Microstructural characterization and quantification Essay

Mixed ionic-electronic conducting (MIEC) membrane provide high energy efficiency in pure oxygen separation[1], oxyfuel coal combustion[2] and petro-chemical process[3] due to its 100% selectivity to oxygen. Typical perovskite-type single phase MIEC membranes, such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ[4] and La0.6Sr0.4Co0.2Fe0.8O3-δ[5], achieve high oxygen fluxes but suffer from a carbonation or sulfating reaction induced phase instability at elevated temperature on exposure to CO2 or SO2[6]",[7]. Dual phase oxygen transport membranes (DP-M) consist of separate ionic and electronic conductors exhibit good chemical stability under flue gas condition with oxygen permeability that can be potentially improved by selecting high performance and stable individual conductors or optimizing the microstructures factors like phase volume fraction, grain size, and phase spatial distribution[8]",[9]",[10]",[11]. The selection of conductors can be flexible and straightforward as plenty of ionic and electronic conductors have been well developed[12]",[13]",[14]",[15]. However, microstructure aspects are more challenging as its complex diversity, what’s more, its influence on properties is not fully understood. For example, it is suggested that a percolative minor phase should possesses a volume fraction above 30% and possesses good homogeneity through the matrix[9], meanwhile, good connectivity of minor phase will be ensured when grain size ratio of matrix phase to minor phase is smaller or equal than 1[11]",[16]. Nevertheless, the minor phase is also possible to create long range free paths with volume fraction lower than 30%. High oxygen permeation was reported for Ce0.8Sm0.2O2--20 vol.% PrBaCo2O5+ since a fiber-shaped electronic conductive skeleton is formed across the membrane[17]. Furthermore, studies on Ce0.8Gd0.2O2--FeCo2O4 (CGO-FCO) composites indicated the highest permeation is achieved with 18.5 vol.% FCO phase, which own to the formation of grain boundary phases[8]. In addition to chemical properties, mechanical properties of DP-M are also expected to be affected by microstructural morphology like homogeneity, even there are currently no related reports, but other ceramic composites like Al2O3-ZrO2 with homogeneous microstructure are reported to show outstanding flexure strength and excellent Weibull modulus[18].

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