| Summary: | 碩士 === 明志科技大學 === 材料工程研究所 === 101 === The microstructure and ionic conductivity of reactively sputtered yttria-stabilized zirconia (YSZ) films in various sputtering modes are systematically studied using a closed-loop controlled system with plasma emission monitoring. A transition-mode sputtering corresponding to 45% of target poisoning produces a microstructure with ultrafine crystallites embedded in the amorphous matrix, which undergoes an abnormal grain growth upon annealing. The ionic conductivity of such a film is found to be higher than that of poisoning mode by about a half order of magnitude, which in turn higher than the YSZ bulk by about one order of magnitude. Formation of an ultra-large grained YSZ film is believed to be responsible for the former comparison due to the suppression of the grain boundary blocking effect, while the latter comparison can be attributed to the interface effect.
The residual stress of YSZ film is evaluated by cos2αsin2ψ methodology. The XRD data result in that an in-plane compressive stress is applied to the as-deposited and annealed film. Oxygen vacancy provides a diffusion site and dictates the diffusivity of oxygen ions, while the radius of oxygen vacancy is dependent on the residual stress surrounding it. When the residual stress is shifted from a compressive to a tensile mode by thermal annealing, the radius of oxygen vacancy will be increased and the ionic conductivity will thus be enhanced.
In addition, variations of ionic conductivities of amorphous Al2O3/ YSZ (30 nm) hetero-structures with different Al2O3 coating were also investigated in this work. We found that aluminum ion bombardment induced enhancement of film crystallinity, eliminate of dislocation and release of interfacial residual stress in a nano-YSZ film near the surface of the quartz substrate during deposition of a-Al2O3 layer. After annealed at 800oC for 2hrs, the ionic conductivity of specimens is observed to increase with an increase of amorphous Al2O3 film thickness from 10 to 30 um, because of increasing of tensile stress at the interface.
However, the ionic conductivities of (Al2O3/YSZ)n nano-films on quartz substrate are found to periodically decrease with an increase of Al2O3 layer amount and all lower than that of single YSZ film. It due to that Al atom diffuses to YSZ layer to restrict oxygen ion migration, based upon analyses of TEM and EDS observations.
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