Diffusion mechanism of cations and anions in cation-exchange process for fabrication of high-T{sub}c superconducting HgBa{sub}2CaCu{sub}2O{sub}(6+δ)films

摘要

We have investigated the diffusion mechanism of Hg and T1 cations in cation-exchange process that has been used successfully for fabrication of Hg-based high-T1 superconducting films. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) was employed to map the distribution of T1 and Hg cations in films quenched at different stages of the cation exchange process. SEMIEDS mappings showed that the nonuniform distribution of Hg is visible in micrometer size on the surface of samples quenched after short-time (～1Omin) 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 T1-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 °C 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 T{sub}c 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.