusion mechanism for anions is also through grain boundaries at a much larger time scale compared to the cations.
INTRODUCTION Hg-based high-Tc superconductors (Hg-HTSs) are of great interest for electrical and electronic applications above the liquid N2 temperature because of their high Tc values up to 135 K [1-3]. In order to fulfill the requirements in practical applications, epitaxy in Hg-HTSs must be achieved for the films to carry high critical current density (Jc), low microwave surface resistance (Rs) and etc. To overcome the difficulties associated with the development of epitaxy in Hg-HTSs in conventional processes in which amorphous precursor contains mixture of Ba, Ca and Cu oxides, and Hg oxide in some cases, we recently developed a cation-exchange process [4,5] in which epitaxial Tl-based HTSs are used as precursor and the formation of Hg-HTSs is through a Tl to Hg cation exchange at lowered annealing temperatures without changing the overall crystalline structure of the precursor. Using this cation-exchange process, high Jc, low Rs and high microwave power handling capability have been obtained in epitaxial HgBa2CaCu2O6+
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