| Bi2Cu0.1V0.9O5.35-δ-based compounds with a layered perovskite-type structurehave superior oxygen-ion conductivity at400-600℃, making them promisingcandidate materials for moderate-temperature electrochemical devices, such asoxygen separation membranes, membrane reactors and oxygen sensor devices.Currently, developing advantageous synthetic methods for preparingBi2Cu0.1V0.9O5.35-δ-based oxides and improving their oxygen-ion conductingproperties remain as open questions. In this work, attempts were made towards theseaims with Bi2Cu0.1-xWxV0.9O5.35-δ(x=0-0.08) solid-solution compositions as thesubject. A facile and convenient wet-chemical method was developed to synthesizeBi2Cu0.1-xWxV0.9O5.35-δpowders. The contributing factors to structure, oxygen-ionconducting properties and order-disorder phase transition behaviors of the ceramicspecimens were examined with respect to synthetic route, preparation condition andchemical composition. Based on results derived from the experimental inspections,the correlation between oxygen-ion conducting properties and order-disorder phasetransition behaviors were discussed in view of oxygen-ion motion in the vanadatelayers of the layered structure.A citric-acid/EDTA synthetic technology was developed to synthesizeBi2Cu0.1-xWxV0.9O5.35-δpowders. The effects of synthetic conditions on the phasestructure and morphology of the resulting powders were examined. The resultshighlight the merits of employing citric-acid/EDTAcomplex ligands in synthesizingthe powders. The powders with a pure Aurivillius phase together with a superfine anduniform particle morphology (~80nm) were yielded at temperatures as low as450-500℃. The desired particle morphology enabled a substantially improvedsintering activity of the powders. The densification and microstructure of the ceramicspecimens were checked as a function of sintering conditions. The ceramic specimenssintered at640℃showed fine-grained microstructures (~2μm) and satisfactoryrelative densities around94%. The grain sizes of the ceramic specimens prepared by the citric-acid/EDTA method are much smaller than those (~12μm) of the ceramicspecimens prepared by the traditional solid-state reaction method, indicating theadvantage of the wet-chemical route in refining the microstructure ofBi2Cu0.1-xWxV0.9O5.35-δcermaic specimens.AC impedance spectroscopy method was adopted to study the oxygen-ionconductivity of Bi2Cu0.1V0.9O5.35-δceramic specimens prepared by thecitric-acid/EDTA method and traditional solid-state reaction method, respectively.Irrespective of the different preparation methods, a change of the activation energyfor oxygen-ion conductivity occurred within the temperature range of380-420℃insuccessive heating/cooling cycle for the ceramic specimens. The activation energies(~0.3eV) at high temperatures (>420℃) were considerably smaller than those (~0.6eV) in low temperature range (<340℃). On the other hand, the ceramic specimensprepared by the solid-state reaction method displayed an appreciable hysteresis ofoxygen-ion conductivity in successive heating/cooling cycle, whereas the specimensprepared by the citric-acid/EDTA method showed a fairly good consistence inoxygen-ion conductivity without a remarkable hysteresis behavior. The oxygen-ionconductivity values of the ceramic specimens prepared by the two methods,respectively, were basically identical at high temperatures. In the low temperaturerange, however, the specimens prepared by the citric-acid/EDTA method exhibitedlarger oxygen-ion conductivity values as compared with the specimens prepared bythe solid-state reaction method.Differential scanning calorimetry, thermal expansion analysis, X-ray diffractionanalysis and oxygen-ion conductivity analysis were combined to explore theinformation regarding the order-disorder phase transition of Bi2Cu0.1V0.9O5.35-δceramic specimens prepared by different methods and/or with varied thermal histories.It was identified that the ceramic specimens had a tetragonal symmetry at roomtemperature. In a successive heating/cooling cycle, the specimens experienced a γ′-γphase transition, corresponding to a temperature-induced subtle adjustment betweenpartial order and disorder for the distribution of oxygen vacancies in the vanadatelayers of the tetragonal structure. It was confirmed that the order-disorder transitionwas gradually realized in a temperature range near400℃via a position exchangebetween the oxygenions and vacancies in the vanadate layers. Moreover, the order-disorder phase transition behaviors were found to be heavily dependent on themicrostructure and thermal history of the specimens. This dependence is explicated inrelation to the kinetics of oxygen-ion motion in the vanadate layers.Based on the above outcomes, the correlation between the order-disorder phasetransition and oxygen-ion conductivity were discussed. Considering distinctthermodynamic attributes of the order-disorder phase transitions in heating andcooling processes, it is suggested that there is a difference in kinetics between theadjustments of oxygen-vacancy distribution in the two processes, affecting theoxygen-ion conducting properties and order-disorder transition behaviors of thespecimens in successive heating/cooling cycle as a result. Producing a fine-grainedmicrostructure is favorable for enhancing stresses among the fine grains, inducing alattice distortion of the structure and facilitatingthe motion of oxygenions in thevanadate layers. This case benefits the adjustment of oxygen-vacancy distribution inheating process.Meanwhile, this case favors fading the difference in oxygen-ionconductivity between heating and cooling processes, especially at low temperatures.This scenario is believed to be the reason of the consistent oxygen-ion conductivity ina successive heating/cooling cycle for the fine-grained ceramic specimens.The crystal structure and oxygen-ion conductivity of Bi2Cu0.1-xWxV0.9O5.35-δceramic specimens prepared by the citric-acid/EDTA method were investigated. Theresults indicate that the content of W6+is critical to the crystal structure ofBi2Cu0.1-xWxV0.9O5.35-δ. A tetrgonal structure was idetified for the specimens withx≤0.05, while the structure of the specimen with x=0.08turned to be orthorhombic.For the tetragonal ceramic specimen (x≤0.05), the oxygen-ion conducting propertieswere improved with increasing the content of W6+. This trend is explained withrespect to a reduction of the content of low-valence dopant Cu2+, which acts as a trapto the transport of oxygen ions in the vanadate layers. At600℃, the oxygen-ionconductivity of the ceramic specimen with x=0.05attained1.4×10-1S/cm, which isamong the most impressive conductivity data for tetragonal Bi2Cu0.1V0.9O5.35-δ-basedsolid solution compositions. |
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